The Flexibility of an Amorphous Cobalt Hydroxide Nanomaterial Promotes the Electrocatalysis of Oxygen Evolution Reaction

Defect-Driven Evolution of Oxo-Coordinated Cobalt Active Sites with Rapid Structural Transformation for Efficient Water Oxidation

Reconstructing the surface nature of metal-organic frameworks (MOFs) as precatalytic structures is a promising methodology for improving electrocatalytic performance. However, regulating the structural evolution of MOFs during electrolysis remains highly uncontrollable and lacks an in-depth understanding of the role of in situ-derived active sites. Here, we suggest a simple approach to fine-tune the symmetry of Co-MOFs with an oxo-coordinated asymmetric coordination that acts as a prototypical structure motif for the oxygen evolution reaction (OER). Through a facile thermal treatment, the Co-N4 configuration of Co-MOFs transforms to the distorted Co-N3-oxo configuration of defective Co-ligand nanoclusters. By operando spectroscopic characterization, the reconstructed Co-N3-oxo structure enables a rapid structural transition toward homogeneous oxyhydroxides. Moreover, the defective nature of the precatalytic structure regulates the surface Co-O bonding environment with abundant μ2-O-Co3+ sites, thereby exhibiting highly enhanced OER activity with an overpotential of 256 mV at 10 mA cm-2 and excellent durability for 100 h, compared with the pristine Co-MOFs. Atomistic simulations reveal that the effect of OER intermediates on the oxyhydroxides gets distributed among neighboring Co ions, promoting balanced binding of the intermediates. This work highlights an effective strategy to design the MOF-based structure for optimizing the surface nature, thus enhancing the electrocatalytic activity.

Unveiling the potential of amorphous nanocatalysts in membrane-based hydrogen production

Hydrogen, as a clean and renewable energy source, is a promising candidate to replace fossil fuels and alleviate the environmental crisis. Compared with the traditional H-type cells with a finite-gap, the design of membrane electrodes can reduce the gas transmission resistance, enhance the current density, and improve the efficiency of hydrogen production. However, the harsh environment in the electrolyser makes the membrane electrode based water electrolysis technology still limited by the lack of catalyst activity and stability under the working conditions. Due to the abundant active sites and structural flexibility, amorphous nanocatalysts are alternatives. In this paper, we review the recent research progress of amorphous nanomaterials as electrocatalysts for hydrogen production by electrolysis at membrane electrodes, illustrate and discuss their structural advantages in membrane electrode catalytic systems, as well as explore the significance of the amorphous structure for the development of membrane electrode systems. Finally, the article also looks at future opportunities and adaptations of amorphous catalysts for hydrogen production at membrane electrodes. The authors hope that this review will deepen the understanding of the potential of amorphous nanomaterials for application in electrochemical hydrogen production, facilitating future nanomaterials research and new sustainable pathways for hydrogen production.

Fine-tuning nanoflower-like Fe/Co hybrids with high content oxyhydroxide accelerating oxygen evolution kinetics

Iron hydroxide (FeOOH) is a potential active component in iron-based electrocatalysts for water electrolysis. However, its catalytic performance is constrained by its slow oxygen evolution reaction (OER) kinetics. Herein, we synthesized a nanoflower-like FeCo-hydro(oxy)oxides composite with tunable Fe/Co ratios (Fex-Coy) on nickel foam (NF) via a one-step electrodeposition technique. This method allows for precise control over the morphology and composition of the hybrid nanoflowers. The optimized Fe9-Co1 discloses favorable OER performance with a low overpotential of 222 mV at 50 mA cm-2 and demonstrates good stability exceeding 60 h at 10 mA cm-2. Further, an assembled Fe9-Co1(+)||Pt/C(-) dual-electrode configuration achieves a low cell voltage of 1.73 V at the current density of 100 mA cm-2 for water splitting, with long-term stability for 70 h and minimal degradation. Studies indicate that the distinctive nanoflower morphology of Fe9-Co1 enhances active site exposure, while both FeOOH and reconstructed CoOOH serve as catalytic centers, contributing to the observed OER performance. This work introduces a facile approach for synthesizing OER electrocatalysts, underscoring the role of the high-valence state of Fe/Co as active sites in the OER process.

High-Valence Cu Induced by Photoelectric Reconstruction for Dynamically Stable Oxygen Evolution Sites

Oxygen vacancies are generally considered to play a crucial role in the oxygen evolution reaction (OER). However, the generation of active sites created by oxygen vacancies is inevitably restricted by their condensation and elimination reactions. To overcome this limitation, here, we demonstrate a novel photoelectric reconstruction strategy to incorporate atomically dispersed Cu into ultrathin (about 2-3 molecular) amorphous oxyhydroxide (a-CuM, M = Co, Ni, Fe, or Zn), facilitating deprotonation of the reconstructed oxyhydroxide to generate high-valence Cu. The in situ XAFS results and first-principles calculations reveal that Cu atoms are stabilized at high valence during the OER process due to Jahn-Teller distortion, resulting in para-type double oxygen vacancies as dynamically stable catalytic sites. The optimal a-CuCo catalyst exhibits a record-high mass activity of 3404.7 A g-1 at an overpotential of 300 mV, superior to the benchmarking hydroxide and oxide catalysts. The developed photoelectric reconstruction strategy opens up a new pathway to construct in situ stable oxygen vacancies by high-valence Cu single sites, which extends the design rules for creating dynamically stable active sites.

Revealing the structural evolution of CuAg composites during electrochemical carbon monoxide reduction

Comprehending the catalyst structural evolution during the electrocatalytic process is crucial for establishing robust structure/performance correlations for future catalysts design. Herein, we interrogate the structural evolution of a promising Cu-Ag oxide catalyst precursor during electrochemical carbon monoxide reduction. By using extensive in situ and ex situ characterization techniques, we reveal that the homogenous oxide precursors undergo a transformation to a bimetallic composite consisting of small Ag nanoparticles enveloped by thin layers of amorphous Cu. We believe that the amorphous Cu layer with undercoordinated nature is responsible for the enhanced catalytic performance of the current catalyst composite. By tuning the Cu/Ag ratio in the oxide precursor, we find that increasing the Ag concentration greatly promotes liquid products formation while suppressing the byproduct hydrogen. CO2/CO co-feeding electrolysis and isotopic labelling experiments suggest that high CO concentrations in the feed favor the formation of multi-carbon products. Overall, we anticipate the insights obtained for Cu-Ag bimetallic systems for CO electroreduction in this study may guide future catalyst design with improved performance.

Interface-Engineered NiFe/Ni-S Nanoparticles for Reliable Alkaline Oxygen Production at Industrial Current: A Sulfur Source Confinement Strategy

Using powder-based ink appears to be the most suitable candidate for commercializing the membrane electrode assembly (MEA), while research on the powder-based NPM catalyst for anion exchange membrane water electrolyzer (AEMWE) is currently insufficient, especially at high current density. Herein, a sulfur source (NiFe Layered double hydroxide adsorbedSO 4 2 - ${\mathrm{SO}}_4^{2 - }$ ) confinement strategy is developed to integrate Ni3S2 onto the surface of amorphous/crystalline NiFe alloy nanoparticles (denoted NiFe/Ni-S), achieving advanced control over the sulfidation process for the formation of metal sulfides. The constructed interface under the sulfur source confinement strategy generates abundant active sites that increase electron transport at the electrode-electrolyte interface and improve ability over an extended period at a high current density. Consequently, the constructed NiFe/Ni-S delivers an ultra-low overpotential of 239 mV at 10 mA cm-2 and 0.66 mAcm ECSA - 2 ${\mathrm{cm}}_{{\mathrm{ECSA}}}^{ - 2}$ under an overpotential of 300 mV. The AEMWE with NiFe/Ni-S anode exhibits a cell voltage of 1.664 V @ 0.5 A cm-2 and a 400 h stability at 1.0 A cm-2.

Amorphous conversion in pyrolytic symmetric trinuclear nickel clusters trigger trifunctional electrocatalysts

The pursuit of multifunctional electrocatalysts holds significant importance due to their comprehension of material chemistry. Amorphous materials are particularly appealing, yet they pose challenges in terms of rational design due to their structural disorder and thermal instability. Herein, we propose a strategy that entails the tandem (low-temperature/250-350 °C) pyrolysis of molecular clusters, enabling preservation of the local short-range structures of the precursor Schiff base nickel (Ni3[2(C21H24N3Ni1.5O6)]). The temperature-dependent residuals demonstrate exceptional activity and stability for at least three distinct electrocatalytic processes, including the oxygen evolution reaction (η10 = 197 mV), urea oxidation reaction (η10 = 1.339 V), and methanol oxidation reaction (1358 mA cm-2 at 0.56 V). Three distinct nickel atom motifs are discovered for three efficient electrocatalytic reactions (Ni1 and Ni1' are preferred for UOR/MOR, while Ni2 is preferred for OER). Our discoveries pave the way for the potential development of multifunctional electrocatalysts through disordered engineering in molecular clusters under tandem pyrolysis.

Crystalline/Amorphous Mo-Ni(OH)2/FexNiy(OH)3x+2y hierarchical nanotubes as efficient electrocatalyst for overall water splitting

The development of efficient bifunctional catalysts for overall water splitting is highly desirable and essential for the advancement of hydrogen technology. In this work, Mo-Ni(OH)2/FexNiy(OH)3x+2y with hierarchical nanotube structure is constructed on flexible carbon cloth (CC) through simple electrochemical deposition and hydrothermal method. The hollow tube-structure is in favor of both exposing active sites and enhancing mass transfer capability. Moreover, the doping of Mo can enhance the electronic conductivity of heterostructures. The interfacial interaction between amorphous and crystal can enhance effectively the charge transfer kinetics across the interface. Therefore, Mo-Ni(OH)2/FexNiy(OH)3x+2y can achieve a low overpotential of 57 mV for hydrogen evolution reaction (HER) and 229 mV for oxygen evolution reaction (OER) at 10 mA·cm-2. In addition, Mo-Ni(OH)2/FexNiy(OH)3x+2y needs a potential of only 1.54 V at 10 mA·cm-2 for overall water splitting, and retains for a long period of time (60 h) reliable. The work will provide a valuable approach to the construction of highly efficient electrocatalysts for overall water splitting.

Iron doping and interface engineering on amorphous/crystalline Fe-NixSy heterostructures toward high-stability and kinetically accelerated water splitting

It is very important to develop transition metal-based electrocatalysts with excellent activity, high stability and low-cost for overall water splitting. In this work, the Fe-doped NixSy/NF amorphous/crystalline heterostructure nanoarrays (Fe-NixSy/NF) was synthesized by a simple one-step method. The resulting hierarchically structured nanoarrays offer the advantages of large surface area, high structural void fraction and accessible internal surfaces. These advantages not only furnish additional catalytically active sites, but also enhance the stability of the structure and effectively accelerate mass diffusion and charge transport. Experimental and characterization results indicate that Fe doping increases the electrical conductivity of amorphous/crystalline NixSy/NF, and the NiS-Ni3S2 heterojunctions evoke interfacial charge rearrangement and optimize the adsorption free energy of the intermediates, which allows the catalyst to exhibit low overpotential and superior electrocatalytic activity. Especially, the overpotentials of hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) of Fe-NixSy/NF at 10 mA cm-2 in an alkaline environment are 102.4 and 230.5 mV, respectively. When applied as a bifunctional catalyst for overall water splitting, it requires only 1.45 V cell voltage to deliver a current density of 10 mA cm-2, which is preferable to the all-noble metal Pt/C || IrO2 electrocatalyst (1.62 mV @ 10 mA cm-2). In addition, Fe-NixSy/NF has excellent stability, and there is no obvious degradation after 96 h continuous operation at a current density of 100 mA cm-2. This work affords insights into the application of doping strategies and crystalline/amorphous synergistic modulation of the electrocatalytic activity of transition metal-based catalysts in energy conversion systems.

Recent Progress on Phase Engineering of Nanomaterials

As a key structural parameter, phase depicts the arrangement of atoms in materials. Normally, a nanomaterial exists in its thermodynamically stable crystal phase. With the development of nanotechnology, nanomaterials with unconventional crystal phases, which rarely exist in their bulk counterparts, or amorphous phase have been prepared using carefully controlled reaction conditions. Together these methods are beginning to enable phase engineering of nanomaterials (PEN), i.e., the synthesis of nanomaterials with unconventional phases and the transformation between different phases, to obtain desired properties and functions. This Review summarizes the research progress in the field of PEN. First, we present representative strategies for the direct synthesis of unconventional phases and modulation of phase transformation in diverse kinds of nanomaterials. We cover the synthesis of nanomaterials ranging from metal nanostructures such as Au, Ag, Cu, Pd, and Ru, and their alloys; metal oxides, borides, and carbides; to transition metal dichalcogenides (TMDs) and 2D layered materials. We review synthesis and growth methods ranging from wet-chemical reduction and seed-mediated epitaxial growth to chemical vapor deposition (CVD), high pressure phase transformation, and electron and ion-beam irradiation. After that, we summarize the significant influence of phase on the various properties of unconventional-phase nanomaterials. We also discuss the potential applications of the developed unconventional-phase nanomaterials in different areas including catalysis, electrochemical energy storage (batteries and supercapacitors), solar cells, optoelectronics, and sensing. Finally, we discuss existing challenges and future research directions in PEN.

How Amorphous Nanomaterials Enhanced Electrocatalytic, SERS, and Mechanical Properties

There is ever-growing research interest in nanomaterials because of the unique properties that emerge on the nanometer scale. While crystalline nanomaterials have received a surge of attention for exhibiting state-of-the-art properties in various fields, their amorphous counterparts have also attracted attention in recent years owing to their unique structural features that crystalline materials lack. In short, amorphous nanomaterials only have short-range order at the atomic scale, and their atomic packing lacks long-range periodic arrangement, in which the coordinatively unsaturated environment, isotropic atomic structure, and modulated electron state all contribute to their outstanding performance in various applications. Given their intriguing characteristics, we herein present a series of representative works to elaborate on the structural advantages of amorphous nanomaterials as well as their enhanced electrocatalytic, surface-enhanced Raman scattering (SERS), and mechanical properties, thereby elucidating the underlying structure-function relationship. We hope that this proposed relationship will be universally applicable, thus encouraging future work in the design of amorphous materials that show promising performance in a wide range of fields.

Regulating Electronic Structure of Iron Nitride by Tungsten Nitride Nanosheets for Accelerated Overall Water Splitting

Two essential characteristics that are required for hybrid electrocatalysts to exhibit higher oxygen and hydrogen evolution reaction (OER and HER, respectively) activity are a favorable electronic configuration and a sufficient density of active sites at the interface between the two materials within the hybrid. In the present study, a hybrid electrocatalyst is introduced with a novel architecture consisting of coral-like iron nitride (Fe2 N) arrays and tungsten nitride (W2 N3 ) nanosheets that satisfies these requirements. The resulting W2 N3 /Fe2 N catalyst achieves high OER activity (268.5 mV at 50 mA cm-2 ) and HER activity (85.2 mV at 10 mA cm-2 ) with excellent long-term durability in an alkaline medium. In addition, density functional theory calculations reveal that the individual band centers experience an upshift in the hybrid W2 N3 /Fe2 N structure, thus improving the OER and HER activity. The strategy adopted here thus provides a valuable guide for the fabrication of cost-effective multi-metallic crystalline hybrids for use as multifunctional electrocatalysts.

In Situ Surface Restructuring of Amorphous Ni-Doped CoMo Phosphate-Based Three-Dimensional Networked Nanosheets: Highly Efficient and Durable Electrocatalyst for Overall Alkaline Water Splitting

Developing cost-efficient bifunctional electrocatalysts with high efficiency and durability for the production of green hydrogen and oxygen is a demanding and challenging research area. Due to their high earth abundance, transition metal-based electrocatalysts are alternatives to noble metal-based water splitting electrocatalysts. Herein, binder-free three-dimensional (3D) networked nanosheets of Ni-doped CoMo ternary phosphate (Pi) were prepared using a facile electrochemical synthetic strategy on flexible carbon cloth without any high-temperature heat treatment or complicated electrode fabrication. The optimized CoMoNiPi electrocatalyst delivers admirable hydrogen (η₁₀ = 96 mV) and oxygen (η₁₀ = 272 mV) evolution performances in 1.0 M KOH electrolyte. For overall water splitting in a two-electrode system, the present catalyst demands only 1.59 and 1.90 V to reach current densities of 10 and 100 mA/cm², respectively, which is lower than that of the Pt/C||RuO₂ couple (1.61 V @ 10 mA/cm², 2 V > @ 100 mA/cm²) and many other catalysts reported previously. Furthermore, the present catalyst delivers excellent long-term stability in a two-electrode system continuously over 100 h at a high current density of 100 mA/cm², exhibiting nearly 100% faradic efficiency. The unique 3D amorphous structure with high porosity, a high active surface area, and lower charge transfer resistance provides excellent overall water splitting. Notably, the amorphous structure of the present catalyst favors the in situ surface reconstruction during electrolysis and generates very stable surface-active sites capable of long-term performance. The present work provides a route for the preparation of multimetallic-Pi nanostructures for various electrode applications that are easy to prepare and have superior activity, high stability, and low cost.

Magnetic Heating Amorphous NiFe Hydroxide Nanosheets Encapsulated Ni Nanoparticles@Wood Carbon to Boost Oxygen Evolution Reaction Activity

The oxygen evolution reaction (OER) has significant effects on the water-splitting process and rechargeable metal-air batteries; however, the sluggish reaction kinetics caused by the four-electron transfer process for transition metal catalysts hinder large-scale commercialization in highly efficient electrochemical energy conversion devices. Herein, a magnetic heating-assisted enhancement design for low-cost carbonized wood with high OER activity is proposed, in which Ni nanoparticles are encapsulated in amorphous NiFe hydroxide nanosheets (a-NiFe@Ni-CW) via direct calcination and electroplating. The introduction of amorphous NiFe hydroxide nanosheets optimizes the electronic structure of a-NiFe@Ni-CW, accelerating electron transfer and reducing the energy barrier in the OER. More importantly, the Ni nanoparticles located on carbonized wood can function as magnetic heating centers under the effect of an alternating current (AC) magnetic field, further promoting the adsorption of reaction intermediates. Consequently, a-NiFe@Ni-CW demonstrated an overpotential of 268 mV at 100 mA cm^-2 for the OER under an AC magnetic field, which is superior to that of most reported transition metal catalysts. Starting with sustainable and abundant wood, this work provides a reference for highly effective and low-cost electrocatalyst design with the assistance of a magnetic field.

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.

Amorphous NiFe Oxide-based Nanoreactors for Efficient Electrocatalytic Water Oxidation

Synergy engineering is an important way to enhance the kinetic activity of oxygen-evolution-reaction (OER) electrocatalysts. Here, we fabricated NiFe amorphous nanoreactor (NiFe-ANR) oxide as OER electrocatalysts via a mild self-catalytic reaction. Firstly, the amorphousness helps transform NiFe-ANR into highly active hydroxyhydroxides, and its many fine-grain boundaries increase active sites. More importantly, as proved by experiments and finite element analysis, the nanoreactor structure alters the spatial curvature and the mass transfer over the catalyst, thereby enriching OH^- in the catalyst surface and inner part. Thus, the catalyst with the structure of amorphous nanoreactors gained excellent activity, far superior to the NiFe catalyst with the structure of crystalline nanoreactor or amorphous non-nanoreactor. This work provides new insights into the applications and mechanisms of amorphousness and nanoreactors, embodying the "1+1>2" synergy of crystalline state and morphology.

Synthesis of amorphous trimetallic PdCuNiP nanoparticles for enhanced OER

Metal phosphides with multi-element components and amorphous structure represent a novel kind of electrocatalysts for promising activity and durability towards the oxygen evolution reaction (OER). In this work, a two-step strategy, including alloying and phosphating processes, is reported to synthesize trimetallic amorphous PdCuNiP phosphide nanoparticles for efficient OER under alkaline conditions. The synergistic effect between Pd, Cu, Ni, and P elements, as well as the amorphous structure of the obtained PdCuNiP phosphide nanoparticles, would boost the intrinsic catalytic activity of Pd nanoparticles towards a wide range of reactions. These obtained trimetallic amorphous PdCuNiP phosphide nanoparticles exhibit long-term stability, nearly a 20-fold increase in mass activity toward OER compared with the initial Pd nanoparticles, and 223 mV lower in overpotential at 10 mA cm^-2. This work not only provides a reliable synthetic strategy for multi-metallic phosphide nanoparticles, but also expands the potential applications of this promising class of multi-metallic amorphous phosphides.

Photoassisted Electrochemical Hydrogen Evolution Reaction of MFe2O4@Ultrathin Black Phosphorus Amorphous-Crystalline Interface

Exploring highly active, stable, and low-cost catalysts for photoelectrochemical hydrogen evolution reaction (PE-HER) is vital in the field of energy conversion. Herein, we construct a new amorphous crystalline interface that amorphous iron-based spinel oxide (A-MFe₂O₄ (M = Ni, Co, Zn)) is uniformly anchored on the crystalline exfoliated black phosphorus (C-EBP) nanosheets via electrochemical and solvothermal strategies. Among these A-MFe₂O₄@C-EBP catalysts, more oxygen defects of A-NiFe₂O₄@C-EBP interface provide a larger effective electrochemical active area of 32.33 mF cm^-2 as well as a turnover frequency of 0.44 s^-1 and allow for an optimum equilibrium of the hydrogen-containing adsorption intermediates. Furthermore, A-NiFe₂O₄@C-EBP exhibits significant PE-HER performance with an overpotential of 42 mV at 10 mA cm^-2 under visible-light irradiation. Density functional theory (DFT) calculations show that the amorphous-crystalline composite structure causes a large number of oxygen defects enhancing the intrinsic activity of A-NiFe₂O₄@C-EBP, which A-NiFe₂O₄@C-EBP significantly improves its adsorption capacity for H* for HER and has the lowest Gibbs free energy change for HER. This study not only provides a superior multifunctional amorphous-crystalline interface catalysts but also helps to understand the catalytic mechanism of PE-HER.

Unveiling the Accelerated Water Electrolysis Kinetics of Heterostructural Iron-Cobalt-Nickel Sulfides by Probing into Crystalline/Amorphous Interfaces in Stepwise Catalytic Reactions

Amorphization and crystalline grain boundary engineering are adopted separately in improving the catalytic kinetics for water electrolysis. Yet, the synergistic effect and advance in the cooperated form of crystalline/amorphous interfaces (CAI) have rarely been elucidated insightfully. Herein, a trimetallic FeCo(NiS2 )4 catalyst with numerous CAI (FeCo(NiS2 )4 -C/A) is presented, which shows highly efficient catalytic activity toward both hydrogen and oxygen evolution reactions (HER and OER). Density functional theory (DFT) studies reveal that CAI plays a significant role in accelerating water electrolysis kinetics, in which Co atoms on the CAI of FeCo(NiS2 )4 -C/A catalyst exhibit the optimal binding energy of 0.002 eV for H atoms in HER while it also has the lowest reaction barrier of 1.40 eV for the key step of OER. H2 O molecules are inclined to be absorbed on the interfacial Ni atoms based on DFT calculations. As a result, the heterostructural CAI-containing catalyst shows a low overpotential of 82 and 230 mV for HER and OER, respectively. As a bifunctional catalyst, it delivers a current density of 10 mA cm-2 at a low cell voltage of 1.51 V, which enables it a noble candidate as metal-based catalysts for water splitting. This work explores the role of CAI in accelerating the HER and OER kinetics for water electrolysis, which sheds light on the development of efficient, stable, and economical water electrolysis systems by facile interface-engineering implantations.

Boosting Hydrogen Evolution Reaction Activity of Amorphous Molybdenum Sulfide Under High Currents Via Preferential Electron Filling Induced by Tungsten Doping

The lack of highly efficient, durable, and cost-effective electrocatalysts for the hydrogen evolution reaction (HER) working at high current densities poses a significant challenge for the large-scale implementation of hydrogen production from renewable energy. Herein, amorphous molybdenum tungsten sulfide/nitrogen-doped reduced graphene oxide nanocomposites (a-MoWS_x /N-RGO) are synthesized by plasma treatment for use as high-performance HER catalysts. By adjusting the plasma treatment duration and chemical composition, an optimal a-MoWS_x /N-RGO catalyst is obtained, which exhibits a low overpotential of 348 mV at a current density of 1000 mA cm^-2 and almost no decay after 24 h of working at this current density, outperforming commercial platinum/carbon (Pt/C) and previously reported heteroatom-doped MoS₂ -based catalysts. Based on density functional theory (DFT) calculations, it is found that with a reasonable tungsten doping level, the catalytic active site (2S^{2 -} ) shows excellent catalytic performance working at high current densities because extra electrons preferentially fill at 2S^{2 -} . The introduction of tungsten tends to lower the electronic structure energy, resulting in a closer-to-zero positive Δ G H ∗ $\Delta {G}_{{{\rm{H}}}^{\rm{*}}}$ . Excessive tungsten introduction, however, can lead to structural damage and a worse HER performance under high current densities. The work provides a route towards rationally designing high-performance catalysts for the HER at industrial-level currents using earth-abundant elements.

An in situ grown amorphous ZrO2 layer on zeolite for enhanced phosphate adsorption

Zeolite supported amorphous metal oxide nanolayers with high specific surface area, abundant adsorption sites, and excellent reusability hold a bright prospect in the efficient removal of contaminants, yet it is proven to be still challenging to precisely regulate and control their synthesis. Herein, we reported a facile synthetic strategy for rational design and achieving the uniform and firm in situ growth of an amorphous ZrO2 layer decorated on the surface of zeolite (ZEO@AZ) for enhanced phosphate adsorption. The Langmuir isotherm model and pseudo-second order kinetic equation well described the adsorption process towards phosphate solution, and the synthetized ZEO@AZ exhibited an excellent maximum adsorption amount of 24.98 mgP g-1. Furthermore, the adsorption of phosphates on ZEO@AZ was confirmed to be chemisorption, endothermic and spontaneous. This approach for fabricating amorphous metal oxide nanolayers on a robust matrix may provide a new route for constructing composites with superb phosphate adsorption performance.

Electronegativity-Induced Charge Balancing to Boost Stability and Activity of Amorphous Electrocatalysts

Amorphization is an efficient strategy to activate intrinsically inert catalysts. However, the low crystallinity of amorphous catalysts often causes high solubility and poor electrochemical stability in aqueous solution. Here, a different mechanism is developed to simultaneously stabilize and activate the water-soluble amorphous MoS_x O_y via a charge-balancing strategy, which is induced by different electronegativity between the co-dopants Rh (2.28) and Sn (1.96). The electron-rich Sn prefers to stabilize the unstable apical O sites in MoS_x O_y through charge transfer, which can prevent the H from attacking. Meanwhile, the Rh, as the charge regulator, shifts the main active sites on the basal plane from inert Sn to active apical Rh sites. As a result, the amorphous RhSn-MoS_x O_y exhibits drastic enhancement in electrochemical stability (η₁₀ increases only by 12 mV) after 1000 cycles and a distinct activity (η₁₀ : 26 mV and Tafel: 30.8 mV dec^-1 ) for the hydrogen evolution reaction in acidic solution. This work paves a route for turning impracticably water-soluble catalysts into treasure and inspires new ideas to design high-performance amorphous electrocatalysts.

Alkali-Induced In Situ Formation of Amorphous NixFe1-x(OH)2 from a Linear [M3(COO)6]-Based MOF Template for Overall Electrochemical Water Splitting

Amorphous and bifunctional electrocatalysts based on 3d transition metals tend to exhibit better performance than their crystalline counterparts and are a promising choice for efficient overall water splitting yet far from being well explored. A 3,6-net metal-organic framework (MOF) of [Ni₃(bpt)₂(DMF)₂(H₂O)₂]·1.5DMF (Ni-MOF), based on linear [Ni₃(COO)₆] as a node and [1,1'-biphenyl]-3,4',5-tricarboxylic acid (H₃bpt) as a linker, was conveniently prepared via a hydrothermal reaction. Benefitting from the wide compatibility of the octahedral coordination geometry in Ni-MOF for different 3d metal ions, the molecular level and controllable metal doping facilitates the production of the desired Ni/Fe bimetallic MOF. A high-concentration alkali solution of 1 M KOH induced the in situ transformation of the MOF as a precursor to new amorphous electrocatalysts of [Ni(OH)₂(H₂O)_0.6]·H₂O [a-Ni(OH)₂] and its metal-doped derivatives of a-Ni_0.77Fe_0.23(OH)₂ and a-Ni_0.65Fe_0.35(OH)₂. In particular, the costly organic ligand H₃bpt was fully dissolved in the alkaline solution and can be recovered for cyclic utilization by subsequent acidification. The obtained amorphous hydroxide was deduced to be loose and defective layers containing both coordinated and lattice water based on combined characterizations of TG, IR, Raman, XPS, and sorption analysis. As opposed to the crystalline counterpart of Ni(OH)₂ with stacked packing layers and an absent lattice water, the abundant catalytic active sites of the amorphous electrocatalyst endow good performance in both oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). The bifunctional a-Ni_0.65Fe_0.35(OH)₂ coated on nickel foam realizes small overpotentials of 247 and 99 mV for OER and HER, respectively, under a current density of 10 mA cm^-2, which can work with a cell voltage of merely 1.60 V for overall water splitting. This study provides an efficient strategy for widely screening and preparing new functional amorphous materials for electrocatalytic application.

γ-FeO(OH) with Multi-surface Terminations Intrinsically Active for Electrocatalytic Oxygen Evolution Reaction

Due to poor conductivity, electrocatalytic performance of independently prepared iron oxy-hydroxide (FeO(OH)) is inferior whereas in-situ derived FeO(OH) from the iron based electro(pre)catalyst shows superior oxygen evolution reaction (OER). Use...

CO2-assisted synthesis of crystalline/amorphous NiFe-MOF heterostructure for high-efficiency electrocatalytic water oxidation

Modulating the crystalline phase and structure of metal organic frameworks (MOFs) for superior electrocatalytic oxygen evolution reaction (OER) performance is a significant but challenging topic. Herein, a facile CO2-assisted strategy...

Construction of Ni(CN)2 /NiSe2 Heterostructures by Stepwise Topochemical Pathways for Efficient Electrocatalytic Oxygen Evolution

Exploiting effective electrocatalysts based on elaborate heterostructures for the oxygen evolution reaction (OER) has been considered as a promising strategy for boosting water splitting efficiency to produce the clean energy-hydrogen. However, constructing catalytically active heterostructures with novel composition and architecture remains poorly developed due to the synthetic challenge. In this work, it is demonstrated that unique Ni(CN)₂ /NiSe₂ heterostructures, composed of single-crystalline Ni(CN)₂ nanoplates surrounded by crystallographically aligned NiSe₂ nanosatellites, can be created from nickel-based Hofmann-type coordination polymers through stepwise topochemical pathways. When employed as the OER electrocatalyst, the Ni(CN)₂ /NiSe₂ heterostructures show enhanced performance, which could be attributed to optimized geometric and electronic structures of the catalytic sites endowed by the synergy between the two components. This work demonstrates a rational synthetic route for creating a novel Ni-based OER electrocatalyst that possesses nanoscale heterostructure, whose composition, spatial organization, and interface configuration can be finely manipulated.

Metal and metal oxide amorphous nanomaterials towards electrochemical applications

Amorphous nanomaterials have aroused extensive interest due to their unique properties. Their performance is highly related with their distinct atomic arrangements, which have no long-range order but possess short- to medium-range order. Herein, an overview of state-of-the-art synthesis methods of amorphous nanomaterials, structural characteristics and their electrochemical properties is presented. Advanced characterization methods for analyzing and proving the local order of amorphous structures, such as X-ray absorption fine structure spectroscopy, atomic electron tomography and nanobeam electron diffraction, are introduced. Various synthesis strategies for amorphous nanomaterials are covered, especially the salt-assisted metal organic decomposition method to prepare ultrathin amorphous nanosheets. Furthermore, the design and structure-activity relationship of amorphous nanomaterials towards electrochemical applications, including electrocatalysts and battery anode/cathode materials, is discussed.

Designing Self-Supported Electrocatalysts for Electrochemical Water Splitting: Surface/Interface Engineering toward Enhanced Electrocatalytic Performance

Electrochemical water splitting is regarded as the most attractive technique to store renewable electricity in the form of hydrogen fuel. However, the corresponding anodic oxygen evolution reaction (OER) and cathodic hydrogen evolution reaction (HER) remain challenging, which exhibit complex reactions and sluggish kinetic behaviors at the triple-phase interface. Material surface and interface engineering provide a feasible approach to improve catalytic activity. Besides, self-supported electrocatalysts have been proven to be highly efficient toward water splitting, because of the regulated catalyst/substrate interface. In this Review, the state-of-the-art achievements in self-supported electrocatalyst for HER/OER have demonstrated the feasibility of surface and interface engineering strategies to boost performance. The six key effective surface/interface engineering approaches for rational catalysts design are systematically reviewed, including defect engineering, morphology engineering, crystallographic tailoring, heterostructure design, catalyst/substrate interface engineering, and catalyst/electrolyte interface regulation. Finally, the challenges and opportunities on the valuable directions are proposed to inspire future investigation of highly active and durable HER/OER electrocatalysts.

Enhanced activity promoted by amorphous metal oxyhydroxides on CeO2 for alkaline oxygen evolution reaction

Herein, we demonstrate a direct growth of amorphous metal oxyhydroxide (AMO) attached on CeO₂ by a galvanic replacement mechanism as advanced oxygen evolution reaction (OER) catalyst. In this unique structure, the CeO₂ substrate not only offers high specific surface area for the formation of AMO, but also provides high conductivity, guaranteeing the promoted electron transfer for the catalytic reaction. In addition, the AMO on the surface of the CeO₂ exposes abundant active sites for the OER. Benefiting from the above advantages, the as-prepared AMO@CeO₂ supported on nickel foam (AMO@CeO₂/NF) exhibits excellent OER performance with low overpotential of 261 mV at 10 mA cm^-2, high turnover frequency of 0.07 s^-1 at 20 mA cm^-2 and superior stability in 1.0 M KOH.

Assembling and Regulating of Transition Metal-Based Heterophase Vanadates as Efficient Oxygen Evolution Catalysts

Developing efficient, durable, and low-cost earth-abundant elements-based oxygen evolution reaction (OER) catalysts by rapid and scalable strategies is of great importance for future sustainable electrochemical hydrogen production. The earth-abundant high-valency metals, especially vanadium, can modulate the electronic structure of 3d metal oxides and oxyhydroxides and offer the active sites near-optimal adsorption energies for OER intermediates. Here, the authors propose a facile assembling and regulating strategy to controllably synthesize a serial of transition metal (CoFe, NiFe, and NiCo)-based vanadates for efficient OER catalysis. By tuning the reaction concentrations, NiFe-based vanadates with different crystallinities can be facilely regulated, where the catalyst with moderate heterophase (mixed crystalline and amorphous structures) shows the best OER catalytic activity in terms of low overpotential (267 mV at the current density of 10 mA cm^-2 ), low Tafel slope (38 mV per decade), and excellent long-term durability in alkaline electrolyte, exceeding its noble metal-based counterparts (RuO₂ ) and most current existing OER catalysts. This work not only reports a facile and controllable method to synthesize a series of vanadates-based catalysts with heterophase nanostructures for high-performance OER catalysis, but also may expand the scope of designing cost-effective transition metal-based electrocatalysts for water splitting.

Amorphous nickel tungstate films prepared by SILAR method for electrocatalytic oxygen evolution reaction

Development of electrocatalyst using facile way from non-noble metal compounds with high efficiency for effective water electrolysis is highly demanding for production of hydrogen energy. Nickel based electrocatalysts were currently developed for electrochemical water oxidation in alkaline pH. Herein, amorphous nickel tungstate (NiWO₄) was synthesized using the facile successive ionic layer adsorption and reaction method. The films were characterized by X-ray diffraction, Raman spectroscopy, Fourier transfer infrared spectroscopy, scanning electron microscopy, X-ray photoelectron spectroscopy, and transmission electron microscopy techniques. The electrochemical analysis showed 315 mV of overpotential at 100 mA cm^-2 with lowest Tafel slope of 32 mV dec^-1 for oxygen evolution reaction (OER) making films of NiWO₄ compatible towards electrocatalysis of water in alkaline media. The chronopotentiometry measurements at 100 mA cm^-2 over 24 h showed 97% retention of OER activity. The electrochemical active surface area (ECSA) of NW120 film was 25.5 cm^-2.

Catalytic Activity of High-Surface-Area Amorphous MgO Obtained from Upsalite

The first aim of the research was to synthesize a pure Upsalite, which is an amorphous form of MgCO3, by modifying a procedure described in the literature, so that it would be the precursor of a high-surface, amorphous magnesium oxide. The results indicate that within the studied reaction conditions, the type of alcohol used as the reactant has the most pronounced effect on the yield of reaction. From the two alcohols that led to the highest yield of Upsalite, methanol gave a substantially larger surface area (794 vs. 191 m2 g−1). The optimized synthesis conditions of Upsalite were used to obtain MgO via thermolysis, whose activity in the transfer hydrogenation reaction (THR) from ethanol, 2-propanol and 2-pentanol to various carbonyl compounds was determined. The optimal conditions for the thermolysis were as follows: vacuum, T = 673 K as the final temperature, and a heating rate of 2 deg min−1. The high-surface, amorphous magnesia (SBET = 488 m2 g−1) was found to be a very selective catalyst to 4-t-butylcyclohexanone in THR, which led to a diastereoselectivity of over 94% to the E-isomer of 4-t-butylcyclohexanol for more than 3 h, with conversions of up to 97% with either 2-propanol or 2-pentanol as the hydrogen donor. In the case of acrolein and 2-n-propylacrolein being used as the hydrogen acceptors, the unsaturated alcohol (UOL) was the main product of the reaction, with higher UOL yields noted for ethanol than 2-propanol.

Bimetallic-MOF-Derived Amorphous Zinc/Cobalt-Iron-Based Hollow Nanowall Arrays via Ion Exchange for Highly Efficient Oxygen Evolution

Oxygen evolution reaction (OER) is critical for optimizing renewable energy systems, including metal-air batteries and water electrolysis. One major challenge for OER is to develop durable and cost-effective electrocatalysts with high catalytic performance. Herein, a controllable ion-exchange method to synthesize amorphous zinc/cobalt-iron hydroxide-based hollow nanowall arrays (A-Zn/Co-Fe HNAs) derived from bimetallic metal-organic frameworks (MOFs) on carbon cloth is reported. The amorphous characteristic enables the presented materials with more electrocatalytic sites and short diffusion paths for rapid access to the electrolyte, achieving efficient charge transfer for OER. The optimized nanostructure of A-Zn/Co-Fe HNAs via tuning the amount of iron sulfate in the reaction solution delivers a low overpotential of 226 mV to reach a current density of 10 mA cm^-2 with a small Tafel slope of 37.81 mV dec^-1 while exhibiting high durability at varied current densities over 80 h. The remarkable electrochemical performance can be attributed to the synergistic effect from chemical elements of Zn, Co-Fe, and a robust hollow structure. This simple method of fabricating bimetallic-MOF-derived amorphous Zn/Co-Fe HNAs on carbon cloth can be applied as a practical platform for other OER electrocatalysts.

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.

Rejuvenating the Geometric Electrocatalytic OER Performance of Crystalline Co3 O4 by Microstructure Engineering with Sulfate

Despite significant research on its electrocatalytic OER activity, the geometric performance of Co₃ O₄ has remained unsatisfactory compared to relatively amorphous Co-based materials. In particular, the activity of Co₃ O₄ prepared through annealing always gets inferior compared to its amorphous precursor. This demands the development of synthetic techniques to prepare Co₃ O₄ with superior activity as the unpredictable crystal structure of the amorphous materials makes it difficult to understand their structure-activity relationships despite higher geometric activity. In this article, we have shown that incorporation of sulfate in pre-annealed materials plays a pivotal role in boosting the OER activity of annealed Co₃ O₄ irrespective of the pre-annealed phase. In contrast to commonly used nitrate or carbonate that leaves the structure upon annealing and renders the resulting Co₃ O₄ with poor activity, sulfate remains in the annealed structure due to its thermal stability and causes a dramatic enhancement in the geometric electrocatalytic OER activity of resulting Co₃ O₄ compared to the pre-annealed phase. This was due to the "pore-alteration ability" and "crystallization hindrance effect" of sulfate ions that significantly alter the microstructure of the resulting Co₃ O₄ during annealing process by dramatically improving the surface area, pore size, and pore volume. Moreover, sulfate incorporation provided structures with considerably higher mesoporosity that is known to be conducive for reactant and product diffusion within the network. The improved textural properties led to better exposure of the catalytic centres to the electrolyte leading to higher geometric OER activity despite identical intrinsic activity of both sulfate free and incorporated Co₃ O₄ as confirmed from their specific activities. Further, the Co₃ O₄ synthesized by annealing sulfate incorporated precursor was found to be rich with oxygen defects that are known to increase the potency of a material towards electrocatalytic OER. The sulfate ions also etched out in the electrolyte during electrocatalysis leading to complete unblocking of the pores thereby helping in sustaining the high geometric OER activity. To our knowledge, this is the first report where the geometric electrocatalytic OER activity of an annealed Co₃ O₄ is significantly better compared to its pre-annealed phase and is in fact comparable to the activity of amorphous Co-hydroxide based compounds.

Ultrathin amorphous iron-doped cobalt-molybdenum hydroxide nanosheets for advanced oxygen evolution reactions

Developing the highly efficient and low-cost electrocatalysts for the oxygen evolution reactions (OERs), as vital half reactions of water splitting, is crucial for renewable energy technology. The electrocatalysts based on multi-component and hierarchically structured non-noble metal hydr(oxy)oxide materials are of great prospects. Herein, we report an efficient strategy at low temperatures for synthesizing amorphous iron-doped cobalt-molybdenum ultrathin hydroxide (Fe-CoMo UH) nanosheets. Benefiting from the ultrathin amorphous structure and multi-metal coordination, Fe-CoMo UH nanosheets exhibit outstanding performance for OERs with a low overpotential of 245 mV at 10 mA cm^-2, a small Tafel slope of 37 mV dec^-1 and an excellent stability for 90 h. The mass activity of Fe-CoMo UH is higher than that of commercial Ir/C and most of the transition metal hydroxide catalysts. This work provides a feasible consideration for the construction of promising efficient non-noble metal catalysts.

Amorphous Ta2O5-supported Ru as an efficient electrocatalyst for selective hydrogenation of cinnamaldehyde with water as the hydrogen source

Amorphous Ta2O5-supported Ru as an efficient electrocatalyst for electrocatalytic selective hydrogenation of cinnamaldehyde.

Three-Dimensional Cobalt Hydroxide Hollow Cube/Vertical Nanosheets with High Desalination Capacity and Long-Term Performance Stability

Faradaic electrode materials have significantly improved the performance of membrane capacitive deionization, which offers an opportunity to produce freshwater from seawater or brackish water in an energy-efficient way. However, Faradaic materials hold the drawbacks of slow desalination rate due to the intrinsic low ion diffusion kinetics and inferior stability arising from the volume expansion during ion intercalation, impeding the engineering application of capacitive deionization. Herein, a pseudocapacitive material with hollow architecture was prepared via template-etching method, namely, cuboid cobalt hydroxide, with fast desalination rate (3.3 mg (NaCl)·g^-1 (h-Co(OH)₂)·min^-1 at 100 mA·g^-1) and outstanding stability (90% capacity retention after 100 cycles). The hollow structure enables swift ion transport inside the material and keeps the electrode intact by alleviating the stress induced from volume expansion during the ion capture process, which is corroborated well by in situ electrochemical dilatometry and finite element simulation. Additionally, benefiting from the elimination of unreacted bulk material and vertical cobalt hydroxide nanosheets on the exterior surface, the synthesized material provides a high desalination capacity (117 ± 6 mg (NaCl)·g^-1 (h-Co(OH)₂) at 30 mA·g^-1). This work provides a new strategy, constructing microscale hollow faradic configuration, to further boost the desalination performance of Faradaic materials.

Integration of Oxygen-Vacancy-Rich NiFe-Layered Double Hydroxide onto Silicon as Photoanode for Enhanced Photoelectrochemical Water Oxidation

Photoelectrochemical (PEC) water splitting has the potential to efficiently convert intermittent solar energy into storable hydrogen fuel. However, poor charge separation and transfer ability as well as sluggish surface oxygen evolution reaction (OER) kinetics of the photoelectrode severely hinder the advance in PEC performance. Herein, a facile electrodeposition method was used to integrate Mo-doped NiFe-layered double hydroxide onto a NiOx /Ni-protected Si photoanode for enhanced PEC water oxidation. Mo doping contributed to an increased amount of oxygen vacancies, whereas a dynamic surface self-reconstruction was induced by Mo leaching under PEC OER conditions. This led to enhanced PEC performance with an onset potential of 0.87 V vs. reversible hydrogen electrode (RHE), a photocurrent density of 39.3 mA cm-2 at 1.23 V vs. RHE, a fill factor of 0.38, and a solar-to-oxygen conversion efficiency of 5.3 %, along with a stability of 130 h continuous PEC reaction. The performance was superior to that of the undoped NiFe-LDH/NiOx /Ni/Si (4.3 %), which was attributed to the elevated interface charge separation, fast charge transfer, and accelerated OER kinetics.