Heterostructured Ni3S2-Ni3P/NF as a Bifunctional Catalyst for Overall Urea-Water Electrolysis for Hydrogen Generation

A hydrophilic Ni3S2-MoN heterostructure on Ni foam (Ni3S2-MoN/NF) as an electrocatalyst for enhanced hydrogen evolution in alkaline media

In this study, a Ni3S2-MoN heterostructure was directly synthesized on a Ni foam support (Ni3S2-MoN/NF) through a facile one-step annealing process, using MoCl5 and thiourea at 450 °C under an N2 gas flow. The synthesized Ni3S2-MoN/NF catalyst achieved exceptional hydrogen evolution reaction (HER) activity, with low overpotential values of 154.3 mV (η50) at 50 mA cm-2 and 186.3 mV (η100) at 100 mA cm-2 in 1.0 M KOH. The enhanced HER performance was attributed to the synergistic effect between the highly active Ni3S2-MoN heterostructure and the NF support, which offers a large surface area and excellent electrical conductivity. Density functional theory (DFT) calculations reveal that MoN strongly adsorbs H2O and OH, facilitating water dissociation. Additionally, MoN was found to induce structural modifications in Ni3S2, enhancing the hydrogen binding energy and thereby improving the HER activity. Furthermore, the inherently hydrophobic Ni3S2 surface was transformed into a hydrophilic Ni3S2-MoN surface, further enhancing the HER activity. In a 24 h stability test at 100 mA cm-2, the HER activity of Ni3S2-MoN/NF improved, due to the partial conversion of Ni3S2 into amorphous Ni(OH)2 species, resulting in a reduction of overpotential by 30 mV (η50 = 125.5 and η100 = 158.2 mV). This simple and economical synthesis method highlights the potential of Ni3S2-MoN/NF as a promising catalyst for the HER with high activity and stability.

What Is the Mechanism by which the Introduction of Amorphous SeOx Effectively Promotes Urea-Assisted Water Electrolysis Performance of Ni(OH)2?

Nickel hydroxide (Ni(OH)2) is considered to be one of the most promising electrocatalysts for urea oxidation reaction (UOR) under alkaline conditions due to its flexible structure, wide composition and abundant 3D electrons. However, its slow electrochemical reaction rate, high affinity for the reaction intermediate *COOH, easy exposure to low exponential crystal faces and limited metal active sites that seriously hinder the further improvement of UOR activities. Herein it is reported electrocatalyst composed of rich oxygen-vacancy (Ov) defects with amorphous SeOx-covered Ni(OH)2 (Ov-SeOx/Ni(OH)2). Surprisingly, at 100 mA cm-2, compared with Ni(OH)2 (1.46 V (vs RHE)), Ov-SeOx/Ni(OH)2 has a potential of 1.35 V. Meanwhile, Ov-SeOx/Ni(OH)2 catalyst also showed good hydrogen evolution reaction (HER) performance, so it is used as the electrolytic cell assembled by UOR and HER bifunctional catalysts and only 1.57 V could reach 100 mA cm-2. Density functional theory (DFT) study revealed that introduce of amorphous SeOx optimizes the electronic structure of the central active metal, amorphous/crystalline interfaces promote charge-carrier transfer, shift d-band center and entail numerous spin-polarized electrons during the reaction, which speeds up the UOR reaction kinetics.

Dual-doped medium-entropy phosphides for complete urea electrolysis

Developing dual-functional electrocatalysts for urea-water decomposition still faces significant challenges. In this study, the vanadium (V) and cerium (Ce) co-doped FeCoNi medium-entropy phosphide (VCe-FeCoNiP/NF) were effectively fabricated on nickel foam (NF) via "two-step method," which involved hydrothermal treatment followed by phosphorization. Experimental results indicate that, benefiting from dual-ion doping and medium-entropy configuration, VCe-FeCoNiP/NF demonstrates unique electronic effects among the multimetallic elements, thereby exhibited remarkable catalytic activity for both urea oxidation reaction (UOR) and hydrogen evolution reaction (HER). Under urea-water conditions (1 M KOH with 0.33 M urea), the VCe-FeCoNi/NF catalyst merely required 1.338 V (vs RHE) and an overpotential of 173 mV to attain a current density of 100 mA·cm-2 for UOR and HER, respectively. Moreover, it could stably operate at a current density of 20 mA·cm-2 for 225 h in overall urea-water decomposition. This work provides new insights for designing high-performance urea-water electrolysis catalysts.

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

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

Synergistic Inclusion of Reaction Activator and Reaction Accelerator to Ni-MOF Toward Extra-Ordinary Performance of Urea Oxidation Reaction

Recently electrochemical urea oxidation reaction (UOR) has emerged as the technology of demand for commercialization of urea-based energy conversion. However, the nascent idea is limited by the energy burden of threshold voltage and the sluggish reaction kinetics involving a six-electron transfer mechanism. Herein, for the first time, the engineering of electrocatalysts are proposed with simultaneous inclusion of UOR activator and UOR accelerator. Nitrogen-doped carbon-decorated Ni-based Metal Organic Framework (MOF) has been synthesized as the base catalyst material. MoO2 and rGO with varied loading have been attached to the MOF to get the desired MoO2/Ni-MOF/rGO heterostructure incorporating defects and crystal strain within the materials. Investigations reveal that the invoked lattice strain and atomic defects promote plenteous Ni3+ active sites. The optimized sample demonstrates extraordinary performance of UOR having the potential value as low as 1.32 V versus RHE to reach the current density of 10 mA cm-2 and the tafel slope is only 31 mV dec-1 reflecting very fast reaction kinetics. Here MoO2 plays the role of UOR activator whereas optimized loading of rGO proliferates the reaction speed. This work, experimentally and theoretically, presents a new insight to enhance electrocatalytic urea oxidation reaction opening an avenue of urea-based energy-harvesting technology.

Large scalable nickel phosphide/nickel@carbon cloth-based flexible composite electrodes for symmetric supercapacitors

A flexible Ni3P/Ni@CC composite electrode with remarkable mechanical robustness is developed by a scalable and facile synthetic approach. The as-constructed symmetric supercapacitor demonstrates impressive energy density (51.1 W h kg-1) at a high power density (3500 W kg-1) along with excellent cycling stability.

Charge-redistributed Co3O4/Fe0.3Co0.7P heterointerfaces for efficient electrocatalytic urea oxidation

Charge-redistributed Co3O4/Fe0.3Co0.7P heterointerfaces are designed for effective electrocatalytic urea oxidation in alkaline medium, delivering excellent performance with only 1.41 V vs. RHE at 100 mA cm-2, low Tafel slope of 74 mV dec-1 and 36-h robust stability. The fine regulation of charge redistribution through heterointerfaces provides an effective strategy to design highly efficient electrocatalysts.

Assessment of the importance and catalytic role of chromium oxide and chromium carbide for hydrogen generation via hydrolysis of magnesium

Increasing energy demands and low-carbon emission energy carriers are global challenges for renewable energy resources. Regarding the aforementioned issues, magnesium-based composites are promising candidates for energy carriers. However, rapid initial hydrolysis kinetics and higher hydrogen yields are the objectives for practical applications. In this study, chromium-based catalysts (Cr2O3 and Cr3C2) were employed via ball milling to activate Mg. Finally, we used Mg-X wt% Y (X = 1, 3, 5, and 10; Y = Cr2O3 and Cr3C2) composites to produce hydrogen. Mg-10 wt% Cr2O3 can produce 798 and 812 mL g-1 hydrogen, while Mg-10 wt% Cr3C2 can produce 821 and 831.6 mL g-1 hydrogen in seawater and 0.5 M MgCl2 solutions, respectively. Additionally, Cr2O3 and Cr3C2 significantly improve the Mg hydrolysis activation energies. However, by incorporating Cr2O3 and Cr3C2, the activation energies for the hydrolysis of Mg with seawater achieved were 19.5 kJ mol-1 and 17.3 kJ mol-1, while they reduced to 15.7 kJ mol-1 and 14.4 kJ mol-1 with 0.5 M MgCl2 solutions, respectively. In comparison, Mg-10 wt% Cr3C2 composite exhibits superior performance, which is attributed to the higher anode potential value of Cr3C2. This work accelerates the hydrolysis kinetics and provides a sufficient technique to produce hydrogen from Mg composites for application in portable devices.

Unraveling the Oxidation Kinetics Through Electronic Structure Regulation of MnCo2O4.5@Ni3S2 p-n Junction for Urea-Assisted Electrocatalytic Activity

A promising strategy to boost electrocatalytic performance is via assembly of hetero-nanostructured electrocatalysts that delivers the essential specific surface area and also active sites by lowering the reaction barrier. However, the challenges associated with the intricate designs and mechanisms remain underexplored. Therefore, the present study constructs a p-n junction in a free-standing MnCo2O4.5@Ni3S2 on Ni-Foam. The space-charge region's electrical characteristics is dramatically altered by the formed p-n junction, which enhances the electron transfer process for urea-assisted electrocatalytic water splitting (UOR). The optimal MnCo2O4.5@Ni3S2 electrocatalyst results in greater oxygen evolution reactivity (OER) than pure systems, delivering an overpotential of only 240 mV. Remarkably, upon employing as UOR electrode the required potential decreases to 30 mV. The impressive performance of the designed catalyst is attributed to the enhanced electrical conductivity, greater number of electrochemical active sites, and improved redox activity due to the junction interface formed between p-MnCo2O4.5 and n-Ni3S2. There are strong indications that the in situ formed extreme-surface NiOOH, starting from Ni3S2, boosts the electrocatalytic activity, i.e., the electrochemical  surface reconstruction generates the active species. In conclusion, this work presents a high-performance p-n junction design for broad use, together with a viable and affordable UOR electrocatalyst.

Interface engineering of three-phase nickel-cobalt sulfide/nickel phosphide/iron phosphide heterostructure for enhanced water splitting and urea electrolysis

Rational designing efficient transition metal-based multifunctional electrocatalysts is highly desirable for improving the efficiency of hydrogen production from water cracking. Herein, a self-supported three-phase heterostructure electrocatalyst of nickel-cobalt sulfide/nickel phosphide/iron phosphide (CoNi5S8-Ni2P-FeP2) was prepared by a two-step gas-phase sulfurization/phosphorization strategy. The heterostructure in CoNi5S8-Ni2P-FeP2 provides a favorable interfacial environment for electron transfer and synergistic interaction of multiphase active components, while the introduced electronegative P/S not only serves as a carrier for proton capture in the hydrogen evolution reaction (HER) process but also promotes the metal-electron outflow, which in turn accelerates the generation of high-valent Ni3+ species to enhance the catalytic activity of oxygen evolution reaction (OER) and urea oxidation reaction (UOR). As expected, CoNi5S8-Ni2P-FeP2 reveals excellent multifunctional electrocatalytic properties. An overpotential of 35/215 mV is required to reach 10 mA cm-2 for HER/OER. More encouragingly, a current of 100 mA cm-2 requires only 1.36 V for UOR with CoNi5S8-Ni2P-FeP2 as anode, which is much lower as compared to the OER (1.50 V). Besides, a two-electrode water/urea electrolyzer assembled based on CoNi5S8-Ni2P-FeP2 has a voltage of only 1.59/1.48 V when the system reaches 50 mA cm-2. This work provides a new idea for the design of energy-efficient water/urea-assisted water-splitting multifunctional catalysts with multi-component heterostructure synergistic interface engineering.

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.

Phosphide-Based Electrocatalysts for Urea Electrolysis: Recent Trends and Progress

Electrolysis is a trend in producing hydrogen as a fuel for renewable energy development, and urea electrolysis is considered as one of the advanced electrolysis processes, where efficient materials still need to be explored. Notably, urea electrolysis came into existence to counter-part the electrode reactions in water electrolysis, which has hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Among those reactions, OER is sluggish and limits water splitting. Hence, urea electrolysis emerged with urea oxidation reaction (UOR) and HER as their reactions to tackle the water electrolysis. Among the explored materials, noble-metal catalysts are efficient, but their cost and scarcity limit the scaling-up of the Urea electrolysis. Hence, current challenges must be addressed, and novel efficient electrocatalysts are to be implemented to commercialize urea electrolysis technology. Phosphides, as an efficient UOR electrocatalyst, have gained huge attention due to their exceptional lattice structure geometry. The phosphide group benefits the water molecule adsorption and water dissociation, and facilitates the oxyhydrate of the metal site. This review summarizes recent trends in phosphide-based electrocatalysts for urea electrolysis, discusses synthesis strategies and crystal structure relationship with catalytic activity, and presents the challenges of phosphide electrocatalysts in urea electrolysis.

Superhydrophilic Dendritic FeP/Cu3P Electrocatalyst for Urea Splitting via the Intramolecular Mechanism

The electrocatalytic overall urea splitting can achieve the dual goals of urea treatment and hydrogen energy acquisition. Herein, we exploited the principle of precipitation dissolution equilibrium to obtain bimetallic phosphide FeP/Cu3P/CF for the simultaneous oxidation of urea and reduction of water and comprehensively reveal the inherent molecular thermodynamic mechanisms on the surface of catalysts. The excellent electrochemical performance can be derived from the super water affinity and synergistic effect. Especially, the theoretical calculation unveils that the synergistic effect between FeP and Cu3P can lower the activation energy required for urea electrooxidation, thereby promoting urea splitting. In situ differential electrochemical mass spectrometry (in situ DEMS) measurements further demonstrated that urea oxidation on FeP/Cu3P/CF proceeded according to the intramolecular mechanism. This work has laid the foundation for constructing highly efficient superhydrophilic bifunctional electrocatalysts.

Binary and nanostructured NiMn perovskite fluorides as efficient electrocatalysts for urea oxidation reaction

Urea electrolysis holds tremendous promise to provide green and sustainable energy and environmental solutions, because it can simultaneously remedy urea-containing wastewater and provide energy-saving hydrogen. However, the development of this emerging technology remains challenging mainly due to a dearth of high-performance electrocatalysts for efficient urea oxidation reaction (UOR). Perovskite fluorides have the advantages of intrinsic 3D diffusion pathways, robust architecture, and tunable chemical composition, thus receiving increasing attention in many applications. In this work, the UOR performances of a series of ABF3 samples (A = K; B = Ni/Mn, Ni/Co, Co/Mn) with various compositions are investigated in a systematic fashion for the first time. Among the binary samples, KNMF41 (Ni/Mn atomic ratio = 4:1) is the optimal sample with reduced overpotential (reaching 100 mA cm-2 at 1.43 V), low Tafel slope (40 mV dec-1), enhanced reaction rate constant (6.3 × 105 cm3 mol-1 s-1), and high turnover frequency (TOF, 0.19 s-1 at 1.60 V) toward urea oxidation. By comparing with NiCo and CoMn samples, the binary NiMn design is confirmed to endow the perovskite fluoride with higher electrocatalytic activity, thanks to the directed adsorption of urea molecules on the adjacent NiMn active sites. This work presents a targeted synthetic strategy for obtaining efficient electrocatalysts.

Bifunctional vanadium doped mesoporous Co3O4 on nickel foam towards highly efficient overall urea and water splitting in the alkaline electrolyte

Developing more active and stable electrode materials for oxygen evolution reaction (OER) and urea oxidation reaction (UOR) is necessary for electrocatalytic water and urea oxidation which can be used to generate hydrogen. Here, a low-cost vanadium-doped mesoporous cobalt oxide on Ni foam (V/meso-Co/NF) electrodes are obtained via the grouping of an in-situ citric acid (CA)-assisted evaporation-induced self-assembly (EISA) method and electrophoretic deposition process, and work as highly efficient and long-lasting electrocatalytic materials for OER/UOR. In particular, V/meso-Co/NF electrodes require 329 mV overpotential to maintain a 50 mA/cm2, with exceptional long-term durability of 30 h. Interestingly, V/meso-Co/NF also exhibits excellent electrocatalytic UOR performance, reaching 50 and 100 mA/cm2 versus RHE at low potentials of 1.34 and 1.35 V, respectively. By employing the V/meso-Co/NF materials as both the anode and cathode, this urea electrolysis assembly V/meso-Co/NF-5 (+,-) reaches current densities of 100 mA cm-2 at 1.62 V in KOH/urea, which is nearly 340 mV lesser than classical water electrolysis. The V/meso-Co/NF-5 electrocatalysts also exhibit remarkable durability for electrocatalytic OERs and UORs. The obtained findings revealed that the synthesized V/meso-Co/NF might be a promising electrode materials for overall urea-rich wastewater management and H2 generation from wastewater.

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.

Exploring The Synergistic Effect Of CoSeP/CoP Interface Catalyst For Efficient Urea Electrolysis

Electrocatalytic oxidation of urea (UOR) is a potential energy-saving hydrogen production technology that can replace oxygen evolution reaction (OER). Therefore, CoSeP/CoP interface catalyst is synthesized on nickel foam using hydrothermal, solvothermal, and in situ template methods. The strong interaction of tailored CoSeP/CoP interface promotes the hydrogen production performance of electrolytic urea. During the hydrogen evolution reaction (HER), the overpotential can reach 33.7 mV at 10 mA cm-2 . The cell voltage can reach 1.36 V at 10 mA cm-2 in the overall urea electrolytic process. Notably, the overall urine electrolysis performance of the catalyst in the human urine medium can reach 1.40 V at 10 mA cm-2 and can exhibit durable cycle stability at 100 mA cm-2 . Density functional theory (DFT) proves that the CoSeP/CoP interface catalyst can better adsorb and stabilize reaction intermediates CO* and NH* on its surface through a strong synergistic effect, thus enhancing the catalytic activity.

Engineering cobalt molybdate nanosheet arrays with phosphorus-modified nickel as heterogeneous electrodes for highly-active energy-saving water splitting

Electrochemical urea electrolysis has been regarded as a promising strategy to replace traditional water-splitting technology to achieve hydrogen fuel due to its cost savings and high energy efficiency. Designing efficient bifunctional electrocatalysts easily is important but still faces significant challenges. Herein, an interface engineering strategy is used to construct a hybrid material by coupling cobalt molybdate (CoMoO₄) nanosheet arrays with phosphorus-modified nickel (P-Ni) particles on copper foam (P-Ni@CoMoO₄/CF) through the hydrothermal and in-situ electrodeposition process. Benefiting from the abundant catalytic active sites, low charge transfer resistance, and synergistic coupling effect, the optimal P-Ni@CoMoO₄/CF electrocatalyst presents a superior bifunctional activity for urea oxidation reaction (UOR) and hydrogen evolution reaction (HER). In detail, a small overpotential of 125 mV and a low potential of 1.36 V is required to attain the current density of 100 mA cm^-2 for HER and UOR, respectively. In the process of urea electrolysis, the P-Ni@CoMoO₄/CF-based electrolyzer provides a current density of 100 mA cm^-2 with an overall voltage of 1.50 V, about 170 mV less than that in a traditional water electrolyzer. The high performance of P-Ni@CoMoO₄/CF outperforms many recently reported electrodes, suggesting its promising application in energy-saving hydrogen production. Our work proposes a novel idea for the rational design and exploitation of low-cost and robust bifunctional electrodes for electrocatalysis.

Surface engineering of carbon-coated cobalt-doped nickel phosphides bifunctional electrocatalyst for boosting 5-hydroxymethylfurfural oxidation coupled with hydrogen evolution

Multiple surface/interface engineering is an effective approach to develop efficient electrocatalysts for promoting the practical application of electrocatalysis and achieving carbon neutrality. Herein, a deep eutectic liquid precursor containing phosphorus was designed. The self-supported three-dimensional (3D) cobalt-doped Ni12P5/Ni3P nanowire networks coated with a thin layer of carbon (Co-NixP@C) were prepared by using an in-situ one-step pyrolysis method. The as-obtained Co-NixP@C hybrid possesses a superaerophobic/superhydrophilic surface, which could promote electrolyte diffusion and enhance bubble release. Density functional theory (DFT) calculations reveal that Co-doping in NixP@C can promote the adsorption and activation of 5-hydroxymethylfurfural (HMF) molecules, and optimize the energy barrier of H* absorption. The self-supported Co-NixP@C was used as an efficient bifunctional electrocatalyst for HMF oxidation coupled with hydrogen evolution reaction (HER) in a 1.0 M KOH solution. A nearly 100 % yield of 2,5-furandicarboxylic acid (FDCA) was achieved. The self-supported Co-NixP@C displayed high activity and stability for both HER and HMF conversion. The HMF oxidation coupled with HER can be efficiently driven by a 1.5 V commercial photovoltaic panel under sunlight. This study lays the foundation for large-scale industrialization in sustainable fine-chemical and energy engineering.

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.

Recent Development of Nickel-Based Electrocatalysts for Urea Electrolysis in Alkaline Solution

Recently, urea electrolysis has been regarded as an up-and-coming pathway for the sustainability of hydrogen fuel production according to its far lower theoretical and thermodynamic electrolytic cell potential (0.37 V) compared to water electrolysis (1.23 V) and rectification of urea-rich wastewater pollution. The new era of the "hydrogen energy economy" involving urea electrolysis can efficiently promote the development of a low-carbon future. In recent decades, numerous inexpensive and fruitful nickel-based materials (metallic Ni, Ni-alloys, oxides/hydroxides, chalcogenides, nitrides and phosphides) have been explored as potential energy saving monofunctional and bifunctional electrocatalysts for urea electrolysis in alkaline solution. In this review, we start with a discussion about the basics and fundamentals of urea electrolysis, including the urea oxidation reaction (UOR) and the hydrogen evolution reaction (HER), and then discuss the strategies for designing electrocatalysts for the UOR, HER and both reactions (bifunctional). Next, the catalytic performance, mechanisms and factors including morphology, composition and electrode/electrolyte kinetics for the ameliorated and diminished activity of the various aforementioned nickel-based electrocatalysts for urea electrolysis, including monofunctional (UOR or HER) and bifunctional (UOR and HER) types, are summarized. Lastly, the features of persisting challenges, future prospects and expectations of unravelling the bifunctional electrocatalysts for urea-based energy conversion technologies, including urea electrolysis, urea fuel cells and photoelectrochemical urea splitting, are illuminated.

Highly Efficient Water Splitting Catalyst Composed of N,P-Doped Porous Carbon Decorated with Surface P-Enriched Ni2P Nanoparticles

A non-noble-metal hybrid catalyst (Ni₂P/NPC-P), composed of N,P-doped porous carbon decorated with surface P-enriched Ni₂P nanoparticles, is developed to address the urgent challenges associated with mass production of clean hydrogen fuel. The synthesis features one-pot pyrolysis of inexpensive fluid catalytic cracking slurry, graphitic carbon nitride, and inorganic salts, followed by a feasible surface phosphidation process. As a non-noble metal catalyst, Ni₂P/NPC-P demonstrates excellent performance in hydrogen evolution reaction in alkaline electrolytes with a low overpotential of 73 mV at a current density of 10 mA cm^-2 (η₁₀) and a small Tafel slope of 56 mV dec^-1, meanwhile exhibits durability with no significant η₁₀ change after 2000 catalytic cycles. Theoretical calculation reveals that the negatively charged P-enriched surface accelerated the rate-determining transformation and desorption of OH*. In overall water splitting, the electrocatalyst achieves a low η₁₀ of 1.633 V, promising its potential in the cost-effective mass production of hydrogen fuel.

Construction of Fe-doped NiS-NiS2 Heterostructured Microspheres Via Etching Prussian Blue Analogues for Efficient Water-Urea Splitting

Developing efficient and robust non-precious-metal-based catalysts to accelerate electrocatalytic reaction kinetics is crucial for electrochemical water-urea splitting. Herein, Fe-doped NiS-NiS₂ heterostructured microspheres, an electrocatalyst, are synthesized via etching Prussian blue analogues following a controlled annealing treatment. The resulting microspheres are constructed by mesoporous nanoplates, granting the virtues of large surface areas, high structural void porosity, and accessible inner surface. These advantages not only provide more redox reaction centers but also strengthen structural robustness and effectively facilitate the mass diffusion and charge transport. Density functional theory simulations validate that the Fe-doping improves the conductivity of nickel sulfides, whereas the NiS-NiS₂ heterojunctions induce interface charge rearrangement for optimizing the adsorption free energy of intermediates, resulting in a low overpotential and high electrocatalytic activity. Specifically, an ultralow overpotential of 270 mV at 50 mA cm^-2 for the oxygen evolution reaction (OER) is achieved. After adding 0.33 M urea into 1 M KOH, Fe-doped NiS-NiS₂ obtains a strikingly reduced urea oxidation reaction potential of 1.36 V to reach 50 mA cm^-2 , around 140 mV less than OER. This work provides insights into the synergistic modulation of electrocatalytic activity of non-noble catalysts for applications in energy conversion systems.