Blending Cr2O3 into a NiO–Ni Electrocatalyst for Sustained Water Splitting

The rising H₂ economy demands active and durable electrocatalysts based on low‐cost, earth‐abundant materials for water electrolysis/photolysis. Here we report nanoscale Ni metal cores over‐coated by a Cr₂O₃‐blended NiO layer synthesized on metallic foam substrates. The Ni@NiO/Cr₂O₃ triphase material exhibits superior activity and stability similar to Pt for the hydrogen‐evolution reaction in basic solutions. The chemically stable Cr₂O₃ is crucial for preventing oxidation of the Ni core, maintaining abundant NiO/Ni interfaces as catalytically active sites in the heterostructure and thus imparting high stability to the hydrogen‐evolution catalyst. The highly active and stable electrocatalyst enables an alkaline electrolyzer operating at 20 mA cm^?2 at a voltage lower than 1.5 V, lasting longer than 3 weeks without decay. The non‐precious metal catalysts afford a high efficiency of about 15 % for light‐driven water splitting using GaAs solar cells.     

Oxide‐Supported IrNiOx Core–Shell Particles as Efficient,Cost‐Effective,and Stable Catalysts for Electrochemical Water Splitting

Active and highly stable oxide‐supported IrNiO_x core–shell catalysts for electrochemical water splitting are presented. IrNi_x@IrO_x nanoparticles supported on high‐surface‐area mesoporous antimony‐doped tin oxide (IrNiO_x /Meso‐ATO) were synthesized from bimetallic IrNi_x precursor alloys (PA‐IrNi_x /Meso‐ATO) using electrochemical Ni leaching and concomitant Ir oxidation. Special emphasis was placed on Ni/NiO surface segregation under thermal treatment of the PA‐IrNi_x /Meso‐ATO as well as on the surface chemical state of the particle/oxide support interface. Combining a wide array of characterization methods, we uncovered the detrimental effect of segregated NiO phases on the water splitting activity of core–shell particles. The core–shell IrNiO_x /Meso‐ATO catalyst displayed high water‐splitting activity and unprecedented stability in acidic electrolyte providing substantial progress in the development of PEM electrolyzer anode catalysts with drastically reduced Ir loading and significantly enhanced durability.     

A General Route to Prepare Low‐Ruthenium‐Content Bimetallic Electrocatalysts for pH‐Universal Hydrogen Evolution Reaction by Using Carbon Quantum Dots

A challenging but pressing task to design and synthesize novel, efficient, and robust pH‐universal hydrogen evolution reaction (HER) electrocatalysts for scalable and sustainable hydrogen production through electrochemical water splitting. Herein, we report a facile method to prepare an efficient and robust Ru‐M (M=Ni, Mn, Cu) bimetal nanoparticle and carbon quantum dot hybrid (RuM/CQDs) for pH‐universal HER. The RuNi/CQDs catalysts exhibit outstanding HER performance at all pH levels. The unexpected low overpotentials of 13, 58, and 18 mV shown by RuNi/CQDs allow a current density of 10 mA cm^?2 in 1 m KOH, 0.5 m H₂SO₄, and 1 m PBS, respectively, for Ru loading at 5.93 μgRu cm^?2. This performance is among the best catalytic activities reported for any platinum‐free electrocatalyst. Theoretical studies reveal that Ni doping results in a moderate weakening of the hydrogen bonding energy of nearby surface Ru atoms, which plays a critical role in improving the HER activity.     

Hollow Nanotube Ru/Cu2+1O Supported on Copper Foam as a Bifunctional Catalyst for Overall Water Splitting

Hydrogen energy is considered as one of the ideal clean energies for solving the energy shortage and environmental issues, and developing highly efficient electrocatalysts for overall water splitting to produce hydrogen is still a huge challenge. Herein, for the first time, Ru-doped Cu₂₊₁O vertically arranged nanotube arrays in situ grown on Cu foam (Ru/Cu₂₊₁O NT/CuF) are reported and further investigated for their catalytic properties for overall water splitting. The Ru/Cu₂₊₁O NT/CuF presents ultrahigh catalytic activities for both the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in alkaline conditions, and it exhibits a small overpotential of 32 mV at 10 mA cm⁻² in the HER, and only needs 210 mV overpotential to achieve a current density of 10 mA cm⁻² in the OER. Importantly, the alkaline electrolyzer using Ru/Cu₂₊₁O NT/CuF as a bifunctional electrocatalyst only needs 1.53 V voltage to deliver a current density of 10 mA cm⁻², which is much lower than the benchmark of IrO₂(+)/Pt(−) counterpart (1.64 V at 10 mA cm⁻²). The excellent performance of the Ru/Cu₂₊₁O NT/CuF catalyst is attributed to its high conductive substrate and special Ru-doped nanotube structure, which provides a high electrochemical active surface area and 3D gas diffusion channel.     

Ru-Doping Enhanced Electrocatalysis of Metal–Organic Framework Nanosheets toward Overall Water Splitting

An Ru-doping strategy is reported to substantially improve both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) electrocatalytic activity of Ni/Fe-based metal–organic framework (MOF) for overall water splitting. As-synthesized Ru-doped Ni/Fe MIL-53 MOF nanosheets grown on nickel foam (MIL-53(Ru-NiFe)@NF) afford HER and OER current density of 50 mA cm⁻² at an overpotential of 62 and 210 mV, respectively, in alkaline solution with a nominal Ru loading of ≈110 μg cm⁻². When using as both anodic and cathodic (pre-)catalyst, MIL-53(Ru-NiFe)@NF enables overall water splitting at a current density of 50 mA cm⁻² for a cell voltage of 1.6 V without iR compensation, which is much superior to state-of-the-art RuO₂-Pt/C-based electrolyzer. It is discovered that the Ru-doping considerably modulates the growth of MOF to form thin nanosheets, and enhances the intrinsic HER electrocatalytic activity by accelerating the sluggish Volmer step and improving the intermediate oxygen adsorption for increased OER catalytic activity.     

Interfacial Coupling of Graphene with Nickel Nanoparticles for Water Splitting and Urea Oxidation: A Spectroelectrochemical Investigation

Nickel nanoparticle and graphene interfaces of various stoichiometries were created through electrodeposition techniques. The catalytic behavior of the electrodeposited films was investigated through spectro-electrochemical methodologies. UV-vis absorbance spectra of the electrodeposited films are significantly different in the air and alkaline medium. Furthermore, UV-vis and Raman spectroscopy confirmed the coupling of Ni nanoparticles (Ni-NP) with the graphene framework, along with NiO and Ni(OH)₂. A combination of Raman and impedance spectroscopy revealed that the surface adsorption and charge transfer properties of the electrodeposited films are entirely dependent on the defects on graphene structure as well as distribution of Ni-NP on graphene. The electrodeposited films possess heterogeneous catalytic properties with a low overpotential of 50 mV (10 mA/cm⁻²) for hydrogen evolution reaction, as well as 601 mV and 391 mV (at 50 mA/cm⁻²) for the oxygen evolution reaction and urea oxidation reaction, respectively. In addition, eelectrodeposited samples show extraordinary overall water splitting performance by achieving a current density of 10 mA/cm² at a very low applied potential of 1.38 V. This synergistic coupling of Ni and graphene renders the electrodeposited samples promising candidates as electrodes for overall water splitting in alkaline and urea-supplemented solutions.     

NiFe水滑石纳米片阵列负载Ru用于提升碱性电催化析氢和析氧性能

通过离子交换的方式将Ru负载到NiFe水滑石(LDH)纳米阵列表面得到(Ru/NiFe LDH),Ru的引入显著提升了NiFe LDH的活性比表面积,暴露了更多的活性位点,同时调控了其电子结构,大大提升了其本征催化活性。在碱性条件下,催化析氢反应时仅需50 mV的过电位即可达到10 mA·cm^-2的电流密度,Tafel斜率为52.3 mV·dec^-1。而相同条件下原始NiFe LDH达到10mA·cm^-2的电流密度则需要226 mV的过电位,Tafel斜率为157.5 mV·dec^-1。同时制备的Ru/NiFe LDH也展现出了良好的析氧催化活性,在50 mA·cm^-2的电流密度下,过电位仅为231 mV,而NiFe LDH则需237 mV。Ru/NiFe LDH在长时间的电催化条件下依然能保持良好的工作稳定性。     

Towards Long‐Term Photostability of Nickel Hydroxide/BiVO4 Photoanodes for Oxygen Evolution Catalysts via In Situ Catalyst Tuning

Increasing long‐term photostability of BiVO₄ photoelectrode is an important issue for solar water splitting. The NiOOH oxygen evolution catalyst (OEC) has fast water oxidation kinetics compared to the FeOOH OEC. However, it generally shows a lower photoresponse and poor stability because of the more substantial interface recombination at the NiOOH/BiVO₄ junction. Herein, we utilize a plasma etching approach to reduce both interface/surface recombination at NiOOH/BiVO₄ and NiOOH/electrolyte junctions. Further, adding Fe²⁺ into the borate buffer electrolyte alleviates the active but unstable character of etched‐NiOOH/BiVO₄, leading to an outstanding oxygen evolution over 200 h. The improved charge transfer and photostability can be attributed to the active defects and a mixture of NiOOH/NiO/Ni in OEC induced by plasma etching. Metallic Ni acts as the ion source for the in situ generation of the NiFe OEC over long‐term durability.     

NiFe水滑石纳米片阵列负载Ru用于提升碱性电催化析氢和析氧性能

通过离子交换的方式将Ru负载到NiFe水滑石（LDH）纳米阵列表面得到（Ru/NiFe LDH），Ru的引入显著提升了NiFe LDH的活性比表面积，暴露了更多的活性位点，同时调控了其电子结构，大大提升了其本征催化活性。在碱性条件下，催化析氢反应时仅需50 mV的过电位即可达到10 mA·cm^-2的电流密度，Tafel斜率为52.3 mV·dec^-1。而相同条件下原始NiFe LDH达到10mA·cm^-2的电流密度则需要226 mV的过电位，Tafel斜率为157.5 mV·dec^-1。同时制备的Ru/NiFe LDH也展现出了良好的析氧催化活性，在50 mA·cm^-2的电流密度下，过电位仅为231 mV，而NiFe LDH则需237 mV。Ru/NiFe LDH在长时间的电催化条件下依然能保持良好的工作稳定性。     

Facile one-step synthesis of Ru doped NiCoP nanoparticles as highly efficient electrocatalysts for oxygen evolution reaction

Transition metal phosphides (TMPs) as ever-evolving electrocatalytic materials have attracted increasing attention in water splitting reactions owing to their cost-effective, highly active and stable catalytic properties. This work presents a facile synthetic route to NiCoP nanoparticles with Ru dopants which function as highly efficient electrocatalysts for oxygen evolution reaction (OER) in alkaline media. The Ru dopants induced a high content of Ni and Co vacancies in NiCoP nanoparticles, and the more defective Ru doped NiCoP phase than undoped NiCoP ones led to a greater number of catalytically active sites and improved electrical conductivity after undergoing electrochemical activation. The Ru doped NiCoP catalyst exhibited high OER catalytic performance in alkaline media with a low overpotential of 281 mV at 10 mA cm⁻² and a Tafel slope of 42.7 mV dec⁻¹.     

Oxide‐Supported IrNiOx Core–Shell Particles as Efficient,Cost‐Effective,and Stable Catalysts for Electrochemical Water Splitting

Active and highly stable oxide‐supported IrNiO_x core–shell catalysts for electrochemical water splitting are presented. IrNi_x@IrO_x nanoparticles supported on high‐surface‐area mesoporous antimony‐doped tin oxide (IrNiO_x /Meso‐ATO) were synthesized from bimetallic IrNi_x precursor alloys (PA‐IrNi_x /Meso‐ATO) using electrochemical Ni leaching and concomitant Ir oxidation. Special emphasis was placed on Ni/NiO surface segregation under thermal treatment of the PA‐IrNi_x /Meso‐ATO as well as on the surface chemical state of the particle/oxide support interface. Combining a wide array of characterization methods, we uncovered the detrimental effect of segregated NiO phases on the water splitting activity of core–shell particles. The core–shell IrNiO_x /Meso‐ATO catalyst displayed high water‐splitting activity and unprecedented stability in acidic electrolyte providing substantial progress in the development of PEM electrolyzer anode catalysts with drastically reduced Ir loading and significantly enhanced durability.     

Hierarchical Architectures Based on Ru Nanoparticles/Oxygen-Rich-Carbon Nanotubes for Efficient Hydrogen Evolution

Highly active and durable electrocatalysts are essential for producing hydrogen fuel through the hydrogen evolution reaction (HER). Here, a uniform deposition of Ru nanoparticles strongly interacting with oxygen-rich carbon nanotube architectures (Ru-OCNT) through ozonation and hydrothermal approaches has been designed. The hierarchical structure of Ru-OCNT is made by self-assembly of oxygen functionalities of OCNT. Ru nanoparticles interact strongly with OCNT at the Ru/OCNT interface to give excellent catalytic activity and stability of the Ru-OCNT, as further confirmed by density functional theory. Owing to the hierarchical structure and adjusted surface chemistry, Ru-OCNT has an overpotential of 34 mV at 10 mA cm⁻² with a Tafel slope of 27.8 mV dec⁻¹ in 1 M KOH, and an overpotential of 55 mV with Tafel slope of 33 mV dec⁻¹ in 0.5 M H₂SO₄. The smaller Tafel slope of Ru-OCNT than Ru-CNT and commercial Pt/C in both alkaline and acidic electrolytes indicates high catalytic activity and fast charge transfer kinetics. The as-proposed chemistry provides the rational design of hierarchically structured CNT/nanoparticle electrocatalysts for HER to produce hydrogen fuel.     

X-ray and UV photoemission studies of valence electronic structure of NiO

The X-ray and UV photoemission valence band spectra of NiO are interpreted using the molecular orbital theory for the NiO^10?₆ cluster and the sudden approximation (monopole selection rules). They exhibit the effects of crystal field splitting, multiplet splitting, electron shake-up (O 2pe^b_g→ Ni 3de^a_g). relaxation and Ni 3dO 2p hybridization. Shake-up satellite data indicate that the NiO optical absorption edge near 4 eV is associated with an O 2p → Ni 3d transition. The NiO valence electronic structure obtained in this work is compared with band structure models of Wilson and Mattheiss.     

Impact of Surface Defects on LaNiO3 Perovskite Electrocatalysts for the Oxygen Evolution Reaction

Perovskite oxides are regarded as promising electrocatalysts for water splitting due to their cost-effectiveness, high efficiency and durability in the oxygen evolution reaction (OER). Despite these advantages, a fundamental understanding of how critical structural parameters of perovskite electrocatalysts influence their activity and stability is lacking. Here, we investigate the impact of structural defects on OER performance for representative LaNiO₃ perovskite electrocatalysts. Hydrogen reduction of 700 °C calcined LaNiO₃ induces a high density of surface oxygen vacancies, and confers significantly enhanced OER activity and stability compared to unreduced LaNiO₃; the former exhibit a low onset overpotential of 380 mV at 10 mA cm⁻² and a small Tafel slope of 70.8 mV dec⁻¹. Oxygen vacancy formation is accompanied by mixed Ni²⁺/Ni³⁺ valence states, which quantum-chemical DFT calculations reveal modify the perovskite electronic structure. Further, it reveals that the formation of oxygen vacancies is thermodynamically more favourable on the surface than in the bulk; it increases the electronic conductivity of reduced LaNiO₃ in accordance with the enhanced OER activity that is observed.     

Dual-Atom Support Boosts Nickel-Catalyzed Urea Electrooxidation

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

Interfacial Engineering FeOOH/CoO Nanoneedle Array for Efficient Overall Water Splitting Driven by Solar Energy

Interface engineering has been applied as an effective strategy to boost the electrocatalytic performance because of the strong coupling and synergistic effects between individual components. Here, we engineered vertically aligned FeOOH/CoO nanoneedle array with a synergistic interface between FeOOH and CoO on Ni foam (NF) by a simple impregnation method. The synthesized FeOOH/CoO exhibits outstanding electrocatalytic activity and stability for both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in an alkaline medium. For the overall water splitting, the bifunctional FeOOH/CoO nanoneedle catalyst requires only a cell voltage of 1.58 V to achieve a current density of 10 mA cm⁻², which is much lower than that required for IrO₂//Pt/C (1.68 V). The FeOOH/CoO catalyst has been successfully applied for solar cell-driven water electrolysis, revealing its great potential for commercial hydrogen production and solar energy storage.     

Ru纳米团簇/MoO3-x纳米带双功能催化剂的合成及其肼辅助全解水性能

将超小Ru纳米团簇锚定于富含氧空位MoO_3-x纳米带的双功能催化剂（Ru/MoO_3-x）。该催化剂展现出优异的肼氧化（HzOR）和析氢反应（HER）催化性能,10 mA·cm^-2时的过电势分别为-79和-27 mV,所组成的肼辅助电催化全解水（OWS）的电解池电压仅为13 mV,明显优于商业化20% Pt/C和已报道的一些催化剂。如此优异的性能主要归因于Ru纳米团簇有利于HzOR中N₂H₄的脱氢和HER氢中间体的吸/脱附平衡以及MoO_3-x中的氧空位和Ru/MoO_3-x异质结构导致的丰富的电化学活性位点和优化的电子转移动力学。     

Partially Pyrolyzed Binary Metal–Organic Framework Nanosheets for Efficient Electrochemical Hydrogen Peroxide Synthesis

Herein, we developed a partially controlled pyrolysis strategy to create evenly distributed NiO nanoparticles within NiFe‐MOF nanosheets (MOF NSs) for electrochemical synthesis of H₂O₂ by a two‐electron oxygen reduction reaction (ORR). The elemental Ni can be partially transformed to NiO and uniformly distributed on the surface of the MOF NSs, which is crucial for the formation of the particular structure. The optimized MOF NSs‐300 exhibits the highest activity for ORR with near‐zero overpotential and excellent H₂O₂ selectivity (ca. 99 %) in 0.1 m KOH solution. A high‐yield H₂O₂ production rate of 6.5 mol g_cat^?1 h^?1 has also been achieved by MOF NSs‐300 in 0.1 m KOH and at 0.6 V (vs. RHE). In contrast to completely pyrolyzed products, the enhanced catalytic activities of partially pyrolyzed MOF NSs‐300 originates mainly from the retained MOF structure and the newly generated NiO nanoparticles, forming the coordinatively unsaturated Ni atoms and tuning the performance towards electrochemical H₂O₂ synthesis.     

Fabrication of a Hierarchical Ni(OH)2@Ni3S2/Ni Foam Electrode from a Prussian Blue Analogue-Based Composite with Enhanced Electrochemical Capacitive and Electrocatalytic Properties

Developing high-efficiency, cost-effective, and durable electrodes is significant for electrochemical capacitors and electrocatalysis. Herein, a 3D bifunctional electrode consisting of nickel hydroxide nanosheets@nickel sulfide nanocubes arrays on Ni foam (Ni(OH)₂@Ni₃S₂/NF) obtained from a Prussian blue analogue-based precursor is reported. The 3D higher-order porous structure and synergistic effect of different compositions endow the electrode with large specific surface area, facile ion/electron transport path, and improved conductivity. As a result, the Ni(OH)₂@Ni₃S₂/NF electrode exhibits a high specific capacity of 211 mA h g⁻¹ at a current density of 1 A g⁻¹ and 73 % capacity retention after 5000 cycles at 5 A g⁻¹. Moreover, the Ni(OH)₂@Ni₃S₂/NF electrode has superior electrocatalytic activity for the hydrogen evolution reaction with low overpotentials of 140 and 210 mV at current densities of 10 and 100 mA cm⁻², respectively. The synthetic strategy for the unique higher-order porous structure can be extended to fabricate other composite materials for energy storage and conversion.     

Iron-Doped Nickel Molybdate with Enhanced Oxygen Evolution Kinetics

Electrochemical water splitting is one of the potential approaches for making renewable energy production and storage viable. The oxygen evolution reaction (OER), as a sluggish four-electron electrochemical reaction, has to overcome high overpotential to accomplish overall water splitting. Therefore, developing low-cost and highly active OER catalysts is the key for achieving efficient and economical water electrolysis. In this work, Fe-doped NiMoO₄ was synthesized and evaluated as the OER catalyst in alkaline medium. Fe³⁺ doping helps to regulate the electronic structure of Ni centers in NiMoO₄, which consequently promotes the catalytic activity of NiMoO₄. The overpotential to reach a current density of 10 mA cm⁻² is 299 mV in 1 m KOH for the optimal Ni_0.9Fe_0.1MoO₄, which is 65 mV lower than that for NiMoO₄. Further, the catalyst also shows exceptional performance stability during a 2 h chronopotentiometry testing. Moreover, the real catalytically active center of Ni_0.9Fe_0.1MoO₄ is also unraveled based on the ex situ characterizations. These results provide new alternatives for precious-metal-free catalysts for alkaline OER and also expand the Fe-doping-induced synergistic effect towards performance enhancement to new catalyst systems.