Nickel Ferrocyanide as High-Performance Next Generation Electrocatalyst for Urea Oxidation

doi:10.21203/rs.3.rs-67358/v1

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Nickel Ferrocyanide as High-Performance Next Generation Electrocatalyst for Urea Oxidation

https://doi.org/10.21203/rs.3.rs-67358/v1

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published 20 Sep, 2021

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Urea oxidation, a key process in energy and environmental science, faces challenges because of the insufficient understanding of its mechanism and the lack of efficient catalysts. Here we demonstrate that nickel ferrocyanide (Ni₂Fe(CN)₆) molecular catalyst supported on Ni form can drive urea oxidation reaction (UOR) with the record electrochemical activity and stability among all supported catalysts reported so far. A combination of kinetics data, in-situ spectroscopic measurements and energy computations suggests a new UOR pathway that delivers such outstanding performance. Different from most studied Ni-based catalysts with NiOOH derivative as a real catalytically active site for UOR, Ni₂Fe(CN)₆ appears to be a next-generation catalyst able to directly facilitate a two-step reaction pathway involving a critical reaction of intermediate ammonia’s production (on Ni site) and oxidation (on Fe site). Due to the alternative rate-determining step with a more favorable thermal energetics, Ni₂Fe(CN)₆ broke the limiting activity of the reported so far UOR catalysts. As a result, the UOR process on Ni₂Fe(CN)₆ can replace conventional water oxidation process in various energy-saving systems for hydrogen and hydrogen peroxide production.

Materials Chemistry

nickel ferrocyanide

urea oxidation reaction

electrocatalyst

Electrochemical urea oxidation reaction (UOR) is of great importance in a range of energy-related applications and devices.^1-5 Due to a more negative equilibrium potential (E⁰= 0.37 V vs reversible hydrogen electrode, RHE), UOR is an ideal alternative anode half reaction to conventional oxygen evolution reaction (OER, E⁰= 1.23 V vs RHE) for pure hydrogen and hydrogen peroxide production in energy-saving water electrolyzers.^6,7 Practically, compared to precious pure water oxidation and poisoning methanol/hydrazine oxidation reactions, UOR process has significant advantages because urea is globally abundant in human urine, urea-rich wastewater and byproduct of industrial activities. In addition, compared to the emerging seawater oxidation, UOR can avoid chlorine gas generation because of the lower reaction potential of UOR than that for chlorine evolution reaction (E⁰= 1.36 V vs RHE).⁸ Therefore, using UOR to replace OER can not only save the energy input but also reduce the contamination of urea-rich wastewater.^4,6

Compared to OER, UOR suffers from even more sluggish kinetics because of the complicated 6e^- transfer process, which needs high performance catalysts to decrease the overpotential to achieve an efficient device.^{2, 9-11} Initially, some noble metal catalysts such as Ti-Pt, Ti-Pt-Ir, and Ru were used to enhance the activity, however due to their scarcity they are costly.² However, the catalytic performance of the recently developed non-noble metal UOR catalysts based on Ni hydroxides, oxides, sulfides, and phosphides, etc, does not satisfy requirements of practical applications due to the high overpotential, low current density and inadequate stability.^7,12,13For example, the best so far Ni-based UOR catalysts, e.g., Ni_0.9Fe_0.1O_x, can deliver a 10 mA cm^-2 at the potential of 1.34 V (vs RHE) in 1.0 M KOH with 0.33 M urea, which does not represent a significant improvement to the potential on as compared to the state-of-the-art OER catalysts.¹⁴ In addition, the stability of the reported UOR catalysts is questionable; only a few catalysts maintain a relative current density higher than 90 % of the initial value for 50 hour continuous reaction.¹²

This poor performance may be due to the inherent reaction mechanism. Specifically, for the most studied Ni-based materials, before UOR onset (e.g., 1.40 V vs RHE), the catalysts inevitably undergo a self-oxidation process to form Ni³⁺stated NiOOH derivative on the surfaces (always happens at ~1.36 V vs RHE), which then serves as the direct active sites for UOR. Under this scheme, the rate-determining step (RDS) of the overall reaction is desorption of the last *COO intermediate from the active sites to form CO₃²⁻ in the electrolyte. However, because of a strong binding of Ni³⁺ active sites with *COO intermediate, the energetics (~1242.2 kJ mol^-1) for this reaction mechanism is unfavorable, which results in low activity of the catalyst.¹⁵ In addition, this reaction pathway involves a CO intermediate, which can poison the catalysts and make them unstable.^2,15 Even though a wide variety of catalysts have been developed with different compositions and structures, most of them involve NiOOH species, which limits their activity (the best one is ~101 mA cm^-2 at a potential of 1.40 V vs RHE in 1.0 M KOH with 0.33 M urea).^9,16,17 Note that as a 6e^- transfer reaction, the UOR process should have more alternatives with optimal thermodynamics or kinetics beyond the mechanism involving NiOOH species. This has been widely validated for other multiple-electron transfer reactions such as oxygen reduction reaction and carbon dioxide reduction reaction that have multiple reaction pathways.¹⁸ One can image that a new mechanism with more rational reaction intermediates and better adsorption free energy can significantly improve the activity of catalysts.^12,18,19 This improvement can be achieved by employing the atomic-level design of high-performance UOR catalysts and by performing the advanced spectroscopic studies to identify the critical reaction intermediates.

In this work, we propose a new and more energetically favorable UOR pathway triggered by nickel ferrocyanide Ni₂Fe(CN)₆ electrocatalyst, one of Prussian blue analogues.²⁰ With an alternative reaction intermediate, the new reaction mechanism on Ni₂Fe(CN)₆delivers one of the best UOR activity and assures high stability as compared to the existing catalysts. Through an advanced characterization by in-situ techniques (e.g., synchrotron radiation-Fourier transform infrared spectroscopy, Raman spectroscopy, ammonia detection) and density functional theory (DFT) computation, a critical intermediate of *NH₃ was identified, which involves two steps of production and oxidation of NH₃on different sites of the catalyst surface. This mechanism is very different from the currently known mechanism with *NCO, *HN-CO or *COO as the key intermediates. In addition, our experiment demonstrated the UOR process driven by Ni₂Fe(CN)₆ can effectively replace the conventional water oxidation in different energy saving systems for hydrogen or H₂O₂ production.

Physicochemical properties. The typical Ni₂Fe(CN)₆ catalyst was prepared on nickel foam through a self-assembly method by immersing cleaned Ni foams in the mixed solution of polyvinylpyrrolidone, C₆H₅Na₃O₇·2H₂O, NiCl₂·6H₂O and K₃[Fe(CN)]₆ using different aging times and salt concentrations. Scanning electron microscope (SEM) image shows that the as-synthesized product is composed of nanocubes assembled on the surface of nickel foam with a single layer coating (Fig. 1a). Transmission electron microscope (TEM) image shows a highly crystalline cubic nanoparticle with an edge length of 180 ± 10 nm (Fig. 1a inset). Different reaction conditions can be used to tune the nanocube size and coverage on the nickel foam (Supplementary Fig. 1, 2). Analysis of the X-ray powder diffraction (XRD) pattern obtained for Ni₂Fe(CN)₆ powder (Fig. 1b) allowed for identification of a single cubic phase where Fe atom coordinates with carbon atoms in CN⁻ species and Ni atom has two coordination forms: one coordinates with N atoms in CN⁻ species and the other is situated in the center alone.^21,22 Interestingly, both Fe²⁺ and Ni²⁺ species were not oxidized during UOR process (Fig. 1c, d), which differs from the mechanism reported for other Ni-based catalysts. ^{9, 15}

UOR performance evaluation. Since human urine contains 2-2.5 wt. % of urea (equals to a molar concentration of ~ 0.33 M), 0.33 M urea was chosen in the electrolysis.^{2, 12} As shown by the linear sweep voltammetry (LSV), UOR on an optimized Ni₂Fe(CN)₆ catalyst exhibits a more negative onset potential than that of OER (Fig. 2a, Supplementary Fig. 3). To obtain a current density of 100 mA cm^− 2, UOR needs a potential of 1.35 V, much smaller than that for OER (1.68 V). More importantly, as shown in Fig. 2a, the urea oxidation on Ni₂Fe(CN)₆ proceeds before the self-oxidation of Ni²⁺ to Ni³⁺. This is very different from the mechanism reported for other reported Ni-based catalysts, in which a NiOOH phase is formed before UOR and serves as the active sites.¹⁵ Specifically, for Ni₂Fe(CN)₆ catalyst a very small apparent UOR activation energy of 14.0 kJ/mol (at a potential of 1.39 V) is observed (Supplementary Fig. 4). Importantly, the potential of 1.35 V to achieve an UOR current density of 100 mA·cm^− 2 on Ni₂Fe(CN)₆ is the lowest one among all known UOR electrocatalysts reported including nickel hydroxides, metals, phosphides, etc.^{10,13,16,23−33} In addition, the current density at a certain potential (e.g. 1.40 V) on Ni₂Fe(CN)₆ is 2.5 times higher than that obtained for the state-of-the-art nickel-based supported electrocatalysts (Supplementary Fig. 5, Table 2).¹⁴ Interestingly, it was also found that two other cyanides, ferric ferrocyanide (Fe₄[Fe(CN)₆]₃) and nickel cobaltcyanide (Ni₃[Co(CN)₆]₂), showed worse activity compared to Ni₂Fe(CN)₆ (Fig. 2b, Supplementary Fig. 6).^34,35 Therefore, we speculate that the observed high performance of Ni₂Fe(CN)₆ is caused by cooperative action of two active sites of Ni and Fe in Ni₂Fe(CN)₆ catalyst.

The kinetic study shows that the UOR process on Ni₂Fe(CN)₆ is independent of urea concentration (Fig. 2c), which agrees with the reported so far Ni-based electrocatalysts.³⁶ As can be seen in Fig. 2d, UOR shows a strong dependence on the amount of KOH with a reaction order of 1.10 to OH⁻ concentration, which is different from that of about 2.00 for other Ni-based electrocatalysts.³⁶ For example, a reaction order of 1.75 was obtained on a conventional UOR catalyst of NiC₂O₄ (Supplementary Fig. 7,8).³⁷ It was revealed that the UOR’s RDS on NiOOH intermediated electrocatalysts is desorption of CO₂ from NiOOH active sites, which requires a 2 M OH⁻ to form CO₃²⁻ in the electrolyte. ^{15, 38–40} Therefore, the reaction order of 1.10 on the newly developed Ni₂Fe(CN)₆ catalyst indicates an alternative reaction pathway and RDS.

Comparison to the conventional NiC ₂ O ₄ catalyst. The above statement was validated by in-situ Raman spectroscopy studies of two types of catalysts, Ni₂Fe(CN)₆ and conventional NiC₂O₄ (Supplementary Fig. 9). As shown in Fig. 3a, two strong peaks at 2100 and 2140 cm^− 1 are characteristic of cyanide stretching in Ni₂Fe(CN)₆ complex,²⁰ while the peaks at 250 and 348 cm^− 1 belong to the Ni-N stretching vibration and the peak at 510 cm^− 1 belongs to the Fe-C stretching vibration, respectively.²² Importantly, at various potentials and different reaction times (Fig. 3a, Supplementary Fig. 10a), the Raman spectra of Ni₂Fe(CN)₆ are similar to that at the open circuit potential (OCP). However, for NiC₂O₄, the NiOOH doublet peaks at 473 and 560 cm^− 1 appear in a very short time at a high potential during UOR process (Fig. 3b and Supplementary Fig. 10b), indicating it is partially reconstructed to NiOOH, which serves as the active phase for UOR and agrees with the existing literature.^38,39 As expected, Ni₂Fe(CN)₆ without NiOOH species showed a significantly enhanced apparent UOR activity and stability compared to NiC₂O₄ catalyst (Fig. 3c). About 90% of the current density remained after 50 hour operation on Ni₂Fe(CN)₆, which is a record for UOR catalysts.

The new reaction pathway of UOR for Ni₂Fe(CN)₆ was further investigated by the real-time ammonia detection in the electrolyte using an ion ammonia-selective electrode (Orion™ High-Performance Ammonia Electrode 9512HPBNWP, Supplementary Fig. 11).^41− 43 As shown in Fig. 3d, a large amount of ammonia was produced and detected on Ni₂Fe(CN)₆ under a potential of 1.36 V (current density of 70.0 mA cm^− 2), while very little was detected on NiC₂O₄ at similar conditions (current density of 72.0 mA cm^− 2 at a potential of 1.45 V). It was further found that Ni₂Fe(CN)₆ also can efficiently catalyze the oxidation of ammonia (Supplementary Fig. 12). Therefore, we propose that the UOR process on Ni₂Fe(CN)₆ contains *NH₃ as a key reaction intermediate, which is very different from the currently reported mechanisms involving *NCO, *HN-CO, or *COO as the intermediates.^38,39

DFT computations. Based on the above analysis of kinetics and real-time ammonia detection, we propose a new mechanism for UOR, which comprises of two steps, namely production of NH₃ and oxidation of NH₃ (the detailed pathways can be found in the experimental section). The Gibbs free energy of each reaction intermediate was computed by density functional theory (DFT, Supplementary Table 1 and Supplementary Figs. 13–16). For the first step (Fig. 4a), Ni sites were proven to be the primary active sites because the free energy value of RDS ([M·OCO₂]_ads + 2e⁻ → M + CO₃²⁻) on Ni site is smaller than that on Fe site. For the second step (Fig. 4b), the biggest difference is in the formation of *NH·NH₂ intermediates, which reveals that the dehydrogenation reaction ([M·NH₂·NH₂]_ads + OH⁻ → [M·NH·NH₂]_ads + H₂O + e⁻) is harder to occur on Ni sites than Fe sites (free energy of 1.23 vs 0.94 eV). For the overall reaction pathways, the calculated RDS is the formation of *NH + * NH₂ (IMFe9 to IMFe10 as shown in Fig. 4b), in which 1 M *NH₂ + *NH₂ react with 1 M OH⁻, which yields the reaction order of 1.0 with respect to the OH⁻ concentration. This result agrees well with the experimentally determined reaction order of 1.10 on Ni₂Fe(CN)₆ catalyst (Fig. 2d). Overall, it is proposed that Ni is responsible for conversion of urea into ammonia and carbonate, while Fe is responsible for transformation of ammonia into nitrogen (Fig. 4c). This synergistic catalysis between Ni and Fe sites revealed by DFT computation agrees well with experimental observation that Ni₂Fe(CN)₆ exhibits remarkably higher activity than other two types of Fe₄[Fe(CN)₆]₃ and Ni₃[Co(CN)₆]₂ catalysts (Fig. 2b).

Identification of intermediates. We performed in-situ synchrotron radiation-Fourier transform infrared spectroscopy (SR-FTIR) analysis to identify the critical reaction intermediates proposed in the DFT computation and real-time ammonia detection. As shown in Fig. 5a, compared with the spectrum at OCP, two obvious absorption bands appear at 2925 cm^− 1 and 1203 cm^− 1 under the UOR working potential (e.g. 1.35–1.65 V), which can be assigned to the N-H stretching vibration of *N = NH₂⁺ and C-O stretching vibration of *OCONH₂ species, respectively.^{44, 45} In addition, with increasing potential and operating time, these two characteristic peaks become stronger (Fig. 5b-c and Supplementary Fig. 17). The simulated results of harmonic vibrational frequencies (Fig. 5d) also indicate that two peaks can be attributed to *N = NH₂⁺ and *OCONH₂ species. This clearly indicates that these two intermediates are produced in the UOR process, which supports the DFT computation with IMFe12 and IMNi3 intermediates. At this stage, the combination of kinetics analysis, DFT computation and in-situ SR-FTIR spectroscopy data confirms a new UOR mechanism on Ni₂Fe(CN)₆ without NiOOH generation, namely two-step processes of NH₃ production and oxidation at two different active sites.

Energy-saving systems driven by UOR for replacement of OER. To establish an energy-saving system benefiting from the low overpotential of UOR on Ni₂Fe(CN)₆ catalyst, we assembled an UOR//HER electrolyzer using Ni₂Fe(CN)₆ as the anode in an electrolyte containing 1 M KOH and 0.33 M urea. For comparison, an OER//HER electrolyzer with RuO₂ as the anode for OER was performed in 1.0 M KOH (Supplementary Fig. 18). To obtain a current density of 10 and 100 mA cm^− 2, urea electrolysis needs a cell voltage of 1.38 and 1.50 V, respectively, whereas water electrolysis needs the value of 1.56 and 1.85 V (Fig. 6a). This clearly indicates the energy-saving advantage of the UOR process on Ni₂Fe(CN)₆ electrocatalyst to replace OER. In addition, the H₂ production in this UOR//HER cell was very stable with a Faradaic efficiency higher than 90% (Fig. 6b, Supplementary Fig. 19). Besides H₂ production, the energy-saving system can also be applied in a UOR//2e⁻ORR flow cell composed of Ni₂Fe(CN)₆ anode and mesoporous carbon (CMK-3) cathode ⁴⁶ (Supplementary Fig. 20). Nowadays, in-situ electrochemical production of H₂O₂ via 2e⁻ ORR has become a promising way because it can reduce the danger and costs of the transportation of H₂O₂.^{46, 47} As expected, the urea electrolysis needs a smaller energy input than water electrolysis for urea elimination and H₂O₂ generation (Supplementary Fig. 21). Specifically, a H₂O₂ production rate of 225.3 g m^− 2h^− 1 (with a Faradaic efficiency of 82.3%) and an urea elimination rate of 140.1 g m^− 2h^− 1 (with a Faradaic efficiency of 94.9%) were achieved at the cell voltage of only 0.63 V (Fig. 6c, Supplementary Fig. 22). In addition, the urea elimination and H₂O₂ generation were very efficient for urea concentrations varying from 0.0033 to 0.33 M (Fig. 6d, Supplementary Fig. 23), which is in the range of industrial urea-containing wastewater.

In summary, Ni₂Fe(CN)₆ as a next-generation UOR electrocatalyst with excellent efficiency and stability was developed by a simple and readily scalable method. Studies of mechanism by employing the advanced in situ Raman spectroscopy, in situ SR-FTIR techniques and real-time ammonia detection revealed a new and more energetically favorable UOR pathway on Ni₂Fe(CN)₆ as compared to most reported electrocatalysts. DFT results revealed that the highly enhanced electrochemical performance originates from the synergistic effect of Ni and Fe double active sites in Ni₂Fe(CN)₆. The efficient UOR on Ni₂Fe(CN)₆ replacing conventional OER is advantageous because of energy saving and reducing contamination of urea-rich wastewater. This work opens a new avenue to develop alternative electrocatalysts for UOR with boosted activity and stability.

Ni ₂ Fe(CN) ₆ synthesis. 0.1 mmol of polyvinylpyrrolidone (PVP), 1.5 mmol of NiCl₂·6H₂O and 2.1 mmol of C₆H₅Na₃O₇·2H₂O were dissolved in 300 mL of deionized water. Then the mixed solution was added dropwise to 200 mL of solution containing 0.5 mmol of K₃[Fe(CN)₆]. Ni foam was sonicated in acetone, diluted nitric acid solution and deionized water for 15 minutes, respectively. The cleaned Ni foam was placed into the mixed solution and aged for 48 hours to obtain the final Ni₂Fe(CN)₆ electrocatalyst.

Electrochemical measurements. IM6e electrochemical workstation (Zahner-Electrik, Germany) was used to test the UOR activity in a three-electrode system, in which the as-prepared free standing electrocatalyst was directly used as the working electrode. Hg/HgO electrode and carbon rod were used as the reference and counter electrodes, respectively. 1.0 M KOH was used as the electrolyte with 0.33 M urea. The LSV curves were obtained at a scan rate of 5 mV s^− 1. All curves were corrected manually with iR compensation and the potential was converted to RHE. The UOR//HER system was carried out in a two-electrode mode separated by an anion membrane. The UOR//2e-ORR system was carried out in a commercial flow cell, in which the prepared Ni₂Fe(CN)₆ on nickel foam was used as the anode, and the mesoporous carbon coated gas diffusion electrode was used as the cathode.⁴⁶ The anode contained 1.0 mol L^− 1 KOH with 0.0033-0.33 mol L^− 1 urea and the cathode contained 1.0 mol L^− 1 KOH. The areas of anode and cathode are both 4 cm².

Electrochemical in situ Raman measurements. The Raman spectroscopy was carried out using a Via-Reflex spectrometer (Renishaw) with a laser excitation wavelength of 532 nm and the measured potential for UOR was in the range of 1.2–1.6 V controlled by an electrochemical workstation (CHI750E Instruments). The in-situ electrochemical three-electrode cell contained Ni₂Fe(CN)₆ electrocatalyst as the working electrode, Ag/AgCl as the reference electrode and Pt wire as the counter electrode. 1.0 M KOH was used as the electrolyte with 0.33 M urea solution. All Raman spectra at various applied potentials were obtained after a constant potential was applied to the catalyst’s electrode for 20 min.

In situ synchrotron radiation FTIR measurements. In situ SR-FTIR measurements were made at the infrared beamline BL01B of National Synchrotron Radiation Laboratory (NSRL, China) through a homemade top-plate cell reflection IR setup with a ZnSe crystal as the infrared transmission window (cutoff energy of ~ 625 cm^− 1).⁴⁸ This end-station was equipped with an FTIR spectrometer (Bruker 66v/s) with a KBr beam splitter and liquid nitrogen cooled MCT detector. The system is coupled with an IR microscope (Bruker Hyperion 3000) with a 16x objective. It is capable to perform infrared spectroscopy measurements over a broad range of 15–4000 cm^− 1 with high spectral resolution of 0.25 cm^− 1. The catalyst electrode is tightly pressed against the ZnSe crystal window with a micron-scale gap in order to reduce the loss of infrared light. To ensure the quality of the obtained SR-FTIR spectra, the apparatus adopts a reflection mode with a vertical incidence of infrared light. Each infrared absorption spectrum was acquired by averaging 514 scans at are solution of 2 cm^− 1. All infrared spectra were obtained after a constant potential was applied to the catalyst’s electrode for 30 min.

DFT calculations. Spin-polarized DFT calculations were carried out using the DMol3 quantum chemical module.^49,50 The gradient-corrected density-functional PW91 (Perdew-Wang generalized-gradient approximation) was applied to predict the structures, single-point energies, zero-point energies, as well as thermodynamic parameters.⁵¹ The Tkatchenko-Scheffler term of semiempirical dispersion-correction for DFT (DFT-D) was considered to estimate the bond energy of the σ-π coordination between Ni²⁺ and CN⁻.⁵² Infrared spectra of active intermediates were derived from harmonic vibrational frequencies calculations⁵³, and the vibrational analysis was performed at the final geometry using the identical parameters with the geometry optimizations. More details on the DFT calculations are provided in Supporting Information.

Gibbs free energy changes of reactions were calculated as follows.

$$\varDelta G=\varDelta E+\varDelta ZPE+\varDelta {\int }_{0}^{T}{C}_{p}dT-T\varDelta S+\varDelta {E}_{sol}$$

where E is single-point energy, ZPE means zero-point energy, $\varDelta {\int }_{0}^{T}{C}_{p}dT$ and $-T\varDelta S$ stand for the correction factors of enthalpy and entropy, E_sol is the solvation energy. A new reaction mechanism comprised of two steps, namely the production of NH₃ and oxidation of NH₃, is proposed:

Step 1: Production of NH₃

Step 2: Oxidation of NH₃

Competing Interests

The authors declare no competing interests.

Authors’ contribution

P.C. and Y.Z. conceived the project. S.Z.Q. supervised the project. S.K.G. performed the experiments of materials preparation, characterizations and activity tests, with the assistance of J.H. and H.B.S. X.Z. and Q.H.L. performed the in-situ SR-FTIR tests. S.Q.L. performed DFT calculations. P.C., Y.Z. and S.Z.Q. wrote the manuscript. M. J. revised the manuscript.

Acknowledgements

We acknowledge the funding from National Natural Science Foundation of China (21771002, U1932212), Key Project of Educational Department of Anhui Province (KJ2019A0861) and Australian Research Council (DP160104866, FL170100154 and DP190103472).

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Nickel Ferrocyanide as High-Performance Next Generation Electrocatalyst for Urea Oxidation

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