Efficient Hydrogen Evolution Electrocatalysis Using Cobalt Nanotubes Decorated with Titanium Dioxide Nanodots

Activating TiO2 through the Phase Transition-Mediated Hydrogen Spillover to Outperform Pt for Electrocatalytic pH-Universal Hydrogen Evolution

Endowing conventional materials with specific functions that are hardly available is invariably of significant importance but greatly challenging. TiO2 is proven to be highly active for the photocatalytic hydrogen evolution while intrinsically inert for electrocatalytic hydrogen evolution reaction (HER) due to its poor electrical conductivity and unfavorable hydrogen adsorption/desorption behavior. Herein, the first activation of inert TiO2 for electrocatalytic HER is demonstrated by synergistically modulating the positions of d-band center and triggering hydrogen spillover through the dual doping-induced partial phase transition. The N, F co-doping-induced partial phase transition from anatase to rutile phase in TiO2 (AR-TiO2|(N,F)) exhibits extraordinary HER performance with overpotentials of 74, 80, and 142 mV at a current density of 10 mA cm-2 in 1.0 M KOH, 0.5 M H2SO4, and 1.0 M phosphate-buffered saline electrolytes, respectively, which are substantially better than pure TiO2, and even superior to the benchmark Pt/C catalysts. These findings may open a new avenue for the development of low-cost alternative to noble metal catalysts for electrocatalytic hydrogen production.

Metal-organic framework derived transition metal sulfides grown on carbon nanofibers as self-supported catalysts for hydrogen evolution reaction

Metal-organic framework (MOF) derived transition metal-based electrocatalysts have received great attention as substitutes for noble metal-based hydrogen evolution catalysts. However, the low conductivity and easy detachments from electrodes of raw MOF have seriously hindered their applications in hydrogen evolution reaction. Herein, we report the facile preparation of Co-NSC@CBC84, a porous carbon-based and self-supported catalyst containing Co9S8 active species, by pyrolysis and sulfidation of in-situ grown ZIF-67 on polydopamine-modified biomass bacterial cellulose (PDA/BC). As a binder-free and self-supported electrocatalyst, Co-NSC@CBC84 exhibits superior electrocatalytic properties to other reported cobalt-based sulfide catalytic materials and has good stability in 0.5 M H2SO4 electrolyte. At the current density of 10 mA cm-2, only an overpotential of 138 mV was required, corresponding to a Tafel slope of 123 mV dec-1, owing to the strong synergy effect between Co-NSC nanoparticles and CBC substrate. This work therefore provides a feasible approach to prepare self-supported transition metal sulfides as HER catalysts, which is helpful for the development of noble metal-free catalysts and biomass carbon materials.

Bifunctional Ni Foam Supported TiO2 @Ni3 S2 core@shell Nanorod Arrays for Boosting Electrocatalytic Biomass Upgrading and H2 Production Reactions

Replacing traditional oxygen evoltion reaction (OER) with biomass oxidation reaction (BOR) is an advantageous alternative choice to obtain green hydrogen energy from electrocatalytic water splitting. Herein, a novel of extremely homogeneous Ni3 S2 nanosheets covered TiO2 nanorod arrays are in situ growth on conductive Ni foam (Ni/TiO2 @Ni3 S2 ). The Ni/TiO2 @Ni3 S2 electrode exhibits excellent electrocatalytic activity and long-term stability for both BOR and hydrogen evolution reaction (HER). Especially, taking glucose as a typical biomass, the average hydrogen production rate of the HER-glucose oxidation reaction (GOR) two-electrode system reached 984.74 µmol h-1 , about 2.7 times higher than that of in a common HER//OER two-electrode water splitting system (365.50 µmol h-1 ). The calculated power energy saving efficiency of the GOR//HER system is about 13% less than that of the OER//HER system. Meanwhile, the corresponding selectivity of the value-added formic acid produced by GOR reaches about 80%. Moreover, the Ni/TiO2 @Ni3 S2 electrode also exhibits excellent electrocatalytic activity on a diverse range of typical biomass intermediates, such as urea, sucrose, fructose, furfuryl alcohol (FFA), 5-hydroxymethylfurfural (HMF), and alcohol (EtOH). These results show that Ni/TiO2 @Ni3 S2 has great potential in electrocatalysis, especially in replacing OER reaction with BOR reaction and promoting the sustainable development of hydrogen production.

Nanoscale Metal Particle Modified Single-Atom Catalyst: Synthesis, Characterization, and Application

Single-atom catalysts (SACs) have attracted considerable attention in heterogeneous catalysis because of their well-defined active sites, maximum atomic utilization efficiency, and unique unsaturated coordinated structures. However, their effectiveness is limited to reactions requiring active sites containing multiple metal atoms. Furthermore, the loading amounts of single-atom sites must be restricted to prevent aggregation, which can adversely affect the catalytic performance despite the high activity of the individual atoms. The introduction of nanoscale metal particles (NMPs) into SACs (NMP-SACs) has proven to be an efficient approach for improving their catalytic performance. A comprehensive review is urgently needed to systematically introduce the synthesis, characterization, and application of NMP-SACs and the mechanisms behind their superior catalytic performance. This review first presents and classifies the different mechanisms through which NMPs enhance the performance of SACs. It then summarizes the currently reported synthetic strategies and state-of-the-art characterization techniques of NMP-SACs. Moreover, their application in electro/thermo/photocatalysis, and the reasons for their superior performance are discussed. Finally, the challenges and perspectives of NMP-SACs for the future design of advanced catalysts are addressed.

In situ formed nickel tungsten oxide amorphous layer on metal-organic framework derived ZnxNi1-xWO4 surface by self-reconstruction for acid hydrogen evolution reaction

Noble metal free electrocatalysts for hydrogen evolution reaction (HER) in acid play an important role in proton exchange membrane-based electrolysis. Here, we develop an in situ surface self-reconstruction strategy to construct excellent acidic HER catalysts. Firstly, free-standing zinc nickel tungstate nanosheets inlaid with nickel tungsten alloy nanoparticles were synthesized on carbon cloth as pre-catalyst via metal-organic framework derived method. Amorphous nickel tungsten oxide (Ni-W-O) layer is in situ formed on surface of nanosheet as actual HER active site with the dissolution of NiW alloy nanoparticles and the leaching of cations. While the morphology of the free-standing structure remains the same, keeping the maximized exposure of active sites and serving as the electron transportation framework. As a result, benefiting from disordered arrangement of atoms and the synergistic effect between Ni and W atoms, the amorphous Ni-W-O layer exhibits an excellent acidic HER activity with only an overpotential of 46 mV to drive a current density of 10 mA cm-2 and a quite good Tafel slope of 36.4 mV dec-1 as well as an excellent durability. This work enlightens the exploration of surface evolution of catalysts during HER in acidic solution and employs it as a strategy for designing acidic HER catalysts.

Highly efficient MoS2/WS2 heterojunctions for the CO2 reduction reaction: strong electronic transmission

Transition metal dichalcogenides (TMDs) possess several advantages, such as high conductivity, stable structure, and low cost, making them promising catalysts for carbon dioxide electroreduction. However, the high overpotential and the desorption characteristics of the reaction products during the reduction of carbon dioxide present significant challenges in the field of catalysis. In this study, we have further enhanced the catalytic activity of the original WS2 structure by constructing a heterojunction. We systematically investigate the catalytic activity of MoS2/WS2 heterojunctions supported by transition metals using density functional theory (DFT) calculations. The findings of this study are as follows: (1) the unique multiphase structure enhances the catalytic performance for CO2 reduction. (2) After constructing the MoS2/WS2 heterojunction, the electronic properties and conductivity of the heterojunction can be significantly enhanced, thereby facilitating the catalytic reduction of carbon dioxide. The Cu loading on the Cu@MoS2/WS2 heterojunction significantly reduces the overpotential, with a very low limit potential of -0.58 V. The adsorption behavior of CO on the Cu@MoS2/WS2 heterojunction was evaluated using adsorption energy, desorption energy, and density of states (DOS). The appropriate interaction between CO and Cu@ MoS2/WS2 promotes the reduction of CO2 to CO and facilitates smooth desorption of CO, demonstrating a strong catalytic effect on the CO2 reduction reaction (CO2RR). Therefore, it can be seen that Cu@MoS2/WS2 may be considered as potential single-atom catalysts (SACs) for CO2 reduction electrocatalysts. Finally, it is hoped that our results will provide theoretical support for the development of efficient CO2 reduction catalysts.

Adsorption Site Regulations of [W-O]-Doped CoP Boosting the Hydrazine Oxidation-Coupled Hydrogen Evolution at Elevated Current Density

Hydrazine oxidation reaction (HzOR) assisted hydrogen evolution reaction (HER) offers a feasible path for low power consumption to hydrogen production. Unfortunately however, the total electrooxidation of hydrazine in anode and the dissociation kinetics of water in cathode are critically depend on the interaction between the reaction intermediates and surface of catalysts, which are still challenging due to the totally different catalytic mechanisms. Herein, the [W-O] group with strong adsorption capacity is introduced into CoP nanoflakes to fabricate bifunctional catalyst, which possesses excellent catalytic performances towards both HER (185.60 mV at 1000 mA cm-2) and HzOR (78.99 mV at 10,00 mA cm-2) with the overall electrolyzer potential of 1.634 V lower than that of the water splitting system at 100 mA cm-2. The introduction of [W-O] groups, working as the adsorption sites for H2O dissociation and N2H4 dehydrogenation, leads to the formation of porous structure on CoP nanoflakes and regulates the electronic structure of Co through the linked O in [W-O] group as well, resultantly boosting the hydrogen production and HzOR. Moreover, a proof-of-concept direct hydrazine fuel cell-powered H2 production system has been assembled, realizing H2 evolution at a rate of 3.53 mmol cm-2 h-1 at room temperature without external electricity supply.

Ni/TiO2 heterostructures derived from phase separation for enhanced electrocatalysis of hydrogen evolution and biomass oxidative upgrading in anion exchange membrane electrolyzers

The construction of heterostructures is an effective strategy to enhance electrocatalysis for hydrogen evolution reactions (HERs) and biomass oxidative upgrading. In this work, a Ni/TiO2 heterostructure prepared by a phase-separation strategy was adopted as a bifunctional electrocatalyst for HERs and biomass oxidation in alkaline media. Due to the optimized hydrogen adsorption energetics as well as the interfacial water structure and hydrogen bond connectivity in the electrical double layer, Ni/TiO2 exhibited high activity for HERs with an overpotential of 28 mV at 10 mA cm-2 and good durability at 1000 mA cm-2 for over 100 h in an anion exchange membrane (AEM) electrolyzer. In addition, Ni/TiO2 showed high catalytic performance for the oxidation of biomass-based platform compound 5-hydroxymethylfurfural (HMF) to high-value added compound 2,5-furandicarboxylic acid (FDCA). Continuous production of FDCA with a yield >95% was achieved in the AEM electrolyzer for over 50 h. The superior HMF oxidation performance on the Ni/TiO2 heterostructure compared to Ni resulted from stronger HMF adsorption, lower Ni3+-O formation potential, longer Ni3+-O bond and smaller Ni crystal size.

Zr-MOF/NiS2 hybrids on nickel foam as advanced electrocatalysts for efficient hydrogen evolution

As a typical transition-metal sulfides (TMS), nickel disulfide (NiS₂) has attracted great attention in terms of hydrogen evolution reaction (HER). Howbeit, owing to the poor conductivity, slow reaction kinetics and instability of NiS₂, its HER activity is still necessary to be improved. In this work, we designed hybrid structures consisting of the nickel foam (NF) as a self-supporting electrode, NiS₂ derived from the sulfuration of NF and Zr-MOF grown on the surface of NiS₂@NF (Zr-MOF/NiS₂@NF). Due to the synergistic effect between the different constituents, the obtained Zr-MOF/NiS₂@NF demonstrates ideal electrochemical hydrogen evolution ability in acidic and alkalescent environment, reaching a standard current density of 10 mA cm^-2 at overpotentials of 110 and 72 mV in 0.5 M H₂SO₄ and 1 M KOH electrolytes, respectively. What is more, it also maintains excellent electrocatalytic durability for 10 h in both electrolytes. This work could provide a useful guidance on effectively combining metal sulfide with MOF for high-performance HER electrocatalysts.

Modification of TiO2 nanotubes by graphene-strontium and cobalt molybdate perovskite for efficient hydrogen evolution reaction in acidic medium

Herein, we demonstrate that modification of TiO₂ nanotubes with graphene-strontium and cobalt molybdate perovskite can turn them into active electrocatalysts for hydrogen evolution reaction (HER). For this purpose, a simple method of hydrothermal synthesis of perovskites was developed directly on the TiO₂ nanotubes substrate. Moreover, the obtained hybrids were also decorated with graphene oxide (GO) during one-step hydrothermal synthesis. The obtained materials were characterized by scanning electron microscopy with energy dispersive X-ray analysis, Raman spectroscopy, and X-ray diffraction analysis. Catalytic properties were verified by electrochemical methods (linear voltammetry, chronopotentiometry). The obtained hybrids were characterized by much better catalytic properties towards hydrogen evolution reaction compared to TiO₂ and slightly worse than platinum. The optimized hybrid catalyst (decorated by GO) can drive a cathodic current density of 10 mA cm^-2 at an overpotential of 121 mV for HER with a small Tafel slope of 90 mV dec^-1 in 0.2 M H₂SO₄.

Modeling of the Simultaneous Hydrogen Evolution and Cobalt Electrodeposition

A mathematical model of simultaneous cobalt deposition and hydrogen evolution was developed and applied to the electroreduction process of 5 mM Co 2+ ions investigated by cyclic voltammetry (CV) technique at different hydrogen ion concentrations (pH = 2, 3, 4). The kinetic parameters of such a complex process were determined, and the validity of the model and its sensitivity to changes in individual parameters were verified. The relative value of the approximate standard deviation (ASD % ) was used to determine the degree of fit of the model to the experimental data. The catalytic effect of cobalt on the hydrogen evolution process was comprehensively confirmed.

Trimetallic Sulfide Hollow Superstructures with Engineered d-Band Center for Oxygen Reduction to Hydrogen Peroxide in Alkaline Solution

High-performance transition metal chalcogenides (TMCs) as electrocatalysts for two-electron oxygen reduction reaction (2e-ORR) in alkaline medium are promising for hydrogen peroxide (H2 O2 ) production, but their synthesis remains challenging. In this work, a titanium-doped zinc-cobalt sulfide hollow superstructure (Ti-ZnCoS HSS) is rationally designed as an efficient electrocatalyst for H2 O2 electrosynthesis. Synthesized by using hybrid metal-organic frameworks (MOFs) as precursors after sulfidation treatment, the resultant Ti-ZnCoS HSS exhibits a hollow-on-hollow superstructure with small nanocages assembled around a large cake-like cavity. Both experimental and simulation results demonstrate that the polymetallic composition tailors the d-band center and binding energy with oxygen species. Moreover, the hollow superstructure provides abundant active sites and promotes mass and electron transfer. The synergistic d-band center and superstructure engineering at both atomic and nanoscale levels lead to the remarkable 2e-ORR performance of Ti-ZnCoS HSS with a high selectivity of 98%, activity (potential at 1 mA cm-2 of 0.774 V vs reversible hydrogen electrode (RHE)), a H2 O2 production rate of 675 mmol h-1 gcat -1 , and long-term stability in alkaline condition, among the best 2e-ORR electrocatalysts reported to date. This strategy paves the way toward the rational design of polymetallic TMCs as advanced 2e-ORR catalysts.

Conversion of Catalytically Inert 2D Bismuth Oxide Nanosheets for Effective Electrochemical Hydrogen Evolution Reaction Catalysis via Oxygen Vacancy Concentration Modulation

Oxygen vacancies (V_o) in electrocatalysts are closely correlated with the hydrogen evolution reaction (HER) activity. The role of vacancy defects and the effect of their concentration, however, yet remains unclear. Herein, Bi₂O₃, an unfavorable electrocatalyst for the HER due to a less than ideal hydrogen adsorption Gibbs free energy (ΔG_H*), is utilized as a perfect model to explore the function of V_o on HER performance. Through a facile plasma irradiation strategy, Bi₂O₃ nanosheets with different V_o concentrations are fabricated to evaluate the influence of defects on the HER process. Unexpectedly, while the generated oxygen vacancies contribute to the enhanced HER performance, higher V_o concentrations beyond a saturation value result in a significant drop in HER activity. By tunning the V_o concentration in the Bi₂O₃ nanosheets via adjusting the treatment time, the Bi₂O₃ catalyst with an optimized oxygen vacancy concentration and detectable charge carrier concentration of 1.52 × 10²⁴ cm^-3 demonstrates enhanced HER performance with an overpotential of 174.2 mV to reach 10 mA cm^-2, a Tafel slope of 80 mV dec^-1, and an exchange current density of 316 mA cm^-2 in an alkaline solution, which approaches the top-tier activity among Bi-based HER electrocatalysts. Density-functional theory calculations confirm the preferred adsorption of H* onto Bi₂O₃ as a function of oxygen chemical potential (∆μ_O) and oxygen partial potential (P_O2) and reveal that high V_o concentrations result in excessive stability of adsorbed hydrogen and hence the inferior HER activity. This study reveals the oxygen vacancy concentration-HER catalytic activity relationship and provides insights into activating catalytically inert materials into highly efficient electrocatalysts.

Copper mesh supported nickel nanowire array as a catalyst for the hydrogen evolution reaction in high current density water electrolysis

Hydrogen generation by water splitting using various renewable energy sources will play an important role in the sustainable green energy supply of the future. Unfortunately, wide industrial adoption of this process is currently impeded by the necessity to use noble metal based electrolysis catalysts. In this study, a low-cost and highly efficient water electrolysis catalyst active in the hydrogen evolution reaction (HER) taking place in alkaline medium is developed. The catalyst preparation procedure consists of electrodeposition of a rough nickel layer onto a smooth copper mesh, followed by the growth of hierarchical Ni nanowires on its surface. The rough nickel layer provides plenty of active sites for nanowire formation, resulting in a synergetic effect between copper and nickel in the copper mesh supported nickel nanowire array, which effectively enhances the HER electrocatalytic performance of this novel material. The catalyst demonstrated an HER overpotential as low as 317 mV in 1 M KOH electrolyte at a current density of 1 A cm^-2. The copper substrate's superior electrical conductivity to that of nickel is responsible for the excellent HER performance of the catalyst at high current density, making it a promising candidate to replace highly expensive noble metal based electrodes.

Fluoride-incorporated cobalt-based electrocatalyst towards enhanced hydrogen evolution reaction

We report an electrocatalyst, Co bases (metallic Co and Co(OH)₂) with fluoride-incorporated CoO coating on the surface of (CoO-F/Co), was synthesized by the electro-deposition method. The porous network architecture of CoO-F/Co on the glassy carbon electrode exhibited an ultra-low overpotential of 15 mV, achieving the geometric current density of 10 mA cm^-2 in 1.0 M KOH, which were comparable with the HER performance of numerous reported noble metal electrocatalysts. It is demonstrated that fluoride incorporation improved the electrodeposition particle size, electronic density, conductivity and hydrophilicity of CoO-F/Co the HER performance.

A Pacman-Like Titanium-Doped Cobalt Sulfide Hollow Superstructure for Electrocatalytic Oxygen Evolution

Transition-metal sulfides (TMSs) are attractive oxygen evolution reaction (OER) electrocatalysts. Developing new strategies to improve their electrochemical performance of TMSs is of great significance. Herein, a unique pacman-like titanium-doped cobalt sulfide hollow superstructure (Ti-CoSx HSS) is fabricated as an OER electrocatalyst. Using a prearranged metal-organic framework (MOF)-on-MOF heterostructure as a precursor treated by one-pot sulfidation, a sequential structural conversion process leads to the formation of Ti-CoSx HSS, which is assembled by interconnected Ti-doped CoSx nanocages around a cake-like cavity. Benefiting from the architecture and compositional advantages, Ti-CoSx HSS exhibits excellent OER performance with an overpotential of 249 mV at 10 mA cm^-2 and Tafel slope of 45.5 mV dec^-1 due to increased active site exposure, enhanced electron and mass transfer. This strategy enabled by MOF-on-MOF paves the way toward innovative MOF derivatives for various applications.

Enhancing Hydrogen Evolution by Optimizing the Hydrogen Adsorption on Titanium Monoxide Nanodot-Decorated Cobalt Sulfide Nanosheets

Modulating the local electronic state of metal compounds through interfacial interaction has become a key method for manufacturing high-performance hydrogen evolution reaction (HER) electrocatalysts. The electron-rich active sites can promote the adsorption of hydrogen, which accelerates the Volmer step and thereby enhances the electrocatalytic performance of HER. Here, we found that the strong interfacial interaction between TiO nanodots (TiO/Co-S) and Co-S nanosheets could advantageously improve the performance toward HER of electrocatalyst. Meanwhile, XPS results showed that modulating the local electronic structure of the TiO nanodots produces electron-rich regions on Co. As a result, the overpotential of the TiO/Co-S nanocomposite at 10 mA cm^-2 was 107 mV, and the Tafel slope was 83.3 mV dec^-1 . This study focused on the effect of the solid-solid interface on the local electronic structure of the catalytic metal active sites and successfully improved the catalytic activity of transition metal materials in HER catalysis.

Dual-electrocatalysis behavior of star-like zinc-cobalt-sulfide decorated with cobalt-molybdenum-phosphide in hydrogen and oxygen evolution reactions

Although important advances have been acquired in the field of electrocatalysis, the design and fabrication of highly efficient and stable non-noble earth-abundant metal catalysts for the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) remain a significant challenge. In this study, we have designed a superior bifunctional catalyst for OER and HER in alkaline media based on the Co-Mo-P/Zn-Co-S multicomponent heterostructure. The as-prepared multicomponent heterostructure was successfully obtained via a simple three-step hydrothermal-sulfidation-electrodeposition process consisting of star-like Co-Zn-S covered with Co-Mo-P. The structure and morphology evaluation of the prepared catalysts were performed via Fourier transform infrared spectroscopy, X-ray diffraction spectroscopy, field emission scanning electron microscopy, X-ray photoelectron spectroscopy, energy-dispersive X-ray spectroscopy, and elemental mapping techniques. The electrochemical tests show that Co-Mo-P/Co-Zn-S exhibits outstanding activity toward both OER and HER with OER overpotentials of 273 mV and 312 mV to drive the benchmark current densities of 10 and 50 mA cm^-2, respectively, with a Tafel slope of 41 mV dec^-1. In addition, the HER overpotentials of 120 mV and 165 mV were required to reach the benchmark current densities of 10 and 50 mA cm^-2, respectively, with a Tafel slope of 61.7 mV dec^-1 that outperforms most other state-of-the-art catalysts. In the case of HER, the prepared catalyst required an overpotential of 202 mV to reach the current density of 200 mA cm^-2 that was much lower than the overpotential of Pt/C (286 mV) to achieve the same current density. Co-Mo-P/Co-Zn-S also exhibits a suitable stability length of 10 h for OER and HER during the chronoamperometric tests. The superior performance of the Co-Mo-P/Co-Zn-S multicomponent heterostructure toward OER and HER may be related to the large specific surface area, accelerated mass and electron transport, and synergistic effect of multiple hybrid materials. These merits suggest that Co-Mo-P/Co-Zn-S can be considered as a promising catalyst for bi-functional OER and HER, and can be offered a great promise for future applications.

High-Density Ruthenium Single Atoms Anchored on Oxygen-Vacancy-Rich g-C3N4-C-TiO2 Heterostructural Nanosphere for Efficient Electrocatalytic Hydrogen Evolution Reaction

Designing and constructing high-density single-atom catalysts (SACs) is vital for electrochemical hydrogen evolution to meet the demand for fundamental research and practical applications of electrocatalysis. However, it is challenging to synthesize atomically dispersed electrocatalysts with high density and high performance. Herein, an integrated g-C₃N₄-C-TiO₂ heterostructural nanosphere with oxygen-rich vacancies is constructed by a multicomponent assembly-calcination strategy. Abundant single Ru atoms (12.4 wt %) are then anchored via the occupation of partial oxygen vacancies, forming a unique Ru/g-C₃N₄-C-TiO₂ heterostructure. A reasonable configuration is developed including single Ru atoms bonded with two oxygens and two nitrogens and coupled with jacent oxygen vacancies on the g-C₃N₄-C-TiO₂ nanosphere. Density functional theory calculations reveal that the remaining oxygen vacancies are beneficial for water dissociation, while single Ru atoms facilitate hydrogen adsorption. As expected, the result exhibits high electrocatalytic activity, delivering overpotentials of 112 and 107 mV at 10 mA cm^-2, Tafel slopes of 83 and 65 mV dec^-1 in H₂SO₄ and KOH, and a turnover frequency of 0.28 H₂ s^-1 at -100 mV toward the hydrogen evolution reaction (HER). Benefiting from the outstanding electrocatalytic performance, such a unique heterostructure with dense single Ru sites and oxygen vacancies could serve as a prominent alternative HER catalyst for renewable energy applications.

Implanted metal-nitrogen active sites enhance the electrocatalytic activity of zeolitic imidazolate zinc framework-derived porous carbon for the hydrogen evolution reaction in acidic and alkaline media

Developing electrocatalysts with excellent catalytic performance and superior durability for hydrogen evolution reaction (HER) remains a challenge. Herein, metal-nitrogen sites (M-N_x, M = Ni and Cu) are successfully implanted into zeolitic imidazolate zinc framework (ZIF-8)-derived nitrogen-doped porous carbon (ZIF/NC) to prepare Ni-ZIF/NC and Cu-ZIF/NC electrocatalysts for the HER. These M-N_x active sites significantly enhanced the electrocatalytic activities of Ni-ZIF/NC and Cu-ZIF/NC. Metal Ni acted as a catalyst for catalysis of Ni-ZIF/NC to form carbon nanotubes-like structures, which provided convenient ion transmission pathways. Owing to its special morphology and an increased number of defects, Ni-ZIF/NC displayed superior electrocatalytic activity in the HER compared to those of Cu-ZIF/NC and ZIF/NC. In an alkaline environment, Ni-ZIF/NC exhibited an overpotential at the current density of 10 mA cm^-2 (η₁₀) of 163.0 mV and Tafel slope of 85.0 mV dec^-1, demonstrating an electrocatalytic property equivalent to that of Pt/C. In an acidic environment, Ni-ZIF/NC yielded a η₁₀ of 177.4 mV and Tafel slope of 83.9 mV dec^-1, which were comparable to those of 20 wt.% Pt/C. Moreover, Ni-ZIF/NC and Cu-ZIF/NC also exhibited superior stabilities in alkaline environments. This work offers a valuable strategy for controlling the morphology and implanting M-N_x active sites into carbon for designing novel catalysts for use in alternative new energy applications.

Solvent-free microwave synthesis of ultra-small Ru-Mo2C@CNT with strong metal-support interaction for industrial hydrogen evolution

Exploring a simple, fast, solvent-free synthetic method for large-scale preparation of cheap, highly active electrocatalysts for industrial hydrogen evolution reaction is one of the most promising work today. In this work, a simple, fast and solvent-free microwave pyrolysis method is used to synthesize ultra-small (3.5 nm) Ru-Mo₂C@CNT catalyst with heterogeneous structure and strong metal-support interaction in one step. The Ru-Mo₂C@CNT catalyst only exhibits an overpotential of 15 mV at a current density of 10 mA cm⁻², and exhibits a large turnover frequency value up to 21.9 s⁻¹ under an overpotential of 100 mV in 1.0 M KOH. In addition, this catalyst can reach high current densities of 500 mA cm⁻² and 1000 mA cm⁻² at low overpotentials of 56 mV and 78 mV respectively, and it displays high stability of 1000 h. This work provides a feasible way for the reasonable design of other large-scale production catalysts.

While H₂ could be a renewable fuel, the large-scale preparation of cheap, active electrocatalysts for large-scale production remains a challenge. Here, authors use a rapid, solvent-free microwave pyrolysis method to synthesize nanostructured catalysts for large-scale industrial H₂ production.

Cobalt and nitrogen co-doped Ni3S2 nanoflowers on nickel foam as high-efficiency electrocatalysts for overall water splitting in alkaline media

The development of high-performance and cost-effective bifunctional water splitting catalysts has enormous significance in the hydrogen production industry from water electrolysis. Herein, an in situ Co and N co-doping method was developed to improve the electrocatalytic performance of Ni3S2 catalysts. The Co-N-Ni3S2/NF is successfully synthesized for the first time by a one-step hydrothermal method, wherein nickel foam, thioacetamide and Co(NO3)2·6H2O are used as the nickel source, sulfur source, nitrogen source and cobalt source. Co-N-Ni3S2/NF exhibits excellent oxygen evolution reaction activity (an overpotential of 285 mV@50 mA cm-2) and hydrogen evolution reaction activity (an overpotential of 215 mV@10 mA cm-2) in 1 M KOH solution. The electrolytic cell displayed a low cell voltage of 1.50 V when the Co-N-Ni3S2/NF material was used as the bifunctional water splitting electrocatalyst, which is one of the best catalysts reported so far. Density functional theory calculations show that Co-N-Ni3S2/NF exhibits stronger water adsorption energy than those of N-Ni3S2/NF, Co-Ni3S2/NF and Ni3S2/NF. It is proved that the doping of Co and N can effectively regulate the electron cloud density of Ni, thus enhancing the electrochemical activity of Co-N-Ni3S2/NF.

Promoting Bifunctional Water Splitting by Modification of the Electronic Structure at the Interface of NiFe Layered Double Hydroxide and Ag

Electrochemical water splitting is a promising method for the renewable production of high-purity hydrogen via the hydrogen evolution reaction (HER). Ni-Fe layered double hydroxides (Ni-Fe LDHs) are highly efficient materials for mediating the oxygen evolution reaction (OER), a half-reaction for water splitting at the anode, but LDHs typically display poor HER performance. Here, we report the preparation of self-organized Ag@NiFe layered double hydroxide core-shell electrodes on Ni foam (Ag@NiFe/NF) prepared by galvanic etching for mediating both the HER and OER (bifunctional water-splitting electrocatalysis). This synthetic strategy allowed for the preparation of organized hierarchical architectures which displayed improved the electrochemical performance by tuning the electronic structure of the catalyst and increasing the surface area utilization. X-ray photoelectron spectroscopy (XPS) and theoretical calculations revealed that electron transfer from the Ni-Fe LDH to Ag influenced the adsorption of the reaction intermediates leading to enhanced catalytic activity. The Ag@NiFe/NF electrode displayed overpotentials as low as 180 and 80 mV for oxygen and hydrogen evolution, respectively, at a current density of 10 mA cm^-2, and improvements in the specific activity by ∼5× and ∼1.5× for the oxygen and hydrogen evolution reaction, respectively, compared to benchmark NiFe hydroxide materials. Additionally, an integrated water-splitting electrolyzer electrode can be driven by an AA battery.

Induction of Co2P Growth on a MXene (Ti3C2Tx)-Modified Self-Supporting Electrode for Efficient Overall Water Splitting

It still is a challenge to create a superior and easily coupled bifunctional electrocatalyst for water splitting impelled by a low voltage. In this work, the controlled growth of Co₂P NAs on the surface of a MXene (Ti₃C₂T_x)-modified self-supporting electrode is demonstrated as a competent and reliable bifunctional electrocatalyst for efficient water splitting. The heterointerface in Co₂P@Ti₃C₂T_x with an optimized adsorption free energy of H*, H₂O, and better conductivity can give enhanced HER (hydrogen evolution reaction) activity, with a low overpotential (42 mV) at 10 mA cm^-2. Additionally, the OER (oxygen evolution reaction) activity has also been similarly strengthened by the synergy of Co₂P and MXene with an overpotential of 267 mV to arrive at 10 mA cm^-2. Furthermore, the excellent bifunctional electrode (Co₂P@Ti₃C₂T_x∥Co₂P@Ti₃C₂T_x) exhibits efficient engineering water-splitting performance (1.46 V@10 mA cm^-2) in alkaline solution. This simple design can propose a promising approach to exploit precious-metal-free catalysts for energy conversion.

Size-controlled Ag quantum dots decorated on binder-free hierarchical NiCoP films by magnetron sputtering to boost electrochemical performance for supercapacitors

This paper reports novel binder-free and self-supported electrodes of hierarchical nickel-cobalt phosphide (NiCoP) films decorated with size-controlled Ag quantum dots by magnetron sputtering (Ag/NiCoP). Ag quantum dots with an average particle size of 7.90 nm uniformly distribute over the nanosheet-assembled architecture of NiCoP films. Benefitting from the good ohmic contact in the interfaces between Ag quantum dots and NiCoP nanosheets, Ag/NiCoP exhibits an ultrahigh specific capacitance of 6150 mF cm-2 (3050 F g-1 at 1 A g-1) higher than the 3445 mF cm-2 (1722 F g-1 at 1 A g-1) of bare NiCoP at 2 mA cm-2. The specific areal capacitance has been increased by 78.5% after introducing Ag quantum dots. 34% capacitance retention rate is achieved while the current density increases from 2 to 30 mA cm-2. The cycling stability displays a remarkable capacitance retention of 73% for 4000 cycles at 30 mA cm-2. These boosted electrochemical performances are mainly attributed to the synergistic effects of enough electroactive sites, high electronic conductivity, and easy electrolyte ion diffusion. An asymmetric supercapacitor is fabricated using hierarchical Ag/NiCoP as the positive electrode and activated carbon as the negative electrode. The supercapacitor delivers an energy density of 0.254 mW h cm-2 (1.81 mW h cm-3) at a power density of 1.88 mW cm-2 (13.4 mW cm-3). At a power density of 18.8 mW cm-2 (134 mW cm-3), an energy density of 0.115 mW h cm-2 (0.82 mW h cm-3) can still be maintained. This study provides an avenue to design a novel generation of supercapacitors for energy storage devices.

NiCo-Layered Double Hydroxide-Derived B-Doped CoP/Ni2P Hollow Nanoprisms as High-Efficiency Electrocatalysts for Hydrogen Evolution Reaction

Rational design and controllable synthesis of multiple metal components according to chemical composition and morphology are essential for obtaining desirable electrochemical performance for efficient hydrogen production because of the morphology and synergistic effects of different components. Herein, we report an approach to facilely fabricate bimetal compounds with a well-defined hollow nanoprism structure using a self-templated strategy to synthesize novel hierarchical NiCo-layered double hydroxide (NiCo-LDH) nanosheets as precursors followed by in situ phosphorization. Among the as-synthesized products of different mole ratios of Ni/Co, the NiCo₂-B-P nanoprisms that integrate the advantages of a hollow structure, an optimal Ni-Co synergistic effect, and a unique B-doped CoP/Ni₂P bimetallic phosphide derived from NiCo-LDH nanosheets exhibit excellent hydrogen evolution reaction (HER) activity in an alkaline solution at 10 mA cm^-2 with the lowest overpotential of 78 mV and long-term stability. This study may offer an appropriate structure and compositional design of bimetallic alkaline HER catalysts.

Modification of Black Phosphorus Nanosheets with a Ni-Containing Carbon Layer as Efficient and Stable Hydrogen Production Electrocatalysts

Few-layered black phosphorus (FP) has recently attracted extensive research in the energy and materials fields. However, because of its chemically unstable nature under ambient conditions, very positive hydrogen adsorption energy and less active sites, FP has not been an efficient catalyst for the hydrogen evolution reaction (HER). In this research, we have developed a new strategy to overcome FP's drawbacks and to make it an active and stable HER catalyst. Our approach is to deposit a Ni²⁺-anchored thin carbon layer onto the surface of FP via controlled decarboxylation of Ni ethylenediaminetetraacetate (Ni-EDTA). The carbon layer on the surface of FP prevents it from making direct contact with its external environment, thereby greatly improving its stability. At the same time, transition-metal Ni that is dispersed in its carbon layer changes its hydrogen adsorption energy so as to improve its electrocatalytic activity. The prepared FP@Ni-C shows an outstanding HER performance with an overpotential of only 284 mV to obtain 10 mA cm^-2 current density with excellent electrocatalytic stability. The FP@Ni-C catalyst showed almost no activity loss during a 12 h catalyst life test. This study provides a new approach to the synthesis of highly efficient and stable electrocatalysts based on two-dimensional materials, using a facile catalyst preparation method.

A multi-interfacial FeOOH@NiCo2O4 heterojunction as a highly efficient bifunctional electrocatalyst for overall water splitting

Electrocatalytic water decomposition is the key to sustainable energy, and the design and synthesis of cost-effective electrocatalysts is the main objective of electrocatalytic water splitting. In this paper, multi-interfacial FeOOH@NiCo2O4 hybrid nanoflowers are prepared through a two-step hydrothermal reaction. In such heterostructures, NiCo2O4 nanoflowers are coated with a layer of FeOOH nanoparticles. In addition, the obtained electrocatalyst could provide abundant electroactive sites and the formation of FeOOH@NiCo2O4 nanointerfaces can also improve the charge transfer rate. As a result, under the HER and OER conditions, the prepared catalysts show an outstanding electrocatalytic performance. Moreover, in a two-electrode water splitting system, the FeOOH@NiCo2O4 heterostructure, as a dual-function electrocatalyst, needs a cell voltage of only 1.58 V at a current density of 10 mA cm-2. This study provides a facile and feasible method to construct different kinds of heterostructures as bifunctional electrocatalysts with multiple interfaces by a simple hydrothermal method.

Stable boron and cobalt co-doped TiO2 nanotubes anode for efficient degradation of organic pollutants

Anode materials are crucial to anodic oxidation for wastewater treatment. In this regard, stable boron and cobalt co-doped TiO₂ nanotube (B, Co-TNT) was prepared for the first time, and its lifetime was found increased significantly while electrocatalytic activity decreased with the increase of Co(NO₃)₂ in preparation from 1 to 10 mM. Characterized by scanning electron microscope (SEM), X-Ray Diffraction (XRD) and X-ray Photo-electronic Spectroscopy (XPS), B and Co content were optimized and successfully doped on TNT, which was more smooth without ripple with Co content of 0.038 mg/cm² in a valence of +2, and B atomic content of 2.17 at.% in form of Ti-B-O. This optimized anode enhanced electrode lifetime 122.8 times while the electrochemical activity decreased slightly when compared to the undoped TNT. The effects of current density, initial pH and initial 2,4-dichlorophenoxyacetic acid (2,4-D) concentration were investigated, and the mainly responsible radical for degradation was confirmed to be the surface OH on B, Co-TNT anode. This anode had better performance on the TOC removal, mineralization current density (MCE) and energy consumption (Ec) when compared with BDD, PbO₂, DSA and Pt anodes, and it also presented a very stable degradation for 10 cycles oxidation of 20 mg/L 2,4-D with allowable Co leaching. Therefore, B, Co-TNT anode is a promising, stable, safety and cost-effective anode for application in electrochemical advanced oxidation processes (EAOPs).

Atomic Layer Deposition-Assisted Fabrication of Co-Nanoparticle/N-Doped Carbon Nanotube Hybrids as Efficient Electrocatalysts for the Oxygen Evolution Reaction

Transition metal (TM)-based carbon hybrids have numerous applications in the field of regenerative electrochemical energy. The synergetic effects of high conductivity of carbon supports and abundant catalytic active sites in TMs make these hybrids promising oxygen evolution reaction (OER) electrocatalysts. However, strategies for modulating the catalytic active species in the above hybrids are limited despite being highly sought after. Furthermore, the exact roles of chemical species in the hybrids (e.g., N, C, or TM) mainly responsible for this high OER performance remain unknown. Herein, an innovative approach based on atomic layer deposition is developed to tune the true active species in Co nanoparticle/N-doped carbon nanotube (Co/N-CNT) hybrids. Specifically, the configuration predominantly promoting water oxidation in an alkaline medium is identified as pyridinic N-Co-C. Furthermore, a physicochemical intact interface between metallic Co nanoparticles and conductive N-CNTs is demonstrated to induce synergetic effects for accelerating charge transfer and enhancing electrocatalytic activity as well as stability in the hybrid catalysts. The optimized hybrid catalyst is revealed to exhibit outstanding alkaline OER activity and stability, outperforming RuO₂ , a benchmark novel OER electrocatalyst.

MOF-Derived Sulfide-Based Electrocatalyst and Scaffold for Boosted Hydrogen Production

Metal-organic frameworks (MOFs) can act as precursors or templates to a myriad of nanostructured materials that are difficult to prepare. In this study, Co-MOF nanorods (NRs) were prepared at room temperature followed by a calcination and hydrothermal sulfurization strategy to transform the MOF into CoS NRs on carbon cloth (CoS/CC). Intriguingly, the resultant 3D sulfide NRs can serve as scaffolds to electrodeposit layered double hydroxides (LDHs) on the surfaces. Through combining the advantages of structure and composition, the as-fabricated CoS@CoNi-LDH/CC exhibits remarkable electrocatalytic activity for the hydrogen evolution reaction (HER). An overpotential of 124 mV is needed to reach a current density of 10 mA cm^-2 with a Tafel slope of only 89 mV dec^-1, which is superior to that of pure CoS/CC (141 mV along with 103 mV dec^-1) and other reported cobalt-based catalysts. Notably, after the chronopotentiometry test for 50 h, the overpotential of CoS@CoNi-LDH/CC increased by 17 mV only.

A Universal Process: Self-Templated and Orientated Fabrication of XMoO4 (X: Ni, Co, or Fe) Nanosheets on MoO2 Nanoplates as Electrocatalysts for Efficient Water Splitting

Fabrication of superior nonprecious electrocatalysts is essential for water electrolysis. Herein, the epitaxial growth of the XMoO₄ (X = Ni, Co, Fe) nanosheets on the hexagonal MoO₂ nanoplates are carried out. The preoxidation of MoO₂ nanoplate is fatal to the epitaxial growth of a nanosheets array on MoO₂ nanoplates. The hierarchical heterostructure of the vertically aligned NiMo nanosheets on MoO₂ nanoplate (NiMo/MoO₂) is well-maintained in the process of in situ topotactic reduction transformation from NiMoO₄·xH₂O/MoO₂. Attributing it to the rich electroactive sites from nanosheets array, together with the intrinsic electrocatalytic performance of NiMo alloy, the as-engineered NiMo/MoO₂ as electrocatalyst exhibits admirable hydrogen evolution reaction (HER) activity with a small onset potential of -12 mV vs RHE (1 mA cm^-2) and a tafel value of 43.6 mV dec^-1 at alkaline media. Furthermore, the obtained CoMoO₄/MoO₂ possesses excellent oxygen evolution performance, which is verified by an ultralow overpotential of 230 mV@10 mA cm^-2, small Tafel slope (51 mV dec^-1), and robust durability. The developed NiMo/MoO₂ and CoMoO₄/MoO₂ electrocatalysts are assembled into an alkaline electrolyzer, which affords a cell potential of 1.51 V at 10 mA cm^-2, as well as outstanding operational durability, which is superior to the typically constructed 20 wt % Pt/C-RuO₂ system (1.59 V at 10 mA cm^-2). Hence, the universal strategy using MoO₂ nanoplates as Mo source and epitaxial substrate may be extended to explore and construct economical and superior Mo-based electrocatalysts for water electrolysis.

Self-Supported Vanadium Carbide by an Electropolymerization-Assisted Method for Efficient Hydrogen Production

Exploring efficient electrodes toward the hydrogen evolution reaction (HER) remains a great challenge for large-scale hydrogen production. Owing to its high earth abundance, low electrical resistivity, and small density, vanadium carbide (VC) is a promising HER electrode candidate but has been rarely explored. In this work, VC nanoparticles encased in nitrogen-doped carbon matrix on carbon cloth (VC@NC/CC) were prepared as a binder-free HER cathode through electropolymerization followed by carbothermal reduction under argon. In the first step of pyrrole electropolymerization, the VO₄ ^3- anions, serving as both vanadium source and supporting electrolyte, were homogeneously incorporated in the positively charged polypyrrole (PPy) framework through coulombic interaction. The electropolymerization was effective for preparation of binder-free metal carbide materials with various polymer monomers as carbon source, which was favorable for the high performance of metal carbide electrodes. During the pyrolysis process, the polymeric hybrids were converted to VC nanoparticles and entrapped in the PPy-derived N-doped carbon matrix. The optimized VC@NC/CC electrode exhibited high catalytic activity and durability in both acidic and alkaline media. The use of VC for efficient HER is remarkable, and such a convenient and versatile electropolymerization-assisted method is appealing for the fabrication of industrially scalable large-area VC electrodes for efficient hydrogen production.

Aerobic Oil-Phase Cyclic Magnetic Adsorption to Synthesize 1D Fe2O3@TiO2 Nanotube Composites for Enhanced Visible-Light Photocatalytic Degradation

In this work, Fe₂O₃@TiO₂ nanostructures with staggered band alignment were newly designed by an aerobic oil-phase cyclic magnetic adsorption method. XRD and TEM analyses were performed to verify the uniform deposition of Fe₂O₃ nanoparticles on the nanotube inner walls of TiO₂. The steady-state degradation experiments exhibited that 1FeTi possessed the most superior performance, which might be ascribable to the satisfying dark adsorption capacity, efficient photocatalytic activity, ease of magnetic separation, and economic efficiency. These results indicated that the deposition of Fe₂O₃ into TiO₂ nanotubes significantly enhanced the activity of Fe₂O₃, which was mainly ascribed to the Fe₂O₃-induced formation of staggered iron oxides@TiO₂ band alignment and thus efficient separation of h⁺ and e^-. Furthermore, the PL intensity and lifetime of the decay curve were considered as key criterions for the activity's evaluation. Finally, the leaching tests and regeneration experiments were also performed, which illustrated the inhibited photodissolution compared with TiO₂/Fe₃O₄ and stable cycling ability, enabling 1FeTi to be a promising magnetic material for photocatalytic water remediation.

Molybdenum Nitride Electrocatalysts for Hydrogen Evolution More Efficient than Platinum/Carbon: Mo2N/CeO2@Nickel Foam

To produce hydrogen economically by electrolysis of water, one needs to develop a non-precious-metal catalyst that is as efficient as platinum metal. Here, we prepare such a catalyst by growing a layer of Mo₂N over a layer of CeO₂ deposited on nickel foam (NF) [hereafter, Mo₂N /CeO₂@NF] and show that the activity of this self-supported catalyst for hydrogen evolution in 1.0 M KOH is more efficient than that of the Pt/C electrode, achieving a current density of 10 mA/cm² at a fairly low overpotential of 26 mV. Furthermore, after a long-time electrochemical stability test for 24 h at a fixed current density, the overpotential needed to attain a current density of 10 mA/cm² is increased only by 6 mV, implying the huge potential of this method to prepare a super HER activity electrode for water splitting.

Pt nanoparticle-decorated two-dimensional oxygen-deficient TiO2 nanosheets as an efficient and stable electrocatalyst for the hydrogen evolution reaction

Developing novel hydrogen evolution reaction (HER) catalysts with high activity, high stability and low cost is of great importance for the ever-broader applications of hydrogen energy. Among the conventionally used platinum-based heterogeneous catalysts, the high consumption and low utilization efficiency of precious platinum are the most crucial issues. Herein we present a facile approach to prepare an effective HER catalyst with platinum nanoparticles dispersed on oxygen-deficient TiO2-x nanosheets (NSs). The fabricated Pt-TiO2-x NS electrocatalyst shows an overpotential of 35 mV at 10 mA cm-2 for the HER in 0.5 M H2SO4, which is highly comparable to that of commercial Pt/C (34 mV). More attractively, the Pt-TiO2-x NS electrocatalyst largely enhanced the mass activity (MA) of Pt and electrochemical stability compared to commercial Pt/C. The excellent HER performance of Pt-TiO2-x NSs is attributed to the synergetic effect between highly dispersed Pt species and TiO2-x NSs with oxygen vacancies, which enhances both electrocatalytic activity and durability over a wide pH range. This strategy can provide insights into constructing highly efficient catalysts and their support for different energy-related applications.