Synergistic engineering of heterostructure and oxygen vacancy in cobalt hydroxide/aluminum oxyhydroxide as bifunctional electrocatalysts for urea-assisted hydrogen production.

Designing inexpensive, high-efficiency and durable bifunctional catalysts for urea oxidation reaction (UOR) and hydrogen evolution reaction (HER) is an encouraging tactic to produce hydrogen with reduced energy expenditure. Herein, oxygen vacancy-rich cobalt hydroxide/aluminum oxyhydroxide heterostructure on nickel foam (denoted as Co(OH)2/AlOOH/NF-100) has been fabricated using one step hydrothermal process. Theoretical calculation and experimental results indicate the electrons transfer from Co(OH)2 to highly active AlOOH results in the interfacial charge redistribution and optimization of electronic structure. Abundant oxygen vacancies in the heterostructure could improve the conductivity and simultaneously serve as the active sites for catalytic reaction. Consequently, the optimal Co(OH)2/AlOOH/NF-100 demonstrates excellent electrocatalytic performance for HER (62.9 mV@10 mA cm-2) and UOR (1.36 V@10 mA cm-2) due to the synergy between heterointerface and oxygen vacancies. Additionally, the in situ electrochemical impedance spectrum (EIS) for UOR suggests that the heterostructured catalyst exhibits rapid reaction kinetics, mass transfer and current response. Importantly, the urea-assisted electrolysis composed of the Co(OH)2/AlOOH/NF-100 manifests a low cell voltage (1.48 V @ 10 mA cm-2) in 1 M KOH containing 0.5 M urea. This work presents a promising avenue to the development of HER/UOR bifunctional electrocatalysts.

Engineering of novel vanadium-doped Ni4N@CuCoN0.6 heterostructure for enhancing urea oxidation reaction at high current density.

Exploring highly effective nickel-based catalytic materials for urea oxidation reaction (UOR) at high current density remains a challenging task for urea electrolysis. Herein, the vanadium-doped Ni4N@CuCoN0.6 heterostructure with brush shape structure supported on copper foam (denoted as V-doped-Ni4N@CuCoN0.6/CF) is prepared to serve as a highly-performing UOR electrocatalyst at high current density (E10/500/1000 = 1.26/1.40/1.45 V). Thanks to the construction of heterojunction, the local charge density undergoes redistribution at the V-doped-Ni4N@CuCoN0.6/CF interface between Ni4N and CuCoN0.6, thereby significantly enriching the active site and facilitating the adsorption process of urea molecules in a 1.0 M KOH + 0.5 M urea solution. Notably, V-doping exerts an additional influence by modulating the electronic structure of the catalyst, potentially optimizing the urea adsorption/CO2 desorption process and reducing the energy barrier for the rate-determining step. Operando electrochemical impedance spectroscopy results show direct oxidation for urea molecules on the surface of V-doped-Ni4N@CuCoN0.6/CF, indicating that V-doping and heterostructure can effectively improve electron transfer. Furthermore, because the brush shape structure can be beneficial for bubble release and liquid transport on the catalyst's surface, for UOR stability, V-doped-Ni4N@CuCoN0.6/CF  can maintain at 500 mA cm-2 for 80 h. Overall, this work suggests a highly-performing UOR electrocatalyst and provides a promising strategy for developing highly-efficient catalysts for UOR applications.

Tuning of Oxygen Vacancies in Co3O4 Electrocatalyst for Effectiveness in Urea Oxidation and Water Splitting.

The development of an excellent multifunctional electrocatalyst that is based on non-precious metal is critical for improving the electrochemical processes of the hydrogen evolution reaction (HER), the oxygen evolution reaction (OER), and the urea oxidation reaction (UOR) in alkaline media. This study demonstrates that incorporating Mo into Co3O4 facilitated the formation of rich oxygen vacancies (Vo), which promotes effective nitrate adsorption and activation in urea electrolysis. Subsequently, in situ/operando X-ray absorption spectroscopy is used to explore the active sites in Mo-Co3O4-3 under OER, indicating the oxygen vacancies are first filled with OH• in Mo-Co3O4; facilitated the pre-oxidation of low-valence Co, and promoted the reconstruction/deprotonation of intermediate Co-OOH•. Mo-Co3O4-3 electrocatalysts show impressive HER, OER, and UOR with low overpotentials of 141 mV, 220 mV, and 1.32 V, respectively, at 10 mA cm-2 in an alkaline medium. Furthermore, in situ/Operando Raman spectroscopy results reveal the importance of CoOOH active sites for enhanced electrochemical performance in Mo-Co3O4-3 compared to the pure Co3O4. The urea electrolyzer with Mo-Co3O4 electrocatalysts acts as an anode and the cathode delivers 1.42 V at 10 mA cm-2. A viable approach to creating effective UOR electrocatalysts involves synergistic engineering exploiting doping and oxygen vacancies.

Recent Developments in Membrane-Free Hybrid Water Electrolysis for Low-Cost Hydrogen Production Along with Value-Added Products.

Water electrolysis using renewable energy is considered as a promising technique for sustainable and green hydrogen production. Conventional water electrolysis has two components - hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) occurring at the cathode and anode respectively. However, electrolysis of water suffers from high overpotential due to the slow kinetics of OER. To overcome this hybrid water electrolysis has been developed by replacing conventional anode oxidation producing oxygen with oxidation of cost-effective materials producing value-added chemicals. This review summarizes recent advances in organic oxidative reactions such as alcohols, urea, hydrazine, and biomass at the anode instead of OER. Furthermore, the review also highlights the use of membrane-free hybrid water electrolysis as a method to overcome the cost and complexity associated with conventional membrane-based electrolyzer thereby improving overall efficiency. This approach holds promise for scalable and cost-effective large-scale hydrogen production along with value-added products. Finally, current challenges and future perspectives are discussed for further development in membrane-free hybrid water electrolysis.

A Rechargeable Urea-Assisted Zn-Air Battery with High Energy Efficiency and Fast-Charging Enabled by Engineering High-Energy Interfacial Structures.

Electrochemical urea oxidation reaction (UOR) offers a promising alternative to the oxygen evolution reaction (OER) in clean energy conversion and storage systems. Nickel-based catalysts are highly regarded as promising electrocatalysts for the UOR. However, their effectiveness is significantly hindered by the unavoidable self-oxidation reaction of nickel species during UOR. To address this challenge, we proposed an interface chemistry modulation strategy to boost UOR kinetics by creating a high-energy interfacial heterostructure. This heterostructure features the incorporation of Ag at the CoOOH@NiOOH heterojunction interface. Strong interactions significantly promote the electron exchanges in the heterointerface between the -OH and -O. Consequently, the improved electron delocalization led to the formation of stronger bonds between Co sites and urea CO(NH2)2, promoting a preference for urea to occupy Co active sites over OH*. The resulting catalyst, Ag-CoOOH@NiOOH, affords an ultrahigh UOR activity with a low potential of 1.33 V at 100 mA cm-2. The fabricated catalyst exhibits a mass activity exceeding that of initial cobalt oxyhydroxide by over 11.9 times. The rechargeable urea-assisted zinc-air batteries (ZABs) achieves a record-breaking energy efficiency of 74.56% at 1 mA cm-2, remarkable durability (1000 hours at even a current density of 50 mA cm-2), and quick charge performances.

Harnessing the Unconventional Cubic Phase in 2D LaNiO3 Perovskite for Highly Efficient Urea Oxidation.

Phase engineering is a critical strategy in electrocatalysis, as it allows for the modulation of electronic, geometric, and chemical properties to directly influence the catalytic performance. Despite its potential, phase engineering remains particularly challenging in thermodynamically stable perovskites, especially in a 2D structure constraint. Herein, we report phase engineering in 2D LaNiO3 perovskite using the strongly non-equilibrium microwave shock method. This approach enables the synthesis of conventional hexagonal and unconventional trigonal and cubic phases in LaNiO3 by inducing selective phase transitions at designed temperatures, followed by rapid quenching to allow precise phase control while preserving the 2D porous structure. These phase transitions induce structural distortions in the [LaO]+ layers and the hybridization between Ni 3d and O 2p states, thus modifying local charge distribution and enhancing electron transport during the six-electron urea oxidation reaction (UOR). The cubic LaNiO3 offers optimal electron transport and active site accessibility due to its high structural symmetry and open interlayer spacing, resulting in a low onset potential of 1.27 V and a Tafel slope of 33.1 mV dec-1 for UOR, outperforming most current catalysts. Our strategy features high designability in phase engineering, enabling various electrocatalysts to harness the power of unconventional phases.

Boosting the Production of Hydrogen from an Overall Urea Splitting Reaction Using a Tri-Functional Scandium-Cobalt Electrocatalyst.

The creation of highly efficient and economical electrocatalysts is essential to the massive electrolysis of water to produce clean energy. The ability to use urea reaction of oxidation (UOR) in place of the oxygen/hydrogen evolution process (OER/HER) during water splitting is a significant step toward the production of high-purity hydrogen with less energy usage. Empirical evidence suggests that the UOR process consists of two stages. First, the metal sites undergo an electrochemical pre-oxidation reaction, and then the urea molecules on the high-valence metal sites are chemically oxidized. Here, the use of scandium-doped CoTe supported on carbon nanotubes called Sc@CoTe/CNT is reported and CoTe/CNT as a composite to efficiently promote hydrogen generation from highly durable and active electrocatalysts for the OER/UOR/HER in urea and alkali solutions. Electrochemical impedance spectroscopy indicates that the UOR facilitates charge transfer across the interface. Furthermore, the Sc@CoTe/CNT nanocatalyst has high performance in KOH and KOH-containing urea solutions as demonstrated by the HER, OER, and UOR (215 mV, 1.59, and 1.31 V, respectively, at 10 mA cm-2 in 1 m KOH) and CoTe/CNT shows 195 mV, 1.61 and 1.3 V, respectively. Consequently, the total urea splitting system achieves 1.29 V, whereas the overall water splitting device obtaines 1.49 V of Sc@CoTe/CNT and CoTe/CNT shows 1.54, 1.48 V, respectively. This work presents a viable method of combining HER with UOR for maximally effective hydrogen production.

Insights into the Understanding of the Nickel-Based Pre-Catalyst Effect on Urea Oxidation Reaction Activity.

Nickel-based catalysts are regarded as the most excellent urea oxidation reaction (UOR) catalysts in alkaline media. Whatever kind of nickel-based catalysts is utilized to catalyze UOR, it is widely believed that the in situ-formed Ni3+ moieties are the true active sites and the as-utilized nickel-based catalysts just serve as pre-catalysts. Digging the pre-catalyst effect on the activity of Ni3+ moieties helps to better design nickel-based catalysts. Herein, five different anions of OH-, CO32-, SiO32-, MoO42-, and WO42- were used to bond with Ni2+ to fabricate the pre-catalysts ß-Ni(OH)2, Ni-CO3, Ni-SiO3, Ni-MoO4, and Ni-WO4. It is found that the true active sites of the five as-fabricated catalysts are the same in situ-formed Ni3+ moieties and the five as-fabricated catalysts demonstrate different UOR activity. Although the as-synthesized five catalysts just serve as the pre-catalysts, they determine the quantity of active sites and activity per active site, thus determining the catalytic activity of the catalysts. Among the five catalysts, the amorphous nickel tungstate exhibits the most superior activity per active site and can catalyze UOR to reach 158.10 mA·cm-2 at 1.6 V, exceeding the majority of catalysts. This work makes for a deeper understanding of the pre-catalyst effect on UOR activity and helps to better design nickel-based UOR catalysts.

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

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

Boosting urea-assisted water splitting over P-MoO2@CoNiP through Mo leaching/reabsorption coupling CoNiP reconstruction.

Combining the urea oxidation reaction (UOR) with the hydrogen evolution reaction (HER) is an effective technology for energy-saving hydrogen production. Herein, a bifunctional electrocatalyst with CoNiP nanosheet coating on P-doped MoO2 nanorods (P-MoO2@CoNiP) is obtained via a two-step hydrothermal followed a phosphorization process. The catalyst demonstrates exceptional alkaline HER performance due to the formation of MoO2 and the dissolution/absorption of Mo. Meanwhile, the inclusion of Co and P in the P-MoO2@CoNiP catalyst facilitated the formation of NiOOH, enhancing UOR performance. Density functional theory calculations reveal that the hydrogen adsorption Gibbs free energy (ΔGH*) of P-MoO2@CoNiP is closer to 0 eV than CoNiP, favoring the HER. The catalyst only needs -0.08 and 1.38 V to reach 100 mA cm-2 for catalyzing the HER and UOR, respectively. The full urea electrolysis system driven by P-MoO2@CoNiP requires 1.51 V to achieve 100 mA cm-2, 120 mV lower than the traditional water electrolysis.

Ru nanocrystals modified porous FeOOH nanostructures with open 3D interconnected architecture supported on NiFe foam as high-performance electrocatalyst for oxygen evolution reaction and electrocatalytic urea oxidation.

The construction of binder-free electrodes with well-defined three-dimensional (3D) morphology and optimized electronic structure represents an efficient strategy for the design of high-performance electrocatalysts for the development of efficient green hydrogen technologies. Herein, Ru nanocrystals were modified on 3D interconnected porous FeOOH nanostructures with open network-like frameworks on NiFe foam (Ru/FeOOH@NFF), which were used as an efficient electrocatalyst. In this study, a 3D interconnected porous FeOOH with an open network structure was first electrodeposited on NiFe foam and served as the support for the in-situ modification of Ru nanocrystals. Subsequently, the Ru nanocrystals and abundant oxygen vacancies were simultaneously incorporated into the FeOOH matrix via the adsorption-reduction method, which involved NaBH4 reduction. The Ru/FeOOH@NFF electrocatalyst shows a large specific surface area, abundant oxygen vacancies, and modulated electronic structure, which collectively result in a significant enhancement of catalytic properties with respect to the oxygen evolution reaction (OER) and urea oxidation reaction (UOR). The Ru/FeOOH@NFF catalyst exhibits an outstanding OER performance, requiring a low overpotential (360 mV) at 200 mA cm-2 with a small Tafel slope (58 mV dec-1). Meanwhile, the Ru/FeOOH@NFF catalyst demonstrates more efficient UOR activity for achieving 200 mA cm-2 at a lower overpotential of 272 mV. Furthermore, an overall urea electrolysis cell using the Ru/FeOOH@NFF as the anode and Pt as the cathode (Ru/FeOOH@NFF||Pt) reveals a cell voltage of 1.478 V at 10 mA cm-2 and a prominent durability (120 h at 50 mA cm-2). This work will provide a valuable understanding of the construction of high-performance electrocatalysts with 3D microstructure for promoting urea-assisted water electrolysis.

Structural Design of Nickel Hydroxide for Efficient Urea Electrooxidation.

Urea stands as a ubiquitous environmental contaminant. However, not only does urea oxidation reaction technology facilitate energy conversion, but it also significantly contributes to treating wastewater rich in urea. Furthermore, urea electrolysis has a significantly lower theoretical potential (0.37 V) compared to water electrolysis (1.23 V). As an electrochemical reaction, the catalytic efficacy of urea oxidation is largely contingent upon the catalyst employed. Among the plethora of urea oxidation electrocatalysts, nickel-based compounds emerge as the preeminent transition metal due to their cost-effectiveness and heightened activity in urea oxidation. Ni(OH)2 is endowed with manifold advantages, including structural versatility, facile synthesis, and stability in alkaline environments. This review delineates the recent advancements in Ni(OH)2 catalysts for electrocatalytic urea oxidation reaction, encapsulating pivotal research findings in morphology, dopant incorporation, defect engineering, and heterogeneous architectures. Additionally, we have proposed personal insights into the challenges encountered in the research on nickel hydroxide for urea oxidation, aiming to promote efficient urea conversion and facilitate its practical applications.

Regulating Reconstruction-Engineered Active Sites for Accelerated Electrocatalytic Conversion of Urea.

Reconstruction-engineered electrocatalysts with enriched high active Ni species for urea oxidation reaction (UOR) have recently become promising candidates for energy conversion. However, to inhibit the over-oxidation of urea brought by the high valence state of Ni, tremendous efforts are devoted to obtaining low-value products of nitrogen gas to avoid toxic nitrite formation, undesirably causing inefficient utilization of the nitrogen cycle. Herein, we proposed a mediation engineering strategy to significantly boost high-value nitrite formation to help close a loop for the employment of a nitrogen economy. Specifically, platinum-loaded nickel phosphides (Pt-Ni2P) catalysts exhibit a promising nitrite production rate (0.82 mol kWh-1 cm-2), high stability over 66 h of Zn-urea-air battery operation, and 135 h of co-production of nitrite and hydrogen under 200 mA cm-2 in a zero-gap membrane electrode assembly (MEA) system. The in situ spectroscopic characterizations and computational calculations demonstrated that the urea oxidation kinetics is facilitated by enriched dynamic Ni3+ active sites, thus augmenting the "cyanate" UOR pathway. The C-N cleavage was further verified as the rate-determining step for nitrite generation.

Improved Urea Oxidation Performance via Interface Electron Redistributions of the NiFe(OH)x/MnO2/NF p-p Heterojunction.

The development of highly efficient urea oxidation reaction (UOR) electrocatalysts is the key to simultaneously achieving green hydrogen production and the treatment of urea-containing wastewater. Ni-based electrocatalysts are expected to replace precious metal catalysts for UOR because of their high activity and low cost. However, the construction of Ni-based electrocatalysts that can synergistically enhance UOR still needs further in-depth study. In this study, highly active electrocatalysts of NiFe(OH)x/MnO2 p-p heterostructures are constructed on nickel foam (NF) by electrodeposition (NiFe(OH)x/MnO2/NF), illustrating the effect of electronic structure changes at heterogeneous interfaces on UOR and revealing the catalytic mechanism of UOR. The NiFe(OH)x/MnO2/NF only needs 1.364 V (vs Reversible Hydrogen Electrode, RHE) to reach 10 mA cm-2 for UOR. Structural characterizations and theoretical calculations indicate that energy gap leads to directed charge transfer and redistribution at the heterojunction interface, forming electron-rich (MnO2) and electron-poor (NiFe(OH)x) regions. This enhances the catalyst's adsorption of urea and reaction intermediates, reduces thermodynamic barriers during the UOR process, promotes the formation of Ni3+ phases at lower potentials, and thus improves UOR performance. This work provides a new idea for the development of Ni-based high-efficiency UOR electrocatalysts.

Synergistic modulation of the d-band center in Ni3S2 by selenium and iron for enhanced oxygen evolution reaction (OER) and urea oxidation reaction (UOR).

Efficient production of green hydrogen energy is crucial in addressing the energy crisis and environmental concerns. The oxygen evolution reaction (OER) poses a challenge in conventional overall water electrolysis due to its slow thermodynamically process. Urea oxidation reaction (UOR) offers an alternative anodic oxidation method that is highly efficient and cost-effective, with favorable thermodynamics and sustainability. Recently, there has been limited research on bifunctional catalysts that exhibit excellent activity for both OER and UOR reactions. In this study, we developed a selenium and iron co-doped nickel sulfide (SeFe-Ni3S2) catalyst that demonstrated excellent Tafel slopes of 53.9 mV dec-1 and 16.4 mV dec-1 for OER and UOR, respectively. Density Functional Theory (DFT) calculations revealed that the introduction of metal (iron) and nonmetallic elements (selenium) was found to coordinate the d-band center, resulting in improved adsorption/desorption energies of the catalysts and reduced the overpotentials and limiting potentials for OER and UOR, respectively. This activity enhancement can be attributed to the altered electronic coordination structure after the introduction of selenium (Se) and iron (Fe), leading to an increase in the intrinsic activity of the catalyst. This work offers a new strategy for bifunctional catalysts for OER and UOR, presenting new possibilities for the future development of hydrogen production and novel energy conversion technologies. It contributes towards the urgent search for technologies that efficiently produce green hydrogen energy, providing potential solutions to mitigate the energy crisis and protect the environment.

Coordinated d-p hybridized hcp@fcc NiRu alloy doped by interstitial atoms for boosting urea-assisted simulated seawater electrolysis at industrial current densities.

The production of hydrogen through seawater electrolysis has recently garnered increasing concern. However, hydrogen evolution reaction (HER) by alkaline seawater electrocatalysis is severely impeded by the slow H2O adsorption and H* binding kinetics at industrial current densities. Herein, a face-centered cubic/hexagonal close packed (fcc/hcp) NiRu alloy heterojunction was fabricated on Ni foam (N doped NiRu-inf/NF) by a low-temperature nitrogen plasma activation. Simultaneously, nitrogen atoms are introduced into the alloy to facilitate d-p hybridization. When N doped NiRu-inf/NF is integrated into a dual-electrode cell for urea-assisted seawater electrolysis, it achieves 100 mA cm-2 with an ultra-low voltage of 1.36 V and excellent stability. Density functional theory (DFT) verifies that the robust d-p hybridization among Ni, Ru and N exhibits more energy level matching for H2O molecule adsorption at the Ru sites, while simultaneously reducing the interaction between H* and Ni sites in N-doped NiRu-inf.

Superhydrophilic/superaerophobic CoP/CoMoO4 multi-level hierarchitecture electrocatalyst for urea-assisted hydrogen evolution reaction in alkaline media.

Utilizing the thermodynamically favorable urea oxidation reaction instead of the anodic oxygen precipitation reaction is an alternative pathway for the energy-saving hydrogen production. Therefore, it is significant to explore advanced electrocatalysts for both HER and UOR. In this work, a dendritic heteroarchitectures of 2D CoMoO4 nanosheets deposited on 1D CoP nanoneedles (CoP/CoMoO4-CC) was fabricated as bifunctional electrocatalyst. 1D CoP nanostructure with fast charge transport pathways and 2D CoMoO4 nanostructure with large specific surface area and short paths for electron/mass transport. The unique morphology endows the superhydrophilic and superaerophobic properties, allowing for the rapid contact with the reactants and rapid removal of surface-generated gases. As a result, the CoP/CoMoO4-CC shows efficient bifunctional activity. This work offers a new avenue to rationally design bifunctional electrocatalysts for large-scale practical hydrogen production.

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.

Highly-dispersed 2D NiFeP/CoP heterojunction trifunctional catalyst for efficient electrolysis of water and urea.

The electrocatalytic production of "green hydrogen", such as through the electrolysis of water or urea has been vigorously advocated to alleviate the energy crisis. However, their electrode reactions including oxygen evolution reaction (OER), urea oxidation reaction (UOR), and hydrogen evolution reaction (HER) all suffer from sluggish kinetics, which urgently need catalysts to accelerate the processes. Herein, we design and prepare an OER/UOR/HER trifunctional catalyst by transforming the homemade CoO nanorod into a two-dimensional (2D) ultrathin heterojunction nickel-iron-cobalt hybrid phosphides nanosheet (NiFeP/CoP) via a hydrothermal-phosphorization method. Consequently, a strong electronic interaction was found among the Ni2P/FeP4/CoP heterogeneous interfaces, which regulates the electronic structure. Besides the high mass transfer property of 2D nanosheet, Ni2P/FeP4/CoP displays improved OER/UOR/HER performance. At 10 mA cm-2, the OER overpotential reaches 274 mV in 1.0 M KOH, and the potential of UOR is only 1.389 V in 1.0 M KOH and 0.33 M urea. More strikingly, the two-electrode systems for electrolysis water and urea-assisted electrolysis water assembled by NiFeP/CoP could maintain long-term stability for 35 h and 12 h, respectively. This work may help to pave the way for upcoming research horizons of multifunctional electrocatalysts.

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.