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

Shi-Kui Geng Anhui University Yao Zheng University of Adelaide https://orcid.org/0000-0002-2411-8041 Shan-Qing Li Chizhou University Xu Zhao University of Science and Technology of China Jun Hu Anhui University Hai-Bo Shu Anhui University Mietek Jaroniec Department of Chemistry and Biochemistry https://orcid.org/0000-0002-1178-5611 Ping Chen Anhui University Qinghua Liu University of Science and Technology of China https://orcid.org/0000-0003-4090-3311 Shizhang Qiao (  s.qiao@adelaide.edu.au ) University of Adelaide https://orcid.org/0000-0002-4568-8422


Introduction
Electrochemical urea oxidation reaction (UOR) is of great importance in a range of energy-related applications and devices. [1][2][3][4][5] Due to a more negative equilibrium potential (E 0 = 0.37 V vs reversible hydrogen electrode, RHE), UOR is an ideal alternative anode half reaction to conventional oxygen evolution reaction (OER, E 0 = 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 signi cant 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 (E 0 = 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 6etransfer process, which needs high performance catalysts to decrease the overpotential to achieve an e cient device. 2,[9][10][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, sul des, and phosphides, etc, does not satisfy requirements of practical applications due to the high overpotential, low current density and inadequate stability. 7,12,13 For example, the best so far Ni-based UOR catalysts, e.g., Ni 0.9 Fe 0.1 O 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 signi cant 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. Speci cally, for the most studied Ni-based materials, before UOR onset (e.g., 1.40 V vs RHE), the catalysts inevitably undergo a selfoxidation process to form Ni 3+ stated NiOOH derivative on the surfaces (always happens at ~1. 36 9,16,17 Note that as a 6etransfer 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 signi cantly 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 Ni 2 Fe(CN) 6 electrocatalyst, one of Prussian blue analogues. 20 With an alternative reaction intermediate, the new reaction mechanism on Ni 2 Fe(CN) 6 delivers one of the best UOR activity and assures high stability as compared to the existing catalysts. Through an advanced characterization by insitu techniques (e.g., synchrotron radiation-Fourier transform infrared spectroscopy, Raman spectroscopy, ammonia detection) and density functional theory (DFT) computation, a critical intermediate of *NH 3 was identi ed, which involves two steps of production and oxidation of NH 3 on 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 Ni 2 Fe(CN) 6 can effectively replace the conventional water oxidation in different energy saving systems for hydrogen or H 2 O 2 production.

Results And Discussion
Physicochemical properties. The typical Ni 2 Fe(CN) 6 catalyst was prepared on nickel foam through a selfassembly method by immersing cleaned Ni foams in the mixed solution of polyvinylpyrrolidone, 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 Ni 2 Fe(CN) 6 powder ( Fig. 1b) allowed for identi cation 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 Fe 2+ and Ni 2+ 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 Ni 2 Fe(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 Ni 2 Fe(CN) 6 proceeds before the self-oxidation of Ni 2+ to Ni 3+ . 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 Speci cally, for Ni 2 Fe(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 Ni 2 Fe(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 Ni 2 Fe(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 (Fe 4 [Fe(CN) 6 ] 3 ) and nickel cobaltcyanide (Ni 3 [Co(CN) 6 ] 2 ), showed worse activity compared to Ni 2 Fe(CN) 6 ( Fig. 2b, Supplementary Fig. 6). 34,35 Therefore, we speculate that the observed high performance of Ni 2 Fe(CN) 6 is caused by cooperative action of two active sites of Ni and Fe in Ni 2 Fe(CN) 6 catalyst.
The kinetic study shows that the UOR process on Ni 2 Fe(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 NiC 2 O 4 ( Supplementary Fig. 7,8). 37 Fig. 9). As shown in Fig. 3a, two strong peaks at 2100 and 2140 cm − 1 are characteristic of cyanide stretching in Ni 2 Fe(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 Ni 2 Fe(CN) 6 are similar to that at the open circuit potential (OCP). However, for NiC 2 O 4 , 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, Ni 2 Fe(CN) 6 without NiOOH species showed a signi cantly enhanced apparent UOR activity and stability compared to NiC 2 O 4 catalyst (Fig. 3c). About 90% of the current density remained after 50 hour operation on Ni 2 Fe(CN) 6 , which is a record for UOR catalysts.
The new reaction pathway of UOR for Ni 2 Fe(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 Fig. 12). Therefore, we propose that the UOR process on Ni 2 Fe(CN) 6 contains *NH 3 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 NH 3 and oxidation of NH 3 (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 rst step (Fig. 4a), Ni sites were proven to be the primary active sites because the free energy value of RDS ([M·OCO 2 ] ads + 2e − → M + CO 3 2− ) on Ni site is smaller than that on Fe site. For the second step (Fig. 4b) Fig. 4b), in which 1 M *NH 2 + *NH 2 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 Ni 2 Fe(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 Ni 2 Fe(CN) 6 (Fig. 2b).
Identi cation 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 = NH 2 + and C-O stretching vibration of *OCONH 2 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 = NH 2 + and *OCONH 2 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 con rms a new UOR mechanism on Ni 2 Fe(CN) 6 without NiOOH generation, namely two-step processes of NH 3 production and oxidation at two different active sites.
Energy-saving systems driven by UOR for replacement of OER. To establish an energy-saving system bene ting from the low overpotential of UOR on Ni 2 Fe(CN) 6 catalyst, we assembled an UOR//HER electrolyzer using Ni 2 Fe(CN) 6 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 Ni 2 Fe(CN) 6 electrocatalyst to replace OER. In addition, the H 2 production in this UOR//HER cell was very stable with a Faradaic e ciency higher than 90% (Fig. 6b, Supplementary Fig. 19). Besides H 2 production, the energysaving system can also be applied in a UOR//2e − ORR ow cell composed of Ni 2 Fe(CN) 6 anode and mesoporous carbon (CMK-3) cathode 46 (Supplementary Fig. 20). Nowadays, in-situ electrochemical production of H 2 O 2 via 2e − ORR has become a promising way because it can reduce the danger and costs of the transportation of H 2 O 2 . 46,47 As expected, the urea electrolysis needs a smaller energy input than water electrolysis for urea elimination and H 2 O 2 generation (Supplementary Fig. 21). Speci cally, a  (Fig. 6c, Supplementary Fig. 22). In addition, the urea elimination and H 2 O 2 generation were very e cient 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.

Conclusions
In summary, Ni 2 Fe(CN) 6 as a next-generation UOR electrocatalyst with excellent e ciency 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 Ni 2 Fe(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 Ni 2 Fe(CN) 6 . The e cient UOR on 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 twoelectrode mode separated by an anion membrane. The UOR//2e-ORR system was carried out in a commercial ow cell, in which the prepared Ni 2 Fe(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 Electrochemical in situ Raman measurements. The Raman spectroscopy was carried out using a Via-Re ex 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 Ni 2 Fe(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 re ection 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 re ection 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-Sche er term of semiempirical dispersion-correction for DFT (DFT-D) was considered to estimate the bond energy of the σ-π coordination between Ni 2+ and CN − . 52 Infrared spectra of active intermediates were derived from harmonic vibrational frequencies calculations 53 , and the vibrational analysis was performed at the nal 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.
where E is single-point energy, ZPE means zero-point energy, and 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 NH 3 and oxidation of NH 3 , is proposed: Step 1: Production of NH 3 Step 2: Oxidation of NH 3

Competing Interests
The authors declare no competing interests.
Authors' contribution

Supplementary Files
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