Amorphous NiMS (M: Co, Fe or Mn) holey nanosheets derived from crystal phase transition for enhanced oxygen evolution in water splitting

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.

Defect-Induced Atomic Arrangement in CoFe Bimetallic Heterostructures with Boosted Oxygen Evolution Activity

Three CoFe-bimetallic oxides with different compositions (termed as CoFeO_x -A/N/H) are prepared by thermally treating metal-organic-framework (MOF) precursors under different atmospheres (air, N_2, and NaBH₄ /N₂ ), respectively. With the aid of vast oxygen vacancies (O_v ), cobalt at tetrahedral sites (Co²⁺ (Th)) in spinel Co₃ O₄ is diffused into interstitial octahedral sites (Oh) to form rocksalt CoO and ternary oxide CoFe₂ O₄ has been induced to give the unique defective CoO/CoFe₂ O₄ heterostructure. The resultant CoFeO_x -H exhibits superb electrocatalytic activity toward water oxidation: overpotential at 10 mA cm^-2 is 192 mV, which is 122 mV smaller than that of CoFeO_x -A. The smaller Tafel slope (42.53 mV dec^-1 ) and higher turnover frequency (785.5 h^-1 ) suggest fast reaction kinetics. X-ray absorption spectroscopy, ex situ characterizations, and theoretical calculations reveal that defect engineering effectively tunes the electronic configuration to a more active state, resulting in the greatly decreased binding energy of oxo intermediates, and consequently much lower catalytic overpotential. Moreover, the construction of hetero-interface in CoFeO_x -H can provide rich active sites and promote efficient electron transfer. This work may shed light on a comprehensive understanding of the modulation of electron configuration of bimetallic oxides and inspire the smart design of high-performance electrocatalysts.

Recent advances in amorphous electrocatalysts for oxygen evolution reaction

Oxygen evolution reaction (OER) has attracted great attention as an important half-reaction in the electrochemical splitting of water for green hydrogen production. However, the inadequacy of highly efficient and stable electrocatalysts has impeded the development of this technology. Amorphous materials with long-range disordered structures have exhibited superior electrocatalytic performance compared to their crystalline counterparts due to more active sites and higher structural flexibility. This review summarizes the preparation methods of amorphous materials involving oxides, hydroxide, phosphides, sulfides, and their composites, and introduces the recent progress of amorphous OER electrocatalysts in acidic and alkaline media. Finally, the existing challenges and future perspectives for amorphous electrocatalysts for OER are discussed. Therefore, we believe that this review will guide designing amorphous OER electrocatalysts with high performance for future energy applications.

A facile method to synthesize 3D nanosheets of Fe/S dopedα-Ni(OH)2 as an electrocatalyst for improved oxygen evolution reaction

S-doped Fe/Ni oxide and Fe/Ni hydride oxide catalysts exhibit good oxygen evolution reaction (OER) performance. Nevertheless, the over-doping of S and the agglomeration of active sites still hinder the improvement of the performance of these catalysts. The S/O ratio regulation can optimize the electronic structure effectively so as to improve the OER performance of the catalysts, but few studies have focused on this study. Here, we find a facile room-temperature method to synthesize the unique 3D ultra-thin FeNiOS nanosheets with an adjustable S/O ratio for OER. The FeNiOS-NS catalysts exhibit excellent OER performance with an overpotential of 235 mV at 10 mA cm^-2and a small Tafel slope of 64.2 mV dec^-1in 0.1 M KOH, which originated from the sufficient exposure of the active Fe-Ni component and the optimized electronic structure due to the tunable S/O ratio. This study demonstrates a novel strategy to optimize the OER performance of Ni-based catalysts.

Critical Review, Recent Updates on Zeolitic Imidazolate Framework-67 (ZIF-67) and Its Derivatives for Electrochemical Water Splitting

Design and construction of low-cost electrocatalysts with high catalytic activity and long-term stability is a challenging task in the field of catalysis. Metal-organic frameworks (MOF) are promising candidates as precursor materials in the development of highly efficient electrocatalysts for energy conversion and storage applications. This review starts with a summary of basic concepts and key evaluation parameters involved in the electrochemical water-splitting reaction. Then, different synthesis approaches reported for the cobalt-based Zeolitic imidazolate framework (ZIF-67) and its derivatives are critically reviewed. Additionally, several strategies employed to enhance the electrocatalytic activity and stability of ZIF-67-based electrocatalysts are discussed in detail. The present review provides a succinct insight into the ZIF-67 and its derivatives (oxides, hydroxides, sulfides, selenides, phosphide, nitrides, telluride, heteroatom/metal-doped carbon, noble metal-supported ZIF-67 derivatives) reported for oxygen evolution reaction (OER), hydrogen evolution reaction (HER), and overall water splitting applications. Finally, this review concludes with the associated challenges and the perspectives on developing the best economic, durable electrocatalytic materials.

Manipulation on Two-Dimensional Amorphous Nanomaterials for Enhanced Electrochemical Energy Storage and Conversion

Low-carbon society is calling for advanced electrochemical energy storage and conversion systems and techniques, in which functional electrode materials are a core factor. As a new member of the material family, two-dimensional amorphous nanomaterials (2D ANMs) are booming gradually and show promising application prospects in electrochemical fields for extended specific surface area, abundant active sites, tunable electron states, and faster ion transport capacity. Specifically, their flexible structures provide significant adjustment room that allows readily and desirable modification. Recent advances have witnessed omnifarious manipulation means on 2D ANMs for enhanced electrochemical performance. Here, this review is devoted to collecting and summarizing the manipulation strategies of 2D ANMs in terms of component interaction and geometric configuration design, expecting to promote the controllable development of such a new class of nanomaterial. Our view covers the 2D ANMs applied in electrochemical fields, including battery, supercapacitor, and electrocatalysis, meanwhile we also clarify the relationship between manipulation manner and beneficial effect on electrochemical properties. Finally, we conclude the review with our personal insights and provide an outlook for more effective manipulation ways on functional and practical 2D ANMs.

Defect-Engineered NiCo-S Composite as a Bifunctional Electrode for High-Performance Supercapacitor and Electrocatalysis

Defect engineering is a reasonable solution to improve the surface properties and electronic structure of nanomaterials. However, how to introduce dual defects into nanomaterials by a simple way is still facing challenge. Herein, we propose a facile two-step solvothermal method to introduce Fe dopants and S vacancies into metal-organic framework-derived bimetallic nickel cobalt sulfide composites (NiCo-S). The as-prepared Fe-doped NiCo-S (Fe-NiCo-S) possesses improved charge storage kinetics and activities as electrode material for supercapacitors and the oxygen evolution reaction (OER). The obtained Fe-NiCo-S nanosheet has a high specific capacitance (2779.6 F g^-1 at 1 A g^-1) and excellent rate performance (1627.2 F g^-1 at 10 A g^-1). A hybrid supercapacitor device made of Fe-NiCo-S as the positive electrode and reduced graphene oxide (rGO) as the negative electrode presents a high energy density of 56.0 Wh kg^-1 at a power density of 847.1 W kg^-1 and excellent cycling stability (capacity retention of 96.5% after 10,000 cycles at 10 A g^-1). Additionally, the Fe-NiCo-S composite modified by Fe doping and S vacancy has an ultralow oxygen evolution overpotential of 247 mV at 10 mA cm^-2. Based on the density functional theory (DFT) calculation, defects cause more electrons to appear near the Fermi level, which is conducive to electron transfer in electrochemical processes. Our work provides a rational strategy for facilely introducing dual defects into metal sulfides and may provide a novel idea to prepare electrode materials for energy storage and energy conversion application.

Hierarchical sheet-on-sheet heterojunction array of a β-Ni(OH)2/Fe(OH)3 self-supporting anode for effective overall alkaline water splitting

Rationally designing high-performance non-noble metal electrocatalysts is of essence to improve energy conversion efficiency in water splitting. Herein, a unique 3D hierarchical sheet-on-sheet heterojunction between Fe(OH)₃ and β-Ni(OH)₂ on pretreated Ni foam (NiFe-HD/pre-NF) was fabricated by a two-step strategy involving the interfacial hydrolysis-deposition of Fe²⁺ and electrodeposition of Ni²⁺. The presence of the Ni-O-Fe bridge at the Fe(OH)₃/β-Ni(OH)₂ heterointerface can induce interfacial electronic redistribution to form Ni³⁺ in NiFe-HD/pre-NF, and further strengthen the adsorption of OH^- and weaken the O-H bond to change the rate-determining step (RDS) for accelerating OER kinetics. Benefiting from the sheet-on-sheet architecture and dual-phase synergism on NiFe-HD/pre-NF, the optimal NiFe-HD/pre-NF exhibits excellent OER performance with a lower overpotential of 256 mV at 100 mA cm^-2, a small Tafel slope of 81 mV dec^-1, high intrinsic activity and robust stability. Alkaline water-splitting using NiFe-HD/pre-NF as the anode requires ultralow cell voltages of 1.62 V and 1.83 V at current densities of 100 mA cm^-2 and 400 mA cm^-2, respectively, which are comparable with commercial alkaline water electrolysis, and operates steadily at a current density of 100 mA cm^-2 for 85 h without decay. This work proposes a facile strategy for constructing heterojunctions and modulating electronic interaction to develop electrocatalysts with new architectures.

Flower-like Fe-Co-M (M=S, O, P and Se) Nanosheet Arrays Grown on Nickel Foam as High-efficiency Bifunctional Electrocatalysts

The development of highly efficient, inexpensive, abundant and non-precious metal electrocatalysts is the lifeblood of the hydrogen production industry, especially the hydrogen production industry by electrolysis of water. A Fe-Co-S/NF bifunctional electrocatalyst with nanoflower-like structure was synthesized on three-dimensional porous nickel foam through one-step hydrothermal and one-step high-temperature sulfuration operations, and the material displays high-efficiency electrocatalytic performance. As a catalyst for the hydrogen evolution reaction, Fe-Co-S/NF can drive a current density of 10 mA/cm² at an overpotential of 143 mV with a Tafel slope of 80.2 mV/dec. When it was used as an oxygen evolution reaction catalyst, it exhibits good OER reactivity with a low Tafel slope (82.6 mV/dec) and with requiring only 117 mV overpotential to drive current densities up to 50 mA/cm² . In addition, the Fe-Co-S/NF//Fe-Co-S/NF electrolytic cell was assembled, an electrolysis voltage of 1.64 V is required to drive a current density of 50 mA/cm² , which is one of the most active catalysts reported so far. This work indicates that the introduction of S, P and Se treating processes could effectively improve electrical conductivity of the material and enhance the catalytic activity of the material. This work offers an effective and convenient method for improving the morphology of the catalyst, increasing the surface area of the catalyst and developing high-efficiency and low-cost catalysts.

A macroporous titanium oxynitride-supported bifunctional oxygen electrocatalyst for zinc–air batteries

A macroporous titanium oxynitride-supported bifunctional oxygen electrocatalyst was fabricated for the development of rechargeable zinc–air batteries.

Ultrathin 2D flower-like CoP@C with the active (211) facet for efficient electrocatalytic water splitting

Due to the more exposed (211) facet, the electrocatalytic water splitting activity of 2D CoP@C is superior to that of its counterpart 3D structure.

A three-dimensional porous CoSnS@CNT nanoarchitecture as a highly efficient bifunctional catalyst for boosted OER performance and photocatalytic degradation

It is urgent and significant to develop competent, inexpensive transition metal-based catalysts with multifunctional catalytic properties for wide applications. To meet this requirement, herein, for the first time, we present a novel bifunctional CoSnS@CNT hybrid via a simple one-pot surfactant-free hydrothermal method. The CoSnS@CNT hybrid has a unique three-dimensional (3D) porous nanoarchitecture, which is constructed by ultrathin CoSnS homogenously and compactly anchored on a highly conductive CNT skeleton. The porous nanoarchitecture of CoSnS@CNT provides abundant catalytic sites and facilitates ion diffusion, and the CNT skeleton accelerates electron transfer. Benefitting from these merits, the CoSnS@CNT hybrid acted as a bifunctional catalyst with boosted electrocatalytic and photocatalytic performance, where it delivered a tremendous oxygen evolution reaction (OER) performance with a low overpotential of 330 mV at a current density of 10 mA cm^-2 and excellent outstanding stability. Moreover, it showed 91.72% photocatalytic degradation for Rhodamine B dye, which is 2-times higher than that of bare CoSnS. This study presents a systematic approach to judiciously design nanostructures and simply synthesize non-noble metal-based bifunctional catalysts with boosted electrocatalytic and photocatalytic activities.