# Double Hydroxide Nanocatalysts for Urea Electrooxidation Engineered toward Environmentally Benign Products

Yuwei Yang, Jodie A Yuwono, Todd Whittaker, Marc Manyé Ibáñez, Bingliang Wang, Changmin Kim, Albina Y Borisevich, Stephanie Chua, Jhair Pena Prada, Xichu Wang, et al.

► To cite this version:

Yuwei Yang, Jodie A Yuwono, Todd Whittaker, Marc Manyé Ibáñez, Bingliang Wang, et al. Double Hydroxide Nanocatalysts for Urea Electrooxidation Engineered toward Environmentally Benign Products. Advanced Materials, 2024, 36 (35), pp.2403187. 10.1002/adma.202403187. hal-05128587

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# Double Hydroxide Nanocatalysts for Urea Electrooxidation Engineered toward Environmentally Benign Products

Yuwei Yang, Jodie A Yuwono, Todd Whittaker, Marc Manyé Ibáñez, Bingliang Wang, Changmin Kim, Albina Y Borisevich, Stephanie Chua, Jhair Pena Prada, Xichu Wang, Pierre-Olivier Autran, Raymond R Unocic, Liming Dai, Adam Holewinski, and Nicholas M Bedford*

Recent advancements in the electrochemical urea oxidation reaction (UOR) present promising avenues for wastewater remediation and energy recovery. Despite progress toward optimized efficiency, hurdles persist in steering oxidation products away from environmentally unfriendly products, mostly due to a lack of understanding of structure-selectivity relationships. In this study, the UOR performance of Ni and Cu double hydroxides, which show marked differences in their reactivity and selectivity is evaluated. CuCo hydroxides predominantly produce N₂, reaching a current density of 20 mA cm⁻² at 1.04 V – 250 mV less than NiCo hydroxides that generate nitrogen oxides. A collection of in-situ spectroscopies and scattering experiments reveal a unique in situ generated Cu(2-x)+-OO⁻* active sites in CuCo, which initiates nucleophilic substitution of NH₂ from the amide, leading to N-N coupling between *NH on Co and Cu. In contrast, the formation of nitrogen oxides on NiCo is primarily attributed to the presence of high-valence Ni³⁺ and Ni⁴⁺, which facilitates N-H activation. This process, in conjunction with the excessive accumulation of OH⁻ ions on Jahn-Teller (JT) distorted Co sites, leads to the generation of NO₂⁻ as the primary product. This work underscores the importance of catalyst composition and structural engineering in tailoring innocuous UOR products.

## 1. Introduction

Urea is an important and highly used chemical in a range of industrial applications and is the primary component of nearly half of all fertilizers used globally. The hydrolysis of urea in agricultural and industrial effluents is a major environmental concern, often resulting in substantial environmental hazards, including the proliferation of toxic algal blooms. Given these detrimental environmental impacts, the treatment of aqueous urea waste is paramount.[1] The electrochemical urea oxidation reaction (UOR) stands out as an innovative and environmentally sustainable solution for treating urea-enriched wastewater, promoting both nitrogen removal and green hydrogen production at the cathode of an electrolysis cell.[2] UOR and the oxygen evolution reaction (OER) have notable distinctions between the standard equilibrium potential with values of 0.07 and 1.23 V versus RHE, respectively, and the required operation

Y. Yang, B. Wang, C. Kim, S. Chua, J. P. Prada, X. Wang, L. Dai, N. M Bedford
School of Chemical Engineering
University of New South Wales
Sydney, NSW 2052, Australia
E-mail: n.bedford@unsw.edu.au, nicholas.bedford@mines.edu

Y. Yang, L. Dai, N. M Bedford
Australian Research Council Centre of Excellence in Carbon Science and Innovation
University of New South Wales
Sydney, NSW 2052, Australia

J. A Yuwono
School of Chemical Engineering
The University of Adelaide
Adelaide, SA 5005, Australia

T. Whittaker, M. M. Ibáñez, A. Holewinski
Department of Chemical and Biological Engineering
University of Colorado
Boulder, CO 80309, USA

T. Whittaker, M. M. Ibáñez, A. Holewinski
Renewable and Sustainable Energy Institute
University of Colorado
Boulder, CO 80309, USA

B. Wang, C. Kim, X. Wang, L. Dai
Australian Carbon Materials Centre (A-CMC)
University of New South Wales
Sydney, NSW 2052, Australia

A. Y Borisevich, R. R Unocic
Center for Nanophase Materials Sciences
Oak Ridge National Laboratory
Oak Ridge, TN 37831, USA

The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adma.202403187

© 2024 The Author(s). Advanced Materials published by Wiley-VCH GmbH. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

DOI: 10.1002/adma.202403187
potential in practical is generally much higher than the theoretical potential, underscoring the opportunity for high energy efficiency of urea electrolysis compared with conventional water electrolysis.[1a] Further, UOR presents a straightforward and eco-friendly approach for treating urea-contaminated wastewater by converting urea directly into nitrogen and CO₂, simplifying the treatment process compared to urea recycling methods. The integration of renewable energy sources with the electrolysis system enhances its sustainability and allows for easy incorporation into existing treatment infrastructures. Furthermore, the process facilitates the production of hydrogen at the cathode, contributing to a circular economy by providing a valuable by-product for industrial use, thereby increasing the overall economic and environmental benefits of the system in large-scale applications.

Ni-based catalysts have been extensively studied for a range of anodic nucleophile electrooxidation reactions, which generally involves an active hydrogen in the substrate's reaction sites, as well as for UOR.[3] Numerous studies have highlighted the efficacy of Ni-based catalysts as UOR electrocatalysts, particularly noting their ability to exhibit substantial current densities at low potentials.[4] This characteristic is advantageous for both energy conservation and enhancing hydrogen production at the cathode. The primary research driver to date has been a focus on achieving low overpotentials to avoid the competing OER at elevated current densities.[5] Conversely, research toward steering the selectivity of UOR catalysts toward non-toxic products remains limited to date. In alkaline UOR, Ni-based catalysts predominantly yield NO₂⁻ or NO₃⁻ over the thermodynamically favored N₂.[6] This is likely related to their excellent alkaline OER performance, promoting a competitive adsorption behavior between oxygen-containing intermediates (O*, OH*, OOH*) and N-terminated urea molecules and leading to *NH oxidation to NO₂⁻/NO₃⁻*.[6,7] One solution strategy for this issue is to use an electrocatalyst with undesirable OER reactivity, ideally having a higher affinity to urea rather than OH⁻ to selectively sidestep the NO₂⁻/NO₃⁻ pathway.

Engineering catalysts' UOR selectivity holds substantial importance for further reducing the environmental hazards related to nitrogen oxides while producing green hydrogen efficiently at the cathode. Recent work from our group has demonstrated that Cu-based materials display excellent reactivity in hydroxymethylfurfural oxidation reaction (HMFOR), and importantly, a notably reduced OER performance.[8] Further, recent report by Shen et al. has shown that Cu-engineered nickel boride catalysts exhibit significantly enhanced N₂ selectivity for UOR as well.[9] In this paper, CuCo hydroxides are evaluated for UOR against NiCo hydroxides, an established OER catalyst. Through comprehensive electrochemical analyses, including on-line electrochemical mass spectrometry (OLEMS), the selectivity of the UOR toward N₂ for CuCo hydroxides is observed over oxidized

nitrogen. Conversely, NiCo hydroxides exhibit a preference for generating nitrite (NO₂⁻) as the principal nitrogenous product. Notably, CuCo hydroxide exhibits a much lower onset potential of 0.87 V versus compared to NiCo hydroxides at 1.21 V versus RHE, while further achieving a current density of 20 mA cm⁻² at 1.04 V, which is 250 mV lower than that observed for NiCo hydroxide. Both hydroxides in this work demonstrate superior performance compared with the other researched electrocatalysts (Table S1, Supporting Information). In situ X-ray absorption spectroscopy (XAS), high-energy X-ray diffraction (HE-XRD), Raman spectroscopy, and infrared (IR) spectroscopy were utilized to investigate the active structures of the two hydroxides during the UOR and OER, suggesting structure-selectivity relationships for UOR. Additionally, density functional theory (DFT) calculations were performed to probe reaction mechanisms and align with empirically elucidated reaction pathways and reactive site structure, further confirms the significant differences the energy barriers present for each reaction pathways on the two hydroxides. Collectively, this work provides insights for the rational development and design of highly reactive catalytic materials with enhanced UOR selectivity, which is expected to be translatable to other NORs as well.

## 2. Hydroxide Engineering and Electrocatalyst Performance Evaluation

NiCo and CuCo double hydroxides were synthesized using a co-precipitation technique in solutions of formamide and water, followed by a subsequent ultrasonic treatment to obtain ultrathin hydroxides.[10] Aberration corrected scanning transmission electron microscopy (AC-STEM) reveals that NiCo adopts an ultra-thin nanosheet morphology, whereas CuCo exhibits a more intricate narrower ultrathin nanobelt morphology, aligning with previously documented morphologyies of Cu-based hydroxides (Figures S1 and S2, Supporting Information).[11] Uniform distribution of elements in both NiCo and CuCo is confirmed by energy dispersive X-ray spectroscopy (EDS) results (Figures S1 and S2, Supporting Information), showcasing metal uniformity along the entire catalyst that is expected in double hydroxides. Electron energy-loss spectroscopy (EELS) line profiles were analyzed to determine the oxidation states in the two hydroxides. The Co L-edge and Cu L-edge spectra of CuCo (Figure S3, Supporting Information) revealed integrated intensity ratios of the L3 to L2 peaks at 4.53 and 3.23, respectively, indicating the average oxidation states of Co and Cu ions are approximately Co²⁺ and Cu²⁺. For NiCo, the Co L-edge and Ni L-edge spectra showed L3 to L2 peak intensity ratios of 3.88 and 3.61 respectively (Figure S4, Supporting Information), suggesting the average oxidation states of Co and Ni ions to be Co²³⁺ and Ni²⁺. The specific lattice fringe spacing of 0.26 nm observed in NiCo corresponds to the (009) crystal plane of Ni-based hydroxides (Figure 1a).[12] In contrast, CuCo displays a more disordered polycrystalline structure as compared to NiCo, with notable fringe spacings of 0.24 nm corresponding to the (002) plane of Cu hydroxides.[13] Furthermore, a distinct lattice fringe with a spacing of 0.21 nm, identified in Figure 1b, is attributed to the CuO (111) planes.[14] The partial formation of the CuO may be due to the heat applied during the synthesis procedure, which causes dehydration of the Cu(OH)₂. The characteristic hydroxide structure of CuCo and NiCo is confirmed by

<footnote>P.-O. Autran
European Synchrotron Radiation Facility
Grenoble, CS 40220, France</footnote>

<footnote>N. M. Bedford
Department of Chemistry
Colorado School of Mines
Golden, CO 80401, USA</footnote>
<figure><img src="image_3.png" /><figcaption>**Figure 1.** AC-STEM image of a) NiCo, and b) CuCo; c) PDF analysis of CuCo and NiCo (inset: zoomed-in PDF plot); d) Co K edge XANES spectrum of CuCo, NiCo, Co(OH)₂, Co₃O₄, CoO, and CoOOH; e) NEXAFS spectra and the optimal Gaussian fitting results of Co L-edge of e) CuCo and f) NiCo; g) Cu K edge R-space EXAFS spectrum of CuCo, Cu(OH)₂, CuO, and Cu₂O; h) Ni K edge R-space EXAFS spectrum of NiCo, Ni(OH)₂, NiO, and Ni₂O₃; i) Co K edge EXAFS spectrum of CuCo, NiCo, Co(OH)₂, Co₃O₄, CoOOH.</figcaption></figure>

HE-XRD paired with pair distribution function (PDF) analysis (Figure 1c; Figures S5 and S6, Supporting Information), which yields atomic-scale structural information in real space by transforming the total scattering data from HE-XRD into a function that represents coordination spheres extending up to > ≈30 Å.[15] The amplitude of the peaks in the PDFs exhibits a marked reduction at ≈15 Å, signifying that both NiCo and CuCo manifest significantly diminished long-range order beyond the theoretical thickness of two layers of comparable hydroxide at ≈16 Å.[16] This observation is indicative of a substantial reduction in domain size and thickness of the ultrafine hydroxides in comparison to conventional hydroxide materials.[17] Further, in contrast to NiCo, CuCo double hydroxides possess a reduced metal-metal (M-M) distance at 3.06 Å (Figure 1c inset) and correspondingly shorter coordination lengths at extended ranges, attributable to

the significantly Jahn-Teller (JT) distorted octahedral structure of Cu-based hydroxides.

The local atomic structure and oxidation state of NiCo and CuCo were examined by X-ray absorption fine structure (XANES) spectroscopy using the K and L edges of Ni, Co, and Cu, along O K edge XANES. Upon comparison with established standards, both Cu and Ni show an oxidation state of ≈+2, while Co in both hydroxides exhibit an average oxidation state ranging from +2 to +3 (Figure 1d; Figure S7, Supporting Information). The results derived from linear combination fitting (LCF) indicate that Ni and Co possess a respective average oxidation state of +1.89 and +2.40 in NiCo, while Cu and Co have an average oxidation state of +1.99 and +2.12 in CuCo, respectively. The obtained oxidation states are in good agreement with the previous EELS analysis results. Additionally, fitting the XANES of the Co L edges
and corresponding O K edges of two hydroxides provides additional insights into the d orbital electron configurations of the transition metals and their corresponding local geometric structure (Figure 1e and f; Figures S8 and S9, Supporting Information). The Co L edge XANES spectra in both NiCo and CuCo exhibit split $t_{2g}$ and $e_g$ contributions from $Co^{2+}$, indicative of a JT-distorted Co-O framework. Additionally, $t_{2g}$ and $e_g$ contribution from $Co^{3+}$ are observed at higher binding energies. Typically, a symmetrical Co octahedron adopts a low spin state (LS $t_{2g}^6e_g^0$); however, the observed $e_g$ contribution in trivalent Co suggests a shift toward an intermediate spin state (IS $t_{2g}^5e_g^1$) or a high spin state (HS $t_{2g}^4e_g^2$) in both hydroxides due to Jahn-Teller distortion. These electronic configurations, characterized by an unoccupied $t_{2g}$ orbital, are advantageous for accepting the valence electron from $OH^-$, thereby facilitating the $OH^-$-assisted initial dehydrogenation of the urea molecule ($H_2NCoNH_2$ to $NHCoNH_2$), which initiates the UOR process.[18]

The local structure of the individual chemistries in NiCo and CuCo were then examined through extended X-ray absorption fine structure (EXAFS) analysis (Figure 1g-i; Figure S10, Supporting Information). In the Ni K-edge EXAFS spectra, two prominent peaks are observed at 1.6 and 2.7 Å (without phase correction), corresponding to the Ni-O and Ni-Co(Ni) interactions respectively. The coexistence of Ni and Co in the NiCo is substantiated through wavelet transform (WT) analysis of the Ni K-edge EXAFS spectra (Figure S11, Supporting Information), whereby two distinct hotspots in high k-space at 2.7 Å confirm the presence of both metals at this distance. Compared with the general hydroxide structure, The EXAFS spectra at the Cu K edge for CuCo reveal a reduced Cu-O distance of 1.4 Å, accompanied by doublet features indicative of Cu-Co(Cu) and second Cu-O interactions at different distances, mirroring the inherent JT distorted $Cu(OH)_2$ structure. The simultaneous presence of Cu and Co in the system is further substantiated by the WT contour analysis of the Cu K-edge EXAFS (Figure S12, Supporting Information). Compared to the Cu K-edge EXAFS spectra of standard CuO and $Cu_2O$, the Cu atoms within the CuCo hydroxides likely have additional Cu-related phases within the system. The EXAFS spectra at the Co K edge for NiCo and CuCo reveal a broadened Co-O peak, indicative of the existence of varying Co-O coordination within this region. EXAFS fitting of the Co K edge reveals the existence of 5 ± 1 Co-O1 coordination with a length of 1.99 ± 0.03 Å, and 2.31 ± 0.76 Co-O2 coordination with a length of 2.18 ± 0.05 Å in NiCo (Table S1 and Figure S13, Supporting Information). Similarly, CuCo displays 1 ± 0 Co-O1 coordination with a length of 1.88 ± 0.03 Å, and 4 ± 1 Co-O2 coordination with a length of 2.07 ± 0.02 Å (Table S2 and Figure S14, Supporting Information). These findings are in alignment with the previously hypothesized JT-distorted structure derived from Co L edge and O K edge analysis.

The electrocatalytic efficacy of NiCo and CuCo catalysts in UOR was first assessed using linear sweep voltammetry (LSV) in 1 M KOH with and without 0.33 M urea (Figure 2a,b; Figure S15, Supporting Information). It is evident that UOR manifests a markedly lower onset potential in comparison to OER for both materials. To achieve a current density of 20 mA cm$^{-2}$, the requisite potential for UOR on CuCo and NiCo is 1.04 RHE and 1.29 V RHE, respectively, in contrast to 1.47 RHE and 1.56 V RHE for OER, resulting in voltage differences of 430 and 270 mV, respectively.

This finding corroborates the effectiveness of both hydroxides in catalyzing UOR as opposed to OER. Moreover, both catalysts demonstrate commendable onset potentials for UOR, with CuCo being particularly noteworthy for its exceptionally low onset potential of 0.86 V RHE. LSV plots for CuCo and NiCo reveal distinct oxidation features attributed to different redox processes: for CuCo, the features correspond to $Co^{2+}/Co^{3+}$ transitions dominated UOR; similarly, for NiCo, the oxidation peaks are associated with $Ni^{2+}$ to $Ni^{3+}/Ni^{4+}$ transitions dominated UOR processes. The subsequent sections will delve into operating characteristics to provide detailed structural and electronic evidence of these observations.

Further, the electrochemical response of both electrocatalysts under urea-concentration-dependent conditions and KOH-concentration-dependent conditions are also studied, as demonstrated in Figures S16 and S17 (Supporting Information). A range of urea concentrations (0-1 M) are tested to approximate practical wastewater conditions, among which the 0.33 M urea was chosen to reflect the typical urea content found in urine, while 1 M urea corresponds to concentrations commonly observed in industrial urea-containing wastewater.[19] In the varied urea concentration, an increase of urea concentration from 0.1 to 1 M significantly enhances current density and catalytic activity for both CuCo and NiCo catalysts due to the increased urea molecule coverage on the catalyst surface. The above trend of increased current density toward the increasing urea concentration indicates that the reaction operates within a diffusion-controlled regime over this concentration range. On the other hand, as illustrated in Figure S17 (Supporting Information), the polarization curves for both NiCo and CuCo in 0.01 M KOH exhibit diminished catalyst oxidation features and significantly reduced current densities, suggesting that the in situ phase transformation of the catalysts may be restricted due to insufficient $OH^-$ availability in the 0.01 M KOH environment.

A combination of OLEMS, discrete multi-chemistry analysis, and NMR (nuclear magnetic resonance) was used to identify the evolved gaseous and aqueous N-products from urea electrolysis (Figure 2c-j). Calibration curves for $NO_2^-$ and $NO_2^-+NO_3^-$, as well as total liquid N product, were established using standard solutions (Figure S18, Supporting Information). The liquid phase N product analysis shows that the NiCo catalyst consistently demonstrated a high selectivity of ≈80% toward liquid $NO_x$ products across all urea concentrations tested (Figure 2c,d; Table S4, Supporting Information). Conversely, the CuCo catalyst exhibited significantly lower selectivity, maintaining a level below 2% at each concentration. The above results suggest that the concentration of urea is not a determining factor of the reaction pathway. Additionally, in the KOH-concentration-dependent conditions, as depicted in Figure 2e,f, and Table S5 (Supporting Information), while the urea conversion rates remain consistent across different KOH concentrations, the selectivity toward liquid $NO_x$ products by NiCo is notably reduced to only 9.65% of the total urea oxidation products in 0.01 M KOH, indicating a shift in the reaction pathway on NiCo under these conditions. The above finding suggests the high selectivity for liquid $NO_x$ products observed with NiCo may be influenced by the $Ni^{2+}$ to $Ni^{3+}/Ni^{4+}$ transformation and the accumulation of $OH^-$ on $Co^{3+}$ sites, as the product selectivity is drastically different when electrocatalyst phase transformation is inhibited in low KOH concentrations.
<figure><img src="image_4.png" /><figcaption>**Figure 2.** a) LSV curves of CuCo in 1 M KOH and 1 M KOH with 0.33 M urea b) LSV curves of CuCo in 1 M KOH and 1 M KOH with 0.33 M urea at a scan rate of 20 mV s⁻¹; c) Discrete analysis result of liquid phase NO₂⁻, NO₃⁻ and total N recorded for 1 h on NiCo and CuCo electrodes at different urea concentrations in 1 M KOH; d) Liquid N product (NO₂⁻/NO₃⁻) rate; e) Discrete analysis result of liquid phase NO₂⁻, NO₃⁻ and total N recorded for 1 h on NiCo and CuCo electrodes of 0.033 M urea in 0.1, 0.5, and 1 M KOH; f) Liquid N product (NO₂⁻/NO₃⁻) rate; g) CuCo and h) NiCo catalysts to monitor N₂, O₂, NO, and N₂O during UOR (0.33 M urea in 1 M KOH). i) NMR spectra recorded for 1 h UOR (0.33 M urea in 1 M KOH) products on CuCo (magenta trace) and CuCo (navy blue trace) anodes; j) The corresponding FEs toward urea oxidation with error bars obtained from NiCo and CuCo electrodes at 1.5 V versus RHE.</figcaption></figure>

OLEMS enables real-time analysis of gaseous products during electrochemical reactions, effectively identifying individual gases in complex mixtures through precise mass-to-charge (m/z) ratios and detailed fragmentation patterns, minimizing interference among different gas products. OLEMS is employed as a powerful tool for studying the gaseous N-product during electrochemical UOR. OLEMS analysis of the gas phase shows the generation of N₂ on both catalysts as the potential increased to 1.2 V versus RHE (Figure 2g,h; Figures S19 and S20, Supporting Information), with the N₂ signal for CuCo ~ 4× higher than NiCo. Besides N₂ and O₂, traces of N₂O and NO were also observed on both catalysts at elevated operation potentials, starting at 1.5 V for CuCo and 1.35 V for NiCo. The detection of a signal at m/z = 44 is attributed to N₂O rather than CO₂, as CO₂ rapidly converts into CO₃⁻ as an aqueous product at pH 14;[20] a control experiment conducted on the Pt-catalyzed oxidation of formic acid did not yield a CO₂ signal even at 1.5 V RHE (Figure S21, Supporting Information). The results obtained from ¹³C NMR analysis (Figure 2i) indicated the presence of urea and CO₃²⁻ as products for both NiCo and CuCo catalysts. CO₃²⁻ generation is attributed to the hydration and dissociation of CO₂ in alkaline electrolytes. Interestingly, an additional peak corresponding to cyanate (CNO⁻) is observed as a product only for UOR

from NiCo, which is an intermediate product observed during the early cleavage of C-N bonds. Further, the generated N₂ is further quantified by onlin-GC measurement (Figure 1j). Prior to conducting the experiments, calibration profiles were established using the same gas-tight H-cell setup, with identical calibration achieved in over ten instances (Figure S22, Supporting Information). Figure 1j and Figure S23 (Supporting Information) shows that the CuCo electrode exhibited a significantly stronger N₂ response, indicating higher selectivity for urea oxidation compared to the NiCo electrode. Faradaic efficiencies reflect this, with CuCo achieving 83.48% and NiCo only 27.83%, highlighting the superior catalytic performance of urea-to-N₂ conversion of the CuCo electrode in urea oxidation. The analysis clearly demonstrated different reaction pathways on Ni-based hydroxides and Cu-based hydroxides: NiCo catalyst followed the urea-to-nitrite pathway, while the CuCo catalyst follows the urea-to-nitrogen pathway.

In regard to oxidized nitrogen produced by NiCo hydroxides, our results are consistent with reported literature on Ni-based catalyst for UOR, wherein oxidized nitrogenous products pose potential secondary hazards to the environment.[7b,21] The generation of overoxidized products is predominantly associated with the competitive adsorption dynamics between urea molecules and abundant hydroxide ions in the alkaline electrolyte. This
dynamic suggests an OH⁻-assisted C-N cleavage pathway, involves a nucleophilic OH⁻ attack on the electron-deficient carbonyl carbon and subsequent early cleavage of the C-N bond. After C-N cleavage, the remaining surface *NH₂* reacts with the adjacent OH⁻, undergoing oxidation into overoxidized nitrogenous products, namely, NO₂⁻, NO₃⁻, and N₂O.[¹a,⁷b] Efforts have been made to manipulate Ni hydroxide-based catalysts to achieve improved urea-to-nitrogen conversion, including surface engineering with polymers and introduction of vacancies.[⁷b] Nevertheless, the overoxidized nitrogenous products is only halved due to the inherent high performance of Ni hydroxide-based catalysts toward Ni²⁺/Ni³⁺ mediated-nucleophilic oxidation reactions.[¹a,³b] Conversely, in the Cu-based hydroxides, the Cu is postulated to maintain its Cu²⁺ oxidation state under the influence of the applied potential, thereby mitigating the likelihood of excessive OH⁻ accumulation at the metal sites and achieving different N-product selectivity. The subsequent sections implementing in-situ characterization techniques and DFT showcase the differences in the structure induced UOR reaction pathways. Finally, the robustness of both electrocatalysts is explored by analyzing the post-reaction electrolytes inductively coupled plasma mass spectrometry (ICP-MS). The results revealed that the electrolyte from the NiCo reaction contained 13.39 µg L⁻¹ of Ni and 54.24 µg L⁻¹ of Co. In contrast, the electrolyte from the CuCo reaction showed considerably higher metal leaching with 538.7 µg L⁻¹ of Cu and 77.75 µg L⁻¹ of Co. These findings suggest a superior robustness of the NiCo catalyst compared to the CuCo, which is likely due to the Cu-based hydroxides attaining a more distorted structure that is prone to metal leaching during long-term reactions.

### 3. Structure-Driven Mechanistic Understanding of UOR using In Situ XANES

UOR involves several important steps, including N-H bond activation; C-N bond cleavage; N-O coupling, as well as N-N coupling. In-situ XANES was employed to investigate NiCo and CuCo materials under OER and UOR conditions, elucidating the changes in the electronic structure of specific metals and providing empirical evidence for the distinct UOR selectivity behavior exhibited by the two catalysts. In addition, LCF was performed on the in-situ XANES spectra of Ni K-edge, Cu K-edge, and Co K-edge to acquire the weighted average oxidation states (Figure 3a–h; Figures S24–S31, Supporting Information). In 1 M KOH electrolyte, the Ni valence state in NiCo shows a gradual increase in response to the rising applied potential and ultimately achieving a steady oxidation state of +3.54 at a potential of 1.50 V (Figure 3a,c). The observed oxidation state signifies the formation of γ-NiOOH, which approximates an oxidation state of +3.6. A sudden surge in the oxidation state from +2.28 at 1.2 V to +3.23 at 1.3 V correlates with the metal oxidation signature observed in the LSV curve beginning at 1.21 V (Figure 2b, Figure 3a,c). In contrast, the oxidation process of Co within the catalyst is delayed, with a significant change in oxidation state becoming apparent only after the applied potential exceeds 1.5 V, and the oxidation state peaks at +3 at 1.6 V (Figure S32, Supporting Information). This lag in the oxidation of Co is likely due to the notably distorted octahedral Co-O structure, of which the elongated-typed

Co-O JT distortions. The axial bonds of the octahedron are elongated (2.18 ± 0.05 Å), and the equatorial bonds are shortened (1.99 ± 0.03 Å), which consequently elevates the energy of the *d*₂ orbital, making it more accessible to the ligands and leading to stronger bonding. Thus, the transition to higher oxidation states from Co²⁺ to Co³⁺ is slower and kinetically less favorable due to the reorganized electron density and altered orbital alignment. These changes make it energetically challenging for the cobalt to lose electrons required for oxidation. With the presence of urea, the Ni demonstrates a relatively stable oxidation state, fluctuating between +2.00 and +2.18 from 1.1 to 1.5 V, followed by an uptick to an oxidation state of +2.61 at 1.6 V (Figure 3b,d). The Co exhibits a marginal increase in oxidation state, moving from +2.36 to +2.52 across the applied potential range (Figure S32, Supporting Information). Overall, both Ni and Co maintain a stabilized oxidation state compared with the OER condition, suggesting the UOR mechanism on the NiCo relies on the established Ni²⁺/Ni³⁺ or Ni⁴⁺ redox-mediated indirect oxidation mechanism as proposed for the nucleophile oxidation reactions.[⁶] Additionally, the abrupt shift in Ni oxidation state at 1.6 V coincides with the detrimental competition region of UOR and OER as illustrated in the LSV plot, indicating the likely initiation of OER at this voltage. The influence of Ni³⁺ and Ni⁴⁺ oxidation states on the selectivity for nitrogen oxides during UOR was rigorously investigated using a potential-dependent product analysis with NiCo catalysts. As depicted in Figure S33 and Table S6 (Supporting Information), although there is a noticeable reduction in the yield, the nitrogen oxides still represent 59.19% of the total urea converted. The data confirms that Ni³⁺ is the active site for UOR, while the higher oxidation state of Ni⁴⁺ further facilitates the production of nitrogen oxides.

The electronic structure of post-reacted NiCo was examined by soft X-ray XANES. The Ni L edge and Co L edge XANES show a NiCo oxidation state as a mix of Ni³⁺/Ni⁴⁺, while Co³⁺ is after OER (Figure S34, Supporting Information).[²²] Importantly, the pronounced pre-edge peak at the O K-edge, situated at 528 eV, denotes a substantial concentration of oxygen ligand holes within the structure post-OER (Figure S35, Supporting Information).[²³] Typically, Ni³⁺/Ni⁴⁺ is encountered as a metastable intermediate and only appears during the electrooxidation of Ni hydroxide.[²⁴] Here, the stabilization of Ni³⁺/Ni⁴⁺ is facilitated by the trapped oxygen ligand holes created in adjacent oxygen atoms.[²⁵] These ‘trapped oxygen holes’, are a consequence of the octahedral rotation, which arises from the mismatch in symmetry between Jahn-Teller (JT) distorted 3d7 Co²⁺ octahedra and JT-distorted 3d7 Ni³⁺ octahedra during the oxidation process.[²⁶] This intricate structural change induced electronic structure changes facilitate the stabilization of the otherwise unstable Ni⁴⁺ oxidation state. Concurrently, this electronic structural change also defers the Co²⁺ to Co³⁺ oxidation in OER, with in-situ XANES showing the stabilization of the Ni oxidation state precedes for the subsequent oxidation of Co. After UOR, Ni reverts to its initial Ni²⁺ oxidation state, whereas Co has a mixed valence state of Co²⁺/Co³⁺, further supporting that the UOR on the NiCo involves an indirect redox mechanism mediated by Ni²⁺/Ni³⁺ or Ni⁴⁺ redox couples, with partial participation of the Co²⁺/Co³⁺ redox.[³b,²⁷]

The Cu K-edge XANES of CuCo under OER reveals an intermediate feature (feature 2) situated between the 1s→3d transition characteristic of Cu²⁺ (feature 1) and the 1s→4p transition
<figure><img src="image_5.png" /><figcaption>**Figure 3.** In situ XANES spectrum of a) Ni K-edge of NiCo in 1 M KOH; b) Ni K-edge of NiCo in 1 M KOH + 0.33 M urea; Optimal linear combination fitting result of the XANES spectra and average oxidation state of Ni K-edge of NiCo in c) 1 M KOH and d) 1 M KOH + 0.33 M urea; In situ XANES spectrum of e) Cu K-edge of CuCo in 1 M KOH; f) Cu K-edge of CuCo in 1 M KOH + 0.33 M urea; Optimal linear combination fitting result of the XANES spectra and average oxidation state of Cu K-edge of CuCo in g) 1 M KOH and h) 1 M KOH + 0.33 M urea.</figcaption></figure>
of Cu²⁺ (feature 3) (Figure 3e).[28] This intermediate feature is indicative of the presence of Cu superoxo species (Cu(2-x)+.OO⁻·).[29] Notably, this feature initially increases in intensity and subsequently decreases with increased applied potential. A detailed analysis of the first derivative spectra of the Cu K edge accentuates this trend for forming reductive Cu(2-x)+.OO⁻·, particularly with the peak at 8982.1 eV, which begins to intensify at a low applied potential of 1.1 V (Figure S36, Supporting Information).[30] A decline in the intensity of this peak is observed at 1.5 V, alongside a pronounced shift in the main peak to 8987.22 eV from 1.6 V, suggesting a transformation from Cu(OH)₂ to CuO-like structure at high voltages. LCF of the Cu K edge further corroborates these observations as shown in Figure 3g,h. Under OER, there is a rise in the Cu⁺ contribution at 1.1 V, which decreases at 1.5 V. Notably, there is a significant increase in the Cu²⁺ contribution from CuO simultaneously, indicating a structural shift from Cu(OH)₂ to CuO commencing at this potential. This observation of a 'transient reduction,' where part of the Cu²⁺ is reduced to Cu⁺, parallels findings in other catalytic systems such as Cu-MOFs or Cu-zeolite catalyzed reactions for methane conversion to methanol, or C-H bond activation in the presence of O₂. In these systems, the reduced Cu⁺ species acts as an electron donor, activating adsorbed O₂ on the surface (Cu⁺ + O₂ → Cu²⁺ + .OO⁻·), which forms an active Cu-oxo species. This Cu-oxo species then facilitates the C-H activation process, essential for the conversion of CH₄ to methanol.[30c,d,31] Herein, in our case, the dynamic transformation between Cu(OH)₂ to CuO₂⁻ in alkaline conditions under electrochemical condition has also been previously reported,[32] and our above findings suggest the below structural transformation: Cu(OH)₂ + 2OH⁻ ↔ Cu(2-x)+OO⁻· + 2H₂O ↔ CuO + H₂O + 2OH⁻·.

This pattern supports the proposed hypothesis that Cu²⁺ in Cu(OH)₂ undergoes a progressive conversion to Cu<sub>superoxo</sub> species then to Cu²⁺ in CuO during the OER. However, under the UOR condition, the portions of contributions remains stable and mostly to be Cu(OH)₂ in the tested potential ranges. In contrast, the Co K edge reveals an immediate alteration in the oxidation state, escalating from +2.09 to +2.94 at 1.1 V and gradually approaching +3 with an increased applied potential (Figure S37, Supporting Information). The simultaneous occurrence of reduction at the Cu sites and oxidation at the Co sites suggests a potential interdependence between these two processes.

Typically, Cu-based hydroxides exhibit a pronounced JT distortion in their octahedral structures, with two apical Cu-O coordination of varying lengths, or alternatively, a square pyramidal geometry with a single apical Cu-O coordination.[33] In the specific case of CuCo, the fitting results of ex-situ Cu K edge EXAFS indicate local coordination around the Cu sites consisting of ≈4 ± 1 Cu-O coordination in the equatorial plane with an average length of 1.95 ± 0.01 Å, and ≈1 ± 0 apical Cu-O coordination at a longer distance of 2.29 ± 0.03 Å (Table S7, Supporting Information). Additionally, the fitting results for the Co site reveal the presence of about 1 ± 0 Co-O1 coordination at a length of 1.88 ± 0.03 Å, and ≈4 ± 1 Co-O2 coordination at a length of 2.07 ± 0.02 Å (Table S7, Supporting Information). Thus, the Co sites undergo a rapid transition to a higher oxidation state with applied potential, to achieve a more symmetrical octahedral configuration to stabilize the overall hydrotalcite structure. Contrasting

with the high-valence Ni⁴⁺ formation in NiCo, the observed reduction to Cu(2-x)+ in CuCo implies an exchange effect that induces ligand-to-metal charge transfer (LMCT) from the bridging oxygen to Cu²⁺.[34] This is attributed to the increased structural distortion near the Cu sites under applied potential, which in turn promotes the formation of the Cu superoxo species (Cu(2-x)+.OO⁻·). The post-reaction analysis of CuCo shows the Co L edge spectra reflecting a coexistence of Co²⁺ and Co³⁺ after both reactions (Figure S38, Supporting Information), while the Cu L edge spectra demonstrate a more stabilized electronic structure. The O K edge spectra offer more differentiated insight than the metal L edges. The newly emerged spectral features at 530.6 and 533.6 eV are attributed to Cu²⁺ in CuO and Cu⁺, respectively (Figure S39, Supporting Information).[35] More importantly, compared with post-OER, the pre-edge features of the post-UOR O K edge spectra display a diminished Cu²⁺ peak intensity alongside an amplified Cu⁺ signal, suggesting that the UOR process is predominantly facilitated by the redox dynamics between Cu(2-x)+ and Cu²⁺.

## 4. Reaction-Dependent Structure Evolution of NiCo and CuCo During OER and UOR

The structural-selectivity relationship of NiCo toward urea-to-nitrite conversion is further analyzed through in situ EXAFS, in situ Raman and in situ HE-XRD paired with PDF analysis. Under OER, in situ EXAFS at the Ni K edge reveals two peaks at 1.58 and 2.67 Å, corresponding to Ni-O and Ni-M (M = Ni, Co) coordination in Ni-based hydroxide (Figure S40, Supporting Information). At a potential of 1.4 V, a clear shift to shorter lengths indicates a transition from hydroxide to oxyhydroxide (Figure 4a; Figure S41 and Table S7, Supporting Information).[36] Conversely, the Co K edge EXAFS spectrum shows a consistent rise in Co-O peak intensity with potential without shift in peak positions until 1.5 V, likely due to ongoing OH⁻ accumulation on Co (Figure S41 and Table S7, Supporting Information). A noticeable shift in the Co-M peak to a shorter length at 1.5 V indicates a transition from Co hydroxide to oxyhydroxide, aligning with the increased Co oxidation state observed in in-situ XANES spectrum (Figure 4b; Figure S32, Supporting Information). Detailed EXAFS fitting analysis was conducted to clarify the dynamic transitions of the Co-O sites at different lengths in the originally JT-distorted structure. The analysis indicates a contraction in the Co-O1 coordination length from 1.99 ± 0.03 to 1.92 ± 0.03 Å under open circuit voltage (OCV) conditions, coupled with an increase in the coordination number (CN), indicative of the rapid surface adsorption of OH⁻ ions even at OCV (Figure 4b). This CN persists in increasing until a complete transition to a symmetrical octahedral CoOOH structure is achieved at 1.6 V, characterized by a total of 5 ± 1 Co-O coordination at a length of 1.88 ± 0.04 Å (Figure 4b). Under UOR, the Ni K edge EXAFS spectrum indicates that Ni maintains a more stable local structure than it under OER conditions, with notable structural changes commencing only at a potential of 1.6 V (Figure 4c; Table S8, Supporting Information). This stabilized structure supports the mechanism of UOR at the Ni site being governed by a redox process involving Ni²⁺/Ni³⁺ and Ni⁴⁺ states. Concurrently, the coordination structure of Co also exhibits stabilization, yet the observation of a reduced Co-O coordination length and a consistent increase in the intensity of
<figure><img src="image_6.png" /><figcaption>**Figure 4.** Optimal in situ EXAFS fitting result of coordination number (CN) and length of a) Ni K-edge first shell coordination in 1 M KOH; b) Co K-edge first shell coordination in 1 M KOH; c) Ni K-edge first shell coordination in 1 M KOH + 0.33 M urea; d) Co K edge first shell coordination in 1 M KOH + 0.33 M urea; in situ Raman spectrum of e) NiCo in 1 M KOH; f) NiCo in 1 M KOH +0.33 M urea; Atomic PDFs of g) NiCo in 1 M KOH; h) NiCo in 1 M KOH +0.33 M urea.</figcaption></figure>

the Co-O peak suggests ongoing OH⁻ adsorption, despite slightly suppressed by the presence of urea molecules (Figure 4d).

Under OER conditions at lower potentials (1.1 and 1.2 V), Raman spectra of NiCo exhibit two distinct peaks at 465 and 528 cm⁻¹, which correspond to the A1g stretching of Ni-OH and Ni-O vibrations, respectively,[³⁷] and the peaks at 488 and 607 cm⁻¹ are associate with the Co-O vibrations (Figure 4e).[³⁸] Distinct change of the spectra is observed at 1.3 V, with two new predominant peak generated at 474 and 551 cm⁻¹, respectively, which are typical features of γ-NiOOH.[³⁸] Additionally, the broad doublet features in the grey highlighted region ranging from 900 to 1150 cm⁻¹, appeared at 1.3 V, corresponds to features active oxygen intermediate prior to oxygen release during OER process.[³⁹] As the potential increases, the peak at 607 cm⁻¹ gradually vanishes, and a new shoulder peak ≈565 cm⁻¹ becomes evident from 1.5 V, signifying the transformation from

Co(OH)₂ to CoOOH,[³⁸] in agreement with the in-situ XAS results. Under UOR conditions, Raman spectroscopy reveals that NiCo maintains a stable Ni-based hydroxide structure (Figure 4f). Notably, there is an absence of Ni-peroxo and Ni-superoxo species at higher reaction potentials, while an increased ratio of Ni-OH to Ni-O features is observed, suggesting a continuous build-up of OH⁻ on the catalyst surface during UOR. This overaccumulation of surface OH⁻ is reported to interfere with urea adsorption, prompting early cleavage of the C-N bond and potentially leading to the formation of overoxidized nitrogen products.[⁷b] Further, the appearance of the peak at 1056 cm⁻¹ at 1.5 V is assigned to NO₃⁻, further confirming the generation of the overoxidized products on NiCo.[⁴⁰] A minor NO₃⁻ peak observed at 1.2 V, which may stem from the diffusion of free NO₃⁻ from the interlayers of the NiCo hydroxides.[⁴¹] In situ PDF analysis was conducted on the NiCo catalyst to investigate the comprehensive
<figure><img src="image_7.png" /><figcaption>**Figure 5.** Optimal in situ EXAFS fitting result of coordination number (CN) and length of a) Cu K-edge first shell coordination in 1 M KOH; b) Cu K-edge first shell coordination in 1 M KOH + 0.33 M urea; c) Co K-edge first shell coordination in 1 M KOH; d) Co K-edge first shell coordination in 1 M KOH + 0.33 M urea; e) CuCo in 1 M KOH; f) CuCo in 1 M KOH + 0.33 M urea; g) CuCo in 1 M KOH; h) CuCo in 1 M KOH + 0.33 M urea.</figcaption></figure>

structural changes during the reaction (Figure 4g,h). Under OER conditions with applied potential, both the M-O and M-M peaks exhibit a negative shift, indicating a complete structural transition from Ni-based hydroxides to oxyhydroxides. In comparison to the original NiCo, three distinct M-M peaks at 2.78, 3.03, and 3.27 Å are observed. These peaks can be respectively attributed to M-M in oxyhydroxide, M-M in hydroxide (owing to the delayed structural transformation at the Co sites), and the JT distorted M-M sites. The overall structure stabilizes in the presence of urea, further corroborating the previously discussed mechanism of electrogenerated oxyhydroxide/hydroxide redox-mediated nucleophile oxidation reaction.

The in-situ EXAFS analysis at the Cu K edge of CuCo indicates a consistent coordination number and length for the equatorial Cu-O coordination. Notably, there is a decrease in the in-

tensity of the multiple scattering Cu-O peak at 3.07 Å (without phase correction), coupled with an increase in peak intensity at a shorter length of 2.57 Å (Figure 5a; Figure S42 and Table S9, Supporting Information). The lack of a peak shift in the Cu-M coordination implies that the transformation into a complete CuO structure is not fully realized, but rather alterations in the oxygen coordination environment near Cu sites (Figure S43 and Table S9, Supporting Information). The initial Jahn-Teller (JT) distorted Cu(OH)₂ structure, characterized by shared edges between apical and equatorial OH⁻ ions, likely transforms into a CuO-like structure due to the partial loss of protons at the Cu-OH edge sites.[42] This transformation is accompanied by the emergence of Cu superoxo species, observed from a low applied bias of 1.1 V at the in situ XANES spectrum of Cu K edge (Figure 3e, feature 2). Further, the analysis of the Cu K edge EXAFS fitting
indicates a prompt extension in the apical Cu-O length, expanding from 2.31 to 2.37 Å, even at OCV (Figure 5a). The increased apical Cu-O coordination length is reported to significantly impact the initiation of superexchange processes within the material and enhance charge transfer between Cu and O.[43] This mechanism is likely responsible for the formation of the unique Cu superoxo species in Cu-based hydroxides. Conversely, under UOR conditions, these changes in the apical Cu-O coordination are inhibited, implying the consumption of Cu superoxo species in the UOR process (Figure 5b; Table S10, Supporting Information). Additionally, the EXAFS analysis at the Co K edge reveals an immediate structural alteration at 1.1 V. Importantly, there is no significant increase in the CN of Co-O, suggesting a lack of pronounced OH⁻ accumulation on the Co sites, especially when compared to the Co in NiCo (Figure 5c,d; Table S10, Supporting Information).

The Raman spectra for CuCo under both OER and UOR conditions display several similarities (Figure 5e,f). The distinct peak at 491 cm⁻¹, accompanied by a minor peak at 1059 cm⁻¹ emerging from 1.1 V, is attributed to the Co-O and O-O stretching vibrations in the end-on Co superoxo species. Further, the peak at 606 cm⁻¹ assigned to Co-OH gradually decreases with increased intensity of the Co superoxo species with further applied potential. These observations align with the immediate structural changes detected in the Co K edge EXAFS analysis under both conditions.[44] Additionally, the grey highlighted zone ranging from ≈280 – 450 cm⁻¹ are representative features of Cu⁺/Cu²⁺-O stretching.[45] Under the UOR condition, CuCo shows the decreased intensity of both the features of Cu⁺/Cu²⁺-O stretching, as well as the Co superoxo species, suggesting both sites are involved in the redox reaction during UOR, which is consistent with the in situ XAS results. In-situ PDF analysis was also conducted on the CuCo catalyst to investigate the comprehensive structural changes during the reaction (Figure 5g,h). Under OER conditions, the M-O and M-M peaks, initially at 2.06 and 3.06 Å, shifted to slightly shorter coordination lengths of 1.92 and 3.02 Å, respectively. This shift is attributed to the conversion of Co(OH)₂ to CoOOH and Cu(OH)₂ to a CuO-like structure. The emergence of an additional peak ≈3.36 Å could be associated with the JT-distorted Cu-Co interactions. Concurrently, the second M-O peak at 3.76 Å decreased in intensity with increasing potential under the OER condition, aligning with the diminished multiple scattering M-O peak observed in in-situ EXAFS results. Under UOR conditions, similar alterations in the M-O peaks were noted due to changes in the local structure at the Co-O sites. The M-M peak and the second M-O peak maintained relatively stable coordination lengths, further confirming that UOR on CuCo likely encompasses an indirect oxidation mechanism, involving interactions between in situ generated catalyst surface and the reactants. Additionally, we performed in situ IR spectroscopy to help uncover reaction pathways of the surface-bound species. As shown in Figures S44 and S45 (Supporting Information), there are clear catalyst dependent and materials dependent differences in the IR that indicate different surface interactions with each catalyst. From previous reports,[46] the in situ IR spectroscopy aligns with our proposed mechanistic understanding of UOR on CuCo and NiCo (detailed in the following DFT section), noting that distinguishing between specific vibrational modes is not trivial.

## 5. DFT Calculations for Understanding UOR Mechanisms

DFT analysis was performed to comprehend the distinct selectivity of CuCo and NiCo catalysts toward nitrite and nitrogen, respectively. Representative models of the catalysts were developed based on the simulation outcomes derived from EXAFS spectra of the individual metal K edges until reaching a stable convergence of surface energies (Figures S46 and S47, Table S11, Supporting Information). The urea adsorption behavior on NiCo and CuCo surfaces was explored by exploring all potential adsorption sites and configurations utilize the computational hydrogen electrode (CHE) method, which is widely recognized and employed in the field to simulate reaction pathways effectively. In CuCo systems, a stable adsorption behavior is observed when a single N-terminated urea molecule is adsorbed onto the Co site. Conversely, for NiCo systems, stabilization is achieved when an O-terminated urea molecule is absorbed across two adjacent metal sites as bridging oxygen (Figure S48, Supporting Information). These observations, combined with previous in-situ structural and electronic structural analyses, facilitate the proposal of potential reaction mechanisms for the conversion of urea to nitrogen on CuCo and the transformation of urea to nitrite on NiCo. For the CuCo, the in-situ generated Cu(2-x)+-OO⁻• acts as an active electrophile to initiate the nucleophilic substitution of NH₂ from the amide, followed by the oxidization of the Cu(2-x)+ to Cu²⁺ and simultaneous N-H bond activation that converts NH₂ to NH. Consequently, bimolecular N-N coupling occurs between the *NH adsorbed on adjacent Co and Cu, resulting in the formation of N₂ and CO₂ as the final products (Figure 6a; Figure S49 and Table S12, Supporting Information).*[28,29–47] In terms of NiCo, the interaction of urea is characterized by O-terminated adsorption at the double Ni site, accompanied by an over-accumulation of OH⁻ ions at the Co sites. This accumulation triggers an extensive OH⁻ attack on the carbon center of the urea molecule, resulting in C-N cleavage. Both cleavage products *NH and *OCNH₂ undergo further oxidation processes, which leads to the formation of overoxidized nitrogen products and CNO⁻, as confirmed by NMR results. The N-H activation reaction is characterized by significant proton and electron transfer processes, which are primarily facilitated by a Ni⁴⁺ and Ni³⁺/Ni²⁺ mediated indirect oxidation scheme. Additionally, a direct oxidation scheme involving the accumulated OH⁻ ions on the Co sites plays a crucial role in the dehydrogenation of N-H species (Figure 6b; Figure S50 and Table S13, Supporting Information).

To validate the suggested mechanisms for the formation of key products in the NiCo and CuCo catalysts, additional DFT calculations were conducted, analyzing the energy profiles of the early stages in the UOR (Figure 6c,d). The emphasis was placed on exploring the reaction pathways that result in the production of NOₓ + NCO and N₂ + CO₂, offering a comprehensive and quantitative insight into the reaction mechanisms occurring on these catalysts. Regarding pathway selectivity, DFT calculations indicated distinct preferences based on the catalyst surface. On the CuCo surface, the formation of CNO⁻ is energetically less favorable, as illustrated in Figure 6c. Conversely, on the NiCo surface, the *NHCONH₂ intermediate more readily undergoes C-N cleavage and subsequent oxidation to CNO⁻, as shown in Figure 6d. Therefore, DFT findings suggest a preference for the NOₓ + NCO
<figure><img src="image_8.png" /><figcaption>**Figure 6.** a) A schematic illustration of urea oxidation pathway and structural transformation on CuCo; b) A schematic illustration of urea oxidation pathway and structural transformation on NiCo; Comparison of energetic availability of C N cleavage (leading to NCO formation) and no C-N cleavage pathways on c) CuCo and d) NiCo using DFT; e) DFT simulation and mechanism for UOR to N₂ and CO₂ on CuCo and NiCo.</figcaption></figure>

formation pathway on NiCo, while CuCo is more inclined toward the N₂ + CO₂ formation pathway. Additionally, CuCo exhibits a greater number of downward steps in the urea-to-N₂ conversion process compared to the urea-to-nitrite conversion on NiCo, aligning with the superior electrochemical performance of CuCo over NiCo in urea oxidation (Figure 6e).

## 6. Conclusion

Employing a suite of in-situ characterization methods, our investigation reveals that the structure-induced mechanistic origin for the selectivity differences in CuCo and NiCo double hydroxides for UOR. While Ni-based catalysts exhibit excellent electrochemical UOR, the predominantly products are nitrogen oxides (NO₂⁻/NO₃⁻) due to presence of high valence Ni⁴⁺ that promotes N-H activation, while JT-distorted Co sites accumulate OH⁻. In contrast, CuCo hydroxides exhibit a preference for N₂ formation,

a phenomenon linked to the action of in-situ generated Cu(2-x)+-OO⁻* as an active center. This facilitates the nucleophilic substitution of NH₂ from the amide, followed by bimolecular N-N coupling between *NH* adsorbed on adjacent Co and Cu sites. The observed lower onset potential and distinct selectivity toward N₂ in CuCo hydroxides, compared to NiCo hydroxides, underscore the critical role of catalyst composition and structural engineering in influencing product selectivity. These insights not only deepen our understanding of the catalytic behaviors of Ni- and Cu-based materials but also guide the rational design of catalysts with bespoke selectivity for targeted applications. The outcomes of this research present viable strategies for reducing environmental hazards associated with urea waste. Advancing catalyst development to favor the production of environmentally benign products like N₂ promises to enhance the sustainability and economic viability of urea electrolysis processes. It should be noted that although the CuCo electrocatalyst exhibits an ultra-low
onset potential, the operational potentials of currently studied catalysts remain higher than the theoretical thermodynamic potential, suggesting the necessity for ongoing research in this field. Ultimately, the knowledge acquired from this study is expected to spur further innovation in electrocatalysis, leading to the creation of high-performance materials tailored to the diverse requirements of industrial applications.

## Supporting Information

Supporting Information is available from the Wiley Online Library or from the author.

## Acknowledgements

Y.Y. and N.M.B would like to acknowledge support from the Faculty of Engineering at UNSW and the Australian Renewable Energy Agency (ARENA) Hydrogen Program (2018/RND015). L.D, N.M.B, and Y.Y would like to acknowledge support from ARC CoE-CSI (CE230100032). A.H would like to acknowledge the support from DOE BES Grant #DE-SC0023322. XAS measurements were performed at the XAS (Ni, Co, and Cu K-edge) and SXR (O K-edge, metal L-edges) beamlines of the Australian Synchrotron, part of ANSTO. The authors would like to thank Drs Jessica Hamilton and Burce Cowie for their assistance with experiments at XAS and SXR respectively. The authors acknowledge the European Synchrotron Radiation Facility (ESRF) for the provision of synchrotron radiation facilities for in-situ HE-XRD/PDF measurements at the ID11 beamline. The authors acknowledge the use of the facilities and the scientific and technical assistance of the ORNL's Center for Nanophase Materials Sciences (CNMS), Mark Wainwright Analytical Centre (MWAC), and the Electron Microscope Unit (EMU) at UNSW, Sydney. N.M.B. would like to acknowledge support from the UNSW Digital Grid Futures Institute. The research was undertaken with the assistance of computational resources provided by National Computational Infrastructure (NCI) Australia.

Open access publishing facilitated by University of New South Wales, as part of the Wiley - University of New South Wales agreement via the Council of Australian University Librarians.

## Conflict of Interest

The authors declare no conflict of interest.

## Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.

## Keywords

Cu-based hydroxide, electron and proton transfer processes, Ni-based hydroxide, structural evolution, urea electrooxidation, X-ray absorption spectroscopy

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