In Situ Electrochemically Derived Nanoporous Oxides from Transition Metal Dichalcogenides for Active Oxygen Evolution Catalysts

Regulating interfacial microenvironment via anion adsorption to boost oxygen evolution reaction

The often-overlooked anions in raw materials have been less explored in terms of their promotion of the oxygen evolution reaction (OER) from the perspective of interfacial microenvironment regulation. In this study, we obtained the catalyst (CoOOH-NO3-) by a one-step electrochemical reconstruction method, in which anion adsorption onto the active catalyst can optimize the interfacial microenvironment to promote OER under alkaline conditions. This is because the issue of excessive OH- adsorption within the inner compact layer of CoOOH can be ameliorated by the adsorption of anions, making it easier for active sites to branch OH-, thereby regulating the interfacial microenvironment. It was validated through a series of experiments that after tuning the interfacial microenvironment, reduction in the contact angle on the electrode surface facilitates the release of O2, promotes Co transformation to a higher oxidation state (Co(IV)) and lowers the onset potential for the reaction. Furthermore, the one-step synthesis method, along with the strategy of microenvironment regulation, applies to various metal salts (chlorides, acetates). Our research introduces a non-chemical synthesis method for the direct use of metal salts, providing a rational approach and insight for understanding how anion adsorption regulates interfacial microenvironment to enhance catalytic activity.

One-Step Mechanochemical Synthesis of Bulk-Sized Electroactive MoS2/Oxalic Acid Dihydrate Composite for Catalytic Nitrophenol Reductions

Nitrophenols are used in fabricating explosives and reported to be carcinogenic in nature, dictating a necessity for their efficient and eco-friendly reduction. The majority of such reductions involve multistep protocols that use expensive reagents. Herein, a one-step greener approach has been developed to mechanochemically prepare a cost-effective molybdenum disulfide (MoS2) and oxalic acid dihydrate-based composite to catalytically reduce nitrophenols into their corresponding amine in aqueous medium. The catalyst works in situ by virtue of the electroactivation of the morphologically rough MoS2 surface, which generates nascent active sites. The active sites are composed of paramagnetic Mo5+ centers generated during the mechanical grinding process. The peritectoidically transformed oxalic acid dihydrate, which is physisorbed on the MoS2 surface during the catalyst's preparation process, gets released in aqueous medium, lowering the pH and accelerating the hydrolysis of BH4 -. The BH4 - quickly interacts with the nascent active sites, propagating the reduction at a faster rate. We establish an Eley-Rideal mechanism that is responsible for a remarkably high rate constant. These findings are based on a thorough analysis using UV-visible spectroscopy, powder X-ray diffraction, X-ray photoelectron spectroscopy, Raman and electron paramagnetic resonance spectroscopies, high-resolution mass spectrometry, along with first-principles quantum mechanical solid-state calculations.

Bifunctional Nanoelectrocatalyst: Semi-Amorphous Cobalt Tungstate-Based Nanocomposites Surpassing Amorphous and Crystalline Counterparts

Electrocatalytic water splitting to hydrogen and oxygen is considered to be one of the significant routes for future renewable energy conversion. Therefore, the development of unique, efficient, and cost-effective bifunctional electrocatalysts, which can perform both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), is critical for the accessible utilization of renewable energy. The increasing overall efficiency of the water-splitting reaction is pivotal for sustainable energy conversion and hydrogen production. Herein, we present a facile solution-based synthesis route to control the crystallinity of the cobalt tungstate (CoWO4) nanomaterials to amorphous, semiamorphous, and crystalline phases and further exploit these nanomaterials as bifunctional electrocatalysts for the HER, OER, and overall electrochemical water splitting reaction. Notably, the semiamorphous cobalt tungstate nanomaterials show a remarkable surface area of approximately 150 m2/g, the highest reported for CoWO4-based materials, and demonstrated superior activity for both the OER and HER and outstanding stability over the amorphous and crystalline counterparts. Notably, the SemiAmp||SemiAmp electrolyzer demonstrated a better performance for overall water splitting than the commercial RuO2||Pt/C electrolyzer.

Nanoscale-Confined Synthesis of 2D Metal Compounds for Electrochemical Applications

2D metal compounds, such as transition metal dichalcogenides (TMDs), layered double hydroxides (LDHs), and MXenes, are emerging as important electrocatalyst materials in the transition to a sustainable energy future. Aided by their high surface area, electrical conductivity, and tunable electronic properties, these materials have provided a crucial research thrust in enhancing the efficiency of green hydrogen production, fuel cells, and carbon reduction processes. Most importantly, the synthesis of nanostructured 2D compounds, while challenging, is the key to optimizing their catalytic performance. Recent advancements in this field have highlighted the potential of 2D metal compounds in revolutionizing energy conversion technologies, which entails the discovery of new material compositions, the development of novel synthetic routes, and the integration of these materials into practical energy conversion systems. This review presents an overview of the distinctive characteristics of nanoscale-confined 2D metal compounds, the challenges encountered in their synthesis, and electrochemical applications.

Pyridinic-N Seized Co in Biphasic Nanoarchitecture for Reversible Oxygen Electrocatalysis Enabling Longevous (>1200 h) Aqueous and Dual-Anion Kosmotropic Electrolyte Stabilized High Power Quasisolid-State Zn-Air Battery

Integration of different active sites by heterostructure engineering is pivotal to optimize the intrinsic activities of an oxygen electrocatalyst and much needed to enhance the performance of rechargeable Zn-air batteries (ZABs). Herein, a biphasic nanoarchitecture encased in in situ grown N-doped graphitic carbon (MnO/Co-NGC) with heterointerfacial sites are constructed. The density functional theory model reveals formation of lattice oxygen bridged heterostructure with pyridinic nitrogen atoms anchored Co species, which facilitate adsorption of oxygen intermediates. Consequently, the well-designed catalyst with accessible active sites, abundant oxygen vacant sites, and heterointerfacial coupling effects, simultaneously accelerate the electron/mass transfer and thus promotes the trifunctional electrocatalysis. The assembled aqueous ZAB delivers maximum power density of ≈268 mW cm-2 and a specific capacity of 797.8 mAh gzn -1 along with excellent rechargeability and extremely small voltage gap decay rate of 0.0007 V h-1. Further, the fabricated quasisolid-state ZAB owns a remarkable power density of 163 mW cm-2 and long cycle life, outperforming the benchmark air-electrode and many recent reports, underlining its robustness and suitability for practical utilization in diverse portable applications.

Nickel Phosphide: The Effect of Phosphorus Content on the Activity and Stability toward Oxygen Evolution Reaction in Alkaline Medium

In this work a systematic study of the effect of the metal to phosphorus ratio in Ni-P nanoparticles on their catalytic activity with respect to the OER is reported. To this end, nickel phosphide nanoparticles are synthesized through two different synthesis routes, one involving in-situ phosphidation and one involving ex-situ phosphidation. In-situ phosphidation is performed via two steps route in a one-pot synthesis, in which Ni nanoparticles are formed at 220 °C, but not isolated, and then transformed to phase-pure either Ni12P5 or Ni2P nanocrystallites. In the second synthesis method (ex-situ phosphidation), nickel nanoparticles with an excess amount of trioctylphosphine (TOP) as a capping agent are synthesized and separated from the solution, then subsequently annealed in three different atmospheres, leading to the formation of three types of NixPy viz. [NixPy-H2/Ar], [NixPy-Ar], and [NixPy-air]. [NixPy-air] nanoparticles shows the best electrocatalytic activity among the annealed nanoparticles in Ar and H2/Ar but lower than Ni12P5 nanoparticles. However, [NixPy-air] shows very high stability in comparison with other synthesized nanoparticles. Moreover, the effect of the adventitious and spiked Fe in the electrolyte is studied on the electrocatalytic activity of all synthesized nanoparticles.

N-Doped Carbon Modified (NixFe1-x)Se Supported on Vertical Graphene toward Efficient and Stable OER Performance

NiFe-based nanomaterials are extensively studied as one of the promising candidates for the oxygen evolution reaction (OER). However, their practical application is still largely impeded by the unsatisfied activity and poor durability caused by the severe leaching of active species. Herein, a rapid and facile combustion method is developed to synthesize the vertical graphene (VG) supported N-doped carbon modified (NixFe1-x)Se composites (NC@(NixFe1-x)Se/VG). The interconnected heterostructure of obtained materials plays a vital role in boosting the catalytic performance, offering rich active sites and convenient pathways for rapid electron and ion transport. The incorporation of Se into NiFe facilitates the formation of active species via in situ surface reconstruction. According to density functional theory (DFT) calculations, the in situ formation of a Ni0.75Fe0.25Se/Ni0.75Fe0.25OOH layer significantly enhances the catalytic activity of NC@(NixFe1-x)Se/VG. Furthermore, the surface-adsorbed selenoxide species contribute to the stabilization of the catalytic active phase and increase the overall stability. The obtained NC@(NixFe1-x)Se/VG exhibits a low overpotential of 220 mV at 20 mA cm-2 and long-term stability over 300 h. This work offers a novel perspective on the design and fabrication of OER electrocatalysts with high activity and stability.

Design and Optimization of Nanoporous Materials as Catalysts for Oxygen Evolution Reaction-A Review

With the growing demand for new energy sources, electrochemical water splitting for hydrogen production is a technology that must be vigorously promoted. Therefore, to improve the efficiency of the oxygen evolution reaction (OER) at the anode, high-performance OER catalysts are essential. Given their advantages in electrocatalysis, nanoporous materials have garnered considerable attention in previous studies for OER applications. This review provides a comprehensive overview of various strategies to optimize active site utilization in nanoporous materials. These strategies include regulating pore size and porosity, constructing hierarchical nanoporous structures, and enhancing material conductivity. Additionally, it covers approaches to boost the intrinsic OER activity of nanoporous materials, such as tuning the composition of anions and cations, creating vacancies, constructing interfaces, and forming boundary active sites. While nanoporous materials offer significant potential for advancing OER, challenges remain, including difficulties in quantifying activity within nanopores, the unclear impact of nanoporous material morphology, challenges in accessing nanopore interiors with in situ techniques, and a lack of theoretical calculations on pore structure. However, these challenges also present opportunities, and we hope this review provides a fresh perspective to inspire future research.

Metal Doping Regulates Electrocatalysts Restructuring During Oxygen Evolution Reaction

High-efficiency and low-cost catalysts for oxygen evolution reaction (OER) are critical for electrochemical water splitting to generate hydrogen, which is a clean fuel for sustainable energy conversion and storage. Among the emerging OER catalysts, transition metal dichalcogenides have exhibited superior activity compared to commercial standards such as RuO2, but inferior stability due to uncontrolled restructuring with OER. In this study, we create bimetallic sulfide catalysts by adapting the atomic ratio of Ni and Co in CoxNi1-xSy electrocatalysts to investigate the intricate restructuring processes. Surface-sensitive X-ray photoelectron spectroscopy and bulk-sensitive X-ray absorption spectroscopy confirmed the favorable restructuring of transition metal sulfide material following OER processes. Our results indicate that a small amount of Ni substitution can reshape the Co local electronic structure, which regulates the restructuring process to optimize the balance between OER activity and stability. This work represents a significant advancement in the development of efficient and noble metal-free OER electrocatalysts through a doping-regulated restructuring approach.

Crystal Structure Regulation of CoSe2 Induced by Fe Dopant for Promoted Surface Reconstitution toward Energetic Oxygen Evolution Reaction

Most nonoxide catalysts based on transition metal elements will inevitably change their primitive phases under anodic oxidation conditions in alkaline media. Establishing a relationship between the bulk phase and surface evolution is imperative to reveal the intrinsic catalytic active sites. In this work, it is demonstrated that the introduction of Fe facilitates the phase transition of orthorhombic CoSe2 into its cubic counterpart and then accelerates the Co-Fe hydroxide layer generation on the surface during electrocatalytic oxygen evolution reaction (OER). As a result, the Fe-doped cubic CoSe2 catalyst exhibits a significantly enhanced activity with a considerable overpotential decrease of 79.9 and 66.9 mV to deliver 10 mA·cm-2 accompanied by a Tafel slope of 48.0 mV·dec-1 toward OER when compared to orthorhombic CoSe2 and Fe-doped orthorhombic CoSe2, respectively. Density functional theory (DFT) calculations reveal that the introduction of Fe on the surface hydroxide layers will tune electron density around Co atoms and raise the d-band center. These findings will provide deep insights into the surface reconstitution of the OER electrocatalysts based on transition metal elements.

A Layered Bi2 Te3 @PPy Cathode for Aqueous Zinc-Ion Batteries: Mechanism and Application in Printed Flexible Batteries

Low-cost, safe, and environmental-friendly rechargeable aqueous zinc-ion batteries (ZIBs) are promising as next-generation energy storage devices for wearable electronics among other applications. However, sluggish ionic transport kinetics and the unstable electrode structure during ionic insertion/extraction hamper their deployment. Herein, a new cathode material based on a layered metal chalcogenide (LMC), bismuth telluride (Bi2 Te3 ), coated with polypyrrole (PPy) is proposed. Taking advantage of the PPy coating, the Bi2 Te3 @PPy composite presents strong ionic absorption affinity, high oxidation resistance, and high structural stability. The ZIBs based on Bi2 Te3 @PPy cathodes exhibit high capacities and ultra-long lifespans of over 5000 cycles. They also present outstanding stability even under bending. In addition, here the reaction mechanism is analyzed using in situ X-ray diffraction, X-ray photoelectron spectroscopy, and computational tools and it is demonstrated that, in the aqueous system, Zn2+ is not inserted into the cathode as previously assumed. In contrast, proton charge storage dominates the process. Overall, this work not only shows the great potential of LMCs as ZIB cathode materials and the advantages of PPy coating, but also clarifies the charge/discharge mechanism in rechargeable ZIBs based on LMCs.

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.

Boosting the Electrochemical 5-Hydroxymethylfurfural Oxidation by Balancing the Competitive Adsorption of Organic and OH- over Controllable Reconstructed Ni3 S2 /NiOx

The electrocatalytic oxidation of 5-hydroxymethylfurfural (HMF) is a promising method for the efficient production of biomass-derived high-value-added chemicals. However, its practical application is limited by: 1) the low activity and selectivity caused by the competitive adsorption of HMF and OH- and 2) the low operational stability caused by the uncontrollable reconstruction of the catalyst. To overcome these limitations, a series of Ni3 S2 /NiOx -n catalysts with controllable compositions and well-defined structures are synthesized using a novel in situ controlled surface reconstruction strategy. The adsorption behavior of HMF and OH- can be continuously adjusted by varying the ratio of NiOx to Ni3 S2 on the catalysts surface, as indicated by in situ characterizations, contact angle analysis, and theoretical simulations. Owing to the balanced competitive adsorption of HMF and OH- , the optimized Ni3 S2 /NiOx -15 catalyst exhibited remarkable HMF electrocatalytic oxidation performance, with the current density reaching 366 mA cm-2 at 1.5 VRHE and the Faradaic efficiency of the product, 2,5-furanedicarboxylic acid, reaching 98%. Moreover, Ni3 S2 /NiOx -15 exhibits excellent durability, with its activity and structure remaining stable for over 100 h of operation. This study provides a new route for the design and construction of catalysts for value-added biomass conversion and offers new insights into enhancing catalytic performance by balancing competitive adsorption.

Phase-Selective Synthesis of Cobalt Sulfide Heterostructure Catalysts as Efficient Counter Electrodes in Dye-Sensitized Solar Cells

Developing a cost-saving, high-efficiency, and simple synthesis of counter electrode (CE) material to replace pricy Pt for dye-sensitized solar cells (DSSCs) has become a research hotspot. Owing to the electronic coupling effects between various components, semiconductor heterostructures can significantly enhance the catalytic performance and endurance of counter electrodes. However, the strategy to controllably synthesize the same element in several phase heterostructures used as the CE in DSSCs is still absent. Here, we fabricate well-defined CoS2 /CoS heterostructures and use them as CE catalysts in DSSCs. The as-designed CoS2 /CoS heterostructures display high catalytic performance and endurance for the triiodide reduction in DSSCs thanks to the combined and synergistic effects. As a result, a DSSC with CoS2 /CoS achieves a high energy conversion with an efficiency of 9.47 % under standard simulated solar radiation, surpassing that of pristine Pt-based CE (9.20 %). Besides, the CoS2 /CoS heterostructures possess a quick activity initiation process and extended stability, broadening their potential applications in various areas. Therefore, our proposed synthetic approach could offer new insights for synthesizing functional heterostructure materials with improved catalytic activities in DSSCs.

Transition metal chalcogenides carbon-based as bifunctional cathode electrocatalysts for rechargeable zinc-air battery: An updated review

The rechargeable alkaline aqueous zinc-air batteries (ZABs) are prospective candidates to supply the energy demand for their high theoretical energy density, inherent safety, and environmental friendliness. However, their practical application is mainly restricted by the unsatisfactory efficiency of the air electrode, leading to an intense search for high-efficient oxygen electrocatalysts. In recent years, the composites of carbon materials and transition metal chalcogenides (TMC/C) have emerged as promising alternatives because of the unique properties of these single compounds and the synergistic effect between them. In this sense, this review presented the electrochemical properties of these composites and their effects on the ZAB performance. The operational fundamentals of the ZABs were described. After elucidating the role of the carbon matrix in the hybrid material, the latest developments in the ZAB performance of the monometallic structure and spinel of TMC/C were detailed. In addition, we report topics on doping and heterostructure due to the large number of studies involving these specific defects. Finally, a critical conclusion and a brief overview sought to contribute to the advancement of TMC/C in the ZABs.

Carbon-incorporated Ni2P-Fe2P hollow nanorods as superior electrocatalysts for the oxygen evolution reaction

A rational design and cost-effective transition metal-based hollow nanostructures are important for sustainable energy materials with high efficiency. This study reports on carbon-incorporated Ni₂P-Fe₂P hollow nanorods ((Ni,Fe)₂P/C HNRs) derived from a self-template approach as efficient electrocatalysts. Initially, a Ni₂(BDC)₂(DABCO)-MOF (Ni-MOF) is converted to NiFe-PBA hollow nanorods (HNRs) through facile ion exchange which was further converted to (Ni,Fe)₂P/C HNRs via a subsequent phosphidation process. The resulting (Ni,Fe)₂P/C HNRs exhibit remarkable activity for the oxygen evolution reaction in an alkaline solution requiring a small overpotential of 258 mV to drive a current density of 10 mA cm^-2 and long-term stability with little deactivation after 40 h. (Ni,Fe)₂P/C HNRs outperform (Ni,Fe)₂P/C NPs and commercial RuO₂. The unique hollow morphology and interfacial electronic structure substantially increase the active site and charge transfer rate of our electrocatalyst, resulting in excellent OER activity and stability.

Transition Metal Non-Oxides as Electrocatalysts: Advantages and Challenges

The identification of hydrogen as green fuel in the near future has stirred global realization toward a sustainable outlook and thus boosted extensive research in the field of water electrolysis focusing on the hydrogen evolution reaction (HER), oxygen evolution reaction (OER), and oxygen reduction reaction (ORR). A huge class of compounds consisting of transition metal-based nitrides, carbides, chalcogenides, phosphides, and borides, which can be collectively termed transition metal non-oxides (TMNOs), has emerged recently as an efficient class of electrocatalysts in terms of performance and longevity when compared to transition metal oxides (TMOs). Moreover, the superiority of TMNOs over TMOs to effectively catalyze not only OERs but also HERs and ORRs renders bifunctionality and even trifunctionality in some cases and therefore can replace conventional noble metal electrocatalysts. In this review, the crystal structure and phases of different classes of nanostructured TMNOs are extensively discussed, focusing on recent advances in design strategies by various regulatory synthetic routes, and hence diversified properties of TMNOs are identified to serve as next-generation bi/trifunctional electrocatalysts. The challenges and future perspectives of materials in the field of energy conversion and storage aiding toward a better hydrogen economy are also discussed in this review.

Reinforced Layered Double Hydroxide Oxygen-Evolution Electrocatalysts: A Polyoxometallic Acid Wet-Etching Approach and Synergistic Mechanism

Nickel-iron-based layered double hydroxides (NiFe LDHs) have attracted tremendous research and industrial interests for oxygen evolution reaction (OER) electrocatalysis. However, methodologies on simultaneous regulation of performance-influencing factors remain scarce and their real synergistic effects are not clear enough. Herein, a versatile polyoxometallic acids (POMs) etching approach is reported to ingeniously reconstruct NiFe LDH, including 3D morphological nanotailoring, Fe³⁺ and α-Ni(OH)₂ active species reconfiguration, creation of multiple Ni, Fe, and O vacancies, and intercalation of POM polyanionic clusters. The experimental and theoretical data collaboratively unveil that control of the key performance-influencing factors and their multiple synergistic mechanisms dominantly contribute to the step-like OER performance enhancement. The reinforced electrocatalyst is further produced with low cost and high performance up to Ф180 mm in diameter, showing its feasibility and advancement of the promising configuration of NiFe LDH-PMo12(+) II Ni@NiFe LDH(-) for alkaline anion-exchange-membrane electrode stack cells. Furthermore, to mathematically evaluate the evolution, a novel empirical formula is proposed for quantitative identification of structure-activity correlations, which offers an opportunity for first and fast reliability on materials screening.

Zn-Doped CoS2 Nanoarrays for an Efficient Oxygen Evolution Reaction: Understanding the Doping Effect for a Precatalyst

Development of low-cost, efficient, and durable electrocatalysts for the oxygen evolution reaction (OER) is crucial for multiple energy conversions and storage devices. Herein, Zn-doped CoS₂ nanoarrays supported on carbon cloth, Co(Zn)S₂/CC, are fabricated through a facile sulfidization of CoZn metal-organic frameworks. This precatalyst, Co(Zn)S₂/CC, with a well-defined nanoarray structure affords excellent OER catalytic activity (η = 248 mV at 10 mA/cm²) and long-term durability in 1 M KOH. X-ray photoelectron and in situ Raman spectroscopic studies indicate that Co(Zn)S₂ undergoes surface reconstruction with the generation of Co(Zn)OOH adsorbed with SO₄^2- at the surface during the OER process. The Zn dopant is calculated to impact on the electronic structure of Co species and further the adsorption of intermediates. This work not only provides a novel method for the synthesis of bimetallic sulfides but also gives insights into the doping effect on the OER performance of transition metal sulfides.

Electrochemically Derived Crystalline CuO from Covellite CuS Nanoplates: A Multifunctional Anode Material

In the present era, electrochemical water splitting has been showcased as a reliable solution for alternative and sustainable energy development. The development of a cheap, albeit active, catalyst to split water at a substantial overpotential with long durability is a perdurable challenge. Moreover, understanding the nature of surface-active species under electrochemical conditions remains fundamentally important. A facile hydrothermal approach is herein adapted to prepare covellite (hexagonal) phase CuS nanoplates. In the covellite CuS lattice, copper is present in a mixed-valent state, supported by two different binding energy values (932.10 eV for Cu^I and 933.65 eV for Cu^II) found in X-ray photoelectron spectroscopy analysis, and adopted two different geometries, that is, trigonal planar preferably for Cu^I and tetrahedral preferably for Cu^II. The as-synthesized covellite CuS behaves as an efficient electro(pre)catalyst for alkaline water oxidation while deposited on a glassy carbon and nickel foam (NF) electrodes. Under cyclic voltammetry cycles, covellite CuS electrochemically and irreversibly oxidized to CuO, indicated by a redox feature at 1.2 V (vs the reversible hydrogen electrode) and an ex situ Raman study. Electrochemically activated covellite CuS to the CuO phase (termed as CuS_EA) behaves as a pure copper-based catalyst showing an overpotential (η) of only 349 (±5) mV at a current density of 20 mA cm^-2, and the TOF value obtained at η₃₄₉ (at 349 mV) is 1.1 × 10^-3 s^-1. A low R_ct of 5.90 Ω and a moderate Tafel slope of 82 mV dec^-1 confirm the fair activity of the CuS_EA catalyst compared to the CuS precatalyst, reference CuO, and other reported copper catalysts. Notably, the CuS_EA/NF anode can deliver a constant current of ca. 15 mA cm^-2 over a period of 10 h and even a high current density of 100 mA cm^-2 for 1 h. Post-oxygen evolution reaction (OER)-chronoamperometric characterization of the anode via several spectroscopic and microscopic tools firmly establishes the formation of crystalline CuO as the active material along with some amorphous Cu(OH)₂ via bulk reconstruction of the covellite CuS under electrochemical conditions. Given the promising OER activity, the CuS_EA/NF anode can be fabricated as a water electrolyzer, Pt(-)//(+)CuS_EA/NF, that delivers a j of 10 mA cm^-2 at a cell potential of 1.58 V. The same electrolyzer can further be used for electrochemical transformation of organic feedstocks like ethanol, furfural, and 5-hydroxymethylfurfural to their respective acids. The present study showcases that a highly active CuO/Cu(OH)₂ heterostructure can be constructed in situ on NF from the covellite CuS nanoplate, which is not only a superior pure copper-based electrocatalyst active for OER and overall water splitting but also for the electro-oxidation of industrial feedstocks.

Two-Dimensional Cobalt Sulfide/Iron-Nitrogen-Carbon Holey Sheets with Improved Durability for Oxygen Electrocatalysis

Transition-metal sulfide as a promising bifunctional oxygen electrocatalyst alternative to scarce platinum-group metals has attracted much attention, but it suffers activity loss over time owing to poor structural/compositional stability during catalysis. Herein, we report a self-template method for preparing a two-dimensional cobalt sulfide holey sheet superstructure with hierarchical porosity followed by the encapsulation of thin iron-nitrogen-carbon as a protective layer. The iron-nitrogen-carbon layer to some degree precludes the phase transition of cobalt sulfide underneath and preserves the structural integrity during catalysis, therefore rendering an exceptional durability in terms of no obvious activity loss after 10,000 cycles of the accelerated durability test. It also noticeably enhances the intrinsic activity of cobalt sulfide and does not influence its exposure into the electrolyte, resulting in showing an extraordinary electrochemical performance in terms of a potential difference of 0.69 V for the overall oxygen redox. A rechargeable zinc-air battery assembled by a cobalt sulfide/iron-nitrogen-carbon air cathode delivers approximately 4.2 times higher power density than that without an iron-nitrogen-carbon layer and stably operates for 300 h with a high voltaic efficiency. This work gives a facile and effective strategy for improving the long-term durability of transition-metal sulfide electrocatalysts.

Electrochemical incorporation of heteroatom into surface reconstruction induced Ni vacancy of NixO nanosheet for enhanced water oxidation

Surface reconstruction of non-oxide oxygen evolution reaction (OER) electrocatalysts has been intensively studied to improve their catalytic performances. However, further modification of the reconstructed active surfaces for better catalytic performances has not been reported. In this work, NiSe nanorods are prepared on nickel foam (NiSe@NF) as the pre-catalyst for electrochemical OER. It is revealed that non-stoichiometric NiO nanosheets with abundant Ni vacancies (Ni_xO) are formed on the surfaces of NiSe nanorods (Ni_xO/NiSe@NF) via in-situ electrochemical oxidation. Furthermore, the OER performances are obviously improved after heteroatom Fe is incorporated electrochemically into Ni_xO nanosheets ((FeNi)O/NiSe@NF). For OER to have a current density of 20 mA cm^-2 in 1 M KOH solution, the as-prepared (FeNi)O/NiSe@NF electrode only needs an overpotential of 268 mV. Density functional theory (DFT) calculations reveal that the formation of Ni vacancy can increase the free energy of *OH. More importantly, the incorporation of heteroatom Fe into Ni vacancy can significantly decrease the free energy of *O, which enables Fe-NiO to have the lowest theoretical overpotential for OER in this work. The present work provides a facile and universal strategy to modify the reconstructed active oxides' surfaces for higher electrocatalytic performances.

Catalytic Activity Trends of Pyrite Transition Metal Dichalcogenides for Oxygen Reduction and Evolution

Transition metal dichalcogenides (TMDs) have been considered as promising materials for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) electrocatalysis. While there have been numerous studies focusing on layered...

Microflower-like Co9S8@MoS2 heterostructure as an efficient bifunctional catalyst for overall water splitting

The development of a distinguished and high-performance catalyst for H₂ and O₂ generation is a rational strategy for producing hydrogen fuel via electrochemical water splitting. Herein, a flower-like Co₉S₈@MoS₂ heterostructure with effective bifunctional activity was achieved using a one-pot approach via the hydrothermal treatment of metal-coordinated species followed by pyrolysis under an N₂ atmosphere. The heterostructures exhibited a 3D interconnected network with a large electrochemical active surface area and a junctional complex with hydrogen evolution reaction (HER) catalytic activity of MoS₂ and oxygen evolution reaction (OER) catalytic activity of Co₉S₈, exhibiting low overpotentials of 295 and 103 mV for OER and HER at 10 mA cm⁻² current density, respectively. Additionally, the catalyst-assembled electrolyser provided favourable catalytic activity and strong durability for overall water splitting in 1 M KOH electrolyte. The results of the study highlight the importance of structural engineering for the design and preparation of cost-effective and efficient bifunctional electrocatalysts.

A flower-like Co₉S₈@MoS₂ heterostructure was prepared as efficient bifunctional electrocatalyst for overall water splitting by a sample one-pot approach.

Structural Transformation of Heterogeneous Materials for Electrocatalytic Oxygen Evolution Reaction

Electrochemical water splitting for hydrogen generation is a promising pathway for renewable energy conversion and storage. One of the most important issues for efficient water splitting is to develop cost-effective and highly efficient electrocatalysts to drive sluggish oxygen-evolution reaction (OER) at the anode side. Notably, structural transformation such as surface oxidation of metals or metal nonoxide compounds and surface amorphization of some metal oxides during OER have attracted growing attention in recent years. The investigation of structural transformation in OER will contribute to the in-depth understanding of accurate catalytic mechanisms and will finally benefit the rational design of catalytic materials with high activity. In this Review, we provide an overview of heterogeneous materials with obvious structural transformation during OER electrocatalysis. To gain insight into the essence of structural transformation, we summarize the driving forces and critical factors that affect the transformation process. In addition, advanced techniques that are used to probe chemical states and atomic structures of transformed surfaces are also introduced. We then discuss the structure of active species and the relationship between catalytic performance and structural properties of transformed materials. Finally, the challenges and prospects of heterogeneous OER electrocatalysis are presented.

Intercalation-induced partial exfoliation of NiFe LDHs with abundant active edge sites for highly enhanced oxygen evolution reaction

Edge sites and interlayer space of NiFe layered double hydroxides (LDHs) play an important role in water oxidation. However, the combined effect of interlayer expansion and partial exfoliation on the catalytic activity is yet to be investigated. Herein, scalable synthesis of partially exfoliated citrate-intercalated NiFe LDHs with tunable interlayer space have been achieved. The effect of citrate concentration on the phase, morphology, surface elemental composition, electronic states of surface metals, and electrochemical properties are comprehensively studied. The unique structure results in improved intrinsic catalytic activity and abundant active edge sites for oxygen evolution reaction. The optimal NiFe LDHs show an overpotential of 225 mV at 10 mA cm^-2, which is much smaller than that (∼305 mV) of the single-layer NiFe LDH nanosheets reported in the literature. The high catalytic activity can be mainly attributed to the combined effect between the enlarged interlayer space and the partial exfoliation/nanosheet thickness. That is, the interlayer space is related to the reaction kinetics/mechanism, while the degree of exfoliation affects the magnitude of the current density at a certain potential.

Hydrogen-etched CoS2 to produce a Co9S8@CoS2 heterostructure electrocatalyst for highly efficient oxygen evolution reaction

There is a pressing requirement for developing high-efficiency non-noble metal electrocatalysts in oxygen evolution reactions (OER), where transition metal sulfides are considered to be promising electrocatalysts for the OER in alkaline medium. Herein, we report the outstanding OER performance of Co9S8@CoS2 heterojunctions synthesized by hydrogen etched CoS2, where the optimized heterojunction shows a low η 50 of 396 mV and a small Tafel slope of 181.61 mV dec-1. The excellent electrocatalytic performance of this heterostructure is attributed to the interface electronic effect. Importantly, the post-stage characterization results indicate that the Co9S8@CoS2 heterostructure exhibits a dynamic reconfiguration during the OER with the formation of CoOOH in situ, and thus exhibits a superior electrocatalytic performance.

Advanced Transition Metal-Based OER Electrocatalysts: Current Status, Opportunities, and Challenges

Oxygen evolution reaction (OER) is an important half-reaction involved in many electrochemical applications, such as water splitting and rechargeable metal-air batteries. However, the sluggish kinetics of its four-electron transfer process becomes a bottleneck to the performance enhancement. Thus, rational design of electrocatalysts for OER based on thorough understanding of mechanisms and structure-activity relationship is of vital significance. This review begins with the introduction of OER mechanisms which include conventional adsorbate evolution mechanism and lattice-oxygen-mediated mechanism. The reaction pathways and related intermediates are discussed in detail, and several descriptors which greatly assist in catalyst screen and optimization are summarized. Some important parameters suggested as measurement criteria for OER are also mentioned and discussed. Then, recent developments and breakthroughs in experimental achievements on transition metal-based OER electrocatalysts are reviewed to reveal the novel design principles. Finally, some perspectives and future directions are proposed for further catalytic performance enhancement and deeper understanding of catalyst design. It is believed that iterative improvements based on the understanding of mechanisms and fundamental design principles are essential to realize the applications of efficient transition metal-based OER electrocatalysts for electrochemical energy storage and conversion technologies.

Metallic S-CoTe with Surface Reconstruction Activated by Electrochemical Oxidation for Oxygen Evolution Catalysis

Developing highly active electrocatalysts toward oxygen evolution reaction (OER) is critical for the application of water splitting for hydrogen production and can further alleviate the energy crisis problem, but still remaining challenging. Especially, unlocking the catalytic site, in turn, helps design the available catalysts. Herein, the nanorod cobalt telluride with sulfur incorporation grown on a carbon cloth (S-CoTe/CC) as catalysts for OER, which displays extraordinary catalytic activity, is reported. Significantly, the in situ formed CoOOH species on the surface of S-CoTe merited from the structure evolution during the OER process serves as the active species. Furthermore, density functional theory calculations demonstrate that sulfur incorporation can tailor the electronic structure of active species and substantially optimize the free energy, accelerating the OER kinetics. This work provides an in-depth understanding of enhanced OER mechanism through foreign elements incorporating into precatalysts and is beneficial for the guiding design of more efficient catalysts.

Comprehensive Understandings into Complete Reconstruction of Precatalysts: Synthesis, Applications, and Characterizations

Reconstruction induced by external environment (such as applied voltage bias and test electrolytes) changes catalyst component and catalytic behaviors. Investigations of complete reconstruction in energy conversion recently receive intensive attention, which promote the targeted design of top-performance materials with maximum component utilization and good stability. However, the advantages of complete reconstruction, its design strategies, and extensive applications have not achieved the profound understandings and summaries it deserves. Here, this review systematically summarizes several important advances in complete reconstruction for the first time, which includes 1) fundamental understandings of complete reconstruction, the characteristics and advantages of completely reconstructed catalysts, and their design principles, 2) types of reconstruction-involved precatalysts for oxygen evolution reaction catalysis in wide pH solution, and origins of limited reconstruction degree as well as design strategies/principles toward complete reconstruction, 3) complete reconstruction for novel material synthesis and other electrocatalysis fields, and 4) advanced in situ/operando or multiangle/level characterization techniques to capture the dynamic reconstruction processes and real catalytic contributors. Finally, the existing major challenges and unexplored/unsolved issues on studying the reconstruction chemistry are summarized, and an outlook for the further development of complete reconstruction is briefly proposed. This review will arouse the attention on complete reconstruction materials and their applications in diverse fields.

Metal-Organic Frameworks and Metal-Organic Gels for Oxygen Electrocatalysis: Structural and Compositional Considerations

Increasing demand for sustainable and clean energy is calling for the next-generation energy conversion and storage technologies such as fuel cells, water electrolyzers, CO₂ /N₂ reduction electrolyzers, metal-air batteries, etc. All these electrochemical processes involve oxygen electrocatalysis. Boosting the intrinsic activity and the active-site density through rational design of metal-organic frameworks (MOFs) and metal-organic gels (MOGs) as precursors represents a new approach toward improving oxygen electrocatalysis efficiency. MOFs/MOGs afford a broad selection of combinations between metal nodes and organic linkers and are known to produce electrocatalysts with high surface areas, variable porosity, and excellent activity after pyrolysis. Some recent studies on MOFs/MOGs for oxygen electrocatalysis and their new perspectives in synthesis, characterization, and performance are discussed. New insights on the structural and compositional design in MOF/MOG-derived oxygen electrocatalysts are summarized. Critical challenges and future research directions are also outlined.

Anomalous self-optimization of sulfate ions for boosted oxygen evolution reaction

Broadly, the oxygen evolution reaction (OER) has been deeply understood as a significant part of energy conversion and storage. Nevertheless, the anions in the OER catalysts have been neglected for various reasons such as inactive sites, dissolution, and oxidation, amongst others. Herein, we applied a model catalyst s-Ni(OH)₂ to track the anionic behavior in the catalyst during the electrochemical process to fill this gap. The advanced operando synchrotron radiation Fourier transform infrared (SR-FTIR) spectroscopy, synchrotron radiation photoelectron spectroscopy (SRPES) depth detection and differential X-ray absorption fine structure (Δ-XAFS) spectrum jointly point out that some oxidized sulfur species (SO₄^2-) will self-optimize new Ni-S bonds during OER process. Such amazing anionic self-optimization (ASO) behavior has never been observed in the OER process. Subsequently, the optimization-derived component shows a significantly improved electrocatalytic performance (activity, stability, etc.) compared to reference catalyst Ni(OH)₂. Theoretical calculation further suggests that the ASO process indeed derives a thermodynamically stable structure of the OER catalyst, and then gives its superb catalytic performance by optimizing the thermodynamic and kinetic processes in the OER, respectively. This work demonstrates the vital role of anions in the electrochemical process, which will open up new perspectives for understanding OER and provide some new ideas in related fields (especially catalysis and chemistry).

In situ fabrication of a Ni–Fe–S hollow hierarchical sphere: an efficient (pre)catalyst for OER and HER

Mixed Ni–Fe–S hierarchical hollow microspheres as highly active and stable bifunctional catalysts for full water splitting show favourably comparable performance to Pt/C and IrO₂.

Ferrocene-Incorporated Cobalt Sulfide Nanoarchitecture for Superior Oxygen Evolution Reaction

Here, ferrocene(Fc)-incorporated cobalt sulfide (Co_x S_y ) nanostructures directly grown on carbon nanotube (CNT) or carbon fiber (CF) networks for electrochemical oxygen evolution reaction (OER) using a facile one-step solvothermal method are reported. The strong synergistic interaction between Fc-Co_x S_y nanostructures and electrically conductive CNTs results in the superior electrocatalytic activity with a very small overpotential of ≈304 mV at 10 mA cm^-2 and a low Tafel slope of 54.2 mV dec^-1 in 1 m KOH electrolyte. Furthermore, the Fc-incorporated Co_x S_y (FCoS) nanostructures are directly grown on the acid pretreated carbon fiber (ACF), and the resulting fabricated electrode delivers excellent OER performance with a low overpotential of ≈315 mV at 10 mA cm^-2 . Such superior OER catalytic activity can be attributed to 3D Fc-Co_x S_y nanoarchitectures that consist of a high concentration of vertical nanosheets with uniform distribution of nanoparticles that afford a large number of active surface areas and edge sites. Besides, the tight contact interface between ACF substrate and Fc-Co_x S_y nanostructures could effectively facilitate the electron transfer rate in the OER. This study provides valuable insights for the rational design of energy storage and conversion materials by the incorporation of other transition metal into metal sulfide/oxide nanostructures utilizing metallocene.

Modulation engineering of in situ cathodic activation of FePx based on W-incorporation for the hydrogen evolution reaction

In situ electrochemical activation as a new pretreating method to adjust electrocatalytic performance attracts extensive attention. However, the activation mechanisms of electrocatalysts are still ambiguous. Herein, we propose a facile modulation strategy of in situ cathodic activation of FeP_x based on W-incorporation (W-FeP_x/IF) for the hydrogen evolution reaction (HER). The activated W-FeO_x with obvious surface reconstruction demonstrates the role of W-incorporation for driving the cathodic activation of FeP_x, which suggests the larger surface area and more active sites. In fact, W incorporation can not only accelerate the cathodic activation process but also act as the adsorption sites for H_ad to form the synergistic effect with FeO_x for water dissociation. The obtained W-FeO_x/IF exhibits greatly enhanced HER activity featuring decreased overpotential from 237.7 to 154.0 mV at 100 mA cm^-2, which may be ascribed to W-FeO_x with double catalytic active sites after cathodic activation. Additionally, the modulation effects of cathodic activation can be exactly achieved by changing electrochemical parameters such as CV cycles. W-FeO_x/IF also shows excellent long-term stability for at least 100 h at 100 mA cm^-2. This modulation engineering based on metal doping is expected to provide inspiration for the understanding of the cathodic activation process for efficient electrocatalysts.

Effect of Selenium Content on Nickel Sulfoselenide-Derived Nickel (Oxy)hydroxide Electrocatalysts for Water Oxidation

An efficient and inexpensive electrocatalyst for the oxygen evolution reaction (OER) must be found in order to improve the viability of hydrogen fuel production via water electrolysis. Recent work has indicated that nickel chalcogenide materials show promise as electrocatalysts for this reaction and that their performance can be further enhanced with the generation of ternary, bimetallic chalcogenides (i.e., Ni_1-aM_aX₂); however, relatively few studies have investigated ternary chalcogenides created through the addition of a second chalcogen (i.e., NiX_2-aY_a). To address this, we studied a series of Se-modified Ni₃S₂ composites for use as OER electrocatalysts in alkaline solution. We found that the addition of Se results in the creation of Ni₃S₂/NiSe composites composed of cross-doped metal chalcogenides and show that the addition of 10% Se reduces the overpotential required to reach a current density of 10 mA/cm² by 40 mV versus a pure nickel sulfide material. Chemical analysis of the composites' surfaces shows a reduction in the amount of nickel oxide species with Se incorporation, which is supported by transmission electron microscopy; this reduction is correlated with a decrease in the OER overpotentials measured for these samples. Together, our results suggest that the incorporation of Se into Ni₃S₂ creates a more conductive material with a less-oxidized surface that is more electrocatalytically active and resistant to further oxidation. Importantly, oxidation does still occur, and the active catalyst is most likely a nickel (oxy)hydroxide surrounding a crystalline, conductive Ni₃S_2-xSe_x core.

Optimized Metal Chalcogenides for Boosting Water Splitting

Electrocatalytic water splitting (2H2O → 2H2 + O2) is a very promising avenue to effectively and environmentally friendly produce highly pure hydrogen (H2) and oxygen (O2) at a large scale. Different materials have been developed to enhance the efficiency for water splitting. Among them, chalcogenides with unique atomic arrangement and high electronic transport show interesting catalytic properties in various electrochemical reactions, such as the hydrogen evolution reaction, oxygen evolution reaction, and overall water splitting, while the control of their morphology and structure is of vital importance to their catalytic performance. Herein, the general synthetic methods are summarized to prepare metal chalcogenides and different strategies are designed to improve their catalytic performance for water splitting. The remaining challenges in the research and development of metal chalcogenides and possible directions for future research are also summarized.

A Ni/Fe-based heterometallic phthalocyanine conjugated polymer for the oxygen evolution reaction

A Ni/Fe-based heterometallic phthalocyanine polymer Fe_0.5Ni_0.5Pc-CP has been prepared, exhibiting high catalytic activity towards the oxygen evolution reaction due to the electronic interactions between the neighboring Fe and Ni atoms.

Non-precious-metal catalysts for alkaline water electrolysis: operando characterizations, theoretical calculations, and recent advances

Advances of non-precious-metal catalysts for alkaline water electrolysis are reviewed, highlighting operando techniques and theoretical calculations in their development.