Selenide-Based Electrocatalysts and Scaffolds for Water Oxidation Applications

Flexible 3D carbon cloth as a high-performing electrode for energy storage and conversion

High-performance energy storage and conversion devices with high energy density, power density and long-term cycling life are of great importance in current consumer electronics, portable electronics and electric vehicles. Carbon materials have been widely investigated and utilized in various energy storage and conversion devices due to their excellent conductivity, mechanical and chemical stability, and low cost. Abundant excellent reviews have summarized the most recent progress and future outlooks for most of the current prime carbon materials used in energy storage and conversion devices, such as carbon nanotubes, fullerene, graphene, porous carbon and carbon fibers. However, the significance of three-dimensional (3D) commercial carbon cloth (CC), one of the key carbon materials with outstanding mechanical stability, high conductivity and flexibility, in the energy storage and conversion field, especially in wearable electronics and flexible devices, has not been systematically summarized yet. In this review article, we present a careful investigation of flexible CC in the energy storage and conversion field. We first give a general introduction to the common properties of CC and the roles it has played in energy storage and conversion systems. Then, we meticulously investigate the crucial role of CC in typical electrochemical energy storage systems, including lithium-ion batteries, sodium-ion batteries, lithium-sulfur batteries and supercapacitors. Following a description of the wide application potential of CC in electrocatalytic hydrogen evolution, oxygen evolution/reduction, full-water splitting, etc., we will give a brief introduction to the application of CC in the areas of photocatalytically and photoelectrochemically induced solar energy conversion and storage. The review will end with a brief summary of the typical superiorities that CC has in current energy conversion and storage systems, as well as providing some perspectives and outlooks on its future applications in the field. Our main interest will be focused on CC-based flexible devices due to the inherent superiority of CC and the increasing demand for flexible and wearable electronics.

Study of the Phase-Evolution Mechanism of an Fe-Se System at the Nanoscale: Optimization of Synthesis Conditions for the Isolation of Pure Phases and Their Controlled Growth

The iron selenide (Fe-Se) family of nanoparticles (Fe_xSe_y-where x/y ranges from 1:2 to 1:1) has been fabricated by a thermal decomposition method. The control over solution chemistry has been developed by intensively investigating the effect of reaction parameters by means of wide-angle X-ray scattering, leading to the rich insights into the phase-evolution mechanism of the Fe-Se system. The phase transformation followed the FeSe₂ → Fe₃Se₄ → Fe₇Se₈ → FeSe sequence in the temperature range of 110-300 °C. The deep mechanistic insight helped in the identification of optimized conditions needed to crystallize the individual phase of the Fe-Se system as well as control of the morphology, crystalline phase purity, and thermal stability of the obtained Fe-Se nanoparticles.

Flexible Co-Mo-N/Au Electrodes with a Hierarchical Nanoporous Architecture as Highly Efficient Electrocatalysts for Oxygen Evolution Reaction

Designing highly active and robust electrocatalysts for oxygen evolution reaction (OER) is crucial for many renewable energy storage and conversion devices. Here, self-supported monolithic hybrid electrodes that are composed of bimetallic cobalt-molybdenum nitride nanosheets vertically aligned on 3D and bicontinuous nanoporous gold (NP Au/CoMoN_x ) are reported as highly efficient electrocatalysts to boost the sluggish reaction kinetics of water oxidation in alkaline media. By virtue of the constituent CoMoN_x nanosheets having large accessible CoMoO_x surface with remarkably enhanced electrocatalytic activity and the nanoporous Au skeleton facilitating electron transfer and mass transport, the NP Au/CoMoN_x electrode exhibits superior OER electrocatalysis in 1 m KOH, with low onset overpotential (166 mV) and Tafel slope (46 mV dec^-1 ). It only takes a low overpotential of 370 mV to reach ultrahigh current density of 1156 mA cm^-2 , ≈140-fold higher than free CoMoN_x nanosheets. The electrocatalytic performance makes it an attractive candidate as the OER catalyst in the water electrolysis.

A bifunctional nanoporous Ni-Co-Se electrocatalyst with a superaerophobic surface for water and hydrazine oxidation

The sluggish kinetics of the oxygen evolution reaction (OER) has severely hindered the energetic convenience of water splitting. Thus, developing a highly efficient catalyst for the OER and replacing the OER with hydrazine oxidation (HzOR) are effective strategies for water electrolysis to achieve sustainable hydrogen production. Herein, bifunctional nanosheet arrays Ni0.6Co0.4Se with a porous structure were fabricated on Ni foam (NF) by the bubble dynamic template method during electrodeposition. Compared with CoSe2 and NiSe2, Ni0.6Co0.4Se exhibits excellent electrocatalytic performance for both the OER and HzOR. A low overpotential of only 249 mV is required to drive 10 mA cm-2, and a retention rate of nearly 100% after 24 h at 10 mA cm-2 is observed for Ni0.6Co0.4Se towards the OER. By substituting the OER by HzOR, an extremely high current density of 300 mA cm-2 at 0.4 V vs. RHE and a retention rate of 86.8% at 200 mA cm-2 after 12 h can be achieved. Interestingly, the mechanistic reason for the enhanced catalytic ability of Ni0.6Co0.4Se was studied, which is associated with the synergistic effects of Ni and Co, large ECSA, high electrical conductivity and most importantly the superaerophobic nature induced by the porous structure of Ni0.6Co0.4Se. The non-noble metal bifunctional electrocatalyst demonstrates a promising potential for application in both the OER and HzOR.

Developing Indium-based Ternary Spinel Selenides for Efficient Solid Flexible Zn-Air Batteries and Water Splitting

Exploring economic, efficient, and corrosion-resistant oxygen electrocatalysts contributes to further development of zinc-air batteries and overall water splitting. Herein, a novel ternary spinel CoIn₂Se₄ nanomaterial, with Co²⁺ and In³⁺ occupying the tetrahedral and octahedral sites of the crystalline structure, has been fabricated using a facile and environment-friendly method. Moreover, CoIn₂Se₄ nanosheets outperform pristine CoSe₂ and In₂Se₃ in catalyzing both oxygen evolution reaction (an overpotential of 315 mV at 10 mA cm^-2) and oxygen reduction reaction (an onset overpotential of 0.88 V). The reduced charge resistance and increased active site exposure ratio contribute to the superior performance of CoIn₂Se₄. Moreover, density of theory (density functional theory-DFT) calculations suggest a significantly reduced reaction energy barrier after introducing indium into the spinel, and therefore the reaction kinetics are facilitated. Based on the advantages described above, CoIn₂Se₄ is used as the air cathode for a solid flexible zinc-air battery (FZAB). The system displays an efficient performance that outperforms the Pt@Ir/C catalyst: a significantly enhanced specific capacity of 733 mAh g_Zn^-1, a high energy density of 931 Wh kg^-1, and an excellent flexibility with long cycle life performance (over 400 cycles). The CoIn₂Se₄-based two-series-connected FZABs successfully power light-emitting diode (LED) screens for over 10 h. Meanwhile, CoIn₂Se₄ also shows a significantly enhanced hydrogen evolution reaction (HER) performance than other samples. Finally, the CoIn₂Se₄-based electrolyzer shows an efficient overall water-splitting performance with high stability. This work demonstrates that the indium-based ternary selenides show promising applications in developing renewable energy and water-splitting devices.

In Situ Crystallization of Active NiOOH/CoOOH Heterostructures with Hydroxide Ion Adsorption Sites on Velutipes-like CoSe/NiSe Nanorods as Catalysts for Oxygen Evolution and Cocatalysts for Methanol Oxidation

Hydroxide ion (OH^-) adsorption process is critical for accelerating the half-reactions of both metal-air batteries and direct methanol fuel cells in alkaline media. This study designs a rational catalyst/cocatalyst by constructing the readily available OH^-adsorption sites to boost oxygen evolution reaction (OER) and methanol oxidation reaction (MOR). Cobalt selenide-coated nickel selenide nanorods are in situ grown on nickel foam to obtain CoSe/NiSe-nrs/NF via a one-pot solvothermal synthesis route. CoSe-0.2/NiSe-nrs/NF (Co/Ni molar ratio of 0.26) exhibits an excellent OER activity(an overpotential of 310 mV at 100 mA cm^-2 and a Tafel slope of 58.3 mV dec^-1). The differently oriented CoSe/NiSe-nrs with a velutipes-like structure and metallic property provide a promising electrical conductivity for charge transfer. In situ X-ray diffraction tests verify the crystallization of active β-NiOOH during OER, and the crystallized NiOOH/CoOOH contributes to the excellent OER cycling stability in alkaline media. Synergistic effects between CoSe and NiSe-nrs/NF can balance the formation of NiOOH/CoOOH heterostructures to govern the exposure of available active sites. NiOOH/CoOOH as a highly active component can energetically adsorb OH^- to promote OER. CoSe/NiSe-nrs/NFs as a low Pt-loading (0.5 wt%) support offer the mutually beneficial interactions for promoting cocatalytic and CO_ads (poisonous intermediate) co-oxidation activities toward MOR. The electrochemically active surface area and mass activity of Pt/CoSe-0.2/NiSe-nrs/NF are 85 m² g_pt^-1 and 1437.1 mA mg_pt^-1, respectively, which are much higher than those of commercial Pt/C (10.0 wt%). OH^- absorbed on the NiOOH/CoOOH structure eliminates CO_ads on the Pt surface via bifunctional mechanisms to improve the MOR activity. This study provides a promising reference for designing the versatile catalysts for energy conversion.

Self-Supported Transition-Metal-Based Electrocatalysts for Hydrogen and Oxygen Evolution

Electrochemical water splitting is a promising technology for sustainable conversion, storage, and transport of hydrogen energy. Searching for earth-abundant hydrogen/oxygen evolution reaction (HER/OER) electrocatalysts with high activity and durability to replace noble-metal-based catalysts plays paramount importance in the scalable application of water electrolysis. A freestanding electrode architecture is highly attractive as compared to the conventional coated powdery form because of enhanced kinetics and stability. Herein, recent progress in developing transition-metal-based HER/OER electrocatalytic materials is reviewed with selected examples of chalcogenides, phosphides, carbides, nitrides, alloys, phosphates, oxides, hydroxides, and oxyhydroxides. Focusing on self-supported electrodes, the latest advances in their structural design, controllable synthesis, mechanistic understanding, and strategies for performance enhancement are presented. Remaining challenges and future perspectives for the further development of self-supported electrocatalysts are also discussed.

Fe-doped Co3O4 polycrystalline nanosheets as a binder-free bifunctional cathode for robust and efficient zinc–air batteries

Self-standing Fe-Co₃O₄@CC bifunctional electrocatalysts enabled high-performance Zn–air batteries with a power density of 268.6 mW cm⁻² and superior cycling stability.

Electrocatalytic activity sites for the oxygen evolution reaction on binary cobalt and nickel phosphides

The catalytic activity of CoNiP originates from the synergistic effect of CoOOH and NiOOH, rather than exclusively from CoOOH or NiOOH.

Multifunctional Electrocatalysis on a Porous N-Doped NiCo2O4@C Nanonetwork

Developing a multifunctional electrocatalyst with eminent activity, strong durability, and cheapness for the hydrogen/oxygen evolution reaction (HER/OER) and oxygen reduction reaction (ORR) is critical to overall water splitting and regenerative fuel cells. Herein, a nitrogen-doped nanonetwork assembled by porous and defective NiCo₂O₄@C nanowires grown on nickel foam (N-NiCo₂O₄@C@NF) is crafted via biomimetic mineralization and following carbonization of phase-transited lysozyme (PTL)-coupled NiCo₂O₄. The as-obtained N-NiCo₂O₄@C@NF electrocatalysts exhibit an exceptional catalytic activity with ultralow overpotentials for the HER (42 mV) and OER (242 mV) to afford 10 mA cm^-2 while maintaining good stability in alkaline media. Meanwhile, the N-NiCo2O4@C electrocatalysts presents a superior catalytic activity for ORR and a favorable four-electron pathway. The unprecedented catalytic performance arises from a highly porous structure and abundant defects and synergistic effects of components. This work may offer a new possibility in the exploration of multifunctional electrocatalysts for various energy-related electrocatalytic reactions.

MOF-derived cobalt-nickel phosphide nanoboxes as electrocatalysts for the hydrogen evolution reaction

The development of high-efficiency nonprecious electrocatalysts based on inexpensive and Earth abundant elements is of great significance for renewable energy technologies. Group VIII transition metal phosphides (TMPs) gradually stand out due to their intriguing properties including low resistance and superior catalytic activity and stability. Herein, we adopt a unique MOF-derived strategy to synthesize transition metal phosphide nanoboxes which can be employed as electrocatalysts for the hydrogen evolution reaction. During this process, we converted a Co-MOF to a CoNi-MOF by ion exchange and low-temperature phosphating to achieve CoNiP nanoboxes. The CoNiP nanoboxes can reach a current density of 10 mA cm^-2 at a low overpotential of 138 mV with a small Tafel slope of 65 mV dec^-1.

Nickel-molybdenum nitride nanoplate electrocatalysts for concurrent electrolytic hydrogen and formate productions

Hydrogen production by electrocatalytic water splitting is an efficient and economical technology, however, is severely impeded by the kinetic-sluggish and low value-added anodic oxygen evolution reaction. Here we report the nickel-molybdenum-nitride nanoplates loaded on carbon fiber cloth (Ni-Mo-N/CFC), for the concurrent electrolytic productions of high-purity hydrogen at the cathode and value-added formate at the anode in low-cost alkaline glycerol solutions. Especially, when equipped with Ni-Mo-N/CFC at both anode and cathode, the established electrolyzer requires as low as 1.36 V of cell voltage to achieve 10 mA cm^-2, which is 260 mV lower than that in alkaline aqueous solution. Moreover, high Faraday efficiencies of 99.7% for H₂ evolution and 95.0% for formate production have been obtained. Based on the excellent electrochemical performances of Ni-Mo-N/CFC, electrolytic H₂ and formate productions from the alkaline glycerol solutions are an energy-efficient and promising technology for the renewable and clean energy supply in the future.

Chiral Induction and Remote Chiral Communication in Quinoline Oligoamide Foldamers for Determination of Enantiomeric Excess and Absolute Configuration of Chiral Amines and Their Derivatives

Two pentameric foldamers, Q5 and Q5C-S, containing a C-F bond were synthesized based on quinoline oligamide foldamers for the measurement of enantiomeric excess and for the determination of absolute configuration of chiral amines, diamines, amino alcohols, and α-amino acid esters. Chiral induction of Q5 was triggered in situ when the chiral analytes reacted with the C-F bond in Q5 by a N-nucleophilic substitution reaction, leading to a linear correlation between the CD amplitude at the region of quinoline chromophores and the ee values of the chiral analytes, which can be used for the ee determination of chiral analytes. Furthermore, the CD intensity of Q5C-S containing a chiral motif at its C-terminus enhances via remote, favorable chiral communication when the chiral induction was triggered in situ by chiral analytes at the N-terminus matches the original helicity of Q5C-S, but decreases via remote, conflicted chiral communication when the chiral induction is triggered in situ by chiral molecules at the N-terminus mismatches the original one. The system can thus be used for determination of the absolute configuration of chiral analytes, given that the chirality of the chiral motif at the C-terminus of Q5C-S is known.

CeOx-Decorated Hierarchical NiCo2S4 Hollow Nanotubes Arrays for Enhanced Oxygen Evolution Reaction Electrocatalysis

Developing hollow self-supported nanotube arrays with hierarchical microporous and abundant multiactive sites shows great promise for oxygen evolution reaction (OER) electrocatalysis. Herein, a facile and low-cost strategy of NiCo₂S₄ hollow nanotubes arrays decorated with CeO_x nanoparticles (NPs) assembled on a flexible support carbon cloth (CC) for enhanced OER performance is reported. The obtained hierarchical nanoarrays CeO_x/NiCo₂S₄/CC exhibited excellent activity toward OER with an overpotential (270 mV) at 10 mA cm^-2, relatively weak Tafel slope, and distinguished durability. CeO_x/NiCo₂S₄/CC nanoarrays not only provide fast electronic transmission and well-defined connection to the substrate but also defective sites and electron transfer by the introduction of CeO_x NPs. This new strategy was offered to construct low-cost and effectively hierarchical structural electrocatalysts containing rare-earth species.

Surface-Modified Hollow Ternary NiCo2Px Catalysts for Efficient Electrochemical Water Splitting and Energy Storage

Generally, a cost-effective electrocatalytic process that offers an efficient electrochemical energy conversion and storage necessitates a rational design and selection of structure as well as composition of active catalytic centers. Herein, we achieved an unprecedented surface morphology and shape tuning to obtain hollow NiCo₂P_x with a continuum of active sharp edges (spiked) on a hollow spherical surface by means of facile hydrothermal treatments. The highly exposed, branched spike-covered hollow structure of NiCo₂P_x shows remarkable performance enhancement for hydrogen evolution reaction and oxygen evolution reaction in a wide range of Ph solutions. This catalytic performance was utilized to assemble a water electrolyzer working in an alkaline environment. In particular, this electrolyzer only requires an output voltage of 1.62 V to deliver a current density of 10 mA cm^-2 and shows almost no decrease in this value even after a continuous run for 50 h. The new surface-engineered NiCo₂P_x establishes to be highly active, cost-effective, and robust toward electrochemical energy conversion. Additionally, the charge storage capabilities of spike-covered hollow NiCo₂P_x structures is also investigated, and it shows a specific capacitance of 682 and 608 F g^-1 at a current density of 1 A g^-1 with excellent rate capacitance retention. Thus, the importance of surface engineering of nanocrystalline morphologies in design toward the development of a multifunctional electrocatalyst for efficient water splitting and charge storage applications is demonstrated.

3D Co-Ni Nanocone Array Shielded with Conducting Amorphous Carbon Used as Fused, Separable, and Stable Mimicking Peroxidases for RGB-Color Intensiometric pH Indication

We demonstrate an in situ synthesis for preparing a carbon-shielded three-dimensional Co-Ni nanocone array on Ni foam (CoNi NCs/NF@C) via the solvothermal and thermal annealing processes. It is found that the easily separable CoNi NCs/NF@C possesses high peroxidase/catalase dual-mimic activity and good catalytic stability. The fusion of the amorphous carbon sheath with the Co-Ni nanocones (1) effectively improves interfacial electron transfer and catalytic stability of the Co-Ni nanocone array because of the excellent conductivity of amorphous carbon and (2) protects the Co-Ni nanocone array in the catalysis process from exposing to the harsh chemical environment, dramatically escaping the catalytic activity loss of Co-Ni (hydro)oxide. Interestingly, when CoNi NCs/NF@C mimics peroxidase using 3,3',5,5'-tetramethylbenzidine (TMB) as a substrate, the color of the TMB-H₂O₂-CoNi NC/NF@C system changes at different pH values. Based on this property, a facile strategy was developed for semiquantitative and qualitative determination of pH using the Eyedropper function in Microsoft's PowerPoint software, where the RGB (red, green, and blue) value of the sample can be conveniently measured by using a standard colorimetric card without the requirement of complicated instrumentation. Moreover, the relationship between the color of the reaction system and the pH was investigated, which was demonstrated by the total Euclidean distance (ED), that is, the square root of the sum of the squares of the ΔRGB values. The ED change of the reaction system is reversible and occurs in the pH range from 0.64 to 8.4, which is useful for indicating the pH of strongly acidic environments. The colorimetric system exhibits a linear range from 0.64 to 2.38 and 2.5 to 6.5. A colorimetric card was designed based on the color changes of this system as a function of pH values. This work provides a colorimetric assay method for the simple, rapid, and visual indication of pH which can be used to understand the biological processes in physiology and pathology fields.

Valence Engineering via Dual-Cation and Boron Doping in Pyrite Selenide for Highly Efficient Oxygen Evolution

Valence engineering has been proved an effective approach to modify the electronic property of a catalyst and boost its oxygen evolution reaction (OER) activity, while the limited number of elements restricts the structural diversity and the active sites. Also, the catalyst performance and stability are greatly limited by cationic dissolution, ripening, or crystal migration in a catalytic system. Here we employed a widely used technique to fabricate heteroepitaxial pyrite selenide through dual-cation substitution and a boron dopant to achieve better activity and stability. The overpotential of Ni-pyrite selenide catalyst is decreased from 543 mV to 279.8 mV at 10 mA cm^-2 with a Tafel slope from 161 to 59.5 mV dec^-1. Our theoretical calculations suggest both cation and boron doping can effectively optimize adsorption energy of OER intermediates, promote the charge transfer among the heteroatoms, and improve their OER property. This work underscores the importance of modulating surface electronic structure with the use of multiple elements and provides a general guidance on the minimization of activity loss with valence engineering.

NiFe Hydroxide Supported on Hierarchically Porous Nickel Mesh as a High-Performance Bifunctional Electrocatalyst for Water Splitting at Large Current Density

The preparation of efficient and low-cost bifunctional catalysts with superior stability for water splitting is a topic of significant current interest for hydrogen generation. A facile strategy has been developed to fabricate highly active electrodes with hierarchical porous structures by using a two-step electrodeposition method, in which NiFe layered double hydroxide is grown in situ on a three-dimensional hierarchical Ni mesh (NiFe/Ni/Ni). The as-prepared NiFe/Ni/Ni electrodes demonstrate remarkable structural stability with high surface areas, effective gas transportation, and fast electron transfer. Benefiting from the unique structure, the self-supported NiFe/Ni/Ni electrodes exhibit overpotentials of 190 mV and 300 mV for the oxygen evolution reaction (OER) at current densities of 10 and 500 mA cm^-2 , respectively. Furthermore, the self-supported NiFe/Ni/Ni electrodes also exhibit high performance in the hydrogen evolution reaction (HER) and excellent stability at a current density of 500 mA cm^-2 for both OER and HER. Remarkably, using NiFe/Ni/Ni as both the cathode and anode for alkaline water electrolysis, a current density of 500 mA cm^-2 is attained at a cell voltage of 1.96 V. Additionally, the water electrolyzer demonstrates superior stability even at a large current density (500 mA cm^-2 ) when subjected to high temperatures.

Enhanced photocatalytic H2 production under visible light on composite photocatalyst (CdS/NiSe nanorods) synthesized in aqueous solution

Cocatalysts play a critical role in the activity and stability of photocatalytic systems. Currently, efficient cocatalysts mainly comprise of expensive noble metals. Herein we report a composite photocatalyst consisting of CdS nanorods (NRs) and noble-metal-free cocatalyst NiSe, which efficiently enhances the hydrogen production activity of CdS NRs under visible light. NiSe was synthesized through a facile aqueous solution method and CdS/NiSe NRs composites were prepared by in situ deposition of NiSe on CdS NRs. This provides increased contact between cocatalyst and photosensitizer leading to enhanced electron transfer at the interface of NiSe and CdS. The current photocatalytic system gave the highest hydrogen evolution rate of 340 µmol h^-1 under optimal conditions. The enhanced stability of the system was observed for 30 h of irradiation resulting in 14 mmol of hydrogen evolution. The highest AQY of 12% was observed using the 420 nm monochromatic light. In addition, CdS/NiSe NRs showed significant higher H₂ evolution rate than that of 1.0 wt% loaded CdS/Pt NRs proving NiSe as highly efficient cocatalyst. Photoluminescence spectra and the photocurrent response were used to confirm the efficient charge transfer at the interface of NiSe and CdS nanorods. The work presented here demonstrates the successful use of an inexpensive, non-noble-metal cocatalyst for enhanced photocatalytic hydrogen production.

Hierarchical Cobalt-Doped Molybdenum-Nickel Nitride Nanowires as Multifunctional Electrocatalysts

Herein, we demonstrate hierarchically porous Co-doped MoNi nitride nanowires for multifunctional electrocatalysts. After the Co incorporation for water electrolysis and zinc-air systems, the active surface area is enhanced, whereas the charge-transfer and mass-transfer resistances are reduced significantly. Due to the dual modulation in the electric conductivity and active surface area induced by the Co-doping, the hierarchically porous trimetal nitrides show high activity and good stability for the hydrogen evolution reaction, oxygen evolution reaction, and oxygen reduction reaction. The two-electrode electrolyzer assembled by the bifunctional electrocatalysts can deliver 10 mA cm^-2 at a voltage of merely 1.57 V, compared to the best reported electrocatalysts. Meanwhile, two all-solid-state zinc-air batteries in series can power more than 50 red light-emitting diodes and the two-electrode electrolyzer catalyzed by the multifunctional electrocatalysts with excellent operation stability.

Promoting Electrocatalysis upon Aerogels

Electrocatalysis plays a prominent role in renewable energy conversion and storage, enabling a number of sustainable processes for future technologies. There are generally three strategies to improve the efficiency (or activity) of the electrocatalysts: i) increasing the intrinsic activity of the catalyst itself, ii) improving the exposure of active sites, and iii) accelerating mass transfer during catalysis (both reactants and products). These strategies are not mutually exclusive and can ideally be addressed simultaneously, leading to the largest improvements in activity. Aerogels, as featured by large surface area, high porosity, and self-supportability, provide a platform that matches all the aforementioned criteria for the design of efficient electrocatalysts. The field of aerogel synthesis has seen much progress in recent years, mainly thanks to the rapid development of nanotechnology. Employing precursors with different properties enables the resulting aerogel with targeted catalytic properties and improved performances. Here, the design strategies of aerogel catalysts are demonstrated, and their performance for several electrochemical reactions is reviewed. The common principles that govern electrocatalysis are further discussed for each category of reactions, thus serving as a guide to the development of future aerogel electrocatalysts.

Surface states in bulk single crystal of topological semimetal Co3Sn2S2 toward water oxidation

The band inversion in topological phase matters bring exotic physical properties such as the topologically protected surface states (TSS). They strongly influence the surface electronic structures of the materials and could serve as a good platform to gain insight into the surface reactions. Here we synthesized high-quality bulk single crystals of Co₃Sn₂S₂ that naturally hosts the band structure of a topological semimetal. This guarantees the existence of robust TSS from the Co atoms. Co₃Sn₂S₂ crystals expose their Kagome lattice that constructed by Co atoms and have high electrical conductivity. They serves as catalytic centers for oxygen evolution process (OER), making bonding and electron transfer more efficient due to the partially filled orbital. The bulk single crystal exhibits outstanding OER catalytic performance, although the surface area is much smaller than that of Co-based nanostructured catalysts. Our findings emphasize the importance of tailoring TSS for the rational design of high-activity electrocatalysts.

Recent Progress on Surface Reconstruction of Earth-Abundant Electrocatalysts for Water Oxidation

As one important electrode reaction in electrocatalytic and photoelectrochemical cells for renewable energy circulation, oxygen catalysis has attracted considerable research in developing efficient and cost-effective catalysts. Due to the inevitable formation of oxygenic intermediates on surface sites during the complex reaction steps, the surface structure dynamically evolves toward reaction-preferred active species. To date, transition metal compounds, here defined as TM-Xides, where "X" refers to typical nonmetal elements from group IIIA to VIA, including hydroxide as well, are reported as high-performance oxygen evolution reaction (OER) electrocatalysts. However, more studies observe at least exterior oxidation or amorphization of materials. Thus, whether the TM-Xides can be defined as OER catalysts deserves further discussion. This Review pays attention to recent progress on the surface reconstruction of TM-Xide OER electrocatalysts with an emphasis on the identification of the true active species for OER, and aims at disseminating the real contributors of OER performance, especially under long-duration electrocatalysis.

α-Ni(OH)2 Originated from Electro-Oxidation of NiSe2 Supported by Carbon Nanoarray on Carbon Cloth for Efficient Water Oxidation

Development of effective oxygen evolution reaction (OER) electrocatalysts has been intensively studied to improve water splitting efficiency and cost effectiveness in the last ten years. However, it is a big challenge to obtain highly efficient and durable OER electrocatalysts with overpotentials below 200 mV at 10 mA cm^-2 despite the efforts made to date. In this work, the successful synthesis of supersmall α-Ni(OH)₂ is reported through electro-oxidation of NiSe₂ loaded onto carbon nanoarrays. The obtained α-Ni(OH)₂ shows excellent activity and long-term stability for OER, with an overpotential of only 190 mV at the current density of 10 mA cm^-2 , which represents a highly efficient OER electrocatalyst. The excellent activity could be ascribed to the large electrochemical surface area provided by the carbon nanoarray, as well as the supersmall size (≈10 nm) of α-Ni(OH)₂ which possess a large number of active sites for the reaction. In addition, the phase evolution of α-Ni(OH)₂ from NiSe₂ during the electro-oxidation process was monitored with in situ X-ray absorption fine structure (XAFS) analysis.

Metal Organic Framework-Templated Synthesis of Bimetallic Selenides with Rich Phase Boundaries for Sodium-Ion Storage and Oxygen Evolution Reaction

Two-phase or multiphase compounds have been evidenced to exhibit good electrochemical performance for energy applications; however, the mechanism insights into these materials, especially the performance improvement by engineering the high-active phase boundaries in bimetallic compounds, remain to be seen. Here, we report a bimetallic selenide heterostructure (CoSe₂/ZnSe) and the fundamental mechanism behind their superior electrochemical performance. The charge redistribution at the phase boundaries of CoSe₂/ZnSe was experimentally and theoretically proven. Benefiting from the abundant phase boundaries, CoSe₂/ZnSe exerts low Na⁺ adsorption energy and fast diffusion kinetics for sodium-ion batteries and high activity for oxygen evolution reaction. As expected, excellent sodium storage capability, specifically a superb cyclic stability of up to 800 cycles for the Na₃V₂(PO₄)₃∥CoZn-Se full cell, and efficient water oxidation with a small overpotential of 320 mV to reach 10 mA cm^-2 were obtained. This work demonstrates the importance of phase boundaries in bimetallic compounds to boost the performance in various fields.

Evaluating DNA Derived and Hydrothermally Aided Cobalt Selenide Catalysts for Electrocatalytic Water Oxidation

Electrocatalysts with engaging oxygen evolution reaction (OER) activity with lesser overpotentials are highly desired to have increased cell efficiency. In this work, cobalt selenide catalysts were prepared utilizing both wet-chemical route (CoSe and CoSe-DNA) and hydrothermal route (Co_0.85Se-hyd). In wet-chemical route, cobalt selenide is prepared with DNA (CoSe-DNA) and without DNA (CoSe). The morphological results in the wet-chemical route had given a clear picture that, with the assistance of DNA, cobalt selenide had formed as nanochains with particle size below 5 nm, while it agglomerated in the absence of DNA. The morphology was nano networks in the hydrothermally assisted synthesis. These catalysts were analyzed for OER activity in 1 M KOH. The overpotentials required at a current density of 10 mA cm^-2 were 352, 382, and 383 mV for Co_0.85Se-hyd, CoSe, and CoSe-DNA catalysts, respectively. The Tafel slope value was lowest for Co_0.85Se-hyd (65 mV/dec) compared to CoSe-DNA (71 mV/dec) and CoSe (80 mV/dec). The chronoamperometry test was studied for 24 h at a potential of 394 mV for Co_0.85Se-hyd and was found to be stable with a smaller decrease in activity. From the OER study, it is clear that Co_0.85Se was found to be superior to others. This kind of related study can be useful to design the catalyst with increased efficiency by varying the method of preparation.

Core-Shell Prussian Blue Analogs with Compositional Heterogeneity and Open Cages for Oxygen Evolution Reaction

Here, a reduction-cation exchange (RCE) strategy is proposed for synthesizing Fe-Co based bimetallic Prussian blue analogs (PBAs) with heterogeneous composition distribution and open cage nanocage architecture. Specially, bivalent cobalt is introduced into a potassium ferricyanide solution containing hydrochloric acid and polyvinyl pyrrolidone. The uniform PBAs with opened cages are formed tardily after hydrothermal reaction. Time-dependent evolution characterization on composition elucidating the RCE mechanism is based on the sequential reduction of ferric iron and cation exchange reaction between divalent iron and cobalt. The PBA structures are confirmed by electron tomography technology, and the heterogeneous element distribution is verified by energy-dispersive X-ray spectroscopy elemental analysis, leading to the formation of core-shell PBAs with compositional heterogeneity (Fe rich shell and Co rich core) and open cage architecture. When the PBA catalysts are used to boost the oxygen evolution reaction (OER), superior OER activity and long-term stability (low overpotential of 271 mV at 10 mA cm-2 and ≈5.3% potential increase for 24 h) are achieved, which is attributed to the unique compositional and structural properties as well as high special surface areas (576.2 m2 g-1). The strategies offer insights for developing PBAs with compositional and structural multiplicity, which encourages more practical catalytic applications.

Rational construction of self-supported triangle-like MOF-derived hollow (Ni,Co)Se2 arrays for electrocatalysis and supercapacitors

In this work, we have adopted a facile three-step method for constructing an intriguing bifunctional electrode of self-supported hollow (Ni,Co)Se2 arrays with a metal-organic framework (MOF) precursor. The triangle-like cobalt-based MOF arrays are first grown on a carbon cloth at room temperature, which is followed by an ion exchange/etching process with Ni(NO3)2 to form a critical hollow nanostructure with an incorporated hetero-metal element. The intermediate is then transformed into the final product through solvothermal selenization treatment. Taking advantages of the structural and compositional merits as well as the self-supporting nature, the resultant (Ni,Co)Se2 electrode exhibits excellent electrochemical activity and stability. When tested as an electrocatalyst for the oxygen evolution reaction (OER), the (Ni,Co)Se2 array electrode displayed a low onset overpotential of 226 mV and a small overpotential of 256 mV to afford a current density of 10 mA cm-2. The (Ni,Co)Se2 electrode is also utilized in a supercapacitor, which delivers a high specific capacitance of 2.85 F cm-2 at 2 mA cm-2 and exhibits excellent cycling stability with a capacitance retention of 80.8% after 2000 charge-discharge cycles at 20 mA cm-2. These results demonstrate the significance of the rational design of electrode materials and disclose the potential of our MOF-derived hollow (Ni,Co)Se2 array electrode for a variety of practical applications in energy conversion and storage.

Highly Active Oxygen Evolution on Carbon Fiber Paper Coated with Atomic-Layer-Deposited Cobalt Oxide

In this work, we evaluated the oxygen evolution performance of cobalt oxide (CoO _x)-coated carbon fiber paper in electrochemical water splitting. For a uniform coating of CoO _x layers along the carbon fiber paper, the atomic layer deposition (ALD) technique was applied. We achieved a uniform and conformal coating of atomic-layer-deposited CoO _x (ALD-CoO _x) on the carbon fiber paper. The overpotential for oxygen evolution measured for the optimized ALD-coated carbon fiber paper was as low as 343 mV at 10 mA cm^-2, which is competitive with the activity of state-of-the-art CoO _x prepared on electrodes with large surface areas. Oxygen evolution is not enhanced after a critical thickness, about 28 nm in our study, is reached. The optimal thickness of the ALD-CoO _x film is dependent on two competing effects: the high oxidation state of cobalt ions in thicker CoO _x helps the oxygen evolution, whereas the introduction of a thick oxide coating decelerates the rate of charge transfer at the surface.