Abstract
The present invention is concerned with the epitaxial growth of unconventional 2H Cu on hexagonal close-packed (hcp) IrNi template, leading to forming of IrNiCu@Cu nanostructures as electrocatalyst. IrNiCu@Cu-20 shows superior catalytic performance, with NH.sub.3 Faradaic efficiency (FE) of 86% at 0.1 (vs reversible hydrogen electrode (RHE)) and NH.sub.3 yield rate of 687.3 mmol gCu1 h1, far better than common face-centered cubic (fcc) Cu. IrNiCu@Cu-30 and IrNiCu@Cu-50 covered by hcp Cu shell display high selectivity towards nitrite (NO.sup.2), with NO2 FE above 60% at 0.1 (vs RHE). IrNiCu@Cu-20 has the optimal electronic structures for NO.sub.3RR due to the highest d-band center and strongest reaction trend with the lowest energy barriers. The electrocatalysts are effective in electrochemical nitrate reduction NO.sub.3RR.
Claims
1. A method of manufacture of templates of IrNi nanobranches (NBs) in hexagonal close-packed phase (hcp), comprising the steps in the sequential order of: adding Iridium acetylacetonate [Ir(acac).sub.3] and nickel acetylacetonate [Ni(acac).sub.2] to a combination of oleylamine (OAm)/oleic acid (OA) to form a first mixture, subjecting the first mixture to ultrasonication to obtain a homogenous solution, adding formaldehyde (HCHO) to the homogenous solution to form a second mixture, the second mixture being a growth solution, placing or transferring the growth solution into a container made with an inert material or with an inert lining, and subjecting the growth solution to heating in a reactor, cooling the growth solution, harvesting the IrNi NB templates by subjecting the growth solution to centrifugation and washing by an oil removing agent, and collecting the IrNi NB templates by re-dispersing the IrNi NB templates in a solvent of ethanol.
2. A method as claimed in claim 1, wherein: the HCHO solution is added to the homogenous solution dropwise under stirring or agitation.
3. A method as claimed in claim 1, wherein the oil removing agent is a mixture of ethanol and hexane with a volume ratio of 2:1 to 1:2 and the centrifugation and washing are conducted 3-4 times.
4. A method as claimed in claim 3, wherein the oil removing agent is a mixture of ethanol and hexane with a volume ratio of 1:2 and the centrifugation and washing are conducted 3 times.
5. A method as claimed in claim 1, wherein: the weight ratio of the Ir(acac).sub.3 and Ni(acac).sub.2 is 1.1:1 to 1:1.1, the volume ratio of the OAm and OA is 7.3:1 to 9:1, the ultrasonication lasts for 1.5-3 hours, the amount of the HCHO solution used is 90-00 L, and the growth solution is heated in the reactor from room temperature to a temperature of 200 C.-220 C. and the temperature is maintained for 10-16 hours.
6. A method as claimed in claim 5, wherein: the weight ratio of the Ir(acac)3 and Ni(acac)2 is 1:1, the volume ratio of the OAm and OA is 7.3:1, the ultrasonication lasts for 2 hours, the amount of the HCHO solution used is 100 L, and the growth solution is heated in the reactor from room temperature to a temperature of 220 C. and the temperature is maintained for 14 hours.
7. A method of manufacture of IrNiCu@Cu nanostructures with Cu in a hexagonal close-packed phase (hcp), comprising firstly, in the sequential order of: preparing templates of hexagonal close-packed phase (hcp) of IrNi nanobranches (NBs), the preparing including the steps in the sequential order of: adding Iridum acetylacetonate [Ir(acac).sub.3] and nickel acetylacetonate [Ni(acac).sub.2] to a combination of oleylamine (OAm)/oleic acid (OA) to form a first mixture, subjecting the first mixture to ultrasonication to obtain a homogenous solution, adding formaldehyde (HCHO) to the homogenous solution to form a second mixture, the second mixture being a growth solution, placing or transferring the growth solution into a container made with an inert material or with an inert lining, and subjecting the growth solution to heating in a reactor, cooling the growth solution, harvesting the IrNi NB templates by subjecting the growth solution to centrifugation and washing by an oil removing agent, and collecting the IrNi NB templates by re-dispersing the IrNi NB templates in a solvent of ethanol; and further comprising, secondly, in the sequential order of: obtaining a predetermined quantity of the IrNi NB templates, removing the ethanol solvent in which the IrNi NB templates are suspended by way of centrifugation, adding OAm and copper acetylacetonate [Cu(acac).sub.2] to the IrNi NB templates and forming a homogenous solution, mixing a reducing agent to the homogenous solution to reduce the Cu(acac).sub.2 solution to Cu, wherein the mixing is conducted by way of oscillation and not ultrasonification, heating the homogenous solution to a predetermined temperature for a predetermined heating duration, allowing growth of the IrNiCu@Cu nanostructures, and isolating reaction products from the homogenous solution by way of centrifugation and/or washing with an oil removing agent, the reaction products being the IrNiCu@Cu nanostructures.
8. A method as claimed in claim 7, wherein: the IrNi NB templates has a mass concentration of 1.8-2 mg mL.sup.1, and the amount of the IrNi NB templates is 180-200 L, the centrifugation to remove the ethanol solvent is conducted with a speed of 90,000-10,000 rpm for 2-3 mins, the quantity of OAm is 1.4-1.5 mL and the concentration and quantity of Cu(acac).sub.2 solution are 80-120 UL and 8-10 mM respectively, the quantity of the reducing agent is 80-100 L, the predetermined temperature to which the homogenous solution is heated is 150-120 C. and the predetermined heating duration of 20-50 mins the volume ratio of ethanol and n-hexane in the ethanol and n-hexane is 8:1 to 9:1.
9. A method as claimed in claim 8, wherein: the IrNi NB templates has a mass concentration of 2 mg mL.sup.1, and the amount of the IrNi NB templates is 200 L, the centrifugation to remove the ethanol solvent is conducted with a speed of 10,000 rpm for 2 mins, the quantity of OAm is 1.5 mL, and the concentration and quantity of Cu(acac).sub.2 solution are 100 L and 10 mM, respectively, the quantity of the reducing agent is 100 L, and the volume ratio of ethanol and n-hexane in the ethanol and n-hexane is 9:1.
10. A method as claimed in claim 8, wherein the predetermined heating time is 20 mins, 30 mins, or 40 mins.
11. A method as claimed in claim 8, wherein the predetermined heating time is 20 mins.
12. A method as claimed in claim 8, wherein the predetermined heating time is 30 mins.
13. A method as claimed in claim 8, wherein the predetermined heating time is 40 mins.
14. A method of making an electrode provided with an electrocatalyst of IrNiCu@Cu nanostructures made from a method as claimed in claim 8, comprising a step of coating the IrNiCu@Cu nanostructure electrocatalyst on the electrode.
15. A method of enhancing the performance of electrochemical nitrate reduction reaction (NO.sub.3RR), comprising a step of effecting the NO.sub.3RR by using an electrode made from a method of claim 14.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
[0047] The patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawing(s) will be provided by the Office upon request and payment of the necessary fee.
[0048] Some embodiments of the present invention will now be explained, with reference to the accompanied drawings, in which:
[0049] FIGS. 1A-1I illustrate synthesis and structural characterization of IrNiCu@Cu-20. Specifically, FIG. 1A is a schematic illustration for the epitaxial growth of 2H Cu on hcp IrNi nanobranches; FIGS. 1B-D are HAADF-STEM images of IrNiCu@Cu-20; FIGS. 1E-1F illustrate FFT patterns derived from the marked areas with green and red squares in FIG. 1D; FIG. 1G is an enlarged HAADF-STEM image of IrNiCu@Cu-20 from the marked area in white square in FIG. 1D; FIG. 1H are images of HAADF-STEM (scale bar, 20 nm) and the corresponding EDS elemental mappings of a typical IrNiCu@Cu-20 nano-branch; and FIG. 1I is a graph showing the line-scan profiles of IrNiCu@Cu-20 acquired along the light-yellow line in FIG. 1H.
[0050] FIGS. 2A-2L illustrate structural characterization of IrNiCu@Cu-30 and IrNiCu@Cu-50. Specifically, FIG. 2A is an SEM image and FIGS. 2B-2C are HAADF-STEM images of IrNiCu@Cu-30; FIG. 2D illustrates the FFT pattern of FIG. 20; FIG. 2E is a graph illustrating line-scan profiles of IrNiCu@Cu-30 acquired along the light-yellow (horizontal) line in FIG. 2F; FIG. 2F is an HAADF-STEM image (scale bar, 10 nm) and the corresponding EDS elemental mappings of a typical IrNiCu@Cu-30 nano-branch; FIG. 2G is a SEM image and FIGS. 2H-2I are HAADF-STEM images of IrNiCu@Cu-50; FIG. 2J illustrate the FFT pattern of FIG. 2I; FIG. 2K are line-scan profiles of IrNiCu@Cu-50 acquired along the light-yellow (horizontal) line in FIG. 2L; and FIG. 2L is a HAADF-STEM image (scale bar, 10 nm) and the corresponding EDS elemental mappings of a typical IrNiCu@Cu-50 nano-branch.
[0051] FIGS. 3A-3G illustrate an X-ray spectral analysis. Specifically, FIG. 3A is a graph showing the Cu 2p XPS spectra of IrNiCu@Cu-20, IrNiCu@Cu-30, IrNiCu@Cu-50 and Cu NPs; FIG. 3B is a graph showing the normalized Cu K-edge XANES spectra of IrNiCu@Cu-20, IrNiCu@Cu-30, IrNiCu@Cu-50 and Cu NPs with the data for reference samples of Cu foil and CuO are included here for comparison; FIG. 3C is a graph showing fourier transform of k.sup.2-weighted Cu K-edge EXAFS spectra of IrNi@Cu-20, IrNi@Cu-30, IrNi@Cu-50 and Cu NPs with the data for reference samples of Cu foil and CuO are included here for comparison; FIGS. 1D to 1G are charts showing wavelet transform for the Cu K-edge EXAFS spectra of (d) IrNiCu@Cu-20, (e) IrNiCu@Cu-30, (f) IrNiCu@Cu-50, and (g) Cu NPs, respectively.
[0052] FIGS. 4A-4I illustrate the electrochemical nitrate reduction performance; Specifically, FIG. 4A is a graph showing the LSV curves of IrNiCu@Cu-20, IrNiCu@Cu-30 and IrNiCu@Cu-50 at 5 mV s.sup.1 with 85% IR compensation; FIGS. 4B-4C are graphs showing the FEs of NHs and NO.sub.2.sup., respectively, and the NH.sub.3 yield rates of IrNiCu@Cu-20, IrNiCu@Cu-30 and IrNiCu@Cu-50 based on constant potential electrolysis, respectively; FIG. 4D is a graph showing the NHs FEs of IrNiCu@Cu-20 quantified by UV-vis and NMR methods at 0.1 V (vs RHE); FIG. 4E is a graph showing the 1H NMR spectra of electrolytes after NO.sub.3RR using nitrate-free solution, K.sup.14NO.sub.3 and K.sup.15NO.sub.3; FIG. 4F is a graph showing the NH.sub.3 FEs and yield rates of IrNiCu@Cu-20 and IrNiCu@Cu-30 at 0.1 V (vs RHE) during 15 consecutive cycles; FIG. 4G is a graph showing the real-time concentration changes of NO.sub.3.sub., NO.sub.2.sub. and NH.sub.3 during nitrate reduction using IrNiCu@Cu-20 and IrNiCu@Cu-30 as the electrocatalysts; FIGS. 4H-4I are graphs showing the in-situ DEMS patterns of IrNiCu@Cu-20 and IrNiCu@Cu-30, respectively, during electrocatalytic nitrate reduction based on 5 consecutive LSV scans.
[0053] FIGS. 5A-5N illustrate theoretical studies of IrNiCu@Cu nanostructures of the presentation invention. Specifically, FIGS. 5A-5C are schematic diagrams showing the electronic distributions of bonding and anti-bonding orbitals near the Fermi level of fcc Cu, IrNiCu@Cu-20, and IrNiCu@Cu-30, respectively, with orange balls representing Cu, blue balls presenting Ni, and purple balls representing Ir, and blue iso-surfaces representing bonding orbitals and green iso-surfaces representing anti-bonding orbitals. FIGS. 5D-5F are graphs showing the PDOS of fcc Cu, IrNiCu@Cu-20, and IrNiCu@Cu-30, respectively; FIG. 5G is a graph showing the d-band center comparison of fcc Cu, IrNiCu@Cu-20, and IrNiCu@Cu-30; FIG. 5H is a graph showing the site-dependent PDOSs of Cu-3d in fcc Cu and IrNiCu@Cu-20, with INC@C-20 and INC@C-30 represent IrNiCu@Cu-20 and IrNiCu@Cu-30, respectively; FIG. 5I is a graph showing the site-dependent PDOS of Cu-3d in IrNiCu@Cu-30; FIGS. 5J-5K showing the PDOS of key intermediates of NO.sub.3RR in IrNiCu@Cu-20 and IrNiCu@Cu-30, respectively; FIG. 5I is an graph showing the adsorption energies of NO.sub.3.sub. and H*, FIG. 5M is a graph showing the reaction energy changes of NO.sub.3RR; and FIG. 5N is a graph showing the reaction energy changes of HER.
[0054] FIGS. 6A-6D shows Zn-NO.sub.3.sub. battery demonstration. Specifically, FIG. 6A is a graph illustrating an open circuit potential of Zn-NO.sub.3.sub. batteries constructed with IrNiCu@Cu-20 and IrNiCu@Cu-30 (Insets: the digital photographs elucidating the battery device and its ability to power an electronic timer); FIG. 6B is a graph illustrating discharging polarization profiles and the corresponding power density curves; FIG. 6C is a graph showing the rate capabilities of Zn-NO.sub.3.sub. batteries during discharging; FIG. 6D is a graph showing a galvanostatic discharge profiles of Zn-NO.sub.3.sub. batteries from OCV to 0.005 V (vs Zn.sup.2+/Zn) at the current density of 1 and 1.5 mA cm.sup.2 using IrNiCu@Cu-20 as cathode.
[0055] FIGS. 7A-7F illustrate characterization of fcc Cu nanoparticles (Cu NPs). Specifically, FIGS. 7A-7B are SEM image and relevant EDS spectrum of Cu NPs, respectively, FIGS. 7C-7E are TEM image, size distribution and SAED pattern of Cu NPs, respectively; and FIG. 7F is a graph showing the powder XRD pattern of Cu NPs.
[0056] FIG. 8A is a TEM image of hcp IrNi template; and FIG. 8B is an image showing a SAED of the hcp IrNi template.
[0057] FIGS. 9A-9B are graphs showing the average Cu shell thickness of IrNiCu@Cu-30 and IrNiCu@Cu-50, respectively, based on data obtained from TEM images and Gauss fitting results.
[0058] FIG. 10 is a graph showing Ir 4f XPS spectra of IrNi, IrNiCu@Cu-20, IrNiCu@Cu-30, and IrNiCu@Cu-50.
[0059] FIG. 1I is a graph showing Ni 2p XPS spectra of IrNi, IrNi@Cu-20, IrNi@Cu-30, and IrNi@Cu-50.
[0060] FIG. 12A is a graph showing normalized Ir Ls-edge XANES spectra of IrNiCu@Cu-20, IrNiCu@Cu-30, and IrNiCu@Cu-50 in reference to Ir foil and IrO.sub.2; and FIG. 12B is a showing Fourier transform of k.sup.2-weighted EXAFS spectra of IrNiCu@Cu-20, IrNiCu@Cu-30, and IrNiCu@Cu-50 in reference to Ir foil and IrO.sub.2.
[0061] FIG. 12C is a graph showing normalized Ni K-edge XANES spectra of IrNiCu@Cu-20, IrNiCu@Cu-30, and IrNiCu@Cu-50 in reference to Ni foil and NiO; and FIG. 12D is a graph showing fourier transform of k.sup.2-weighted EXAFS spectra of IrNiCu@Cu-20, IrNiCu@Cu-30, and IrNiCu@Cu-50 in reference to Ni foil and NiO.
[0062] FIG. 13 is a graph showing Cu LMM AES spectra of IrNiCu@Cu-20, IrNiCu@Cu-30, IrNiCu@Cu-50, and Cu NPs.
[0063] FIG. 14 is a chart showing EXAFS fitting results of k2-weighted R space Cu K-edge of IrNiCu@Cu-20, IrNiCu@Cu-30, IrNiCu@Cu-50, and fcc Cu NPs.
[0064] FIG. 15 is a chart showing wavelet transform for the Cu K-edge EXAFS spectra of standard Cu foil.
[0065] FIGS. 16A-16B are graphs showing LSV profiles of IrNiCu@Cu-20 and IrNiCu@Cu-30, respectively, tested in 0.1 M NO3+0.1 M KOH with different mass loadings of Cu; and FIGS. 16C-16D are graphs showing LSV profiles of IrNiCu@Cu-20 and IrNiCu@Cu-30, respectively, tested in different pH with the Cu mass loading of 200 g cm2; in that the LSV measurements were conducted without iR compensation.
[0066] FIGS. 17A & 17C are graphs showing the effect of Cu mass loading (50, 100, 200 g) on NO.sub.3RR performance using IrNiCu@Cu-20 as electrocatalysts in 0.1 M KOH with 0.1 M KNO.sub.3; and FIGS. 17B & 17D are graphs showing the effect of Cu mass loading (50, 100, 200 g) on NO.sub.3RR performance using IrNiCu@Cu as electrocatalysts in 0.1 M KOH with 0.1 M KNO.sub.3; in that measurements were carried out for three times and the mean values are plotted.
[0067] FIGS. 18A & 18C are graphs showing the effect of pH (12, 13, 14) on NO.sub.3RR performance using IrNiCu@Cu-20 as electrocatalysts with 0.1 M KNO.sub.3; and FIGS. 18B & 18D are graphs showing the effect of pH (12, 13, 14) on NO.sub.3RR performance using IrNiCu@Cu-30 as electrocatalysts with 0.1 M KNO.sub.3; in that measurements were carried out for three times and the mean values are plotted.
[0068] FIGS. 19A-19B are graphs of calibration curve of NH.sub.4.sub.+ using NH.sub.4Cl as standards. Specifically, FIG. 19A illustrates UV-vis absorption spectra of different NH.sub.4.sub.+ concentrations and FIG. 19B illustrates linear fitting results of the calibration curve.
[0069] FIGS. 20A-20B are graphs of calibration curve of NO.sub.2.sub. using KNO.sub.2 as standards. Specifically, FIG. 20A illustrates UV-vis absorption spectra of different NO.sub.2.sub. concentrations and FIG. 20B illustrates linear fitting results of the calibration curve.
[0070] FIGS. 21A-21B are graphs showing calibration curve of NO.sub.3.sub. using KNO.sub.3 as standards. Specifically, FIG. 21A illustrates UV-vis absorption spectra of different NO.sub.3.sub. concentrations and FIG. 21B illustrates linear fitting results of the calibration curve.
[0071] FIGS. 22A and 22B are graphs showing the NH.sub.3 and NO.sub.2.sub. FEs using pristine IrNi and fcc Cu NPs, respectively, as electrocatalysts in 0.1 M KOH with 0.1 M KNO.sub.3, in that measurements were carried out for three times and the mean values are plotted.
[0072] FIG. 23 is a graph of EDS spectra of IrNiCu@Cu-20, IrNiCu@Cu-30, and IrNiCu@Cu-50.
[0073] FIG. 24 is a graph showing Cu atomic percentage of IrNiCu@Cu-20, IrNiCu@Cu-30, and IrNiCu@Cu-50 based on EDS results.\
[0074] FIGS. 25A & 25B are graphs showing the chronoamperometric curves during electrolysis under different electrode potentials using IrNiCu@Cu-20 and IrNiCu@Cu-30, respectively, as electrocatalysts.
[0075] FIGS. 26A, 26B and 26C are graphs showing, respectively, a) LSV profiles with 85% iR compensation, b) Faradaic efficiencies, and c) NH.sub.3 yield rates of IrNiCu@Cu-20 and IrNiCu@Cu-30 in 0.01 M NO.sub.3.sub. with 0.1 M KOH electrolyte; and FIGS. 26D, 26E and 26F are graphs, showing, respectively, d) LSV profiles with 85% iR compensation, e) Faradaic efficiencies, and f) NH.sub.3 yield rates of IrNiCu@Cu-20 and IrNiCu@Cu-30 in 1 M NO.sub.3.sub. with 0.1 M KOH electrolyte.
[0076] FIGS. 27A & 27B are heatmaps of NH.sub.3 FE over a) IrNiCu@Cu-20 and b) IrNiCu@Cu-30, respectively; FIGS. 27C & 27D are heatmaps of NHs yield rate over c) IrNiCu@Cu-20 and d) IrNiCu@Cu-30, respectively; and FIGS. 27E & 27F are heatmaps of NO.sub.2.sub. FE over e) IrNiCu@Cu-20 and f) IrNiCu@Cu-30, respectively.
[0077] FIG. 28A is a graph of the .sup.1H NMR spectra of .sup.14NH.sub.4.sub.+ with a series of .sup.14NH.sub.4Cl concentrations using C.sub.4H.sub.4O.sub.4 as internal standards; and FIG. 28B is a graph showing the calibration curve according to the integral area of .sup.14NH.sub.4.sub.+/C.sub.4H.sub.4O.sub.4 versus the concentration of .sup.14NH.sub.4.sub.+.
[0078] FIG. 29A is a graph of the .sup.1H NMR spectra of .sup.15NH.sub.4.sub.+ with a series of .sup.15NH.sub.4Cl concentrations using C.sub.4H.sub.4O.sub.4 as internal standards; and FIG. 29B is a graph showing the calibration curve according to the integral area of .sup.15NH.sub.4.sub.+/C.sub.4H.sub.4O.sub.4 versus the concentration of .sup.15NH.sub.4.sub.+.
[0079] FIG. 30 is a chart showing in-situ DEMS patterns of IrNiCu@Cu-50 during electrocatalytic nitrate reduction based on 4 consecutive LSV scans from 0.2 to 0.7 V (vs RHE).
[0080] FIG. 31A is a graph showing working electrode potential profile of continuous 24-h electrolysis at constant current of 100 mA cm-2 using IrNiCu@Cu-20 as the catalyst; and FIG. 31B is a graph showing the concentration of NO.sub.3.sub., NO.sub.2.sub., and NH.sub.3 concentration change during the 24-h electrolysis process.
[0081] FIG. 32 is a graph showing the discharging polarization profile and the corresponding power density curve of Zn-NO.sub.3.sub. battery using IrNiCu@Cu-20 as the cathode (catholyte composition: 0.1 M NO.sub.3.sub.+0.1 M KOH+0.2 M K.sub.2SO.sub.4).
[0082] FIG. 33A and FIG. 33B are graphs, respectively, showing discharging polarization profiles and the corresponding power density curves of Zn-NO.sub.3.sub. battery using IrNiCu@Cu-20 as the cathode in different catholytes with a) 0.01 M NO.sub.3.sub. and b) 1 M NO.sub.3.sub..
[0083] FIG. 34 is a schematic diagram showing an unconventional phase (hcp) 2H Cu synthesized through epitaxial growth on hop IrNi template, and the distribution of 2H Cu is well regulated by controlling reaction time, resulting in three kinds of IrNiCu@Cu nanostructures. Notably, IrNiCu@Cu-20 with low Cu coverage demonstrates superior ammonia yield rate toward nitrate electroreduction, which is 3.6 times that of IrNiCu@Cu-50 with high Cu coverage.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION
[0084] Electrochemical nitrate reduction reaction (NO.sub.3RR) is emerging as a promising strategy for nitrate removal and ammonia (NH.sub.3) production using renewable electricity. Although there has been progress in developing strategies in NO.sub.3RR, the crystal phase effect of electrocatalysts on NO.sub.3RR remains rarely explored and largely unknown. Research and development leading to the present invention shows that the epitaxial growth of unconventional 2H Cu on hexagonal close-packed (hcp) IrNi template, resulting in the formation of three IrNiCu@Cu nanostructures. For example, IrNiCu@Cu-20 shows superior catalytic performance, with NH.sub.3 Faradaic efficiency (FE) of 86% at 0.1 (vs reversible hydrogen electrode (RHE)) and NH.sub.3 yield rate of 687.3 mmol g.sub.Cu.sup.1 h.sup.1, far better than conventional phase (common face-centered cubic-fcc) Cu. In sharp contrast, IrNiCu@Cu-30 and IrNiCu@Cu-50 covered by hcp Cu shell display high selectivity towards nitrite (NO.sub.2.sub.), with NO.sub.2.sub. FE above 60% at 0.1 (vs RHE). Theoretical calculations have demonstrated that the IrNiCu@Cu-20 has the optimal electronic structures for NO.sub.3RR due to the highest d-band center and strongest reaction trend with the lowest energy barriers. The high electroactivity of IrNiCu@Cu-20 originates from the abundant low coordination of Cu sites on the surface, which guarantees the fast electron transfer to accelerate the intermediate conversions. The present invention provides a feasible tactic to regulate the product distribution of NO.sub.3RR by crystal phase engineering of electrocatalysts.
[0085] Extensive experiments and studies leading to the present invention are described below.
Materials and Methods
Chemicals and Reagents
[0086] Iridium (III) acetylacetonate (Ir(acac).sub.3, 99%) and sodium nitroprusside (C.sub.5H.sub.4FeN.sub.6Na.sub.2O.sub.3) were purchased from Alfa Aesar. Oleic acid (OA, 99%) was purchased from Sigma-Aldrich. Solution of sodium hypochlorite (NaClO, 0.1 M) was purchased from Macklin. Nickel (II) acetylacetonate (Ni(acac).sub.2, 95%), oleylamine (OAm, 80-90%), formaldehyde (HCHO, 37 wt. % in H.sub.2O), isopropyl alcohol (IPA, AR, 99.5%), potassium hydroxide (KOH, AR, 99%), potassium nitrate (KNO.sub.3, AR, 99%), ammonium chloride (ACS, 99.5%), salicylic acid (AR, 99.5%), trisodium citrate dihydrate (98%), sodium hydroxide (NaOH, AR, 96%), maleic acid (AR, 99.0% (HPLC)), and deuterium oxide (D.sub.2O, AR, 99%) were supplied by Shanghai Aladdin Biochemical Technology Co., Ltd. Ethanol (absolute, 99.9%) and hexane (99%) were obtained from Anaqua Global International Inc. Limited. All the chemicals and reagents were used as received without further purification. Deionized (DI) water used in the experiments was obtained from the Milli-Q Plus System with a resistance of 18.2 M cm.
Synthesis of hcp IrNi Template
[0087] Preparation of IrNi firstly involved the use of 4 mg of Ir(acac).sub.3 and 4 mg of Ni(acac).sub.2 added into 12 mL glass vial, and then 4.5 mL of OAm and 0.5 mL of OA were added into the system. Until a homogeneous solution formed, 100 L of formaldehyde were added. After stirring for about 15 mins, 2.5 mL of mixed solution were added into 4 mL glass vial, which was subsequently moved to autoclave. The reaction last for 12 h under 220 C., and the sample was collected by centrifugation. After rinsing by mixed solvent of ethanol and n-hexane for several times, the sample was obtained and stored in ethanol.
Synthesis of Unconventional Phase (hcp) IrNiCu@Cu Nanostructures
[0088] Typically, 200 L of hcp IrNi template with a mass concentration of 2 mg mL.sup.1 were taken and the solvent was discarded after centrifugation. Then, 1.5 mL of OAm and 100 L of Cu(acac).sub.2 solution (10 mM) were added into the glass bottle to make a homogeneous solution. Afterwards, 100 L of 1,2-butanediol were added. Using the oscillator instead of ultrasonication to mix the solution to avoid the pre-reduction of Cu.sup.2+. The glass bottle was sealed and put into the oil bath under the temperature of 120 C., and the reaction time was controlled as 20, 30, and 50 mins, respectively. After reaction, the sample was acquired by centrifugation and washed with the mixture of ethanol/n-hexane (v/v=9/1) for several times. The obtained samples were denoted as IrNiCu@Cu-20, IrNiCu@Cu-30, and IrNiCu@Cu-50 based on reaction periods and stored in ethanol for further use.
Synthesis of fcc Cu Nanoparticles
[0089] 2 mL of Cu(acac).sub.2 solution (40 mM) were put into a glass bottle, and then 500 L of 1,2-butanediol were added to make a homogeneous solution. After reaction under 170 C. for 12 h, the product was separated by centrifugation, and washed with ethanol for several times. Then, the obtained fcc Cu nanoparticles were stored in ethanol for further use.
Characterization
[0090] The transmission electron microscope (TEM) and 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 microscope (SEM) measurements were conducted on QUANTA 250. X-ray photoelectron spectroscopy (XPS) test was performed on Thermo Scientific Nexsa spectrophotometer with Al-K radiation system. The calibration of the data was performed by using the C 1s peak at 284.8 eV. X-ray absorption spectroscopy (XAS) measurement was conducted in a transmission mode at beamline X-ray absorption fine structure for catalysis (XAFCA) of Singapore Synchrotron Light Source. The data processing was performed with the Athena and Artemis software packages. In-situ differential electrochemical mass spectrometry (DEMS) test was performed on the Linglu DEMS analysis system from Shanghai Linglu Instrument Co., Ltd.
Electrochemical Nitrate Reduction Tests
[0091] The preparation of working electrode. The catalysts dispersed in ethanol were further washed with ethanol to remove the remaining surfactants. Then, 2 mg of the catalysts (Note that the mass was based on Cu) were re-dispersed into 1 mL of isopropanol, followed by adding 20 L of Nafion solution to make a homogeneous suspension (2 mg.sub.Cu mL.sup.1). Subsequently, it was ultrasonicated for about 2 h to enable the well dispersion of catalyst inks. Afterwards, certain amounts (e.g., 25, 50, and 100 L) of catalyst inks were dropped onto the carbon paper with an area of 1 cm.sup.2 (1 cm1 cm).
[0092] NO.sub.3RR test. The electrochemical NO.sub.3RR performance test was performed in a H-type cell separated by a proton exchange membrane (Nafion 117). The catalyst-modified carbon paper, Pt plate, and Ag/AgCl (filled with saturated KCl) were used as the working, counter, and reference electrodes, respectively. All the potentials were converted to the reversible hydrogen electrode (RHE) based on the equation: E (vs RHE)=E (vs Ag/AgCl)+0.197 V+0.059pH. The solution containing different concentrations of KOH (e.g., 0.01 M, 0.1 M, and 1 M) and 0.1 M KNO.sub.3 were used as the electrolyte, which was purged with high purity argon (Ar) for at least 30 mins before the test. Then, 25 mL of electrolyte were added into both the anode and cathode compartments of the H-type cell. The linear sweep voltammetry (LSV) curves were acquired at a scan rate of 5 mV s.sup.1. The chronoamperometry test was conducted for 1 h at each potential under a stirring rate of 600 rpm with 85% iR compensation. All electrochemical tests were done with Ivium-n-Stat electrochemical workstation.
Detection of Products and Reactants
[0093] NH.sub.3/NH.sub.4.sub.+ detection. Indophenol blue method was used. In detail, 1 mL of electrolyte was taken out after test and diluted with distilled water at suitable folds. Firstly, 2.5 mL of solution A (composed of 0.625 M NaOH, 0.36 M salicylic acid and 0.17 M sodium citrate) were added. Then 300 L of solution B (sodium nitroferricyanide, 10 mg mL.sup.1) and 150 L of solution C (NaClO, active chlorine 6-14 wt. %) were added, successively. After homogeneous mixing, the solution was kept without disturbance for 2 h under dark environment. Next, UV-vis spectrophotometry (Shimadzu-UV1700) was used to examine the absorbance values at 660 nm of these mixed solutions, and the NH.sub.3 concentrations can be obtained according to the calibration curves. The amount of generated NH.sub.3 was also calculated by 1H NMR method. 1 mL of electrolyte after NO.sub.3RR was added with 10 L of C.sub.4H.sub.4O.sub.4 which acted as the internal standards. After that, 50 L of 4 M H.sub.2SO.sub.4 were further introduced to provide a weak acid environment. Subsequently, 450 L of the above solution were mixed with 50 L of D.sub.2O for NMR tests. The integral peak area ratios between NH.sub.4.sub.+ and C.sub.4H.sub.4O.sub.4 were calculated and the corresponding NH.sub.4.sub.+ concentrations can be determined according to the standard curve. The standard NH.sub.4.sub.+ solutions with given concentrations of (NH.sub.4).sub.2SO.sub.4 in 0.05 M H.sub.2SO.sub.4 were prepared to establish the calibration curves for UV-vis and NMR methods. As for the .sup.15N-labeling experiments, all the electrochemical operations and quantitative analysis were the same except for using .sup.15KNO.sub.3 as the nitrogen sources.
[0094] NO.sub.2.sub. detection. Firstly, 4 g of p-aminobenzenesulfonamide, 0.2 g of N-(1-Naphthyl)ethylenediamine dihydrochloride, and 10 mL of phosphoric acid (density=1.70 g mL.sup.1) were added into 50 mL of ultrapure water. After ultrasonication, the obtained transparent solution was used as the coloring reagent. Then, 5 mL of diluted electrolyte were mixed with 0.1 mL of coloring reagent. After 20 mins, the absorption spectrum was taken at the wavelength of 540 nm. A series of standard potassium nitrite solutions were prepared to obtain the calibration curve.
[0095] NO.sub.3.sup. detection. Firstly, a certain amount of electrolyte was taken out from the electrolytic cell and diluted to 5 mL for measurement. Then, 0.1 mL of 1 M HCl solution and 0.01 mL of 0.8 wt. % sulfamic acid solution were added into the solution to be tested. The absorption spectra were measured at a wavelength of 220 nm and 275 nm, and the final absorbance value was calculated according to the equation: A=A.sub.220 nm2*A.sub.275 nm. The concentration-absorbance curve was calibrated using a series of standard potassium nitrate solutions and potassium nitrate was dried before use.
Calculation of Faradaic Efficiency (FE) and Yield Rate
[0096] Herein, the FEs of NH.sub.3 and NO.sub.2.sub. were calculated based on the following equations:
[00001]
[0097] The yield rate of NH.sub.3 was calculated based on the following equation:
[00002]
where F is the Faraday constant (96485 C mol.sup.1), C.sub.NO.sub.2.sub. and C.sub.NH.sub.3 represent the concentration of NO.sub.2.sub. and NH.sub.3 (mg L.sup.1), V is the volume of the electrolyte (L), M.sub.NO.sub.2.sub. and MNH, are the molar mass of NO.sub.2.sub. and NH.sub.3 (g mol.sup.1), Q is the total amount of charge (C), mcu is the mass of Cu loading (mg), and t is the electrolysis time (h).
Assembly of Zn-Nitrate Battery
[0098] A typical H-type cell separated by a bipolar membrane was utilized to assemble the Zn-NO.sub.3.sub. battery. The catalyst supported on carbon paper and a polished Zn foil were used as the working and counter electrodes, respectively. 25 mL of electrolyte composed of 0.1 M KOH and 0.1 M KNO.sub.3 were added into the cathode compartment, while 25 mL of electrolyte composed of 1 M KOH and 0.02 M Zn(Ac).sub.2 (Ac=acetate) were added into the anode compartment. The discharging curve was recorded by an Ivium-n-Stat electrochemical workstation with the sweep rate of 5 mV s.sup.1. The galvanostatic discharge-charge curves were collected with the constant current. The galvanostatic tests with different current densities were performed for 1 h at room temperature using LAND battery test system (CT2001A, Wuhan LAND Electronic Co. Ltd).
Calculation Setup
[0099] In the present invention, density functional theory (DFT) calculations based on the CASTEP package were applied to investigate electronic structures and reaction trends of NO.sub.3RR. For the functionals, the generalized gradient approximation (GGA) and Perdew-Burke-Ernzerhof (PBE) functionals were applied, thus allowing the supplying of accurate descriptions of the exchange-correlation interactions. For all the geometry optimizations, the plane-wave cutoff energy has been set to 440 eV, and the ultrasoft pseudopotentials were applied. Broyden-Fletcher-Goldfarb-Shannon (BFGS) has been selected as the algorithm for energy minimization. In addition, the coarse quality for the k-point was chosen. The IrNi@Cu.sub.20 and IrNi@Cu.sub.30 models have been built based on the hcp IrNi structures with 7-layer thickness. For all the surfaces, 20 vacuum space has been introduced in the z-axis to guarantee complete relaxation. The following convergence criteria were applied to guarantee the geometry optimizations including: 1) the Hellmann-Feynman forces should be converged to less than 0.001 eV/; 2) the total energy difference should be converged to smaller than 5105 eV/atom; and 3) the maximum displacement for each atom should be smaller than 0.005 .
Results and Discussion
Structural Characterizations of Unconventional Phase IrNiCu@Cu Catalysts
[0100] Unconventional phase Cu was obtained through epitaxial growth using hexagonal close-packed (hcp) IrNi nanobranches as the templates in oil phase, and 1,2-butanediol was utilized as the reductant (FIG. 1A, and see more details in below from Materials and Methods section). By controlling the reaction time, the surface distribution of unconventional phase Cu was greatly altered, and the obtained samples were denoted as IrNiCu@Cu-20, IrNiCu@Cu-30 and IrNiCu@Cu-50 based on different reaction times of 20, 30 and 50 mins, respectively. To uncover the difference between common and unconventional phase of Cu, Cu nanoparticles (Cu NPs) with pure fcc phase were also prepared with an average diameter of 18.2 nm (FIGS. 7A-7E). IrNiCu@Cu-20 displays similar morphology as pristine hcp IrNi (FIG. 1B and FIGS. 8A-8B, while the surface of nanobranches becomes rougher (FIG. 1C), which indicates the overgrowth of Cu. The enlarged high-angle annular dark-field scanning transmission electron microscope (HAADF-STEM) image shows obvious twisty surface, and areas with low Z-contrast, which suggests the existence of Cu due to its smaller atomic number (FIG. 1D). The surface and core of IrNiCu@Cu-20 nanobranches display clear hcp diffraction patterns (FIGS. 7E-7F). Besides, the interplanar spacings of 2.17 and 2.31 are assigned to the (002).sub.h and (010).sub.h facets of hep phase, respectively (FIG. 7G), and the observation of atoms in low Z-contrast indicates the formation of few-layer 2H Cu nanostructures. The elemental mappings show that unconventional phase Cu nano-islands are uniformly dispersed on the surface nanobranches (FIG. 1H), and the corresponding line scan reveals the Cu/Ir-rich shell and Ni-rich core (FIG. 1I).
[0101] As the reaction time increases to 30 mins, no nanoparticles are observed (FIG. 2A), while the surface of nanobranches becomes more uneven (FIG. 2B). A thin unconventional phase 2H Cu shell is found on IrNiCu@Cu-30 sample as indicated by HAADF-STEM image and the corresponding fast Fourier transform (FFT) pattern (FIGS. 2C-2D). Moreover, the thickness of 2H Cu increases to over four atomic layers, and it inherits AB stacking sequence from the template (FIG. 2C). IrNiCu@Cu-30 displays similar line scan profiles as IrNiCu@Cu-20, but the intensity of Cu increases (FIG. 2E), and the thin Cu shell is well characterized by elemental mappings (FIG. 2F). When the reaction time prolongs further to 50 mins, a thick Cu shell appears and encapsulates the template completely, without the formation of Cu nanoparticles (FIGS. 2G-2H). As can be seen from FIG. 2I, Cu atoms follow the atomic arrangement of template, and the atomic-resolved image exhibits the characteristic stacking sequence of AB proved by the corresponding selected-area FFT pattern along the [100].sub.h zone axis, (FIG. 2J). In addition, the interplanar spacings of 2.14 and 2.28 are ascribed to the (002).sub.h and (010).sub.h facets of 2H Cu, respectively, which are quite close to the corresponding interplanar spacings of template. As can be seen in FIG. 2k, the intensity of Cu K-edge increases markedly, and Cu shell gradually forms with increasing growth time. Specifically, the thickness of Cu shell evolved from 1.80.1 nm for IrNiCu@Cu-30 to 7.80.1 nm for IrNiCu@Cu-50 (FIGS. 9A-9B, Supporting Information). Based on elemental mappings, IrNiCu@Cu-50 demonstrates a sandwich-like structure, consisting of Ni-rich core, Ir-rich middle layer, and Cu shell (FIG. 2I). Therefore, unconventional phase Cu was successfully obtained and its distribution on the template can be well adjusted.
X-Ray Spectral Analysis
[0102] The chemical state and coordination environment of IrNiCu@Cu nanostructures were analyzed by using X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS). Ir 4f XPS spectra mainly show metallic Ir peaks at around 61 eV (FIG. 410, Supporting Information), while the Ni 2p XPS spectra can only be detected in IrNiCu@Cu-20 with a metallic Ni peak at about 853.3 eV (FIG. 1I), probably arising from the signal shield by thick atomic layer of Ir and Cu in IrNiCu@Cu-30 and IrNiCu@Cu-50. Notwithstanding, Ni mainly exists in metallic state for these samples, proved by normalized Ni K-edge X-ray absorption near-edge structures (XANES) spectra (FIGS. 12A-12D). In addition, the bond lengths of IrIr and NiNi are quite different from Ir and Ni foils with conventional fcc phase (FIGS. 2A-12D and Table 1). According to Cu 2p XPS spectra, the peaks located at around 931.8 and 951.5 eV represent the 2p3/2 and 2p1/2 doublets of metallic Cu, respectively. Apparently, oxidized Cu species are observed in all samples, especially for IrNiCu@Cu-20 and IrNiCu@Cu-30 with lower Cu contents (FIG. 3A). The Cu LMM Auger electron spectroscopy (AES) spectra further reveal that the proportion of metallic Cu is lower in IrNiCu@Cu-20 and IrNiCu@Cu-30 (FIGS. 7A-7F), while the peak intensities for both Cu.sup.0 (568 eV) and Cu.sup.+ (570.3 eV) are comparable for IrNiCu@Cu-50. As can been seen from Cu K-edge XANES spectra, the oxidation state of Cu is between 0 and +2 (FIG. 3B). The lower the amount of Cu, the higher the oxidation state. Based on the corresponding Fourier transformed (FT) k.sup.2-weighted extended X-ray absorption fine structure (EXAFS) spectra and fitting results (FIG. 3C, FIGS. 14A-14 and Table 1), all samples display two dominant peaks located at around 2.54 and 1.92 , which are ascribed to the CuCu and CuO scattering paths of the first shell, respectively. Besides, the coordination number (CN) of CuO bond in IrNiCu@Cu-20 is 2.3, higher than that of IrNiCu@Cu-30 (1.1) and IrNiCu@Cu-50 (1.4). Moreover, the wavelet transform (WT) of Cu EXAFS oscillations present a strong CuO center in IrNiCu@Cu-20 (FIGS. 3D-3G and FIG. 15), and the relatively high oxidation degree might construct a coordination environment suitable for multi-step nitrate reduction.
Electrochemical Nitrate Reduction Performance
[0103] The NO.sub.3RR performance of IrNiCu@Cu nanostructures was evaluated by H-type cell in alkaline electrolyte, and the optimum Cu mass loading was determined to be 200 g (FIGS. 16A-16D and 17A-17D). Besides, the pH effect was also checked, and 0.1 M KOH is more suitable to obtain optimized NH.sub.3 FE and yield rate as it can suppress hydrogen evolution and offer adequate adsorbed hydrogen (FIGS. 16A-16D and 18A-18D). After adding nitrate into 0.1 M KOH, the current density increases markedly based on linear sweep voltammetry (LSV) curves, and IrNiCu@Cu-20 presents the highest activity towards nitrate reduction with the onset potential of ca. 0.2 V (vs RHE), implying a relatively low overpotential and high energy efficiency (FIG. 4A). Besides, the overpotential decreases by about 366 mV at 40 mA cm.sup.2, indicating that IrNiCu@Cu-20 favors nitrate reduction other than HER at relevant potentials. The products (mainly NO.sub.2.sub. and NH.sub.3) after potentiostatic electrolysis at several electrode potentials were analyzed by colorimetric methods based on calibration curves (FIGS. 19A-21B. Obviously, IrNiCu@Cu with different unconventional phase Cu distributions demonstrate distinct NO.sub.3RR performance. IrNiCu@Cu-20 shows higher NH.sub.3 FE, while the other two counterparts hold higher NO.sub.2.sub. FE. At 0 V (vs RHE), the total FE of IrNiCu@Cu-20 reaches up to 98.8%, and the highest NH.sub.3 FE of 86% is obtained at 0.1 V (vs RHE) (FIG. 4B). Then, the total FE drops at more negative potentials due to the competitive HER. In contrast, IrNiCu@Cu-30 presents the highest NO.sub.2.sub. FE of 61.8% at 0.1 V (vs RHE), and NO.sub.2.sub. FE gradually decreases with decreasing potentials. IrNiCu@Cu-50 follows the same trend as IrNiCu@Cu-30 as the surface of both catalysts is mostly covered by Cu, and the pure fcc Cu generates NO.sub.2.sub. as the primary product with a high NO.sub.2.sub. FE of 58% at 0.15 V (vs RHE) (FIGS. 22A-22B).
[0104] Considering the poor NO.sub.3RR performance of pristine IrNi template (FIGS. 21A-21B, NH.sub.3 yield rate was calculated based on the mass loading of Cu, and the atomic ratio of Cu was confirmed by both energy dispersive X-ray spectroscopy (EDS) (FIGS. 23 and 24). IrNiCu@Cu-20 exhibits significantly faster NH.sub.3 generation rate, with the maximum value of 687.3 mmol g.sub.Cu.sup.1 h.sup.1 (FIG. 4C). However, the highest NH.sub.3 yield rates of IrNiCu@Cu-30 and IrNiCu@Cu-50 are 370.3 and 235.1 mmol g.sub.Cu.sup.1 h.sup.1, respectively, and the value for fcc Cu NPs is almost negligible. Correspondingly, IrNiCu@Cu-20 presents higher current density at the same electrode potential during electrolysis (FIGS. 25A-25B). This indicates that the utilization rate of Cu atoms becomes lower in the occasion of thick Cu aggregation, and manipulating the configuration of unconventional phase Cu is effective in regulating NO.sub.3RR performance. Afterward, the effect of NO.sub.3.sub. level was investigated, and IrNiCu@Cu-20 performs better than IrNiCu@Cu-30 in both 0.01 M and 1 M NO.sub.3.sub.. 1 M NO.sub.3.sub. brings greater current density, and a higher NHs yield rate of 2233.61 mmol g.sub.Cu.sup.1 h.sup.1 at 0.2 V (vs RHE) was achieved for IrNiCu@Cu-20 (FIGS. 26-26F). By integrating the performance data acquired in different media, heatmaps of NH.sub.3 FE, NH.sub.3 yield rate and NO.sub.2.sub. FE over IrNiCu@Cu-20 and IrNiCu@Cu-30 were constructed (FIGS. 27A-27F). Generally, NO.sub.3.sub. level is a key factor, while OH.sup. concentration is not a significant performance enhancer. At relatively high NO.sub.3.sub. level, the hydrogenation rate might not keep up with the NO.sub.3.sub.-to-NO.sub.2.sub. conversion rate for IrNiCu@Cu-30 with a higher surface Cu coverage, thus leading to NO.sub.2.sub. accumulation. For IrNiCu@Cu-20, more exposed Ir atoms provide enough active sites to convert NO.sub.2.sub. to NH.sub.3. Simply put, the distribution of unconventional phase Cu affects the reaction endpoint by controlling the hydrogenation rate. In addition, IrNiCu@Cu-20 shows a competitive electrochemical nitrate-to-ammonia performance in alkaline media with low overpotential compared to other reported catalysts (Table 2).
[0105] To confirm the accuracy of NH.sub.3 quantification, 1H nuclear magnetic resonance (NMR) spectroscopy was applied to check the concentration of NH.sub.4.sub.+ (FIGS. 28A-28B and 29A-29B). The FEs determined by colorimetric tests and NMR tests are comparable using K.sup.14NO.sub.3 or K.sup.15NO.sub.3 as nitrate sources, and trace amount of NH.sub.3/NH.sub.4.sub.+ is produced under electrolysis without nitrate (FIG. 4D). In addition, .sup.1H NMR spectra using .sup.14KNO.sub.3 as the electrolyte showed the typical triplet of .sup.14NH.sub.4.sub.+, whereas the expected double peaks of .sup.15NH.sub.4.sub.+ were observed in reactions with .sup.15KNO.sub.3 (FIG. 4E). In addition, no peaks for NH.sub.4.sub.+ showed after electrolysis without nitrate at 0.1 V (vs RHE). Subsequently, the stability of IrNiCu@Cu nanostructures was checked during 15 consecutive cycles (FIG. 4F). IrNiCu@Cu-20 shows around 80% NH.sub.3 FE, and the NH.sub.3 yield rate of IrNiCu@Cu-20 slightly fluctuates at ca. 500 mmol g.sub.Cu.sup.1 h.sup.1, always above those of IrNiCu@Cu-30. In comparison, NH.sub.3 FE of IrNiCu@Cu-30 is at the level of 40%, with about 35% FE towards NO.sub.2.sub.. Therefore, it is speculated that the configuration of IrNiCu@Cu-20 is suitable for deep hydrogenation of NO.sub.2.sub. to generate NH.sub.3, as more Ir atoms with the ability to produce active hydrogen can be exposed at the catalytic interface.
[0106] Real-time monitoring of N-species (e.g., NO.sub.3.sub., NO.sub.2.sub., NH.sub.3) concentration was conducted to reveal the conversion process during nitrate reduction (FIG. 4G). Over a 7.5-h electrolysis period, the removal rate of NO.sub.3.sub. is about 50% using IrNiCu@Cu-20 and IrNiCu@Cu-30 as electrocatalysts. Significantly, the cumulative NO.sub.2.sub. concentration reaches the maxima after 2 h, and then NO.sub.2.sub. will be transformed to NH.sub.3 through a series of proton-coupled electron transfer steps. For IrNiCu@Cu-20, the NH.sub.3 level gradually increases to 30.9 mmol L.sup.1, while the main product for IrNiCu@Cu-30 after NO.sub.3.sub. reduction is apparently NO.sub.2.sub., with the final NO.sub.2.sub. level of 39.2 mmol L.sup.1. Thus IrNiCu@Cu-20 shows better NO.sub.3.sub.to-NH.sub.3 conversion capability, which is further proved by in-situ differential electrochemical mass spectroscopy (DEMS). NH.sub.3 and its fragments (e.g., N, NH, NH.sub.2) were detected for both IrNiCu@Cu-20 and IrNiCu@Cu-30. Importantly, the signal of nitroxyl (HNO) and hydroxylamine (NH.sub.2OH), as the iconic intermediate on the pathway towards NH.sub.3, is observed for IrNiCu@Cu-20, indicating that hydrogenation reactions occur to form NH.sub.3 (FIG. 4H). However, these intermediates are negligible for IrNiCu@Cu-30 and IrNiCu@Cu-50 (FIG. 4I, and FIG. 30), and a weak peak of NO.sub.2.sub. byproduct is discernable for IrNiCu@Cu-50. Furthermore, IrNiCu@Cu-20 was applied in continuous 24-h electrolysis under constant current density of 0.1 A cm.sup.2, and the residual NO.sub.3.sub. level dropped below the drinking water standard (10 mg L.sup.1 NO.sub.3.sub.N) with negligible amount of NO.sub.2.sub. produced (FIGS. 31A-31B). This proves that IrNiCu@Cu-20 is potential in removing and converting NO.sub.3.sub. to NHs toward practical treatments.
Theoretical Studies
[0107] To reveal the origins of high NO.sub.3RR performances of IrNiCu@Cu-20 for the generation of NH.sub.3, DFT calculations have been further carried out to investigate the electronic structures and reaction trends. First of all, the present invention has demonstrated the surface electronic distributions regarding the bonding and anti-bonding orbitals near the Fermi level (E.sub.F). For the fcc Cu surfaces, the surface is mainly dominated by the anti-bonding orbitals, indicating the weak capability of electron transfer (FIG. 5A). In contrast, the surface IrNiCu@Cu-20 shows much higher contributions of the bonding orbitals, especially near the edge sites of the surface Cu (FIG. 5B). This reveals that the surface low-coordinated Cu sites are highly electroactive, which contributes to the adsorption of key intermediates with efficient electron transfer. The exposed surface of IrNi surfaces also has strong distributions of bonding orbitals, potentially facilitating the electron transfer to enhance the water dissociation for efficient generation of protons during NO.sub.3RR. As the surface Cu layers further grow in IrNiCu@Cu-30, the electroactivity of the surfaces reduces owing to the decreases of low coordinated sites, where the bonding orbitals are concentrated at the limited edge sites (FIG. 5C). The projected partial density of states (PDOS) of fcc Cu has revealed that Cu-3d orbitals are mainly located between E.sub.V-1.0 eV to E.sub.V-5.0 eV (E.sub.V denotes 0 eV) with low electron density near the E.sub.F, indicating the limited electron transfer efficiency (FIG. 5D). For IrNiCu@Cu-20, Cu-3d orbitals display a sharp peak with an upshifted location towards the E.sub.F, revealing the evidently improved electroactivity for NO.sub.3RR (FIG. 5G). Both Ir-5d and Ni-3d orbitals have significantly contributed to the largely increased electron density near E.sub.F, which not only improves the site-to-site electron transfer efficiency but also reduces the resistance of the IrNiCu@Cu-20, leading to the optimized electroactivity. As the concentration of Cu further increases in IrNiCu@Cu-30, the Cu-3d orbitals have exhibited a downshift, resulting in reduced electroactivity (FIG. 5F). Meanwhile, electronic structures of the IrNi have not been significantly affected, where both the Ir-5d and Ni-3d orbitals remain similar to that in IrNiCu@Cu-20. These results demonstrate that the different electroactivity towards NO.sub.3RR is induced by the electronic structures of surface Cu in IrNiCu@Cu-20 and IrNiCu@Cu-30. Based on the d-band center comparisons, it is noted that IrNiCu@Cu-20 has shown the highest d-band center of Cu and the overall structure, supporting the optimal electroactivity towards the NO.sub.3RR (FIG. 5G). Compared to fcc Cu, the IrNiCu@Cu-30 displays a lower d-band center of Cu sites but a higher overall d-band center than fcc Cu. To explore the electronic structures of Cu sites, we have compared the site-dependent PDOS of Cu-3d orbitals in fcc Cu and IrNiCu@Cu-20 (FIG. 5H). Notably, the Cu-3d orbitals gradually upshift from fcc Cu bulk to the surface of IrNiCu@Cu-20. In particular, the surface Cu sites with low CN have displayed much sharper PDOS than those middle and interface Cu sites with higher coordination, which play as the main active sites to guarantee the efficient NO.sub.3RR. In comparison, the Cu sites in IrNiCu@Cu-30 show overall broadened 3d orbitals and a downshifted position than the IrNiCu@Cu-20 (FIG. 5I). Even for the surface and step Cu sites, the 3d orbitals only slightly upshift towards the Er, leading to the evidently reduced electroactivity than the IrNiCu@Cu-20. For the adsorptions of key intermediates from NO.sub.3* to NO*, the PDOS shows a good linear relationship of the orbitals, which guarantees fast electron transfer during the reduction process (FIG. 5J). Such a linear relationship of the PDOS is absent in the IrNiCu@Cu-30, indicating the conversion from NO.sub.2* to NO* potentially meets higher barriers (FIG. 5K). This determines the high yield of NO.sub.2.sub. with the limited generation of NH.sub.3 for the NO.sub.3RR on IrNiCu@Cu-30.
[0108] The adsorption energies of NOs and protons are important for the NO.sub.3RR, which are compared among fcc Cu, IrNiCu@Cu-20, and IrNiCu@Cu-30 (FIG. 5L). The adsorption of both NO.sub.3.sub. and H* are most energetically preferred on IrNiCu@Cu-20, benefiting the subsequent reduction of NO.sub.3RR. In addition, the much stronger binding of the proton on IrNiCu@Cu-20 suppresses the unfavored HER process to guarantee a high selectivity and yield of NH.sub.3 generation. For NO.sub.3RR, IrNiCu@Cu-20 has the strongest reaction trend with the smallest energy barrier for NO.sub.3RR, where the conversion from NO.sub.2* to NO* is the rate-determining step (RDS) with an energy barrier of 0.42 eV (FIG. 5M). Meanwhile, both fcc Cu and IrNi@Cuso meet much larger RDS barriers at the reduction of NO.sub.2* to NO* of 0.99 and 1.29 eV, respectively, limiting the reaction at the generation of NO.sub.2.sub. as the main products of NO.sub.3RR. In addition, for the competition generation of both NH.sub.2OH* and NH* from NHOH*, both pathways are energetically preferred on IrNiCu@Cu-20, supporting the experimental characterizations. However, fcc Cu and IrNiCu@Cu-30 only prefer the reaction pathway through NH* due to the higher energy costs for the formation of NH.sub.2OH*, decreasing the formation trends towards NH.sub.3. As the competitive reaction, the reaction trend of HER is also investigated, where IrNiCu@Cu-20 meets the largest energy barrier due to the overbinding of protons (FIG. 5n). The energy barrier for HER is reduced on IrNiCu@Cu-30 and fcc Cu, which also affects the selectivity towards the NO.sub.3RR.
Demonstration of Zn-Nitrate Battery
[0109] Benefited from the positive onset potential of IrNiCu@Cu nanostructures, the assembled Zn-NO.sub.3.sub. batteries using Zn as the anode demonstrate relatively high open-circuit voltage (OCV) of around 1.4 V (vs Zn.sup.2+/Zn) for IrNiCu@Cu-20 and IrNiCu@Cu-30, and the OCV is stable for 24 h without disturbance (FIG. 6A). Meanwhile, Zn-NO.sub.3.sub. battery holds potential to be used as a power source to drive electronic devices, such as electronic timer. As shown in FIG. 6B, the typical discharging polarization curve of this battery system reveals a peak power density (P.sub.max) of 1.21 mW cm.sup.2 at 0.3 V (vs Zn.sup.2+/Zn) when using IrNiCu@Cu-20 as the cathode. By further optimizing the catholyte composition to lower the solution resistance, a higher Pmax of 2.54 mW cm.sup.2 is achieved (FIG. 32), and the P.sub.max of 3.3 mW cm.sup.2 is obtained in catholyte containing 1 M NO.sub.3.sub. with enhanced mass transfer (FIGS. 33A-33B). The Zn-NO.sub.3.sub. battery performance using IrNiCu@Cu-20 as the cathode is also comparable to other emerging electrocatalysts (Table 3). After replacing IrNiCu@Cu-20 with IrNiCu@Cu-30, P.sub.max drops to 1 mW cm.sup.2 at 0.29 V (vs Zn.sup.2+/Zn). Furthermore, the rate performance of IrNiCu@Cu nanostructures during discharging is quite different. IrNiCu@Cu-20 brings higher discharge plateaus at the same current density in comparison with IrNiCu@Cu-30. To be specific, the discharge plateaus are about 1.29, 1.25, 1.20, 1.12, 1.02, 0.76, and 0.51 V (vs Zn.sup.2+/Zn) at 0.1, 0.2, 0.5, 0.8, 1.0, 1.5 and 2.0 mA cm 2 for IrNiCu@Cu-20, respectively (FIG. 6C). When the current density decreases to the original level, the working voltage also recovers to the initial state, indicating that IrNiCu@Cu-20 can tolerate high current impact. Besides, for IrNiCu@Cu-20 cathode, the constructed Zn-NO.sub.3.sub. battery can release a total electrical energy of 41.86 mWh at the discharging current density of 1 mA cm.sup.2, which corresponds to a high energy density of 71555.6 Wh kg.sub.cat.sup.1, and full discharging at 1.5 mA cm 2 leads to a lower output voltage with lower electrical energy of 23.4 mWh (FIG. 6D). In general, Zn-NO.sub.3.sub. galvanic cell can act as a power supply with a delicate design of cathode materials.
Discussions
[0110] In the present invention, unconventional phase (hexagonal close-packed phase-hcp) Cu, i.e., 2H Cu structure, has been successfully obtained through epitaxial growth on a hcp IrNi template. Importantly, the distribution of unconventional phase Cu evolved into different configurations by adjusting the growth time, resulting in different performance towards nitrate reduction and Zn-nitrate batteries. In particular, IrNiCu@Cu-20 with Cu nano-islands on the template displayed a better selectivity toward NH.sub.3, with the highest NH.sub.3 FE of 86% at 0.1 V (vs RHE) in alkaline media, proved by in-situ DEMS which captured the signals of important intermediates (i.e., HNO and NH.sub.2OH). Meanwhile, the NH.sub.3 yield rate reached up to 687.3 mmol g.sub.Cu.sup.1 h.sup.1. In contrast, the main product for IrNiCu@Cu-30 and IrNiCu@Cu-50 with a larger Cu coverage on IrNi surface was NO.sub.2.sub., with NO.sub.2.sub. FE up to 61.8% and 71.7% at 0.1 V (vs RHE), respectively. Furthermore, DFT calculations have unraveled distinct electronic structures induced by the structure of the surface Cu layers, where IrNi@Cu.sub.20 has the highest d-band center for surface Cu sites to guarantee the strong adsorptions of key intermediates. The optimal electronic structures of IrNi@Cu.sub.20 supply the fast conversion of key intermediates, which reduces the energy barriers towards the generation of NH.sub.3. IrNi@Cu.sub.20 with low-coordinated Cu sites also suggest the potential of coordination environment regulation toward Cu sites for enhanced nitrate reduction. In all, controlling the distribution of unconventional phase Cu or Cu coverage at the catalytic interface provides an effective strategy to regulate the NO.sub.3RR performance towards practical applications.
[0111] As mentioned above, electrocatalysts play a key role in optimizing the performance of NO.sub.3RR to realize high NH.sub.3 generation rate, high NHs selectivity and high energy efficiency. Currently, metal-based catalysts have been extensively studied owing to their superior activity toward nitrate reduction, with several materials factors (e.g., defect, crystallinity, strain, and facet) explored to uncover the structure-property relationship of electrocatalysts. For example, oxygen vacancies were introduced into TiO.sub.2 to obtain an enhanced NH.sub.3 Faradaic efficiency (FE) of 85%, as the oxygen vacancy could accommodate the oxygen atom in nitrate to weaken the NO bond.[.sup.23] Besides, metal vacancies were created in WSe.sub.2-x, and unsaturated W sites showed stronger adsorption towards nitrate. In addition, amorphous RuO.sub.2 with a modified d-band center holds a lower reaction energy barrier of *NO hydrogenation than crystallized RuO.sub.2. In another study, strained Ru nanoclusters reported by Yu et al. indicate a strain-induced hydrogen radicals (H) formation, which is important for nitrate protonation. Generally, regulating the structural parameters of metal-based catalysts will alter the interaction between reactant and catalysis interface, thus leading to a promoted NO.sub.3.sub.-to-NH.sub.3 conversion.
[0112] Crystal phase, which refers to the atomic arrangement in a material to form a long-range ordered structure, is also an important material factor that can greatly affect the catalytic reactions. Metal nanomaterials with unconventional phases or heterophases have displayed lower overpotential and higher specific activity for a series of reactions, including hydrogen evolution reaction (HER), alcohol oxidation, carbon dioxide reduction reaction (CO.sub.2RR). 4H/face-centered cubic (fcc) Au@Cu exhibits enhanced overall activity and better ethylene selectivity in CO.sub.2RR. Recently, 4H/fcc Ir nanostructures were reported to exhibit enhanced performance in electrochemically reversible CO.sub.2 conversion and coupled into LiCO.sub.2 battery to achieve a high energy efficiency up to 84%. However, the effect of phase has seldomly been explored in NO.sub.3RR. Copper (Cu) has partially filled d orbital, resulting in strong affinity toward nitrate molecule and activation of NO bond. Considering Cu is effective in catalyzing the potential determining step of NO.sub.3RR, i.e., nitrate reduction to nitrite (NO.sub.2.sub.), and inhibiting the formation of H.sub.2, Cu-based electrocatalysts have been proved efficient towards NO.sub.3RR..sup.[42-44] Nevertheless, given the harsh synthesis condition and easy oxidation of Cu under ambient environment, catalysts with unconventional phase Cu have not been reported for nitrate reduction.
[0113] In the present invention, unconventional phase 2H Cu is obtained via epitaxial growth on hexagonal close-packed (hcp) IrNi nanobranches, and explicit structure-performance relation is presented to uncover the importance of rationally designed Cu sites in NO.sub.3RR. During the reduction process, Cu atom will diffuse into the template and form a ternary IrNiCu alloy, and the distribution of 2H Cu on IrNi surface can be modulated elaborately by reaction time, leading to controllable product distribution after nitrate reduction. IrNi nanobranches with dispersed 2HCu nano-islands (IrNiCu@Cu-20) display the highest NH.sub.3 FE of 86% at 0.1 V (vs reversible hydrogen electrode (RHE)), with a NH.sub.3 yield rate of 687.3 mmol g.sub.Cu.sup.1 h.sup.1. However, the main product of IrNi nanobranches with almost fully covered Cu (IrNiCu@Cu-30 and IrNiCu@Cu-50) is NO.sub.2.sub. with FE up to 61.8% at 0.1 V (vs RHE). Furthermore, a tandem catalysis mechanism is discovered on IrNiCu@Cu-20, where NO.sub.2.sub. produced by Cu sites is subsequently hydrogenated on IrNi sites. Density functional theory (DFT) calculations have shown that the control of Cu growth has significant influences on the NO.sub.3RR performance, where IrNiCu@Cu-20 has shown the highest electroactivity due to the abundant electroactive low-coordinated sites. The upshifted d-band center in IrNiCu@Cu-20 guarantees fast electron transfer for efficient generation of NH.sub.3 by decreasing the barriers of the rate-determining step. Last but not the least, zinc (Zn)-nitrate battery is constructed, indicating that crystal phase engineering of metal-based nanostructures provides an effective strategy to regulate the performance of catalytic reactions and energy devices.
[0114] It should be understood that certain features of the invention, which are, for clarity, described in the content of separate embodiments, may be provided in combination in a single embodiment. Conversely, various features of the invention which are, for brevity, described in the content of a single embodiment, may be provided separately or in any appropriate sub-combinations. It is to be noted that certain features of the embodiments are illustrated by way of non-limiting examples. Further, when specific values or discrete parameters in, for example, an experiment are indicated, the values or parameters in the same ratio or proportion will be considered as understood by a skilled person as equally workable.
Tables
TABLE-US-00001 TABLE 1 A summary of EXAFS fitting results of IrNi, IrNiCu@Cu-20, IrNiCu@Cu-30, IrNiCu@Cu-50, and fcc Cu NPs. Scattering Sample path CN .sup.2 (.sup.2) E.sub.0 (eV) R () R-factor IrNi NiNi 8.9 0.0109 2.78 2.62 0.011 NiIr 0.7 0.0065 2.78 2.59 IrNi 5.1 0.0065 9.51 2.59 IrIr 4.9 0.0065 9.51 2.66 IrNiCu@Cu- NiNi 10.6 0.0111 3.10 2.62 0.009 20 NiIr 0.6 0.0057 3.10 2.59 IrNi 4.9 0.0057 8.26 2.59 IrIr 6.0 0.0057 8.26 2.66 CuO 2.3 0.0057 6.48 1.95 0.007 CuCu1 2.5 0.0076 2.87 2.54 CuCu2 0.9 0.0076 2.87 2.93 IrNiCu@Cu- NiNi 11.2 0.0113 3.60 2.62 0.019 30 NiIr 0.6 0.0053 3.60 2.61 IrNi 4.9 0.0053 9.37 2.61 IrIr 7.1 0.0053 9.37 2.68 CuO 1.1 0.0078 5.22 1.92 0.002 CuCu 6.9 0.0085 0.92 2.54 IrNiCu@Cu- NiNi 11.4 0.0114 3.55 2.62 0.014 50 NiIr 0.6 0.0051 3.55 2.59 IrNi 5.5 0.0051 6.76 2.59 IrIr 5.3 0.0051 6.76 2.65 CuO 1.4 0.0078 5.79 1.93 0.002 CuCu 5.9 0.0082 0.38 2.55 Cu NPs CuO 0.9 0.0054 5.63 1.90 0.002 CuCu 5.1 0.0083 0.69 2.54 Ni foil NiNi 12 0.0061 0.01 2.48 0.001 Ir powder IrIr 12 0.0034 7.75 2.71 0.007 Cu foil CuCu 12 0.0085 1.00 2.54 0.001
TABLE-US-00002 TABLE 2 A comparison of the electrochemical nitrate reduction performance of IrNiCu@Cu-20 with other reported electrocatalysts in alkaline and neutral media. Potential NH.sub.3 (V vs FE NH.sub.3 yield Catalysts Electrolytes RHE) (%) rate Refs. IrNiCu@Cu-20 0.1M KOH + 0.1M 0.1 86 687.3 mmol This KNO.sub.3 g.sub.Cu.sup.1 h.sup.1 work 3.07 mg cm.sup.2 h.sup.1 IrNiCu@Cu-20 0.1M KOH + 1M 0.2 94.14 10.52 mg cm.sup.2 This KNO.sub.3 h.sup.1 work Alkaline media Ru.sub.15Co.sub.85 0.1M KOH + 0.1M 0 97 54.4 mg mg.sup.1 [16] KNO.sub.3 h.sup.1 CuCoO.sub.x 0.1M KOH + 0.01M 0.1 97.8 3.86 mg cm.sup.2 [17] KNO.sub.3 h.sup.1 CuPd nanocubes 1M KOH + 1M 0.5 92.5 106.2 mg h.sup.1 [18] KNO.sub.3 mg.sup.1 (0.6 V) CuCo SP 0.1M KOH + 0.01M 0.175 93.3 2.64 mg cm.sup.2 [19] NO.sub.3.sup. h.sup.1 CuNC SAC 0.1M KOH + 0.1M 1.0 84.7 4.5 mg cm.sup.2 [20] KNO.sub.3 h.sup.1 FePPy SACs 0.1M KOH + 0.1M 0.7 Nearly 100 2.75 mg cm.sup.2 [21] KNO.sub.3 h.sup.1 Au.sub.1Cu SAAs 0.1M KOH + 7.14 0.2 98.7 0.555 mg cm.sup.2 [22] mM NO.sub.3.sup. h.sup.1 Cu@C 1M KOH +1 mM 0.3 72 0.31 mg cm.sup.2 [23] NO.sub.3.sup. h.sup.1 -CD-K.sup.+ 0.1M KOH + 0.1M 0.9 79.3 4.66 mg cm.sup.2 [24] KNO.sub.3 h.sup.1 Cu nanodisks 0.1M KOH + 10 0.5 81.1 0.91 mg mg.sup.1 [25] mM KNO.sub.3 h.sup.1 Cu NBs (100) 1M KOH + 0.1M 0.15 95 650 mmol g.sub.cat.sup.1 [26] KNO.sub.3 h.sup.1 Bi nanocrystals 1M KOH + 0.5M 0.5 90.6 ca. 12 g g.sub.cat.sup.1 [27] KNO.sub.3 h.sup.1 Gd SA on O-defect 1M KOH + 1M 0.2 ca. 68% 628 g mg.sub.cat.sup.1 [28] rich NiO KNO.sub.3 h.sup.1 Fe.sub.3C on N-doped C 1M KOH + 75 0.5 96.7 1.19 mmol [29] nanosheet mM KNO.sub.3 mg.sup.1 h.sup.1 FeB.sub.2 1M KOH + 0.1M 0.6 96.8 25.5 mg cm.sup.2 [30] KNO.sub.3 h.sup.1 CoPCNS on Cu foam 1M NaOH + 1M 1.03 88.6 8.47 mmol [31] NaNO.sub.3 cm.sup.2 h.sup.1 Neutral media NiO.sub.4CCP 0.5M NaNO.sub.3 + 1M 0.7 94.7 1.83 mmol g.sup.1 [32] Na.sub.2SO.sub.4 h.sup.1 Fe@NC 500 ppm NaNO.sub.3 + 0.75 91.8 ca. 2.25 mg [33] 0.5M Na.sub.2SO.sub.4 cm.sup.2 h.sup.1 Pd NA on nickel foam 0.5M Na.sub.2SO.sub.4 + 1.2 78 1.52 mmol [34] 0.1M NaNO.sub.3 cm.sup.2 h.sup.1 Mn incorporated Co.sub.3O.sub.4 0.5M K.sub.2SO.sub.4 + 0.1M 1.2 99.5 35 mg cm.sup.2 h.sup.1 [35] KNO.sub.3 Cu-doped Co.sub.3O.sub.4 0.1M Na.sub.2SO.sub.4 + 0.6 86.5 36.71 mmol g.sup.1 [36] 500 ppm NO.sub.3.sup. h.sup.1 FeSA (FeN.sub.4) 0.5M KNO.sub.3 + 0.1M 0.66 75 5.245 mg [37] K.sub.2SO.sub.4 mg.sub.cat.sup.1 h.sup.1 Cu-SA (Cu-cis-N.sub.2O.sub.2) 0.5M Na.sub.2SO.sub.4 + 1.6 ca. 80 27.84 mg cm.sup.2 [38] 1000 ppm NO.sub.3.sup.N h.sup.1 FeNC 0.1M K.sub.2SO.sub.4 + 0.5M 0.9 68 18.8 mg mg.sup.1 [39] KNO.sub.3 h.sup.1 LaCoO.sub.3 1M Na.sub.2SO.sub.4 + 0.5M 1.0 91.5 4.18 mmol [40] KNO.sub.3 mg.sup.1 h.sup.1 Pd-nanodot/Zr-MOF 0.1M Na.sub.2SO.sub.4 + 1.3 58.1 287.31 mmol [41] 500 ppm NO.sub.3.sup.N g.sub.cat.sup.1 h.sup.1
TABLE-US-00003 TABLE 3 A comparison of the Zn-nitrate battery performance of IrNiCu@Cu-20 with other reported electrocatalysts. Open Peak circuit power voltage density Cathodes Catholytes (V) (mW cm.sup.2) Refs. IrNiCu@Cu-20 0.1M NO.sub.3.sup. + 0.1M 1.4 2.54 This KOH + 0.2M K.sub.2SO.sub.4 work 1M NO.sub.3.sup. + 0.01M 1.39 3.3 KOH Pd doped TiO.sub.2 0.25M LiNO.sub.3 + 5M 0.81 0.87 [7] LiCl NiCOPV.sub.P 0.1M NO.sub.3.sup. + 1M 1.39 1.14 [42] KOH RhCu M-tpp 3000 ppm NO.sub.3.sup. + ca. 1.52 1.54 [43] 0.5M Na.sub.2SO.sub.4 RuFe NFs 0.1M NaNO.sub.3 + 0.5M 1.37 1.9 [44] Na.sub.2SO.sub.4 Fe doped Ni.sub.2P 0.05M KNO.sub.3 + 0.2M 1.22 3.25 [45] K.sub.2SO.sub.4 Fe.sub.2TiO.sub.5 0.1M NaNO.sub.3 + PBS 1.5 5.6 [46] nanofibers Metastable 0.05M KNO.sub.3 + 1M 1.27 7.56 [47] phase Cu KOH Ni SA 200 ppm NO.sub.3.sup.N + 1.51 12.7 [48 alloyed Cu 0.5M K.sub.2SO.sub.4 Cu nanowire 4000 ppm NO.sub.3.sup. + 0.93 14.1 [49] 0.1M KOH Ru/-Co(OH).sub.2 0.1M KNO.sub.3 + 1M 1.48 29.87 (flow [50] KOH cell)
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