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.

Artificial Heterointerfaces with Regulated Charge Distribution of Ni Active Sites for Urea Oxidation Reaction

In contrast to the thermodynamically unfavorable anodic oxygen evolution reaction, the electrocatalytic urea oxidation reaction (UOR) presents a more favorable thermodynamic potential. However, the practical application of UOR has been hindered by sluggish kinetics. In this study, hierarchical porous nanosheet arrays featuring abundant Ni-WO3 heterointerfaces on nickel foam (Ni-WO3/NF) is introduced as a monolith electrode, demonstrating exceptional activity and stability toward UOR. The Ni-WO3/NF catalyst exhibits unprecedentedly rapid UOR kinetics (200 mA cm-2 at 1.384 V vs. RHE) and a high turnover frequency (0.456 s-1), surpassing most previously reported Ni-based catalysts, with negligible activity decay observed during a durability test lasting 150 h. Ex situ X-ray photoelectron spectroscopy and density functional theory calculations elucidate that the WO3 interface significantly modulates the local charge distribution of Ni species, facilitating the generation of Ni3+ with optimal affinity for interacting with urea molecules and CO2 intermediates at heterointerfaces during UOR. This mechanism accelerates the interfacial electrocatalytic kinetics. Additionally, in situ Fourier transform infrared spectroscopy provides deep insights into the substantial contribution of interfacial Ni-WO3 sites to UOR electrocatalysis, unraveling the underlying molecular-level mechanisms. Finally, the study explores the application of a direct urea fuel cell to inspire future practical implementations.

Modulating Electronic Structure by Etching Strategy to Construct NiSe2 /Ni0.85 Se Heterostructure for Urea-Assisted Hydrogen Evolution Reaction

Exploring and developing novel strategies for constructing heterostructure electrocatalysts is still challenging for water electrolysis. Herein, a creative etching treatment strategy is adopted to construct NiSe2 /Ni0.85 Se heterostructure. The rich heterointerfaces between NiSe2 and Ni0.85 Se emerge strong electronic interaction, which easily induces the electron transfer from NiSe2 to Ni0.85 Se, and tunes the charge-state of NiSe2 and Ni0.85 Se. In the NiSe2 /Ni0.85 Se heterojunction nanomaterial, the higher charge-state Ni0.85 Se is capable of affording partial electrons to combine with hydrogen protons, inducing the rapid formation of H2 molecule. Accordingly, the lower charge-state NiSe2 in the NiSe2 /Ni0.85 Se heterojunction nanomaterial is more easily oxidized into high valence state Ni3+ during the oxygen evolution reaction (OER) process, which is beneficial to accelerate the mass/charge transfer and enhance the electrocatalytic activities towards OER. Theoretical calculations indicate that the heterointerfaces are conducive to modulating the electronic structure and optimizing the adsorption energy toward intermediate H* during the hydrogen evolution reaction (HER) process, leading to superior electrocatalytic activities. To expand the application of the NiSe2 /Ni0.85 Se-2h electrocatalyst, urea is served as the adjuvant to proceed with the energy-saving hydrogen production and pollutant degradation, and it is proven to be a brilliant strategy.

Ferric Oxide Nanocrystals-Embedded Co/Fe-MOF with Self-Tuned d-Band Centers for Boosting Urea-Assisted Overall Water Splitting

A novel semiconductive Co/Fe-MOF embedded with Fe2 O3 nanocrystals (Fe2 O3 @CoFe-MOF) is developed as a trifunctional electrocatalyst for the urea oxidation reaction (UOR), oxygen evolution reaction (OER), and hydrogen evolution reaction for enhancing the efficiency of the hydrogen production via the urea-assisted overall water splitting. Fe2 O3 @CoFe-TPyP-MOF comprises unsaturated metal-nitrogen coordination sites, affording enriched defects, self-tuned d-band centers, and efficient π-π interaction between different layers. Density functional theory calculation confirms that the adsorption of urea can be optimized at Fe2 O3 @CoFe-TPyP-MOF, realizing the efficient adsorption of intermediates and desorption of the final product of CO2 and N2 characterized by the in situ Fourier transform infrared spectroscopy. The two-electrode urea-assisted water splitting device-assembled with Fe2 O3 @CoFe-TPyP-MOF illustrates a low cell voltage of 1.41 V versus the reversible hydrogen electrode at the current density of 10 mA cm-2 , attaining the hydrogen production rate of 13.13 µmol min-1 in 1 m KOH with 0.33 m urea. The in situ electrochemical Raman spectra and other basic characterizations of the used electrocatalyst uncover that Fe2 O3 @CoFe-TPyP-MOF undergoes the reversible structural reconstruction after the UOR test, while it demonstrates the irreversible reconstruction after the OER measurement. This work redounds the progress of urea-assisted water spitting for hydrogen production.

Manipulation of Electron Spins with Oxygen Vacancy on Amorphous/Crystalline Composite-Type Catalyst

By substituting the oxygen evolution reaction (OER) with the anodic urea oxidation reaction (UOR), it not only reduces energy consumption for green hydrogen generation but also allows purification of urea-rich wastewater. Spin engineering of the d orbital and oxygen-containing adsorbates has been recognized as an effective pathway for enhancing the performance of electrocatalysts. In this work, we report the fabrication of a bifunctional electrocatalyst composed of amorphous RuO2-coated NiO ultrathin nanosheets (a-RuO2/NiO) with abundant amorphous/crystalline interfaces for hydrogen evolution reaction (HER) and UOR. Impressively, only 1.372 V of voltage is required to attain a current density of 10 mA cm-2 over a urea electrolyzer. The increased oxygen vacancies in a-RuO2/NiO by incorporation of amorphous RuO2 enhance the total magnetization and entail numerous spin-polarized electrons during the reaction, which speeds up the UOR reaction kinetics. The density functional theory study reveals that the amorphous/crystalline interfaces promote charge-carrier transfer, and the tailored d-band center endows the optimized adsorption of oxygen-generated intermediates. This kind of oxygen vacancy induced spin-polarized electrons toward boosting HER and UOR kinetics and provides a reliable reference for exploration of advanced electrocatalysts.

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.

Amorphous Ni(OH)2 -Ni3 S2 /NF nano-flower heterostructure catalyst promotes efficient urea assisted overall water splitting

Urea assisted overall water splitting represents a cost-effective and efficient technology for hydrogen production, which not only obviates the generation of explosive H2 and O2 gas mixture but also minimizes the energy cost for the water splitting. In this study, we employed a one-pot hydrothermal method to directly synthesize Ni(OH)2 -Ni3 S2 /NF hybrid nanoflowers on a nickel foam (NF) substrate, resulting in efficient and stable bi-functional electrocatalysts for urea oxidation reaction (UOR) and hydrogen evolution reaction (HER). Under alkaline conditions, the Ni(OH)2 -Ni3 S2 /NF catalyst exhibits low voltage requirements of 1.346 V and -0.014 V vs. RHE with a current density of 10 mA cm-2 for UOR and HER, respectively. Furthermore, when employing the Ni(OH)2 -Ni3 S2 /NF catalyst as both anode and cathode for urea-assisted overall water splitting, it requires a cell voltage of merely 1.396 V with a current density of 10 mA cm-2 , which is notably lower than the voltage required for complete water decomposition at the same current density (1.568 V vs. RHE). The one-step synthesis of the Ni(OH)2 -Ni3 S2 /NF catalyst lays a foundation for further exploration of other transition metal complexes as dual-function electrocatalysts, enabling energy-efficient electrolytic hydrogen production and the treatment of urea-rich wastewater.

Engineering Morphology and Electron Redistribution of a Ni/WO3 Mott-Schottky Bifunctional Electrocatalyst for Efficient Alkaline Urea Splitting

Construction of the desired morphology and nanointerface to expose the active sites and modulate the electronic structure offers an effective approach to boosting urea splitting for energy-saving hydrogen generation. Herein, we fabricate a Ni/WO3 Mott-Schottky heterojunction electrocatalyst with a hedgehog-like structure supported on Ni foam toward alkaline urea splitting. Different Ni/WO3 morphologies, such as microspheres, hedgehog-like structures, octahedrons, and cubes, were obtained when various ratios of Ni/W feeds were used. The Mott-Schottky nanointerfaces between Ni and WO3 domains are visually confirmed by high-resolution transmission electron microscopy images, which also accelerated the charge transfer rate. Benefiting from the high electrochemically active surface area and enhanced charge transferability, the optimal Ni/WO3 electrode exhibits outstanding catalytic activity toward hydrogen generation with a low overpotential of 163 mV at 100 mA cm-2 in alkaline solution and reduced cell voltage of 1.67 V when coupled with urea oxidation reaction. Theoretical calculations reveal that the Ni sites in Ni/WO3 optimize the H adsorption energy (ΔGH*) with the |ΔGH*| value of 0.097 eV, much lower than that of Ni (0.35 eV) and WO3 (0.235 eV). This work demonstrates important guidance in designing an efficient electrocatalyst for urea splitting.

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.

Anchoring Polydopamine on ZnCo2 O4 Nanowire To Facilitate Urea Water Electrolysis

To overcome the sluggishness of the oxygen evolution reaction (OER), the urea oxidation reaction was developed. In the case of OER application studies ZnCo2 O4 is an excellent electrocatalyst, towards the UOR has been performed with surface-grown polydopamine (PDA) with surface-grown polydopamine (PDA). ZnCo2 O4 @PDA is produced over the surface of nickel foam by a hydrothermal method followed by self-polymerization of dopamine hydrochloride. Dopamine hydrochloride was varied in solution to study the optimal growth of PDA necessary to enhance the electrochemical activity. Prepared ZnCo2 O4 @PDA was characterized by X-ray diffraction, electronic structural, and morphology/microstructure studies. With successful confirmation, the developed electrode material was applied to UOR and ZnCo2 O4 @PDA-1.5, delivering an excellent low overpotential of 80 mV at 20 mA cm-2 in the electrolyte mixture of 1 M potassium hydroxide+0.33 M urea. To support the excellent UOR activity, other electrochemical properties such as the Tafel slope, electrochemical surface active sites, and electrochemical impedance spectroscopy were also studied. Furthermore, a schematic illustration explaining the UOR mechanism is shown to allow a clear understanding of the obtained electrochemical activity. Finally, urea water electrolysis was carried out in a two-electrode symmetrical cell and compared with water electrolysis. This clearly showed the potential of the developed material for efficient electrochemical hydrogen production.

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.

Copper Phosphide Nanowires as High-Performance Catalysts for Urea-Assisted Hydrogen Evolution in Alkaline Medium

Water electrolysis represented a promising avenue for the large-scale production of high-purity hydrogen. However, the high overpotential and sluggish reaction rates associated with the anodic oxygen evolution reaction (OER) posed significant obstacles to efficient water splitting. To tackle these challenges, the urea oxidation reaction (UOR) emerged as a more favorable thermodynamic alternative to OER, offering both the energy-efficient hydrogen evolution reaction (HER) and the potential for the treating of urea-rich wastewater. In this work, a two-step methodology comprising nanowire growth and phosphating treatment was employed to fabricate Cu₃P nanowires on Cu foam (Cu₃P-NW/CF) catalysts. These novel catalytic architectures exhibited notable efficiencies in facilitating both the UOR and HER in alkaline solutions. Specifically, within urea-containing electrolytes, the UOR manifested desirable operational potentials of 1.43 V and 1.65 V versus the reversible hydrogen electrode (vs. RHE) to reach the current densities of 10 and 100 mA cm^-2, respectively. Concurrently, the catalyst displayed a meager overpotential of 60 mV for the HER at a current density of 10 mA cm^-2. Remarkably, the two-electrode urea electrolysis system, exploiting the designed catalyst as both the cathode and anode, demonstrated an outstanding performance, attaining a low cell voltage of 1.79 V to achieve a current density of 100 mA cm^-2. Importantly, this voltage is preferable to the conventional water electrolysis threshold in the absence of urea molecules. Moreover, our study shed light on the potential of innovative Cu-based materials for the scalable fabrication of electrocatalysts, energy-efficient hydrogen generation, and the treatment of urea-rich wastewater.

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

Urea oxidation reaction (UOR) is an ideal replacement of the conventional anodic oxygen evolution reaction (OER) for efficient hydrogen production due to the favorable thermodynamics. However, the UOR activity is severely limited by the high oxidation potential of Ni-based catalysts to form Ni³⁺ , which is considered as the active site for UOR. Herein, by using in situ cryoTEM, cryo-electron tomography, and in situ Raman, combined with theoretical calculations, a multistep dissolution process of nickel molybdate hydrate is reported, whereby NiMoO₄ ·xH₂ O nanosheets exfoliate from the bulk NiMoO₄ ·H₂ O nanorods due to the dissolution of Mo species and crystalline water, and further dissolution results in superthin and amorphous nickel (II) hydroxide (ANH) flocculus catalyst. Owing to the superthin and amorphous structure, the ANH catalyst can be oxidized to NiOOH at a much lower potential than conventional Ni(OH)₂ and finally exhibits more than an order of magnitude higher current density (640 mA cm^-2 ), 30 times higher mass activity, 27 times higher TOF than those of Ni(OH)₂ catalyst. The multistep dissolution mechanism provides an effective methodology for the preparation of highly active amorphous catalysts.

Direct Urea/H2O2 Fuel Cell with a Hierarchical Porous Nanoglass Anode for High-Efficiency Energy Conversion

Direct urea/H₂O₂ fuel cells (DUFCs) constitute a sustainable bifunctional energy conversion technique devoted to simultaneously eliminating environmental wastewater with urea and generating clean energy. However, exploring an efficient anode material for DUFCs still remains a huge challenge. In this work, a Ni-P hierarchical porous nanoglass (HPNG) catalytic electrode was developed via a low-cost, industrially available electrodeposition technique, which exhibits one of the best performances reported so far in the urea oxidation reaction (UOR), with a potential of 1.330 V at a current density of 10 mA cm^-2 and a Tafel slope of 9.77 mV dec^-1. The superior UOR performance of the HPNG electrode is attributed to the excellent intrinsic catalytic activity of NG with a high-energy state and an extremely enlarged surface area from the unique 3D hierarchical porous structure. Furthermore, a DUFC system with the HPNG anode shows a performance breakthrough as indicated by the maximum power density of 38.15 mW cm^-2 for 0.5 M urea, representing one of the best yet reported DUFCs. Our work demonstrates the feasibility of the scalable production of HPNG electrodes and is expected to be a great contribution to the development of the practical use of DUFCs in the near future for bifunctional energy conversion.

3D Hierarchical-Architectured Nanoarray Electrode for Boosted and Sustained Urea Electro-Oxidation

Exploring active and durable Ni-based materials with optimized electronic and architectural engineering to promote the urea oxidation reaction (UOR) is pivotal for the urea-related technologies. Herein a 3D self-supported hierarchical-architectured nanoarray electrode (CC/MnNi@NC) is proposed in which 1D N-doped carbon nanotubes (N-CNTs) with 0D MnNi nanoparticles (NPs) encapsulation are intertwined into 2D nanosheet aligned on the carbon cloth for prominently boosted and sustained UOR electrocatalysis. From combined experimental and theoretical investigations, Mn-alloying can regulate Ni electronic state with downshift of the d-band center, facilitating active Ni³⁺ species generation and prompting the rate-determining step (*COO intermediate desorption). Meanwhile, the micro/nano-hierarchical nanoarray configuration with N-CNTs encapsulating MnNi NPs can not only endow strong operational durability against metal corrosion/agglomeration and enrich the density of active sites, but also accelerate electron transfer, and more intriguingly, promote mass transfer as a result of desirable superhydrophilic and quasi-superaerophobic characteristics. Therefore, with such elegant integration of 0D, 1D and 2D motifs into 3D micro/nano-hierarchical architecture, the resulting CC/MnNi@NC can deliver admirable UOR performance, favorably comparable to the best-performing UOR electrocatalysts reported thus far. This work opens a fresh prospect in developing advanced electrocatalysts via electronic manipulation coupled with architectural engineering for various energy conversion technologies.

Phase Segregation in Cu0.5 Ni0.5 Alloy Boosting Urea-Assisted Hydrogen Production in Alkaline Media

Coupling urea oxidation reaction (UOR) and hydrogen evolution reaction (HER) is promising for energy-efficient hydrogen production. However, developing cheap and highly active bifunctional electrocatalysts for overall urea electrolysis remains challenging. In this work, a metastable Cu_0.5 Ni_0.5 alloy is synthesized by a one-step electrodeposition method. It only requires the potentials of 1.33 and -28 mV to obtain the current density of ±10 mA cm^-2 for UOR and HER, respectively. The metastable alloy is considered to be the main reason causing the above excellent performances. In the alkaline medium, the as-prepared Cu_0.5 Ni_0.5 alloy exhibits good stability for HER; and conversely, NiOOH species can be rapidly formed during the UOR due to the phase segregation of Cu_0.5 Ni_0.5 alloy. In particular, for the energy-saving hydrogen generation system coupled with HER and UOR, only 1.38 V of voltage is needed at 10 mA cm^-2 ; and at 100 mA cm^-2 , the voltage decreases by ≈305 mV compared with that of the routine water electrolysis system (HER || OER). Compared with some catalysts reported recently, the Cu_0.5 Ni_0.5 catalyst owns superior electrocatalytic activity and durability. Furthermore, this work provides a simple, mild, and rapid method for designing highly active bifunctional electrocatalysts toward urea-supporting overall water splitting.

Dual-Atom Support Boosts Nickel-Catalyzed Urea Electrooxidation

Nickel-based catalysts have been regarded as one of the most promising electrocatalysts for urea oxidation reaction (UOR), however, their activity is largely limited by the inevitable self-oxidation reaction of Ni species (NSOR) during the UOR. Here, we proposed an interface chemistry modulation strategy to trigger the occurrence of UOR before the NSOR via constructing a 2D/2D heterostructure that consists of ultrathin NiO anchored Ru-Co dual-atom support (Ru-Co DAS/NiO). Operando spectroscopic characterizations confirm this unique triggering mechanism on the surface of Ru-Co DAS/NiO. Consequently, the fabricated catalyst exhibits outstanding UOR activity with a low potential of 1.288 V at 10 mA cm-2 and remarkable long-term durability for more than 330 h operation. DFT calculations and spectroscopic characterizations demonstrate that the favorable electronic structure induced by this unique heterointerface endows the catalyst energetically more favorable for the UOR than the NSOR.

Preparation of 3D Nd2O3-NiSe-Modified Nitrogen-Doped Carbon and Its Electrocatalytic Oxidation of Methanol and Urea

Developing renewable energy sources and controlling water pollution are critical but challenging problems. Urea oxidation (UOR) and methanol oxidation (MOR), both of which have high research value, have the potential to effectively address wastewater pollution and energy crisis problems. A three-dimensional neodymium-dioxide/nickel-selenide-modified nitrogen-doped carbon nanosheet (Nd₂O₃-NiSe-NC) catalyst is prepared in this study by using mixed freeze-drying, salt-template-assisted technology, and high-temperature pyrolysis. The Nd₂O₃-NiSe-NC electrode showed good catalytic activity for MOR (peak current density ~145.04 mA cm^-2 and low oxidation potential ~1.33 V) and UOR (peak current density ~100.68 mA cm^-2 and low oxidation potential ~1.32 V); the catalyst has excellent MOR and UOR characteristics. The electrochemical reaction activity and the electron transfer rate increased because of selenide and carbon doping. Moreover, the synergistic action of neodymium oxide doping, nickel selenide, and the oxygen vacancy generated at the interface can adjust the electronic structure. The doping of rare-earth-metal oxides can also effectively adjust the electronic density of nickel selenide, allowing it to act as a cocatalyst, thus improving the catalytic activity in the UOR and MOR processes. The optimal UOR and MOR properties are achieved by adjusting the catalyst ratio and carbonization temperature. This experiment presents a straightforward synthetic method for creating a new rare-earth-based composite catalyst.

Formation and Stabilization of NiOOH by Introducing α-FeOOH in LDH: Composite Electrocatalyst for Oxygen Evolution and Urea Oxidation Reactions

NiOOH is considered as the most active intermediate during electrochemical oxidation reaction, however, it is hard to directly synthesize due to high oxidation energy. Herein, theoretical calculations predict that α-FeOOH enables a decline in formation energy and an improvement in stabilization of NiOOH in NiFe-based layered double hydroxide (LDH). Inspiringly, a composite composed of α-FeOOH and LDH is well-designed and successfully fabricated in hydrothermal treatment by adding extra Fe³⁺ resource, and stable NiOOH is obtained by the following electro-oxidation method. Benefiting from strong electron-capturing capability of α-FeOOH, it efficiently promotes charge redistribution around the Ni/Fe sites and activates Ni atoms of LDH, verified by X-ray photoelectron spectra (XPS) and X-ray absorption spectra (XAS). The d-band center is optimized that balances the absorption and desorption energy, and thus Gibbs free energy barrier is lowered dramatically toward oxygen evolution reaction (OER) and urea oxidation reaction (UOR), and finally showing an outstanding overpotential of 195 mV and a potential of 1.35 V at 10 mA cm^-2 , respectively. This study provides a novel strategy to construct highly efficient catalysts via the introduction of a new phase for complex multiple-electron reactions.

Self-Supporting Metal-Organic Framework-Based Nanoarrays for Electrocatalysis

The replacement of powdery catalysts with self-supporting alternatives for catalyzing various electrochemical reactions is extremely important for the large-scale commercial application of renewable energy storage and conversion technologies. Metal-organic framework (MOF)-based nanoarrays possess tunable compositions, well-defined structure, abundant active sites, effective mass and electron transport, etc., which enable them to exhibit superior electrocatalytic performance in multiple electrochemical reactions. This review presents the latest research progress in developing MOF-based nanoarrays for electrocatalysis. We first highlight the structural features and electrocatalytic advantages of MOF-based nanoarrays, followed by a detailed summary of the design and synthesis strategies of MOF-based nanoarrays, and then describe the recent progress of their application in various electrocatalytic reactions. Finally, the challenges and perspectives are discussed, where further exploration into MOF-based nanoarrays will facilitate the development of electrochemical energy conversion technologies.

Ni-Doped CuO Nanoarrays Activate Urea Adsorption and Stabilizes Reaction Intermediates to Achieve High-Performance Urea Oxidation Catalysts

Urea oxidation reaction (UOR) with a low equilibrium potential offers a promising route to replace the oxygen evolution reaction for energy-saving hydrogen generation. However, the overpotential of the UOR is still high due to the complicated 6e^- transfer process and adsorption/desorption of intermediate products. Herein, utilizing a cation exchange strategy, Ni-doped CuO nanoarrays grown on 3D Cu foam are synthesized. Notably, Ni-CuO NAs/CF requires a low potential of 1.366 V versus a reversible hydrogen electrode to drive a current density of 100 mA cm^-2 , outperforming various benchmark electrocatalysts and maintaining robust stability in alkaline media. Theoretical and experimental studies reveal that Ni as the driving force center can effectively enhance the urea adsorption and stabilize CO*/NH* intermediates toward the UOR. These findings suggest a new direction for constructing nanostructures and modulating electronic structures, ultimately developing promising Cu-based electrode catalysts.

Hollow NiCoP Nanoprisms Derived from Prussian Blue Analogues as Bifunctional Electrocatalysts for Urea-Assisted Hydrogen Production in Alkaline Media

Integrating the hydrogen evolution reaction (HER) and urea oxidation reaction (UOR) is an energy-saving approach for electrolytic H₂ production. Here, hollow NiCoP nanoprisms are derived from Prussian blue analogues by a combined self-template coordination reaction and gas-phase phosphorization strategy. Benefiting from the strong electron interaction, unique hollow nanostructure, and enhanced mass/charge transfer, NiCoP nanoprisms display outstanding alkaline HER and UOR performance. Specifically, low potentials of -0.052, -0.115, and -0.159 V for HER and ultralow potentials of 1.30, 1.36, and 1.42 V for UOR at current densities of 10, 50, and 100 mA cm^-2 are obtained. Moreover, in a urea-assisted water electrolysis system, NiCoP nanoprisms only require cell voltages of 1.36, 1.49, and 1.57 V to offer current densities of 10, 50, and 100 mA cm^-2 , about 170, 180, and 200 mV less than the traditional water electrolysis. Theoretical calculations indicate the Co substitution in Ni₂ P promotes the adsorption and dissociation of water molecules, optimizes the desorption energy of active hydrogen atoms, and enhances the adsorption of urea molecules, thus accelerating the kinetics of HER and UOR. This work facilitates the application of hollow bimetallic phosphides in electrochemical preparation of clean energy and provides a successful paradigm for urea-rich wastewater electrolysis.

Crystalline-Amorphous Ni3S2-NiMoO4 Heterostructure for Durable Urea Electrolysis-Assisted Hydrogen Production at High Current Density

Developing bifunctional catalysts with good performance at a high current density for the urea oxidation reaction (UOR) and the hydrogen evolution reaction (HER) can effectively relieve the severe environmental and energy pressures. Herein, amorphous NiMoO₄ decorated Ni₃S₂ grown on nickel foam (Ni₃S₂-NiMoO₄/NF) is prepared to accelerate UOR and HER. The crystalline-amorphous heterostructure could regulate the interfacial electron structure to reduce the electron density near Ni₃S₂ for optimizing UOR and HER. The decoration of NiMoO₄ enhances its anti-poisoning ability for CO-intermediate species to show good stability at high current densities. Meanwhile, the nano-/microstructure with high hydrophilicity improves mass transfer and the accessibility of electrolyte. Driving high current densities of ±1000 mA cm^-2, it merely needs 1.38 V (UOR) and -263 mV (HER). For urea electrolysis, it can deliver 1000 mA cm^-2 at 1.73 V and stably operate at 500 mA cm^-2 for 120 h. Therefore, this study provides new ideas for durable urea electrolysis-assisted H₂ production.

High-Entropy Oxide Derived from Metal-Organic Framework as a Bifunctional Electrocatalyst for Efficient Urea Oxidation and Oxygen Evolution Reactions

High-entropy oxides (HEOs) offer unique features through a combination of incompatible metal cations to a single crystalline lattice. Owing to their special characteristics such as abundant cation compositions, high entropy stabilization, chemical and thermal stability, and lattice distortion effect, they have drawn ever-increasing attention for various applications. However, very few studies have been reported for catalytic application, and developing HEOs with large surface areas for efficient catalytic application is still in infancy. Herein, we design nanostructured HEO of (FeNiCoCrCu)₃O₄ using metal-organic frameworks (MOFs) as sacrificial templates to achieve a large surface area, high density of exposed active sites, and more oxygen vacancies. Single-crystalline phase HEOs with surface area as large as 206 m² g^-1 are produced and further applied as bifunctional electrocatalysts for the urea oxidation reaction (UOR) and oxygen evolution reaction (OER). Benefiting from enhanced oxygen vacancies and a large surface area with abundant exposed active sites, the optimized HEO exhibited excellent electrocatalytic activity toward UOR with a very low potential of 1.35 V at the current density of 10 mA cm^-2 and showed long-term stability for 36 h operation, making a significant catalytic performance over previously reported HEOs. Moreover, the HEO demonstrated an efficient catalytic performance toward OER with a low overpotential of 270 mV at 10 mA cm^-2 and low Tafel slope of 49 mV dec^-1. The excellent catalytic activity is ascribed to the starting MOF precursor and favorable high-entropy effect.

Nanostructured Metallic Glass in a Highly Upgraded Energy State Contributing to Efficient Catalytic Performance

Metallic glasses (MGs), with high density of low coordination sites and high Gibbs free energy state, are novel promising and competitive candidates in the family of electrochemical catalysts. However, it remains a grand challenge to modify the properties of MGs by control of the disordered atomic structure. Recently, nanostructured metallic glasses (NGs), consisting of amorphous nanometer-sized grains connected by amorphous interfaces, have been reported to exhibit tunable properties compared to the MGs with identical chemical composition. Here, it is demonstrated that electrodeposited Ni-P NG is characterized by an extremely high energy state due to its heterogeneous structure, which significantly promotes the catalytic performance. Moreover, the Ni-P NG with a heterogeneous structure is a perfect precursor for the fabrication of unique honey-like nanoporous structure, which displays superior catalytic performance in the urea oxidation reaction (UOR). Specifically, modified Ni-P NG requires a potential of mere 1.36 V at 10 mA cm^-2 , with a Tafel slope of 13 mV dec^-1 , which is the best UOR performance in Ni-based alloys. The present work demonstrates that the nanostructurization of MGs provides a universal and effective pathway to upgrade the energy state of MGs for the design of high-performance catalysts in energy conversion.

CoP Nanoparticle Confined in P, N Co-Doped Porous Carbon Anchored on P-Doped Carbonized Wood Fibers with Tailored Electronic Structure for Efficient Urea Electro-Oxidation

Electronic structure optimization and architecture modulation are widely regarded as rational strategies to enhance the electrocatalysts catalytic performance. Herein, a hybridization of ZIF-67-derived CoP nanoparticles embedded in P, N co-doped carbon matrix (PNC) and anchored on P-doped carbonized wood fibers (PCWF) is constructed using a simple simultaneous phosphorization and carbonization strategy. Benefiting from the optimized surface/interface electronic structures, abundant exposed active sites, and outstanding conductivity, the CoP@PNC/PCWF can drive the urea oxidation reaction (UOR) with greater activity and better stability than most recently reported electrocatalysts, in which a potential as low as 1.32 V (vs reversible hydrogen electrode, RHE) is needed to reach 50 mA cm^-2 and shows excellent durability. Furthermore, for overall urea splitting, using the CoP@PNC/PCWF electrocatalyst as the anode and commercial Pt/C supported on nickel foam as the cathode, an ultralow cell voltage of 1.50 V (vs RHE) is expected to achieve the 50 mA cm^-2 and operate continuously for more than 50 h at 20 mA cm^-2 . The reported strategy may shed light on the use of renewable resources to design and synthesize high-performance non-Ni-based phosphides UOR electrocatalysts for energy-saving H₂ production.

Hierarchical Structure of CuO Nanowires Decorated with Ni(OH)2 Supported on Cu Foam for Hydrogen Production via Urea Electrocatalysis

Owing to the low theoretical potential of the urea oxidation reaction (UOR), urea electrolysis is an energy-saving technique for the generation of hydrogen. Herein, a hierarchical structure of CuO nanowires decorated with nickel hydroxide supported on 3D Cu foam is constructed. Combined theoretical and experimental analyses demonstrate the high reactivity and selectivity of CuO and Ni(OH)₂ toward the UOR instead of the oxygen evolution reaction. The hierarchical structure creates a synergistic effect between the two highly active sites, enabling an exceptional UOR activity with a record low potential of 1.334 V (vs the reversible hydrogen electrode) to reach 100 mA cm^-2 and a low Tafel slope of 14 mV dec^-1 in 1 m KOH and 0.5 m urea electrolyte. Assembling full urea electrolysis driven by this developed UOR electrocatalyst as the anode and a commercial Pt/C electrocatalyst as the cathode provides a current density of 20 mA cm^-2 at a cell voltage of ≈1.36 V with promising operational stability for at least 150 h. This work not only enriches the UOR material family but also significantly advances energy-saving hydrogen production.

Ni3S2/Ni Heterostructure Nanobelt Arrays as Bifunctional Catalysts for Urea-Rich Wastewater Degradation

Urea electrolysis is a cost-effective method for urea-rich wastewater degradation to achieve a pollution-free environment. In this work, the Ni₃S₂/Ni heterostructure nanobelt arrays supported on nickel foam (Ni₃S₂/Ni/NF) are synthesized for accelerating the urea oxidation reaction (UOR) and hydrogen evolution reaction (HER). It only needs ultralow potentials of 1.30 V and -54 mV to achieve the current density of ±10 mA cm^-2 for UOR and HER, respectively. Meanwhile, the overall urea oxidation driven by Ni₃S₂/Ni/NF only needs 1.36 V to achieve 10 mA cm^-2, and it can remain at 100 mA cm^-2 for 60 h without obvious activity attenuation. The superior performance could be attributed to the heterostructure between Ni₃S₂ and Ni, which can promote electron transfer and form electron-poor Ni species to optimize urea decomposition and hydrogen production. Moreover, the nanobelt self-supported structure could expose abundant active sites. This work thus provides a feasible and cost-effective strategy for urea-rich wastewater degradation and hydrogen production.

Coupling Interface Constructions of FeNi3-MoO2 Heterostructures for Efficient Urea Oxidation and Hydrogen Evolution Reaction

Urea electrolysis has prospects for urea-containing wastewater purification and hydrogen (H₂) production, but the shortage of cost-effective catalysts restricts its development. In this work, the tomentum-like FeNi₃-MoO₂ heterojunction nanosheets array self-supported on nickel foam (NF) as bifunctional catalyst is prepared by facile hydrothermal and annealing method. Only 1.29 V and -50.8 mV is required to obtain ±10 mA cm^-2 for urea oxidation and hydrogen evolution reaction (UOR and HER), respectively, showing great bifunctional catalytic activity. For overall urea electrolysis, it only needs 1.37 V to reach 10 mA cm^-2 and can last at 100 mA cm^-2 for 70 h without obvious activity attenuation, showing outstanding durability. Coupling interface constructions of FeNi₃-MoO₂ heterostructures, novel morphology with a mesoporous and self-supporting structure could be the reason for this good performance. This work thus proposes a promising catalyst for boosting UOR and HER to realize efficient overall urea electrolysis.

Maximized Schottky Effect: The Ultrafine V2 O3 /Ni Heterojunctions Repeatedly Arranging on Monolayer Nanosheets for Efficient and Stable Water-to-Hydrogen Conversion

The Mott-Schottky heterojunction formed at the interface of ultrafine metallic Ni and semiconducting V₂ O₃ nanoparticles is constructed, and the heterojunctions are "knitted" into the tulle-like monolayer nanosheets on nickel foam (NF). The greatly reduced particle sizes of both Ni and V₂ O₃ on the Mott-Schottky heterojunction highly enhance the number of Schottky heterojunctions per unit area of the materials. Moreover, arranging the heterojunctions into the monolayer nanosheets makes the heterojunctions repeat and expose to the electrolyte sufficiently. The Schottky heterojunctions are like countless self-powered charge transfer workstations embedded in the tulle-like monolayer nanosheets, promoting maximum of the materials to participate into the electron transfer and become catalytic active sites. In addition, the tulle-like monolayer nanosheet structure can assist in pumping liquid phase electrolyte to the surface of catalysts, owing to the capillary force. The V₂ O₃ /Ni/NF Mott-Schottky catalyst exhibits excellent hydrogen evolution reaction (HER) performance with a low η₁₀ of 54 mV and needs -107 mV to get the current density of -100 mA cm^-2 . Furthermore, V₂ O₃ /Ni/NF Schottky electrocatalyst exhibits excellent urea oxidation reaction activity: 1.40, 1.51, and 1.61 V versus reversible hydrogen electrode (RHE) voltage are required to reach a current density of 100, 500, and 1000 mA cm^-2 , respectively.

Regulating the Local Charge Distribution of Ni Active Sites for the Urea Oxidation Reaction

In electrochemical energy storage and conversion systems, the anodic oxygen evolution reaction (OER) accounts for a large proportion of the energy consumption. The electrocatalytic urea oxidation reaction (UOR) is one of the promising alternatives to OER, owing to its low thermodynamic potential. However, owing to the sluggish UOR kinetics, its potential in practical use has not been unlocked. Herein, we developed a tungsten-doped nickel catalyst (Ni-WO_x ) with superior activity towards UOR. The Ni-WO_x catalyst exhibited record fast reaction kinetics (440 mA cm^-2 at 1.6 V versus reversible hydrogen electrode) and a high turnover frequency of 0.11 s^-1 , which is 4.8 times higher than that without W dopants. In further experiments, we found that the W dopant regulated the local charge distribution of Ni atoms, leading to the formation of Ni³⁺ sites with superior activity and thus accelerating the interfacial catalytic reaction. Moreover, when we integrated Ni-WO_x into a CO₂ flow electrolyzer, the cell voltage is reduced to 2.16 V accompanying with ≈98 % Faradaic efficiency towards carbon monoxide.

Metal-Organic Polymer-Derived Interconnected Fe-Ni Alloy by Carbon Nanotubes as an Advanced Design of Urea Oxidation Catalysts

The electrochemical urea oxidation reaction (UOR) is considered as a promising renewable source for harvesting energy from waste. We report a new synthetic design approach to produce an iron-nickel alloy nanocatalyst from a metal-organic polymer (MOP) by a single-step carbonization process at 500 °C, thus forming a core-shell of iron-nickel-coated carbon (C@FeNi) nanostructures wired by embedded carbon nanotubes (CNTs) (CNT/C@FeNi). Powder X-ray diffraction confirmed the formation of metallic FeNi₃ alloy nanoparticles (∼20 to 28 nm). Our experimental results showed that MOP containing CNTs acquired an interconnected hierarchical topology, which prevented the collapse of its microstructure during pyrolysis. Hence, CNT/C@FeNi shows higher porosity (10 times) than C@FeNi. The electrochemical UOR in alkaline electrolytes on these catalysts was studied using cyclic voltammetry (CV). The result showed a higher anodic current (3.5 mA cm^-2) for CNT/C@FeNi than for C@FeNi (1.1 mA cm^-2) at 1.5 V/RHE. CNT/C@FeNi displayed good stability in chronoamperometry experiments and a lower Tafel slope (33 mV dec^-1) than C@FeNi (41.1 mV dec^-1). In this study, CNT/C@FeNi exhibits higher exchange current density (3.2 μA cm^-2) than does C@FeNi (2 μA cm^-2). The reaction rate orders of CNT/C@FeNi and C@FeNi at a kinetically controlled potential of 1.4 V/RHE were 0.5 and 0.9, respectively, higher than the 0.26 of β-Ni(OH)₂, Ni/Ni(OH)₂ electrodes. The electrochemical impedance result showed a lower charge-transfer resistance for CNT/C@FeNi (61 Ω·cm^-2) than for C@FeNi (162 Ω·cm^-2), due to faster oxidation kinetics associated with the CNT linkage. Moreover, CNT/C@FeNi exhibited a lower Tafel slope and resistance and higher heterogeneity (25.2 × 10^-5 cm s^-1), as well as relatively high faradic efficiency (68.4%) compared to C@FeNi (56%). Thus, the carbon-coated FeNi₃ core connected by CNT facilitates lower charge-transfer resistance and reduces the UOR overpotential.

Coaxial Ni-S@N-Doped Carbon Nanofibers Derived Hierarchical Electrodes for Efficient H2 Production via Urea Electrolysis

Electrochemical water splitting into hydrogen is a promising strategy for hydrogen production powered by solar energy. However, the cell voltage of an electrolyzer is still too high for practical application, which is mainly limited by the sluggish oxygen evolution reaction process. To this end, hybrid water electrolyzers have drawn tremendous attention. Herein, coaxial Ni/Ni₃S₂@N-doped nanofibers are directly grown on nickel foam (NF), which is highly active for hydrogen evolution reaction. Meanwhile, the Ni₃S₂@N-doped nanofibers on NF prepared in an Ar atmosphere display superior urea oxidation reaction performance to previously reported catalysts. The cell voltage is about 1.50 V in urea electrolysis to deliver a current density of 20 mA cm^-2, lower than that of a traditional water electrolyzer (1.82 V). The current density is around 77% relative to its initial value of 20 mA cm^-2 after 20 h, superior to Pt/C|Ir/C-based urea electrolysis (14%). It is found that the synergistic effect between metallic Ni and Ni₃S₂, as well as the interfacial effect between metal centers and N-doped carbon, favors the initial dissociation of H₂O and the adsorption/desorption of H* with thermal neutral Gibbs free energy. Meanwhile, the in-situ generated NiOOH on the outer surface of Ni₃S₂ possessed lower electrochemical activation energy for urea decomposition. Meanwhile, the abundant oxygen vacancies in electrodes could expose more active sites for the adsorption of intermediates, including H* and OOH*. It is also found that the hierarchical nanostructure of densely packed nanowires provides ideal electronic and ionic transport paths for fast electrocatalytic kinetics. The present work indicated that the modulation of compositions and hierarchical nanostructure is effective to prepare efficient catalysts for H₂ production via urea electrolysis.

Oxygen Vacancy-rich Ni/NiO@NC Nanosheets with Schottky Heterointerface for Efficient Urea Oxidation Reaction

H₂ production via electrocatalytic water splitting is greatly hindered by the sluggish oxygen evolution reaction (OER). The urea oxidation reaction (UOR) draws specific attention not only because of its lower theoretical voltage of 0.37 V compared with OER (1.23 V), but also for treating sewage water. Herein, Ni/NiO nanosheets with an ultrathin N-doped C layer containing a Schottky Ni and NiO heterointerface is constructed. Because of the self-driven charge redistribution at the heterointerface, janus charge domains are successfully created to drive the cleavage of urea molecules. Meanwhile, the synergistic effect between N-doped C and Ni/NiO restrains the deactivation of active sites in alkaline solution. The catalyst displays 1.35 V for UOR at 10 mA/cm² , 0.27 V lower than that of OER. The final potential increase is only 2 mV after long-term stability test of 12 h for UOR, much smaller than the uncoated sample (38 mV). The present work shows that C-coated transition metal nanomaterials with oxygen vacancies and a Schottky heterointerface are promising candidates for simultaneously boosting UOR with both high activity and long-term stability.

Manganese-Modulated Cobalt-Based Layered Double Hydroxide Grown on Nickel Foam with 1D-2D-3D Heterostructure for Highly Efficient Oxygen Evolution Reaction and Urea Oxidation Reaction

Hydrogen production by energy-efficient water electrolysis is a green avenue for the development of contemporary society. However, the oxygen evolution reaction (OER) and the urea oxidation reaction (UOR) occurring at the anode are impeded by the sluggish reaction kinetics during the water-splitting process. Consequently, it is promising to develop bifunctional anodic electrocatalysts consisting of nonprecious metals. Herein, a bifunctional CoMn layered double hydroxide (LDH) was grown on nickel foam (NF) with a 1D-2D-3D hierarchical structure for efficient OER and UOR performance in alkaline solution. Owing to the significant synergistic effect of Mn doping and heterostructure engineering, the obtained Co₁ Mn₁ LDH/NF exhibits satisfactory OER activity with a low potential of 1.515 V to attain 10 mA cm^-2 . Besides, the potential of the Co₁ Mn₁ LDH/NF catalyst for UOR at the same current density is only 1.326 V, which is much lower than those of its counterparts and most reported electrocatalysts. An urea electrolytic cell with a Co₁ Mn₁ LDH/NF anode and a Pt-C/NF cathode was established, and a low cell voltage of 1.354 V at 10 mA cm^-2 was acquired. The optimized strategy may result in promising candidates for developing a new generation of bifunctional electrocatalysts for clean energy production.

Designing Advanced Catalysts for Energy Conversion Based on Urea Oxidation Reaction

Urea oxidation reaction (UOR) is the underlying reaction that determines the performance of modern urea-based energy conversion technologies. These technologies include electrocatalytic and photoelectrochemical urea splitting for hydrogen production and direct urea fuel cells as power engines. They have demonstrated great potentials as alternatives to current water splitting and hydrogen fuel cell systems with more favorable operating conditions and cost effectiveness. At the moment, UOR performance is mainly limited by the 6-electron transfer process. In this case, various material design and synthesis strategies have recently been reported to produce highly efficient UOR catalysts. The performance of these advanced catalysts is optimized by the modification of their structural and chemical properties, including porosity development, heterostructure construction, defect engineering, surface functionalization, and electronic structure modulation. Considering the rich progress in this field, the recent advances in the design and synthesis of UOR catalysts for urea electrolysis, photoelectrochemical urea splitting, and direct urea fuel cells are reviewed here. Particular attention is paid to those design concepts, which specifically target the characteristics of urea molecules. Moreover, challenges and prospects for the future development of urea-based energy conversion technologies and corresponding catalysts are also discussed.

Low-Cost Ni2P/Ni0.96S Heterostructured Bifunctional Electrocatalyst toward Highly Efficient Overall Urea-Water Electrolysis

Water splitting is a sustainable approach for production of hydrogen to fuel some clean energy technologies. This process, unfortunately, has been significantly impeded by the puzzles in either the efficient but economically unaffordable noble-metal-based catalysts or the low-cost but kinetically sluggish abundant-element-based catalysts. Particularly, the discovery of efficient bifunctional catalysts that can simultaneously trigger the reactions of both anode and cathode for overall water splitting still remains as a grand challenge. Herein, a novel low-cost bifunctional Ni₂P/Ni_0.96S heterostructured electrocatalyst, which is active for both the urea oxidation reaction at the anode and the hydrogen evolution reaction at the cathode, is innovated for high-efficiency overall splitting of urea-rich wastewater. A systematic configuration of a Ni foam (NF)-supported Ni₂P/Ni_0.96S catalyst electrode exhibits superior catalytic activity and stability. The Ni₂P/Ni_0.96S/NF||Ni₂P/Ni_0.96S/NF cell needs only 1.453 V to reach a current density of 100 mA/cm² in basic urea-containing water, while it is 1.693 V for a reference noble-based Pt/C/NF||IrO₂/NF electrolysis cell. This work therefore not only contributes to develop a low-cost, high-efficiency, bifunctional electrocatalyst but also provides a practically feasible approach for urea-rich wastewater treatment.

Nanostructured Nickel Nitride with Reduced Graphene Oxide Composite Bifunctional Electrocatalysts for an Efficient Water-Urea Splitting

A three-dimensional nickel nitride with reduced graphene oxide composite on nickel foam (s-X, where s represents Ni3N/rGO@NF and the annealing temperature X can be 320, 350, or 380) electrode has been fabricated through a facile method. We demonstrate that s-350 has excellent urea oxidation reaction (UOR) activity, with a demanded potential of 1.342 V to reach 10 mA/cm2 and bears high hydrogen evolution reaction (HER) activity. It provides a low overpotential of 124 mV at 10 mA/cm2, which enables the successful construction of its two-electrode alkaline electrolyzer (s-350||s-350) for water-urea splitting. It merely requires a voltage of 1.518 V to obtain 100 mA/cm2 and is 0.145 V lower than that of pure water splitting. This noble metal-free bifunctional electrode is regarded as an inexpensive and effective water-urea electrolysis assisted hydrogen production technology, which is commercially viable.

A Lattice-Oxygen-Involved Reaction Pathway to Boost Urea Oxidation

The electrocatalytic urea oxidation reaction (UOR) provides more economic electrons than water oxidation for various renewable energy-related systems owing to its lower thermodynamic barriers. However, it is limited by sluggish reaction kinetics, especially by CO₂ desorption steps, masking its energetic advantage compared with water oxidation. Now, a lattice-oxygen-involved UOR mechanism on Ni⁴⁺ active sites is reported that has significantly faster reaction kinetics than the conventional UOR mechanisms. Combined DFT, ¹⁸ O isotope-labeling mass spectrometry, and in situ IR spectroscopy show that lattice oxygen is directly involved in transforming *CO to CO₂ and accelerating the UOR rate. The resultant Ni⁴⁺ catalyst on a glassy carbon electrode exhibits a high current density (264 mA cm^-2 at 1.6 V versus RHE), outperforming the state-of-the-art catalysts, and the turnover frequency of Ni⁴⁺ active sites towards UOR is 5 times higher than that of Ni³⁺ active sites.

[MoS4]2--Intercalated NiCo-Layered Double Hydroxide Nanospikes: An Efficiently Synergized Material for Urine To Direct H2 Generation

Substituting the energy-uphill water oxidation half-cell with readily oxidizable urea-rich urine, a ground-breaking bridge is constructed, combining the energy-efficient hydrogen generation and environmental protection. Hence, designing a robust multifunctional electrocatalyst is desirable for widespread implementation of this waste to fuel technology. In this context, here, we report a simple tuning of the electrocatalytically favorable characteristics of NiCo-layered double hydroxide by introducing [MoS₄]^2- in its interlayer space. The [MoS₄]^2- insertion as well as its effect on the electronic structure tuning is thoroughly studied via X-ray photoelectron spectroscopy in combination with electrochemical analysis. This insertion induces overall electronic structure tuning of the hydroxide layer in such a way that the designed catalyst exhibited favorable kinetics toward all the required reactions of hydrogen generation. This is why our homemade catalyst, when utilized both as a cathode and anode to fabricate a urea electrolyzer, required a mere ∼1.37 V cell potential to generate sufficient H₂ by reaching the benchmark 10 mA cm^-2 in 1 M KOH/0.33 M urea along with long-lasting catalytic efficiency. Other indispensable reason of selecting [MoS₄]^2- is its high-valent nature making the catalyst highly selective and insensitive to common catalyst-poisoning toxins of urine. This is experimentally supported by performing the real urine electrolysis, where the nanospike-covered Ni foam-based catalyst showed a performance similar to that of synthetic urea, offering its industrial value. Other intuition of selecting [MoS₄]^2- was to provide a ligand-based mechanism for hydrogen evolution half-cell [hydrogen evolution reaction (HER)] to preclude the HER-competing oxygen reduction. Another crucial point of our work is its potential to avoid the mixing of two explosive product gases, that is, H₂ and O₂.

Ni3N/NF as Bifunctional Catalysts for Both Hydrogen Generation and Urea Decomposition

Oxygen evolution reaction (OER) has a high overpotential, which can significantly reduce the energy efficiency in water decomposition. Using urea oxidation reaction (UOR) to replace OER has been a feasible and energy-saving approach because of its lower electrode potential. Furthermore, UOR is also an important process in wastewater treatment. This paper successfully synthesizes a high-performance bifunctional catalyst for urea electrolysis. The catalyst is nickel nitride bead-like nanospheres array supported on Ni foam (Ni₃N/NF). Several characterization methods are used to analyze the catalyst's morphology, structure, and composition as well as catalytic activity/stability, including X-ray diffraction, scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, and electrochemical methods (cyclic voltammetry, linear sweep voltammetry, electrochemical impedance spectroscopy, and CAM). A concurrent two-electrode electrolyzer (Ni₃N/NF∥Ni₃N/NF) is constructed and used to validate the catalyst performance, and the results show that the cell achieves 100 mA·cm^-2 at 1.42 V, while the cell voltage of Pt/C∥IrO₂ is 1.60 V, indicating that the Ni₃N/NF catalyst is superior to precious metals.

Multivariate MOF-Templated Pomegranate-Like Ni/C as Efficient Bifunctional Electrocatalyst for Hydrogen Evolution and Urea Oxidation

The production of hydrogen through electrolysis is considered as a feasible strategy to quench the world's clean-energy thirst. Compared with water electrolysis, urea electrolysis presents a more promising prospect in the way that it could carry out sewage treatment as well as energy-efficient hydrogen production at the same time. Herein, highly porous pomegranate-like Ni/C was synthesized from multivariate metal-organic frameworks and exhibits excellent hydrogen evolution activity with an unprecedented low overpotential of 40 mV at the current density of 10 mA cm^-2 in 1 M KOH, ranking among the best earth-abundant electrocatalysts deposited on glassy carbon electrodes reported to date. In addition, it also displays superb anodic urea oxidation activity with an onset potential of 1.33 V vs RHE. Furthermore, a two-electrode urea electrolyzer with Ni/C as both the cathode and anode electrocatalyst was fabricated and generates 52 times more hydrogen than the water electrolyzer under the same conditions.