Patent Application
20210079534
The present invention relates to an electrochemical apparatus and method for the conversion of dinitrogen (N2) into ammonia.
In one form, the invention relates to the cathodic reduction of dinitrogen.
In one particular aspect, the present invention is suitable for use in industrial production of ammonia.
It is to be appreciated that any discussion of documents, devices, acts or knowledge in this specification is included to explain the context of the present invention. Further, the discussion throughout this specification comes about due to the realisation of the inventor and/or the identification of certain related art problems by the inventor. Moreover, any discussion of material such as documents, devices, acts or knowledge in this specification is included to explain the context of the invention in terms of the inventor's knowledge and experience and, accordingly, any such discussion should not be taken as an admission that any of the material forms part of the prior art base or the common general knowledge in the relevant art in Australia, or elsewhere, on or before the priority date of the disclosure and claims herein.
Ammonia production is a highly energy intensive process, consuming 1-3% of the world electrical energy and about 5% of the world natural gas production. World production is currently around 200 million tonnes annually, reflecting the vast need for this chemical in agriculture, pharmaceutical production and many other industrial processes.
Ammonia is also being considered as a carbon-free solar energy storage material, due to its useful characteristics as a chemical energy carrier. Compared to other chemicals that could be used to store solar energy (such as hydrogen), ammonia is safe, eco-friendly and, most importantly, produces no CO2 emissions. Once stored in this form, the energy is readily recovered via the ammonia fuel cell.
For more than a hundred years, ammonia has been produced from dinitrogen and hydrogen in the presence of an iron based catalyst at high pressures and high temperatures according to the following nitrogen reduction reaction (NRR):
This process, known as the Haber-Bosch process has been of key importance in producing the inexpensive fertilisers that have supported the large global population growth over the past century. The Haber-Bosch process uses very high temperatures and pressures, and requires substantial amounts of energy in the form of natural gas, oil or coal for the production of the required hydrogen. Despite these drawbacks, the Haber Bosch process remains the predominant method for industrial ammonia synthesis.
Given the need to feed a growing world population, whilst simultaneously reducing global carbon emissions, it is highly desirable to break the link between industrial nitrogen-based fertiliser production and the use of fossil fuels. Therefore, there is intense interest in alternative pathways for ammonia synthesis, particularly those that have a reduced carbon footprint.
In recent years, electrochemical conversion of dinitrogen to ammonia has attracted particular interest because it can be economically feasible, easily scalable, would operate at ambient condition and could be coupled to renewable energy sources such as wind, hydro or solar. The electrochemical reduction process involves dinitrogen gas as a starting material, which is reduced to form ammonia, and uses various aqueous electrolytes or H2 gas as a proton (H+) source.
In the past, it has been hypothesised that the electrochemical reduction of dinitrogen largely depends on the structure, components, and surface morphology of the electrocatalyst. (van der Ham et al., Chemical Society Reviews 43, 5183-5191 (2014)). In general, prior art NRR studies conducted at ambient temperature and pressure have suffered the drawbacks of low Faradaic efficiency (FE<10%) and/or low ammonia yield rate (10−14 to 10−11 mol s−1cm−2). One problem lies in the low selectivity for NRR from hydrogen evolution reaction (HER) due to the competing proton reduction to hydrogen (H++2e−↔H2) such that the efficiency of electrons contributing to the dinitrogen reduction reaction is low. In principal, the efficiency can be improved by (i) limiting the proton transfer rate by reducing the proton concentration in bulk solution and/or increasing the barrier for the proton to the electrode surface and/or (ii) limiting the electron transfer by lowering the electron stream. (Singh et al., ACS Catalysis, 2017, 7, 706-709). In addition, although it remains debatable, the standard reduction potential of N2 is close (˜350 mV) to that for HER. Therefore, the H+ adsorption is greatly favoured due to the generation of an electric field during electrochemical reduction, thus rendering NRR challenging.
One effective way of limiting the proton rate is to use aprotic electrolytes such as aprotic liquid salts, which not only significantly increases the dinitrogen solubility but also reduce the protons present in the electrolytes. (Armand et al., Nat Mater, 2009, 8, 621-629). Introducing aprotic liquid salts such as ([P6,6,6,14][eFAP] and [C4mpyr][eFAP]) with high dinitrogen solubility can significantly increase the selectivity for dinitrogen reduction. These salts, however, have quite high viscosities (400 mPa·s and 204 mPa·s at 298K) and low conductivities, which limits the mass transfer and results in a low current density.
One of the most notable early attempts to carry out electrochemical NRR at ambient temperature and pressure was conducted by Kordali et al., using a Pt electrode as a cathode. The study achieved an ammonia yield rate of 3.12×10−12 mol s−1 cm−2, with a FE of 0.28% at 20° C. The reason for the low FE was the high catalytic activity of Pt electrode for HER. Therefore, a number of more recent studies have been directed towards the development of novel advanced catalysts for NRR at ambient temperature and pressure based on Sabatier's principle to disfavour H+ adsorption. Materials that exhibit weak adsorption for H+ (ΔGM-H>0) have been reported to yield relatively improved NRR FE and ammonia yield rates. For example, Bao et al. demonstrated the use of high-index faceted gold nanorods (ΔGM-H>0.3 eV), as NRR catalyst, delivering a maximum ammonia yield rate of 2.69×10−11 mol s−1 cm−2 with a FE of 4.00%. (D. Bao et al., Advanced Materials, 2017, 29, 1604799.)
Accordingly, there is an ongoing need to improve the process for electrochemical reduction of dinitrogen to ammonia in terms of improved yield and improved FE.
An object of the present invention is to provide an improved electrochemical process for production of ammonia.
Another object of the present invention is to provide an improved method for cathodic dinitrogen reduction.
A further object of the present invention is to alleviate at least one disadvantage associated with prior art processes for ammonia production by dinitrogen reduction.
It is an object of the embodiments described herein to overcome or alleviate at least one of the above noted drawbacks of related art systems or to at least provide a useful alternative to related art systems.
In a first aspect of embodiments of the present invention there is provided a method for the electrochemical reduction of dinitrogen to ammonia, the method comprising the steps of:
wherein the dinitrogen is reduced to ammonia at the cathodic working electrode.
Where used herein the term ‘low viscosity’ refers to viscosity values between 0.6 and 40.0 mPa/S measured by the falling ball technique at 25° C. Furthermore, where used herein the term ‘low viscosity’ refers to viscosity values between 0.4 and 25.0 mPa/S measured by the falling ball technique at 50° C.
As suitable solvents for use with the present invention do not have high ion conductivity on their own, the term ‘supporting high conductivity’ refers to the combination of solvent with the liquid salt.
Where used herein the term ‘low ionic conductivity’ refers to a salt or salt/solvent mixture having conductivity values between 1×10−4 and 1×10−2 S/cm measured by AC impedance spectroscopy at 25° C. Where used herein the term ‘high ionic conductivity’ refers to a salt or salt/solvent mixture having conductivity values between 2×10−4 and 4×10−2 S/cm measured by AC impedance spectroscopy at 50° C. Hewlett Packard 4284 LCR meter was used to measure conductivity by using AC impedance spectroscopy over a range of 20 Hz to 1 MHz.
Preferably, the one or more liquid salts is selected from the one or more liquid salts described herein below.
The method may additionally include a step (3) comprising collecting ammonia generated at the cathodic working electrode, separating the ammonia from other liquids and gases present by using a separate trap or separation unit.
In a second aspect of embodiments described herein there is provided a cell for electrochemical reduction of dinitrogen to ammonia, the cell comprising:
wherein dinitrogen introduced to the cell is reduced to ammonia at the cathodic working electrode in the presence of a source of hydrogen.
The counter electrode may be placed in the same electrolyte as the cathodic working electrode or alternatively, it may be separated by some means such as an electrolyte membrane or separator material. In another embodiment the counter electrode may be located in a compartment, which optionally contains a different electrolyte medium, such as an aqueous solution.
The counter electrode reaction may comprise water oxidation or another advantageous oxidation reaction well known to the person skilled in the art such as sulphite oxidation.
In a further aspect of embodiments described herein there is provided a cell for electrochemical reduction of dinitrogen to ammonia, the cell comprising:
In a further aspect of embodiments described herein there is provided a cell for electrochemical reduction of dinitrogen to ammonia, the cell comprising:
In a preferred embodiment, the electrolyte further comprises one or more solvents, preferably one or more organic solvents having low viscosity and high conductivity as herein defined.
Preferably the cations are selected from the group comprising: 1-(3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluorooctyl)-2,3-dimethylimidazolium, 1-(3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluorooctyl)-3-methylimidazolium, 1-ethyl-3-methylimidazolium, 1-butyl-methyl pyrrolidinium, trihexyl tetradecylphosphonium, tributyl-(4,4,5,5,6,6,7,7,8,8,9,9, 10,10,11,11,11-heptadecafluoro undecyl)-phosphonium, tributyl-(3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluoroctyl) phosphonium, N-ethyl-N,N,N-tris(2-(2-methoxyethoxy)ethyl)ammonium and 1-(2-methoxyethyl)-1-methyl pyrrolidinium, 1-methyl-pyrrolidinium, 1-(4,4,5,5,6,6,7,7,8,8,9,9,10,10,11,11,11-heptadecafluoroundecyl-1-methylpyrrolidinium, and trihexyl (4,4,5,5,6,6,7,7,8,8,9,9,10,10,11,11,11-heptadecaluoroundecyl) ammonium.
Preferably the anion is selected from the group comprising: tris(pentafluoroethyl) trifluorophosphate, tris(perfluoroethyl)trifluoro phosphate, bis(trifluorosulfonyl)imide, nonafluorobutane sulfanoate, nonafluorobutane sulphonate, tridecafluorohexane sulfonate, heptadecafluorooctane sulfonate, 1,1,2,2,-tetrafluoroethane sulfonate, trifluoromethane sulphonate, nonafluoropentanoate, pentadecafluoro octanoate, and tetrakis((1,1,1,3,3,3-hexafluoropropan-2-yl)oxy)borate, tetrakis((1,1,1,3,3,3,-hexafluoropropan-2-yl)oxy) borate, and heptadecafluorononanoate.
In a particularly preferred embodiment the liquid salt is selected from one or more of the group comprising:
In a particularly preferred embodiment of the cell for electrochemical reduction of dinitrogen to ammonia, the reduction of dinitrogen to ammonia occurs principally in a region adjacent a three phase boundary on the working surface of the cathodic working electrode. Typically, the nanostructured electrocatalyst is applied to the working surface of the cathodic working electrode to create the gas/electrolyte/metal three-phase boundary region where electrolysis principally takes place.
Typically, a continuous current will pass between the cathodic working electrode and the counter electrode, however in some applications such as a wind power photovoltaic panel power driven process an intermittent or pulsed current may be suitable.
The cell for electrochemical reduction of dinitrogen to ammonia may include other features well known to those in the art for carrying out electrolytic reactions and controlling the current between the electrodes. For example, the cell may be adapted to control the temperature or pressure of operation using well known means such as heaters, cooling units or pressurising means. In addition, the cell may include an ultrasonic generator for generating sound waves of energy greater than 20 kHz.
The cell for electrochemical reduction of dinitrogen to ammonia preferably includes gas flow layers having the function of allowing introduction to the cell of a stream of gas comprising nitrogen and hydrogen or water vapour, and exit of gas containing ammonia. The ammonia may optionally be collected external to the cell.
In a further embodiment an assembly may be formed when two or more cells according to the present invention are stacked in series. One or more of the stacked cells may additionally be folded or rolled. Gas can be introduced at any convenient location including from the “end” of the longest dimension of the assembly or from either side of the assembly.
Using the method and cell of the present invention, it is possible to achieve high-efficiency electrochemical reduction of nitrogen into ammonia at ambient conditions. Without wishing to be bound by theory it is believed that by careful choice of certain organic solvents, the viscosity of liquid salts can be suitably decreased while the mass transportation in the liquid salt can be increased.
The electrolyte typically comprises (a) one or more liquid salts, preferably in combination with (b) one or more organic solvents having low viscosity and high ionic conductivity. The electrolyte may also comprise a controlled amount of water.
The electrolyte is typically a liquid or gelled liquid at the temperature at which the dinitrogen reduction is performed.
When the electrolyte according to the present invention includes a solvent, it will be readily apparent to the person skilled in the art that certain species will be unsuitable. For example, a highly fluorinated hydrocarbon such as a straight perfluoroalkane (e.g. perfluorooctane) would not readily dissolve the liquid salts of the present invention. It will also be apparent to the skilled person that small fluorinated species such as perfluoroalkyl chains would also be unsuitable because they only exist as gases at ambient temperature. Furthermore, reactive solvents, such as acids, alcohols and those having other halogen substituents would be clearly unsuitable.
The preferred solvents for use in the present invention have low viscosity and provide high ionic conductivity. Preferably, the boiling point of the solvent should not be so low that volatility is an issue when bubbling dinitrogen through the electrolyte.
In general, the solvent has an appropriate balance of organic moieties and polarity. Hence fluoroalkyl chains, fluorinated esters, ketones, ethers, sulfoxides and phenyls are potentially suitable or can be adapted. For example, the proportion of fluorine anions in the solvent could be reduced, by introducing more functional moieties (e.g. oxygen) to the solvent structure. Suitable solvents would include the following (and their variants): 1,1,1,6,6,6-hexafluorohexane, methyltrifluoroacetate, ethyltrifluoroacetate, octafluorotoluene, trifluorotoluene, (2,2,2-trifluoroethoxy)pentafluorobenzene, 1,2,4,5-tetrafluorobenzene, 1,3,5-tris(trifluoromethyl)benzene, 1,3-bis(1,1,2,2-tetrafluoroethoxy)benzene, 1,3-bis(trifluoromethyl)benzene, 1-fluoro-4-(trifluoromethoxy)benzene, 2-fluorobenzotrifluoride and pentafluorobenzene.
The solvents may be at least partially fluorinated, preferably fully fluorinated. In a preferred embodiment the solvent is 1H,1H,5H-octafluoropentyl 1,1,2,2-tetrafluoroethyl ether (FPEE) or 1,1,2,2,3,3,4-heptafluorocyclopentane (HFCP) or trifluorotoluene (TFT).
Preferably, the solvent is present in the liquid salt at a level between 0.1 mol % and 90.0 mol %, more preferably between 0.2 mol % and 20.0 mol % and most preferably between 0.5 mol % and 50.0 mol %.
In a preferred embodiment both the solvent and the liquid salt are fluorinated.
In one embodiment, the external source of hydrogen is a controlled amount of water that is continuously introduced into the electrolyte or gas stream. In another embodiment the hydrogen source may also be H2 gas that is introduced as an anode reactant, producing protons in the electrolyte. In a further embodiment, the source of hydrogen is an acid such as sulphuric acid where the product is intended to be separated as an ammonium slat such as ammonium sulphate.
The thermodynamic activity of hydrogen (as protons) in the electrolyte can be controlled, for example, by additions of amounts of acid or alkaline components to the electrolyte formulation. One preferred method of doing so is to add the acid of the liquid salt anion or the hydroxide salt of the liquid salt cation to respectively raise of lower the electrolyte proton activity.
Where used herein the term liquid salt is intended to refer to an ion conductive medium that is liquid or that can be rendered liquid by mixing with one or more solvents at the temperature of use and that contains one or more salts (each of which may be solid or liquid in their pure states). The salts may be selected from any suitable metal salts, organic salts, protic salts, complex ion salts or the like.
The liquid salt can also be formed by mixing several salts to create the desired characteristics.
The electrolyte comprising the one or more liquid salts provides an ion conductive medium in which the process reactions occur. The electrolyte of the present invention offers the advantage that some gases are more soluble in these electrolytes comprising the one or more liquid salts than in water. In particular, some electrolytes comprising the one or more liquid salts can provide an elevated solubility for N2 gas (compared to aqueous and other electrolytes) thereby increasing the concentration of N2 at the electrolyte/electrode interface.
In a preferred embodiment the cation and/or anion of the liquid salt is fluorinated or perfluorinated. The preferred solvent preferably (i) dissolves the liquid salt at levels between 0.1 mol % and 90.0 mol % and (ii) is sufficiently electrochemically stable in the potential range where the nitrogen reduction reaction takes place.
It is also particularly preferred that the liquid salt and/or the solvent has a relatively high nitrogen solubility (compared, for example, to liquid salts of the prior art such as imidazolium salts and nitrile based anions such as dicyanamide). Preferably the nitrogen solubility greater than about 100 mg/L.
The electrolyte is typically in the form of a layer. Preferably the electrolyte includes a spacer or electrolyte membrane (which itself may act as an electrolyte), for example a polymer electrolyte such as Nafion™ or a Nafion™-liquid salt blend, or a gelled liquid salt electrolyte, or is an electrolyte soaked into a porous separator such as paper or Celeguard™.
In a further embodiment of the present invention, there is provided an electrolyte membrane comprising a thin layer of material combined with one or more liquid salts as herein described for use in the cell of the present invention.
In a further embodiment of the present invention, the electrolyte flows through a porous electrode or over the surface of a non-porous electrode such that the N2 is carried continuously to the electrode and the ammonia produced is continuously removed from the cell, to be separated from the electrolyte in a subsequent process.
Preferably, the catalyst for electrochemical reduction according to the present invention comprises nanostructured materials having a high electrochemical working surface area, as indicated by a double layer capacity, measured in an adjacent electrolyte layer of greater than 0.1 mF/cm2 and preferably greater 1 mF/cm2.
Preferably, the nanostructured catalyst comprises one or more metals in the form of elemental metal or inorganic compounds comprising one or more metals. The nanostructured catalyst may be in the form of discrete particles or sheet or film or three dimensional structure. The nanostructured catalyst embodies morphological features that may be of any shape with at least one dimension in the range of 1 nm to 1000 nm.
Suitable metals include any of the transition metals or lanthanide metals including Fe, Ru, Mo, Cu, Pd, Ti, Ce and La as well as their alloys with other metals and semimetals.
The aforementioned metals may be surface decorated with an oxide or a sulfide of the metal, or a composite may be formed of the metal with its oxides or sulfides.
The catalyst may also comprise a metal complex consisting of two metals bridged by sulfides. Preferably, the metals are Fe and Mo.
For example, the catalyst may be a nanoparticle film prepared as a composite material with binder to form a film. As such, the nanoparticle film may be prepared by a cyclic voltammetry or a pulsed voltammetry electrodeposition method.
In another preferred embodiment the catalyst may comprise conductive polymer materials such as poly(3,4-ethylenedioxythiophene) (PEDOT).
In yet another preferred embodiment the catalyst may comprise doped carbon materials, particularly carbons doped with N and/or S or metal atoms or particles.
The catalyst is preferably supported or decorated on an electrically conductive, chemically inert support. Suitable supports include fluorine-doped tin oxide, graphene, reduced graphene oxide, porous carbons, carbon cloth, carbon nanotubes, conducting polymers and porous metals.
Faradaic efficiency is a particular deficiency of related processes of the prior art. Faradaic efficiency can be used to describe the fraction of electric current that is utilised in the N2 reduction reaction. The remaining fraction that is, (100−Faradaic efficiency) %, is consumed in undesirable side reactions including the production of H2 and hydrazine. These bi-products represent wasted energy and may also require complex separation methods from the desired product. It is one of the purposes of the present invention to provide a method of relatively high Faradaic efficiency preparation of ammonia.
Other aspects and preferred forms are disclosed in the specification and/or defined in the appended claims, forming a part of the description of the invention.
In essence, embodiments of the present invention stem from the realization that the efficiency of ammonia production can be improved by use of a hybrid electrolyte, that is, a combination of specific liquid salts and organic solvents that increase mass transport during the electrochemical reduction reaction.
Advantages provided by the present invention compared with the processes of the prior art comprise the following:
Further scope of applicability of embodiments of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the disclosure herein will become apparent to those skilled in the art from this detailed description.
Further disclosure, objects, advantages and aspects of preferred and other embodiments of the present application may be better understood by those skilled in the relevant art by reference to the following description of embodiments taken in conjunction with the accompanying drawings, which are given by way of illustration only, and thus are not limitative of the disclosure herein, and in which:
318K; and
328K);
318K;
328K and
338K) for a range of mixtures of HFCP/[C4mpyr][eFAP];
318K;
1328K and
338K) for a range of mixtures of FPEE/[C4mpyr][eFAP];
318K;
328K and
338K) for a range of mixtures of HFCP/[C4mpyr][eFAP];
318K;
328K and
338K) for a range of mixtures of FPEE/[C4mpyr][eFAP];
Where used herein the abbreviations refer to the following chemical species:
The present invention will be further described with reference to the following non-limiting examples. These examples explore a range of organic solvents for the development of a new electrolyte, based on a combination of certain liquid salts and organic solvents, having low viscosity, high conductivity and high N2 solubility.
The method of the present invention utilising an electrolyte based on the combination of certain liquid salts and solvents cannot only significantly increase the solubility of dinitrogen but also increase mass transport during the reduction reaction and at the same time lower the rate of undesirable competing reactions such as H2 production.
A series of organic solvents as listed in Table 1 were examined as electrolyte solvents. The ionic conductivity was measured by electrochemical impedance spectroscopy (EIS) using a dip-cell connected to temperature controller. Among the organic solvents, triflourotoluene shows a good compatibility with NRR system which was systematically studied with different mole fraction additions (XTFT=0˜0.96) for the electrochemical dinitrogen reduction reaction.
All chemicals were used without any further purification unless otherwise stated. 1-butyl-1-methylpyrrolidinium tris(pentafluoroethyl) trifluorophosphate ([C4mpyr][eFAP]) was purchased from Merck. The liquid salt was pre-treated by known procedures. The [C4mpyr][eFAP] was preconditioned to be slightly alkaline by washing with 1 mM KOH three times, then it was dried in vacuum for 8 hrs followed by further drying with molecular sieves. The dried liquid salt was then transferred to a vial through which dinitrogen was bubbled (10 mL min−1) for at least 12 h to fully equilibrate the liquid with respect to dinitrogen and water before use in experiments.
An iron (Fe) catalyst electrode was prepared by an electrodeposition method. In a typical experiment, the electrolyte for the deposition containing 10 mM iron sulfate (FeSO4), 10 mM citric acid and 20 mM sodium hydroxide (NaOH). Stainless steel (SS) cloth was used as the substrate. The deposition was conducted in a three-electrode system by using a SS cloth, a saturated calomel electrode (SCE) and a titanium mesh as the working, reference and counter electrodes, respectively. The Fe nanostructured catalyst was electrodeposited by cycling the potential for 10 times between −1.8 V and −0.8 V at a sweep rate of 0.02 V s-1. After deposition, the sample was rinsed with distilled water thoroughly and dried with nitrogen.
Electrochemical reduction of dinitrogen was conducted in a three-electrode configuration with dinitrogen gas flowing over the working electrode. Cyclic voltammograms were measured in a single compartment cell, while ammonia production at fixed potential was conducted by isolating the platinum counter electrode with a glass frit in a typical H-cell arrangement. The ionic conductivity was measured by EIS using a dip-cell connected to temperature controller. All electrochemical deposition and electrochemical experiments were carried out at ambient condition.
For the examined aprotic solvents, octafluorotoluene (OFT) exhibited only partial miscibility (up to 0.8 mole fraction), while trifluorotoluene (TFT) showed good miscibility at any mole fraction. The compatibility of these miscible solvents was tested using the ammonia detection method.
Most of the protic solvents, except 3-methoxypropionitrile (MPN), interfere with the ammonia detection method. Those skilled in the art would understand that these solvents can be examined further if alternative ammonia detection methods are adopted to avoid the interference. The aprotic solvents, TFT and OFT both exhibited good compatibility with the ammonia detection method.
Another important criterion is that the solvent meets the requirement of electrochemical stability in the potential range where dinitrogen reduction is conducted. Although MPN has been used in many electrochemical reactions as a stable solvent, it decomposed when a large amount of ammonia was detected in blank argon gas experiments. However, the TFT exhibited good electrochemical stability. Thus, based on the overall performance of the examined solvents, TFT was selected for further study.
Viscosity and conductivity are important factors for electrochemical reactions, as they affect mass transport and electron transfer performance. A range of mixtures of liquid salt ([C4mpyr][eFAP]) and TFT were prepared with different TFT solvent mole fractions (xTFT=nTFT/(nTFT+nLS)). The density and viscosity of the mixtures were measured as shown in
The addition of TFT into the liquid salt also substantially changed the conductivity as shown in
The general method used for determining N2 solubility is as follows: N2 solubility is measured using dual-volume apparatus based on the isochoric saturation method. In this method, a ballast chamber is used to deliver a known amount of gas to the equilibrium chamber containing the degassed liquid sample. When pressure equilibrium is established between the liquid sample and its headspace, the solubility of N2 in the liquid sample can then be determined.
Solution 1 (
From the obtained data, plotting mole fraction versus the nitrogen solubility does not show a consistent, steady trend. However, plotting mass fraction versus the nitrogen solubility shows a very clear linear trend. This observation could suggest that in this system, nitrogen solubility is dominated by physical absorption rather than chemical absorption.
Cyclic voltammetric (CV) measurements were carried out on the stainless steel cloth supported Fe electrodes in the hybrid electrolytes with different composition. The results are shown in
Dinitrogen electrochemical reductions were conducted on the stainless steel supported Fe electrode in the above electrolytes and
The Faradic efficiency for ammonia synthesis increased with the addition of TFT until up to xTFT=0.83, but further increases in the TFT concentration reduced the efficiency. This means that reasonable TFT addition can improve the selectivity for dinitrogen reduction. It is worth noting that the yield of ammonia increased significantly in the presence of TFT. For example, the yield rate at xTFT=0.83 is 48 mg h−1m−2, which is more than three time of the number in pure liquid salt (14 mg h−1 m−2). This indicates that the addition of TFT can significantly improve the ammonia electrochemical synthesis.
In an electrochemical dinitrogen reduction reaction, two main performances are important for practical application: (i) the ammonia selectivity or Faradic efficiency and (ii) the yield rate. The Faradic efficiency reflects the energy conversion efficiency from electricity to chemicals, while the yield rate is more important for industry as it directly relates to the production capability. Previous inventions, such as those described in International Patent Application PCT/AU2017/000036 have successfully increased the Faradic efficiency by introducing fluorine-based liquid salt electrolytes. The present invention significantly improves the ammonia yield rate of those electrolytes by introducing solvents into the liquid salt electrolyte.
As illustrated in
In addition, it should be noted that the Faradic efficiency for ammonia synthesis increased rather than decreased. This may be due to TFT also having high dinitrogen solubility, given that TFT is a fluorous organic liquid that has strong interaction with dinitrogen to promote the dinitrogen solubility. Other fluorous solvents such as octafluorotoluene (listed in Table 1A) are also promising solvents for dinitrogen reduction. Table 1B further illustrates the N2 solubilities of various fluorous solvents.
As the temperature increases, the viscosity of the solvent/liquid salt mixture decreases. Solvent/liquid salt mixtures that exhibit higher viscosities are affected by temperature more than those mixtures with low viscosity. For example, the viscosities of HFCP/[C4mpyr][eFAP] (χ=0.56) are 26.5 mPa s and 8.0 mPa s at 298K and 338K respectively, showing a decrease of 18.5 mPa s. On the other hand, the viscosities of HFCP/[C4mpyr][eFAP] (χ=0.96) are 2.1 mPa s and 1.0 mPa s at 298K and 338K respectively, showing a decrease of 1.1 mPa/s.
The addition of fluorous solvent affects the conductivity when added to [C4mpyr][eFAP]. Increasing the amount of fluorous solvent enhances the conductivity which may be the result of the viscosity decreasing. However through the addition of more solvent, the conductivity decreases as a result of a decline in the number of species in the electrolyte to support fast charge transfers. There is an observed peak at which the mole fraction of solvent/[C4mpyr][eFAP] reaches an optimum. For the HFCP/[C4mpyr][eFAP] mixtures, this optimum peak occurs at a mole fraction of 0.87. On the other hand, the peak conductivity for FPEE/[C4mpyr][eFAP] occurs at a mole fraction of 0.80 at 298K; furthermore this peak shifts upon increasing temperature suggesting a change in interactions and contributions to conductivity.
Table 2 lists the compounds described herein as examples and their solubility in a series of exemplary solvents.
With particular reference to Table 2 the general method used for determining the solubility limit of a salt in a solvent is as follows. To make up a salt/solvent electrolyte, an aliquot of salt was added, in steps, to a known mass of solvent. After each aliquot addition, the salt/solvent electrolyte was shaken in a vortex mixer and allowed to settle. Dissolution of a salt in solvent was determined by observing a single phase and the lack of any liquid/solid particles in this phase after the electrolyte is shaken. Further aliquots of salt were added until dissolution no longer occurred. Unless described otherwise, the solubility of a salt in a solvent is represented by Max(Xα) reported as mole fraction of the salt (Xα) in the mixture.
Where a salt/solvent mixture is noted as “insoluble” this indicates that the solubility is lower than of interest in electrochemical applications (where Max(Xα))<0.01).
Where a solubility limit was not found but is at least sufficient for electrochemical use, the word “soluble” is noted in Table 2 and the Example. This includes systems where the two components are soluble in all proportions.
It is noted that some salt/solvent electrolytes were not electrochemically tested at their determined solubility limit. In these cases, the mole fraction used is recorded as “Xα” in Table 2 below and in the Examples.
Other solvents tested include octafluorotoluene (OFT), perfluoro-1-butanesulfonyl fluoride (PBSF), perfluoro-1-octanesulfonyl fluoride (POSF), perfluorohexane (PFHex), perfluorooctane (PFOct), perfluoromethyldecalin (PFMD) and 1,1,1,5,5,6,6,6-Octafluoro-2,4-hexanedione (OFHD).
The following examples set out the synthetic methods, characterisation and test results for the following compounds used in this study:
In some of the following examples, solubility tests are carried out. Examples marked ‘A’ relate to mixtures with FPEE and Examples marked ‘B’ relate to mixtures with HFCP.
Abbreviation: [P6,6,6,14][C4P9CO2].
Synthetic Procedure: The commercially available starting material, [P6,6,6,14][Cl] (3.87 g, 7.45 mmol) was dissolved in 40 ml of distilled water at room temperature and nonafluoropentanoic acid (1.94 g, 7.42 mmol) was added. The reaction mixture was stirred for 24 h under nitrogen gas. The white cloudy solution was extracted with dichloromethane three times. The dichloromethane extract was washed with water five times followed by a single wash with potassium hydroxide (1 mM). Dichloromethane was removed in vacuo to afford a colourless liquid (4.91 g, 95%).
Characterisation—1H NMR (400 MHz, CDCl3) δ ppm: 0.85-0.93 (12H, t, 4CH3), 1.25 (20H, m, 10CH2), 1.27-1.35 (12H, m, 6CH2), 1.39-1.55 (16H, m, 8CH2), 2.12-2.24; 19F NMR (368 MHz, CDCl3) δ ppm: −81.5 (3F, m, CF3), −115.1 (2F, t, CF2), −122.0 (2F, m, CF2), −126.5 (2F, t, CF2). ES-MS: ES+ m/z 483.5 P6,6,6,14+, ES− m/z 263 C5F9O2−, 219 C5F9−.
Solubility: The solubility of [P6,6,6,14][C4F9CO2] in FPEE is Xα=>0.40.
Electrochemistry: The electrochemical method was the same as Example 5A except that the electrolyte was Xα 0.13 [P6,6,6,14][C4F9CO2] in FPEE. The reference electrode was Ag/Ag triflate dissolved in [C4mpyr][eFAP]. A constant potential of −2V vs the reference electrode was applied for two hours to determine the NH3 formation rate while the solution was bubbled with N2 gas. The gas was then bubbled through two 1 mM H2SO4 traps to collect ammonia.
A yield rate of 2.14×10−12 mol/cm2/s was found corresponding to a faradaic efficiency of 2.5%.
Solubility: The solubility of [P6,6,6,14][C4F9CO2] in HFCP is Xα=>0.28.
Electrochemistry: The electrochemical method was the same as Example 5A except that the electrolyte was Xα 0.08 [P6,6,6,14][C4F9CO2] in HFCP. The reference electrode was Ag/Ag triflate dissolved in [C4mpyr][eFAP]. A constant potential of −2V vs the reference electrode was applied for two hours to determine the NH3 formation rate while the solution was bubbled with N2 gas. The gas was then bubbled through two 1 mM H2SO4 traps to collect ammonia.
A yield rate of 2.2×10−12 mol/cm2/s was found corresponding to a faradaic efficiency of 1.5%.
Abbreviation: [P6,6,6,14][C6P13SO3].
Synthetic Procedure: see procedure for [P6,6,6,14][C5F9O2]. Potassium tridecafluorohexane sulfonate (3.04 g, 6.94 mmol) was added to [P6,6,6,14][Cl] (3.59 g, 6.91 mmol), and water (40 mL), and after purification, afforded a colourless liquid (5.76 g, 96%).
Characterisation—1H NMR (400 MHz, CDCl3) δ ppm: 0.87-0.92 (12H, t, 4CH3), 1.25 (20H, m, 10CH2), 1.29-1.35 (12H, m, 6CH2), 1.43-1.56 (16H, m, 8CH2), 2.16-2.26; 19F NMR (368 MHz, CDCl3) δ ppm: −81.3 (3F, t, CF3), −114.9 (2F, t, CF2), −121.1 (2F, m, CF2) −122.3 (2F, m, CF2), −123.3 (2F, m, CF2), −126.6 (2F, m, CF2). ES-MS: ES+ m/z 483.5 P6,6,6,14+, ES− m/z 399 C6F13SO3−.
Solubility and Electrochemistry: Not tested, see Example 2B.
Solubility: The solubility of [P6,6,6,14][C6F13SO3] in HFCP is Xα=>0.36.
Electrochemistry: The electrochemical method was the same as Example 5A except that the electrolyte was Xα 0.07. [P6,6,6,14][C6F13SO3] in HFCP. The reference electrode was Ag/Ag triflate dissolved in [C4mpyr][eFAP]. A constant potential of −2V vs the reference electrode was applied for two hours to determine the NH3 formation rate while the solution was bubbled with N2 gas. The gas was then bubbled through two 1 mM H2SO4 traps to collect ammonia.
A yield rate of 2.5×10−12 mol/cm2/s was found corresponding to a faradaic efficiency of 4.1%.
On the basis of this performance, a similar electrochemical result is anticipated for Example 2A.
Abbreviation: [C8H4F13mim][C4F9SO3] also known as [C8H4F13mim][NfO].
[C8H4F13mim][I] was synthesised according to literature methods (Almantariotis et al., The Journal of Physical Chemistry B 2010, 114 (10), 3608-3617).
Synthetic procedure (Quaternisation): Under inert conditions, [C8H4F13mim][I] was prepared by dissolving a slight excess of methyl imidazole (2.64 g, 32.2 mmol) in dry toluene (40 mL) at room temperature. 1H,1H,2H,2H-perfluorooctyl iodide (14.6 g, 30.7 mmol) was added dropwise and shielded from light, over 30 minutes. The reaction mixture was stirred for 2 days at between 60° C. and 110° C. (reflux) after which it was cooled to 0° C. causing the formation of an orange solid. The solid was isolated via filtration and either washed with toluene and diethyl ether or recrystallised from acetonitrile and ethyl acetate at −20° C. The resulting pale yellow solid was dried in vacuo at 40° C. for 4 hours. Analysis showed the formation and isolation of [C8H4F13mim][I] in (5.51 g, 33%).
Characterisation: 1H NMR (400 MHz, (CD3)2CO) δ ppm: 3.12-3.24 (2H, m, CH2), 4.12 (3H, s, CH3), 4.90 (2H, t, CH2), 7.82 (1H, t, CH), 8.01 (1H, t, CH), 9.56 (1H, s, NC(H)N); 19F NMR (368 MHz, (CD3)2CO) δ ppm: −81.7 (3F, d, CF3), −122.4 (2F, s, CF2), −123.4 (2F, s, CF2), −124.0 (2F, s, CF2), −126.8 (2F, s, CF2). ES-MS: ES+ m/z 429 C8H4F13mim+, ES− m/z 127 I—.
Synthetic Procedure (Metathesis): [C8H4F13mim][I] (1.25 g, 2.26 mmol), was dissolved in 40 mL of distilled water at 60° C. and potassium nonfluorobutane sulfonate (0.80 g, 2.37 mmol) was added slowly. The stirring reaction mixture was heated to 85° C. for 2 h and then allowed to cool to room temperature for 12 h. A white precipitate formed and was isolated by filtration followed by drying in vacuo and analysis to show the formation of [C8H4F13mim][C4F9SO3] in (1.13 g, 68%).
Characterisation: 1H NMR (400 MHz, (CD3)2CO) δ ppm: 3.06-3.19 (2H, m, CH2), 4.10 (3H, s, CH3), 4.85 (2H, t, CH2), 7.78 (1H, t, CH), 7.96 (1H, t, CH), 9.28 (1H, s, NC(H)N); 19F NMR (368 MHz, (CD3)2CO) δ ppm: −81.7 (3F, t, CF3), −81.9 (3F, t, CF3), −114.4 (2F, t, CF2), −115.6 (2F, t, CF2), −122.1 (2F, s, CF2), −122.4 (2F, s, CF2), −123.4 (2F, s, CF2), −124.1 (2F, s, CF2), 126.3 (2F, t, CF2), 126.8, (2F, s, CF2). ES-MS: ES+ m/z 429 C8H4F13mim+, ES− m/z 299 C4F9SO3—.
Abbreviation: [C8H4F13mim][NTf2].
[C8H4F13mim][NTf2] was synthesised according to literature methods (Almantariotis, D.; Gefflaut, T.; Pádua, A. A. H.; Coxam, J. Y.; Costa Gomes, M. F., Effect of Fluorination and Size of the Alkyl Side-Chain on the Solubility of Carbon Dioxide in 1-Alkyl-3-methylimidazolium Bis(trifluoromethylsulfonyl)amide Ionic Liquids. The Journal of Physical Chemistry B 2010, 114 (10), 3608-3617).
Synthetic Procedure: [C8H4F13mim][I] (4.00 g, 7.19 mmol, prepared via the synthetic procedure detailed in [C8H4F13mim][C4F9SO3]), was dissolved in 40 mL of distilled water at 60° C. and lithium NTf2 (2.17 g, 7.55 mmol) was dissolved in 10 mL of water and added dropwise. The reaction mixture was allowed to cool to room temperature and stirred for 20 h after which an orange phase and a light yellow phase were observed. The water layer was decanted and the orange layer dissolved in dichloromethane and washed with water three times. After concentration of the dichloromethane layer to dryness and 2 h of drying in vacuo at 40° C. the orange liquid was analysed and found to be [C8H4F13mim][NTf2] (2.35 g, 46%).
Characterisation: 1H NMR (400 MHz, (CD3)2CO) δ ppm: 3.06-3.19 (2H, m, CH2), 4.11 (3H, s, CH3), 4.86 (2H, t, CH2), 7.78 (1H, t, CH), 7.95 (1H, t, CH), 9.22 (1H, s, NC(H)N); 19F NMR (368 MHz, (CD3)2CO) δ ppm: −80.0 (6F, s, (CF3)2), −81.7 (3F, s, CF3), −114.5 (2F, t, CF2), −114.4 (2F, t, CF2), −122.4 (2F, s, CF2), −123.4 (2F, s, CF2), −124.1 (2F, s, CF2), 126.8, (2F, s, CF2). ES-MS: ES+ m/z 429 C8H4F13mim+, ES− m/z 280 NTf2−.
In a non-optimised electrochemical cell using this liquid salt as an electrolyte the ammonia yield rate was found to be 0.89 mg h−1 m−2.
Abbreviation: [C8H4F13dmim][eFAP]
Synthetic procedure (Quaternisation): [C8H4F13dmim][I] was synthesised according to literature methods replacing 1-methylimidazole with 1,2-dimethylimidazole. (Almantariotis et al., The Journal of Physical Chemistry B 2010, 114 (10), 3608). Analysis showed the formation and isolation of [C8H4F13dmim][I] as a white powder in 5.4 g, 31% yield.
Characterisation: 1H NMR (400 MHz, (CD3)2SO) δ ppm: 2.62 (3H, s, CH3), 2.88-2.94 (2H, m, CH2), 3.76 (3H, s, CH3), 4.48-4.52 (2H, t CH2), 7.66 (1H, d, CH), 7.75 (1H, d, CH); 19F NMR (368 MHz, (CD3)2SO, CClF3) δ ppm: −125.28, (2F, s, CF2), −122.57 (2F, s, CF2), −121.17, (2F, s, CF2), −121.22 (2F, s, CF2), −112.57 (2F, s, CF2), −79.76 (3F, t, CF3). ES-MS: ES+ m/z 443 C8H4F13dmim+, ES− m/z 127 I—
Synthetic procedure (Metathesis): [C8H4F13dmim][I] (3.60 g, 6.33 mmol) was dissolved in 20 mL of acetonitrile. [C2,0,1mpyr][eFAP] (3.73 g, 6.33 mmol), dissolved in 3 mL of acetonitrile and added slowly. The reaction solution was stirred at 74° C. for 5 days then stored at −28° C. overnight. A fine precipitate formed, the filtrate was isolated concentrated in vacuo and mostly dissolved in DCM. The DCM reaction mixture was filtered, the filtrate was washed 3 times with distilled water in a separation funnel. The DCM layer was concentrated in vacuo resulting in yellow and colourless crystalline solid found to be a mixture of [C8H4F13dmim][eFAP] and approximately 25 mol % [C2,0,1mpyr][eFAP] starting material in 1.72 g, 29% yield. Due to the absence of iodide, this mixture was deemed suitable for nitrogen reduction reaction trials without further purification.
Characterisation: 1H NMR (400 MHz, (CD3)2SO) δ ppm: 2.06 (1H, br, CH2CH2 ring) 2.61 (3H, s, CH3), 2.86-2.96 (2H, m, CH2), 3.02 (0.8H, s, CH3), 3.31 (0.8H, s, CH3), 3.47-3.51 (1H, m, CH2CH2 ring) 3.54-3.57 (0.5H, m, CH2), 3.76 (3H, s, CH3), 4.48-4.52 (2H, t CH2), 7.64 (1H, d, CH), 7.74 (1H, d, CH). (The integration ratios show [C2,0,1mpyr] present in approximately 25 mol %). 19F NMR (368 MHz, (CD3)2SO) δ ppm: −125.88 (2F, s, CF2), −123.15 (2F, s, CF2), −122.74 (2F, s, CF2), −121.78 (2F, s, CF2), −116.23-−115.41 (7.6F, m, (CF2)3), −113.11 (2F, s, CF2), −87.50 (2F, d, (F)2), −81.18 (7.8F, s, (CF3)2), −80.40 (3F, t, CF3), −79.62 (3F, m, CF3), −44.23 (1.1F, dm, F). (The integration ratios show approximately 25 mol % more eFAP than [C8H4F13dmim] owing to the remaining [C2,0,1mpyr] cation remaining. ES-MS: ES+ m/z 144 [C2,0,1mpyr]+, 443 C8H4F13dmim+, ES−m/z 445 [eFAP]−
Solubility: [C8H4F13dmim][eFAP] shows a max Xα of 0.17 in FPEE at room temperature.
Electrochemistry: A three electrode electrochemical cell was used to perform N2 reduction in an electrolyte of Xα=8.45×10−2 [C8H4F13dmim][eFAP] in FPEE.
The working electrode was electrodeposited Fe on FTO glass (surface area: 0.25 cm2), the counter electrode was a coiled platinum wire separated from the working electrode by a frit and the reference electrode was Ag/Ag triflate in the same electrolyte. To determine NH3 formation rates a constant potential of −1.2V vs the reference electrode was applied for two hours while the solution was bubbled with N2 gas. The gas was then bubbled through a 1 mM H2SO4 trap to collect ammonia. The trap and the electrolyte were then tested for ammonia.
A yield rate of 1.34×10−11 moles of NH3/cm2/s was found corresponding to a faradaic efficiency of 48%.
On the basis of this performance, we anticipate a similar electrochemical result for Example 5B (similar solubility), Examples 6A and 6B (identical cation and higher solubility).
Solubility: [C8H4F13dmim][eFAP] shows a maximum Xα 0.15 in HFCP at room temperature.
Electrochemistry: Not tested, see Example 5A.
Abbreviation: [C8H4F13dmim][NTf2]
Synthetic procedure (Metathesis): [C8H4F13dmim][I] (0.92 g, 1.61 mmol, prepared via the quaternisation synthetic procedure detailed in [C8H4F13dmim][NTf2], and LiNTf2 (0.48 g, 1.69 mmol, 5% excess) was dissolved in 20 mL of methanol, heated to 50° C. and stirred for 2 days. The reaction solvent was removed in vacuo leaving a gooey, off white solid. The solid was dissolved in DCM, the solution heated to 37° C. overnight, followed by stirring at room temperature for 5 d affording a white solid in a yellow solution. The yellow filtrate was isolated and concentrated in vacuo leaving a pale yellow liquid found to be [C8H4F13dmim][NTf2] in 0.58 g, 50% yield.
Characterisation: 1H NMR (400 MHz, (CD3)2SO) δ ppm: 2.61 (3H, s, CH3), 2.84-2.98 (2H, m, CH2), 3.76 (3H, s, CH3), 4.47-4.51 (2H, t CH2), 7.63 (1H, d, CH), 7.73 (1H, d, CH); 19F NMR (368 MHz, (CD3)2SO) δ ppm: −125.76 (2F, s, CF2), −123.07 (2F, s, CF2), −122.65, (2F, s, CF2), −121.70, (2F, s, CF2), −113.06 (2F, s, CF2) −80.24 (3F, s, CF3), −78.76 (6F, s, (CF3)2). ES-MS: ES+ m/z 443 C8H4F13dmim+, ES− m/z 280 [NTf2]−.
Solubility: [C8H4F13dmim][NTf2] shows a max Xα of 0.11 or 0.23 mol/L in FPEE at room temperature.
Electrochemistry: Not tested, see Example 5A.
Solubility: The solubility of [C8H4F13dmim][NTf2] in HFCP is Xα=>0.22 or >2.4 mol/L.
Electrochemistry: Not tested, see Example 5A.
Abbreviation: [C8H4F13dmim][C4F9SO3].
Synthetic procedure (Metathesis): [C8H4F13dmim][I] (1.40 g, 2.52 mmol, prepared via the quaternisation synthetic procedure detailed in [C8H4F13dmim][eFAP]) and K[C4F9SO3] (0.90 g, 2.65 mmol) were stirred together in 30 ml of DCM at 35° C. After 3 d, the reaction mixture was cooled to room temperature and a white powder precipitated from the pale yellow DCM solution. The powder was isolated via filtration, washed with distilled water, dried under vacuum and found to be [C8H4F13dmim][C4F9SO3] in 0.55 g, 29% yield.
Characterisation: 1H NMR (400 MHz, (CD3)2SO) δ ppm: 2.62 (3H, s, CH3), 2.87-2.96 (2H, m, CH2), 3.76 (3H, s, CH3), 4.48-4.52 (2H, t CH2), 7.64 (1H, d, CH), 7.74 (1H, d, CH); 19F NMR (368 MHz, (CD3)2SO) δ ppm: −125.26, (4F, m, CF2), −122.62, (2F, s, CF2), −122.21 (2F, s, CF2), −121.25, (2F, s, CF2), −120.94 (2F, s, CF2), −114.39, (2F, s, CF2), −112.61 (2F, s, CF2) −80.05 (3F, s, CF3), −79.84 (3F, s, CF3). ES-MS: ES+ m/z 443 C8H4F13dmim+, ES− m/z 299 [C4F9SO3]—
Solubility: [C8H4F13dmim][C4F9SO3] shows a max Xα of 0.04 in FPEE at room temperature.
Electrochemistry: The electrochemical method was the same as Example 5A except that the electrolyte was Xα=4.2×10−2 [C8H4F13dmim][C4F9SO3] in FPEE. No significant amount of ammonia was detected in either the trap or the electrolyte.
On the basis of this performance, the inventors anticipate a similar electrochemical result for Examples 7B and 8A.
Solubility: [C8H4F13dmim][C4F9SO3] shows max Xα of 0.017 in HFCP at room temperature.
Electrochemistry: Not tested; see Example 7A.
Abbreviation: [Hmpyr][C8F15O2]
Synthetic Procedure: Perfluorooctanoic acid (1.90 g, 4.59 mmol) was added to N-methylpyrrolidine (0.45 g, 5.28 mmol). Once the exothermic reaction was complete, the reaction mixture was allowed to warm to room temperature and stirred for 24 hours under nitrogen. The product was concentrated in vacuo to afford a light green gel-like liquid (2.00 g, 50%).
Characterisation: 1H NMR (400 MHz, CDCl3) δ ppm: 2.07-2.20 (4H, m, 2CH2), 2.75-2.80 (2H, m, CH2), 2.86-2.87 (3H, d, CH3), 3.82-3.85 (2H, m, CH2), 13.12-13.18 (H, m, NH); 19F NMR (368 MHz, CDCl3) δ ppm: −126.1 (2F, m, CF2), −122.7 (2F, m, CF2), −122.0 (4F, m, 2CF2), −121.8 (2F, m, CF2), −117.2 (2F, m, CF2), −80.8 (3F, t, CF3). ES-MS: ES+ m/z 86 Hmpyr+, ES− m/z 412 C8F15O2−, 369 C7F15−, 219 C4F9−, 169 C3F7−, 119 C2F5
Abbreviation: [Hmpyr][C2H2F4SO3].
Synthetic Procedure: 2-H-perfluoroethylsulfonic acid (2.27 g, 12.5 mmol) was added to N-methylpyrrolidine (1.11 g, 13.0 mmol) neat, and after drying in vacuo, afforded a white solid (2.0 g, 50%).
Characterisation: 1H NMR (400 MHz, CDCl3) δ ppm: 2.13-2.22 (4H, m, 2CH2), 2.87-2.92 (2H, m, CH2), 2.94-2.95 (3H, d, CH3), 3.79-3.85 (2H, m, CH2), 9.55 (H, m, NH); 19F NMR (368 MHz, CDCl3) δ ppm: −135.7 (2F, (doublet of)t, CF2), −123.1 (2F, m, CF2). MS: ES+ m/z 87 Hmpyr+, ES− m/z 181 C2H2F4SO3−
Abbreviation: [C4mpyr][B(otfe)4].
Synthetic Procedure: [Na][B(otfe)4] was synthesised according to literature (Rupp et al, ChemPhysChem 2014, 15 (17), 3729-3731). NaBH4 (2.00 g, 53 mmol) was added to dimethoxyethane (22 mL) and toluene (50 mL). Trifluoroethanol (31.6 g, 316 mmol) was added dropwise over one hour under an acetone/dry ice bath (−15° C.). The mixture was heated to reflux overnight under N2. After purification, afforded a white powder (18 g, 83%).
Characterisation: 1H NMR (400 MHz, CD3CN) δ ppm: 3.67-3.77 (8H, q, 4CH2); 19F NMR (368 MHz, (CD3)2SO) δ ppm: −74.3 (12F, t, 4CF3). ES-MS: ES− m/z no anion detected.
Synthetic Procedure (Metathesis): [C4mpyr][Br] (2.06 g, 9.3 mmol) was dissolved in dry acetonitrile (120 mL) and [Na][B(otfe)4] (4.00 g, 9.3 mmol) was added. Under a nitrogen atmosphere, the reaction mixture was stirred at room temperature for 70 h. The acetonitrile was removed in vacuo and DCM (100 mL) was added to the white solid and left to stir for one hour. The white solid was filtered under N2 and dried in vacuo to afford a white powder (4.03 g, 79%).
Characterisation: 1H NMR (400 MHz, CD3CN) δ ppm: 0.95-0.99 (3H, t, CH3), 1.23-1.42 (2H, m, CH2), 1.70-1.74 (2H, t, CH2), 2.15 (4H, m, 2CH2), 2.93 (3H, s, N—CH3), 3.20-3.24 (2H, m, N—CH2), 3.37-3.41 (4H, m, N—CH2), 3.68-3.76 (5H, q, 4CH2); 19F NMR (368 MHz, (CD3)2SO) δ ppm: −74.3 (12F, t, 4CF3). ES-MS: ES+ m/z 142 C4mpyr+, ES− no anion detected.
Abbreviation: [C11H6F17mpyr][CF3SO3].\
Synthetic Procedure (Quaternisation): Under inert conditions, 1-(4,4,5,5,6,6,7,7,8,8,9,9,10,10,11,11,11-heptadecafluoroundecyl-1-methylpyrrolidiniumiodide ([C11H6F17mpyr][I]) was prepared by dissolving methyl pyrrolidine (0.263 g, 3.09 mmol) in dry acetonitrile at 50° C. Commercially available 1,1,1,2,2,3,3,4,4,5,5,6,6,7,7,8,8-heptadecafluoro-11-iodoundecane (2.00 g, 3.40 mmol) was added in 10% excess, dropwise and shielded from light, over 30 minutes. The reaction mixture was stirred for 48 h at 50° C., still being shielded from light, after which it was cooled to room temperature. The acetonitrile was removed in vacuo, and the resulting solid was dissolved in dichloromethane (200 mL). The solution was then wash with distilled water (100 mL) three times and the dichloromethane was removed in vacuo yielding a white solid. Analysis showed the formation of [C11H6F17mpyr][I] (1.28 g, 61.7%).
Characterisation: 1H NMR (400 MHz CDCl3): δ (ppm) 2.22 (m, 8H), 3.38 (s, 3H), 3.80 (m, 2H), 4.00 (m, 4H); 19F NMR (376.5 MHz CDCl3): δ (ppm) −80.9 (s, 3H), −113.8 (s, 2H), −122.0 (s, 6H), −122.8 (s, 2H), −123.2 (s, 2H), −126.2 (s, 2H). ES-MS: ES+ m/z 546 C11H6F17mpyr+, ES− m/z 127 I—.
Synthetic Procedure (Metathesis): [C11H6F17mpyr][I] (1.28 g, 1.91 mmol), was dissolved in 30 ml of dry acetone at reflux. Silver trifluoromethane sulfonate (0.490 g, 1.91 mmol) was added in equal molar ratio. The reaction mixture was stirred for 4h, following this the solution was filtered. The acetone was removed from the filtrate in vacuo to yield a pale yellow to white solid that was washed with 20 mL of distilled water three times. The solid was then dried to remove excess water, and was shown by analysis to be [C11H6F17mpyr][CF3SO3] (1.21 g, 91.2%).
Characterisation: 1H NMR (400 MHz, (CD3)2CO): δ (ppm) 2.40 (m, 8H), 3.40 (s, 3H), 3.87 (m, 6H); 19F NMR (368 MHz, (CD3)2CO): δ (ppm) −78.9 (s, 3F), −81.6 (s, 3F), −114.5 (s, 2F), −122.4 (s 6F), −123.2 (s, 2F), −124.0 (2, 2F), −127.0 (s, 2F). ES-MS: ES+ m/z 546 C11H6F17mpyr+, ES− m/z 149 CF3SO3−.
Abbreviation: [C11H6F17mpyr][C4F9SO3] also known as [C11H6F17mpyr][NfO].
Synthetic Procedure: see procedure for [C11H6F17mpyr][CF3SO3]. Potassium nonafluorobutane sulfonate (0.646 g, 1.91 mmol) was added to [C11H6F17mpyr][I] (1.283 g, 1.91 mmol) dissolved in 30 ml of dry acetone at reflux. After purification a pale yellow to white solid was afforded and shown by analysis to be [C11H6F17mpyr][C4F9SO3] (1.44 g, 89.0%).
Characterisation: 1H NMR (400 MHz, (CD3)2CO): δ (ppm) 2.39 (m, 8H), 3.40 (s, 3H), 3.87 (m, 6H); 19F NMR (368 MHz, (CD3)2CO): δ (ppm) −81.6 (s, 3F), −81.9 (s, 3F), −114.5 (s, 2F), −115.5 (s, 2F), −122.1 (s, 8F), −123.2 (s, 2F), −124.0 (s, 2F), −126.7 (s, 4F). ES-MS: ES+ m/z 546 C11H6F17mpyr+, ES− m/z 299 C4F9SO3−.
Abbreviation: [C11H6F17mpyr][C6F13SO3].
Synthetic Procedure: see procedure for [C11H6F17mpyr][CF3SO3]. Potassium tridecafluorohexane sulfonate (0.835 g, 1.91 mmol) was added to [C11H6F17mpyr][I] (1.283 g, 1.91 mmol) dissolved in 30 ml of dry acetone at reflux. After purification a pale yellow to white solid was afforded and shown by analysis to be [C11H6F17mpyr][C6F13SO3] (1.23 g, 68.4%).
Characterisation: 1H NMR (400 MHz, (CD3)2CO): δ (ppm) 2.40 (m, 8H), 3.41 (s, 3H), 3.88 (m, 6H); 19F NMR (368 MHz, (CD3)2CO): δ (ppm) −81.6 (s, 3F), −81.7 (s, 3F), −114.5 (s, 2F), −115.2 (s, 2F), −121.1 (s, 2F), −122.4 (s, 8F), −123.4 (s, 4F), −124.1 (s, 2F), −126.7 (s, 4F). ES-MS: ES+ m/z 546 C11H6F17mpyr+, ES− m/z 399 C6F13SO3−.
Abbreviation: [C11H6F17mpyr][C8F17SO3] also known as [C11H6F17mpyr][PFO].
Synthetic Procedure: see procedure for [C11H6F17mpyr][CF3SO3]. Potassium heptadecafluorooctane sulfonate (1.03 g, 1.91 mmol) was added to [C11H6F17mpyr][I] (1.283 g, 1.91 mmol) dissolved in 30 ml of dry acetone at reflux. After purification a pale yellow to white solid was afforded and shown by analysis to be [C11H6F17mpyr][C8F17SO3] (1.55 g, 77.5%).
Characterisation: 1H NMR (400 MHz, (CD3)2CO): δ (ppm) 2.40 (m, 8H), 3.40 (s, 3H), 3.88 (m, 6H); 19F NMR (368 MHz, (CD3)2CO): δ (ppm) −81.4 (s, 3F), −81.9 (s, 3F), −114.6 (s, 2F), −115.4 (s, 2F), −121.1 (s, 2F), −122.7 (s, 12F), −123.5 (s, 4F), −124.1 (s, 2F), −126.7 (s, 4F). ES-MS: ES+ m/z 546 C11H6F17mpyr+, ES− m/z 499 C8F17SO3−.
Abbreviation: [C8H4F13dmim][C4F9CO2].
Synthetic procedure (Metathesis): [C8H4F13dmim][I] (2.29 g, 4.02 mmol prepared via the quaternisation synthetic procedure detailed in [C8H4F13dmim][eFAP]), was dissolved in 10 mL of MeOH and slowly passed through a freshly prepared amberlyst ion exchange column. 100 percent conversion to [C8H4F13dmim][OH] was assumed. Nonafluoropentanoic acid (1.06 g, 4.02 mmol) was added to the [C8H4F13dmim][OH] in MeOH solution and the reaction mixture was stirred at room temperature overnight followed by heating to 52° C. for 2 h. A yellow viscous layer was observed at the bottom of the reaction flask, isolated with a pasteur pipette, concentrated in vacuo leaving an off white power that was found to be [C8H4F13dmim][C4F9CO2] in 1.08 g, 38% yield.
Characterisation: 1H NMR (400 MHz, (CD3)2SO) δ ppm: 2.62 (3H, s, CH3), 2.85-2.98 (2H, m, CH2), 3.76 (3H, s, CH3), 4.48-4.52 (2H, t CH2), 7.65 (1H, d, CH), 7.75 (1H, d, CH); 19F NMR (368 MHz, (CD3)2SO) δ ppm: −125.37 (2F, s, CF2), −125.05 (2F, s, CF2), −122.64, (2F, s, CF2), −122.18, (4F, d, CF2), −121.27 (2F, s, CF2), −114.72 (2F, s, CF2), −112.64 (2F, t, CF2) −80.19 (3F, t, CF3), −78.87 (3F, s, (CF3)2). ES-MS: ES+ m/z 443 C8H4F13dmim+, ES− m/z 263 [C4F9CO2]—, 219 [C4F9]—.
Solubility: [C8H4F13dmim][C4F9CO2] shows max Xα of 0.032 in FPEE at room temperature.
Electrochemistry: Not tested. See example 15B.
Solubility: [C8H4F13dmim][C4F9CO2] shows a max Xα of 0.14 in HFCP at room temperature.
Electrochemistry: The electrochemical method was the same as Example 5A except that the electrolyte was XA 0.8 [C8H4F13dmim][C4F9COO] in HFCP. A constant potential of −0.8 V vs the reference electrode was applied for 1 hour and 45 minutes to determine the NH3 formation rate. On the basis of this and subsequent performances at lower concentrations, the inventors anticipate a similar electrochemical result for Example 15A.
A rate of 7.2×10−12 moles of NH3/cm2/s was found corresponding to a faradaic efficiency of 48%.
Abbreviation: [P6,6,6,14][C9F17O2].
Synthetic Procedure: see procedure for [P6,6,6,14][C5F9O2]. Heptadecanonanoic acid (2.54 g, 5.47 mmol) was added to [P6,6,6,14][Cl] (2.84 g, 5.47 mmol) and water (40 mL), and after purification, afforded a colourless liquid (5.04 g, 84%).
Characterisation—1H NMR (400 MHz, CDCl3) δ ppm: 0.85-0.92 (12H, t, 4CH3), 1.25 (20H, m, 10CH2), 1.28-1.35 (12H, m, 6CH2), 1.42-1.56 (16H, m, 8CH2), 2.26-2.36; 19F NMR (368 MHz, CDCl3) δ ppm: −81.3 (3F, t, CF3), −116.7 (2F, t, CF2), −122.1 (2F, m, CF2) −122.4 (4F, m, CF2CF2), −122.6 (2F, m, CF2), −123.2 (2F, m, CF2), −126.6 (2F, m, CF2). ES-MS: ES+ m/z 483.5 P6,6,6,14+, ES− m/z 463 C9F17O2—, 419 C9F17—, 269 C5F11—, 219 C4F9—, 169 C3F7—.
Abbreviation: [C4mpyr][eFAP].
This compound was commercially available.
Solubility: [C4mpyr][eFAP] was tested at Xα of 0.35 in FPEE.
Electrochemistry: The electrochemical method was the same as Example 5A except that the electrolyte was Xα 2.3×10−1 [C4mpyr][eFAP] in FPEE. The working electrode was α-Fe@Fe3O4 NR on CFP (surface area: 0.25 cm2). A constant potential of −1.85V vs the reference electrode was applied for one hour to determine the NH3 formation rate.
A yield rate of 2.35×10−11 moles of NH3/cm2/s was found corresponding to a faradaic efficiency of 32%.
On the basis of this performance, a similar is anticipated electrochemical result for Example 28A.
Solubility: [C4mpyr][eFAP] was tested at Xα of 0.35 in HFCP.
Electrochemistry: The electrochemical method was the same as Example 5A except that the electrolyte was Xα 1.0×10−1 [C4mpyr][eFAP] in HFCP. The working electrode was α-Fe@Fe3O4 NR on CFP (surface area: 0.25 cm2). A constant potential of −2.0V vs the reference electrode was applied for one hour to determine the NH3 formation rate.
A yield rate of 1.28×10−11 moles of NH3/cm2/s was found corresponding to a faradaic efficiency of 10.35%.
On the basis of this performance, a similar electrochemical result is anticipated for Example 28B.
Abbreviation: [C2,0,1mpyr][eFAP].
This compound was commercially available.
Solubility and electrochemistry: Not tested, see Example 18B
Solubility: [C2,0,1mpyr][eFAP] was tested at Xα of 0.35 in HFCP.
Electrochemistry: The electrochemical method was the same as Example 5A except for the following parameters. The electrolyte was Xα 3.5×10−1 [C2,O,1mpyr][eFAP] in HFCP. The working electrode was α-Fe@Fe3O4 NR on CFP (surface area: 0.25 cm2). A constant potential of −1.85V vs the reference electrode was applied for one hour to determine the NH3 formation rate.
A yield rate of 1.90×10−11 moles of NH3/cm2/s was found corresponding to a faradaic efficiency of 12%.
On the basis of this performance, the inventors anticipate a similar electrochemical result for Example 18A.
Abbreviation: [P6,6,6,14][C4P9SO3].
Synthesis (metathesis): See procedure for [P6,6,6,14][C4F9CO2]. [[P6,6,6,14][Cl] (3.84 g, 6.76 mmol) was added to potassium nonafluorobutanesulfonate (2.32 g, 6.86 mmol), DCM (˜40 mL) and water (40 mL). After extraction, DCM was removed in vacuo to afford a colourless oil (2.93 g, 74%).
Characterisation: 1H NMR (400 MHz, CDCl3) δ ppm: 1H NMR (400 MHz, CDCl3) δ ppm: 0.84-0.88 (12H, t, 4CH3), 1.23 (20H, m, 10CH2), 1.26-1.29 (12H, m, 6CH2), 1.33-1.40 (8H, m, 4CH2), 2.19-2.27 (8H, m, 4CH2), 3.72-3.85, (8H, m, 4CH2); 19F NMR (400 MHz, DMSO) δ ppm: 125.68-(−125.57) (2F, t, CF2), −121.39-(−121.31) (2F, m, CF2), −114.83-(−114.76) (2F, t, CF2), 80.42 (80.37) (3F, m, CF3). ES-MS: ES+ m/z 483 P6,6,6,14+, ES− m/z 499 [C4F9SO3]—
Solubility: The solubility of [P6,6,6,14][C4F9SO3] in FPEE is Xα=>0.39.
Electrochemistry: The electrochemical method was the same as Example 5A except that the electrolyte was Xα 0.17 [P6,6,6,14][C4F9SO3] in FPEE. The reference electrode was Ag/Ag triflate in [C4mpyr][eFAP]. A constant potential of −2V vs the reference electrode was applied for two hours to determine the NH3 formation rate. Two 1 mM H2SO4 traps were used to collect ammonia.
A yield rate of 5.3×10−12 mol/cm2/s was found corresponding to a faradaic efficiency of 15%.
On the basis of this performance, the inventors anticipate a similar electrochemical result for Example 26B.
Solubility and Electrochemistry: Not tested. See Example 26B.
Abbreviation: [P6,6,6,14][B(Otfe)4].
Synthetic Procedure: see procedure for [C4mpyr][B(otfe)4]. [Na][B(otfe)4] (1.94 g, 4.5 mmol) was added to trihexyltetradecylphosphonium chloride (2.34 g, 4.5 mmol) and acetonitrile (50 mL) and after purification, afforded an opaque gel-like solid (3.74 g, 93%).
Characterisation: 1H NMR (400 MHz, CDCl3) δ ppm: 0.85-0.93 (12H, t, 4CH3), 1.26 (20H, m, 10CH2), 1.29-1.35 (12H, m, 6CH2), 1.41-1.55 (16H, m, 8CH2), 2.17-2.29 (8H, m, 4CH2), 3.71-3.86, (8H, m, 4CH2); 19F NMR (368 MHz, CDCl3) δ ppm: −77.0 (12F, t, 4CF3). ES-MS: ES+ m/z 483.5 P6,6,6,14+, ES− anion not observed.
Abbreviation: [P4,4,4,Rf][C4F9SO3], Rf═C8H4F13.
Synthetic Procedure—[P4,4,4,Rf][I] was synthesised according to literature methods (Tindale et al Canadian Journal of Chemistry 2007, 85, 660+). Tributylphosphine (2.44 g, 12.1 mmol) was added to 1H,1H,2H,2H-perfluorooctyl iodide (5.71 g, 12.3 mmol) and stirred under N2 for 48 hours. Any residual starting material was removed in vacuo at 40° C.
[P4,4,4,Rf][C4F9SO3] see procedure for [P6,6,6,14][C5F9O2]. [P4,4,4,Rf][I] (3.01 g, 4.44 mmol) was added to potassium nonafluorobutanesulfonate (1.60 g, 4.70 mmol) and water (40 mL), and after purification, afforded a light yellow liquid (3.22 g, 85%).
Characterisation: 1H NMR (400 MHz, (CD3)2SO) δ ppm: 0.88-0.99 (9H, t, 3CH3), 1.36-1.58 (12H, m, 6CH2), 1.60-1.70 (2H, m, CH2), 2.12-2.56 (8H, m, 4CH2). 19F NMR (368 MHz, CDCl3) δ ppm: [P4,4,4,Rf]−80.8 (3F, m, CF3), −114.3 (2F, m, CF2), −121.8 (2F, m, CF2) −122.8 (4F, m, CF2CF2), −123.3 (2F, m, CF2), −126.1 (2F, m, CF2). [C4F9SO3]— −81.0 (3F, t, CF3), −114.7 (2F, t, CF2), −121.6 (2F, m, CF2) −126.1 (2F, m, CF2). ES-MS: ES+ m/z 549 P4,4,4,Rf+, ES− m/z 299 C4F9SO3—.
In a non-optimised electrochemical cell using this liquid salt as an electrolyte the ammonia yield rate was found to be 1.1 mg h−1 m−2.
Abbreviation: [P4,4,4,Rf][eFAP], Rf═C8H4F13. Synthetic Procedure: see procedure for [P6,6,6,14][C5F9O2]. [P4,4,4,Rf][I] (2.64 g, 3.90 mmol) was added to 1-(2-Methoxyethyl)-1-methylpyrrolidinium. Tris(pentafluoroethyl)trifluorophosphate (2.31 g, 3.92 mmol), and after purification, afforded a light yellow liquid (3.71 g, 96%).
Characterisation—1H NMR (400 MHz, (CD3)2SO) δ ppm: 0.86-1.01 (9H, t, 3CH3), 1.36-1.56 (12H, m, 6CH2), 1.63-1.71 (2H, m, CH2), 1.98-2.40 (8H, m, 4CH2); 19F NMR (368 MHz, (CD3)2SO) δ ppm: [P4,4,4,Rf]− −80.4 (3F, m, CF3), −113.9 (2F, m, CF2), −121.8 (2F, m, CF2) −122.4 (4F, m, CF2CF2), −122.7 (2F, m, CF2), −125.9 (2F, m, CF2). [eFAP]− −113.8 (4F, m, 2CF2), −115.7 (2F, m, CF2), −88.7+86.2) (2F, m, 2PF), −81.1 (6F, m, 2CF3), −79.6 (3F, m, C2F5), −43.5-(−45.0) (1F, m, PF). ES-MS: ES+ m/z 483 P6,6,6,14+, ES− m/z 445 eFAP−.
Abbreviation: [C2mim][C4F9SO3] also known as [C2mim][NfO].
Synthetic Procedure: see procedure for [P6,6,6,14][C5F9O2]. 1-ethyl-3-methylimidazolium bromide (1.98 g, 10.36 mmol) was added to potassium nonafluorobutanesulfonate (3.54 g, 10.47 mmol) and water (40 mL), and after purification, to afford an opaque liquid (2.3 g, 54%).
Characterisation: 1H NMR (400 MHz, CDCl3) δ ppm: 1.54-1.57 (3H, t, CH3), 3.97 (3H, s, CH3), 4.24-4.30 (2H, q, CH2), 7.31-7.32 (2H, m, 2CH), 9.17 (H, s, CH); 19F NMR (368 MHz, CDCl3) δ ppm: −126.0 (2F, m, CF2), −121.7 (2F, m, CF2), −114.9 (2F, m, CF2), −80.9 (3F, t, CF3). ES-MS: ES+ m/z 111 C2mim+, ES− m/z 299 C4F9SO3—.
Abbreviation: [C2mim][C8F17SO3] also known as [C2mim][PFO].
Synthetic Procedure: see procedure for [P6,6,6,14][C5F9O2]. 1-ethyl-3-methylimidazolium bromide (2.25 g, 4.18 mmol) was added to potassium heptadecafluorooctanesulfonate (0.82 g, 4.19 mmol) and water (40 mL), and after purification, afforded a colourless solid. (0.50 g, 25%).
1H NMR (400 MHz, CDCl3) δ ppm: 1.58-1.62 (3H, t, CH3), 4.02 (3H, s, CH3), 4.29-4.34 (2H, q, CH2), 7.17-7.18 (H, m, CH), 7.20-7.21 (H, m, CH), 9.49 (H, s, CH); 19F NMR (368 MHz, CDCl3) δ ppm: −126.12-(−126.01) (2F, m, CF2), −122.71-(−122.59) (2F, m, CF2), −121.91-(−121.69) (4F, m, 2CF2), −121.61-(−121.48) (2F, m, CF2), −120.69-(−120.59) (2F, m, CF2), −114.58-(−114.49) (2F, m, CF2), −80.77-(−80.72) (3F, t, CF3). ES-MS: ES+ m/z 111 C2mim+, ES− m/z 499 C8F17SO3—.
Abbreviation: [P6,6,6,14][C4F9CO2].
Synthetic Procedure: The commercially available starting material, [P6,6,6,14][Cl] (3.87 g, 7.45 mmol) was dissolved in 40 ml of distilled water at room temperature and nonafluoropentanoic acid (1.94 g, 7.42 mmol) was added. The reaction mixture was stirred for 24 h under nitrogen gas. The white cloudy solution was extracted with DCM three times. The dichloromethane extract was washed with water five times followed by a single wash with potassium hydroxide (1 mM). DCM was removed in vacuo to afford a colourless liquid (4.91 g, 95%).
Characterisation—1H NMR (400 MHz, CDCl3) δ ppm: 0.85-0.93 (12H, t, 4CH3), 1.25 (20H, m, 10CH2), 1.27-1.35 (12H, m, 6CH2), 1.39-1.55 (16H, m, 8CH2), 2.12-2.24; 19F NMR (368 MHz, CDCl3) δ ppm: −81.5 (3F, m, CF3), −115.1 (2F, t, CF2), −122.0 (2F, m, CF2), −126.5 (2F, t, CF2). ES-MS: ES+ m/z 483.5 P6,6,6,14+, ES− m/z 263 C5F9O2−, 219 C5F9−.
Solubility: The solubility of [P6,6,6,14][C4P9CO2] in FPEE is Xα=>0.40.
Electrochemistry: The electrochemical method was the same as Example 5A except that the electrolyte was Xα 0.13 [P6,6,6,14][C4F9CO2]] in FPEE. The reference electrode was Ag/Ag triflate in [C4mpyr][eFAP]. A constant potential of −2V vs the reference electrode was applied for two hours to determine the NH3 formation rate. Two 1 mM H2SO4 traps were used to collect ammonia.
A yield rate of 2.14×10−12 mol/cm2/s corresponding to a faradaic efficiency of 0.5%.
Solubility: The solubility of [P6,6,6,14][C4P9CO2] in HFCP is Xα=>0.28.
Electrochemistry: The electrochemical method was the same as Example 5A except that the electrolyte was Xα 0.08 [P6,6,6,14][C4F9CO2] in HFCP. The reference electrode was Ag/Ag triflate in [C4mpyr][eFAP]. A constant potential of −2V vs the reference electrode was applied for two hours to determine the NH3 formation rate. Two 1 mM H2SO4 traps were used to collect ammonia.
A yield rate of 2.2×10−12 mol/cm2/s was found corresponding to a faradaic efficiency of 1.5%.
Abbreviation: [P6,6,6,14][C6P13SO3].
Synthetic Procedure: see procedure for [P6,6,6,14][C4P9CO2]. Potassium tridecafluorohexane sulfonate (3.04 g, 6.94 mmol) was added to [P6,6,6,14][Cl] (3.59 g, 6.91 mmol), and water (40 mL), and after purification, afforded a colourless liquid (5.76 g, 96%).
Characterisation—1H NMR (400 MHz, CDCl3) δ ppm: 0.87-0.92 (12H, t, 4CH3), 1.25 (20H, m, 10CH2), 1.29-1.35 (12H, m, 6CH2), 1.43-1.56 (16H, m, 8CH2), 2.16-2.26; 19F NMR (368 MHz, CDCl3) δ ppm: −81.3 (3F, t, CF3), −114.9 (2F, t, CF2), −121.1 (2F, m, CF2) −122.3 (2F, m, CF2), −123.3 (2F, m, CF2), −126.6 (2F, m, CF2). ES-MS: ES+ m/z 483.5 P6,6,6,14+, ES− m/z 399 C6F13SO3−.
Solubility and Electrochemistry: Not tested, see Example 26B.
Solubility: The solubility of [P6,6,6,14][C6F13SO3] in HFCP is Xα=>0.36.
Electrochemistry: The electrochemical method was the same as Example 5A except that the electrolyte was Xα 0.07 [P6,6,6,14][C6F13SO3] in HFCP. The reference electrode was Ag/Ag triflate in [C4mpyr][eFAP]. A constant potential of −2V vs the reference electrode was applied for two hours to determine the NH3 formation rate.
A yield rate of 2.5×10−12 mol/cm2/s was found corresponding to a faradaic efficiency of 4.1%.
On the basis of this performance, the inventors anticipate a similar electrochemical result for Example 19A and 19B.
Abbreviation: [P4,4,4,Rf][C4F9SO3] where Rf═C11H6F17.
Synthetic Procedure (Quaternisation): [P4,4,4,Rf][I] was synthesised similarly to literature methods (Tindale et al. Canadian Journal of Chemistry 2007, 85, 660+). Tributylphosphine (1.08 g, 5.34 mmol) was added to 4,4,5,5,6,6,7,7,8,8,9,9,10,10,11,11,11-Heptadecafluoroundecyl iodide (3.09 g, 5.25 mmol) and stirred under N2 for 48 hours. Any residual starting material was removed in vacuo at 40° C. to afford a colourless liquid (4.15 g, 99%).
Characterisation: 1H NMR (400 MHz, CDCl3) δ ppm: 0.97-1.01 (9H, t, 3CH3), 1.52-1.61 (12H, m, 6CH2), 1.90-2.00 (2H, m, CH2), 2.43-2.50 (8H, m, 4CH2), 2.77-2.84 (2H, m, CH2); 19F NMR (368 MHz, CDCl3) δ ppm: [P4,4,4,Rf] −80.8 (3F, m, CF3), −113.8 (2F, m, CF2), −121.6 (2F, m, CF2) −121.9 (4F, m, CF2CF2), −122.7 (2F, m, CF2), −123.2 (2F, m, CF2), −126.1 (2F, m, CF2). ES− MS: ES+ m/z 663.1 P4,4,4,Rf+, ES− m/z 126.9 I—.
Synthetic procedure (Metathesis): [P4,4,4,Rf][I] (1.89 g, 2.39 mmol) was added to potassium nonafluorobutanesulfonate (0.84 g, 2.48 mmol), DCM (˜40 mL) and water (40 mL). After stirring under N2 overnight, purification afforded a colourless liquid (1.93 g, 84%).
Characterisation: 1H NMR (400 MHz, CDCl3) δ ppm: 0.96-1.00 (9H, t, 3CH3), 1.49-1.57 (12H, m, 6CH2), 1.84-1.92 (2H, m, CH2), 2.20-2.27 (8H, m, 4CH2), 2.46-2.54 (2H, m, CH2); 19F NMR (368 MHz, CDCl3) δ ppm: [P4,4,4,Rf]-80.8 (3F, m, CF3), −114.1 (2F, m, CF2), −121.7 (2F, m, CF2) −122.0 (4F, m, CF2CF2), −122.7 (2F, m, CF2), −123.6 (2F, m, CF2), −126.1 (2F, m, CF2). [C4F9SO3]— −81.1 (3F, t, CF3), −114.7 (2F, t, CF2), −121.7 (2F, m, CF2) −126.1 (2F, m, CF2) ES-MS: ES+ m/z 663.1 P4,4,4,Rf+, ES− m/z 299—C4F9SO3
Solubility: The solubility of [P4,4,4 C11H6F17][C4F9SO3] in FPEE is Xα=>0.64.
Electrochemistry: The electrochemical method was the same as Example 5A except that the electrolyte was Xα 0.10 [P4,4,4 C11H6F17][C4F9SO3] in FPEE. The reference electrode was Ag/Ag triflate in [C4mpyr][eFAP]. To determine NH3 formation rates a constant potential of −2V vs the reference electrode was applied for two hours to determine the NH3 formation rate. Two 1 mM H2SO4 traps were used to collect ammonia.
A yield rate of 2.10×10−12 mol/cm2/s was found corresponding to a faradaic efficiency of 5.3%.
On the basis of this performance, the inventors anticipate a similar electrochemical result for Examples 17A.
Solubility and Electrochemistry: Not tested, see Example 27A.
Abbreviation: [P4,4,4,Rf][eFAP], Rf═C11H6F17.
[P4,4,4,Rf][I] (1.88 g, 2.38 mmol) was added to 1-(2-Methoxyethyl)-1-methylpyrrolidinium tris(perfluoroethyl) trifluorophosphate (1.40 g, 2.38 mmol), DCM (˜40 mL) and water (40 mL). After stirring under N2 overnight, purification afforded a colourless liquid (2.33 g, 88%).
Characterisation: 1H NMR (400 MHz, CDCl3) δ ppm: 0.96-0.99 (9H, t, 3CH3), 1.47-1.54 (12H, m, 6CH2), 1.74-1.87 (2H, m, CH2), 2.02-2.09 (8H, m, 4CH2), 2.20-2.31 (2H, m, CH2); 19F NMR (368 MHz, CDCl3) δ ppm: [P4,4,4,Rf] −80.8 (3F, m, CF3), −114.3 (2F, m, CF2), −121.7 (2F, m, CF2) −122.7 (4F, m, CF2CF2), −122.8 (2F, m, CF2), −123.8 (2F, m, CF2), −126.1 (2F, m, CF2). [eFAP]− −43.4-(−45.8) (1F, m, PF), −80.3 (3F, m, C2F5), −81.9 (6F, m, 2CF3), −89.2-(−86.8) (2F, m, 2PF), −115.4 (2F, m, CF2), −115.9 (4F, m, 2CF2). ES-MS: ES+ m/z 663.1 P 4,4,4,Rf+, ES− m/z 445-eFAP.
Solubility: The solubility of [P4,4,4,C11H6F17][eFAP] in FPEE is Xα=>0.65.
Electrochemistry: Not tested, see Example 17A and 27A.
Solubility and Electrochemistry; Not tested see Example 17B and 27A.
Abbreviation: [N2(2,O,2,O,1)3][B(hfip)4].
Synthetic procedure: The synthesis of starting material [Li(hfip)4] was adapted from literature (Bulut et al., Dalton Transactions, 2011, 40, 8114). 11,1,1,3,3,3-hexafluoropropan-2-ol (16.40 g, 0.098 moles) was added dropwise to a solution of LiBH4 (0.5 g, 0.023 moles) in 1,2-dimethoxyethane ((DME), 30 mL) at −15° C. under Nitrogen. The reaction was stirred at room temperature for 4 h followed by reflux (65° C.) for 12 h. The product was concentrated under high vacuum and further dried at 50° C. for 6 h to afford a white solid (13 g, 87%).
Characterisation: 1H NMR (400 MHz, (d-THF) δ ppm: 4.70 (4H, m, CH). 11B NMR (128 MHz) (d-THF) ppm: δ1.80 (quin).
Synthesis (Quaternisation): The synthesis of [N2(20201)3][Br] was adapted from literature (Kar et al., Chem. Commun., 2016, 52, 4033, and Ferrero Vallana et al., 2017, New J Chem., 41(3), 1037-1045). Tris(2-(2-methoxyethoxy)ethyl)amine (10 g, 0.03 moles) and ethyl bromide (5.5 g, 0.05 moles) were mixed in acetonitrile (ACN) (50 mL) and stirred overnight at 50° C. under N2 to give a yellow oil. The crude product was further purified through a column (20 g basic Al2O3, eluent: dichloromethane (DCM)). The solution was extracted with water (6×40 mL) and conc. in vacuo to give a pale yellow oil (9.2 g, 70%).
Characterisation: 1H NMR (400 MHz, (d-CDCl3) δ ppm: 3.99 (6H, m, (OCH2)3), 3.90 (6H, m, (OCH2)3), 3.74 (2H, q, NCH2, J=7.12 Hz), 3.68 (m, 6H, (OCH2)3), 3.52 (6H, m, (NCH2)3), 3.35 (9H, s, (NCH3)3), 1.40 (3H, t, CH3, J=7.06 Hz). 13C NMR (d-DMSO) δ ppm: 101.97, 71.36, 70.02, 64.33, 59.43, 58.65, 57.06, and 19.71. MS [ES]+=352.4 MS [ES] 79.9.
Synthesis (Metathesis): [Li(hfip)4] (2.0 g, 0.003 moles) and [N2(20201)3][Br] (1.24 g, 0.0028 moles) were mixed in water and the solution was stirred for 12 h at room temperature. The crude product was extracted with DCM (5×25 mL) and concentrated in vacuo to give a pale yellow oil (1.70 g, 60%).
Characterisation: 1H NMR (400 MHz, (d-DMSO) δ ppm: 7.78-7.76, (1H, d), 7.33-7.31 (1H, d), 4.70 (4H, m), 3.67-3.65 (2H, t), 3.58-3.56 (2H, t), 3.50-3.48 (4H, q),3.39 (3H, s), 2.4 (3H, s). 11B NMR (128 MHz) (d-DMSO) δ ppm: 1.50 (quin). MS [ES]+=352.4 MS [ES]=678.9 (small fragmentation peak observed at m/z 167).
Solubility: Not tested, see Example 29B.
Electrochemistry: Not tested, see Example 29B.
Solubility: The solubility of [N2(2,O,2,O,1)3][B(hfip)4] in HFCP is >2.4 mol/L.
Electrochemistry: The electrochemical method was the same as Example 5A except that the electrolyte was 3.7×10−1 mol/L [N2(2,O,2,O,1)3][B(hfip)4] in HFCP. The reference electrode was a pseudo reference electrode of a Pt wire in the same electrolyte. A constant potential of −0.7V vs the reference electrode was applied for 2 hours to determine the NH3 formation rate.
A rate of 1.09×10−11 moles of NH3/cm2/s was found corresponding to a faradaic efficiency of 15%.
On the basis of this performance, the inventors anticipate a similar electrochemical result for Example 29A.
Abbreviation: [C4mpyr][eFAP].
The compound was commercially available.
Solubility: [C4mpyr][eFAP] was tested at Xα of 0.18 in HFCP and FPEE (1:1).
Electrochemistry: The electrochemical method was the same as Example 5A except that the electrolyte was Xα 0.18 [C4mpyr][eFAP] in FPEE and HFCP (1:1). A constant potential of −2V vs the reference electrode was applied for two hours to determine the NH3 formation rate. Two 1 mM H2SO4 traps were used to collect ammonia.
A yield rate of 3.3×10-12 mol/cm2/s was found corresponding to a faradaic efficiency of 4.7%.
Abbreviation: [N4,4,4,Rf][C4F9SO3], Rf═C11H6F17 also known as [N4,4,4,Rf][NfO], Rf═C11H6F17.
[N4,4,4,Rf][I] was synthesised according to known literature methods (Alhanash et al, Journal of Fluorine Chemistry 2013, 156, 152-157.). 1,1,1,2,2,3,3,4,4,5,5,6,6,7,7,8,8-heptadecafluoro-11-iodoundecane (2.49 g, 4.23 mmol) was added to tributylamine (0.78 g, 4.23 mmol) and acetonitrile (30 mL), and after purification, afforded a white solid (1.43 g, 43%).
Synthetic Procedure: see procedure for [P6,6,6,14][C5F9O2]. [N4,4,4,Rf][I] (1.17 g, 1.51 mmol) was added to potassium nonafluorobutanesulfonate (0.53 g, 1.57 mmol) and water (40 mL) at 60° C., and after purification, afforded a white solid (1.00 g, 70%).
Characterisation—1H NMR (400 MHz, CDCl3) δ ppm: 0.96-1.05 (9H, t, 3CH3), 1.39-1.48 (6H, m, 3CH2), 1.29-1.35 (12H, m, 6CH2), 1.62-1.73 (6H, m, 3CH2), 1.97-2.08 (2H, p, CH2), 2.25-2.40 (2H, (triplet of)t, CH2), 3.19-3.30 (6H, m, CH2), 3.46-3.54 (2H, m, CH2); 19F NMR (368 MHz, CDCl3) δ ppm: [N4,4,4,Rf]+−81.1 (3F, t, CF3), −113.6 (2F, t, CF2), −121.7 (2F, m, CF2) −121.9 (4F, m, CF2CF2), −122.7 (2F, m, CF2), −123.4 (2F, m, CF2), −126.1 (2F, m, CF2). [C4F9SO3−]— −80.8 (3F, t, CF3), −114.7 (2F, m, CF2), −121.7 (2F, m, CF2), −126.1 (2F, m, CF2) ES-MS: ES+ m/z 646 N4,4,4,Rf+, ES− m/z 299 C4F9SO3−
Abbreviation: [N2(2,O,2,O,1)3][eFAP].
Synthesis (Quaternisation): see procedure for [N2(2,O,2,O,1)3][Br].
Synthesis (Metathesis): 1-(2-methoxyethyl)-1-methylpyrrolidinium [eFAP] (6.8 g, 0.012 moles) and [N2(20201)3][Br] (5 g, 0.012 moles) were mixed in dichloromethane (DCM) and the solution was stirred for 12 h at room temperature. The crude product was washed with water (3×30 mL) and concentrated in vacuo to give a colourless oil (8.2 g, 89%).
Characterisation: 1H NMR (400 MHz, (d-CDCl3) δ ppm: 3.82-3.85 (6H, m), 4.70 (4H, m), 3.61-3.56 (14H, m), 3.50-3.48 (6H, q), 3.34 (9H, s), 1.33 (3H, t); 19F NMR (368 MHz, (CDCl3) δ ppm: [eFAP]− −115.5 (4F, m, 2CF2), −116.6 (2F, m, CF2), −89.6-(−87.1) (2F, m, 2PF), −81.8 (6F, m, 2CF3), −80.3 (3F, m, C2F5), −43.8-(−46.3) (1F, m, PF). MS [ES]+=352.3 MS [ES]=444.9.
Solubility: [N2(2,O,2,O,1)3][eFAP] is miscible with FPEE in all proportions.
Electrochemistry: Not tested, see Example 32B.
Solubility: [N2(2,O,2,O,1)3][eFAP] is miscible with HFCP in all proportions.
Electrochemistry: The electrochemical method was the same as Example 5A except that the electrolyte was 0.1 mol/L [N2(2,O,2,O,1)3][eFAP] in HFCP. The reference electrode was Ag/Ag+ in [C4mpyr][eFAP]. A constant potential of −0.6V vs the reference electrode was applied for two hours to determine the NH3 formation rate.
On the basis of this performance, the inventors anticipate a similar electrochemical result for Example 32A.
Abbreviation: [P6,6,6,14][eFAP]+[P6,6,6,14][C8F17SO3].
Synthetic procedure (metathesis [P6,6,6,14][eFAP]): See procedure for [P6,6,6,14][C4F9CO2]. [P6,6,6,14][Cl] (7.39 g, 14.2 mmol) was added to [C4mpyr][eFAP] (8.59 g. 14.6 mmol) and water (˜90 mL). After extraction, DCM was removed in vacuo to afford a colourless viscous oil (11.7 g, 88%).
Characterisation: 1H NMR (400 MHz, DMSO) δ ppm: 0.84-0.88 (12H, t, 4CH3), 1.23 (20H, m, 10CH2), 1.25-1.29 (12H, m, 6CH2), 1.33-1.40 (8H, m, 4CH2), 1.42-1.50 (8H, m, 4CH2), 2.10-2.18 (8H, m, 4CH2); 19F NMR (400 MHz, DMSO) δ ppm: 116.27 (115.76) (4F, m, 2CF2), 115.67 (115.26) (2F, m, CF2), −88.71-(−86.10) (2F, dm, 2PF), −81.13-(−81.02) (6F, m, 2CF3), −79.61-(−79.40) (3F, m, C2F5), −45.50-(−42.86) (1F, m, PF). ES-MS: ES+ m/z 483 P6,6,6,14+, ES− m/z 445 eFAP−.
Synthetic procedure (metathesis [P6,6,6,14][C8F17SO3]): See procedure for [P6,6,6,14][C4F9CO2]. [P6,6,6,14][Cl] (2.32 g, 4.47 mmol) was added to potassium heptadecafluorooctanesulfonate (2.34 g, 4.35 mmol) and water (40 mL). After extraction, DCM was removed in vacuo to afford a colourless oil (4.33 g, 99%).
Characterisation: 1H NMR (400 MHz, DMSO) δ ppm: 0.84-0.88 (12H, t, 4CH3), 1.23 (20H, m, 10CH2), 1.35-1.39 (12H, m, 6CH2), 1.42-1.46 (8H, m, 4CH2), 2.11-2.18 (8H, m, 4CH2), 3.72-3.85, (8H, m, 4CH2); 19F NMR (400 MHz, DMSO) δ ppm: 80.42 (80.37) (3F, m, CF3), −114.83-(−114.76) (2F, t, CF2), −121.39-(−121.31) (2F, m, CF2), −125.68-(−125.57) (2F, t, CF2). ES− MS: ES+ m/z 483 P 6,6,6,14+, ES− m/z 499 [C8F17SO3]—.
Solubility & Electrochemistry: The electrochemical method was the same as Example 5A except that the electrolyte was Xα 0.07 [P6,6,6,14][eFAP] and Xα 0.08 [P 6,6,6,14][C8F17SO3] in FPEE. The reference electrode was Ag/Ag triflate in [C4mpyr][eFAP]. A constant potential of −2V vs the reference electrode was applied for two hours to determine the NH3 formation rate. Two 1 mM H2SO4 traps were used to collect ammonia.
A yield rate of 3.4×10−12 mol/cm2/s was found corresponding to a faradaic efficiency of 5.4%.
On the basis of this performance, the inventors anticipate a similar electrochemical result for Example 33B.
Solubility and Electrochemistry: Not tested, see Example 33A.
The following further example illustrate enhanced faradaic efficiency and yield rate of ammonia in NRR carried out in ambient conditions. Surface area enhanced α-Fe@Fe3O4 nanorods grown on carbon fibre paper (CFP) were used as a NRR catalyst in an aprotic perfluorinated solvent-liquid salt mixture.
At room temperature and pressure, a moderate ammonia yield rate of 2.35×10−11 mol s−1 cm−2 with a high Faradaic efficiency of 32% was achieved in a XIL=0.23 electrolyte mixture of liquid salt/perfluorinated solvent (1-butyl-1-methy pyrrolidinium tris(pentafluoroethyl)trifluorophosphate/1H,1H,5H-octafluoropentyl 1H,1H,5H-octafluoro pentyl 1,1,2,2-tetrafluoroethyl ether). This study reveals that the ability to limit the availability of proton while increasing the N2 solubility from using aprotic fluorinated electrolyte media could effectively suppress HER. Therefore, the use of fluorinated solvent could be essential in furthering the development of electrochemical NRR technology.
The aforesaid further experiments were performed in respect of the ionic salt/solvent electrolyte combinations set out in Table 2.
Materials: 1-butyl-1-methy pyrrolidinium tris(pentafluoroethyl) trifluorophosphate ([C4mpyr][eFAP]) was purchased from Merck. Hydrofluoroether, 1H,1H,5H-Octafluoropentyl 1H,1H,5H-Octafluoropentyl 1,1,2,2-tetrafluoroethyl ether (FPEE) was purchased from Synquest. Anhydrous Iron(III)chloride was purchased from Sigma-Aldrich. Sodium sulfate was purchased from Ajax finechem. Carbon fibre paper (CFP) was purchased from Fuel Cell Store.
The gases (Argon and N2) were supplied by Air Liquide. Ultra-high purity grade Alphagaz™ (H2O<3 ppm; O2<2 ppm; CnHm<0.5 ppm) N2 and Ar were used in all experiments.
Iron nanorod synthesis: Prior to the modification of CFP with Fe@Fe3O4 NR, CFP was treated overnight with a piranha solution (3:1 v/v, H2SO4:10% H2O2) to introduce oxygen functional groups important for metal nucleation. In a glass beaker, 0.95 g of the anhydrous FeCl3 was dissolved in 70 ml of 0.5 M Na2SO4 using a magnetic stirrer for 5 minutes. The solution was transferred into a Teflon lined autoclave containing 3 cm×2 cm of the piranha treated CFP. The autoclave was sealed and kept at 160° C. for 6 hours. Following the hydrothermal reaction, a yellow film was formed (β-FeOOH) on the surface of the CFP. The film was rinsed thoroughly with Milli-Q water and ethanol and dried overnight in a vacuum oven at 60° C.
To synthesize the α-Fe@Fe3O4, the β-FeOOH on CFP was annealed at 300° C. for 2 hours achieved with a ramping rate of 5° C. min′ under a constant H2 flow of 5 ml min−1. Following the annealing, the initially yellow film was transformed into a black film and exhibited a strong ferromagnetism. The loading of the α-Fe@Fe3O4 NR on CFP was determined to be 0.5 mg cm−2.
Electrode preparation: To prepare the electrode, the CFP modified with α-Fe@Fe3O4 nanorods were cut into pieces with size of 0.5 cm×0.5 cm. The unused portion of the CFP was then sealed with Cu tape and soldered to a Cu wire. All of the Cu portions were electrochemically passivated.
Electrochemical cell: Three electrodes electrochemical cell composed of working electrode (W.E., CFP@Fe NR), reference electrode (R.E.) and counter electrode (C.E., Pt wire) was used. To prepare the R.E., silver trifluoromethanesulfonate was dissolved in [C4mpyr][eFAP] to form 10 mM Ag+ electrolyte. The reference electrode was calibrated against normal hydrogen electrode (NHE) with ferrocene/ferrocenium couple (Fc/Fc+) in XIL, =0.23 electrolyte mixture, with a basis of E0(Fc/Fc+)=0.64 V vs NHE. The C.E. used in this experiment was separated using a glass fritted anode chamber filled with the corresponding electrolyte.
Gas purification and treatment and NRR set-up: Gases used in this study (unless specifically mentioned) is further purified from NOx, O2 and H2O by passing the gas through a 10 mM H2SO4 Milli-Q trap, O2 trap column (Agilent) and a H2O trap column (Agilent), respectively. For wet N2 gas, the columns were not used.
The flow of the wet and dry N2 gas were regulated with separate gas flow meter. Before entering the electrochemical cell, the gases were mixed in a mixing chamber. The reacted N2 was then passed through a final 3 ml, 1 mM H2SO4 ammonia trap to capture the as-formed ammonia during the NRR.
Ammonia detection with indophenol blue method: Ammonia was extracted from the reaction vessel containing the hydrophobic electrolyte mixture using 1 ml of Milli-Q washing solution. From the wash solution, 0.5 ml of Milli-Q was taken and transferred into a 1 ml sample tube. Into the tube 0.5 ml of 0.5 M NaClO4, 50 μL of 1M NaOH solution (with 5 wt. % salicylic acid and 5 wt. % sodium citrate) and 10 μL of 0.5 wt. % C5FeN6Na2O (sodium nitroferricyanide) in water. The mixture was then incubated in the dark at room temperature for 3 hours before the UV-vis test.
The concentration of ammonia is determined by a calibration plot. The calibration plot was prepared by dissolving a known amount of NH4Cl in Milli-Q water. Subsequently the solutions were reacted with the indophenol blue method reagents and the ammonia content was determined using UV-Vis. The calibration plot was collected at least three times to determine the measurement errors. Calibration plot for the 1 mM H2SO4 traps were also collected separately according to the described method.
Carbon fibre paper (CFP) was selected as an electrode substrate to grow Fe nanorods (NR) due to its electrochemical inertness of conductive carbon substrate and high porosity to provide enhanced active surface area. Prior to the surface functionalisation, CFP was treated with piranha solution (a mixture of sulphuric acid and hydrogen peroxide) to create surface-bound oxygen functionalities, important for the initial heteronucleation step of the metal cations. In typical synthesis, 0.95 g of anhydrous FeCl3 is dissolved in 70 ml of 0.5 M Na2SO4. The mixture was transferred into a 100 ml Teflon-lined autoclave and hydrothermally treated at 160° C. for 6 hours. Following, hydrothermal reaction, a uniform layer of bright-yellow β-FeOOH coating was formed, confirmed by X-ray diffraction (XRD) analysis.
Thermal annealing under Hz-atmosphere was employed to reduce the as-synthesised β-FeOOH. The successful synthesis of α-Fe@Fe3O4 NR was validated by both X-ray diffraction (XRD) and scanning electron microscopy characterization techniques. XRD reveals the presence of an intense α-Fe peak at 44.8°, arising from the (110) crystal plane from an α-Fe body centred cubic system with Im-3m space group (JCPDS 06-0696). (Vayssieres et al., Nano letters, 2001, 2, 1393-1395). Relatively weaker peaks observed at 30.0°,33.8° and 43.7° corresponds to (220), (311) and (400) crystal planes in Fe3O4. The presence of Fe3O4 can be attributed to the formation of passivating oxides layer from atmospheric exposure of Fe to oxygen. (Konishi et al., Materials Transactions, 2005, 46, 329-336).
As a proof of concept and based on precedent reports, Fe-based NRR catalyst was used. The Fe catalyst was directly grown on carbon fibre paper (CFP) substrate through hydrothermal method to achieve a high surface area array of nanorods. The electrolyte used in this study is composed of a fluorinated ionic liquid (1-butyl-1-methypyrrolidinium tris(pentafluoroethyl)trifluorophosphate; ([C4mpyr][eFAP])) and a hydrofluoroether (1H,1H,5H-octafluoropentyl 1H,1H,5H-octafluoropentyl 1,1,2,2-tetrafluoroethyl ether; (FPEE)). Both compounds have been shown to exhibit significantly enhanced N2 solubility at RTP. In a previous study employing [C4mpyr][eFAP] as electrolyte on planar Fe-cathode deposited on FTO, a high FE of 60% and ammonia yield rate of 4.7×10−12 mol s−1 cm−2. In this study, the use of FPEE is expected to increase the subsequent adsorption to the electrode active sites.
Scanning electron microscopy (SEM) reveals the morphology and direction of growth of the synthesized β-FeOOH and α-Fe@Fe3O4 nanorods. The β-FeOOH grows in a perpendicular direction against the carbon fiber substrate forming a dense array of Fe nanorods. The β-FeOOH exhibits average diameter of 100-150 nm and length of ˜500-1000 nm. Following, the thermal annealing in H2, the overall morphology of the array is maintained. Most noticeably, the diameter of the individual nanorods is significantly reduced to ˜40-60 nm. The initially tubular nanorods transformed into morphology that resembles interconnected spherical particles. The significant reduction of the nanorod diameter validated the reduction of the β-FeOOH to α-Fe from the removal of structural oxygen from the crystal structure. This efficient reduction by H2 was also assisted by the existence of direct crystallographic transformation pathway between β-FeOOH and α-Fe.
The electrochemical studies were conducted using a standard 3-electrodes in a cell set-up as illustrated in
The physicochemical and electrochemical properties of FPEE, [C4mpyr][eFAP], (
As displayed by the plots of conductance vs. IL mole fraction (XIL) in FPEE in
As previously reported, although most ionic liquid exhibit large electrochemical windows, the introduction of solvent could alter the electrochemical windows of the ionic liquids. (Buzzeo et al., ChemPhysChem, 2006, 7, 176-180) The cyclic voltammograms (CVs) in
Further investigation of the NRR were carried out with controlled potential electrolysis (CPE) technique in XIL=0.16. Nitrogen gas feedstock with controlled amount of moisture (CH2O=˜100 ppm) was used as proton source for the formation of NH3. Prior to CPE experiments, the α-Fe@Fe3O4 NR cathodes were subjected to electrochemical activation at −1.35 V vs NHE for 60 s. During this period, NH3 yield rate is ˜8.0×10−12 mol s−1 cm−21, which corresponds to a NH3 total yield of ˜0.2 nmol, which is significantly lower value than the average yield reported herein (10-30 nmols). CPE with different applied potentials ranging from −0.45 V to −0.75 V vs NHE were carried out and the current transients are shown in
The highest FE and NH3 yield rate of 11.0±0.6% and 7.4×10−12 mol s−1 cm−2 mg m−2 h−1, respectively, were achieved at an applied potential of −0.65 V. This potential is lower than the previously reported optimum NRR potential of −0.8 V vs NHE on electrodeposited Fe cathode in pure [C4mpyr][eFAP]. The application of more negative potential of −0.75 V resulted in diminished FE and ammonia yield rate of 6.6±0.7% and 6.5×10−12 mol s−1 cm−2. The decreases could be ascribed to the increased selectivity towards proton reduction/hydrogen evolution reaction (HER) at more negative potentials. (Zhou et al., Energy & Environ. Sci., 2017, DOI:10.1039/C7EE02716H; Singh et al. ACS Catalysis, 2017, 7, 706-709).
In contrast to recent reports, (Chen et al., Angew. Chem. Int. Ed., 2017, 56(10), 2699-2703; Kong et al., ACS Sus. Chem. Eng., 2017, 5(11), 10986-10955) both experimental and theoretical investigations have indicated that at RTP, Fe2O3 is not the catalytically active centre for NRR in this electrolyte system, rather it is the metallic/reduced Fe-species.
Accordingly, a control Fe2O3 cathode was tried. The Fe2O3 was validated by XRD characterizations, showing that the peak at 2θ=45° for α-Fe (110) has disappeared. Although at the optimised potential of −0.65 V Fe2O3 NR cathode exhibits higher cathodic j, the NRR activity was significantly lower. The ammonia was formed with FE of <1.00% and a yield rate of less than 4.75×10−12 mol s−1 cm−2.
Although a number of Fe2O3 based catalysts have been previously reported for electrochemical NRR, high FE and NH3 have only been typically reported for elevated temperature and operated in reductive H2 environment. (Licht et al., Science, 2014, 345, 637-640; Cui et al., Green chemistry, 2017, 19, 298-304). Therefore, there is a strong indication that Fe2O3 does not provide catalytically active centres for NRR, but rather the active centres are associated with metallic/reduced Fe species.
As an example, Chen et al. has reported the use of Fe2O3/carbon nanotube hybrid cathode for NRR at RTP in aqueous media. They reported a maximum FE of 0.15% and a maximum NH3 yield rate of 3.6×10−12 mol s−1 cm−2. (Chen et al., Angewandte Chemie International Ed., 2017, 56, 2699-2701). Therefore, it is shown that the core-shell α-Fe@Fe3O4 NR structure is important for NRR. The Fe3O4 shell provides a protection to the metallic α-Fe core against further oxidation. Additionally, in contrast to the bulk reduction of Fe3O4 particles, the combination of a highly conductive α-Fe core and Fe3O4 shell should theoretically lower the energy cost for its reduction.
Furthermore, in recognising the role of liquid salt mole fraction on the physicochemical properties of the electrolyte mixture, the NRR performance of the system is further optimised for XIL
The significant drop of NH3 yield rate signifies the important role of FPEE in dissolving a high amount of N2. This observation was further confirmed by the NRR performance at lowest XIL of 0.12 tested in the series. At lower liquid salt mole fraction, the conductance is shown to decrease, leading to a lowered FE by a factor of 2, however the NH3 yield rate remains significantly higher than that in XIL of 0.46, indicating a definitive role of FPEE in increasing the ammonia yield rate. However, the factors correlating FE to XIL are harder to define.
These observations indicate that for each salt/solvent combination there is an optimum yield rate composition that combines high nitrogen solubility as well as high conductivity in the electrolyte as well as low viscosity.
Based on a previous study, it has been shown that in a pure IL system a significantly higher FE of 60% is achievable. (Zhou et al, Energy & Environ. Sci., 2017, DOI: 10.1039/C7EE02716H). The possible explanation to this is the presence of complex molecular interaction and/or different diffusion behaviour of neutral N2 molecule and polar H2O with the mixed electrolyte system. (Araque et al., The Journal of Physical Chemistry B, 2015, 119, 7015-7029). Therefore, as proposed by a precedent viewpoint by Singh et al., (ACS Catalysis, 2017, 7, 706-709), this study has proven the ability of aprotic solvents in enhancing the NRR efficiency by limiting the availability of protons in the electrochemical system. In addition, the use of FPEE further supports NRR, due to the lowered viscosity and significantly enhanced N2 solubility, which could dramatically increase N2 mass transport towards the cathode.
Notwithstanding, protons are also an important element for NH3 synthesis, further investigation on the role of moisture concentration in the optimised system of XIL=0.23 was carried out. By altering the CH2O amount in the system, FE as high as 32% and NH3 yield rate of 2.35×10−11 mol s−1 cm−2 could be achieved. The reported FE is the highest reported in aprotic solvent/liquid salt system, as well as compared to the previously reported NRR catalyst for aqueous solution to date (Table 3).
Finally, time dependent NRR studies were also carried out. The amount of NH3 produced was found to continuously increase as the electrolysis period was increased. This result provides unambiguous evidence of the formation of NH3 from NRR.
In summary, it is clear that metallic Fe sites are the electrocatalytically active NRR centres under ambient conditions. It is shown that a significantly enhanced NRR FE of 32% has been achieved under ambient conditions by choosing the appropriate electrolyte-catalyst system. The ability to control the amount of proton supply in an aprotic solvent is shown to greatly enhance the FE and NH3 yield rate at RTP due to the improved selectivity for NRR over HER. In addition, it is shown that the ability of fluorination to solubilise larger amount of N2 while reducing the viscosity of liquid salts is key to achieving systems with high NRR activity.
From a catalyst design point of view, with a core-shell structure of α-Fe@Fe3O4 NR, the loss of energy in the initial reduction of the Fe3O4 passivating shell could be minimized.
This example relates to the use of one embodiment of the invention in which a flowing liquid electrolyte is used. Preferably, the liquid flow cell for N2 reduction to ammonia consists of two electrodes, a cathode and an anode which are separated by a polymer membrane as depicted in
The cathode (the working electrode on which N2 reduction takes place) is preferably a porous, conductive, three-dimensionally structured substrate, which is coated with a high surface area, N2 reduction catalyst. Electrolyte, saturated with dissolved N2 by bubbling, is pumped from the bubbler and through the cathode. As it passes through the cathode, the electrolyte delivers N2 to the catalyst where it is adsorbed and reduced to NH3. Protons required for the reduction are produced at the anode where H2 gas is oxidised to H+, completing the anodic half of the total electrochemical reaction.
The anode (counter electrode) preferably consists of a platinised carbon catalyst on carbon paper and operates as a gas flow electrode. H2 gas is introduced to the anode through a diffusion layer of sintered stainless steel foam. There is a layer of proton conducting Nafion™ carbon catalyst and the polymer membrane, which serves to aid in proton diffusion towards the cathode and to prevent the electrolyte from flooding the anode. Protons are delivered to the cathode via diffusion through the membrane, which is a porous polymer that has been flooded with the electrolyte. After the NH3 is produced it is carried away from the reaction site by the flowing electrolyte and into the product separation vessel.
The catalyst (for example nanostructured iron, iron oxides or ruthenium) may be deposited on the substrate in several ways including direct electrodeposition, drop casting of catalyst/carbon/conducting-polymer slurries or oleate-mediated hydrothermal deposition. Its purpose is to provide a high density of electrochemically active sites for the reduction of dissolved N2 molecules to ammonia.
Ideally, the cathodic substrate must be highly conductive, porous, wettable by the electrolyte when coated with the catalyst, and must have a high surface area. Examples of such substrates include carbon fibre paper, graphitic carbon felt, 3D-printed metals (iron, stainless steel, nickel etc.), sintered metal foams, stainless steel, steel or iron wool, and multilayered, metallic meshes or grids. These materials allow the unhindered flow of electrolyte to a greater or lesser extent while still providing a high internal surface area on which the catalyst may be deposited. This flow of electrolyte is important as it both delivers dissolved N2 to the catalyst and removes NH3 from the active sites which otherwise would hinder further NH3 production. Once the electrolyte has left the cathode the NH3 can be removed and the electrolyte can be recycled through the cell again. Hydrogen evolved at the cathode along with the NH3 are separated in the product separation vessel.
In a further experiment relating to the present invention, a cell was designed as described above which included a cathode of metallic iron electrodeposited on graphitic carbon felt and a Solupor™ polyethylene membrane.
A potential bias of 1V was applied between the anode and the cathode for 1 hr while an electrolyte of a ratio of 1:2 [C4mpyr][eFAP] to trifluorotoluene was flowed through the cathode at a rate of approximately 10 mL/min. The current was measured and the N2 gas bubbled through the electrolyte was captured by a 1 mM H2SO4 trap to be analysed for NH3. The electrolyte was washed with 1 mM H2SO4 and the aqueous phase was also analysed using the indophenol method for ammonium determination. A rate of ammonia production of 3.2×10−11 mol/cm2/s with a Faradaic efficiency of 5.8%.
In another embodiment, the liquid flow cell of Example 34 was set up such that the evolved hydrogen collected from the separation vessel was introduced into the anode hydrogen stream. This enables the hydrogen collected to be usefully consumed in the anode reaction as depicted in
In another embodiment, the liquid flow cell of Example 34 was used with H2O oxidation as the anode reaction. In this case, the introduced H2 was replaced by H2O vapour in a nitrogen stream.
While this invention has been described in connection with specific embodiments thereof, it will be understood that it is capable of further modification(s). This application is intended to cover any variations uses or adaptations of the invention following in general, the principles of the invention and including such departures from the present disclosure as come within known or customary practice within the art to which the invention pertains and as may be applied to the essential features hereinbefore set forth.
As the present invention may be embodied in several forms without departing from the spirit of the essential characteristics of the invention, it should be understood that the above described embodiments are not to limit the present invention unless otherwise specified, but rather should be construed broadly within the spirit and scope of the invention as defined in the appended claims. The described embodiments are to be considered in all respects as illustrative only and not restrictive.
Various modifications and equivalent arrangements are intended to be included within the spirit and scope of the invention and appended claims. Therefore, the specific embodiments are to be understood to be illustrative of the many ways in which the principles of the present invention may be practiced. In the following claims, means-plus-function clauses are intended to cover structures as performing the defined function and not only structural equivalents, but also equivalent structures.
“Comprises/comprising” and “includes/including” when used in this specification is taken to specify the presence of stated features, integers, steps or components but does not preclude the presence or addition of one or more other features, integers, steps, components or groups thereof. Thus, unless the context clearly requires otherwise, throughout the description and the claims, the words ‘comprise’, ‘comprising’, ‘includes’, ‘including’ and the like are to be construed in an inclusive sense as opposed to an exclusive or exhaustive sense; that is to say, in the sense of “including, but not limited to”.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2017902960 | Jul 2017 | AU | national |
| 2018900370 | Feb 2018 | AU | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/AU2018/000122 | 7/26/2018 | WO | 00 |
