Multistep Dissolution of Lamellar Crystals Generates Superthin Amorphous Ni(OH)2 Catalyst for UOR

Interfacial molybdate-enabled electric field deconfinement to passivate water oxidation for wide-potential biomass electrooxidation

The priority adsorption of OH- in the anodic refining process typically compromises the accessibility of organic reactants and propels their competing oxygen evolution reaction (OER), inevitably generating inactive areas of organics electrooxidation. In this work, an electric field deconfinement strategy enabled by self-originated MoO42- was unveiled to confine the mass diffusion of OH- over a Mo-modulated Ni-based electrode (NiMoOx/NF). The reconstructable NiMoOx/NF catalyst was high-efficiency for selective electrooxidation of various biomass derivatives, especially for electrocatalytic 5-hydroxymethylfurfural (HMF) oxidation reaction (e-HMFOR) to afford 2,5-furanedicarboxylic acid (FDCA, a versatile bioplastic monomer). In-situ tests and finite element analyses evidenced that NiOOH-MoO42- in-situ reconstructed from NiMoOx/NF is responsible for e-HMFOR, where the surface-adsorbed MoO42- can trigger a negative electric field to restrict OH- affinity by electrostatic repulsion but facilitate HMF adsorption, thereby leading to the deteriorated OER and enhanced e-HMFOR. Theoretical calculations further elaborated that introduced MoO42- boosts HMF adsorption to accelerate the reaction kinetics but elevates the energy barrier of O* coupling into OOH* to passivate OER. As a result, a wide potential interval (1.35-1.55 VRHE) was applicable to produce FDCA via e-HMFOR with admirable productivity (95.4-97.8% faradaic efficiencies), rivaling the state-of-the-art Ni-based electrodes. In addition, the established membrane electrode assembly electrolyzer could be operated stably for 40 h at least, with high efficiency in electrosynthesis of gram-grade FDCA. This study underlines the viability and criticality of electric field deconfinement for manipulating the OH- adsorption to facilitate organics electrooxidation and biorefinery while getting rid of competing reactions.

Rapid B-Ni charge transfer pathway induced Ni3+/Ni2+ sites reversible conversions enabling efficient urea oxidation assisted hydrogen production

The high energy consumption induced by the sluggish anodic oxygen evolution reaction (OER) severely limits the efficiency of hydrogen production in water splitting. Replacing OER with urea oxidation reaction (UOR) with lower theoretical voltage with nickel-based layered double hydroxides (NiM-LDHs) as the electrocatalyst enables highly efficient hydrogen production. Herein, a reversible Ni3+/Ni2+ conversion mechanism through rapid B-Ni charge transfer for efficient UOR is first reported. The introduction of the B sites accelerates the surface reconstruction of Ni2+ into Ni3+ in B-NiCo-LDH, ensuring the rapid generation of the active Ni3+ species. In the presence of urea, the rapid B-Ni charge transfer accelerates the reduction process of partial Ni3+ into Ni2+ species, avoiding the overaccumulation of Ni3+ and the over-adsorption of *COO on the catalyst, thereby effectively reducing the energy barrier of UOR. Thus, B-NiCo-LDH demonstrates an ultra-low voltage of 1.39 V vs. RHE to deliver 100 mA cm-2 for UOR. More importantly, the constructed urea-assisted water electrolysis electrolyzer by coupling B-NiCo-LDH at the anode and an HER catalyst (Pt/C) at the cathode achieves 100 mA cm-2 with cell voltages of only 1.55 V and 1.57 V, in alkaline freshwater and seawater, respectively, also exhibiting excellent stability for at least 100 h.

Highly amorphized nickel-based ternary metal catalysts for efficient urea oxidation reaction

Developing efficient catalysts for the urea oxidation reaction (UOR) is of critical importance in advancing sustainable energy. The transient loss of high-valence Ni active sites is one of the key factors limiting the reaction kinetics of UOR. To addressing this challenge, we successfully synthesize transition metal M (M = Mn, Co, Fe)-doped NiMoO4 amorphized self-supported electrodes (NiMoO4/M) through a two-step hydrothermal method. The NiMoO4/Mn shows superior stability and selectivity, achieving a current density of 100 mA cm-2 at an overpotential of 1.48 V (vs. RHE). The results reveal that the catalytic activity improvement stems from amorphized surface structure and the synergistic effects among Ni-Mo-M. During electrochemical reconstruction, ternary Ni-Mo-M active sites are formed, with Mo acting as an electron acceptor to regulate the electronic configuration of Ni. The doped metals further enhance the electron-accepting ability of Mo, facilitating electron transfer between Ni and Mo, optimizing the hydroxide-coupled electron transfer process, thereby accelerating UOR kinetics and advancing the process's efficiency. This study offers a strategic framework for designing highly efficient multi-metallic UOR catalysts through precise modulation of electronic and structural properties.

Pairing N-Vacancy and Adjacent Ni-Sites in the Local Microenvironment to Regulate the Urea Oxidation Reaction Pathway With Enhanced Kinetics

The urea oxidation reaction (UOR) is a promising approach for replacing the oxygen evolution reaction in hydrogen production, offering lower energy consumption. However, the kinetics of Ni-based catalysts for UOR are hindered by the high formation potential of NiOOH and its repeated transition with Ni(OH)2. In this study, a local microenvironment featuring electron-deficient N-vacancies (VN) paired with adjacent electron-rich Ni-sites on Ni3N (Ni3N-VN) to enhance UOR kinetics is constructed. The electron-rich Ni-sites significantly reduce the energy barrier for NiOOH formation and promote the conversion of Ni(OH)2 to NiOOH. Meanwhile, the VN sites induce low charge transfer resistance in Ni3N, facilitating efficient electron transfer and boosting UOR performance while ensuring the stability of the active NiOOH phase. The VN sites promote the adsorption of the urea N atom at the active site, favoring the reaction pathway toward "NCO⁻" formation without requiring complete urea dissociation. This pathway alleviates the NiOOH/Ni(OH)2 conversion cycle, lowers charge transfer resistance, and improves reaction kinetics. Ni3N-VN demonstrates excellent UOR activity (low potential of 1.46 V at 1000 mA cm-2) and industrial prospects (integrating into an anion exchange membrane flow electrolyzer with 20% Pt/C, producing 600 mA cm-2 at 1.84 V), highlighting its potential for practical applications.

Phase Transition Engineering of Metal-Organic Frameworks Induces Multiphase Complexation for Enhancing the Oxygen Evolution Reaction

Phase transition engineering of metal-organic frameworks (MOFs) presents a promising strategy for enhancing electrocatalytic performance in water splitting applications. In this study, we demonstrate a controlled phase transition strategy to synthesize a multiphase composite (op&cp) composed of open phase (op) and closed phase (cp) through precise desolvation treatment. When used as an alkaline water electrocatalyst, op&cp exhibits exceptional oxygen evolution reaction (OER) performance, achieving a remarkably low overpotential of 140 mV under 10 mA cm-2 and maintaining stable operation for over 75 h at 100 mA cm-2. In situ Raman spectroscopy and X-ray photoelectron spectroscopy show that the catalytically active substance NiOOH is formed on the engineered phase with a lower potential (1.2 V vs RHE) than the single-phase material (1.3 V vs RHE). This work establishes phase transition engineering as a viable strategy for improving MOF-based catalysis and explores the fundamental mechanism of the dynamic evolution of active sites during the OER.

Heterogeneity During the Formation of Waterborne Barrier Coating Revealed by Cryogenic Transmission Electron Microscopy

Imaging the film formation process of waterborne barrier coatings in situ with nanoscopic resolution is very challenging, which limits the understanding of the underlying mechanisms and rational design of the materials. Here this challenge is tackled using in situ cryogenic transmission electron microscopy (cryoTEM) in combination with electron tomography (cryoET), which allows 3D imaging of the process with <1 nm resolution. By monitoring the film formation process of poly(ethylene-co-methacrylic acid) (EMAA) ionomer dispersion, onion-like nano-aggregates are captured. These aggregates can be removed by weakening the interactions between EMAA particles via adding amino alcohol coalescing agents or increasing the EMAA neutralization degree, which improves the barrier property of the coating simultaneously, indicating the importance of these heterogeneities to the material performance. The study benefits a better understanding of the formation kinetics of waterborne coatings, and demonstrates cryoTEM as an efficient method for studying the film formation process in situ.

Harnessing Adsorbate-Adsorbate Interaction to Activate C-N Bond for Exceptional Photoelectrochemical Urea Oxidation

The photoelectrochemical (PEC) urea oxidation reaction (UOR) presents a promising half-reaction for green hydrogen production, but the stable resonance structure of the urea molecule results in sluggish kinetics for breaking the C-N bond. Herein, we realize the record PEC UOR performance on a NiO-modified n-Si photoanode (NiO@Ni/n-Si) by harnessing the adsorbate-adsorbate interaction. We quantificationally unveil a dependence of the UOR activation barrier on the coverage of photogenerated surface high-valent Ni-oxo species (NiIV=O) by employing operando PEC spectroscopic measurements and theoretical simulations. The strong attraction between NiIV=O and adsorbed urea facilitates their N-O coupling while weakening the C-N bonding within urea, manifesting as the decreased UOR activation energy from 0.74 to 0.41 eV when the surface coverage of NiIV=O is enhanced from zero to full, corresponding to more than two orders of magnitude enhancement for the UOR rate. Furthermore, an industrial-grade photocurrent density of 100 mA cm-2 is achieved at a potential as low as 1.08 VRHE by stimulating the NiIV=O accumulation under 10 Suns, which is 300 mV lower than the potential required for most reported electrochemical counterparts. This work opens new prospects for achieving high-performance PEC urea oxidation via adsorbate-adsorbate interaction.

Enhancement of the urea oxidation reaction by constructing hierarchical CoFe-PBA@S/NiFe-LDH nanoboxes with strengthened built-in electric fields

The slow kinetics of the oxygen evolution reaction (OER) present a major obstacle for efficient hydrogen production via water electrolysis. In contrast, the urea oxidation reaction (UOR), with its lower thermodynamic barrier, presents a promising alternative to OER. In this study, we designed and synthesized hierarchical CoFe- PBA@S/NiFe-LDH nanoboxes. Sulfur doping in nickel-iron layered double hydroxides (S/NiFe-LDH) introduces a weak built-in electric field (BIEF), which is further strengthened when combined with cobalt-iron Prussian blue analogue (CoFe-PBA) to form a heterojunction. This heterojunction created localized charge polarization at the interface, facilitating efficient electron transfer and reducing the adsorption energy of reaction intermediates, thereby significantly improving intrinsic catalytic activity. Under conditions of 1 M KOH and 0.33 M urea, the CoFe-PBA@S/NiFe-LDH catalyst achieved a current density of 50 mA cm-2 at a relatively low potential of 1.321 V, accompanied by a low Tafel slope (53 mV dec-1). Additionally, it maintained stability at 30 mA cm-2 for 40 h. This work provides vital insights for the strategic design of highly effective heterojunction catalysts for the UOR.

Amorphous Ni(OH)2 Coated Cu Dendrites with Superaerophobic Interface for Bipolar Hydrogen Production Assisted with Formaldehyde Oxidation

Since formaldehyde oxidation reaction (FOR) can release H2, it is attractive to construct a bipolar hydrogen production system consisting of FOR and hydrogen evolution reaction (HER). Although copper-based catalysts have attracted much attention due to their low cost and high FOR activity, the performance enhancement mechanism lacks in-depth investigation. Here, an amorphous-crystalline catalyst of amorphous nickel hydroxide-coated copper dendrites on copper foam (Cu@Ni(OH)2/CF) is prepared. The modification of Ni(OH)2 resulted in hydrophilic and aerophobic states on the Cu@Ni(OH)2/CF surface, facilitating the transport of liquid-phase species on the electrode surface and accelerating the release of H2. The Open circuit potential (OCP) and density functional theory (DFT) calculations indicate that this core-shell structure facilitates the adsorption of HCHO and OH-. In addition, the catalytic mechanism and reaction pathway of FOR are investigated through in situ FTIR and DFT calculations, and the results showed that the modification of Ni(OH)2 lowered the energy barrier for C─H bond breaking and H─H bond formation. In the HER//FOR system, Pt/C//Cu@Ni(OH)2/CF can provide a current density of 0.5 A cm-2 at 0.36 V and achieve efficient and stable H2 production. This work offers new ideas for designing electrocatalysts for bipolar hydrogen production system assisted with formaldehyde oxidation.

Defect Engineering of Metal-Based Atomically Thin Materials for Catalyzing Small-Molecule Conversion Reactions

Recently, metal-based atomically thin materials (M-ATMs) have experienced rapid development due to their large specific surface areas, abundant electrochemically accessible sites, attractive surface chemistry, and strong in-plane chemical bonds. These characteristics make them highly desirable for energy-related conversion reactions. However, the insufficient active sites and slow reaction kinetics leading to unsatisfactory electrocatalytic performance limited their commercial application. To address these issues, defect engineering of M-ATMs has emerged to increase the active sites, modify the electronic structure, and enhance the catalytic reactivity and stability. This review provides a comprehensive summary of defect engineering strategies for M-ATM nanostructures, including vacancy creation, heteroatom doping, amorphous phase/grain boundary generation, and heterointerface construction. Introducing recent advancements in the application of M-ATMs in electrochemical small molecule conversion reactions (e.g., hydrogen, oxygen, carbon dioxide, nitrogen, and sulfur), which can contribute to a circular economy by recycling molecules like H2, O2, CO2, N2, and S. Furthermore, a crucial link between the reconstruction of atomic-level structure and catalytic activity via analyzing the dynamic evolution of M-ATMs during the reaction process is established. The review also outlines the challenges and prospects associated with M-ATM-based catalysts to inspire further research efforts in developing high-performance M-ATMs.

Hollow Mo/MoSVn Nanoreactors with Tunable Built-in Electric Fields for Sustainable Hydrogen Production

Constructing built-in electric field (BIEF) in heterojunction catalyst is an effective way to optimize adsorption/desorption of reaction intermediates, while its precise tailor to achieve efficient bifunctional electrocatalysis remains great challenge. Herein, the hollow Mo/MoSVn nanoreactors with tunable BIEFs are elaborately prepared to simultaneously promote hydrogen evolution reaction (HER) and urea oxidation reaction (UOR) for sustainable hydrogen production. The BIEF induced by sulfur vacancies can be modulated from 0.79 to 0.57 to 0.42 mV nm-1, and exhibits a parabola-shaped relationship with HER and UOR activities, the Mo/MoSV1 nanoreactor with moderate BIEF presents the best bifunctional activity. Theoretical calculations reveal that the moderate BIEF can evidently facilitate the hydrogen adsorption/desorption in the HER and the breakage of N─H bond in the UOR. The electrolyzer assembled with Mo/MoSV1 delivers a cell voltage of 1.49 V at 100 mA cm-2, which is 437 mV lower than that of traditional water electrolysis, and also presents excellent durability at 200 mA cm-2 for 200 h. Life cycle assessment indicates the HER||UOR system possesses notable superiority across various environment impact and energy consumption. This work can provide theoretical and experimental direction on the rational design of advanced materials for energy-saving and eco-friendly hydrogen production.

Pyrazole-Mediated On-Surface Synthesis of Nickel/Nickel Oxide Hybrids for Efficient Urea-Assisted Hydrogen Production

Creating densely functionalized supported materials without aggregation has been one of the ultimate goals for heterogeneous catalysts. Direct conversion of readily available bulk materials into highly dispersed supported materials could be highly beneficial for real applications. In this work, we invented an on-surface synthetic strategy for generating highly loaded and well-dispersed nickel nanoparticles on nickel oxide supports (Ni/NiO). This on-surface synthesis is a two-step process involving the surface transformation of Ni(OH)2 into a layer of chain-like nickel pyrazolate [Ni(pz)2] and an ultra-high-vacuum annealing process that evacuates the pyrazole and decomposes the Ni(OH)2 into the Ni/NiO hybrid. The highly dispersed Ni/NiO catalyst exhibited superior activities and long-term stability for both the urea oxidation reaction and the hydrogen evolution reaction, which enables efficient urea electrolysis with a single catalyst. This approach presents a novel on-surface synthetic strategy for metal oxide-supported materials and offers efficient catalysts for advanced urea-assisted hydrogen production.

Accelerating the reaction kinetics of Ni1-xO/Ni(OH)2/NF by defect engineering for urea-assisted water splitting

Developing electrocatalysts with fast reaction kinetics for the urea oxidation reaction (UOR) in the field of sustainable hydrogen production through urea-assisted water splitting remains challenging. Here, Ni1-xO/Ni(OH)2 supported on nickel foam (Ni1-xO/Ni(OH)2/NF) is prepared via a defect engineering strategy by combining Zn doping and acid etching. The doped Zn species are partially removed, facilitating the formation of NiOOH during acid etching. Residual Zn species modulate the electronic structure of nickel sites, which intrinsically accelerate the reaction kinetics of Ni1-xO/Ni(OH)2/NF. Ni1-xO/Ni(OH)2/NF exhibits excellent performance for the UOR with a low potential of 1.346 V versus the reversible hydrogen electrode to attain 100 mA cm-2, fast reaction kinetics (18.7 mV dec-1), and excellent stability in an alkaline electrolyte. The enhanced reaction kinetics of Ni1-xO/Ni(OH)2/NF are clearly elucidated by operando electrochemical impedance spectroscopy and in situ Raman spectroscopy. Our study offers an effective approach for designing promising Ni-based UOR catalysts for the practical application of urea-assisted water splitting.

An Fe-doped Ni-based oxalate framework with a favorable electronic structure for electrocatalytic water and urea oxidation

An Fe-doped Ni-based oxalate framework, synthesized via a facile co-precipitation method, is applied as an excellent bi-functional electrocatalyst for water and urea oxidation reactions. The obtained framework achieved a large current density of 100 mA cm-2 at 1.497 V and 1.375 V (vs. RHE) for the OER and UOR, highlighting its potential for practical hydrogen production.

Electrodegradation of nitrogenous pollutants in sewage: from reaction fundamentals to energy valorization applications

The excessive accumulation of nitrogen pollutants (mainly nitrate, nitrite, ammonia nitrogen, hydrazine, and urea) in water bodies seriously disrupts the natural nitrogen cycle and poses a significant threat to human life and health. Electrolysis is considered a promising method to degrade these nitrogenous pollutants in sewage, with the advantages of high efficiency, wide generality, easy operability, retrievability, and environmental friendliness. For particular energy devices, including metal-nitrate batteries, direct fuel cells, and hybrid water electrolyzers, the realization of energy valorization from sewage purification processes (e.g., valuable chemical generation, electricity output, and hydrogen production) becomes feasible. Despite the progress in the research on pollutant electrodegradation, the development of electrocatalysts with high activity, stability, and selectivity for pollutant removal, coupled with corresponding energy devices, remains a challenge. This review comprehensively provides advanced insights into the electrodegradation processes of nitrogenous pollutants and relevant energy valorization strategies, focusing on the reaction mechanisms, activity descriptors, electrocatalyst design, and actuated electrodes and operation parameters of tailored energy conversion devices. A feasibility analysis of electrodegradation on real wastewater samples from the perspective of pollutant concentration, pollutant accumulation, and electrolyte effects is provided. Challenges and prospects for the future development of electrodegradation systems are also discussed in detail to bridge the gap between experimental trials and commercial applications.

Electrochemical reconstruction of metal-organic gels into crystalline oxy-hydroxide heterostructures for efficient oxygen evolution electrocatalysis

Metal-organic gels (MOGs) are emerging soft materials with distinct metal active centers, multifunctional ligands and hierarchical porous structures, showing promising potential in the field of electrocatalysis. However, the reconfiguration of MOGs during the electrocatalytic process remains underexplored, with current studies in early developmental stages. To deeply investigate the application of MOG materials in electrocatalysis, the compositional transformations and structural changes under an electrochemical activation method were studied in detail, leading to high-performance OER pre-electrocatalysts. XRD and HRTEM results demonstrate the complete reconfiguration of amorphous Fe5Ni5-MOG into crystalline NiOOH/FeOOH heterostructures. The synergistic effect of the bimetallic center and the rich NiOOH-FeOOH interface in the reconstituted Re-Fe5Ni5-MOG exhibit excellent OER activity in alkaline electrolytes, with low overpotentials (205 mV at 10 mA cm-2) and a Tafel slope of 58 mV dec-1. In situ characterization during the electrocatalytic process reveals the gradual transformation of the metal center into metal hydroxyl oxides upon increasing the voltage to 1.55 V. DFT analysis suggests that in the Fe-Ni double site reaction pathway, active substances preferentially adsorb on the Fe site before the Ni sites.

Rational Design of Ultrahigh-Loading Ir Single Atoms on Reconstructed Mn─NiOOH for Enhanced Catalytic Performance in Urea-Water Electrolysis

Investigating advanced electrocatalysts is crucial for improving the efficacy of water splitting to generate environmentally friendly fuel. The discovery of highly effective electrocatalysts, capable of driving oxygen evolution reaction (OER) and urea oxidation reaction (UOR) in urea-alkaline environments, is pivotal for advancing large-scale hydrogen production. This study aims to introduce a new method that involves creating nanosheets of high-loading iridium single atoms embedded in a manganese-containing nickel oxyhydroxide matrix (Ir@Mn─NiOOH). These nanostructures are derived from self-supported hydrate pre-catalyst nanosheets grown on nickel foam and then activated through electrochemical etching pretreatment. The Ir@Mn─NiOOH nanoarchitecture displays outstanding electrocatalytic activity, having a low overpotential of just 258 mV and a potential of 1.319 V (at 10 mA cm-2) for OER and UOR, respectively. Such extraordinary catalytic characteristics of Ir@Mn─NiOOH is mainly owing to the strong synthetic electronic interaction between Ir single atoms and Mn─NiOOH, which can change its electronic characteristics and boost electrochemical catalytic sites. This research presents a new way to produce exceptionally efficient catalysts by adding a synergistic effect to complex multi-electron processes.

Photo-Excited High-Spin State Ni (III) Species in Mo-Doped Ni3S2 for Efficient Urea Oxidation Reaction

Designing robust catalysts for increasing the sluggish kinetics of the urea oxidation reaction (UOR) is challenging. Herein, the regulation of spin states for metal active sites by photoexcitation to facilitate the adsorption of urea and intermediates is demonstrated. Mo-doped nickel sulfide nanoribbon arrays (Mo-Ni3S2@NMF) with excellent light-trapping capacity are successfully prepared. Under AM 1.5G illumination, the activity of the Mo-Ni3S2@NMF exhibits a 50% improvement in the UOR current. Compared with those under dark conditions, Mo-Ni3S2@NMF achieve 10 mA cm-2 at 1.315 VRHE for UOR and 1.32 Vcell for urea electrolysis, which are decreases of 15 and 80 mV, respectively. The electron spin resonance, in situ Fourier transform infrared spectroscopy analysis and density functional theory calculations reveal that illumination led to the formation of Ni3+ active sites in a high-spin state, which strengthens the d-p orbital hybridization of Ni-N, hence facilitating the adsorption of urea. C─N cleavage of the *CONN intermediate is further inhibited, which promotes the oxidation of urea molecules via the active N2 pathway, thereby accelerating the UOR rate.

Wood-Structured Nanomaterials as Highly Efficient, Self-Standing Electrocatalysts for Water Splitting

Electrocatalytic water splitting (EWS) driven by renewable energy is widely considered an environmentally friendly and sustainable approach for generating hydrogen (H2), an ideal energy carrier for the future. However, the efficiency and economic viability of large-scale water electrolysis depend on electrocatalysts that can efficiently accelerate the electrochemical reactions taking place at the two electrodes. Wood-derived nanomaterials are well-suited for serving as EWS catalysts because of their hierarchically porous structure with high surface area and low tortuosity, compositional tunability, cost-effectiveness, and self-standing integral electrode configuration. Here, recent advancements in the design and synthesis of wood-structured nanomaterials serving as advanced electrocatalysts for water splitting are summarized. First, the design principles and corresponding strategies toward highly effective wood-structured electrocatalysts (WSECs) are emphasized. Then, a comprehensive overview of current findings on WSECs, encompassing diverse structural designs and functionalities such as supported-metal nanoparticles (NPs), single-atom catalysts (SACs), metal compounds, and heterostructured electrocatalysts based on engineered wood hosts are presented. Subsequently, the application of these WSECs in various aspects of water splitting, including the hydrogen evolution reaction (HER), oxygen evolution reaction (OER), overall water splitting (OWS), and hybrid water electrolysis (HWE) are explored. Finally, the prospects, challenges, and opportunities associated with the broad application of WSECs are briefly discussed. This review aims to provide a comprehensive understanding of the ongoing developments in water-splitting catalysts, along with outlining design principles for the future development of WSECs.

Vertical Cross-Alignments of 2D Semiconductors with Steered Internal Electric Field for Urea Electrooxidation via Balancing Intermediates Adsorption

Single-component electrocatalysts generally lead to unbalanced adsorption of OH- and urea during urea oxidation reaction (UOR), thus obtaining low activity and selectivity especially when oxygen evolution reaction (OER) competes at high potentials (>1.5 V). Herein, a cross-alignment strategy of in situ vertically growing Ni(OH)2 nanosheets on 2D semiconductor g-C3N4 is reported to form a hetero-structured electrocatalyst. Various spectroscopy measurements including in situ experiments indicate the existence of enhanced internal electric field at the interfaces of vertical Ni(OH)2 and g-C3N4 nanosheets, favorable for balancing adsorption of reaction intermediates. This heterojunction electrocatalyst shows high-selectivity UOR compared to pure Ni(OH)2, even at high potentials (>1.5 V) and large current density. The computational results show the vertical heterojunction could steer the internal electric field to increase the adsorption of urea, thus efficiently avoiding poisoning of strongly adsorbed OH- on active sites. A membrane electrode assembly (MEA)-based electrolyzer with the heterojunction anode could operate at an industrial-level current density of 200 mA cm-2. This work paves an avenue for designing high-performance electrocatalysts by vertical cross-alignments of active components.

Nickel Hydroxide-Based Electrocatalysts for Promising Electrochemical Oxidation Reactions: Beyond Water Oxidation

Transition metal hydroxides have attracted significant research interest for their energy storage and conversion technique applications. In particular, nickel hydroxide (Ni(OH)2), with increasing significance, is extensively used in material science and engineering. The past decades have witnessed the flourishing of Ni(OH)2-based materials as efficient electrocatalysts for water oxidation, which is a critical catalytic reaction for sustainable technologies, such as water electrolysis, fuel cells, CO2 reduction, and metal-air batteries. Coupling the electrochemical oxidation of small molecules to replace water oxidation at the anode is confirmed as an effective and promising strategy for realizing the energy-saving production. The physicochemical properties of Ni(OH)2 related to conventional water oxidation are first presented in this review. Then, recent progress based on Ni(OH)2 materials for these promising electrochemical reactions is symmetrically categorized and reviewed. Significant emphasis is placed on establishing the structure-activity relationship and disclosing the reaction mechanism. Emerging material design strategies for novel electrocatalysts are also highlighted. Finally, the existing challenges and future research directions are presented.

Double Hydroxide Nanocatalysts for Urea Electrooxidation Engineered toward Environmentally Benign Products

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 cmgeo -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.

Tailoring competitive adsorption sites of hydroxide ion to enhance urea oxidation-assisted hydrogen production

Alloys with bimetallic electron modulation effect are promising catalysts for the electrooxidation of urea. However, the side reaction oxygen evolution reaction (OER) originating from the competitive adsorption of OH- and urea severely limited the urea oxidation reaction (UOR) activity on the alloy catalysts. This work successfully constructs the defect-rich NiCo alloy with lattice strain (PMo-NiCo/NF) by rapid pyrolysis and co-doping. By taking advantage of the compressive strain, the d-band center of NiCo is shifted downward, inhibiting OH- from adsorbing on the NiCo site and avoiding the detrimental OER. Meanwhile, the oxygenophilic P/Mo tailored specific adsorption sites to adsorb OH- preferentially, which further released the NiCo sites to ensure the enriched adsorption of urea, thus improving the UOR efficiency. As a result, PMo-NiCo/NF only requires 1.27 V and -57 mV to drive a current density of ±10 mA cm-2 for UOR and hydrogen evolution reaction (HER), respectively. With the guidance of this work, reactant competing adsorption sites could be tailored for effective electrocatalytic performance.

Low-Temperature Plasma-Constructed Ni-Doped W18O49 Nanorod Arrays for Enhanced Electrocatalytic Oxygen Evolution and Urea Oxidation

Surface engineering by doping and amorphization is receiving widespread attention from the perspective of the regulation of the electrocatalytic activities of electrocatalysts. However, the effective modulation of active sites on catalysts is still challenging. Herein, a straightforward and efficient method combining hydrothermal treatment with low-temperature plasma processing is presented to synthesize Ni-doped W18O49 nanorod arrays on carbon cloth with abundant oxygen vacancies (CC/WO-Ni-x). Mild plasma doping with Ni modifies the electronic structure of the W18O49 nanorod arrays, resulting in the formation of an amorphous structure that significantly reduces the electron transfer resistance. Additionally, the coupling with high-valent W6+ (derived from W18O49) leads to the partial preoxidation of doped Ni to form active Ni3+ species and oxygen vacancies. These features are collectively responsible for the remarkable oxygen evolution reaction (OER) and urea oxidation reaction (UOR) properties of CC/WO-Ni-4, for example, 10 mA cm-2 current density, an overpotential of 265 mV required for the OER under 1.0 M KOH solution. The addition of 500 mM urea to the 1.0 M KOH solution decreases the overpotential required for the same current density from 265 to 93 mV. This study provides insights into the modification of surface structures and presents an effective strategy to optimize the electrocatalytic active sites and enhance the efficiency of multifunctional electrocatalysts.

Recent progress in energy-saving electrocatalytic hydrogen production via regulating the anodic oxidation reaction

Hydrogen energy with its advantages of high calorific value, renewable nature, and zero carbon emissions is considered an ideal candidate for clean energy in the future. The electrochemical decomposition of water, powered by renewable and clean energy sources, presents a sustainable and environmentally friendly approach to hydrogen production. However, the traditional electrochemical overall water-splitting reaction (OWSR) is limited by the anodic oxygen evolution reaction (OER) with sluggish kinetics. Although important advances have been made in efficient OER catalysts, the theoretical thermodynamic difficulty predetermines the inevitable large potential (1.23 V vs. RHE for the OER) and high energy consumption for the conventional water electrolysis to obtain H2. Besides, the generation of reactive oxygen species at high oxidation potentials can lead to equipment degradation and increase maintenance costs. Therefore, to address these challenges, thermodynamically favorable anodic oxidation reactions with lower oxidation potentials than the OER are used to couple with the cathodic hydrogen evolution reaction (HER) to construct new coupling hydrogen production systems. Meanwhile, a series of robust catalysts applied in these new coupled systems are exploited to improve the energy conversion efficiency of hydrogen production. Besides, the electrochemical neutralization energy (ENE) of the asymmetric electrolytes with a pH gradient can further promote the decrease in application voltage and energy consumption for hydrogen production. In this review, we aim to provide an overview of the advancements in electrochemical hydrogen production strategies with low energy consumption, including (1) the traditional electrochemical overall water splitting reaction (OWSR, HER-OER); (2) the small molecule sacrificial agent oxidation reaction (SAOR) and (3) the electrochemical oxidation synthesis reaction (EOSR) coupling with the HER (HER-SAOR, HER-EOSR), respectively; (4) regulating the pH gradient of the cathodic and anodic electrolytes. The operating principle, advantages, and the latest progress of these hydrogen production systems are analyzed in detail. In particular, the recent progress in the catalytic materials applied to these coupled systems and the corresponding catalytic mechanism are further discussed. Furthermore, we also provide a perspective on the potential challenges and future directions to foster advancements in electrocatalytic green sustainable hydrogen production.

Highly selective urea electrooxidation coupled with efficient hydrogen evolution

Electrochemical urea oxidation offers a sustainable avenue for H2 production and wastewater denitrification within the water-energy nexus; however, its wide application is limited by detrimental cyanate or nitrite production instead of innocuous N2. Herein we demonstrate that atomically isolated asymmetric Ni-O-Ti sites on Ti foam anode achieve a N2 selectivity of 99%, surpassing the connected symmetric Ni-O-Ni counterparts in documented Ni-based electrocatalysts with N2 selectivity below 55%, and also deliver a H2 evolution rate of 22.0 mL h-1 when coupled to a Pt counter cathode under 213 mA cm-2 at 1.40 VRHE. These asymmetric sites, featuring oxygenophilic Ti adjacent to Ni, favor interaction with the carbonyl over amino groups in urea, thus preventing premature resonant C⎓N bond breakage before intramolecular N-N coupling towards N2 evolution. A prototype device powered by a commercial Si photovoltaic cell is further developed for solar-powered on-site urine processing and decentralized H2 production.

Agglomeration inhibition engineering of nickel-cobalt alloys by a sacrificial template for efficient urea electrolysis

Designing efficient non-precious metal-based catalysts for urea oxidation reaction (UOR) is essential for achieving energy-saving hydrogen production and the treatment of wastewater containing ammonia. In this study, sodium dodecyl sulfate (SDS) is employed as a sacrificial template to synthesize NiCo alloy nanowires (NiCo(SDS)/CC), and the instinct formation mechanism is investigated. It is found that SDS can inhibit the Ostwald ripening during hydrothermal and calcination processes, which could release abundant active cobalt, thereby modulating the electronic structure to promote the catalytic reaction. Moreover, SDS as a sacrificial template can induce the deposition of metal atoms and increase the specific surface area of the catalyst, providing abundant active sites to accelerate the reaction kinetics. As expected, the NiCo(SDS)/CC exhibits good activity for both UOR and hydrogen evolution reactions (HER) and it requires only 1.31 V and -86 mV to obtain a current density of ±10 mA cm-2, respectively. This work provides a new strategy for reducing the agglomeration of transition metals to design high-performance composite catalysts for urea oxidation.

Mechanistic Analysis of Urea Electrooxidation Pathways: Key to Rational Catalyst Design

Urea electrolysis is an emerging approach to treating urea-enriched wastewater and an attractive alternative anodic process to the oxygen evolution reaction (OER) in electrochemical clean energy conversion and storage technologies (e. g., hydrogen production and CO2 electroreduction). While the thermodynamic potential for urea oxidation to dinitrogen is quite low compared to that of the OER, the catalysts reported to date require high overpotentials that far exceed those for the OER. Consequently, there is much room for improvement and rational catalyst design for the urea oxidation reaction (UOR). At the same time, due to the urea molecule having a more complex structure than water, UOR can lead to the formation of various products beyond the commonly assumed N2 and CO2. This concept article will critically assess recent efforts of the research community to decipher the formation mechanisms of UOR products focusing on the systematic analysis of the reaction selectivity. This work aims to analyze the current state of the art and identify existing gaps, providing an outlook for the future design of UOR catalysts with superior activity and selectivity by applying the knowledge of the molecular transformation mechanisms.

Accelerated Dynamic Reconstruction in Metal-Organic Frameworks with Ligand Defects for Selective Electrooxidation of Amines to Azos Coupling with Hydrogen Production

Electrosynthesis coupled hydrogen production (ESHP) mostly involves catalyst reconstruction in aqueous phase, but accurately identifying and controlling the process is still a challenge. Herein, we modulated the electronic structure and exposed unsaturated sites of metal-organic frameworks (MOFs) via ligand defect to promote the reconstruction of catalyst for azo electrosynthesis (ESA) coupled with hydrogen production overall reaction. The monolayer Ni-MOFs achieved 89.8 % Faraday efficiency and 90.8 % selectivity for the electrooxidation of 1-methyl-1H-pyrazol-3-amine (Pyr-NH2) to azo, and an 18.5-fold increase in H2 production compared to overall water splitting. Operando X-ray absorption fine spectroscopy (XAFS) and various in situ spectroscopy confirm that the ligand defect promotes the potential dependent dynamic reconstruction of Ni(OH)2 and NiOOH, and the reabsorption of ligand significantly lowers the energy barrier of rate-determining step (*Pyr-NH to *Pyr-N). This work provides theoretical guidance for modulation of electrocatalyst reconstruction to achieve highly selective ESHP.

Heterojunction-Induced Rapid Transformation of Ni3+/Ni2+ Sites which Mediates Urea Oxidation for Energy-Efficient Hydrogen Production

Water electrolysis is an environmentally-friendly strategy for hydrogen production but suffers from significant energy consumption. Substituting urea oxidation reaction (UOR) with lower theoretical voltage for water oxidation reaction adopting nickel-based electrocatalysts engenders reduced energy consumption for hydrogen production. The main obstacle remains strong interaction between accumulated Ni3+ and *COO in the conventional Ni3+-catalyzing pathway. Herein, a novel Ni3+/Ni2+ mediated pathway for UOR via constructing a heterojunction of nickel metaphosphate and nickel telluride (Ni2P4O12/NiTe), which efficiently lowers the energy barrier of UOR and avoids the accumulation of Ni3+ and excessive adsorption of *COO on the electrocatalysts, is developed. As a result, Ni2P4O12/NiTe demonstrates an exceptionally low potential of 1.313 V to achieve a current density of 10 mA cm-2 toward efficient urea oxidation reaction while simultaneously showcases an overpotential of merely 24 mV at 10 mA cm-2 for hydrogen evolution reaction. Constructing urea electrolysis electrolyzer using Ni2P4O12/NiTe at both sides attains 100 mA cm-2 at a low cell voltage of 1.475 V along with excellent stability over 500 h accompanied with nearly 100% Faradic efficiency.

Oxygen vacancies enhanced electrocatalytic water splitting of P-FeMoO4 initiated via phosphorus doping

Transition metal oxides (TMOs) are abundant and cost-effective materials. However, poor conductivity and low intrinsic activity limit their application in electrolyzed water catalysts. Herein, we prepared P-FeMoO4 in situ on nickel foam (P-FMO@NF) by phosphorylation-modified FeMoO4 to optimize its electrocatalytic properties. Interestingly, phosphorus doping is accompanied by the generation of oxygen vacancies and surface phosphates. Oxygen vacancies accelerated Mo dissolution during the oxygen evolution reaction (OER), leading to the rapid reconfiguration of P-FMO@NF to FeOOH and regulating the electronic structure of P-FMO@NF. The formation of phosphates is caused by the substitution of some molybdates with phosphates, which further increases the amount of oxygen vacancies. Hence, the OER overpotential of P-FMO@NF at a current density of 10 mA cm-2 is only 206 mV, and the hydrogen evolution reaction (HER) overpotential is 154 mV. It was assembled into a water splitting cell with a voltage of just 1.59 V at 10 mA cm-2 and shows excellent stability over 50 h. These excellent electrocatalytic properties are mainly attributed to the oxygen vacancies, which improve the interfacial charge transfer properties of the catalysts. This study provides new insights into phosphorus doping and offers a new perspective on the design of electrocatalysts.

Janus electronic state of supported iridium nanoclusters for sustainable alkaline water electrolysis

Metal-support electronic interactions play crucial roles in triggering the hydrogen spillover (HSo) to boost hydrogen evolution reaction (HER). It requires the supported metal of electron-rich state to facilitate the proton adsorption/spillover. However, this electron-rich metal state contradicts the traditional metal→support electron transfer protocol and is not compatible with the electron-donating oxygen evolution reaction (OER), especially in proton-poor alkaline conditions. Here we profile an Ir/NiPS3 support structure to study the Ir electronic states and performances in HSo/OER-integrated alkaline water electrolysis. The supported Ir is evidenced with Janus electron-rich and electron-poor states at the tip and interface regions to respectively facilitate the HSo and OER processes. Resultantly, the water electrolysis (WE) is efficiently implemented with 1.51 V at 10 mA cm-2 for 1000 h in 1 M KOH and 1.44 V in urea-KOH electrolyte. This research clarifies the Janus electronic state as fundamental in rationalizing efficient metal-support WE catalysts.

Urea catalytic oxidation for energy and environmental applications

Urea is one of the most essential reactive nitrogen species in the nitrogen cycle and plays an indispensable role in the water-energy-food nexus. However, untreated urea or urine wastewater causes severe environmental pollution and threatens human health. Electrocatalytic and photo(electro)catalytic urea oxidation technologies under mild conditions have become promising methods for energy recovery and environmental remediation. An in-depth understanding of the reaction mechanisms of the urea oxidation reaction (UOR) is important to design efficient electrocatalysts/photo(electro)catalysts for these technologies. This review provides a critical appraisal of the recent advances in the UOR by means of both electrocatalysis and photo(electro)catalysis, aiming to comprehensively assess this emerging field from fundamentals and materials, to practical applications. The emphasis of this review is on the design and development strategies for electrocatalysts/photo(electro)catalysts based on reaction pathways. Meanwhile, the UOR in natural urine is discussed, focusing on the influence of impurity ions. A particular emphasis is placed on the application of the UOR in energy and environmental fields, such as hydrogen production by urea electrolysis, urea fuel cells, and urea/urine wastewater remediation. Finally, future directions, prospects, and remaining challenges are discussed for this emerging research field. This critical review significantly increases the understanding of current progress in urea conversion and the development of a sustainable nitrogen economy.

High-Valence Metal Doping Induced Lattice Expansion for M-FeNi LDH toward Enhanced Urea Oxidation Electrocatalytic Activities

The precise design of low-cost, efficient, and definite electrocatalysts is the key to sustainable renewable energy. The urea oxidation reaction (UOR) offers a promising alternative to the oxygen evolution reaction for energy-saving hydrogen generation. In this study, by tuning the lattice expansion, a series of M-FeNi layered double hydroxides (M-FeNi LDHs, M: Mo, Mn, V) with excellent UOR performance are synthesized. The hydrolytic transformation of Fe-MIL-88A is assisted by urea, Ni2+ and high-valence metals, to form a hollow M-FeNi LDH. Owing to the large atomic radius of the high-valence metal, lattice expansion is induced, and the electronic structure of the FeNi-LDH is regulated. Doping with high-valence metal is more favorable for the formation of the high-valence active species, NiOOH, for the UOR. Moreover, the hollow spindle structure promoted mass transport. Thus, the optimal Mo-FeNi LDH showed outstanding UOR electrocatalytic activity, with 1.32 V at 10 mA cm-2 . Remarkably, the Pt/C||Mo-FeNi LDH catalyst required a cell voltage of 1.38 V at 10 mA·cm-2 in urea-assisted water electrolysis. This study suggests a new direction for constructing nanostructures and modulating electronic structures, which is expected to ultimately lead to the development of a class of auxiliary electrocatalysts.

Modulation Strategies for the Preparation of High-Performance Catalysts for Urea Oxidation Reaction and Their Applications

Compared with the traditional electrolysis of water to produce hydrogen, urea-assisted electrolysis of water to produce hydrogen has significant advantages and has received extensive attention from researchers. Unfortunately, urea oxidation reaction (UOR) involves a complex six-electron transfer process leading to high overpotential, which forces researchers to develop high-performance UOR catalysts to drive the development of urea-assisted water splitting. Based on the UOR mechanism and extensive literature research, this review summarizes the strategies for preparing highly efficient UOR catalysts. First, the UOR mechanism is introduced and the characteristics of excellent UOR catalysts are pointed out. Aiming at this, the following modulation strategies are proposed to improve the catalytic performance based on summarizing various literature: 1) Accelerating the active phase formation to reduce initial potential; 2) Creating double active sites to trigger a new UOR mechanism; 3) Accelerating urea adsorption and promoting C─N bond cleavage to ensure the effective conduct of UOR; 4) Promoting the desorption of CO2 to improve stability and prevent catalyst poisoning; 5) Promoting electron transfer to overcome the inherent slow dynamics of UOR; 6) Increasing active sites or active surface area. Then, the application of UOR in electrochemical devices is summarized. Finally, the current deficiencies and future directions are discussed.

Bifunctional Al-Doped Cobalt Ferrocyanide Nanocube Array for Energy-Saving Hydrogen Production via Urea Electrolysis

The very slow anodic oxygen evolution reaction (OER) greatly limits the development of large-scale hydrogen production via water electrolysis. By replacing OER with an easier urea oxidation reaction (UOR), developing an HER/UOR coupling electrolysis system for hydrogen production could save a significant amount of energy and money. An Al-doped cobalt ferrocyanide (Al-Co2Fe(CN)6) nanocube array was in situ grown on nickel foam (Al-Co2Fe(CN)6/NF). Due to the unique nanocube array structure and regulated electronic structure of Al-Co2Fe(CN)6, the as-prepared Al-Co2Fe(CN)6/NF electrode exhibited outstanding catalytic activities and long-term stability to both UOR and HER. The Al-Co2Fe(CN)6/NF electrode needed potentials of 0.169 V and 1.118 V (vs. a reversible hydrogen electrode) to drive 10 mA cm-2 for HER and UOR, respectively, in alkaline conditions. Applying the Al-Co2Fe(CN)6/NF to a whole-urea electrolysis system, 10 mA cm-2 was achieved at a cell voltage of 1.357 V, which saved 11.2% electricity energy compared to that of traditional water splitting. Density functional theory calculations demonstrated that the boosted UOR activity comes from Co sites with Al-doped electronic environments. This promoted and balanced the adsorption/desorption of the main intermediates in the UOR process. This work indicates that Co-based materials as efficient catalysts have great prospects for application in urea electrolysis systems and are expected to achieve low-cost and energy-saving H2 production.

In situ grown high-valence Mo-doped NiCo Prussian blue analogue for enhanced urea electrooxidation

Mo-doped NiCo Prussian blue analogue (PBA) electrocatalysts self-supported on Ni foam are elaborately designed, which exhibit a low potential of 1.358 V (vs. RHE) to reach 100 mA cm-2 for catalyzing the urea oxidation reaction (UOR). The incorporation of high-valence Mo (+6) modifies the electronic structure and improves the electron transfer ability. Using X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS) techniques, we confirm the effect of Mo doping on the NiCo PBA electronic structure.

In-Site Grown NiFeOOH Nanosheets Foam Directly as Robust Electrocatalyst for Efficient Urea Oxidation Application

In this work, a series of morphology-controlled NiFeOOH nanosheets were directly developed through a one-step mild in-situ acid-etching hydrothermal process. Benefiting from the ultrathin interwoven geometric structure and most favorable electron transport structure, the NiFeOOH nanosheets synthesized under 120 °C (denoted as NiFe_120) exhibited the optimal electrochemical performance for urea oxidation reaction (UOR). An overpotential of merely 1.4 V was required to drive the current density of 100 mA cm-2 , and the electrochemical activity remains no change even after 5000 cycles' accelerated degradation test. Moreover, the assembled urea electrolysis set by using the NiFe_120 as bifunctional catalysts presented a reduced potential of 1.573 V at 10 mA cm-2 , which was much lower than that of overall water splitting. We believe this work will lay a foundation for developing high-performance urea oxidation catalysts for the large-scale production of hydrogen and purification of urea-rich sewage.