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

Suppressing Mo-Species Leaching in MoOx/A-Ni3S2 Cathode for Stable Anion Exchange Membrane Water Electrolysis at Industrial-Scale Current Density

The development of non-noble metal-based hydrogen evolving reaction (HER) electrocatalysts operating under high current density plays a critical role in the large-scale application of anion exchange membrane water electrolysis (AEM-WE). Herein, a porous and hybrid MoS2/Ni3S2 is synthesized on nickel foam (NF) via a one-step hydrothermal method and studied its reconstruction process during alkaline HER conditions. Experimental results indicated that the MoS2 underwent an oxidative dissolution followed by a dynamic equilibrium between dissolution and redeposition of the amorphous MoOx during HER. Meanwhile, S-vacancy-rich Ni3S2 (A-Ni3S2) is exposed and acts as the real active site for HER. The obtained MoOx/A-Ni3S2 catalyst exhibited high catalytic performance in three-electrode systems and single-cell AEM-WE. Finally, for a long-term durability test in the AEM electrolyzer, a dry cathode method is applied to suppress the Mo species leaching from the MoOx/A-Ni3S2 electrode. Remarkably, the device assembled by MoOx/A-Ni3S2 as the cathode catalyst and NiFe as the anode catalyst demonstrated a high stability of 2500 h at 2 A cm-2 and 40 °C with a small aging rate of 30 µV h-1.

Bifunctional transition-metal catalysts for energy-saving hydrogen generation from nitrogenous wastewater

Wastewater from industrial chemical synthesis, agricultural activities, and domestic sewage usually contains high levels of nitrogenous compounds, endangering environmental health and human well-being. Nitrogenous wastewater electrolysis (NWE), despite its ecological merits, is inherently hampered by sluggish kinetics. To improve process efficiency, lower costs, and avoid cross-contamination between the anode and cathode, a range of bifunctional transition-metal catalysts capable of efficient operation at both electrodes have recently been developed. This review outlines the progress in these catalysts for the energy-saving production of hydrogen from nitrogenous wastewater, including urea, hydrazine, and ammonia. It highlights their dual role in both degrading nitrogenous pollutants and generating hydrogen energy. The review meticulously introduces the key performance metrics of the NWE system and surveys the latest advancements in bifunctional transition-metal catalysts, along with their catalytic mechanisms. It culminates in a detailed summary and comparative analysis of representative bifunctional catalysts, emphasizing their electricity consumption and energy-saving efficiency. Lastly, the existing challenges and research prospects are thoroughly discussed.

Co2P-NiMoN/NF Heterostructure Nanorod Arrays as Efficient Bifunctional Electrocatalysts for Urea Electrolysis

Electrolysis of water represents an effective method for the generation of high-purity hydrogen. Nevertheless, the anodic oxygen evolution reaction (OER) exhibits slow kinetics, which leads to a high electrolytic potential and induces excessive energy consumption. In this work, nickel foam-supported 3D phosphide/bimetal nitride (Co2P-NiMoN/NF) nanorod array catalyst is prepared by calcination of NiMoO4, followed by phosphatization of Co(OH)2. The heterostructure catalyst exhibits excellent catalytic activity for cathodic hydrogen evolution reaction (HER: η100 = 98 mV, η1000 = 297 mV) and anodic OER (η100 = 277 mV, η1000 = 382 mV) of water electrolysis in alkaline electrolyte, indicating its feasibility as a bifunctional catalyst for overall water splitting (OWS). Additionally, at a current density of 100 mA cm-2, the associated oxidation potential is decreased by roughly 160 mV when the anodic OER is replaced with the urea oxidation process (UOR), which has a far lower thermodynamic equilibrium potential. Density functional theory (DFT) calculations reveal that the heterointerface between Co2P and NiMoN enriches the density of electronic states near the Fermi level, thereby enhancing electron transfer and promoting charge redistribution. This modulation precisely tunes the adsorption strengths of reactants during the reaction process, ultimately boosting the electrocatalytic performance. A current density of 100 mA cm-2 can be attained at a cell voltage of 1.51 V when Co2P-NiMoN/NF is used as the anode and cathode in the urea electrolysis cell. Notably, this cell potential is significantly lower compared to that of the water electrolysis cell (1.65 V), as well as the previously published values. The findings demonstrate that the Co2P-NiMoN/NF heterostructure can be used as a bifunctional catalyst for water and urea electrolysis and demonstrate an efficient strategy for the energy-efficient production of hydrogen through substituting UOR for OER at the anode of water electrolysis.

Scalable Electrocatalytic Urea Wastewater Treatment Coupled with Hydrogen Production by Regulating Adsorption Behavior of Urea Molecule

Electrocatalytic urea wastewater treatment technology has emerged as a promising method for environmental remediation. However, the realization of highly efficient and scalable electrocatalytic urea wastewater treatment (SEUWT) is still an enormous challenge. Herein, through regulating the adsorption behavior of urea functional groups, the efficient SEUWT coupled hydrogen production is realized in anion exchange membrane water electrolyzer (AEMWE). Density functional theory calculations indicate that self-driven electron transfer at the heterogeneous interface (NiO/Co3O4) can induce charge redistribution, resulting in electron-rich NiO and electron-deficient Co3O4, which are superior to adsorbing C=O (electron-withdrawing group) and -NH2 (electron-donating group), respectively, regulating the adsorption behavior of urea molecule and accelerating the reaction kinetics of urea oxidation. This viewpoint is further verified by temperature-programmed desorption experiments. The SEUWT coupled hydrogen production in AEMWE assembled with NiO/Co3O4 (anode) and NiCoP (cathode) can continuously treat urea wastewater at an initial current density of 600 mA cm-2, with the average urea treatment efficiency about 53%. Compared with overall water splitting, the H2 production rate (8.33 mmol s-1) increases by approximately 3.5 times. This work provides a cost-effective strategy for scalable purifying urea-rich wastewater and energy-saving hydrogen production.

Superhydrophilicity and Electronic Modulation on Self-Supported Lignin-Derived Carbon Coupled with NiO@MoNi4 for Enhancing Urea Electrolysis

Developing highly efficient biomass-derived carbon-based electrocatalysts remains challenging for urea electrolysis because most of these electrocatalysts show powder morphology, which can lead to Ostwald ripening during the reaction process, and its reaction mechanism should be further verified. Herein, self-supported lignin-derived carbon coupling NiO@MoNi4 heterojunction (NiO@MoNi4/C) possesses superhydrophilic properties and electronic modulation, boosting the performance of urea electrolysis. Electrochemical results show that an indirect oxidation step for urea oxidation reaction (UOR) and Volmer-Heyrovsky mechanism for hydrogen evolution reaction (HER) occurs on the surface of NiO@MoNi4/C. It displays low potentials for UOR (E10/500/1000 = 1.28/1.41/1.47 V) and for HER (E-10/-500/-1000 = -38/-264/-355 mV) in 1.0 M KOH + 0.5 M urea electrolyte solution. The good activity is ascribed to the self-supported lignin-derived carbon and heterojunction, which increases the number of active sites, optimizes electronic structure, and improves electron transfer. Benefiting from the self-supported lignin-derived carbon, NiO@MoNi4/C demonstrates corrosion resistance and superhydrophilicity, which avoids Ostwald ripening and accelerates gas-liquid transfer, thus, maintaining for 100 h at ±1000/±1500 mA cm-2 during the UOR and HER test. This work provides a good catalyst for urea electrolysis and presents a promising way for preparing lignin-derived carbon-based catalysts while expanding the application of lignin-based biomass carbon materials.

Fe-Doped Ni3S2 Induces Self-Reconstruction for Urea-Assisted Water Electrolysis Enhancement

Urea oxidation reaction (UOR) is an attractive alternative anodic reaction to oxygen evolution reaction (OER) for its low thermodynamic potential (0.37 V vs RHE). A major challenge that prohibits its practical application is the six-electron transfer process during UOR, demanding enhancements in the catalytic activity. Herein, a Fe-doped Ni3S2 catalyst with a uniform flower-like structure is synthesized in situ on nickel foam via a simple one-step hydrothermal method. The electrochemical properties of Fe-Ni3S2 are significantly improved since a current density of 10 mA cm-2 only requires a 1.33 V potential and remains stable for 60 h. The structural characterization demonstrates a strong interaction between Fe and Ni3S2. After Fe doping, the active site increases, which promotes the formation of NiOOH on the catalyst surface, thus speeding up the UOR process. These changes are beneficial to charge transfer and optimize the adsorption energy of the intermediates. In situ EIS further confirms that Fe promotes electron transfer during the UOR process, reduces the interface resistance between the catalyst and the electrolyte, and lowers the driving voltage.

Bimetallic NiCo Metal-Organic Frameworks with High Stability and Performance Toward Electrocatalytic Oxidation of Urea in Seawater

The urea oxidation reaction (UOR) is an alternative anodic reaction for hydrogen generation via water splitting. The significance of UOR lies in both H2 production and the decontamination of urea-containing wastewater. Commercial electrocatalysts in this field are generally based on noble metals and show several limitations. Bimetal-organic frameworks (BMOFs) can be excellent candidates for the replacement of noble-metal-based catalysts beacuse of their promising features, such as a tunable structure, high surface area, and abundant sites for electrocatalysis. In this study, a series of nickel-cobalt BMOFs (Nix-Coy-BMOFs: x and y refer to a molar fraction of Ni and Co) were synthesized and applied as electrocatalysts in UOR. In particular, a Ni0.15Co0.85-MOF material with a structure similar to that of its parent Co-MOF, revealed exceptional electrocatalytic performance, as evidenced by low values of overpotential (1.33 V vs RHE at 10 mA cm-2), TOF (0.47 s-1), and Tafel slope (125 mV dec-1). At a 40 mA cm-2 current density, Ni0.15Co0.85-MOF also showed excellent stability during the 72 h tests. This performance of NiCo-BMOF can be assigned to the synergistic effect between Co and Ni, abundant active sites, porosity, and a tunable structure, all of which result in an increased reaction rate due to the acceleration of charge and mass transfers. Thus, the present work introduces an efficient noble-metal-free UOR for energy generation from urea-based wastewater.

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.

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.

Fe-Doped Ni2P/NiSe2 Composite Catalysts for Urea Oxidation Reaction (UOR) for Energy-Saving Hydrogen Production by UOR-Assisted Water Splitting

The development of efficient urea oxidation reaction (UOR) catalysts helps UOR replace the oxygen evolution reaction (OER) in hydrogen production from water electrolysis. Here, we prepared Fe-doped Ni2P/NiSe2 composite catalyst (Fe-Ni2P/NiSe2-12) by using phosphating-selenizating and acid etching to increase the intrinsic activity and active areas. Spectral characterization and theoretical calculations demonstrated that electrons flowed through the Ni-P-Fe-interface-Ni-Se-Fe, thus conferring high UOR activity to Fe-Ni2P/NiSe2-12, which only needed 1.39 V vs RHE to produce the current density of 100 mA cm-2. Remarkably, this potential was 164 mV lower than that required for the OER under the same conditions. Furthermore, EIS demonstrated that UOR driven by the Fe-Ni2P/NiSe2-12 exhibited faster interfacial reactions, charge transfer, and current response compared to OER. Consequently, the Fe-Ni2P/NiSe2-12 catalyst can effectively prevent competition with OER and NSOR, making it suitable for efficient hydrogen production in UOR-assisted water electrolysis. Notably, when water electrolysis is operated at a current density of 40 mA cm-2, this UOR-assisted system can achieve a decrease of 140 mV in the potential compared to traditional water electrolysis. This study presents a novel strategy for UOR-assisted water splitting for energy-saving hydrogen production.

Engineering advanced noble-metal-free electrocatalysts for energy-saving hydrogen production from alkaline water via urea electrolysis

When the anodic oxygen evolution reaction (OER) of water splitting is replaced by the urea oxidation reaction (UOR), the electrolyzer can fulfill hydrogen generation in an energy-economic manner for urea electrolysis as well as sewage purification. However, owing to the sluggish kinetics from a six-electron process for UOR, it is in great demand to design and fabricate high-performance and affordable electrocatalysts. Over the past years, numerous non-precious materials (especially nickel-involved samples) have offered huge potential as catalysts for urea electrolysis under alkaline conditions, even in comparison with frequently used noble-metal ones. In this review, recent efforts and progress in these high-efficiency noble-metal-free electrocatalysts are comprehensively summarized. The fundamentals and principles of UOR are first described, followed by highlighting UOR mechanism progress, and then some discussion about density functional theory (DFT) calculations and operando investigations is given to disclose the real reaction mechanism. Afterward, aiming to improve or optimize UOR electrocatalytic properties, various noble-metal-free catalytic materials are introduced in detail and classified into different classes, highlighting the underlying activity-structure relationships. Furthermore, new design trends are also discussed, including targetedly designing nanostructured materials, manipulating anodic products, combining theory and in situ experiments, and constructing bifunctional catalysts. Ultimately, we point out the outlook and explore the possible future opportunities by analyzing the remaining challenges in this booming field.

Manganese-facilitated dynamic active-site generation on Ni2P with self-termination of surface reconstruction for urea oxidation at high current density

Electrochemical urea oxidation reaction (UOR) suffers from sluggish reaction kinetics due to its complex 6-electron transfer processes combined with conversion of complicated intermediates, severely retarding the overall energy conversion efficiency. Herein, manganese-doped nickel phosphide nanosheets (Mn-Ni2P) are constructed and employed for driving UOR. Comprehensive analysis deciphers that Mn doping could efficiently accelerate the surface reconstruction of Mn-Ni2P electrode, generating highly reactive NiOOH-MnOOH heterostructure with local nucleophilic and electrophilic regions. Such unique structure could accelerate the targeted adsorption and activation of C and N atoms, promoting fracture of CN bond in urea. In addition, moderate Mn doping could efficiently enhance the adsorption capacities of urea molecules and some key intermediates, and minish the energy barrier for *CO2 desorption, accelerating refreshing of the catalyst. Consequently, the Mn-Ni2P electrode exhibits excellent UOR catalytic activity, achieving an industrial-level current density of 1000 mA cm-2 at 1.46 V (vs. RHE).

Water-Stable Pillared Three-Dimensional Zn-V Bimetal-Organic Framework for Promoted Electrocatalytic Urea Oxidation

Urea oxidation reaction (UOR) is one of the potential routes in which urea-rich wastewater is used as a source of energy for hydrogen production. Metal-organic frameworks (MOFs) have promising applications in electrocatalytic processes, although there are still challenges in identifying the MOFs' molecular regulation and obtaining practical catalytic systems. The current study sought to synthesize [Zn6(IDC)4(OH)2(Hprz)2]n (Zn-MOF) with three symmetrically independent Zn(II) cations connected via linear N-donor piperazine (Hprz), rigid planar imidazole-4,5-dicarboxylate (IDC3-), and -OH ligands, revealing the 3,4T1 topology. The optimized noble-metal-free Zn0.33V0.66-MOF/NF electrocatalysts show higher robustness and performance compared to those of the parent Zn monometallic MOF/NF electrode and other bimetallic MOFs with different Zn-V molar ratios. The low potential of 1.42 V (vs RHE) at 50 mA cm-2 in 1.0 M KOH with 0.33 M urea required by the developed Zn0.33V0.66-MOF electrode makes its application in the UOR more feasible. The availability of more exposed active sites, ion diffusion path, and higher conductivity result from the distinctive configuration of the synthesized electrocatalyst, which is highly stable and capable of synergistic effects, consequently enhancing the desired reaction. The current research contributes to introducing a practical, cost-effective, and sustainable solution to decompose urea-rich wastewater and produce hydrogen.

Regulation Strategy of Nanostructured Engineering on Indium-Based Materials for Electrocatalytic Conversion of CO2

Electrochemical carbon dioxide reduction (CO2 RR), as an emerging technology, can combine with sustainable energies to convert CO2 into high value-added products, providing an effective pathway to realize carbon neutrality. However, the high activation energy of CO2 , low mass transfer, and competitive hydrogen evolution reaction (HER) leads to the unsatisfied catalytic activity. Recently, Indium (In)-based materials have attracted significant attention in CO2 RR and a series of regulation strategies of nanostructured engineering are exploited to rationally design various advanced In-based electrocatalysts, which forces the necessary of a comprehensive and fundamental summary, but there is still a scarcity. Herein, this review provides a systematic discussion of the nanostructure engineering of In-based materials for the efficient electrocatalytic conversion of CO2 to fuels. These efficient regulation strategies including morphology, size, composition, defects, surface modification, interfacial structure, alloying, and single-atom structure, are summarized for exploring the internal relationship between the CO2 RR performance and the physicochemical properties of In-based catalysts. The correlation of electronic structure and adsorption behavior of reaction intermediates are highlighted to gain in-depth understanding of catalytic reaction kinetics for CO2 RR. Moreover, the challenges and opportunities of In-based materials are proposed, which is expected to inspire the development of other effective catalysts for CO2 RR.

Fe-induced crystalline-amorphous interface engineering of a NiMo-based heterostructure for enhanced water oxidation

Engineering heterostructures with a unique surface/interface structure is one of the effective strategies to develop highly active noble-metal-free catalysts for the oxygen evolution reaction (OER), because the surface/interface of catalysts is the main site for the OER. Herein, we design a coralloid NiMo(Fe)-20 catalyst with a crystalline-amorphous interface through combining a hydrothermal method and an Fe-induced surface reconfiguration strategy. That is, after Fe3+ impregnation treatment, the Ni(OH)2-NiMoO4 pre-catalyst with a complete crystalline surface is restructured into a trimetallic heterostructure with a crystalline-amorphous interface, which facilitates mass diffusion and charge transfer during the OER. As expected, self-supported NiMo(Fe)-20 exhibits excellent electrocatalytic water oxidation performance (overpotential: η-10 = 220 mV, η-100 = 239 mV) in the alkaline electrolyte, and its electrocatalytic performance hardly changes after maintaining the current density of 50 mA cm-2 for 10 hours. Furthermore, nickel foam (NF) supported commercial Pt/C and self-supported NiMo(Fe)-20 served as the cathode and anode of the Pt/C‖NiMo(Fe)-20 electrolyzer, respectively, which exhibits a lower cell voltage (E-100 = 1.53 V) than that of the Pt/C‖RuO2 electrolyzer (E-100 = 1.58 V) assembled with noble metal-based catalysts. The enhanced electrocatalytic performance of the NiMo(Fe)-20 catalyst is mainly attributed to the synergistic effect between the crystalline-amorphous interface and the coralloid trimetallic heterostructure.

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.

Self-supported spinel nanosphere as bifunctional electrocatalysts for energy-saving hydrogen production via urea-water electrolysis

Electrochemical oxidation of urea is of great importance in the removal and energy exchange and storage of urea from wastewater as well as of potential applications in potable dialysis of end-stage renal disease. However, the lack of economical electrocatalysts hinders its widespread application. In this study, we successfully fabricated ZnCo₂O₄ nanospheres with bifunctional catalysis on nickel foam (NF). The catalytic system has high catalytic activity and durability for urea overall electrolysis. The urea oxidation and hydrogen evolution reactions required only 1.32 V and -80.91 mV to obtain ± 10 mA cm^-2. Only 1.39 V was needed to obtain 10 mA cm^-2 for 40 h without noticeably declining activity. The excellent performance could be attributed to the fact that the material can provide multiple redox couplings and a three-dimensional porous structure to facilitate the release of gases from the surface.

Fabrication of highly efficient Rh-doped cobalt-nickel-layered double hydroxide/MXene-based electrocatalyst with rich oxygen vacancies for hydrogen evolution

The development of nonprecious metal catalysts for producing hydrogen from economical alkaline water electrolysis that is both stable and efficient is crucial but remains challenging. In this study, Rh-doped cobalt-nickel-layered double hydroxide (CoNi LDH) nanosheet arrays with abundant oxygen vacancies (Ov) in-situ grown on Ti₃C₂T_x MXene nanosheets (Rh-CoNi LDH/MXene) were successfully fabricated. The synthesized Rh-CoNi LDH/MXene exhibited excellent long-term stability and a low overpotential of 74.6 ± 0.4 mV at -10 mA cm^-2 for hydrogen evolution reaction (HER) owing to its optimized electronic structure. Experimental results and density functional theory calculations revealed that the incorporation of Rh dopant and Ov into CoNi LDH and the coupling interface between Rh-CoNi LDH and MXene optimized the hydrogen adsorption energy, which accelerated the hydrogen evolution kinetics, thereby accelerating the overall alkaline HER process. This work presents a promising strategy for designing and synthesizing highly efficient electrocatalysts for electrochemical energy conversion devices.