Efficient Electrocatalytic Oxygen Evolution at Extremely High Current Density over 3D Ultrasmall Zero‐Valent Iron‐Coupled Nickel Sulfide Nanosheets

In Situ Transformed CoOOH@Co3S4 Heterostructured Catalyst for Highly Efficient Catalytic OER Application

The deprived electrochemical kinetics of the oxygen evolution reaction (OER) catalyst is the prime bottleneck and remains the major obstacle in the water electrolysis processes. Herein, a facile hydrothermal technique was implemented to form a freestanding polyhedron-like Co3O4 on the microporous architecture of Ni foam, its reaction kinetics enhanced through sulfide counterpart transformation in the presence of Na2S, and their catalytic OER performances comparatively investigated in 1 M KOH medium. The formed Co3S4 catalyst shows outstanding catalytic OER activity at a current density of 100 mA cm-2 by achieving a relatively low overpotential of 292 mV compared to the pure Co3O4 catalyst and the commercial IrO2 catalyst. This enhancement results from the improved active centers and conductivity, which boost the intrinsic reaction kinetics. Further, the optimized Co3S4 catalyst exhibits admirable prolonged durability up to 72 h at varied current rates with insignificant selectivity decay. The energy dispersive X-ray spectroscopy (EDX) and Raman spectra measured after the prolonged OER stability test reveal a partial transformation of the active catalyst into an oxyhydroxide phase (i.e., CoOOH@Co3S4), which acts as an active catalyst phase during the electrolysis process.

A Review of Transition Metal Boride, Carbide, Pnictide, and Chalcogenide Water Oxidation Electrocatalysts

Transition metal borides, carbides, pnictides, and chalcogenides (X-ides) have emerged as a class of materials for the oxygen evolution reaction (OER). Because of their high earth abundance, electrical conductivity, and OER performance, these electrocatalysts have the potential to enable the practical application of green energy conversion and storage. Under OER potentials, X-ide electrocatalysts demonstrate various degrees of oxidation resistance due to their differences in chemical composition, crystal structure, and morphology. Depending on their resistance to oxidation, these catalysts will fall into one of three post-OER electrocatalyst categories: fully oxidized oxide/(oxy)hydroxide material, partially oxidized core@shell structure, and unoxidized material. In the past ten years (from 2013 to 2022), over 890 peer-reviewed research papers have focused on X-ide OER electrocatalysts. Previous review papers have provided limited conclusions and have omitted the significance of "catalytically active sites/species/phases" in X-ide OER electrocatalysts. In this review, a comprehensive summary of (i) experimental parameters (e.g., substrates, electrocatalyst loading amounts, geometric overpotentials, Tafel slopes, etc.) and (ii) electrochemical stability tests and post-analyses in X-ide OER electrocatalyst publications from 2013 to 2022 is provided. Both mono and polyanion X-ides are discussed and classified with respect to their material transformation during the OER. Special analytical techniques employed to study X-ide reconstruction are also evaluated. Additionally, future challenges and questions yet to be answered are provided in each section. This review aims to provide researchers with a toolkit to approach X-ide OER electrocatalyst research and to showcase necessary avenues for future investigation.

Hybrid Heterostructure Ni3 N|NiFeP/FF Self-Supporting Electrode for High-Current-Density Alkaline Water Electrolysis

Exploring earth-abundant and efficient electrocatalysts for oxygen evolution reaction (OER) is an urgent need and significant to water electrolysis. Although great achievements have been made, it is still challenging to achieve industrial current density and stability. Herein, a hybrid heterostructure electrode based on Ni₃ N and NiFeP over Fe foam substrate (Ni₃ N|NiFeP/FF) is reported, along with 3D-interconnected hierarchical porous architecture, achieving the low overpotentials of 287, 178, and 290 mV at 500 mA cm^-2 in 1 m KOH, 30 wt% KOH, and alkaline simulated seawater, respectively, with excellent durability at 800 mA cm^-2 over 120 h, which can satisfy the requirements of industrial water electrolysis. Here, the hybrid heterostructure can ensure the low energy barrier of the catalytic active sites, the 3D-interconnected hierarchical porous architecture can facilitate the fast mass/ions/electrons transformation, which contributes together to boost the superb water splitting performance. Furthermore, the COMSOL simulations confirm the multiple merits of the designed electrode during the water electrocatalysis. The present work provides a new strategy in the design and engineering of high-performance electrodes for industrial water electrolysis.

Colodrero RM, Demadis KD. Exploiting the Multifunctionality of M2+/Imidazole-Etidronates for Proton Conductivity (Zn2+) and Electrocatalysis (Co2+, Ni2+) toward the HER, OER, and ORR

This work deals with the synthesis and characterization of one-dimensional (1D) imidazole-containing etidronates, [M2(ETID)(Im)3]·nH2O (M = Co2+ and Ni2+; n = 0, 1, 3) and [Zn2(ETID)2(H2O)2](Im)2, as well as the corresponding Co2+/Ni2+ solid solutions, to evaluate their properties as multipurpose materials for energy conversion processes. Depending on the water content, metal ions in the isostructural Co2+ and Ni2+ derivatives are octahedrally coordinated (n = 3) or consist of octahedral together with dimeric trigonal bipyramidal (n = 1) or square pyramidal (n = 0) environments. The imidazole molecule acts as a ligand (Co2+, Ni2+ derivatives) or charge-compensating protonated species (Zn2+ derivative). For the latter, the proton conductivity is determined to be ∼6 × 10-4 S·cm-1 at 80 °C and 95% relative humidity (RH). By pyrolyzing in 5%H2-Ar at 700-850 °C, core-shell electrocatalysts consisting of Co2+-, Ni2+-phosphides or Co2+/Ni2+-phosphide solid solution particles embedded in a N-doped carbon graphitic matrix are obtained, which exhibit improved catalytic performances compared to the non-N-doped carbon materials. Co2+ phosphides consist of CoP and Co2P in variable proportions according to the used precursor and pyrolytic conditions. However, the Ni2+ phosphide is composed of Ni2P exclusively at high temperatures. Exploration of the electrochemical activity of these metal phosphides toward the oxygen evolution reaction (OER), oxygen reduction reaction (ORR), and hydrogen evolution reaction (HER) reveals that the anhydrous Co2(ETID)(Im)3 pyrolyzed at 800 °C (CoP/Co2P = 80/20 wt %) is the most active trifunctional electrocatalyst, with good integrated capabilities as an anode for overall water splitting (cell voltage of 1.61 V) and potential application in Zn-air batteries. This solid also displays a moderate activity for the HER with an overpotential of 156 mV and a Tafel slope of 79.7 mV·dec-1 in 0.5 M H2SO4. Ni2+- and Co2+/Ni2+-phosphide solid solutions show lower electrochemical performances, which are correlated with the formation of less active crystalline phases.

Fast and Deep Reconstruction of Coprecipitated Fe Phosphates on Nickel Foams for an Alkaline Oxygen Evolution Reaction

Although there is a general consensus that the electrocatalysts will undergo reconstruction to generate (oxy)hydroxides as real active sites during the electrochemical oxygen evolution reaction (OER), the understanding of this process is still far from satisfactory. In particular, the reconstruction process of most of these electrocatalysts is either slow or occurs only on the surface, which thus restrains the OER performance of the electrocatalysts. Herein, we reveal a fast and deep reconstruction of the coprecipitated Fe phosphates on nickel foam, via in situ Raman spectroscopy together with electron microscopy, X-ray photoelectron spectroscopy, and electrochemical tests. The generated NiFe (oxy)hydroxide nanosheets after reconstruction behave as the real active sites for the OER in the alkaline condition, with a low overpotential and excellent durability. The present work provides deep insights on the reconstruction dynamics of OER electrocatalysts.

Three-Phase Heterojunction NiMo-Based Nano-Needle for Water Splitting at Industrial Alkaline Condition

Constructing heterojunction is an effective strategy to develop high-performance non-precious-metal-based catalysts for electrochemical water splitting (WS). Herein, we design and prepare an N-doped-carbon-encapsulated Ni/MoO2 nano-needle with three-phase heterojunction (Ni/MoO2@CN) for accelerating the WS under industrial alkaline condition. Density functional theory calculations reveal that the electrons are redistributed at the three-phase heterojunction interface, which optimizes the adsorption energy of H- and O-containing intermediates to obtain the best ΔGH* for hydrogen evolution reaction (HER) and decrease the ΔG value of rate-determining step for oxygen evolution reaction (OER), thus enhancing the HER/OER catalytic activity. Electrochemical results confirm that Ni/MoO2@CN exhibits good activity for HER (ƞ-10 = 33 mV, ƞ-1000 = 267 mV) and OER (ƞ10 = 250 mV, ƞ1000 = 420 mV). It shows a low potential of 1.86 V at 1000 mA cm-2 for WS in 6.0 M KOH solution at 60 °C and can steadily operate for 330 h. This good HER/OER performance can be attributed to the three-phase heterojunction with high intrinsic activity and the self-supporting nano-needle with more active sites, faster mass diffusion, and bubbles release. This work provides a unique idea for designing high efficiency catalytic materials for WS.

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.

Cation and Anion Co-doped Perovskite Nanofibers for Highly Efficient Electrocatalytic Oxygen Evolution

Perovskite oxides have been recognized as one of the most attractive oxygen evolution reaction (OER) catalysts because of their low cost, earth abundance, and robust nature. Herein, one-dimensional porous LaFe_1-xNi_xO₃ (LFNO) perovskite oxide nanofibers (LFNO NFs) are fabricated with a feasible electrospinning route and its further post-calcination treatment. By tailoring the atomic percent of Fe and Ni in the perovskite oxide, we determined that LaFe_0.25Ni_0.75O₃ (LFNO-III) NFs afford the best OER activity among all the prepared perovskite oxides. Especially remarkable is that the further selenide-doped LaFe_0.25Ni_0.75O₃ (LFNOSe-III) NFs exhibit outstanding OER activity with a low overpotential of 287 mV at 10 mA cm^-2 and a small Tafel slope of 87 mV dec^-1 in 1 M KOH solution, markedly exceeding that of the parent perovskite oxide and the commercial RuO₂. It also delivers decent durability with no significant degradation after 22 h of stability test. In the meanwhile, density functional theory calculations are also conducted to justify the optimized adsorption features of *OH, *O, and *OOH intermediates and unveil that the electrocatalytic active sites are the Ni atoms adjacent to Fe in the Ni- and Se codoped perovskite. This work provides an effective method for the development of highly efficient perovskite oxide catalysts.

Highly Effective Electrochemical Exfoliation of Ultrathin Tantalum Disulfide Nanosheets for Energy-Efficient Hydrogen Evolution Electrocatalysis

Developing highly efficient transition metal dichalcogenide electrocatalysts would be significantly beneficial for the electrocatalytic hydrogen evolution reaction (HER) from water splitting. Herein, we reported novel ultrathin tantalum disulfide nanosheets (TaS₂ NSs) prepared by electrochemically exfoliating bulk TaS₂ with an alternating voltage in an acidic electrolyte. The obtained TaS₂ NS electrocatalyst possessed an ultrathin structure with a lateral size of 2 μm and a thickness of ∼3 nm. Owing to the unique 2D structure, the achieved TaS₂ NSs displayed remarkable electrocatalytic activity toward the HER by a small overpotential of 197 mV at 10 mA cm^-2 and a small Tafel slope of 100 mV dec^-1 in acidic solution, much lower than those of TaS₂ (>547 mV and 216 mV dec^-1, respectively) and other reported TaS₂-based HER electrocatalysts. Furthermore, highly efficient full water splitting could be realized with two electrodes in which TaS₂ NSs acted as the cathode while Ir/C served as the anode, with help of two AA size batteries or solar cells. By replacing the oxygen evolution reaction with the urea oxidation reaction (UOR), bifunctional TaS₂ NSs enabled an energy-effective HER process in the cathode and UOR process in the anode with decreased applied potential.

A Superaerophobic Bimetallic Selenides Heterostructure for Efficient Industrial-Level Oxygen Evolution at Ultra-High Current Densities

Cost-effective and stable electrocatalysts with ultra-high current densities for electrochemical oxygen evolution reaction (OER) are critical to the energy crisis and environmental pollution. Herein, we report a superaerophobic three dimensional (3D) heterostructured nanowrinkles of bimetallic selenides consisting of crystalline NiSe2 and NiFe2Se4 grown on NiFe alloy (NiSe2/NiFe2Se4@NiFe) prepared by a thermal selenization procedure. In this unique 3D heterostructure, numerous nanowrinkles of NiSe2/NiFe2Se4 hybrid with a thickness of ~ 100 nm are grown on NiFe alloy in a uniform manner. Profiting by the large active surface area and high electronic conductivity, the superaerophobic NiSe2/NiFe2Se4@NiFe heterostructure exhibits excellent electrocatalytic activity and durability towards OER in alkaline media, outputting the low potentials of 1.53 and 1.54 V to achieve ultra-high current densities of 500 and 1000 mA cm-2, respectively, which is among the most active Ni/Fe-based selenides, and even superior to the benchmark Ir/C catalyst. The in-situ derived FeOOH and NiOOH species from NiSe2/NiFe2Se4@NiFe are deemed to be efficient active sites for OER.

Ultrathin tin monosulfide nanosheets with the exposed (001) plane for efficient electrocatalytic conversion of CO2 into formate

Current Sn-based materials are ideal catalysts developed to drive the electrochemical conversion of CO2 to formate, but competing proton reduction to hydrogen is an ever-present drain on catalytic selectivity. Herein, we report a reliable electrochemical exfoliation route, with the assistance of alternating voltage, for large-scale preparation of two-dimensional (2D) ultrathin tin monosulfide nanosheets (SnS NSs), which feature a large lateral size of 1.0 μm with a thickness of ∼5.0 nm. Systematic electrochemical studies demonstrated that the achieved SnS NSs exhibited an outstanding electrocatalytic activity towards the CO2 reduction reaction (CO2RR) to the formate product, as evidenced by a considerable faradaic efficiency (F.E.) of 82.1%, a high partial current density of 18.9 mA cm-2 at -1.1 V, and a low Tafel slope of 180 mV dec-1. Further, using an electrode prepared from the resulting SnS NSs by the particle transfer method, a remarkably high formate F.E. over 91% was achieved in a wide potential window. Such high performance renders the SnS NSs among the best reported tin sulfide-based CO2RR electrocatalysts. Theoretical calculations coupled with comprehensive experimental studies demonstrated that the synergistic effect between the ultrathin layered architecture and dominantly exposed (001) plane of SnS NSs accounted for the uniquely efficient catalytic activity for the CO2RR.

Electrochemical exfoliation of ultrathin ternary molybdenum sulfoselenide nanosheets to boost the energy-efficient hydrogen evolution reaction

Developing low-cost and highly efficient transition metal dichalcogenides as alternative electrocatalysts has become an urgent need for the hydrogen evolution reaction (HER). However, the inert basal planes of transition metal dichalcogenides severely suppress their practical applications. Herein, we developed ultrathin ternary molybdenum sulfoselenide (MoSe_xS_2-x) nanosheets by using the cathodic electrochemical exfoliation approach in non-aqueous electrolytes. The exfoliated MoSe_xS_2-x nanosheets demonstrated high structural integrity with lateral dimensions up to ∼1.5 μm and an average thickness of about 3 nm. Owing to the unique ultrathin structure and immensely exposed active sites, the ternary MoSe_xS_2-x nanosheets supported on Ni foam demonstrated a greatly enhanced electrocatalytic activity for the HER in 1.0 M KOH with an overpotential of 123 mV at a current density of 10 mA cm^-2 and high stability, superior to majority of the previously published MoS₂-based electrocatalysts. Furthermore, the ternary MoSe_xS_2-x nanosheets as a highly active bifunctional electrocatalyst contributed to enhanced energy-efficient hydrogen production and electrocatalytic synthesis of ammonia.

Strongly Coupled 3D N-Doped MoO2/Ni3S2 Hybrid for High Current Density Hydrogen Evolution Electrocatalysis and Biomass Upgrading

Developing noble metal-free electrocatalysts toward hydrogen evolution reaction (HER) that can work well at ultrahigh current density are crucial components in renewable energy technologies. Herein, we have reported a strongly coupled 3D hybrid electrocatalyst, which consists of N-doped MoO₂ with Ni₃S₂ grown on Ni foam (N-MoO₂/Ni₃S₂ NF) through an annealing treatment, followed by a thermal ammonia reaction. This N-MoO₂/Ni₃S₂ with a particle size of ∼50 nm was evenly grown on the Ni substrate in this 3D hybrid system. Benefiting from the strong coupling effect, the N-MoO₂/Ni₃S₂ NF exhibited a high HER performance in basic media, with a small value of the Tafel slope (76 mV dec^-1) and a low potential of 517 mV at 1000 mA cm^-2, which was superior to that of Pt/C (631 mV at 1000 mA cm^-2). Experimental results revealed that constructing a coupling interface between N-MoO₂ and Ni₃S₂ facilitated the absorption and dissociation of water molecules, consequently boosting the HER activity. Additionally, the 3D N-MoO₂/Ni₃S₂ NF hybrid could act as a bifunctional electrode for both anode (biomass upgrading) and cathode (HER), which only required a lower potential of 2.08 V at 100 mA cm^-2 as compared to the overall water splitting (2.25 V) and achieved a high biomass conversion ratio of over 90%. Moreover, substituting oxygen evolution reaction by urea oxidation reaction also can assist energy-saving hydrogen evolution for 3D N-MoO₂/Ni₃S₂ NF.

One-Step Synthesis of NiFe Layered Double Hydroxide Nanosheet Array/N-Doped Graphite Foam Electrodes for Oxygen Evolution Reactions

Developing cost-effective and highly efficient oxygen evolution reaction (OER) electrocatalysts is vital for the production of clean hydrogen by electrocatalytic water splitting. Here, three dimensional nickel-iron layered double hydroxide (NiFe LDH) nanosheet arrays are in-situ fabricated on self-supporting nitrogen doped graphited foam (NGF) via a one-step hydrothermal process under an optimized amount of urea. The as prepared NiFe LDH/NGF electrode exhibits a remarkable activity toward OER with a low onset overpotential of 233 mV and a Tafel slope of 59.4 mV dec-1 as well as a long-term durability. Such good performance is attributed to the synergy among the doping effect, the binder-free characteristic, and the architecture of the nanosheet array.

Integrated System of Solar Cells with Hierarchical NiCo2O4 Battery-Supercapacitor Hybrid Devices for Self-Driving Light-Emitting Diodes

An integrated system has been provided with a-Si/H solar cells as energy conversion device, NiCo₂O₄ battery-supercapacitor hybrid (BSH) as energy storage device, and light emitting diodes (LEDs) as energy utilization device. By designing three-dimensional hierarchical NiCo₂O₄ arrays as faradic electrode, with capacitive electrode of active carbon (AC), BSHs were assembled with energy density of 16.6 Wh kg^-1, power density of 7285 W kg^-1, long-term stability with 100% retention after 15,000 cycles, and rather low self-discharge. The NiCo₂O₄//AC BSH was charged to 1.6 V in 1 s by solar cells and acted as reliable sources for powering LEDs. The integrated system is rational for operation, having an overall efficiency of 8.1% with storage efficiency of 74.24%. The integrated system demonstrates a stable solar power conversion, outstanding energy storage behavior, and reliable light emitting. Our study offers a precious strategy to design a self-driven integrated system for highly efficient energy utilization.

Scalable Production of Few-Layer Niobium Disulfide Nanosheets via Electrochemical Exfoliation for Energy-Efficient Hydrogen Evolution Reaction

Two-dimensional (2D) niobium disulfide (NbS₂) materials feature unique physical and chemical properties leading to highly promising energy conversion applications. Herein, we developed a robust synthesis technique consisting of electrochemical exfoliation under alternating currents and subsequent liquid-phase exfoliation to prepare highly uniform few-layer NbS₂ nanosheets. The obtained few-layer NbS₂ material has a 2D nanosheet structure with an ultrathin thickness of ∼3 nm and a lateral size of ∼2 μm. Benefiting from their unique 2D structure and highly exposed active sites, the few-layer NbS₂ nanosheets drop-casted on carbon paper exhibited excellent catalytic activity for the hydrogen evolution reaction (HER) in acid with an overpotential of 90 mV at a current density of 10 mA cm^-2 and a low Tafel slope of 83 mV dec^-1, which are superior to those reported for other NbS₂-based HER electrocatalysts. Furthermore, few-layer NbS₂ nanosheets are effective as bifunctional electrocatalysts for hydrogen production by overall water splitting, where the urea and hydrazine oxidation reactions replace the oxygen evolution reaction.