Patent Application
20250146151
The present disclosure generally relates to a ruthenium-iron alloy nanoflower particle useful as an electrocatalyst in electrochemical synthesis of ammonia.
Ammonia (NH3), as one of the largest-volume industrial chemicals produced globally, is a carbon-free fuel, green energy carrier, important chemical feedstock, and essential precursor of fertilizers. Traditionally, NH3 is massively generated by the Haber-Bosch method, which involves the energy-intensive reaction between nitrogen (N2) and hydrogen (H2) under high temperature and pressure (e.g., 300-500° C. and 20-25 MPa). As an appealing alternative, NH3 synthesis via electrochemical nitrate reduction reaction (NO3RR) has recently attracted tremendous research interest, due in part to its mild operation conditions and compatibility with renewable electricity resources. Notably, given the much lower energy needed for the cleavage of N═O bonds (204 kJ mol−1) and the large solubility of nitrate (NO3−) in aqueous solutions, NO3RR is capable of delivering relatively higher selectivity and yield rate for NH3 synthesis in comparison to nitrogen reduction. Moreover, NO3− ions are also known as one of the most widespread pollutants in groundwater and surface water, arising from excessive use of fertilizers, wastewater discharge, and other human activities. In light of this, NO3RR provides a promising strategy to remediate NO3− waste and simultaneously produce valuable NH3, which promotes a sustainable nitrogen cycle in the ecosystem. Ammonia electrosynthesis via NO3RR is mainly composed of deoxygenation processes (e.g., NO3− to NO2−) and the subsequent multiple hydrogenation steps (e.g., NO2− to NH3), which involves complicated transfer processes of eight electrons and nine protons (i.e., NO3−+9H++8e−→NH3+3H2O). During NO3RR, hydrogen evolution reaction (HER) is the dominant competing reaction, and meanwhile several other kinds of by-products like NO2−, N2, NH2OH, NH2NH2 and N2O can also form, making the efficient conversion from NO3− to NH3 a significant challenge. Therefore, it is essential to develop electrocatalysts with excellent activity and selectivity for NH3 synthesis from NO3−.
Although several kinds of metal-based electrocatalysts, such as Cu, Ni, Pd and their compounds, have been developed for NO3RR, their catalytic performance is still greatly limited by the kinetic mismatch of multiple transformation steps, leading to the accumulation of undesired N-containing species. Recently, ruthenium (Ru)-based materials have emerged as electrocatalysts for NO3RR. Ru active sites can enhance the adsorption/desorption and activation of the most important intermediate (i.e., NO2− or *NO2) during the reaction process, facilitating the subsequent conversion of NO2- to NH3. However, similar to the other platinum-group metals, Ru suffers from the problem of kinetically favoring competing HER rather than NO3RR, owing to the high coverage of active hydrogen (*H) on its surface. The adsorbed *H will strongly compete for Ru active sites and thus lead to the insufficient adsorption and electron injection to the π* antibonding orbitals of NO3− ions, especially in neutral solutions, resulting in unsatisfactory activity and selectivity toward NH3 generation. Tremendous efforts are still needed to overcome this problem. Generally, the introduction of another metal can modulate the electronic structure, local chemical environment and surface properties of Ru, and also tune the adsorption energies of reaction intermediates, thereby enhancing the activity and selectivity of Ru-based electrocatalysts.
There is thus a need for improved NO3RR Ru-based electrocatalysts.
Among various metals, iron (Fe), which is an earth-abundant and cost-effective transition metal, can effectively adsorb NO3− and convert it to NO2−, which is generally regarded as the rate-determining step in NO3RR for most of metal-based heterogeneous catalysts. Therefore, it is proposed that combining Ru and Fe into one electrocatalyst could not only realize the efficient conversion of NO3− to NO2−, but also ensure the rapid hydrogenation of NO2− to NH3. Moreover, two-dimensional (2D) nanostructures have been identified as highly efficient catalysts for various catalytic applications. Note that the ultrathin 2D structures typically possess large specific surface areas, which can greatly accelerate the reaction kinetics. Based on the above considerations, ultrathin 2D Ru-Fe nanostructures could be an effective approach to boost the electrocatalytic reduction of NO3− to NH3.
Provided herein are RuFe nanoflower (NF) particles comprising a plurality of nanosheets, which can be prepared by a one-pot solvothermal method. Compared with Ru nanosheets (NSs) and RuFe nanodendrites (NDs), RuFe NFs show superior catalytic activity and selectivity for NO3RR toward NH3 synthesis. Impressively, RuFe NFs deliver outstanding Faradaic efficiency (FE) of 92.9% and yield rate of 38.68 mg h−1 mgcat−1(64.47 mg h−1 mgRu−1) at −0.30 and −0.65 V (vs reversible hydrogen evolution (RHE)) for NH3 production, respectively, much higher than those of Ru NSs (80.1% and 5.66 mg h−1 mgcat−1) and RuFe NDs (85.7% and 19.26 mg h−1 mgcat−1). Meanwhile, RuFe NFs also demonstrate excellent electrocatalytic stability during consecutive electrolysis for 20 cycles. Experimental investigations and density functional theory (DFT) calculations reveal that the superior NO3RR performance of RuFe NFs is ascribed to the electroactive low-coordinated Ru sites with increased d-band center and electroactivity. The Fe sites guarantee the stable valence states of Ru sites, which benefits the remarkable durability of RuFe NFs during NO3− electrolysis. The Ru sites strengthen the adsorption of intermediates and accelerate the reaction trend with lower energy barriers of the potential-determining steps during NO3RR. In addition, the fabrication of rechargeable zinc-nitrate (Zn—NO3−) battery with large specific capacity of 160,419 mAh gcat−1 (under the current density of 2.5 mA mgcat−1) is also performed by using RuFe NFs as the catalyst cathode.
In a first aspect, provided herein is a ruthenium-iron (RuFe) nanoflower particle comprising a plurality of RuFe nanosheets, wherein the plurality of RuFe nanosheets are in a form of a nanoflower.
In certain embodiments, the plurality of RuFe nanosheets comprise RuFe in a hexagonal close-packed (hcp) phase.
In certain embodiments, the RuFe nanoflower particle has a diameter of 150-250 nm.
In certain embodiments, the plurality of RuFe nanosheets have an average thickness of 1-3 nm.
In certain embodiments, the plurality of RuFe nanosheets have an average thickness of 1.5-2 nm.
In certain embodiments, the RuFe nanoflower particle comprises Ru and Fe in an atomic ratio of 48:52 to 48.5:51.5, respectively.
In certain embodiments, the RuFe nanoflower particle comprises Ru and Fe in an atomic ratio of 48.5:51.5 to 49.5:50.5, respectively.
In certain embodiments, RuFe nanoflower particle has an electrochemically active surface area of 200-267.5 cm2.
In certain embodiments, the RuFe nanoflower particle has a diameter of 150-250 nm; the plurality of RuFe nanosheets have an average thickness of 1.5-2 nm; and the RuFe nanoflower particle comprises Ru and Fe in an atomic ratio of 48.5:51.5 to 49.5:50.5, respectively.
In certain embodiments, the RuFe nanoflower particle is prepared by a method comprising: contacting Ru3(CO)12, Fe(acac)3, glucose, and citric acid in a solvent comprising oleylamine and n-octanol thereby forming a reaction solution and heating the reaction solution thereby forming the RuFe nanoflower particle.
In certain embodiments, the reaction solution is heated at a temperature of 150-250° C.
In a second aspect, provided herein is an electrode comprising the ruthenium-iron (RuFe) nanoflower particle described herein and a base electrode.
In a third aspect, provided herein is an electrochemical cell comprising: the electrode described herein; a counter electrode; optionally a reference electrode; and an electrolyte solution between and in contact with the electrode, the counter electrode, and optionally the reference electrode.
In a fourth aspect, provided herein is a method of producing ammonia, the method comprising: providing the electrochemical cell described herein, wherein the electrolyte solution comprises a substrate selected from the group consisting of a nitrate salt, a nitrite salt, nitric oxide, nitrogen (N2), and mixtures thereof; and applying a potential between the electrode and the counter electrode resulting in the electrolytic reduction of the substrate thereby forming ammonia.
In certain embodiments, the potential is −0.3 to −0.65 volts vs reversible hydrogen electrode.
In certain embodiments, the nitrate salt is present in the electrolyte solution at a concentration of 0.01 to 0.1 M.
In certain embodiments, the method has a NH3 Faradaic efficiency (FE) of 87.1%-92.9% at −0.10 and −0.65 V vs reversible hydrogen evolution.
In a fifth aspect, provided herein is a method of preparing the RuFe nanoflower particle described herein, the method comprising: contacting Ru3(CO)12, Fe(acac)3, glucose, and citric acid in a solvent comprising oleylamine and n-octanol thereby forming a reaction solution and heating the reaction solution thereby forming the RuFe nanoflower particle.
In certain embodiments, the reaction solution is heated at a temperature of 150-300° C.
In certain embodiments, Ru3(CO)12 and Fe(acac)3 are contacted in a molar ratio less than 1:1, respectively.
The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein:
Throughout the present disclosure, unless the context requires otherwise, the word “comprise” or variations such as “comprises” or “comprising”, will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers. It is also noted that in this disclosure and particularly in the claims and/or paragraphs, terms such as “comprises”, “comprised”, “comprising” and the like can have the meaning attributed to it in U.S. Patent law; e.g., they can mean “includes”, “included”, “including”, and the like; and that terms such as “consisting essentially of” and “consists essentially of” have the meaning ascribed to them in U.S. Patent law, e.g., they allow for elements not explicitly recited, but exclude elements that are found in the prior art or that affect a basic or novel characteristic of the present invention.
Furthermore, throughout the present disclosure and claims, unless the context requires otherwise, the word “include” or variations such as “includes” or “including”, will be understood to imply the inclusion of a stated integer or group of integers, but not the exclusion of any other integer or group of integers.
The use of the singular herein includes the plural (and vice versa) unless specifically stated otherwise. In addition, where the use of the term “about” is before a quantitative value, the present teachings also include the specific quantitative value itself, unless specifically stated otherwise. As used herein, the term “about” refers to a ±10%, ±7%, ±5%, ±3%, ±1%, or ±0% variation from the nominal value unless otherwise indicated or inferred.
The term “substantially crystalline” refers to compositions or compounds with at least 70% by weight, at least 75% by weight, at least 80% by weight, at least 85% by weight, at least 90 by weight, at least 95% by weight, at least 96% by weight, at least 97% by weight, at least 98% by weight, at least 99% by weight, at least 99.5% by weight, or more of the composition or compound is present in crystalline form. The compositions or compounds can exist in a single crystalline form or more than one crystalline form. In certain embodiments, the composition or compound has at least 70% by weight, at least 75% by weight, at least 80% by weight, at least 85% by weight, at least 90 by weight, at least 95% by weight, at least 96% by weight, at least 97% by weight, at least 98% by weight, at least 99% by weight, at least 99.5% by weight, or more of the composition or compound present in a single crystalline form. The degree (%) of crystallinity may be determined by the skilled person using X-ray powder diffraction (XRPD). Other techniques, such as solid-state NMR, FT-IR, Raman spectroscopy, differential scanning calorimetry (DSC) and microcalorimetry, may also be used.
As used herein, the term “nanoflowers” refers to particles exhibiting a characteristic three-dimensional flowerlike morphology.
Provided herein is a RuFe nanoflower particle comprising a plurality of RuFe nanosheets, wherein the plurality of RuFe nanosheets are in a form of a nanoflower. In certain embodiments, each of the plurality of RuFe nanosheets are substantially crystalline.
The RuFe nanoflower particle can range in size between 50-400 nm, 50-350 nm, 50-300 nm, 50-250 nm, 100-250 nm, 150-250 nm, 175-225 nm, 180-210 nm, 180-200 nm, 185-195 nm, 50-200 nm, 100-200 nm, 150-200 nm, or 100-150 nm. A plurality of the RuFe nanoflower particles can have an average size between 50-400 nm, 50-350 nm, 50-300 nm, 50-250 nm, 100-250 nm, 150-250 nm, 175-225 nm, 180-210 nm, 180-200 nm, 185-195 nm, 50-200 nm, 100-200 nm, 150-200 nm, or 100-150 nm. In certain embodiments, a plurality of the RuFe nanoflower particles have an average size of about 190 nm.
Each of the plurality of RuFe nanosheets can have an average thickness of 0.5-5, 0.5-4.5, 0.5-4, 0.5-3.5, 0.5-3, 1-3, 1-2.5, 1.5-2.5, 1.5-2.0, 1-2, 1-1.5, 1.3-2.3, 1.4-2.2, 1.5-2.1, 1.6-2, or 1.7-1.9 nm. In certain embodiments, the each of the plurality of RuFe nanosheets have an average thickness of about 1.8 nm.
Depending on the reaction conditions used to prepare the RuFe nanoflower particle, the atomic ratio of Ru to Fe in the RuFe nanoflower particle can range from 46:54 to 52:48; 46.5:53.5 to 51.5:48.5; 47:53 to 51:49; 47.5:52.5 to 50.5:49.5; 48:52 to 50:50; 48.5:51.5 to 49.5:50.5; 49:51
The atomic ratio of Ru to Fe in the RuFe nanoflower particle can range from 46:54 to 52:48; 46.5:53.5 to 51.5:48.5; 47:53 to 51:49; 47.5:52.5 to 50.5:49.5; 48:52 to 50:50; 48.5:51.5 to 49.5:50.5; 49:51 to 50:50; 48.25:51.75 to 49.75:50.25; 48.5:51.5 to 51.5:48.5; or 48.75 to 51.25 to 51.25 to 49.75, respectively. In certain embodiments, the atomic ratio of Ru to Fe in the RuFe nanoflower particle is about 49:51, respectively. A plurality of the RuFe nanoflower particles can have an average atomic ratio of Ru to Fe in the plurality of RuFe nanoflower particles from 46:54 to 52:48; 46.5:53.5 to 51.5:48.5; 47:53 to 51:49; 47.5:52.5 to 50.5:49.5; 48:52 to 50:50; 48.5:51.5 to 49.5:50.5; 49:51 to 50:50; 48.25:51.75 to 49.75:50.25; 48.5:51.5 to 51.5:48.5; or 48.75 to 51.25 to 51.25 to 49.75, respectively. In certain embodiments, a plurality of the RuFe nanoflower particles have an average atomic ratio of Ru to Fe in the plurality of RuFe nanoflower particles of about 49:51, respectively.
The RuFe nanoflower particle or a plurality of the RuFe nanoflower particles can have an electrochemically active surface area of 200-267.5 cm2, 225267.5 cm2, 250-267.5 cm2, or 260-267.5 cm2. In certain embodiments, the RuFe nanoflower particle or a plurality of the RuFe nanoflower particles have an electrochemically active surface area of about 267.5 cm2.
The present disclosure also provides an electrode comprising a base electrode and the RuFe nanoflower particle or a plurality of the RuFe nanoflower particles described herein. In certain embodiments, the RuFe nanoflower particle or the plurality of RuFe nanoflower particles are coated on a surface of the base electrode. The base electrode can be an inert electrode such as a GCE, a graphite electrode, an indium tin oxide (ITO) electrode, a fluorine doped tin oxide (FTO) electrode, carbon paper electrode, carbon fiber electrode, polycarbonate track etch (PCTE)-based electrode, or a titanium-based electrode. In certain embodiments, the electrode is a cathode.
The electrode can optionally comprise a binder. The binder may optionally be cured to further bind the RuFe nanoflower particle or a plurality of the RuFe nanoflower particles with the base electrode and can increase the conductivity of electrode. Typical binders include, for example polyvinylidene fluoride (PVDF), polyvinyl alcohol (PVA), starch, sodium alginate, hydroxypropyl cellulose, carboxymethyl cellulose (CMC), regenerated cellulose, polyvinylpyrrolidone, polyimide, polyamideimide, polyethylene, polypropylene, an ethylene-propylene-diene terpolymer (EPDM), a sulfonated EPDM, a styrene-butadiene rubber, polytetrafluoroethylene (PTFE), a polyacrylic polymer, and combinations thereof. In certain embodiments, the binder is PVA.
The present disclosure also provides an electrochemical cell comprising: the electrode described herein; a counter electrode (or counter/reference electrode); optionally a reference electrode (e.g., in a three-electrode system); and an electrolyte solution between and in contact with the electrode, the counter electrode, and optionally the reference electrode. In certain embodiments, the electrolyte solution comprises an aqueous solution.
A counter electrode refers to an electrode paired with the working electrode, through which passes a current equal in magnitude and opposite in sign to the current passing through the working electrode. The counter electrode can include counter electrodes which also function as reference electrodes (i.e., a counter/reference electrode). Any suitable counter electrode known in the art can be used in connection with the methods described herein. For example, the counter electrode can comprise carbon (e.g., highly oriented pyrolytic graphite), a metal (e.g., platinum), an alloy (e.g., stainless steel), glassy carbon, a conductive polymer, or the like.
The reference electrode can be selected from a standard hydrogen electrode, calomel electrode, copper-copper (II) sulfate electrode, silver chloride electrode, palladium-hydrogen electrode, mercury-mercurous sulfate electrode, and the like.
In certain embodiments, the electrolyte comprises a nitrate salt. The type of nitrate salt is not particularly limited and can be any nitrate salt that is at least partially soluble in the electrolyte solution. The nitrate salt can include one or more cations selected from alkali metals, such as lithium, sodium, potassium, rubidium, and cesium; alkaline earth metals, such as beryllium, magnesium, calcium, strontium, and barium; Group 3-12 transition metals; and NR4+, wherein R is independently for each instance selected from hydrogen and C1-C6 alkyl. In certain embodiments, the nitrate salt is selected from the group consisting of LiNO3, NaNO3, KNO3, Ca(NO3)2, Mg(NO3)2, NH4NO3, CsNO3, and mixtures thereof.
The concentration of the nitrate salt in the electrolyte solution can range from 0.01 to 1 M, 0.01 to 0.9 M, 0.01 to 0.8 M, 0.01 to 0.7 M, 0.01 to 0.6 M, 0.01 to 0.5 M, 0.01 to 0.4 M, 0.01 to 0.3 M, 0.01 to 0.2 M, 0.01 to 0.1 M, 0.05 to 0.1 M, 0.075 to 0.1 M, 0.01 to 0.075 M, 0.01 to 0.05 M, or 0.05 to 0.75 M.
In certain embodiments, the electrolyte solution further comprises one or more supporting electrolytes. In certain embodiments, the supporting electrolyte is an alkali metal (e.g., lithium, sodium, potassium, rubidium, and cesium), alkaline earth metal (e.g., beryllium, magnesium, calcium, strontium, and barium), or ammonium salt of a halide, acetate, carbonate, perchlorate, phosphate, monohydrogen phosphate, dihydrogen phosphate, or sulfate. Exemplary supporting electrolytes include, but are not limited to, LiClO4, NaClO4, KClO4, Na2SO4, K2SO4, NaCl, KCl, MgCl2, NH4Cl, (NH4)2SO4, Na3PO4, K3PO4, MgSO4, Na2CO3, K2CO3, MgCO3, NaOH, and KOH.
Also provided herein is a method of producing ammonia gas, the method comprising providing the electrochemical cell described herein, wherein the electrolyte solution comprises a substrate selected from the group consisting of a nitrate salt, a nitrite salt, nitric oxide, nitrogen (N2), and mixtures thereof; and applying a potential between the electrode and the counter electrode resulting in the electrolytic reduction of the substrate thereby forming ammonia.
The potential applied to the electrode and the counter electrode can range from −0.1 to 1 volts. In certain embodiments, the potential applied to the electrode and the counter electrode can range from −0.1 to −0.9 volts, −0.1 to −0.8 volts, −0.1 to −0.7 volts, −0.1 to −0.6 volts, −0.1 to −0.5 volts, −0.1 to −0.4 volts, −0.1 to −0.3 volts, −0.1 to −0.2 volts, −0.2 to −0.9 volts, −0.3 to −0.9 volts, −0.4 to −0.9 volts, −0.5 to −0.9 volts, −0.6 to −0.9 volts, −0.7 to −0.9 volts, −0.8 to −0.9 volts, −0.2 to −0.8 volts, −0.3 to −0.7 volts, −0.4 to −0.6 volts, −0.5 to −0.6 volts, −0.4 to −0.5 volts, −0.3 to −0.65 volts, −0.4 to −0.65 volts, −0.5 to −0.65 volts, −0.55 to −0.65 volts, or −0.6 to −0.65 volts.
The electrochemical cell provided herein can reduce nitrate to ammonia with a NH3-to-NO2− ratio between 34.4-211.8 to 1, 65.4-211.8 to 1, 73.8-211.8 to 1, 100-211.8 to 1, 175-211.8 to 1, or 200-211.8 to 1. In certain embodiments, the electrochemical cell can generate ammonia at a yield rate of 9.90 mg h−1 mgcat−1 to 38.68 mg h−1 mgcat−1 at −0.65 volts vs RHE at a concentration of nitrate between 0.01-0.1 M.
The method can have a NH3 Faradaic efficiency (FE) of 92.9% and 85.1% at −0.10 and −0.65 V vs reversible hydrogen evolution, respectively.
The RuFe nanoflower particle described herein can be prepared by a solvothermal method comprising: contacting Ru3(CO)12, Fe(acac)3, glucose, and citric acid in a solvent comprising oleylamine and n-octanol thereby forming a reaction solution and heating the reaction solution thereby forming the RuFe nanoflower particle.
Ru3(CO)12 and Fe(acac)3 can be contacted in a molar ratio of Ru3(CO)12 and Fe(acac)3 equal to or less than 3:2, 1:1, 2:3, or 3:7, respectively. In certain embodiments, Ru3(CO)12 and Fe(acac)3 can be contacted in a molar ratio of Ru3(CO)12 and Fe(acac)3 of about 3:7, respectively.
The reaction solution can be heated at a temperature of 150-400° C., 150-350° C., 150-300° C., 150-250° C., or 175-225° C. In certain embodiments, the reaction solution is heated at about 200° C. The reaction solution can be heated for a period of 2 hours to 144 hours, 12 hours to 144 hours, 12 hours to 120 hours, 12 hours to 96 hours, 12 hours to 72 hours, 24 hours to 72 hours, or 30 hours to 54 hours. In certain embodiments, the reaction solution is heated for a period of about 48 hours.
In certain embodiments, the step of heating the reaction solution is conducted under autogenic pressure, i.e., pressure generated as a result of heating in a closed system. Alternatively or additionally, the pressure can be applied externally, e.g., by mechanical means. In certain embodiments, the step of heating the reaction solution is conducted at a pressure of 0.1 to 10 MPa or 0.1 to 1 MPa.
Synthesis and characterization of RuFe NFs. In a typical experiment, the RuFe NFs were synthesized by a facile solvothermal method with triruthenium dodecacarbonyl (Ru3(CO)12) and iron(III) acetylacetonate (Fe(acac)3) as the metal precursors, glucose and citric acid as the reducing agents, and oleylamine and n-octanol as the solvents (
The time-dependent experiments were conducted to further investigate the formation and growth mechanisms of the assembled RuFe NFs. At the reaction time of 12 h, the RuFe nanowires with Ru/Fe atomic ratio of 61.2/38.8 were formed (
Ru nanosheets (NSs) and RuFe nanodendrites (NDs) were also synthesized as control samples. In a typical synthesis, 8 mg of Ru3(CO)12 and 13.6 mg of Fe(acac)3 were added into 5 mL of OAm. After ultrasonication for around 10 mins, it was transferred to the Teflon-lined autoclave (25 mL capacity), and heated from room temperature to 220° C. and kept at this temperature for 14 h. The final products were collected by centrifugation and washed three times with 3 mL of the mixture of ethanol and hexane (v/v=1/1). The obtained RuFe NDs were re-dispersed into ethanol for further use. The obtained Ru NSs adopt the typical hcp phase, which is proved by the TEM, HRTEM, FFT, selected-area electron diffraction (SAED) and XRD patterns (
X-ray spectral analysis. X-ray photoelectron spectroscopy (XPS) was used to investigate the electronic structure of RuFe NFs.
Electrochemical NO3RR performance. The electrochemical performance test was conducted using a standard three-electrode H-type cell under ambient conditions in neutral electrolyte composed of 0.5 M Na2SO4 and 0.1 M NaNO3. The obtained NH3 and NO2− were quantitively determined by ultraviolet visible (UV-vis) spectrophotometry with the standard method (
To confirm the NO3RR performance and rule out the interference of the electrocatalysts, the testing equipment and/or other environmental factors, control experiments were conducted in 0.5 M Na2SO4 in the absence of NaNO3. In this case, the current density is much lower than that with NaNO3 and the yield rate of NH3 is neglectable (
The influence of nitrate concentration on the NO3RR performance of RuFe NFs was also investigated (
The catalytic durability of RuFe NFs was evaluated by using long-term chronoamperometry test. The chronoamperometry test of RuFe NFs and RuFe NDs was performed under −0.30 V (vs RHE) for 12 h. After refreshing the electrolyte, nitrate electrolysis for another 6 h was conducted. During the first 12 h electrolysis process, the current density was decreased with prolonged reaction time, but it increased to the original level again after refreshing the electrolyte (
Theoretical calculations. In order to better understand the superior NO3RR performance of RuFe NFs, we have further introduced DFT calculations to compare the electronic structures and reaction energy between RuFe NFs and RuFe NDs. The surface electronic distributions near the Fermi level (EF) are demonstrated, where the electroactive bonding orbitals are mainly located on the low-coordinated Ru sites on the surface of RuFe NFs, indicating their high electroactivity (
Then we discuss the electrocatalytic performance from the energy perspective. The adsorption energies of reactants (i.e., NO3− and H2O) are critical to the NO3RR performance (
Demonstration of rechargeable Zn—NO3− battery. Considering the high energy density of NH3 (4.32 kW h L-1), it is also regarded as a potential energy carrier in both chemical engineering and energy storage systems. Noteworthily, the equilibrium potential of NO3RR is 0.69 V (vs RHE), much higher than 0.40 V (vs RHE) of oxygen reduction reaction (ORR) in zinc-air batteries, which means large energy density can be obtained with NO3RR as the cathode reaction in an electrochemical device. In light of this, it is also quite compelling to assemble Zn—NO3-battery system to utilize the electrons originating from the NO3RR process, which can not only exhibit a large theoretical energy density of 1,051 W h Kg−1, but also provide a feasible strategy for ammonia production and sewage disposal in the future. Based on the experimental results, RuFe NFs demonstrate excellent FE and outstanding yield rate toward NH3 synthesis at a relatively low overpotential. As a proof-of-concept application, a Zn—NO3− battery was constructed using RuFe NFs as the catalyst cathode. As shown in
An ultrathin nanosheet-assembled RuFe alloy NFs with low-coordinated Ru sites was synthesized by a simple one-pot solvothermal method for highly efficient NO3RR in neutral electrolyte. It was observed that RuFe NFs demonstrate much superior electrocatalytic performance over Ru NSs and RuFe NDs in NO3RR. Remarkably, compared with Ru NSs (5.66 mg h−1 mgcat−1) and RuFe NDs (19.26 mg h−1 mgcat−1), RuFe NFs delivered outstanding NH3 yield rate of 38.68 mg h−1 mgcat−1 (64.47 mg h−1 mgRu−1) at −0.65 V (vs RHE), surpassing most of the reported heterogeneous NO3RR electrocatalysts, and also a stable NH3 electrosynthesis for 20 cycles without obvious decrease in FE and yield rate. Alternatively, RuFe NFs can still achieve efficient NO3RR even with relatively low nitrate concentration. DFT calculations have investigated the electronic modulations in RuFe NFs induced by the lower C.N. of Ru active sites, which supplies the improved electroactivity and stronger reaction trends than that of RuFe NDs. In particular, the surface Ru-4d and Fe-3d orbitals of RuFe NFs constructed a highly stable electronic structure due to the complementary orbitals to reach efficient electron transfer and robust valence states. The reaction energy changes proved the lower energy barriers of PDS on RuFe NFs, which also suppresses the competitive HER during the NO3RR. Furthermore, the successful demonstration of high-specific capacity Zn—NO3− batteries with RuFe NFs as catalyst cathode suggested their important potential applications in advanced energy devices. This work offers a promising and feasible strategy to enhance the electrochemical NO3RR performance via atomic coordination environment engineering of catalysts for sustainable global nitrogen cycle.
Synthesis of RuFe nanoflowers (NFs). In a typical experiment, 8 mg of Ru3(CO)12, 13.6 mg of Fe(acac)3 and 10 mg of D-(+)-glucose were added into 4 mL of OAm. After ultrasonication for about 0.5 h, the resultant homogenous solution was mixed with 40 mg of citric acid and 2 mL of n-octanol. Then the mixture was ultrasonicated for another 0.5 h to obtain the homogenous solution. Subsequently, it was transferred to the Teflon-lined autoclave (25 mL capacity), which was heated from room temperature to 200° C. and kept at this temperature for 48 h. After the reaction, the final products were obtained by centrifugation and washed with 3 mL of the mixture of ethanol and hexane (v/v=1/1) for several times. The obtained RuFe NFs were redispersed into hexane for further use.
Synthesis of RuFe nanodendrites (NDs). In a typical synthesis, 8 mg of Ru3(CO)12 and 13.6 mg of Fe(acac)3 were added into 5 mL of OAm. After ultrasonication for around 10 mins, it was transferred to the Teflon-lined autoclave (25 mL capacity), and heated from room temperature to 220° C. and kept at this temperature for 14 h. The final products were collected by centrifugation and washed three times with 3 mL of the mixture of ethanol and hexane (v/v=1/1). The obtained RuFe NDs were re-dispersed into ethanol for further use.
Synthesis of Ru nanosheets (NSs). Typically, 4.5 mg of Ru(acac)3 was dissolved into 0.5 mL of OAm and 1.5 mL of benzyl alcohol through vigorous ultrasonication. Then the resultant transparent solution was transferred to the Teflon-lined autoclave (25 mL capacity), and heated from room temperature to 200° C. and kept at this temperature for 12 h. After it was cooled to room temperature naturally, the products were centrifugated and washed by 2 mL of the mixture of ethanol and hexane (v/v=1/3) for three times. Finally, the obtained products were re-dispersed into ethanol for further use.
Electrocatalytic measurements. The electrocatalytic test in this work was carried out in an H-type cell separated by an ion-exchange membrane (i.e., Nafion 117) at room temperature. The Ivium-n-Stat electrochemical workstation with multiple channels was utilized to record the electrochemical data. In a typical three-electrode system, a Pt plate, an Ag/AgCl (filled with saturated KCl solution) and the catalyst supported on carbon paper were used as the counter, reference and working electrodes, respectively. All the potentials of this work were converted to the reversible hydrogen electrode (RHE) scale by the following equation: E (vs RHE)=E (vs Ag/AgCl)+0.197 V+0.059×pH. For the electrocatalytic nitrate reduction, the solution composed of 0.5 M Na2SO4 and 0.1 M NaNO3 was used as the electrolyte. Before the test, the solution was purged with high-purity argon (Ar) for at least 30 mins. Then 25 mL of electrolyte were added into the anode and cathode compartments of the H-type cell, respectively. Before the electrochemical measurements, the cyclic voltammetry (CV) was utilized to further remove the capping agents from the catalysts at a sweep rate of 50 mV s−1. The linear sweep voltammetry (LSV) curve was obtained at a scan rate of 5 mV s−1 with the potential range from 0 to −0.7 V (vs RHE). The chronoamperometry test was conducted for 1 h at each potential under a stirring rate of 600 rpm. After the electrolysis, the electrolyte was analyzed by the UV-vis spectrophotometer.
Characterization. The transmission electron microscopy (TEM) and the high-resolution TEM (HRTEM) images were taken on a JEOL-2100F transmission electron microscope operated at 200 kV. The spherical aberration-corrected high-angle annular dark-field scanning TEM (HAADF-STEM) images were obtained on a high-resolution aberration-corrected TEM (JEOL JEM-ARM200F). Scanning electron microscopy (SEM) measurement was conducted on QUANTA 250. Powder X-ray diffraction (XRD) patterns were recorded on a Rigaku SmartLab X-ray diffractometer with Cu Kα X-ray source (λ=1.5406 Λ). X-ray photoelectron spectroscopy (XPS) test was performed on Thermo Scientific Nexsa spectrophotometer with Al-Kα radiation system. X-ray absorption spectroscopy (XAS) measurements were conducted on the beamline 1W1B Beijing Synchrotron Radiation Facility with the transmission mode. A double-crystal Si (111) monochromator was applied to monochromatize the radiation.
Theoretical calculations setup. To compare the NO3RR performance of the RuFe NFs and RuFe NDs, density functional theory (DFT) calculations have been introduced in this work within the CASTEP packages. The generalized gradient approximation (GGA) and the Perdew-Burke-Ernzerhof (PBE) were applied to accurately express the exchange-correlation interactions. We have set the plane-wave basis cutoff energy to 380 eV with the selection of the ultrasoft pseudopotentials. In particular, we applied the Broyden-Fletcher-Goldfarb-Shannon (BFGS) algorithm for the energy minimization and the k-point has been set to coarse quality. Both structures of RuFe NFs and RuFe NDs were cleaved from the hexagonal Ru unit cell with six atomic layers. The ratios of Ru:Fe are 51:53 and 59:49 for RuFe NFs and RuFe NDs, respectively, which are close to the experimental characterizations of 49:51 for RuFe NFs and 54.8:45.2 for RuFe NDs. More low-coordinated Ru sites were constructed on RuFe NFs surfaces. Meanwhile, a 20 Å vacuum space was applied in the z-axis to guarantee sufficient space for full relaxation. For all the geometry optimizations, we have considered the following convergence criteria: 1) the Hellmann-Feynman forces should not exceed 0.001 eV/Å; 2) the total energy difference should be smaller than 5×105 eV/atom; and 3) the inter-ionic displacement of each atom should not exceed 0.005 Å.
