Synergistic Coupling of Ni Nanoparticles with Ni3 C Nanosheets for Highly Efficient Overall Water Splitting

Integrating NiSe2-MoSe2 heterojunctions with N-doped porous carbon substrate architecture for an enhanced electrocatalytic water splitting device

The development of sustainable energy technologies relies on the exploitation of efficient and durable electrocatalysts for water splitting at high current densities. Our work presents a novel bifunctional catalyst, denoted as NM@NC/CC, which combines the benefits of NiSe2-MoSe2 heterojunctions with nitrogen-enriched porous carbon derived from metal-organic frameworks (MOFs). The integration of these components is designed to harness their combined advantages, which include enhanced electron transfer, improved mass and gas evolution dynamics, and an increased number of catalytically active sites. These features collectively optimize the energetics for both the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). As a result, the catalyst facilitates rapid kinetics for the overall water-splitting process. The NM@NC/CC demonstrates low overpotentials, requiring only 91 mV for the HER and 280 mV for the OER to reach a current density of 10 mA cm-2. Even at higher current densities of 100 mA cm-2 for HER and 50 mA cm-2 for OER, the overpotentials are only 159 mV and 350 mV, respectively. Additionally, a two-electrode setup using this catalyst achieves a current density of 10 mA cm-2 with a minimal cell voltage of 1.56 V. The insights gained from this study will contribute to the advancement of electrocatalysts for energy conversion technologies.

Ru-regulated electronic structure CoNi-MOF nanosheets advance water electrolysis kinetics in alkaline and seawater media

Herein, an ion-exchange strategy is utilized to greatly improve the kinetics of hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) by Ru-modified CoNi- 1,3,5-Benzenetricarboxylic acid (BTC)-metal organic framework nanosheets (Ru@CoNi-MOF). Due to the higher Ni active sites and lower electron transfer impedance, Ru@CoNi-MOF catalyst requires the overpotential as low as 47 and 279 mV, at a current density of 10 mA/cm2 toward HER and OER, respectively. Significantly, the mass activity of Ru@CoNi-MOF for HER and OER are 25.9 and 10.6 mA mg-1, nearly 15.2 and 8.8 times higher than that of Ni-MOF. In addition, the electrolyzer of Ru@CoNi-MOF demonstrates exceptional electrolytic performance in both KOH and seawater environment, surpasses the commercial Pt/C||IrO2 couple. Theoretical calculations prove that introducing Ru atoms in - CoNi-MOF modulates the electronic structure of Ni, optimizes adsorption energy for H* and reduces energy barrier of metal organic frameworks (MOFs). This modification significantly improves the kinetic rate of the Ru@CoNi-MOF during water splitting. Certainly, this study highlights the utilization of MOF nanosheets as advanced HER/OER electrocatalysts with immense potential, and will paves a way to develop more efficient MOFs for catalytic applications.

NiFeRu/C and Ru, Fe-Ni5P4/C as complementary electrocatalyst for highly efficient overall water splitting

Designing and fabricating highly competent and inexpensive electrocatalysts are highly desirable for application in electrocatalytic water splitting. In this study, we synthesized NiFeRu/C nanofibers and Ru, Fe dual-doped Ni5P4 (Ru, Fe-Ni5P4)/C nanofibers as complementary electrocatalysts for overall water splitting through electrospinning, carbonization, and phosphorization treatment, respectively. The NiFeRu/C nanofibers and Ru, Fe-Ni5P4/C nanofibers showed high hydrogen evolution reaction and oxygen evolution reaction activity, respectively, due to the presence of numerous exposed active sites and optimized adsorption capacity for the reaction intermediates contributed by the synergistic interaction among different metal components in the electrocatalysts. Hence, the assembled asymmetrical electrolytic cell effectively promoted overall water splitting, requiring a voltage of 1.569, 1.744, and 1.872 V to achieve a current density of 100, 500, and 1,000 mA cm-2, respectively, and it was better than Pt/C||IrO2. Additionally, the electrolytic cell could work at 500 mA cm-2 for 100 h without any noticeable deterioration in activity, which indicated that it was durable at high current density. In this study, we described a novel method for designing highly efficient electrocatalysts for overall water splitting.

Internal Polarization Field Induced Hydroxyl Spillover Effect for Industrial Water Splitting Electrolyzers

The formation of multiple oxygen intermediates supporting efficient oxygen evolution reaction (OER) are affinitive with hydroxyl adsorption. However, ability of the catalyst to capture hydroxyl and maintain the continuous supply at active sits remains a tremendous challenge. Herein, an affordable Ni2P/FeP2 heterostructure is presented to form the internal polarization field (IPF), arising hydroxyl spillover (HOSo) during OER. Facilitated by IPF, the oriented HOSo from FeP2 to Ni2P can activate the Ni site with a new hydroxyl transmission channel and build the optimized reaction path of oxygen intermediates for lower adsorption energy, boosting the OER activity (242 mV vs. RHE at 100 mA cm-2) for least 100 h. More interestingly, for the anion exchange membrane water electrolyzer (AEMWE) with low concentration electrolyte, the advantage of HOSo effect is significantly amplified, delivering 1 A cm-2 at a low cell voltage of 1.88 V with excellent stability for over 50 h.

Fe2O3/Ni Nanocomposite Electrocatalyst on Cellulose for Hydrogen Evolution Reaction and Oxygen Evolution Reaction

A key challenge in the development of sustainable water-splitting (WS) systems is the formulation of electrodes by efficient combinations of electrocatalyst and binder materials. Cellulose, a biopolymer, can be considered an excellent dispersing agent and binder that can replace high-cost synthetic polymers to construct low-cost electrodes. Herein, a novel electrocatalyst was fabricated by combining Fe2O3 and Ni on microcrystalline cellulose (MCC) without the use of any additional binder. Structural characterization techniques confirmed the formation of the Fe2O3-Ni nanocomposite. Microstructural studies confirmed the homogeneity of the ~50 nm-sized Fe2O3-Ni on MCC. The WS performance, which involves the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER), was evaluated using a 1 M KOH electrolyte solution. The Fe2O3-Ni nanocomposite on MCC displayed an efficient performance toward lowering the overpotential in both the HER (163 mV @ 10 mA cm-2) and OER (360 mV @ 10 mA cm-2). These results demonstrate that MCC facilitated the cohesive binding of electrocatalyst materials and attachment to the substrate surface. In the future, modified cellulose-based structures (such as functionalized gels and those dissolved in various media) can be used as efficient binder materials and alternative options for preparing electrodes for WS applications.

Enhanced Electrocatalytic Water Oxidation by Interfacial Phase Transition and Photothermal Effect in Multiply Heterostructured Co9 S8 /Co3 S4 /Cu2 S Nanohybrids

The rational design of advanced nanohybrids (NHs) with optimized interface electronic environment and rapid reaction kinetics is pivotal to electrocatalytic schedule. Herein, we developed a multiple heterogeneous Co9 S8 /Co3 S4 /Cu2 S nanoparticle in which Co3 S4 germinates between Co9 S8 and Cu2 S. Using high-angle annular-dark-field imaging and theoretical calculation, it was found that the integration of Co9 S8 and Cu2 S tends to trigger the interface phase transition of Co9 S8 , leading to Co3 S4 interlayer due to the low formation energy of Co3 S4 /Cu2 S (-7.61 eV) than Co9 S8 /Cu2 S (-5.86 eV). Such phase transition not only lowers the energy barrier of oxygen evolution reaction (OER, from 0.335 eV to 0.297 eV), but also increases charge carrier density (from 7.76×1014 to 2.09×1015  cm-3 ), and creates more active sites. Compared to Co9 S8 and Cu2 S, the Co9 S8 /Co3 S4 /Cu2 S NHs also demonstrate notable photothermal effect that can heat the catalyst locally, offset the endothermic enthalpy change of OER, and promote carrier migrate, reaction intermediates adsorption/deprotonation to improve reaction kinetics. Profiting from these favorable factors, the Co9 S8 /Co3 S4 /Cu2 S catalyst only requires an OER overpotential of 181 mV and overall water splitting cell voltage of 1.43 V to driven 10 mA cm-2 under the irradiation of near-infrared light, outperforming those without light irradiation and many reported Co-based catalysts.

Nanoflower-like FeVNi3S2-xas efficient electrocatalyst for alkaline oxygen evolution reaction

The development of low cost efficient catalysts for oxygen evolution reaction (OER) is still a obstacle to realize the commercialization of electrocatalytic water splitting. Herein, interface engineering and heteroatom doping is adopted to synthesize iron and vanadium doped nickel sulfide on nickel foam via hydrothermal method followed by hydrogen treatment to create sulfur defects. The optimized nanoflower-like FeVNi3S2-x/NF is an efficient OER electrocatalyst that outperforms many of the reported transition metals catalysts. Benefiting from abundant sulfur defects and the synergistic effect of heteroatom doping, FeVNi3S2-x/NF exhibits an ultralow overpotential of 230 mV to reach a current density of 100 mA cm-2, a rapid reaction kinetics with a small Tafel slope of 46.6 mV dec-1, and a stable long-term durability in 1 M KOH. Experimental results and characterizations confirm that sulfur vacancies together with the synergistic effect from multiple heteroatom doping can effectively regulate the electronic structure, resulting in increased electrical conductivity and electrochemically active surface area, thus enhancing OER performance. Furthermore,in situRaman spectroscopy reveals that, the reconstitution amorphous nickel oxyhydroxide (NiOOH) on the catalyst surface is responsible for catalyzing the OER reaction. This work represents a promising methodology to synthesize low-cost and highly active OER electrocatalysts.

A High-Performance MoS2-Based Visible-Near-Infrared Photodetector from Gateless Photogating Effect Induced by Nickel Nanoparticles

Recent advancements in two-dimensional materials have shown huge potential for optoelectronic applications. It is challenging to achieve highly effective and sensitive broadband photodetection based on MoS₂ devices. Defect engineering, such as introducing vacancies, can narrow the bandgap and boost the separation of photogenerated carriers by defect states but leads to a slow response speed. Herein, we propose a nickel nanoparticle-induced gateless photogating effect with a unique energy band structure to enable the application of defect engineering and achieve high optoelectronic performance. The device based on Ni nanoparticle-decorated MoS₂ with S vacancies exhibited high responsivities of 106.21 and 1.38 A W^-1 and detectivities of 1.9 × 10¹² and 8.9 × 10⁹ Jones under 532 and 980 nm illumination (visible to near infrared), respectively, with highly accelerated response speed. This strategy provides new insight into optimizing defect engineering to design high-performance optoelectronic devices capable of broadband photodetection.

Developing Outstanding Bifunctional Electrocatalysts for Rechargeable Zn-Air Batteries Using High-Purity Spinel-Type ZnCo2 Se4 Nanoparticles

Zinc-air batteries are gaining popularity as viable energy sources for green energy storage technologies. The cost and performance of Zn-air batteries are mostly determined by the air electrodes in combination with an oxygen electrocatalyst. This research aims at the particular innovations and challenges relating to air electrodes and related materials. Here, a nanocomposite of ZnCo₂ Se₄ @rGO that exhibits excellent electrocatalytic activity for the oxygen reduction reaction, ORR (E_1/2  = 0.802 V), and oxygen evolution reaction, OER (η₁₀  = 298 mV@10 mA cm^-2 ) is synthesized. In addition, a rechargeable zinc-air battery with ZnCo₂ Se₄ @rGO as the cathode showed a high open circuit voltage (OCV) of 1.38 V, a peak power density of 210.4 mW cm^-2 , and outstanding long-term cycling stability. The electronic structure and oxygen reduction/evolution reaction mechanism of the catalysts ZnCo₂ Se₄ and Co₃ Se₄ are further investigated using density functional theory calculations. Finally, a perspective for designing, preparing, and assembling air electrodes is suggested for the future developments of high-performance Zn-air batteries.

Interfacial engineering of Co5.47N/Mo5N6 nanosheets with rich active sites synergistically accelerates water dissociation kinetics for Pt-like hydrogen evolution

The development of highly efficient hydrogen evolution electrocatalysts with platinum-like activity requires precise control of active sites through interface engineering strategies. In this study, a heterostructured Co_5.47N/Mo₅N₆ catalyst (CoMoN_x) on carbon cloth (CC) was synthesized using a combination of dip-etching and vapor nitridation methods. The rough nanosheet surface of the catalyst with uniformly distributed elements exposes a large active surface area and provides abundant interface sites that serve as additional active sites. The CoMoN_x was found to exhibit exceptional hydrogen evolution reaction (HER) activity with a low overpotential of 44 mV at 10 mA cm^-2 and exceptional stability of 100 h in 1.0 M KOH. The CoMoN_x^(-)||RuO₂⁽⁺⁾ system requires only 1.81 V cell voltage to reach a current density of 200 mA cm^-2, surpassing the majority of previously reported electrolyzers. Density functional theory (DFT) calculations reveal that the strong synergy between Co_5.47N and Mo₅N₆ at the interface can significantly reduce the water dissociation energy barrier, thereby improving the kinetics of hydrogen evolution. Furthermore, the rough nanosheet architecture of the CoMoN_x catalyst with abundant interstitial spaces and multi-channels enhances charge transport and reaction intermediate transportation, synergistically improving the performance of the HER for water splitting.

Introducing High-Valence Iridium Single Atoms into Bimetal Phosphides toward High-Efficiency Oxygen Evolution and Overall Water Splitting

Single atoms are superior electrocatalysts having high atomic utilization and amazing activity for water oxidation and splitting. Herein, this work reports a thermal reduction method to introduce high-valence iridium (Ir) single atoms into bimetal phosphide (FeNiP) nanoparticles toward high-efficiency oxygen evolution reaction (OER) and overall water splitting. The presence of high-valence single Ir atoms (Ir⁴⁺ ) and their synergistic interaction with Ni³⁺ species as well as the disproportionation of Ni³⁺ assisted by Fe collectively contribute to the exceptional OER performance. In specific, at appropriate Ir/Ni and Fe/Ni ratios, the as-prepared Ir-doped FeNiP (Ir₂₅ -Fe₁₆ Ni₁₀₀ P₆₄ ) nanoparticles at a mass loading of only 35 µg cm^-2 show the overpotential as low as 232 mV at 10 mA cm^-2 and activity as high as 1.86 A mg^-1 at 1.5 V versus RHE for OER in 1.0 m KOH. Computational simulations confirm the vital role of high-valence Ir to weaken the adsorption of OER intermediates, favorable for accelerating OER kinetics. Impressively, a Pt/C||Ir₂₅ -Fe₁₆ Ni₁₀₀ P₆₄ two-electrode alkaline electrolyzer affords a current density of 10 mA cm^-2 at a low cell voltage of 1.42 V, along with satisfied stability. An AA battery with a nominal voltage of 1.5 V can drive overall water splitting with obvious bubbles released.

Fabrication of Ultra-Durable and Flexible NiPx -Based Electrode toward High-Efficient Alkaline Seawater Splitting at Industrial Grade Current Density

Designing nonprecious metal-based electrocatalysts to yield sustainable hydrogen energy by large-scale seawater electrolysis is challenging to global emissions of carbon neutrality and carbon peaking. Herein, a series of highly efficient, economical, and robust Ni-P-based nanoballs grown on the flexible and anti-corrosive hydrophobic asbestos (NiP_x @HA) is synthesized by electroless plating at 25 °C toward alkaline simulated seawater splitting. On the basis of the strong chemical attachment between 2D layered substrate and nickel-rich components, robust hexagonal Ni₅ P₄ crystalline modification, and fast electron transfer capability, the overpotentials during hydrogen/oxygen evolution reaction (HER/OER) are 208 and 392 mV at 200 mA cm^-2 , and the chronopotentiometric measurement at 500 mA cm^-2 lasts for over 40 days. Additionally, the versatile strategy is broadly profitable for industrial applications and enables multi-elemental doping (iron/cobalt/molybdenum/boron/tungsten), flexible substrate employment (nickel foam/filter paper/hydrophilic cloth), and scalable synthesis (22 cm × 22 cm). Density functional theory (DFT) also reveals that the optimized performance is due to the fundamental effect of incorporating O-source into Ni₅ P₄ . Therefore, this work exhibits a complementary strategy in the construction of NiP_x -based electrodes and offers bright opportunities to produce scalable hydrogen effectively and stably in alkaline corrosive electrolytes.

Tube-Sponge-Inspired Hierarchical Electrocatalysts with Boosted Mass and Electron Transfer for Efficient Oxygen Evolution

Hindered gas bubble release and limited electron conducting process represent the major bottlenecks for large-scale electrochemical water splitting. Both the desorption of bubbles and continuous electron transport are achievable on the surfaces of biomimetic catalytic materials by designing multiscale structural hierarchy. Inspired by the tubular structures of the deep-sea sponges, an exceptionally active and binder-free porous nickel tube arrays (PNTA) decorated with NiFe-Zn²⁺ -pore nanosheets (NiFe-P_Zn ) are fabricated. The PNTA facilitate removal of bubbles and electron transfer in the oxygen evolution reaction by reproducing trunks of the sponges, and simultaneously, the NiFe-P_Zn increase the number of catalytic active sites by simulating the sponge epidermis. With improved external mass transfer and interior electron transfer, the hierarchical NiFe-P_Zn @PNTA electrode exhibits superior oxygen evolution reaction performance with an overpotential of 172 mV at 10 mA cm^-2 (with a Tafel slope of 50 mV dec^-1 ). Furthermore, this electrocatalytic system recorded excellent reaction stability over 360 h with a constant current density of 100 mA cm^-2 at the potential of 1.52 V (versus RHE). This work provides a new strategy of designing hierarchical electrocatalysts for highly efficient water splitting.

Low temperature in situ immobilization of nanoscale fcc and hcp polymorphic nickel particles in polymer-derived Si-C-O-N(H) to promote electrocatalytic water oxidation in alkaline media

We synthesized nickel (Ni) nanoparticles (NPs) in a high specific surface area (SSA) p-block element-containing inorganic compound prepared via the polymer-derived ceramics (PDC) route to dispatch the obtained nanocomposite towards oxygen evolution reaction (OER). The in situ formation of Ni NPs in an amorphous silicon carboxynitride (Si-C-O-N(H)) matrix is allowed by the reactive blending of a polysilazane, NiCl2 and DMF followed by the subsequent thermolysis of the Ni : organosilicon polymer coordination complex at a temperature as low as 500 °C in flowing argon. The final nanocomposite displays a BET SSA as high as 311 m2 g-1 while the structure of the NPs corresponds to face-centred cubic (fcc) Ni along with interstitial-atom free (IAF) hexagonal close-packed (hcp) Ni as revealed by XRD. A closer look into the compound through FEG-SEM microscopy confirms the formation of pure metallic Ni, while HR-TEM imaging reveals the occurrence of Ni particles featuring a fcc phase and surrounded by carbon layers; thus, forming core-shell structures, along with Ni NPs in an IAF hcp phase. By considering that this newly synthesized material contains only Ni without doping (e.g., Fe) with a low mass loading (0.15 mg cm-2), it shows promising OER performances with an overpotential as low as 360 mV at 10 mA cm-2 according to the high SSA matrix, the presence of the IAF hcp Ni NPs and the development of core-shell structures. Given the simplicity, the flexibility, and the low cost of the proposed synthesis approach, this work opens the doors towards a new family of very active and stable high SSA nanocomposites made by the PDC route containing well dispersed and accessible non-noble transition metals for electrocatalysis applications.

Self-standing Fe3O4 decorated paper electrode as a binder-free trifunctional electrode for electrochemical ammonia synthesis and Zn-O2 batteries

The conversion of the abundant biodegradable material into electroactive electrode material can be a good resource for sustainable energy conversion and storage applications. Herein, we present a simple, cost-effective and green approach for the fabrication of a flexible cellulose paper electrode using an electroless-electrodeposition method. The one-step electroless deposition route is followed to induce conductivity into a non-conductive cellulose paper substrate without using any expensive activators or sensitisers. The Fe₃O₄ is then electro-deposited as an active catalyst over the conductive paper substrate for use in electrochemical activities. The as-fabricated paper electrode shows promising activity and stability during the dinitrogen reduction reaction (NRR) as well as oxygen bifunctional electrocatalysis. A faradaic efficiency of 4.32% with a yield rate of 245 μg h^-1 mg_cat^-1 at -0.1 V is achieved for NRR whereas a very small overpotential of 180 mV is required to reach 10 mA cm^-2 during OER, and the ORR reaction starts at the onset potential of 0.86 V. The practical applicability of the paper electrode is validated by assembling a Zn-O₂ battery showing a peak power density of 81 mW cm^-2 and a stability up to 35 h during charge-discharge cycles, which can power the NRR to produce NH₃ under full cell conditions.

Interfacial Electronic Rearrangement and Synergistic Catalysis for Alkaline Water Splitting in Carbon-Encapsulated Ni (111)/Ni3C (113) Heterostructures

The realization of efficient water electrolysis is still blocked by the requirement for a high and stable driving potential above thermodynamic requirements. An Ni-based electrocatalyst, is a promising alternative for noble-metal-free electrocatalysts but tuning its surface electronic structure and exposing more active sites are the critical challenges to improving its intrinsic catalytic activity. Here, we tackle the challenge by tuning surface electronic structures synergistically with interfacial chemistry and crystal facet engineering, successfully designing and synthesizing the carbon-encapsulated Ni (111)/Ni3C (113) heterojunction electrocatalyst, demonstrating superior hydrogen evolution reaction (HER) activities, good stabilities with a small overpotential of −29 mV at 10 mA/cm2, and a low Tafel slope of 59.96 mV/dec in alkaline surroundings, approximating a commercial Pt/C catalyst and outperforming other reported Ni-based catalysts. The heterostructure electrocatalyst operates at 1.55 V and 1.26 V to reach 10 and 1 mA cm−2 in two-electrode measurements for overall alkaline water splitting, corresponding to 79% and 98% electricity-to-fuel conversion efficiency with respect to the lower heating value of hydrogen.

Ni-O4 as Active Sites for Efficient Oxygen Evolution Reaction with Electronic Metal-Support Interactions

Precise adjustment of the metal site structure in single-atom catalysts (SACs) plays a key role in addressing the oxygen evolution reaction (OER). Herein, we report the synthesis of O-doped Ni SACs anchored on porous graphene-like carbon (Ni-O-G) using molten salts (ZnCl₂ and NaCl) as templates, in which the unique Ni-O₄ structure serves as the active sites. Ni-O-G, with an overpotential of only 238 mV (@ 10 mA cm^-2), is one of the more advanced catalysts. An array of characterizations and density functional theory calculations show that the Ni-O₄ coordination enables Ni to be closer to the Fermi level compared to traditional Ni-N₄, enhancing the electronic metal-support interaction to facilitate OER kinetics. Thus, this work offers an alternative strategy for the structural modulation of Ni SACs and the effect of different coordination elements with the same atomic coordination structure on the intrinsic OER activity.

Simple Synthesis and Characterization of Shell-Thickness-Controlled Ni/Ni3C Core-Shell Nanoparticles

Ni/Ni₃C core-shell nanoparticles with an average diameter of approximately 120 nm were carburized via a chemical solution method using triethylene glycol. It was found that over time, the nanoparticles were covered with a thin Ni₃C shell measuring approximately 1–4 nm, and each Ni core was composed of poly grains. The saturation magnetization of the core-shell nanopowders decreased in proportion to the amount of Ni₃C. The synthesis mechanism of the Ni/Ni₃C core-shell nanoparticles was proposed through X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) analyses.

Tiny Ni Nanoparticles Embedded in Boron- and Nitrogen-Codoped Porous Carbon Nanowires for High-Efficiency Water Splitting

The integration of nickel (Ni) nanoparticle (NP)-embedded carbon layers (Ni@C) into the three-dimensional (3D) hierarchically porous carbon architectures, where ultrahigh boron (B) and nitrogen (N) doping is a potential methodology for boosting Ni catalysts' water splitting performances, was achieved. In this study, the novel 3D ultrafine Ni NP-embedded and B- and N-codoped hierarchically porous carbon nanowires (denoted as Ni@BNPCFs) were successfully synthesized via pyrolysis of the corresponding 3D nickel acetate [Ni(AC)₂·4H₂O]-hydroxybenzeneboronic acid-polyvinylpyrrolidone precursor networks woven by electrospinning. After optimizing the pyrolysis temperatures, various structural and morphological characterization analyses indicate that the optimal Ni@BNPCFs-900 networks own a large surface area, abundant micro/mesopores, and vast carbon edges/defects, which boost doping a large amount of B (5.81 atom %) and N (5.84 atom %) dopants into carbon frameworks with 6.36 atom % of BC₃, pyridinic-N (pyridinic-N-Ni), and graphitic-N active sites. Electrochemical measurements demonstrate that Ni@BNPCFs-900 reveals the best hydrogen evolution reaction (HER) and oxygen reduction reaction catalytic activities in an alkaline solution. The HER potential at 10 mA cm^-2 [E₁₀ = -164.2 mV vs reversible hydrogen electrode (RHE)] of the optimal Ni@BNPCFs-900 is just 96.2 mV more negative than that of the state-of-the-art 20 wt % Pt/C (E₁₀ = -68 mV vs RHE). In particular, the OER E₁₀ and Tafel slope of the optimal Ni@BNPCFs-900 (1.517 V vs RHE and 19.31 mV dec^-1) are much smaller than those of RuO₂ (1.557 V vs RHE and 64.03 mV dec^-1). For full water splitting, the catalytic current density achieves 10 mA cm^-2 at a low cell voltage of 1.584 V for the (-) Ni@BNPCFs-900||Ni@BNPCFs-900 (+) electrolysis cell, which is 10 mV smaller than that of the (-) 20 wt % Pt/C||RuO₂ (+) benchmark (1.594 V) under the same conditions. The synergistic effects of 3D hierarchically porous structures, advanced charge transport ability, and abundant active centers [such as Ni@BNC, BC₃, pyridinic-N (pyridinic-N-Ni), and graphitic-N] are responsible for the excellent water-splitting catalytic activity of the Ni@BNPCFs-900 networks. Especially, because of the remarkable structural and chemical stabilities of 3D hierarchically porous Ni@BNPCFs-900 networks, the (-) Ni@BNPCFs-900||Ni@BNPCFs-900 (+) water electrolysis cell displays an excellent stability.

A MOF derived bifunctional electrocatalyst Ni3ZnC0.7-Mo2C with enhanced performance for overall water splitting

The efficiency and cost of electrocatalysts are critical factors restricting their application in water electrochemical decomposition. In recent years, transition metal carbides (TMCs) have been highlighted due to their unique characteristics for water splitting: good conductivity and stability. However, their electrochemical performance required further optimization. In this work, a distinct non-solvent method was utilized to achieve a Ni₃ZnC_0.7-Mo₂C/Ni foam (NF) catalyst, which exhibited a nanoflower structure with efficient exposed active sites. Moreover, the synergistic effect between the Mo and Ni species greatly affected its HER and OER performance. Ni₃ZnC_0.7-Mo₂C/NF showed excellent electrocatalytic performance with small overpotentials of 58 mV and 257 mV at 10 mA cm^-2 for the HER and OER, respectively. To our delight, the overall water splitting could be driven by only 1.56 V. This work not only demonstrates an excellent bifunctional electrocatalyst for overall water splitting but also provides another method for polymetallic carbide preparation and activity optimization.

Bifunctional catalysts of Ni nanoparticle coupled MoO2 nanorods for overall water splitting

The development of active and cost-effective bifunctional catalysts is crucial for water dissociation through electrolysis. In this study, bifunctional catalysts with Ni nanoparticles (NPs) anchored on MoO₂ nanorods have been synthesized via in situ dissolution of NiMoO₄-ZIF under an inert atmosphere without using hydrogen gas. The Ni-MoO₂ catalyst exhibits high electrocatalytic activity by modulating the calcination temperature. Benefitingfrom the MOF transformation and accompanying Ni particles' outward diffusion, a precisely designed interface heterostructure between Ni and MoO₂ was constructed. As a result, the optimized Ni-MoO₂ catalyst achieves extremely low overpotentials of only 24 mV and 275 mV at 10 mA cm^-2 for the hydrogen evolution reaction and oxygen evolution reaction, respectively. Furthermore, the catalyst required a small cell voltage of 1.55 V to deliver a current density of 10 mA cm^-2 and remained stable over 20 h for overall water splitting. The proposed MOF-derived heterojunction protocol provides a general approach for designing and fabricating transition metal oxide catalysts for water electrolysis.

Chrysanthemum-like FeS/Ni3S2 heterostructure nanoarray as a robust bifunctional electrocatalyst for overall water splitting

The development of a scalable strategy to prepare highly efficient and stable bifunctional electrocatalysts is the key to industrial electrocatalytic water splitting cycles to produce clean hydrogen. Here, a simple and quick one-step hydrothermal method was used to successfully fabricate a three-dimensional core chrysanthemum-like FeS/Ni₃S₂ heterogeneous nanoarray (FeS/Ni₃S₂@NF) on a porous nickel foam skeleton. Compared with the monomer Ni₃S₂@NF, the chrysanthemum-like FeS/ Ni₃S₂@NF heterostructure nanomaterials have improved catalytic performance in alkaline media, showing low overpotentials of 192 mV (η₁₀) and 130 mV (η_-10) for OER and HER, respectively. This study attests that integrated interface engineering and precise morphology control are effective strategies for activating the Ni³⁺/Ni²⁺ coupling, promoting charge transfer and improving the intrinsic activity of the material to accelerate the OER reaction kinetics and promote the overall water splitting performance. The scheme can be reasonably applied to the design and development of transition metal sulfide-based electrocatalysts to put into industrial practice of electrochemical water oxidation.

Preferential Co substitution on Ni sites in Ni–Fe oxide arrays enabling large-current-density alkaline oxygen evolution

Developing low-cost and high-activity transition metal oxide electrocatalysts for an efficient oxygen evolution reaction (OER) at a large current density is highly demanded for industrial application and remains a big challenge. Herein, we report vertically aligned cobalt doped Ni–Fe based oxide (Co–NiO/Fe₂O₃) arrays as a robust OER electrocatalyst via a simple method combining hydrothermal reaction with heat treatment. Density functional theory calculation and XRD Rietveld refinement reveal that Co preferentially occupies the Ni sites compared to Fe in the Ni–Fe based oxides. The electronic structures of the Co–NiO/Fe₂O₃ could be further optimized, leading to the improvement of the intrinsic electronic conductivity and d-band center energy level and the decrease in the reaction energy barrier of the rate-determining step for the OER, thus accelerating its OER electrocatalytic activity. The Co–NiO/Fe₂O₃ nanosheet arrays display state-of-the-art OER activities at a large current density for industrial demands among Fe–Co–Ni based oxide electrocatalysts, which only require an ultra-low overpotential of 230 mV at a high current density of 500 mA cm⁻², and exhibit superb durability at 500 mA cm⁻² for at least 300 h without obvious degradation. The Co–NiO/Fe₂O₃ nanosheet arrays also have a small Tafel slope of 33.9 mV dec⁻¹, demonstrating fast reaction kinetics. This work affords a simple and effective method to design and construct transition metal oxide based electrocatalysts for efficient water oxidation.

Co–NiO/Fe₂O₃ nanosheets featuring Co substitution on Ni sites can effectively regulate electronic structures and exhibit high OER activities with low overpotential (η₅₀₀ = 230 mV), small Tafel slope (33.9 mV dec⁻¹) and superb durability for 300 h.

Atomic Metal-Support Interaction Enables Reconstruction-Free Dual-Site Electrocatalyst

Real bifunctional electrocatalysts for hydrogen evolution reaction and oxygen evolution reaction have to be the ones that exhibit a steady configuration during/after reaction without irreversible structural transformation or surface reconstruction. Otherwise, they can be termed as "precatalysts" rather than real catalysts. Herein, through a strongly atomic metal-support interaction, single-atom dispersed catalysts decorating atomically dispersed Ru onto a nickel-vanadium layered double hydroxide (LDH) scaffold can exhibit excellent HER and OER activities. Both in situ X-ray absorption spectroscopy and operando Raman spectroscopic investigation clarify that the presence of atomic Ru on the surface of nickel-vanadium LDH is playing an imperative role in stabilizing the dangling bond-rich surface and further leads to a reconstruction-free surface. Through strong metal-support interaction provided by nickel-vanadium LDH, the significant interplay can stabilize the reactive atomic Ru site to reach a small fluctuation in oxidation state toward cathodic HER without reconstruction, while the atomic Ru site can stabilize the Ni site to have a greater structural tolerance toward both the bond constriction and structural distortion caused by oxidizing the Ni site during anodic OER and boost the oxidation state increase in the Ni site that contributes to its superior OER performance. Unlike numerous bifunctional catalysts that have suffered from the structural reconstruction/transformation for adapting the HER/OER cycles, the proposed Ru/Ni₃V-LDH is characteristic of steady dual reactive sites with the presence of a strong metal-support interaction (i.e., Ru and Ni sites) for individual catalysis in water splitting and is revealed to be termed as a real bifunctional electrocatalyst.

Unveiling the mechanism of high-performance hydrogen evolution reaction on noble-metal-free (113)-faceted Ni3C: ab initio calculations

To examine the reactivity of noble-metal-free Ni3C towards hydrogen evolution reaction (HER), we report a comprehensive first-principles density functional theory (DFT) study on the stability, geometric structure, electronic characteristics, and catalytic activity for HER on the Ni3C crystal (113) surfaces with different surface terminations, namely the C-rich and Ni-rich terminated surface of Ni3C (113). The results indicate that C-rich and some stoichiometric surfaces are thermodynamically stable. The bridge-site of C-rich Ni3C (113) is indispensable for HER because it not only displays improved electrocatalytic activity, but also possesses appropriate hydrogen adsorption energy, overpotential and robust stability. The ΔG H (0.02 eV) and overpotential obtained by C-rich Ni3C outperformed that obtained by Pt determined by computation (ΔG H = -0.07 eV). Thus, the bridge-sites of C-rich Ni3C (113) function as both excellent and stable active sites and adsorption/desorption sites. Increasing the density of active sites through doping or enlarging the surface area renders a prospective strategy to ameliorate the HER activity further. Overall, this study elucidates new insights into the surface properties of Ni3C for HER from water splitting and opens up a fascinating avenue to optimize the performance of solar energy conversion devices by synthesizing preferentially exposed catalyst facets.

Vanadium-Doping and Interface Engineering for Synergistically Enhanced Electrochemical Overall Water Splitting and Urea Electrolysis

Fabricating effective non-precious metal-based catalysts for hydrogen production via electrochemical water splitting is of considerable importance but remains challenging. Transition metal nitrides possessing metallic character and corrosion resistance have been considered as potential replacements for precious metals. However, their activities for water electrolysis are impeded by the strong hydrogen adsorption and low water adsorption energies. Herein, V-doped bimetallic nitrides, V-FeNi₃N/Ni₃N heterostructure, are synthesized via a hydrothermal-nitridation protocol and used as electrocatalysts for water splitting and urea electrolysis. The V-FeNi₃N/Ni₃N electrode exhibits superior HER and OER activities, and the overpotentials are 62 and 230 mV to acquire a current density of 10 mA cm^-2, respectively. Moreover, as a bifunctional electrocatalyst for overall water splitting, a two-electrode device needs a voltage of 1.54 V to reach a current density of 10 mA cm^-2. The continuous electrolysis can be run for more than 120 h, surpassing most previously reported electrocatalysts. The excellent performance for water electrolysis is dominantly due to V-doping and interface engineering, which could enhance water adsorption and regulate the adsorption/desorption of intermediates species, thereby accelerating HER and OER kinetic processes. Besides, a urea-assisted two-electrode electrolyzer for electrolytic hydrogen production requires a cell voltage of 1.46 V at 10 mA cm^-2, which is 80 mV lower than that of traditional water electrolysis.

Cu3P@CoO core-shell heterostructure with synergistic effect for highly efficient hydrogen evolution

The sluggish charge transfer and poor intrinsic activity are the obstacles that limit the development for electrocatalysts on hydrogen evolution. A novel core-shell heterostructure composed of Cu₃P nanowires with supported CoO nanosheets was synthesized. Owing to numerous active sites and synergistic effect, the as-prepared Cu₃P@CoO was highly efficient for hydrogen evolution and outperformed the single component. The theoretical calculations demonstrate that Cu₃P@CoO had a zero bandgap for the incorporation of metallic Cu₃P, which can greatly accelerate the charge transfer. Besides, the adsorption free energy of intermediates on Cu₃P@CoO can also be optimized, leading to a small energy barrier in the reaction pathway, and thereby an increased intrinsic activity. This work highlights the significance of exploiting the synergistic effect of the heterostructure on the charge transfer and intrinsic activity when designing highly efficient electrocatalysts for hydrogen evolution.

Ultralow Ru Incorporated Amorphous Cobalt-Based Oxides for High-Current-Density Overall Water Splitting in Alkaline and Seawater Media

Realizing efficiency and stable hydrogen production by water electrolysis under high current densities is essential to the forthcoming hydrogen economy. However, its industrial breakthrough is seriously limited by bifunctional catalysts with slow hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) electrocatalytic processes. Herein, an ultralow Ru incorporated amorphous cobalt-based oxide (Ru-CoO_x /NF), effectively driving the electrolysis of water at high current densities in alkaline water and seawater, is designed and constructed. In 1 m KOH, to reach the current density of 1000 mA cm^-2 for HER and OER, it only needs 252 and 370 mV overpotentials, respectively, beyond commercial Pt/C and RuO₂ catalysts. At the high current density, it also presents outstanding electrochemical stability. Then the electrolyzer apparatus assembled with Ru-CoO_x /NF, just requires the ultra-low voltage of 2.2 and 2.62 V to support the current density of 1000 mA cm^-2 in alkaline water and seawater electrolysis, respectively, for hydrogen production, better than that of the commercial Pt/C and RuO₂ catalysts. This work demonstrates that Ru-CoO_x /NF is one of the most promising catalysts for industrial applications and provides a possibility for exploration of high-current-density water electrocatalysis by changing the crystallinity of the catalyst.

Transition metal-based catalysts for electrochemical water splitting at high current density: current status and perspectives

As a clean energy carrier, hydrogen has priority in decarbonization to build sustainable and carbon-neutral economies due to its high energy density and no pollutant emission upon combustion. Electrochemical water splitting driven by renewable electricity to produce green hydrogen with high-purity has been considered to be a promising technology. Unfortunately, the reaction of water electrolysis always requires a large excess potential, let alone the large-scale application (e.g., >500 mA cm^-2 needs a cell voltage range of 1.8-2.4 V). Thus, developing cost-effective and robust transition metal electrocatalysts working at high current density is imperative and urgent for industrial electrocatalytic water splitting. In this review, the strategies and requirements for the design of self-supported electrocatalysts are summarized and discussed. Subsequently, the fundamental mechanisms of water electrolysis (OER or HER) are analyzed, and the required important evaluation parameters, relevant testing conditions and potential conversion in exploring electrocatalysts working at high current density are also introduced. Specifically, recent progress in the engineering of self-supported transition metal-based electrocatalysts for either HER or OER, as well as overall water splitting (OWS), including oxides, hydroxides, phosphides, sulfides, nitrides and alloys applied in the alkaline electrolyte at large current density condition is highlighted in detail, focusing on current advances in the nanostructure design, controllable fabrication and mechanistic understanding for enhancing the electrocatalytic performance. Finally, remaining challenges and outlooks for constructing self-supported transition metal electrocatalysts working at large current density are proposed. It is expected to give guidance and inspiration to rationally design and prepare these electrocatalysts for practical applications, and thus further promote the practical production of hydrogen via electrochemical water splitting.

Oxygen-deficient Cu doped NiFeO nanosheets hydroxide as electrode material for efficient oxygen evolution reaction and supercapacitor

The development of renewable energy conversion and storage has triggered the development of electrode materials for oxygen evolution reaction (OER) and supercapacitors. Here we report a highly active Cu doped NiFe nanosheets hydroxide electrode with rich oxygen vacancies (OVs) (denoted as H-NiFeCuO/NF) prepared by in situ anodic electrodeposition on the three-dimensional macroporous nickel foam (NF) substrate followed by heat treatment with H₂. The as-prepared H-NiFeCuO/NF electrode showed the initial potential of 1.44 V (versus RHE) for OER and 980 F g^-1 specific capacity as supercapacitor in 1 M KOH. Further investigation suggested that the tuning of composition and structure by doping copper ions and creating OVs helped accelerate the electrochemical reactions. This practice provides an efficient approach for the fabrication of heteromultimetallic hydroxide monolithic electrode with high performance in OER or supercapacitor application.

Co-Doped Ni3N Nanosheets with Electron Redistribution as Bifunctional Electrocatalysts for Efficient Water Splitting

Preparation of high-activity and earth-abundant bifunctional catalysts for efficient electrochemical water splitting are crucial and challenging. Herein, Co-doped Ni₃N nanosheets loaded on nickel foam (Co-Ni₃N) were synthesized. The as-prepared Co-Ni₃N exhibits excellent catalytic activity toward both the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) in alkaline media. Density functional theory (DFT) calculation reveals that Co-Ni₃N with redistribution of electrons not only can facilitate the HER kinetics but also can regulate intermediates adsorption energies for OER. Specifically, the Co-Ni₃N exhibits high efficiency and stable catalytic activity, with an overpotential of only 30 and 270 mV at a current density of 10 mA cm^-2 for the HER and OER in 1 M KOH, respectively. This work provides strong evidence to the merit of Co doping to improve the innate electrochemical performance in bifunctional catalysts, which might have a common impact in many similar metal-metal nitride electrocatalysts.

Recent Advances in Transition Metal Carbide Electrocatalysts for Oxygen Evolution Reaction

The electrolysis of water is considered to be a primary method for the mass production of hydrogen on a large scale, as a substitute for unsustainable fossil fuels in the future. However, it is highly restricted by the sluggish kinetics of the four-electron process of the oxygen evolution reaction (OER). Therefore, there is quite an urgent need to develop efficient, abundant, and economical electrocatalysts. Transition metal carbides (TMCs) have recently been recognized as promising electrocatalysts for OER due to their excellent activity, conductivity, and stability. In this review, widely-accepted evaluation parameters and measurement criteria for different electrocatalysts are discussed. Moreover, five sorts of TMC electrocatalysts—including NiC, tungsten carbide (WC), Fe3C, MoC, and MXene—as well as their hybrids, are researched in terms of their morphology and compounds. Additionally, the synthetic methods are summarized. Based on the existing materials, strategies for improving the catalytic ability and new designs of electrocatalysts are put forward. Finally, the future development of TMC materials is discussed both experimentally and theoretically, and feasible modification approaches and prospects of a reliable mechanism are referred to, which would be instructive for designing other effective noble-free electrocatalysts for OER.