Patent Application
20230407490
The current invention relates to a water electrolyzer system capable of generating hydrogen and oxygen gas simultaneously, but separately, as well as the operation of the system.
The listing or discussion of a prior-published document in this specification should not necessarily be taken as an acknowledgement that the document is part of the state of the art or is common general knowledge.
With rising concerns for energy and environment, there has been an increasing demand for clean and renewable energy sources to replace fossil fuels. Hydrogen is an energy carrier and an important feedstock for the chemical industry, and is considered one of the most important clean fuels. However, a vast majority of hydrogen in the world is obtained by natural gas steam re-forming, which produces CO2 with an inevitable carbon footprint. Electrolytic water splitting by generating hydrogen and oxygen from water without additional emissions has attracted considerable attention due to its inherent advantages of producing relatively pure hydrogen, flexibility for small and large-scale production, and sustainability when the electricity used in the electrolysis comes from renewable sources (Lu, X. & Zhao, C., Nat. Commun. 2015, 6, 6616; Wang, Y. et al., Nano Energy 2018, 48, 590-599; and Wang, X. P. et al., Energy Environ. Sci. 2020, 13, 229-237).
In a conventional water splitting system where hydrogen and oxygen evolution occur simultaneously at the two electrodes separated by a diaphragm or ion-exchange membrane, gas crossover issue can happen, especially at low operating current density or elevated pressures, rendering the need for further purification of the collected hydrogen gas. In addition, the formed OH· radicals deteriorate the membrane and other cell components.
To mitigate the above issues, some novel systems have recently been devised to decouple the formation of H2 and O2. One effective way is to use a third redox system, such as nickel hydroxide (Chen, L. et al., Nat. Commun. 2016, 7, 11741) or anthraquinone-2,7-disulfonic acid (Kirkaldy, N. et al., Chem. Sci. 2018, 9, 1621-1626), the redox potential of which is between the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) potential, so as to divide the water electrolysis into two chronologically separated steps for H2 and O2 production at different times. Another way is to employ an electron-coupled proton buffer which has a high proton-electron storage capacity, with which H2 can be produced at a separate time on demand (Rausch, B. et al., Science 2014, 345, 1326-1330; Chen, J. J.; Symes, M. D. & Cronin, L., Nat. Chem. 2018, 10, 1042-1047; MacDonald, L. et al., Sustain. Energy Fuels 2017, 1, 1782-1787; and Lei, J. et al., Chemistry 2019, 25, 11432-11436). Nevertheless, despite improved separation of high purity gases, H2 and O2 cannot be produced concurrently, which inevitably results in an extended time because of the intermittent operation. In addition, the formation of H2 and/or O2 mostly remains at the electrode/electrolyte interface, which results in an inflated overpotential due to the adsorption of the produced gaseous species on the electrodes (Angulo, A. et al., Joule 2020, 4, 555-579). The bubbles evolved on the electrode may induce blockage of the electrocatalyst surface and decrease the ion conductivity in the electrolyte (Wu, H. et al., J. Mater. Chem. A 2017, 5, 24153-24158; and Belleville, P. et al., Int. J. Hydrog. Energy 2018, 43, 14867-14875). In an alternative approach, Girault and coworkers incorporated a redox-flow battery module, using V2+/3+ and Ce3+/4+ as the redox mediators in strong acid media for both energy storage and task specific H2 production (Peljo, P. et al., Green Chem. 2016, 18, 1785-1797; Dennison, C. R. et al., Chimia (Aarau) 2015, 69, 753-758; and Amstutz, V. et al., Energy Environ. Sci. 2014, 7, 2350-2358). While it allows for concurrent off-electrode H2/O2 generation, it is limited by the inherently large free energy loss and the use of corrosive electrolytes.
Therefore, there is a need to develop new water electrolyzers that can overcome the limitations of existing water electrolyzers.
It has been surprisingly found that it is possible to generate hydrogen and oxygen gas simultaneously, continuously and separately from a water electrolyzer system.
Aspects and embodiments of the invention are now discussed by reference to the following numbered clauses.
1. A spatially decoupled redox flow water electrolyzer, comprising:
2. The electrolyzer according to Clause 1, wherein:
3. The electrolyzer according to Clause 2, wherein the supporting electrolyte comprises a solvent and one or more compounds or salts that provide ions.
4. The electrolyzer according to Clause 3, wherein the ions in the salts that provide ions are selected from:
5. The electrolyzer according to Clause 3 or Clause 4, wherein the solvent is water.
6. The electrolyzer according to any one of Clauses 3 to 5, wherein one or more of the following apply:
7. The electrolyzer according to any one of Clauses 2 to 6, wherein the cathodic redox mediator is selected from one or more of DHPS/DHPS-2H, iron (III) triethanolamine/iron (II) triethanolamine, phenazine and derivatives thereof, and viologen and derivatives thereof, optionally wherein the cathodic redox mediator is DHPS/DHPS-2H.
8. The electrolyzer according to any one of Clauses 2 to 7, wherein the cathodic redox mediator is provided in the liquid catholyte at a total concentration of the redox mediator(s) from 0.05 to 3 M, such as from 0.2 to 2 M, such as from 0.5 to 1 M or from 0.4 to 0.8 M, such as about 0.6 M.
9. The electrolyzer according to any one of Clauses 2 to 8, wherein the anodic redox mediator is selected from one or more of [Fe(CN)6]3−/[Fe(CN6)]4−, [MnO4]2−/[MnO4]−, ferrocene and derivatives thereof, and TEMPO and derivatives thereof, optionally wherein the anodic redox mediator is [Fe(CN)6]3−/[Fe(CN6)]4−.
10. The electrolyzer according to any one of Clauses 2 to 9, wherein the anodic redox mediator is provided in the liquid anolyte at a total concentration of the redox mediator(s) from 0.05 to 3 M, such as from 0.2 to 2 M, such as from 0.5 to 1 M or from 0.4 to 0.8 M, such as about 0.6 M.
11. The electrolyzer according to any one of Clauses 2 to 10, wherein the catalyst in the oxygen generation compartment is selected from one or more of the group consisting of NiFe(OH)2@Ni, IrO2, RuO2, other transition metal oxides (TMOs), carbides (TMCs), nitrides (TMNs), phosphides (TMPs), dichalcogenides (TMDs), and borides (TMBs), optionally wherein the catalyst is NiFe(OH)2@Ni.
12. The electrolyzer according to any one of Clauses 2 to 11, wherein the catalyst in the hydrogen generation compartment is selected from one or more of the group consisting of Pt—Ni(OH)2@Ni, Pt@C, a transition metal (TM), a metal alloy, a transition metal oxide (TMO), a transition metal carbide (TMCs), a transition metal nitride (TMNs), a transition metal phosphide (TMPs), a transition metal dichalcogenide (TMD), a transition metal boride (TMBs), and a noble metal, optionally wherein the catalyst is Pt—Ni(OH)2@Ni.
13. The electrolyzer according to any one of Clauses 2 to 12, wherein the electrolyzer is configured to introduce the liquid catholyte and/or the liquid catholyte to the hydrogen generation compartment and oxygen generation compartment, respectively, as a spray.
14. The electrolyzer according to any one of Clauses 2 to 13, wherein the electrolyzer is able to generate oxygen and/or hydrogen with a purity of greater than or equal to 99.9%, such as 99.99%.
15. The electrolyzer according to any one of Clauses 2 to 13, wherein the weight ratio of the catalyst in the oxygen generation compartment to the catalyst in the hydrogen generation compartment is from 0.1:1 to 10:1, such as from 0.5:1 to 2:1.
16. A method of using a spatially decoupled redox flow water electrolyzer according to Clause 1, involving the steps of:
17. The method according to Clause 16, wherein the supporting electrolyte comprises a solvent and one or more compounds or salts that provide ions.
18. The method according to Clause 17, wherein the ions in the salts that provide ions are selected from:
19. The method according to Clause 17 or Clause 18, wherein the solvent is water.
20. The method according to any one of Clauses 16 to 20, wherein one or more of the following apply:
21. The method according to any one of Clauses 16 to 20, wherein the cathodic redox mediator is selected from one or more of DHPS/DHPS-2H, iron (III) triethanolamine/iron (II) triethanolamine, phenazine and derivatives thereof, and viologen and derivatives thereof, optionally wherein the cathodic redox mediator is DHPS/DHPS-2H.
22. The method according to any one of Clauses 16 to 21, wherein the cathodic redox mediator is provided in the liquid catholyte at a total concentration of the redox mediator(s) from 0.05 to 3 M, such as from 0.2 to 2 M, such as from 0.5 to 1 M or from 0.4 to 0.8 M, such as about 0.6 M.
23. The method according to any one of Clauses 16 to 22, wherein the anodic redox mediator is selected from one or more of [Fe(CN)6]3−/[Fe(CN6)]4−, [MnO4]2−/[MnO4]−, ferrocene and derivatives thereof, and TEMPO and derivatives thereof, optionally wherein the anode redox mediator is [Fe(CN)6]3−/[Fe(CN6)]4−.
24. The method according to any one of Clauses 16 to 23, wherein the anodic redox mediator is provided in the liquid anolyte at a total concentration of the redox mediator(s) from 0.05 to 3 M, such as from 0.2 to 2 M, such as from 0.5 to 1 M or from 0.4 to 0.8 M, such as about 0.6 M.
25. The method according to any one of Clauses 16 to 24, wherein the catalyst in the oxygen generation compartment is selected from one or more of the group consisting of NiFe(OH)2@Ni, IrO2, RuO2, other transition metal oxides (TMOs), carbides (TMCs), nitrides (TMNs), phosphides (TMPs), dichalcogenides (TMDs), and borides (TMBs), optionally wherein the catalyst is NiFe(OH)2@Ni.
26. The method according to any one of Clauses 16 to 25, wherein the catalyst in the hydrogen generation compartment is selected from one or more of the group consisting of Pt—Ni(OH)2@Ni, Pt@C, a transition metal (TM), a metal alloy, a transition metal oxide (TMO), a transition metal carbide (TMCs), a transition metal nitride (TMNs), a transition metal phosphide (TMPs), a transition metal dichalcogenide (TMD), a transition metal boride (TMBs), and a noble metal, optionally wherein the catalyst is Pt—Ni(OH)2@Ni.
27. The method according to any one of Clauses 16 to 26, wherein the liquid catholyte and/or the liquid catholyte is introduced to the hydrogen generation compartment and oxygen generation compartment, respectively, as a spray.
28. The method according to any one of Clauses 16 to 27, wherein the electrolyzer is able to generate oxygen and/or hydrogen with a purity of greater than or equal to 99.9%, such as 99.99%.
29. The method according to any one of Clauses 16 to 28, wherein the weight ratio of the catalyst in the oxygen generation compartment to the catalyst in the hydrogen generation compartment is from 0.1:1 to 10:1, such as from 0.5:1 to 2:1.
30. The method according to any one of Clauses 16 to 29, wherein the electrolyzer provides a stable voltage at any given current density.
It has been surprisingly found that it is possible to generate hydrogen and oxygen gas simultaneously, continuously and separately from a water electrolyzer system. Thus, according to a first aspect of the invention, there is provided a spatially decoupled redox flow water electrolyzer, comprising:
The system described herein uses an electrolytic flow cell integrated with separate gas production reactors. This spatial separation of the generation of H2 and O2 allows one to obtain H2 and O2 with ultrahigh purity. The H2 and O2 can be generated concurrently and continuously in the systems described herein, which results in improved productivity as no time is wasted in having to generate each gas at a separate time. H2 and O2 bubbles are completely removed from the flowing electrolyte, which eases cell stack design, as the H2 and O2 are produced in separate compartments away from the electrode surfaces, thereby avoiding the blocked surface area and overpotential caused by gas bubbles generated on the electrodes of conventional systems. It is also possible to use low-cost and robust redox active mediators in an alkaline electrolyte (e.g. DHPS/DHPS-2H and [Fe(CN)6]3−/[Fe(CN)6]4−). This is less corrosive than directly electrochemical water splitting on the electrode in acidic media, as is conventionally done. Given these advantages, the systems disclosed herein are likely to be more stable than conventional systems, meaning that the costs associated with maintenance will be much less as well.
In embodiments herein, the word “comprising” may be interpreted as requiring the features mentioned, but not limiting the presence of other features. Alternatively, the word “comprising” may also relate to the situation where only the components/features listed are intended to be present (e.g. the word “comprising” may be replaced by the phrases “consists of” or “consists essentially of”). It is explicitly contemplated that both the broader and narrower interpretations can be applied to all aspects and embodiments of the present invention. In other words, the word “comprising” and synonyms thereof may be replaced by the phrase “consisting of” or the phrase “consists essentially of” or synonyms thereof and vice versa.
The phrase, “consists essentially of” and its pseudonyms may be interpreted herein to refer to a material where minor impurities may be present. For example, the material may be greater than or equal to 90% pure, such as greater than 95% pure, such as greater than 97% pure, such as greater than 99% pure, such as greater than 99.9% pure, such as greater than 99.99% pure, such as greater than 99.999% pure, such as 100% pure.
As will be appreciated, the system described above requires consumables and catalysts to operate. As such, the electrolyzer, where prepared for (and in) use will be one in which:
Both the cathode and anode compartments contain electrodes, i.e., the cathode and the anode, which can be a carbon, a metal, or a combination thereof. Preferably, these two electrodes have high surface area, to facilitate the desired electrolysis process. They can be made of a carbon, a metal, or a combination thereof.
A spatially decoupled redox flow water electrolyzer according to the current invention is depicted in
An anion-exchange membrane 130 is disposed between the catholyte 111 and anolyte 121 tanks, which allows anions to move from the cathode tank to the anode tank. A current collector is also attached to the catholyte and anolyte tank.
As depicted in
Both the liquid catholyte and the liquid anolyte require the presence of a supporting electrolyte and the desired redox mediator. The supporting electrolyte referred to above in connection to the liquid catholyte and liquid anolyte may comprise a solvent and one or more compounds or salts that provide ions.
A suitable solvent for use in the supporting electrolyte is water.
As mentioned above, the supporting electrolyte also comprises one or more compounds or salts that provide ions. Any suitable material may be used in this capacity. Suitable ions that may be mentioned herein include, but are not limited to ammonium ions, lithium ions, sodium ions, potassium ions, magnesium ions, calcium ions, chloride ions, and hydroxide ions (e.g. ammonium ions, lithium ions, sodium ions, potassium ions, chloride ions, and hydroxide ions). For example, sodium hydroxide may be used as a source of sodium and hydroxide ions.
The supporting electrolyte may be a material that is basic in nature. For example, the supporting electrolyte may have a pH value of from 11 to 15. The concentration of the one or more compounds or salts that provide ions in the solvent may be from 0.05 to 10 M, such as from 1 to 5 M, such as about 4 M.
A redox mediator refers to a compound present (e.g., dissolved) in the electrolyte (catholyte or anolyte) that acts as a molecular shuttle transporting charges between the respective electrodes and the catalysts. A redox mediator transports charge between the respective electrode and the catalyst.
The cathodic redox mediator is selected from one or more of DHPS/DHPS-2H, iron (III) triethanolamine/iron (II) triethanolamine, phenazine and derivatives thereof, and viologen and derivatives thereof. For example, the cathodic redox mediator may be DHPS/DHPS-2H.
Viologens are 1,1′-disubstituted 4,4′-bipyridinium ions (where the nitrogen atoms of the pyridine rings are substituted by an alkyl group (e.g. C1 to C12 alkyl)), with a suitable counterion (e.g. Cl−, F−, Br and I−). An example of a viologen of this type is paraquat. When used herein viologens may include related compounds, such as diquat and bipolaron. Thus, the term “viologens and derivatives thereof” should be interpreted accordingly.
Phenazine and derivatives thereof may refer to phenazine itself, as well as phenothiazine derivatives and phenoxazine derivatives, which may be used as redox mediators and may have the following structure:
The cathodic redox mediator may be provided in the liquid catholyte at a total concentration of the redox mediator(s) from 0.05 to 3 M, such as from 0.2 to 2 M, such as from 0.5 to 1 M or from 0.4 to 0.8 M, such as about 0.6 M.
The anodic redox mediator may be selected from one or more of [Fe(CN)6]3−/[Fe(CN6)]4−, [MnO4]2−/[MnO4]−, ferrocene and derivatives thereof, and TEMPO and derivatives thereof. For example, the anodic redox mediator may be [Fe(CN)6]3−/[Fe(CN6)]4−.
Derivatives of ferrocene that may be mentioned herein include ferrocene derivatives having the structure:
In the above formulae, X is selected from H, F, Cl, Br, I, NO2, COOR, C1-20 alkyl, CF3, and COR, in which R is H or C1-20 alkyl; n is from 0 to 20.
Specific derivatives of ferrocene that may be mentioned herein include but are not limited to bromoferrocene, ferrocenylmethyl dimethyl ethyl ammonium bis(trifluoromethanesulfonyl)imide (Fc1N112-TFSl), N-(pyridin-2-ylmethylene)-1-(2-(diphenylphosphino) ferrocenyl) ethanamine (FeCp2PPh2RCN), 1,1-dimethylferrocene (DMFc), tetraferrocene, di(ethylsulfonic sodium) ferrocene (C14H16FeS2O6Na2), and di(trimethanesulfonic sodium) ferrocene (C16H22FeS2O6Na2).
In particular embodiments of the invention that may be mentioned herein, the derivative of ferrocene may be di(trimethanesulfonic sodium) ferrocene (C16H22FeS2O6Na2) or di(ethylsulfonic sodium) ferrocene (C14H16FeS2O6Na2).
TEMPO is 2,2,6,6-tetramethylpiperidin-1-yl)oxyl and derivatives thereof that may be mentioned herein include, but are not limited to 4-Hydroxy-TEMPO, 4-oxo-TEMPO, 4-amino-TEMPO, 4-cyano-TEMPO and 4-carboxy-TEMPO.
The anodic redox mediator is provided in the liquid anolyte at a total concentration of the redox mediator(s) from 0.05 to 3 M, such as from 0.2 to 2 M, such as from 0.5 to 1 M or from 0.4 to 0.8 M, such as about 0.6 M.
The catalyst used in the oxygen generation compartment may be selected from one or more of the group consisting of NiFe(OH)2@Ni, IrO2, RuO2, other transition metal oxides (TMOs), carbides (TMCs), nitrides (TMNs), phosphides (TMPs), dichalcogenides (TMDs), and borides (TMBs). For example, the catalyst may be NiFe(OH)2@Ni.
The catalyst in the hydrogen generation compartment may be selected from one or more of the group consisting of Pt—Ni(OH)2@Ni, Pt@C, a transition metal (TM), a metal alloy, a transition metal oxide (TMO), a transition metal carbide (TMCs), a transition metal nitride (TMNs), a transition metal phosphide (TMPs), a transition metal dichalcogenide (TMD), a transition metal boride (TMBs), and a noble metal. For example, the catalyst may be Pt—Ni(OH)2@Ni.
An advantage of the current system is that it allows for both oxygen and hydrogen gas to be generates at separate locations, thereby avoiding the issues associated with conventional devices. However, as will be appreciated, if one pumps a liquid into a container, then this may cause issues if the result of this input involves the generation of a gas. To avoid this, the electrolyte could be introduced into the respective gas generation compartments in the form of a spray to avoid flooding of the catalyst beds and facilitate the removal of the formed gas. Thus, in certain embodiments of the invention, the electrolyzer may be configured to introduce the liquid catholyte and/or the liquid catholyte to the hydrogen generation compartment and oxygen generation compartment, respectively, as a spray.
As noted above, an advantage of the current system is the ability to generate oxygen and hydrogen gas simultaneously at different locations within the system. This may help to produce hydrogen and oxygen gas of very high purity. For example, the electrolyzer of the current invention may be able to generate oxygen and/or hydrogen with a purity of greater than or equal to 99.9%, such as 99.99%.
A further advantage of the decoupling of the oxygen and hydrogen production is that could enhance the overall power performance of the device by loading more catalysts (e.g., into the oxygen generation compartment in particular) without otherwise needing to alter the electrochemical cell. In embodiments of the invention, the weight ratio of the catalyst in the oxygen generation compartment to the catalyst in the hydrogen generation compartment may be from 0.1:1 to 10:1, such as from 0.5:1 to 2:1.
In a second aspect of the invention, there is provided a method of using a spatially decoupled redox flow water electrolyzer as described hereinbefore, involving the steps of:
An advantage of the device and method used herein is that the electrolyzer may provide a stable voltage at any given current density.
As details of the materials are discussed extensively above in relation to these devices, they will not be repeated here again in relation to the method for the sake of brevity. Details of how to operate the system may be obtained from the Examples section below.
Further aspects and embodiments of the invention will now be discussed below by reference to the following non-limiting examples.
All the chemicals and reagents were purchased from commercial suppliers and used without further purification. Benzene-1,2-daimine, 2,5-dihydroxy-1,4-benzoquinone, K2PtCl4, Ni(NO3)2·6H2O, Fe(NO3)3·9H2O, NaOH were purchased from Sigma-Aldrich. HCl and H2SO4 were purchased from Alfa Aesar. 10% wt. platinum on carbon was purchased from Fuel Cell Earth. Carbon felt was purchased from Tieling Shenhe Carbon Fiber Material Company. Anion-exchange membrane (Fumasep® FAAM-15) was purchased from Fuel Cell Store. Anion-exchange membrane (Sustainion® X37-50 membrane) was purchased from Dioxide Materials.
The catalyst was directly used for FESEM, EDX and XPS characterizations without any further preparation.
The morphology and microstructure of the synthesized materials were characterized by a Zeiss Supra 40 FESEM at 5 kV.
EDX was recorded at an acceleration voltage of 15 kV.
XPS analysis was conducted with a Kratos Analytical Axis Ultra DLD spectrometer. Monochromated Al K radiation was used as the radiation source, and all the measurements were carried out in vacuum.
TEM measurement was performed on a JEOL-3010 (300 kV acceleration voltage). The samples were ultrasonically dispersed in ethanol for 1 h and the supernatant was dripped onto a copper grid.
1H NMR spectroscopic measurements were performed on Bruker 400 MHz spectrometers. The NMR spectra were recorded in solutions of deuterated dimethyl sulfoxide (DMSO-d6) with residual DMSO (2.49 ppm for 1H NMR) taken as the internal standard. Where necessary, NMR spectra were recorded on Bruker 500 MHz spectrometer using aqueous samples with 10% D2O and suppressed the water signal. The chemical shifts were expressed in parts per million (δ).
UV-vis spectra were collected with a SHIMADZU UV-1800 spectrometer.
We report an alkaline redox-flow electrolytic cell integrated with separate gas production reactors to spatially decouple the concurrently generated H2 and O2 (
A pair of redox mediators were employed as electrolyte-borne charge carriers circulating between the central electrode compartment and separate catalyst bed. Upon operation, the mediators are electrochemically charged on the electrode and then chemically discharged through catalytic HER and OER reactions in the respective reactor tank. For alkaline water electrolysis, despite the advantage of better durability and feasibility for large-scale production (Wang, Y. et al., Nano Energy 2018, 48, 590-599; and Wei, J. et al., Nano-Micro Lett. 2018, 10, 75), a major challenge is that an additional energy barrier stemming from the sluggish water dissociation step needs to be overcome to generate the essential H* intermediates for hydrogen evolution (Mahmood, N. et al., Adv. Sci. 2018, 5, 1700464; Hong, Y., Choi, C. H. & Choi, S. I., ChemSusChem 2019, 12, 4021-4028; Kou, T. et al., Nat. Commun. 2020, 11, 590; and Wang, Y. et al., Adv. Energy Mater. 2017, 7, 1601390). Herein, the phenazine derivative DHPS was employed as the HER redox mediator, which acts as both proton and electron carriers to circumvent the aforementioned rate-limiting step, as DHPS has been reported to be a robust anodic redox mediator for alkaline flow battery (Hollas, A. et al., Nat. Energy 2018, 3, 508-514). As shown in
For the OER side, [Fe(CN)6]3−/4− serves as an energetic charge carrier and instigates OER reaction when it flows through the NiFe(OH)2 catalyst bed in a separate reactor (steps 3 and 4). With such a flow cell setup, DHPS and [Fe(CN)6]4− are instantaneously regenerated as the electrolyte circulates through the cell while H2 and O2 are uninterruptedly produced in the tank, thus obviating complex gas electrode design and gas mixing. The mechanistic process underlying the DHPS-mediated HER reaction was collectively scrutinized with operando UV-vis, NMR and EPR spectroscopy and computational studies in the following examples.
In a round bottom flask, benzene-1,2-diamine (20.0 g, 185 mmol) was added portion wise to concentrated H2SO4 (110 mL) over a period of 30 min. The resulting turbid solution was stirred for 30 min at room temperature to obtain a limpid solution. After that, the reaction mixture was heated at 140° C. for 20 h. The reaction mixture was cooled to 0° C. and ice-cold water (ca. 200 mL) was added slowly until the product precipitated as an off-white solid. The solid was filtered by vacuum filtration and dried for 2 h. The solid was transferred to a round bottom flask and deionized (DI) water (100 mL) was added. The turbid was stirred for 60 min, filtered and dried to get 3,4-diaminobenzenesulfonic acid as an off-white solid (19.8 g, 58%).
1H NMR (400 MHz, DMSO-d6) δ 7.39 (1H, d, J=1.8 Hz), 7.19 (1H, dd, J1=8.4 Hz, J2=1.8 Hz), 6.86 (1H, d, J=8.4 Hz). (Note: the signals for the amine and sulfonic acid protons were a broad hump and merged with aromatic protons)
DHPS (Hollas, A. Et al., Nat. Energy 2018, 3, 508-514)
In a round bottom flask, DI water (130 mL) was heated to 105° C. in an oil bath. While warming the water, 2,5-dihydroxy-1,4-benzoquinone (8.27 g, 59.0 mmol) was added portion wise and stirred for 5 min at 105° C. Then, 3,4-diaminobenzenesulfonic acid (11.1 g, 59.0 mmol) was added portion-wise over a 5 min period. The resulting dark brown reaction mixture was heated to reflux for ca. 16 h. The reaction mixture was cooled to room temperature and diluted with acetone (150 mL). The solid was filtered by vacuum filtration, washed with water (3×50 mL) and acetone (3×50 mL). The solid was dried using vacuum for two days and subsequently dried under reduced pressure using a rotary evaporator at 50° C. for 2 h to obtain DHPS as a dark gold solid (14.8 g, 86%).
1H NMR (400 MHz, DMSO-d6) δ: 8.30 (s 1H), 8.14 (d, J=8.2 Hz, 1H), 8.06 (d, J=8.2 Hz, 1H), 7.37 (s 1H), 7.35 (s 1H) (Note: The acidic protons (OH, SO3H) were observed as a broad hump from (8.8-8.4). 13C NMR (100 MHz, DMSO-d6) δ:157.4, 156.6, 149.4, 140.3, 138.9, 137.1, 135.1, 128.3, 126.1, 120.8, 105.3, 104.1.
The electrochemical properties of DHPS and [Fe(CN)6]3−/4− were analyzed by CV in 4.0 M of NaOH aqueous solution on an Autolab electrochemical workstation (Metrohm, PSTA30) with a three-electrode cell system. Glassy carbon, graphite rod and Hg/HgO were used as the working, counter and reference electrodes, respectively. The electrolyte was 4 M NaOH aqueous solution bubbled with N2 for 1 h prior to use. For the CV test of DHPS, the cells were sealed and protected by N2.
The peak currents at different scan rate follow a linear relation with the square root of the scan rate for both the oxidation and reduction reactions, indicating a diffusion-controlled and electrochemically reversible process. The diffusion coefficient could be determined with the Randles-Sevcik equation (Pan, F. et al., Chem. Mater. 2016, 28, 2052 2057) below:
I
p=2.69×105n3/2AD1/2Cv1/2
where Ip is the peak current in ampere, n is the number of electrons transferred (assumed to be two for DHPS, one for [[Fe(CN)6]3−/4−), A is the electrode area in cm2, D is the diffusion coefficient in cm2/s, C is the electrolyte concentration in mol/cm3, and v is the scan rate in V/s.
Hierarchical Pt-decorated Ni(OH)2 and NiFe(OH)2 nanosheets were synthesized as the HER and OER catalyst, respectively. Considering that powdery materials are susceptible to flowing with the electrolyte, the materials were thus deposited on Ni foam substrate.
Firstly, 5 pieces of Ni foam (3×2 cm2) were immersed in 1 M HCl under ultrasonic treatment to wash away the oxidation layer on the surface. Then, the Ni substrates were rinsed with DI water and transferred to a sealed glass bottle containing HCl aqueous solution (80 mL, 0.11 mM). The reaction mixture was heated at 80° C. under stirring for 20 h. The substrates were washed with DI water and dried in a vacuum oven at 50° C.
The electrochemical deposition was conducted in a three-electrode electrochemical system, using 20 mL of 1 M KOH solution containing K2PtCl4 (200 μL, 60 mM) as the electrolyte. The Ni(OH)2 substrate, graphite rod, and Hg/HgO electrode were used as the working, counter and reference electrodes, respectively. The electrochemical deposition was performed by CV in a potential range of from −0.9 to −1.9 V vs. Hg/HgO for 50 cycles at a sweep rate of 5 mV s−1.
Porous and nanocrystalline flakes of NiFe(OH)2 was synthesized on Ni foam through a facile self-regulated acid-etching method (Wu, H. et al., J. Mater. Chem. A 2017, 5, 24153-24158). For NiFe(OH)2 growth, a piece of HCl-treated Ni foam was treated for the preparation of Ni(OH)2 nanosheets. Then, the Ni foam was transferred to a sealed glass bottle filled with 20 mL of 0.1 mM HCl and 0.01 mM Fe(NO3)3·9H2O, under stirring and heated at 80° C. for 20 h.
The two catalysts, Pt-decorated Ni(OH)2 and NiFe(OH)2 nanosheets, prepared in Example 4 were taken for characterization studies.
For the synthesis of Pt—Ni(OH)2, a uniform network composed of interwoven ultrathin Ni(OH)2 nanosheets (
In comparison, porous and nanocrystalline flakes of NiFe(OH)2 were synthesized on Ni foam through a facile self-regulated acid-etching method (Wu, H. et al., J. Mater. Chem. A 2017, 5, 24153-24158). SEM images (
The electrochemical-chemical properties of the materials prepared in Examples 2 and 4 were studied. CV tests were carried out by following the protocol in Example 3.
LSV was conducted on an Autolab electrochemical workstation (Metrohm, PSTA30). Potentials were presented versus reversible hydrogen electrode (RHE) based on the following equation:
E(V vs. RHE)=EHg/HgO+0.098 V+0.0591 V×pH
The electrochemical properties of DHPS and [Fe(CN)6]3−/4− were scrutinized with voltammetric measurements. DHPS exhibited exemplary robustness in alkaline solution at a potential of −0.05 V (vs. RHE) which nearly coincides with that of HER, while [Fe(CN)6]3−/4− had a potential of 1.37 V (vs. RHE) which is slightly higher than that of OER reaction (
One essential prerequisite for the decoupled water splitting based on a regenerative electrochemical-chemical cycle is that the electrochemical process of the pair of redox mediators should inherently be faster than that of the electrolytic HER and OER reactions. This is so that the overall kinetics, particularly here, that are in tandem with additional HER and OER catalytic chemical process will not be impaired. A controlled LSV was thus conducted to compare the reaction kinetics of both OER and HER reactions with those of the above two redox mediators, DHPS and [Fe(CN)6]3−/4− (
The catalytic properties of Pt—Ni(OH)2 and NiFe(OH)2 towards HER and OER reactions were examined by LSV and CV measurements in 4 M NaOH electrolyte by following the protocols in Examples 3 and 6, respectively.
The reductive sweep curve of the Pt—Ni(OH)2 catalyst exhibited an onset overpotential of ˜10 mV vs. RHE, followed by a sharp current enhancement corresponding to H2 evolution, which is far superior to the Ni(OH)2 electrode and 10% commercial Pt/C—CC electrode (
Based on the above examples, an electrolytic flow cell integrated with two catalyst packed-bed reactors (see
An anion-exchange membrane (Sustain-ion® X37-50 membrane) was employed to separate the two electrode compartments, through which OH− migrates from the cathodic to anodic side during the electrolytic process. Aqueous solutions of 10 mL of 0.6 M DHPS in 4 M NaOH and 50 mL of 0.6 M K3Fe(CN)6 in 4 M NaOH were respectively used as the catholyte and anolyte circulating between the corresponding electrode compartment and reactor. Here, the anolyte was in excess to ensure that the HER reaction is not limited by the more sluggish OER reaction, and thus the following discussion would be based on the cathodic reaction unless otherwise stated.
The cell was assembled by sandwiching two pieces of carbon felt as cathode and anode. The active area of the electrode was 5 cm2. Each half-cell had a graphite plate as the current collector connected to the external electrical circuit. An anion-exchange membrane (Sustainion® X37-50 membrane) was used as the separator. The anolyte consisted of 50 mL of 0.6 M K3Fe(CN)6 in 4 M NaOH, while the catholyte consisted of 12 mL of 0.6 M DHPS in 4 M NaOH. The electrolytes were circulated through the cell stack and tanks using peristaltic pumps. The Pt—Ni(OH)2 catalyst was loaded in the cathodic tank, while the NiFe(OH)2 catalyst was loaded in the anodic tank. 10 cm2 Pt—Ni(OH)2 catalyst and 15 cm2 NiFe(OH)2 catalyst were used in the test (
An anion-exchange membrane (Fumasep® FAAM-15) was used as the separator. The anolyte consisted of 25 mL of 0.4 M K4Fe(CN)6 and 0.2 M K3Fe(CN)6 in 4 M NaOH while 25 mL of 0.2 M DHPS in 4 M NaOH was used as the catholyte. The electrolytes were circulated through the cell stack and tanks using peristaltic pumps. 5 pieces of Pt—Ni(OH)2 catalyst (1×1 cm2) and 5 pieces of NiFe(OH)2 catalyst (1×1 cm2) were added in the cathodic tank and anodic tank, respectively. The current density was 10 mA/cm2. Gases produced in the tank were collected by water displacement method. Two measuring cylinders were filled with water and placed upside-down in a water bath. The gas produced in the tank was fed into the water-filled measuring cylinder through a silicone tube. The gas production was determined by the volume of displaced water in the measuring cylinder.
The electrolytic flow cell was further studied by GITT, for which a current density of 40 mA/cm2 was repeatedly applied for 3 min, followed by 3 min relaxation (
GC headspace analysis was performed using a Shimadzu GC-2010 Plus system by direct auto-injection of gas from the headspace of the catholyte connected to the GC through a silicone tube. For the decoupled water splitting system, the flow battery and electrolyte were prepared as described in the protocol above. For the conventional water splitting system, the cell was assembled by sandwiching NiFe(OH)2 and Pt—Ni(OH)2 as the anode and cathode, respectively. 50 mL of 4 M NaOH and 12 mL of 4 M NaOH were used as anolyte and catholyte, respectively. Each catholyte was purged with N2 before charging. The electrolysis process was operated at a constant current of 40 mA/cm2.
Similar electrochemical-chemical processes of [Fe(CN)6]3−/4− took place at the anodic side involving constant O2 evolution on NiFe(OH)2 catalyst bed. A voltage plateau at 1.53 V and a faradaic efficiency of nearly 100% were attained at 20 mA/cm2 after the cell reached steady state. Such a high faradaic efficiency was well retained for prolonged test as revealed in
The electrolytic flow cell was further studied by GITT, which determined the IR drop accounts for around 19 mV of the voltage increase for every 10 mA/cm2 of current density increment (
The voltage profile of direct water splitting was evaluated by following the same setup and protocol for decoupled water splitting described in Example 8. The electrolyte used was 50 mL of 4 M NaOH. The overall energy efficiency was calculated as
where 1.23 V corresponds to the theoretical water splitting voltage.
The voltage profile of direct water splitting is shown in
The overall energy efficiency of the spatially decoupled water electrolyzer reported here, and other previously reported decoupled water splitting systems was calculated by using the overall energy efficiency equation in Comparative Example 1.
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A H-cell setup, in which two cell compartments were separated by an anion-exchange membrane, was used to assess and compare the ion permselectivity of Sustainion® X37-50 and Fumasep® FAAM-15 membranes. The concentration of DHPS in the deficiency cell was monitored by UV-vis measurement after different durations.
As shown in
To monitor the HER/OER reactions of the mediators on the respective catalyst tanks in redox-flow cell, operando UV-vis spectroscopic measurement was conducted.
UV-vis spectra of the electrolytes from an operational electrolytic flow cell were collected with a SHIMADZU UV-1800 spectrometer. The setup is shown in
A spectroelectrochemical flow cell (
In the absence of catalyst, the concentration of DHPS monotonously decreased over the charging process (
| Number | Date | Country | Kind |
|---|---|---|---|
| 10202010967R | Nov 2020 | SG | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/SG2021/050667 | 11/3/2021 | WO |
