Lamellar structured CoSe 2 nanosheets directly arrayed on Ti plate as an efficient electrochemical catalyst for hydrogen evolution

Construction of Co-Se-W at Interfaces of Phase-Mixed Cobalt Selenide via Spontaneous Phase Transition for Platinum-Like Hydrogen Evolution Activity and Long-Term Durability in Alkaline and Acidic Media

Cost-effective transition metal chalcogenides are highly promising electrocatalysts for both alkaline and acidic hydrogen evolution reactions (HER). However, unsatisfactory HER kinetics and stability have severely hindered their applications in industrial water electrolysis. Herein, a nanoflowers-shaped W-doped cubic/orthorhombic phase-mixed CoSe2 catalyst ((c/o)-CoSe2-W) is reported. The W doping induces spontaneous phase transition from stable phase cubic CoSe2 (c-CoSe2) to metastable phase orthorhombic CoSe2, which not only enables precise regulation of the ratio of two phases but also realizes W doping at the interfaces of two phases. The (c/o)-CoSe2-W catalyst exhibits a Pt-like HER activity in both alkaline and acidic media, with record-low HER overpotentials of 29.8 mV (alkaline) and 35.9 mV (acidic) at 10 mA cm-2, respectively, surpassing the vast majority of previously reported non-precious metal electrocatalysts for both alkaline and acidic HER. The Pt-like HER activities originate from the formation of Co-Se-W active species on the c-CoSe2 side at the phase interface, which effectively modulates electron structures of active sites, not only enhancing H2O adsorption and dissociation at Co sites but also optimizing H* adsorption to ΔGH* ≈ 0 at W sites. Benefiting from the abundant phase interfaces, the catalyst also displays outstanding long-term durability in both acidic and alkaline media.

Recent Development of Electrodes Construction for HER in Electrocatalytic Water Splitting

Production of hydrogen (H2) fuel using the hydrogen evolution reaction (HER) through electrocatalysis of water splitting is inexpensive, has optimal performance, and offers remarkable stability. Developing electrocatalysts with excellent stability and high efficiency has been a significant and challenging factor for practical applications of HER for decades. Hydrogen generation occurs on the HER electrode due to the emission of bubbles, proton diffusion, and the transfer of electrons. These considerations should be taken into account during the construction and development of the electrode. This review offers a synopsis of recent advancements in various electrodes used as a base for electrocatalysts, such as nickel foam, titanium foil, copper foam, carbon foam, and others, and discusses their HER catalytic activity, with a focus on the emission of bubbles, diffusion of ions, the structure of the electrode, and the formulation and preparation process. In conclusion, we provide an overview of ideas to further improve and address the significant issues in the manufacture of HER electrodes.

Artificially Layered CoSe2 Nanosheets by a Dual-Templating Strategy for High-Performance Lithium-Sulfur Batteries

Owing to the attractive merits of layered transition metal dichalcogenides (LTMDs) with van der Waals interactions, it is significant to modulate electronic structures and endow them with fascinating physiochemical properties by converting a nonlayered metal dichalcogenide into an atomic layered one. Herein, a dual-templating strategy is designed to prepare artificially layered CoSe₂ nanosheets on carbon fiber cloth (L-CoSe₂/CFC). It is found that not only the nanosheet morphology but also the layered structure is well inherited from the precursor of layered Co(OH)₂ nanosheets through a wet-solution ion-exchange approach. The as-prepared L-CoSe₂/CFC serves as an efficient multifunctional interlayer to solve the challenges of "shuttling effect" and slow multistep reaction kinetics in lithium-sulfur batteries (LSBs), thus dramatically improving their electrochemical performance. Benefiting from the L-CoSe₂ nanosheets with large interlayer spacing, strong chemical adsorption, and superior catalytic activity, L-CoSe₂/CFC promotes the anchoring of lithium polysulfides (LiPSs) and their catalytic conversion. Consequently, the L-CoSe₂/CFC cell yields a large reversible capacity of 1584 mAh g^-1 at 0.2C and a high rate capability of 987 mAh g^-1 at 4C. A high areal capacity of 4.38 mAh cm^-2 after 100 cycles at 0.2C is achieved for the high-S-loading LSB (4.6 mg cm^-2) using the L-CoSe₂/CFC interlayer.

Template free-synthesis of cobalt-iron chalcogenides [Co0.8Fe0.2L2, L = S, Se] and their robust bifunctional electrocatalysis for the water splitting reaction and Cr(vi) reduction

The ease of production of materials and showing multiple applications are appealing in this modern era of advanced technology. This paper reports the synthesis of a pair of novel cobalt-iron chalcogenides [Co0.8Fe0.2S2 and Co0.8Fe0.2Se2] with enhanced electro catalytic activities. These ternary metal chalcogenides were synthesized by a one-step template-free approach via a hexamethyldisilazane (HMDS)-assisted synthetic method. Transient photocurrent (TPC) studies and electrochemical impedance spectra (EIS) of these materials showed free electron mobility. Their bifunctional activities were verified in both the electrochemical oxygen evolution reaction (OER) and in the electrochemical reduction of toxic inorganic heavy metal ions [Cr(vi)] in polluted water. The materials showed robust catalytic ability in the oxygen evolution reaction with minimum possible over potential (345 and 350 mV @ η10) as determined by linear sweep voltammetry and the lower Tafel values (52.4 and 84.5 mV dec-1) for Co0.8Fe0.2Se2 and Co0.8Fe0.2S2 respectively. Surprisingly, both the materials also showed an excellent activity towards electrochemical Cr(vi) reduction to Cr(iii). Besides the maximum current achieved for Co0.8Fe0.2S2, a minimum value for the Limit of detection (LOD) was obtained for Co0.8Fe0.2S2 (0.159 μg L-1) compared to Co0.8Fe0.2Se2 (0.196 μg L-1). We tested the durability of catalysts, the critical factor for the prolonged use of catalysts, through the recyclability measurements of these materials as catalysts. Both the catalysts presented outstanding durability and balanced electro catalytic activities for up to 1500 CV cycles, and chronoamperometry studies also confirmed exceptional stability. The enhanced catalytic activities of these materials are ascribed to the free electron movement, evidenced by the increased TPC measured and EIS. Therefore, the template-free synthesis of these electro catalysts containing non-noble metal illustrates the practical approach to develop such types of catalysts for multiple functions.

N-doped carbon nanotubes supported CoSe2 nanoparticles: A highly efficient and stable catalyst for H2O2 electrosynthesis in acidic media

Electrocatalytic oxygen reduction reaction (ORR) provides an attractive alternative to anthraquinone process for H₂O₂ synthesis. Rational design of earth-abundant electrocatalysts for H₂O₂ synthesis via a two-electron ORR process in acids is attractive but still very challenging. In this work, we report that nitrogen-doped carbon nanotubes as a multi-functional support for CoSe₂ nanoparticles not only keep CoSe₂ nanoparticles well dispersed but alter the crystal structure, which in turn improves the overall catalytic behaviors and thereby renders high O₂-to-H₂O₂ conversion efficiency. In 0.1 M HClO₄, such CoSe₂@NCNTs hybrid delivers a high H₂O₂ selectivity of 93.2% and a large H₂O₂ yield rate of 172 ppm·h^-1 with excellent durability up to 24 h. Moreover, CoSe₂@NCNTs performs effectively for organic dye degradation via electro-Fenton process.

ELECTRONIC SUPPLEMENTARY MATERIAL

Supplementary material (SEM images, EDX mapping images, XPS spectrum, XRD patterns, RRDE voltammogram, Tafel plots, cyclic voltammograms, UV-Vis spectra, and Tables S1) is available in the online version of this article at 10.1007/s12274-021-3474-0.

Agaric-like cobalt diselenide supported by carbon nanofiber as an efficient catalyst for hydrogen evolution reaction

We synthesized herein a novel 3D cathode constructed by growing cobalt diselenide in situ on the surface of carbon nanofiber for hydrogen evolution reaction. The cobalt diselenides with two typical morphologies (agaric-like and nanorod-like) were synthesized by precisely controlling reaction time and temperature in the same system. They show excellent electrocatalytic performance for hydrogen evolution reactions. Especially, the agaric-like diselenide cobalt electrode has the low overpotential (187 and 199 mV) to obtain the current density of 50 and 100 mA cm^-2 with a small Tafel slope of 37 mV dec^-1 in acidic medium. The excellent catalytic performance of the agaric-like cobalt diselenide can be attributed to its large specific surface area and fast electron transfer rate. More importantly, the agaric-like cobalt diselenide supported carbon nanofiber electrode has excellent long-term stability in electrolyte. The outstanding electrocatalytic performance and stability of agaric-like cobalt diselenide supported carbon nanofiber indicate that it is a promising electrocatalyst for hydrogen evolution reactions.

Coupling Co2P/CoSe2 heterostructure nanoarrays for boosting overall water splitting

Exploiting environmentally friendly and robust electrocatalysts for overall water splitting is of utmost importance in order to alleviate the excessive global energy consumption and climate change. Herein, a simple phosphoselenization method was used to prepare Co2P and CoSe2 coupled nanosheet and nanoneedle composite materials on nickel foam (Co2P/CoSe2/NF). Density functional theory calculations showed that Co2P had a higher water adsorption energy compared with CoSe2, indicating that H2O molecules are strongly adsorbed on the active sites of Co2P, which speeds up the kinetic process of water splitting. The Co2P/CoSe2-300 material displayed superior electrocatalytic activity for both the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) in an alkaline medium. It's worth noting that the Co2P/CoSe2-300 composite material nanoarrays merely needed an ultralow overpotential of 280 mV to drive a current intensity of 100 mA cm-2 for OER. In addition, when a two-electrode system was constructed for overall water splitting, the current intensity of 20 mA cm-2 could be reached while requiring an ultrasmall cell voltage of 1.52 V, which is one of the best catalytic activities reported up to now. Experimental and density functional theory calculations showed that the superior electrocatalytic performance of Co2P/CoSe2-300 could be attributed to its higher electron-transfer rate, higher water adsorption energy, and the synergistic effect of Co2P and CoSe2. Our work provides a novel approach for the one-step construction of composite materials as environmentally friendly and inexpensive water splitting catalysts.

Sulfur-Doped CoSe2 Porous Nanosheets as Efficient Electrocatalysts for the Hydrogen Evolution Reaction

The electrochemical hydrogen evolution reaction (HER), as a promising route for hydrogen production, demands efficient and robust noble-metal-free catalysts. Doping foreign atoms into an efficient catalyst such as CoSe₂ could further enhance its activity toward the HER. Herein, we developed a solvothermal ion exchange approach to doping S into CoSe₂ nanosheets (NSs). We provide a combined experimental and theoretical investigation to establish the obtained S-doped CoSe₂ (S-CoSe₂) nanoporous NSs as highly efficient and Earth-abundant catalysts for the HER. The optimal S-CoSe₂ catalyst delivers a catalytic current density of 10 mA·cm^-2 for the HER at an overpotential of only 88 mV, demonstrating that S-CoSe₂ is one of the most efficient CoSe- and CoS-based catalysts for the HER. We performed density functional theory (DFT) calculations to determine the stable structural configurations of S-CoSe₂, and on the basis of which, we calculated the hydrogen adsorption Gibbs free energy (ΔG_H) on CoSe₂, CoS₂, and the S-CoSe₂ and the barrier energies of the rate-determining step of the HER on S-CoSe₂. DFT calculations reveal that S-doping not only decreases the absolute value of ΔG_H (move toward zero) but also significantly lowers the kinetic barrier energy of the rate-determining step of the HER on S-CoSe₂, leading to a greatly improved HER performance.

Construction of hierarchical cobalt-molybdenum selenide hollow nanospheres architectures for high performance battery-supercapacitor hybrid devices

Transition metal selenides have aroused widespread attention as a class of emerging electrode materials for high-performance supercapacitors attributed to their featured with high theoretical capacitance and low electronegativity. Nevertheless, their practical applications are seriously restricted by the large volume expansion during high-rate charge/discharge. It is imperative to reasonably construct tunable composition and attractive architectures for electrode materials at nanoscale to mitigate the issues. Herein, hierarchical cobalt-molybdenum selenide (denoted as CoSe₂/MoSe₂-3-1) hollow nanospheres architectures are purposefully prepared via an efficient gas bubble-templated method combined with post-annealing process. Benefiting from the rationally hierarchical hollow structures and maximized utilization ratio of active materials, the novel bimetallic selenides acquire superior electrochemical performance with high specific capacity (211.97 mA h g^-1 at 1 A g^-1) and remarkable cycling stability (94.2% capacity retention over 2000 cycles at 3 A g^-1). Significantly, the assembled CoSe₂/MoSe₂-3-1//activated carbon (AC) battery-supercapacitor hybrid (BSH) device renders a high energy density up to 51.84 W h kg^-1 at a power density of 799.2 W kg^-1 and preeminent cycling stability with 93.4% retention over 10,000 cycles. The present work provides an effective and rational design route to engineer advanced bimetallic selenides with hierarchical hollow structures for electrochemical energy storage and conversion.

Cu-Co-M arrays on Ni foam as monolithic structured catalysts for water splitting: effects of co-doped S-P

Finding new methods to design environmentally friendly, highly stable, and robust oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) catalysts remains an extremely important challenge and affords several opportunities for exploiting the water-splitting process. In our work, Cu-Co-M (M = O, S, P, Se, S-P, and P-S) materials were grown in situ on three-dimensional (3D) porous nickel foam (NF) with good electrical conductivity by hydrothermal synthesis, vulcanization, selenylation, and phosphorization of the Cu-Co-precursor under an Ar atmosphere. Cu-Co-P-S/NF presents an overpotential value of 220 mV at 50 mA cm-2 for OER and that for Cu-Co-P-S/NF of 120 mV at 10 mA cm-2 for HER in an alkaline medium. In addition, considering the superior activities of Cu-Co-P-S/NF for OER and HER, the electrode pairing of Cu-Co-P-SOER//Cu-Co-P-SHER is designed for overall water splitting, and the experimental result shows that only a cell voltage of 1.55 V is needed to obtain a current density of 20 mA cm-2. According to the literature, this cell voltage is also among the lowest values that have been previously reported for electrocatalytic water splitting. Further, Cu-Co-P-S//Cu-Co-P-S exhibited efficient stability during a 20 h durability test without significant attenuation under alkaline conditions. By using XPS spectroscopy characterization, it was shown that Cu-Co-P-S presented the highest catalytic performance and long-term durability owing to the abundance of Co3+. The novelty of selecting the highest activity with the same catalyst for OER and HER from Cu-Co-M (M = O, S, P, Se, S-P, and P-S) in order to obtain a well-matched electrode pair, and therefore, simplifying the water-splitting device affords a wide range of possibilities for further exploitation of environmentally friendly and highly efficient electrode pairs.

Oxygen vacancy-confined CoMoO4@CoNiO2 nanorod arrays for oxygen evolution with improved performance

Experimental and DFT calculation results show that the presence of oxygen vacancies can decrease the adsorption energy of intermediates at active sites and facilitate the adsorption of intermediates, thus improving the catalytic properties.

Recent Advances of Cobalt-Based Electrocatalysts for Oxygen Electrode Reactions and Hydrogen Evolution Reaction

This review summarizes recent progress in the development of cobalt-based catalytic centers as the most potentially useful alternatives to noble metal-based electrocatalysts (Pt-, Ir-, and Ru-based) towards the oxygen reduction reaction (ORR), oxygen evolution reaction (OER), and hydrogen evolution reaction (HER) in acid and alkaline media. A series of cobalt-based high-performance electrocatalysts have been designed and synthesized including cobalt oxides/chalcogenides, Co–Nx/C, Co-layered double hydroxides (LDH), and Co–metal-organic frameworks (MOFs). The strategies of controllable synthesis, the structural properties, ligand effect, defects, oxygen vacancies, and support materials are thoroughly discussed as a function of the electrocatalytic performance of cobalt-based electrocatalysts. Finally, prospects for the design of novel, efficient cobalt-based materials, for large-scale application and opportunities, are encouraged.

Rational Design and Synthesis of Extremely Efficient Macroporous CoSe2 -CNT Composite Microspheres for Hydrogen Evolution Reaction

Uniquely structured CoSe₂ -carbon nanotube (CNT) composite microspheres with optimized morphology for the hydrogen-evolution reaction (HER) are prepared by spray pyrolysis and subsequent selenization. The ultrafine CoSe₂ nanocrystals uniformly decorate the entire macroporous CNT backbone in CoSe₂ -CNT composite microspheres. The macroporous CNT backbone strongly improves the electrocatalytic activity of CoSe₂ by improving the electrical conductivity and minimizing the growth of CoSe₂ nanocrystals during the synthesis process. In addition, the macroporous structure resulting from the CNT backbone improves the electrocatalytic activity of the CoSe₂ -CNT microspheres by increasing the removal rate of generated H₂ and minimizing the polarization of the electrode during HER. The CoSe₂ -CNT composite microspheres demonstrate excellent catalytic activity for HER in an acidic medium (10 mA cm^-2 at an overpotential of ≈174 mV). The bare CoSe₂ powders exhibit moderate HER activity, with an overpotential of 226 mV at 10 mA cm^-2 . The Tafel slopes for the CoSe₂ -CNT composite and bare CoSe₂ powders are 37.8 and 58.9 mV dec^-1 , respectively. The CoSe₂ -CNT composite microspheres have a slightly larger Tafel slope than that of commercial carbon-supported platinum nanoparticles, which is 30.2 mV dec^-1 .