Demystifying the active roles of NiFe-based oxides/(oxy)hydroxides for electrochemical water splitting under alkaline conditions

Ligand and Strain Synergistic Effect in NiFeP0.32 LDH for Triggering Efficient Oxygen Evolution Reaction

Developing efficient water-splitting electrocatalysts to accelerate the slow oxygen evolution reaction (OER) kinetics is urgently desired for hydrogen production. Herein, ultralow phosphorus (P)-doped NiFe LDH (NiFePx LDH) with mild compressive strain is synthesized as an efficient OER electrocatalyst. Remarkably, NiFePx LDH with the phosphorus mass ratio of 0.32 wt.% and compressive strain ratio of 2.53% (denoted as NiFeP0.32 LDH) exhibits extraordinary OER activity with an overpotential as low as 210 mV, which is superior to that of commercial IrO2 and other reported P-based OER electrocatalysts. Both experimental performance and density function theory (DFT) calculation demonstrate that the doping of P atoms can generate covalent Fe─P coordination bonds and lattice distortion, thus resulting in the consequent depletion of electrons around the Fe active center and the downward shift of the d-band center, which can lead to a weaker adsorption ability of *O intermediate to improve the catalytic performance of NiFeP0.32 LDH for OER. This work provides novel insights into the distinctive coordinated configuration of P in NiFePx LDH, which can result in superior catalytic performance for OER.

Zeolitic Imidazolate Framework-Derived Co3S4@NiFe-LDH Core-Shell Heterostructure as Efficient Bifunctional Electrocatalyst for Water Splitting

The development of stable and efficient bifunctional electrocatalysts is of utmost importance for overall water splitting. This study introduces Co3S4@NiFe-LDH core-shell heterostructure prepared via an electrodeposition of ultrathin NiFe-LDH nanosheet on zeolitic imidazolium framework-derived Co3S4 nanosheet arrays. The bifunctional Co3S4@NiFe-LDH/NF exhibits impressive catalytic performance and long-term stability for both the OER and HER with low overpotentials of 100 mA cm-2 at 235 mV and 10 mA cm-2 at 95 mV in 1 M KOH, respectively. The assembled cell with Co3S4@NiFe-LDH/NF as both cathode and anode shows voltages of 1.595 and 1.666 V at current densities of 10 and 20 mA cm-2, respectively, as well as ultralong stability over 500 h. DFT calculations expose a robust electron interaction at the heterogeneous interface of the Co3S4@NiFe-LDH/NF core-shell structure. This interaction promotes electron transfer from NiFe-LDH to Co3S4 and reduces the energy barriers for OER intermediates, thereby enhancing electrocatalytic activity. This research contributes novel insights toward the promising materials for electrochemical water splitting through the construction of heterojunction interfaces.

Carbon Oxyanion Self-Transformation on NiFe Oxalates Enables Long-Term Ampere-Level Current Density Seawater Oxidation

Seawater electrolysis is an attractive way of making H2 in coastal areas, and NiFe-based materials are among the top options for alkaline seawater oxidation (ASO). However, ample Cl- in seawater can severely corrode catalytic sites and lead to limited lifespans. Herein, we report that in situ carbon oxyanion self-transformation (COST) from oxalate to carbonate on a monolithic NiFe oxalate micropillar electrode allows safeguard of high-valence metal reaction sites in ASO. In situ/ex situ studies show that spontaneous, timely, and appropriate COST safeguards active sites against Cl- attack during ASO even at an ampere-level current density (j). Our NiFe catalyst shows efficient and stable ASO performance, which requires an overpotential as low as 349 mV to attain a j of 1 A cm-2 . Moreover, the NiFe catalyst with protective surface CO3 2- exhibits a slight activity degradation after 600 h of electrolysis under 1 A cm-2 in alkaline seawater. This work reports effective catalyst surface design concepts at the level of oxyanion self-transformation, acting as a momentous step toward defending active sites in seawater-to-H2 conversion systems.

Quenching modification of NiFe layered double hydroxides as efficient and highly stable electrocatalysts for the oxygen evolution reaction

Nickel- and iron-containing layered double hydroxides (NiFe-LDHs) are prospective electrocatalysts for the oxygen evolution reaction (OER), but they suffer from poor electrical conductivity and inaccessible active sites. Herein, we employ a facile and efficient quenching strategy to modify the morphology and surface characteristics of NiFe-LDHs by rapid cooling in a series of salt solutions. After quenching in a SnCl4 solution, the modified NiFe-LDHs exhibit a low overpotential of 204 mV at a current density of 10 mA·cm-2 and Tafel slope of 58.0 mV·dec-1 in a 1.0 M KOH solution. The improvement in the oxygen-evolution performance is ascribed to the morphology transformation from agglomerated NiFe-LDHs flowers into dispersed two-dimensional NiFe-LDHs nanosheets, which offers more active sites for the OER. Metal atoms are also introduced to the surface of NiFe-LDHs nanosheets during quenching, thereby changing the chemical coordination environment between Ni and Fe and improving their conductivity. Considering the diversity of LDHs and salt solutions, this quenching strategy may provide a sophisticated approach to preparing stable non-noble-metal electrocatalysts for the OER.

Defective blue titanium oxide induces high valence of NiFe-(oxy)hydroxides over heterogeneous interfaces towards high OER catalytic activity

Nickel-iron (oxy)hydroxides (NiFeOxHy) have been validated to speed up sluggish kinetics of the oxygen evolution reaction (OER) but still lack satisfactory substrates to support them. Here, non-stoichiometric blue titanium oxide (B-TiOx) was directly derived from Ti metal by alkaline anodization and used as a substrate for electrodeposition of amorphous NiFeOxHy (NiFe/B-TiOx). The performed X-ray absorption spectroscopy (XAS) and density functional theory (DFT) calculations evidenced that there is a charge transfer between B-TiOx and NiFeOxHy, which gives rise to an elevated valence at the Ni sites (average oxidation state ∼ 2.37). The synthesized NiFe/B-TiOx delivers a current density of 10 mA cm-2 and 100 mA cm-2 at an overpotential of 227 mV and 268 mV, respectively, which are better than that of pure Ti and stainless steel. It also shows outstanding activity and stability under industrial conditions of 6 M KOH. The post-OER characterization studies revealed that the surface morphology and valence states have no significant change after 24 h of operation at 500 mA cm-2, and also can effectively inhibit the leaching of Fe. We illustrate that surface modification of Ti which has high corrosion resistance and mechanical strength, to generate strong interactions with NiFeOxHy is a simple and effective strategy to improve the OER activity and stability of non-precious metal electrodes.

Intrabasal Plane Defect Formation in NiFe Layered Double Hydroxides Enabling Efficient Electrochemical Water Oxidation

Defect engineering has proven to be one of the most effective approaches for the design of high-performance electrocatalysts. Current methods to create defects typically follow a top-down strategy, cutting down the pristine materials into fragmented pieces with surface defects yet also heavily destroying the framework of materials that imposes restrictions on the further improvements in catalytic activity. Herein, we describe a bottom-up strategy to prepare free-standing NiFe layered double hydroxide (LDH) nanoplatelets with abundant internal defects by controlling their growth behavior in acidic conditions. Our best-performing nanoplatelets exhibited the lowest overpotential of 241 mV and the lowest Tafel slope of 43 mV/dec for the oxygen evolution reaction (OER) process, superior to the pristine LDHs and other reference cation-defective LDHs obtained by traditional etching methods. Using both material characterization and density functional theory (DFT) simulation has enabled us to develop relationships between the structure and electrochemical properties of these catalysts, suggesting that the enhanced electrocatalytic activity of nanoplatelets mainly results from their defect-abundant structure and stable layered framework with enhanced exposure of the (001) surface.

A quaternary heterojunction nanoflower for significantly enhanced electrochemical water splitting

Designing highly-efficient, cost-effective, and stable electrocatalysts for water splitting is essential to producing green hydrogen. In this work, a nanoflower quaternary heterostructured Ni(NO3)2(OH)4/Ni(OH)2/Ni3S2/NiFe-LDH electrocatalyst is successfully synthesized by two-step hydrothermal reactions. The sulfur in the electrocatalyst induces higher valence state metal atoms as active sites to accelerate the formation of O2. As expected, benefiting from the unique structural features and solid electronic interactions, Ni(NO3)2(OH)4/Ni(OH)2/Ni3S2/NiFe-LDH exhibits remarkable oxygen evolution reaction performance with a low overpotential of 223 mV at a current density of 100 mA cm-2, a slight Tafel slope of 65.4 mV dec-1, and outstanding stability in alkaline media. Attractively, using Ni(NO3)2(OH)4/Ni(OH)2/Ni3S2/NiFe-LDH as both a cathode and an anode, the alkaline electrolyzer delivers a current density of 10 mA cm-2 only at a cell voltage of 1.67 V, accompanied by superior durability. This work provides a facile method for the rational design of high-performance quaternary electrocatalysts.

Enhanced membraneless fuel cells by electrooxidation of ethylene glycol with a nanostructured cobalt metal catalyst

The advancement of effective and long-lasting electrocatalysts for energy storage devices is crucial to reduce the impact of the energy crisis. In this study, a two-stage reduction process was used to synthesize carbon-supported cobalt alloy nanocatalysts with varying atomic ratios of cobalt, nickel and iron. The formed alloy nanocatalysts were investigated using energy-dispersive X-ray spectroscopy, X-ray diffraction, and transmission electron microscopy to determine their physicochemical characterization. According to XRD results, Cobalt-based alloy nanocatalysts form a face-centered cubic solid solution pattern, illustrating thoroughly mixed ternary metal solid solutions. Transmission electron micrographs also demonstrated that samples of carbon-based cobalt alloys displayed homogeneous dispersion at particle sizes ranging from 18 to 37 nm. Measurements of cyclic voltammetry, linear sweep voltammetry, and chronoamperometry revealed that iron alloy samples exhibited much greater electrochemical activity than non-iron alloy samples. The alloy nanocatalysts were evaluated as anodes for the electrooxidation of ethylene glycol in a single membraneless fuel cell to assess their robustness and efficiency at ambient temperature. Remarkably, in line with the results of cyclic voltammetry and chronoamperometry, the single-cell test showed that the ternary anode works better than its counterparts. The significantly higher electrochemical activity was observed for alloy nanocatalysts containing iron than for non-iron alloy catalysts. Iron stimulates nickel sites to oxidize cobalt to cobalt oxyhydroxides at lower over-potentials, which contributes to the improved performance of ternary alloy catalysts containing iron.

A Facile Molecular Approach to Amorphous Nickel Pnictides and Their Reconstruction to Crystalline Potassium-Intercalated γ-NiOOHx Enabling High-Performance Electrocatalytic Water Oxidation and Selective Oxidation of 5-Hydroxymethylfurfural

The low-temperature molecular precursor approach can be beneficial to conventional solid-state methods, which require high temperatures and lead to relatively large crystalline particles. Herein, a novel, single-step, room-temperature preparation of amorphous nickel pnictide (NiE; EP, As) nanomaterials is reported, starting from NaOCE(dioxane)n and NiBr2 (thf)1.5 . During application for the oxygen evolution reaction (OER), the pnictide anions leach, and both materials fully reconstruct into nickel(III/IV) oxide phases (similar to γ-NiOOH) comprising edge-sharing (NiO6 ) layers with intercalated potassium ions and a d-spacing of 7.27 Å. Remarkably, the intercalated γ-NiOOHx phases are nanocrystalline, unlike the amorphous nickel pnictide precatalysts. This unconventional reconstruction is fast and complete, which is ascribed to the amorphous nature of the nanostructured NiE precatalysts. The obtained γ-NiOOHx can effectively catalyse the OER for 100 h at a high current density (400 mA cm-2 ) and achieves outstandingly high current densities (>600 mA cm-2 ) for the selective, value-added oxidation of 5-hydroxymethylfurfural (HMF). The NiP-derived γ-NiOOHx shows a higher activity for both processes due to more available active sites. It is anticipated that the herein developed, effective, room-temperature molecular synthesis of amorphous nickel pnictide nanomaterials can be applied to other functional transition-metal pnictides.

Iron-doped bimetallic boride Fe-Ni2B/NF-x nanoparticles toward efficient oxygen evolution reaction at a large current density

Transition metal borides are seen as potential candidates for oxygen evolution reaction (OER) electrocatalysts due to their superconductivity and rich surface-active sites, but monometallic borides only display generic OER catalytic performance. Hence, iron-doped bimetallic boride nanoparticles (Fe-Ni2B/NF-x) on Ni foam are reported and applied as superior OER electrocatalysts with high catalytic activities. Such bimetallic boride electrocatalysts require overpotentials of only 194 and 336 mV to afford current densities of 10 and 500 mA cm-2 toward the OER in 1 M KOH electrolyte, and Fe-Ni2B/NF-3 can retain this catalytic stability for at least 100 h at 1.456 V. The performance of the improved catalyst Fe-Ni2B/NF-3 matches the best nickel-based OER electrocatalysts reported so far. Analysis of X-ray photoelectron spectroscopy (XPS) and Gibbs free energy calculations show that Fe-doping essentially acts to modulate the electronic density of Ni2B and lower the free energy of O adsorption in the OER. The charge density differences and d-band theory proved that Fe sites have a high charge state and can be taken as catalytic sites for the OER. This proposed synthesis strategy provides a different view for preparing efficient bimetallic boride electrocatalysts.

Recent advances in interface engineering of Fe/Co/Ni-based heterostructure electrocatalysts for water splitting

Among various methods of developing hydrogen energy, electrocatalytic water splitting for hydrogen production is one of the approaches to achieve the goal of zero carbon emissions. It is of great significance to develop highly active and stable catalysts to improve the efficiency of hydrogen production. In recent years, the construction of nanoscale heterostructure electrocatalysts through interface engineering can not only overcome the shortcomings of single-component materials to effectively improve their electrocatalytic efficiency and stability but also adjust the intrinsic activity or design synergistic interfaces to improve catalytic performance. Among them, some researchers proposed to replace the slow oxygen evolution reaction at the anode with the oxidation reaction of renewable resources such as biomass to improve the catalytic efficiency of the overall water splitting. The existing reviews in the field of electrocatalysis mainly focus on the relationship between the interface structure, principle, and principle of catalytic reaction, and some articles summarize the performance and improvement schemes of transition metal electrocatalysts. Among them, few studies are focusing on Fe/Co/Ni-based heterogeneous compounds, and there are fewer summaries on the oxidation reactions of organic compounds at the anode. To this end, this paper comprehensively describes the interface design and synthesis, interface classification, and application in the field of electrocatalysis of Fe/Co/Ni-based electrocatalysts. Based on the development and application of current interface engineering strategies, the experimental results of biomass electrooxidation reaction (BEOR) replacing anode oxygen evolution reaction (OER) are discussed, and it is feasible to improve the overall electrocatalytic reaction efficiency by coupling with hydrogen evolution reaction (HER). In the end, the challenges and prospects for the application of Fe/Co/Ni-based heterogeneous compounds in water splitting are briefly discussed.

Ni-Fe Oxides/TiO2 Heterojunction Anodes for Reactive Chlorine Generation and Mediated Water Treatment

Reactive chlorine-mediated electrochemical water treatment necessitates selective chlorine evolution reaction (ClER) versus parallel oxygen evolution reaction (OER) in mild pH (7-10), with minimal deployments of precious electrocatalysts. This study reports Ni_0.33Fe_0.67O_y/TiO₂ heterojunction anode prepared by a straightforward sol-gel coating with thermal decomposition at 425 °C. The ClER current efficiency (CE, 70%) and energy efficiency (2.3 mmol W h^-1) were comparable to benchmarking Ir₇Ta₃O_y/TiO₂ at 30 mA cm^-2 in 50 mM NaCl solutions with near-neutral pH. Correlations of ClER CE of variable Ni_xFe_1-xO_y/TiO₂ (x: 0.33, 0.8-1) with the flat-band potential and p-band center, as experimental descriptors for surface charge density, nominated the outer TiO₂ to be the active ClER center. The underlying Ni_0.33Fe_0.67O_y, characterized as biphasic NiFe₂O₄ and NiO, effectively lowered the O binding energy of TiO₂ by electronic interaction across the junction. The OER activity of Ni_0.33Fe_0.67O_y superior to the other Fe-doped Ni oxides suggested that the conductive OER intermediates generated on Ni_0.33Fe_0.67O_y could also facilitate the ClER as an ohmic contact. Stability tests and NH₄⁺ degradation in synthetic and real wastewater confirmed the feasibility of Ni_0.33Fe_0.67O_y/TiO₂ heterojunction anode for mediated water treatments in mild pH.

Oxygen Evolution/Reduction Reaction Catalysts: From In Situ Monitoring and Reaction Mechanisms to Rational Design

The oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) are core steps of various energy conversion and storage systems. However, their sluggish reaction kinetics, i.e., the demanding multielectron transfer processes, still render OER/ORR catalysts less efficient for practical applications. Moreover, the complexity of the catalyst-electrolyte interface makes a comprehensive understanding of the intrinsic OER/ORR mechanisms challenging. Fortunately, recent advances of in situ/operando characterization techniques have facilitated the kinetic monitoring of catalysts under reaction conditions. Here we provide selected highlights of recent in situ/operando mechanistic studies of OER/ORR catalysts with the main emphasis placed on heterogeneous systems (primarily discussing first-row transition metals which operate under basic conditions), followed by a brief outlook on molecular catalysts. Key sections in this review are focused on determination of the true active species, identification of the active sites, and monitoring of the reactive intermediates. For in-depth insights into the above factors, a short overview of the metrics for accurate characterizations of OER/ORR catalysts is provided. A combination of the obtained time-resolved reaction information and reliable activity data will then guide the rational design of new catalysts. Strategies such as optimizing the restructuring process as well as overcoming the adsorption-energy scaling relations will be discussed. Finally, pending current challenges and prospects toward the understanding and development of efficient heterogeneous catalysts and selected homogeneous catalysts are presented.

Facile method to activate substrate for oxygen evolution by a galvanic-cell reaction

NiFe layered double hydroxide (NiFe LDH) is a promising material with multiple functions. In this communication, a novel method is used to prepare NiFe LDH. This synthesis method is achieved via galvanic-cell corrosion between nickel and iron substrates in aqueous solutions containing a halogen group anion (e.g., Cl) at ambient temperature. The as-prepared NiFe LDH electrodes are developed as electrocatalysts for the oxygen evolution reaction (OER) and exhibit excellent catalytic activities and durability. This work provides an energy-efficient, cost-effective, and scaled-up corrosion engineering approach for manufacturing NiFe LDH materials.

Monodispersed Pt Sites Supported on NiFe-LDH from Synchronous Anchoring and Reduction for High Efficiency Overall Water Splitting

Precise design of low-cost, efficient and definite electrocatalysts is the key to sustainable renewable energy. Herein, this work develops a targeted-anchored and subsequent spontaneous-redox strategy to synthesize nickel-iron layered double hydroxide (LDH) nanosheets anchored with monodispersed platinum (Pt) sites (Pt@LDH). Intermediate metal-organic frameworks (MOF)/LDH heterostructure not only provides numerous confine points to guarantee the stability of Pt sites, but also excites the spontaneous reduction for Pt^II . Electronic structure, charge transfer ability and reaction kinetics of Pt@LDH can be effectively facilitated by the monodispersed Pt moieties. As a result, the optimized Pt@LDH that with the 5% ultra-low content Pt exhibits the significant increment in electrochemical water splitting performance in alkaline media, which only afford low overpotentials of 58 mV at 10 mA cm^-2 for hydrogen evolution reaction (HER) and 239 mV at 10 mA cm^-2 for oxygen evolution reaction (OER), respectively. In a real device, Pt@LDH can drive an overall water-splitting at low cell voltage of 1.49 V at 10 mA cm^-2 , which can be superior to most reported similar LDH-based catalysts. Moreover, the versatility of the method is extended to other MOF precursors and noble metals for the design of ultrathin LDH supported monodispersed noble metal electrocatalysts promoting research interest in material design.

Solar utilization beyond photosynthesis

Natural photosynthesis is an efficient biochemical process which converts solar energy into energy-rich carbohydrates. By understanding the key photoelectrochemical processes and mechanisms that underpin natural photosynthesis, advanced solar utilization technologies have been developed that may be used to provide sustainable energy to help address climate change. The processes of light harvesting, catalysis and energy storage in natural photosynthesis have inspired photovoltaics, photoelectrocatalysis and photo-rechargeable battery technologies. In this Review, we describe how advanced solar utilization technologies have drawn inspiration from natural photosynthesis, to find sustainable solutions to the challenges faced by modern society. We summarize the uses of advanced solar utilization technologies, such as converting solar energy to electrical and chemical energy, electrochemical storage and conversion, and associated thermal tandem technologies. Both the foundational mechanisms and typical materials and devices are reported. Finally, potential future solar utilization technologies are presented that may mimic, and even outperform, natural photosynthesis.

N, F Codoped FeOOH Nanosheets with Intercalated Carbonate Anions Rich in Oxygen Defects for Enhanced Alkaline Electrocatalytic Water Splitting

Alkaline water splitting is a highly efficient and clean technology for hydrogen energy generation. However, in alkaline solutions, most catalysts suffer from extreme instability. Herein, a cross-nanostructured N, F, and CO₃^2- codoped iron oxyhydroxide composite (N,F-FeO(OH)-CO₃-NF) rich in oxygen defects is designed for water splitting in the alkaline solution. X-ray photoelectron spectroscopy (XPS) and density functional theory (DFT) calculations show that the introduction of F and CO₃^2- can induce electron redistribution around the active center Fe, accelerate the four-electron transfer process, and optimize the d-band center, thereby improving the efficiency and stability of HER and OER. In a 1 M KOH solution, N,F-FeO(OH)-CO₃-NF only needs the overpotential of 248 mV for OER and the overpotential of 199 mV for HER to reach the current density of 10 mA·cm^-2. Meanwhile, it can reach 100 mA·cm^-2 current density at 1.55 V vs RHE and maintains a current density of 10 mA·cm^-2 for 120 h in a two-electrode electrolytic water device. Compared with bulk hydroxides, the heteroatom and anion codoped composite hydroxides are more stable and have dual functions in the electrolyte solution. This is of great significance for designing a new stable water-splitting electrocatalyst.

Synthesis of NiFeOx nanocatalysts from metal-organic precursors for the oxygen evolution reaction

Production of hydrogen from a renewable source that is water requires the development of sustainable catalytic processes. This implies, among others, developing efficient catalytic materials from abundant and low-cost resources and investigating their performance, especially in the oxidation of water as this half-reaction is the bottleneck of the water splitting process. For this purpose, NiFe-based nanoparticles with sizes ca. 3-4 nm have been synthesized by an organometallic approach and characterized by complementary techniques (WAXS, TEM, STEM-HAADF, EDX, XPS, and ATR-FTIR). They display a Ni core and a mixed Ni-Fe oxide shell. Once deposited onto FTO electrodes, they have been assessed in the electrocatalytic oxygen evolution reaction under alkaline conditions. Three different Ni/Fe ratios (2/1, 1/1 and 1/9) have been studied in comparison with their monometallic counterparts. The Ni2Fe1 nanocatalyst displayed the lowest overpotential (320 mV at j = 10 mA cm^-2) as well as excellent stability over 16 h.

Interfacial Assemble of Prussian Blue Analog to Access Hierarchical FeNi (oxy)-Hydroxide Nanosheets for Electrocatalytic Water Splitting

Developing facile methods for the synthesis of active and stable electrocatalysts is vitally important to realize overall water splitting. Here, we demonstrate a practical method to obtain FeNiOOH nanosheets on nickel foam (NF) as bifunctional electrocatalyst by growing a FeCo Prussian blue analog with further in situ oxidation under ambient conditions. The binder-free, self-standing FeNiOOH/NF electrode with hierarchical nanostructures requires low overpotentials of 260 mV and 240 mV at a current density of 50 mA cm-2 for oxygen evolution reaction and hydrogen evolution reaction, respectively, in 1.0 M KOH solution. Therefore, an alkaline water electrolyzer constructed by bifunctional FeNiOOH/NF electrode as both anode and cathode delivers 50 mA cm-2 under a cell voltage of 1.74 V with remarkable stability, which outperforms the IrO2-Pt/C-based electrolyzer. The excellent performance could be ascribed to the superior FeNiOOH intrinsic activity and the hierarchical structure. This work provides a cost-efficient surface engineering method to obtain binder-free, self-standing bifunctional electrocatalyst on commercial NF, which could be further extended to other energy and environment applications.

Multilayer Strategy for Photoelectrochemical Hydrogen Generation: New Electrode Architecture that Alleviates Multiple Bottlenecks

Years of research have demonstrated that the use of multiple components is essential to the development of a commercial photoelectrode to address specific bottlenecks, such as low charge separation and injection efficiency, low carrier diffusion length and lifetime, and poor durability. A facile strategy for the synthesis of multilayered photoanodes from atomic-layer-deposited ultrathin films has enabled a new type of electrode architecture with a total multilayer thickness of 15-17 nm. We illustrate the advantages of this electrode architecture by using nanolayers to address different bottlenecks, thus producing a multilayer photoelectrode with improved interface kinetics and shorter electron transport path, as determined by interface analyses. The photocurrent density was twice that of the bare structure and reached a maximum of 33.3 ± 2.1 mA cm^-2 at 1.23 V_RHE. An integrated overall water-splitting cell consisting of an electrocatalytic NiS cathode and Bi₂S₃/NiS/NiFeO/TiO₂ photoanode was used for precious-metal-free seawater splitting at a cell voltage of 1.23 V without degradation. The results and root analyses suggest that the distinctive advantages of the electrode architecture, which are superior to those of bulk bottom-up core-shell and hierarchical architectures, originate from the high density of active sites and nanometer-scale layer thickness, which enhance the suitability for interface-oriented energy conversion processes.

Fe doped NiS nanosheet arrays grown on carbon fiber paper for a highly efficient electrocatalytic oxygen evolution reaction

Developing efficient and low-cost non-noble metal catalysts for the oxygen evolution reaction (OER) is important for hydrogen production through water electrolysis. Herein, Fe doped NiS nanosheets directly grown on conductive carbon fiber paper (Fe-NiS@CFP) were fabricated through a two-step hydrothermal process. The microstructure, interface and electronic states of the prepared sample were modulated by Fe doping, exhibiting small internal and interface charge-transfer resistance. Benefiting from these factors, Fe-NiS@CFP shows superior electrocatalytic performance with an overpotential of 275 mV at 100 mA cm^-2 and maintains the activity for at least 50 h as a working electrode for the OER. This work may provide insights into the design and fabrication of non-noble metal sulfide electrocatalysts.

In Situ/Operando Insights into the Stability and Degradation Mechanisms of Heterogeneous Electrocatalysts

The further commercialization of renewable energy conversion and storage technologies requires heterogeneous electrocatalysts that meet the exacting durability target. Studies of the stability and degradation mechanisms of electrocatalysts are expected to provide important breakthroughs in stability issues. Accessible in situ/operando techniques performed under realistic reaction conditions are therefore urgently needed to reveal the nature of active center structures and establish links between the structural motifs in a catalyst and its stability properties. This review highlights recent research advances regarding in situ/operando techniques and improves the understanding of the stabilities of advanced heterogeneous electrocatalysts used in a diverse range of electrochemical reactions; it also proposes some degradation mechanisms. The review concludes by offering suggestions for future research.

Shining Light on Anion-Mixed Nanocatalysts for Efficient Water Electrolysis: Fundamentals, Progress, and Perspectives

This review introduces recent advances of various anion-mixed transition metal compounds (e.g., nitrides, halides, phosphides, chalcogenides, (oxy)hydroxides, and borides) for efficient water electrolysis applications in detail. The challenges and future perspectives are proposed and analyzed for the anion-mixed water dissociation catalysts, including polyanion-mixed and metal-free catalyst, progressive synthesis strategies, advanced in situ characterizations, and atomic level structure-activity relationship. Hydrogen with high energy density and zero carbon emission is widely acknowledged as the most promising candidate toward world's carbon neutrality and future sustainable eco-society. Water-splitting is a constructive technology for unpolluted and high-purity H₂ production, and a series of non-precious electrocatalysts have been developed over the past decade. To further improve the catalytic activities, metal doping is always adopted to modulate the 3d-electronic configuration and electron-donating/accepting (e-DA) properties, while for anion doping, the electronegativity variations among different non-metal elements would also bring some potential in the modulations of e-DA and metal valence for tuning the performances. In this review, we summarize the recent developments of the many different anion-mixed transition metal compounds (e.g., nitrides, halides, phosphides, chalcogenides, oxyhydroxides, and borides/borates) for efficient water electrolysis applications. First, we have introduced the general information of water-splitting and the description of anion-mixed electrocatalysts and highlighted their complementary functions of mixed anions. Furthermore, some latest advances of anion-mixed compounds are also categorized for hydrogen and oxygen evolution electrocatalysis. The rationales behind their enhanced electrochemical performances are discussed. Last but not least, the challenges and future perspectives are briefly proposed for the anion-mixed water dissociation catalysts.

In situ generation of Ni/Fe hydroxide layers by anodic etching of a Ni/Fe film for efficient oxygen evolution reaction

A Ni/Fe hydroxide electrocatalyst was fabricated via a simple and easily controlled method by combining anodic fluoridation and cyclic voltammetry (CV) treatment as an efficient catalyst for the OER.

Introducing Oxygen Vacancies to NiFe LDH through Electrochemistry Reduction to Promote Oxygen Evolution Reaction

The transition metal hydroxide NiFe LDH is a promising oxygen evolution reaction (OER) catalyst. Surface engineering, such as the introduction of oxygen vacancies into NiFe LDH, has been reported to...

2D Salphen-based heteropore covalent organic frameworks for highly efficient electrocatalytic water oxidation

The construction of heteroporous covalent organic frameworks (COFs) is still a challenge. Herein, a series of 2D COFs with hexagonal and quadrilateral pores were constructed via in situ salphen or metal salphen formation. Metallized salphen-based COFs can be used as electrocatalysts towards water oxidation with an overpotential of 266 mV at 10 mA cm^-2.

Recent Progress on Transition Metal Based Layered Double Hydroxides Tailored for Oxygen Electrode Reactions

The oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), namely, so-called oxygen electrode reactions, are two fundamental half-cell reactions in the energy storage and conversion devices, e.g., zinc–air batteries and fuel cells. However, the oxygen electrode reactions suffer from sluggish kinetics, large overpotential and complicated reaction paths, and thus require efficient and stable electrocatalysts. Transition-metal-based layered double hydroxides (LDHs) and their derivatives have displayed excellent catalytic performance, suggesting a major contribution to accelerate electrochemical reactions. The rational regulation of electronic structure, defects, and coordination environment of active sites via various functionalized strategies, including tuning the chemical composition, structural architecture, and topotactic transformation process of LDHs precursors, has a great influence on the resulting electrocatalytic behavior. In addition, an in-depth understanding of the structural performance and chemical-composition-performance relationships of LDHs-based electrocatalysts can promote further rational design and optimization of high-performance electrocatalysts. Finally, prospects for the design of efficient and stable LDHs-based materials, for mass-production and large-scale application in practice, are discussed.

Stepwise chemical oxidation to access ultrathin metal (oxy)-hydroxide nanosheets for the oxygen evolution reaction

Incorporation of ultrathin nanosheets with dopants/defects shows great potential to enable metal (oxy)-hydroxide electrocatalysts with enhanced oxygen evolution reaction (OER) performance via the regulation of atomic structure and bonding arrangements. However, it remains challenging in synthesis especially for such dual control and at large scale. In this study, we present a stepwise chemical oxidation route, involving phase transition and reconstruction processes, to access ultrathin CoOOH nanosheets with a thickness of ca. 4 nm and abundant oxygen vacancies. Other transition metals were also doped into CoOOH nanosheets through this strategy. Among them, the optimized FeCoOOH nanosheets demonstrated an efficient OER activity with overpotential as low as 252 mV (current density: 10 mA cm^-2) and excellent stability. A high and stable solar-to-hydrogen efficiency of 10.5% was acquired when FeCoOOH nanosheets were used as the anode in a constructed water splitting device driven by solar energy. This study offers a noble and facile strategy for potentially scalable preparation of atom-modulated ultrathin metal (oxy)-hydroxide nanosheets, and also demonstrates the OER applications.

The impact of ultrasonic parameters on the exfoliation of NiFe LDH nanosheets as electrocatalysts for the oxygen evolution reaction in alkaline media

The ultrasonic process has been examined to exfoliate layered materials and upgrade their properties for a variety of applications in different media. Our previous studies have shown that the ultra-sonication treatment in water without chemicals has a positive influence on the physical and electrochemical performance of layered materials and nanoparticles. In this work, we have probed the impact of ultrasonication on the physical properties and the oxygen evolution reaction (OER) of the NiFe LDH materials under various conditions, including suspension concentration (2.5-12.5 mg mL-1), sonication times (3-20 min) and amplitudes (50-90%) in water, in particular, sonication times and amplitudes. We found that the concentration, amplitude and time play significant roles on the exfoliation of the NiFe LDH material. Firstly, the NiFe LDH nanosheets displayed the best OER performance under ultrasonic conditions with the concentration of 10 mg mL-1 (50% amplitude and 15 min). Secondly, it was revealed that the exfoliation of the NiFe LDH nanosheets in a short time (<10 min) or a higher amplitudes (≥80%) has left a cutdown on the OER activity. Comprehensively, the optimum OER activity was displayed on the exfoliated NiFe LDH materials under ultrasonic condition of 60% (amplitude), 10 mg mL-1 and 15 min. It demanded only 250 mV overpotentials to reach 10 mA cm-2 in 1 M KOH, which was 100 mV less than the starting NiFe LDH material. It was revealed from the mechanism of sonochemistry and the OER reaction that, after exfoliation, the promoted OER performance is ascribed to the enriched Fe3+ at the active sites, easier oxidation of Ni2+ to Ni3+, and the strong electrical coupling of the Ni2+ and Fe3+ during the OER process. This work provides a green strategy to improve the intrinsic activity of layered materials.

Constructing Ultrathin W-Doped NiFe Nanosheets via Facile Electrosynthesis as Bifunctional Electrocatalysts for Efficient Water Splitting

Exploring cost-effective and efficient bifunctional electrocatalysts via simple fabrication strategies is strongly desired for practical water splitting. Herein, an easy and fast one-step electrodeposition process is developed to fabricate W-doped NiFe (NiFeW)-layered double hydroxides with ultrathin nanosheet features at room temperature and ambient pressure as bifunctional catalysts for water splitting. Notably, the NiFeW nanosheets require overpotentials of only 239 and 115 mV for the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER), respectively, to reach a current density of 10 mA/cm² in alkaline media. Their exceptional performance is further demonstrated in a full electrolyzer configuration with the NiFeW as both anode and cathode catalysts, which achieves a low cell voltage of 1.59 V at 10 mA/cm², 110 mV lower than that of the commercial IrO₂ (anode) and Pt (cathode) catalysts. Moreover, the NiFeW nanosheets are superior to various recently reported bifunctional electrocatalysts. Such remarkable performances mainly ascribe to W doping, which not only effectively modulates the electrocatalyst morphology but also engineers the electronic structure of NiFe hydroxides to boost charge-transfer kinetics for both the OER and HER. Hence, the ultrathin NiFeW nanosheets with an efficient fabrication strategy are promising as bifunctional electrodes for alkaline water electrolyzers.

Facile Access to an Active γ-NiOOH Electrocatalyst for Durable Water Oxidation Derived From an Intermetallic Nickel Germanide Precursor

Identifying novel classes of precatalysts for the oxygen evolution reaction (OER by water oxidation) with enhanced catalytic activity and stability is a key strategy to enable chemical energy conversion. The vast chemical space of intermetallic phases offers plenty of opportunities to discover OER electrocatalysts with improved performance. Herein we report intermetallic nickel germanide (NiGe) acting as a superior activity and durable Ni-based electro(pre)catalyst for OER. It is produced from a molecular bis(germylene)-Ni precursor. The ultra-small NiGe nanocrystals deposited on both nickel foam and fluorinated tin oxide (FTO) electrodes showed lower overpotentials and a durability of over three weeks (505 h) in comparison to the state-of-the-art Ni-, Co-, Fe-, and benchmark NiFe-based electrocatalysts under identical alkaline OER conditions. In contrast to other Ni-based intermetallic precatalysts under alkaline OER conditions, an unexpected electroconversion of NiGe into γ-NiIII OOH with intercalated OH- /CO3 2- transpired that served as a highly active structure as shown by various ex situ methods and quasi in situ Raman spectroscopy.

The construction of NiFeSx/g-C3N4 composites with high photocatalytic activity towards the degradation of refractory pollutants

In this work, a novel NiFe layered double hydroxide-derived sulfide (NiFeSx)-modified g-C3N4 nanosheet photocatalyst (NiFeSx/g-C3N4) was synthesized, and its morphology, structure and visible light absorption capacity were simultaneously characterized by XRD, SEM, TEM, FT-IR, XPS, UV-Vis DRS, PL techniques and EIS Nyquist plots. Furthermore, it was discovered that at an optimum mass ratio of 3% (NiFeSx to g-C3N4), 3% NiFeSx/g-C3N4 composites exhibited the best degradation efficiency toward tetracycline hydrochloride refractory pollutants. The degradation rate of tetracycline hydrochloride by 3% NiFeSx/g-C3N4 composites was 92.54% under 70 min of visible light illumination, which was about 2.61 times higher than that of pure g-C3N4. The improved degradation activity may be attributed to the synergistic effect between the two constituents of as-synthesized composites, and the formed heterojunction reduced the efficiency of photogenerated carriers. More importantly, this work also gives some inspiration to synthesize some similar photocatalysts for a targeted environmental remediation.

Design Strategies of Transition-Metal Phosphate and Phosphonate Electrocatalysts for Energy-Related Reactions

The key challenge to developing renewable energy conversion and storage devices lies in the exploration and rational engineering of cost-effective and highly efficient electrocatalysts for various energy-related electrochemical reactions. Transition-metal phosphates and phosphonates have shown remarkable performances for these reactions based on their unique physicochemical properties. Compared with transition-metal oxides, phosphate groups in transition-metal phosphates and phosphonates show flexible coordination with diverse orientations, making them an ideal platform for designing active electrocatalysts. Although numerous efforts have been spent on the development of transition-metal phosphate and phosphonate electrocatalysts, some urgent issues, such as low intrinsic catalytic efficiency and low electronic conductivity, have to be resolved in accordance with their applications. In this Review, we focus on the design strategies of highly efficient transition-metal phosphate and phosphonate electrocatalysts, with special emphasis on the tuning of transition-metal-center coordination environment, optimization of electronic structures, increase of catalytically active site densities, and construction of heterostructures. Guided by these strategies, recently developed transition-metal phosphate and phosphonate materials have exhibited excellent activity, selectivity, and stability for various energy-related electrocatalytic reactions, showing great potential for replacing noble-metal-based catalysts in next-generation advanced energy techniques. The existing challenges and prospects regarding these materials are also presented.

Low-cost and multi-level structured NiFeMn alloy@NiFeMn oxyhydroxide electrocatalysts for highly efficient overall water splitting

NiFeMn alloy@NiFeMn oxyhydroxide was fabricated by electrodeposition, which reveals exceptional electrocatalytic property toward overall water splitting owing to the extraordinary multi-level structure.

The role of nickel–iron based layered double hydroxide on the crystallinity, electrochemical performance, and thermal and mechanical properties of the poly(ethylene-oxide) solid electrolyte

The electrochemical performance and physical properties of PEO-based composite electrolytes were improved with the addition of a NILDH filler.

Recent Progress on NiFe-Based Electrocatalysts for the Oxygen Evolution Reaction

The seriousness of the energy crisis and the environmental impact of global anthropogenic activities have led to an urgent need to develop efficient and green fuels. Hydrogen, as a promising alternative resource that is produced in an environmentally friendly and sustainable manner by a water splitting reaction, has attracted extensive attention in recent years. However, the large-scale application of water splitting devices is hindered predominantly by the sluggish oxygen evolution reaction (OER) at the anode. Therefore, the design and exploration of high-performing OER electrocatalysts is a critical objective. Considering their low prices, abundant reserves, and intrinsic activities, NiFe-based bimetal compounds are widely studied as excellent OER electrocatalysts. Moreover, recent progress on NiFe-based OER electrocatalysts in alkaline environments is comprehensively and systematically introduced through various catalyst families including NiFe-layered hydroxides, metal-organic frameworks, NiFe-based (oxy)hydroxides, NiFe-based oxides, NiFe alloys, and NiFe-based nonoxides. This review briefly introduces the advanced NiFe-based OER materials and their corresponding reaction mechanisms. Finally, the challenges inherent to and possible strategies for producing extraordinary NiFe-based electrocatalysts are discussed.

A soft molecular 2Fe-2As precursor approach to the synthesis of nanostructured FeAs for efficient electrocatalytic water oxidation

An unprecedented molecular 2Fe-2As precursor complex was synthesized and transformed under soft reaction conditions to produce an active and long-term stable nanocrystalline FeAs material for electrocatalytic water oxidation in alkaline media. The 2Fe2As-centred β-diketiminato complex, having an unusual planar Fe2As2 core structure, results from the salt-metathesis reaction of the corresponding β-diketiminato FeIICl complex and the AsCO- (arsaethynolate) anion as the monoanionic As- source. The as-prepared FeAs phase produced from the precursor has been electrophoretically deposited on conductive electrode substrates and shown to act as a electro(pre)catalyst for the oxygen evolution reaction (OER). The deposited FeAs undergoes corrosion under the severe anodic alkaline conditions which causes extensive dissolution of As into the electrolyte forming finally an active two-line ferrihydrite phase (Fe2O3(H2O) x ). Importantly, the dissolved As in the electrolyte can be fully recaptured (electro-deposited) at the counter electrode making the complete process eco-conscious. The results represent a new and facile entry to unexplored nanostructured transition-metal arsenides and their utilization for high-performance OER electrocatalysis, which are also known to be magnificent high-temperature superconductors.

Recent advances in application of graphitic carbon nitride-based catalysts for degrading organic contaminants in water through advanced oxidation processes beyond photocatalysis: A critical review

Advanced oxidation processes (AOPs) have attracted much interest in the field of water treatment owing to their high removal efficiency for refractory organic contaminants. Graphitic carbon nitride (g-C₃N₄)-based catalysts with high performance and cost effectiveness are promising heterogeneous catalysts for AOPs. Most research on g-C₃N₄-based catalysts focuses on photocatalytic oxidation, but increasingly researchers are paying attention to the application of g-C₃N₄-based catalysts in other AOPs beyond photocatalysis. This review aims to concisely highlight recent state-of-the-art progress of g-C₃N₄-based catalysts in AOPs beyond photocatalysis. Emphasis is made on the application of g-C₃N₄-based catalysts in three classical AOPs including Fenton-based processes, catalytic ozonation and persulfates activation. The catalytic performance and involved mechanism of g-C₃N₄-based catalysts in these AOPs are discussed in detail. Meanwhile, the effect of water chemistry including pH, water temperature, natural organic matter, inorganic anions and dissolved oxygen on the catalytic performance of g-C₃N₄-based catalysts are summarized. Moreover, the reusability, stability and toxicity of g-C₃N₄-based catalysts in water treatment are also mentioned. Lastly, perspectives on the major challenges and opportunities of g-C₃N₄-based catalysts in these AOPs are proposed for better developments in the future research.