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The urea oxidation reaction (UOR) is considered as an intriguing substitution of the oxygen evolution reaction (OER) to couple the cathodic hydrogen evolution reaction (HER) to produce H2 with reduced energy in recent years. For UOR and HER processes, it is the key to develop low-cost and high-performance bifunctional electrocatalysts. In this work, a series of metal–organic framework (MOF)-derived bimetallic FeNi-sulfides is successfully synthesized with specific nanoscale hollow cubic morphology by a bimetallic strategy, where different sulfuration times and treatment temperatures are discussed in detail. The results show that the optimal sulfuration time is 2 h under a treatment temperature of 400 °C. As-synthesized FeNiS400-2h exhibits the best electrocatalytic activity with very low overpotentials of 269 mV for OER, 264 mV for HER, and 1.43 V for UOR at a current density of 10 mA cm–2. Particularly, the coupled FeNiS400-2h||Pt/C cell for UOR||HER only needs a significantly low cell voltage of 1.57 V at the current density of 10 mA cm–2, which has exhibited remarkable improvement over other related metal sulfides in previous reports. Therefore, this study provides a rational multifunctional electrode structural design strategy for electrocatalytic water and urea splitting with energy-saving H2 production. [ABSTRACT FROM AUTHOR]

It is important for the practical application of water electrolysis to explore stable and earth-rich bifunctional catalysts for oxygen evolution reaction (OER) and urea oxidation reaction (UOR). An immersion-selenization strategy was proposed to prepare CoSe2/Co nanorods anchored on Co foam as bifunctional catalysts for OER and UOR. Due to the self-supported properties of CoSe2/Co and its unique nanorod structure, the OER activity is enhanced, showing an overpotential of 318 mV and a Tafel slope of 91.11 mV dec–1. In addition, the electrode showed excellent electrocatalytic UOR activity with an overpotential of 260 mV and a Tafel slope of 94.83 mV dec–1. The nanorod structure was basically retained after a 28 h durability test. This work provides a broad approach to the development of low-cost bifunctional electrocatalysts. [ABSTRACT FROM AUTHOR]

Urea-assisted electrolytic water splitting is recognized as a high-efficiency and energy-saving hydrogen production technology because of its advantages of reducing the thermodynamic electrode potential and decomposing pollutant. Furthermore, the reasonably designed heterojunction catalyst can significantly accelerate the kinetic rate of the urea oxidation reaction (UOR). In this paper, a four-component (including NiCo, NiCoP, Ni3Se2, and NiSe) NiCo-based heterostructure (expressed as NixSey/NC-NCP) was successfully synthesized by combining the electrodeposition method and high-temperature selenization strategy, which consists of an interlaced chainlike structure and an interconnected nanosphere structure. As expected, the NixSey/NC-NCP heterostructure only requires a low potential of 1.36 V to enable the UOR to deliver a current density of 100 mA cm–2, which is 208 mV lower than that of the corresponding oxygen evolution reaction (OER). The results of physical characterizations confirm that the surface of NixSey/NC-NCP was reconstructed during the electrocatalytic UOR process; that is, active species such as CoOOH and NiOOH were generated on the surface. The Pt/C||NixSey/NC-NCP electrolyzer composed of commercial Pt/C and self-supporting NixSey/NC-NCP is used for the urea-assisted water splitting, which only needs low cell voltages of 1.40 and 1.45 V to drive the current densities of 100 and 200 mA cm–2, respectively. This study promotes research on hierarchical heterojunction catalysts to enhance the electrocatalytic UOR performance, effectively reducing the energy required for electrochemical hydrogen production. [ABSTRACT FROM AUTHOR]

The rational design of advanced transition-metal-based electrocatalysts with a heterostructure is a promising strategy for the promotion of the urea oxidation reaction (UOR) for energy-conservation technologies, but achieving a sufficiently high performance remains a challenge. In this work, we report a dramatic improvement in the UOR performance of a heterostructured electrocatalyst that combines NiMn-layered double hydroxide (LDH) nanosheets with NiCo2S4 arrays via a series of facile hydrothermal fabrication steps. Due to the high-flux electron transfer pathways at the close-contact interface, abundant active sites, and unique three-dimensional (3D) architecture, the NiCo2S4@NiMn LDH heterostructure grown on nickel foam exhibits a low potential of 1.37 V at a current density of 100 mA·cm–2 and a low Tafel slope of 43.8 mV·dec–1. More impressively, the proposed electrocatalyst demonstrates robust stability of more than 25 h at a current density of 50 mA·cm–2 with a negligible decrease in activity. In addition, density functional theory calculations reveal that the interface engineering within the heterostructure is beneficial for the adsorption and activation of urea molecules and the improvement of the sluggish UOR dynamics. The dissociation of adsorbed CO-(NH2)2* into CO* and NH* intermediates on the heterostructured NiMn LDH is also facilitated by electronic coupling with NiCo2S4, resulting in superior UOR performance. [ABSTRACT FROM AUTHOR]

In electrochemical energy storage and conversion systems, urea oxidation reaction (UOR) can produce hydrogen and mitigate pollution from urea-rich wastewater in a low-energy manner, whereas the development of this technique was limited via a lack of economical and cost-effective UOR catalysts. Herein, a unique electrocatalyst of MoO3/Ni-N-C was synthesized from a Mo element-incorporated Ni-MOF by heating under an inert atmosphere. The prepared MoO3/Ni-N-C electrode shows superior activity toward UOR, which only needs a low potential of 1.42 V (vs RHE) at 50 mA cm–2 in 1.0 M KOH with 0.5 M urea. The excellent performance for UOR is attributed to the synergistic effect between molybdenum and nickel, the modulation of the electronic structure for the nickel site by MoO3, accelerating the charge transfer, and tuning the reaction interface adsorption energy to enhance electrocatalytic activity. This work provides strategies and directions for exploring other advanced UOR electrode material designs. [ABSTRACT FROM AUTHOR]

To overcome the high potential required for oxygen evolution reactions (OERs), the use of urea oxidation as an alternative to OER has attracted attention. It not only can reduce energy consumption caused by the high overpotential of OER but also simultaneously removes the urea present in polluted water, having a huge potential in industrial applications. Ni- and Fe-based transition metal hydroxides have been shown to be significantly effective in OER and urea oxidation reactions (UORs). Herein, by using FeCoNi layered double hydroxide (FeCoNi-LDH) nanorods as the precursor, we successfully develop the Prussian blue analogue (PBA)/FeCoNi-LDH nanorods grown uniformly on the surface of nickel foam (NF). The obtained electrocatalyst PBA/FeCoNi-LDH/NF nanorods as the anode could provide a current density of 100 mA cm–2 at the applied potential of 1.383 V in 1.0 M KOH solution with 0.5 M urea. The two-electrode electrolyzer (PBA/FeCoNi-LDH/NF∥Pt/C/NF) needs only 1.52 V (vs reversible hydrogen electrode, RHE) to afford the same current density. The high conductivity and 3D porous structure created by the Ni substrate can lead to superior electrocatalytic activity. Nanoarrays are constructed intimately with the base and allow the rapid escape of newly produced gas molecules, and a large number of active sites and rapid charge transfer can be induced by the introduction of sodium nitroprusside. [ABSTRACT FROM AUTHOR]

The sluggish kinetics of the oxygen evolution reaction (OER) always results in a high overpotential at the anode of water electrolysis and an excessive electric energy consumption, which has been a major obstacle for hydrogen production through water electrolysis. In this study, we present a CoNi-LDH/Fe MOF/NF heterostructure catalyst with nanoneedle array morphology for the OER. In 1.0 M KOH solution, the heterostructure catalyst only required overpotentials of 275 and 305 mV to achieve high current densities of 500 and 1000 mA/cm 2 for OER, respectively. The catalytic activities are much higher than those of the reference single-component CoNi-LDH/NF and Fe MOF/NF catalysts. The improved catalytic performance of the heterostructure catalyst can be ascribed to the synergistic effect of CoNi-LDH and Fe MOF. In particular, when the anodic OER is replaced with the urea oxidation reaction (UOR), which has a relatively lower thermodynamic equilibrium potential and is expected to reduce the cell voltage, the overpotentials required to achieve the same current densities can be reduced by 80 and 40 mV, respectively. The cell voltage required to drive overall urea splitting (OUS) is only 1.55 V at 100 mA/cm 2 in the Pt/C/NF||CoNi-LDH/Fe MOF/NF two-electrode electrolytic cell. This value is 60 mV lower compared with that required for overall water splitting (OWS). Our results indicate that a reasonable construction of a heterostructure catalyst can significantly give rise to higher electrocatalytic performance, and using UOR to replace the anodic OER of the OWS can greatly reduce the electrolytic energy consumption.

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 RuO 2 -coated NiO ultrathin nanosheets (a-RuO 2 /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-RuO 2 /NiO by incorporation of amorphous RuO 2 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.

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/WO 3 Mott-Schottky heterojunction electrocatalyst with a hedgehog-like structure supported on Ni foam toward alkaline urea splitting. Different Ni/WO 3 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 WO 3 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/WO 3 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/WO 3 optimize the H adsorption energy (Δ G H* ) with the |Δ G H* | value of 0.097 eV, much lower than that of Ni (0.35 eV) and WO 3 (0.235 eV). This work demonstrates important guidance in designing an efficient electrocatalyst for urea splitting.

Direct urea / H 2 O 2 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.

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.

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 NiMoO 4 decorated Ni 3 S 2 grown on nickel foam (Ni 3 S 2 -NiMoO 4 /NF) is prepared to accelerate UOR and HER. The crystalline-amorphous heterostructure could regulate the interfacial electron structure to reduce the electron density near Ni 3 S 2 for optimizing UOR and HER. The decoration of NiMoO 4 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 H 2 production.

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) 3 O 4 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 m 2 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.

Urea electrolysis is a cost-effective method for urea-rich wastewater degradation to achieve a pollution-free environment. In this work, the Ni 3 S 2 /Ni heterostructure nanobelt arrays supported on nickel foam (Ni 3 S 2 /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 Ni 3 S 2 /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 Ni 3 S 2 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.

Urea electrolysis has prospects for urea-containing wastewater purification and hydrogen (H 2 ) production, but the shortage of cost-effective catalysts restricts its development. In this work, the tomentum-like FeNi 3 -MoO 2 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 FeNi 3 -MoO 2 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.

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 FeNi 3 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 FeNi 3 core connected by CNT facilitates lower charge-transfer resistance and reduces the UOR overpotential.

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/Ni 3 S 2 @N-doped nanofibers are directly grown on nickel foam (NF), which is highly active for hydrogen evolution reaction. Meanwhile, the Ni 3 S 2 @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 Ni 3 S 2 , as well as the interfacial effect between metal centers and N-doped carbon, favors the initial dissociation of H 2 O and the adsorption/desorption of H* with thermal neutral Gibbs free energy. Meanwhile, the in-situ generated NiOOH on the outer surface of Ni 3 S 2 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 H 2 production via urea 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 Ni 2 P/Ni 0.96 S 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 Ni 2 P/Ni 0.96 S catalyst electrode exhibits superior catalytic activity and stability. The Ni 2 P/Ni 0.96 S/NF||Ni 2 P/Ni 0.96 S/NF cell needs only 1.453 V to reach a current density of 100 mA/cm 2 in basic urea-containing water, while it is 1.693 V for a reference noble-based Pt/C/NF||IrO 2 /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.

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 [MoS 4 ] 2- in its interlayer space. The [MoS 4 ] 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 H 2 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 [MoS 4 ] 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 [MoS 4 ] 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, H 2 and O 2 .

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 (Ni 3 N/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 (Ni 3 N/NF∥Ni 3 N/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∥IrO 2 is 1.60 V, indicating that the Ni 3 N/NF catalyst is superior to precious metals.

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.