CoS(2x)Se(2(1-x)) nanowire array: an efficient ternary electrocatalyst for the hydrogen evolution reaction

Noble metal-free CZTS electrocatalysis: synergetic characteristics and emerging applications towards water splitting reactions

This review provides a comprehensive overview of the production and modification of CZTS nanoparticles (NPs) and their application in electrocatalysis for water splitting. Various aspects, including surface modification, heterostructure design with carbon nanostructured materials, and tunable electrocatalytic studies, are discussed. A key focus is the synthesis of small CZTS nanoparticles with tunable reactivity, emphasizing the sonochemical method's role in their formation. Despite CZTS's affordability, it often exhibits poor hydrogen evolution reaction (HER) behavior. Carbon materials like graphene, carbon nanotubes, and C60 are highlighted for their ability to enhance electrocatalytic activity due to their unique properties. The review also discusses the amine functionalization of graphene oxide/CZTS composites, which enhances overall water splitting performance. Doping with non-noble metals such as Fe, Co., and Ni is presented as an effective strategy to improve catalytic activity. Additionally, the synthesis of heterostructures consisting of CZTS nanoparticles attached to MoS2-reduced graphene oxide (rGO) hybrids is explored, showing enhanced HER activity compared to pure CZTS and MoS2. The growing demand for energy and the need for efficient renewable energy sources, particularly hydrogen generation, are driving research in this field. The review aims to demonstrate the potential of CZTS-based electrocatalysts for high-performance and cost-effective hydrogen generation with low environmental impact. Vacuum-based and non-vacuum-based methods for fabricating CZTS are discussed, with a focus on simplicity and efficiency. Future developments in CZTS-based electrocatalysts include enhancing activity and stability, improving charge transfer mechanisms, ensuring cost-effectiveness and scalability, increasing durability, integrating with renewable energy sources, and gaining deeper insight into reaction processes. Overall, CZTS-based electrocatalysts show great promise for sustainable hydrogen generation, with ongoing research focused on improving performance and advancing their practical applications.

Superionic conductor Ag2Se modulated CoSe2 nanosheets prepared via monometallic cation release for efficient pH-universal water electrolysis into hydrogen

Superionic conductors regulated transition metal chalcogenides are the newly emerged electrocatalyst in water electrolysis into clean hydrogen and oxygen. However, there is still much room for the development of structural design, electronic modulation and heterogeneous interface construction to improve the overall water splitting performance in pH-universal solutions, especially in alkaline and neutral mediums. Herein, using β-cyclodextrin (β-CD) and citric acid (CA) organics with abundant hydroxyl (-OH) and carboxyl (-COOH), a special Ag₂Se nanoparticles-decorated CoSe₂ flower-like nanosheets loaded on porous and conductive nickel foam substrate (Ag₂Se-CoSe₂/NF) was successfully constructed by a new method of monometallic cation release of coordinated cobalt. The Ag₂Se phase exerts the nature characteristics of superionic conductors to modulate the morphological and electronic structures of CoSe₂ as well as improve its conductivity. The generated rich active interfaces and abundant Se vacancy defects facilitate numerous active sites exposure to accelerate the hydrogen ion transport and charge transfer. Compared to the single-phase Ag₂Se/NF-8 and CoSe₂/NF, the prepared Ag₂Se-CoSe₂/NF-8 with a two-phase synergistic effect achieves an outstanding pH-universal electrocatalytic hydrogen production performance by water electrolysis, as evidenced by a lower overpotential (60 mV, 212 mV and 85 mV vs RHE at 10 mA cm^-2 for pH = 0.36, 7.00 and 13.70, respectively). Only a voltage of 1.55 V at 10 mA cm^-2 is required to implement the overall water splitting in an alkaline electrolyzer. This work provides significant guidance for the future designation and practical development of transition metal chalcogenides with superionic conductors applied in the electrocatalytic field.

Shining Light on Anion-Mixed Nanocatalysts for Efficient Water Electrolysis: Fundamentals, Progress, and Perspectives

This review introduces recent advances of various anion-mixed transition metal compounds (e.g., nitrides, halides, phosphides, chalcogenides, (oxy)hydroxides, and borides) for efficient water electrolysis applications in detail. The challenges and future perspectives are proposed and analyzed for the anion-mixed water dissociation catalysts, including polyanion-mixed and metal-free catalyst, progressive synthesis strategies, advanced in situ characterizations, and atomic level structure-activity relationship. Hydrogen with high energy density and zero carbon emission is widely acknowledged as the most promising candidate toward world's carbon neutrality and future sustainable eco-society. Water-splitting is a constructive technology for unpolluted and high-purity H₂ production, and a series of non-precious electrocatalysts have been developed over the past decade. To further improve the catalytic activities, metal doping is always adopted to modulate the 3d-electronic configuration and electron-donating/accepting (e-DA) properties, while for anion doping, the electronegativity variations among different non-metal elements would also bring some potential in the modulations of e-DA and metal valence for tuning the performances. In this review, we summarize the recent developments of the many different anion-mixed transition metal compounds (e.g., nitrides, halides, phosphides, chalcogenides, oxyhydroxides, and borides/borates) for efficient water electrolysis applications. First, we have introduced the general information of water-splitting and the description of anion-mixed electrocatalysts and highlighted their complementary functions of mixed anions. Furthermore, some latest advances of anion-mixed compounds are also categorized for hydrogen and oxygen evolution electrocatalysis. The rationales behind their enhanced electrochemical performances are discussed. Last but not least, the challenges and future perspectives are briefly proposed for the anion-mixed water dissociation catalysts.

Flower-like 1T-MoS2/NiCo2S4 on a carbon cloth substrate as an efficient electrocatalyst for the hydrogen evolution reaction

The 1T-MoS₂/NiCo₂S₄ composite in situ grown on carbon cloth (CC) was successfully prepared by a two-step hydrothermal method as an efficient electrode for the hydrogen evolution reaction. The morphology and composition characterization show that the composite has a flower-like structure with a large number of edges and surfaces exposed, and the content of the 1T phase in MoS₂ is 63%. 1T-MoS₂/NiCo₂S₄/CC exhibits an overpotential of 107 mV at 10 mA cm^-2, and a Tafel slope of 66.4 mV dec^-1 in an alkaline electrolyte. After continuous electrolysis for 24 h at an overpotential of 170 mV, 86% of the original current density was retained in an chronoamperometry measurement. The outstanding catalytic performance of the composite is ascribed to its unique structure, high 1T-MoS₂ content and the synergistic catalysis between 1T-MoS₂ and NiCo₂S₄. This work provides a facile and effective strategy for fabricating the 1T-MoS₂/NiCo₂S₄/CC composite and demonstrates that the composite is expected to be a competitive non-noble HER catalyst.

A “Superaerophobic” Se-Doped CoS2 Porous Nanowires Array for Cost-Saving Hydrogen Evolution

The pursuit of low-cost and high-efficiency catalyst is imperative for the development and utilization of hydrogen energy. Heteroatomic doping which is conducive to the redistribution of electric density is one of the promising strategies to improve catalytic activity. Herein, the Se-doped CoS2 porous nanowires array with a superaerophobic surface was constructed on carbon fiber. Due to the electronic modulation and the unique superaerophobic structure, it showed improved hydrogen evolution activity and stability in urea-containing electrolyte. At a current density of 10 mA cm−2, the overpotentials are 188 mV for hydrogen evolution reaction (HER) and 1.46 V for urea oxidation reaction (UOR). When it was set as a cell, the voltage is low as 1.44 V. Meanwhile, the current densities of HER and UOR, as well as of cell remained basically unchanged after a continuous operation for 48 h. This work opens up a new idea for designing of cost-saving hydrogen evolution electrocatalysts.

Surface-Induced 2D/1D Heterostructured Growth of ReS2/CoS2 for High-Performance Electrocatalysts

Two-dimensional/one-dimensional (2D/1D) heterostructures have received much attention from researchers for their abundant catalytically active sites and low contact resistance due to formation of chemical bonds at the interface. The investigation of such heterostructures, however, is confined to lattice-matched materials, which severely limits the material candidates. Herein, we demonstrate a lattice-mismatched 2D/1D heterostructured electrocatalyst consisting of 2D ReS₂ nanosheets and 1D CoS₂ nanowires. We propose that the higher surface energy of the CoS₂ nanowire and the lattice mismatch between 1D and 2D units are crucial for the growth process of ReS₂ nanosheets. More importantly, the terminal S^2- exposed on the surface of CoS₂ nanowires serves not only as the nucleus of ReS₂ nanosheets but also as a bridge to enhance electron transport efficiency. Thus, the ReS₂/CoS₂ heterostructures show outstanding hydrogen evolution reaction performance. This work is of general interest for the design of complex multidimensional nano-heterostructures with outstanding functionalities.

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.

Phosphorus-doped CoTe2/C nanoparticles create new Co-P active sites to promote the hydrogen evolution reaction

Doping has been widely recognized as an effective method for adjusting the performance of electrocatalysts. It can cause changes in the electronic structure of substances. Thereby, it can affect the intrinsic catalytic performance. Herein, we report a facile doping method in which phosphorus can be simultaneously doped into both CoTe2 and C. In the acidic solution, the hydrogen evolution reaction (HER) performance of the obtained P-CoTe2/C nanoparticles was significantly improved compared with that of undoped nanoparticles. At a current density of 10 mA cm-2, the overpotential decreased from 430 mV to 159 mV. Density functional theory (DFT) calculations show that phosphorus doping can produce new high activity Co-P catalytic sites. In addition, phosphorus can be doped into the carbon in the composite at the same time, which enhances the electrical conductivity of the composite. Moreover, in the process of calcination and doping, the electric double layer capacitance (Cdl) of the composite is significantly increased, which helps in exposing more active sites. This work has developed a multi-effect doping method that simultaneously increases the intrinsic activity, conductivity and active sites of the material. This method provides a new strategy for the performance regulation of other electrocatalysts.

Nanostructured Ni2SeS on Porous-Carbon Skeletons as Highly Efficient Electrocatalyst for Hydrogen Evolution in Acidic Medium

Nickel dichalcogenides have received extensive attention as promising noble-metal-free nanocatalysts for a hydrogen evolution reaction. Nonetheless, their catalytic performance is restricted by the sluggish reaction kinetics, limited exposed active sites, and poor conductivity. In this work, we report on an effective strategy to solve those problems by using an as-designed new porous-C/Ni₂SeS nanocatalyst with the Ni₂SeS nanostubs anchored on with porous-carbon skeletons process. On the basis of three advantages, as the enhancement of the intrinsic activity using the ternary sulfoselenide, increased number of exposed active sites due to the 3D hollow substrate, and increased conductivity caused by porous-carbon skeletons, the resulting porous-C/Ni₂SeS requires an overpotential of only 121 mV at a current density of 10 mA cm^-2 with a Tafel slope of 78 mV dec^-1 for hydrogen evolution in acidic media and a good long-term stability. Density functional theory calculations also show that the Gibbs free energy of hydrogen adsorption of the Ni₂SeS was -0.23 eV, which not only is close to the ideal value (0 eV) and Pt reference (-0.09 eV) but also is lower than those of NiS₂ and NiSe₂; large electrical states exist in the vicinity of the Fermi level, which further improves its electrocatalytic performance. This work provides new insights into the rational design of ternary dichalcogenides and hollow structure materials for practical applications in HER catalysis and energy fields.

Direct Growth of CNTs@CoSx Se2(1-x) on Carbon Cloth for Overall Water Splitting

Searching for low-cost, high-efficiency, bifunctional, non-noble-metal electrocatalysts for overall water splitting is crucial to renewable energy conversion. Herein, a series of component-controllable CC/CNTs@CoS_x Se_2(1-x) (CC: carbon cloth, CNT: carbon nanotube) with excellent bifunctional properties in the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) were obtained by chemical vapor deposition. In this strategy, the Zif-67 precursor served as a structural inducer, which was directly grown on CC and pyrolyzed with the assistance of melamine to form multi-walled CNT-encapsulated CoS_x Se_2(1-x) hierarchical nanostructures. Subsequently, the electrocatalytic properties of the as-prepared materials were optimized by adjusting the S/Se molar ratio. Of note is that the lattice distortion caused by the different radii of Se and S generated a polarized electric field for easy adsorption of the intermediate products. The CoOOH generated in situ on the surface of CoS_x Se_2(1-x) , as well as n- and p-type domains in carbon, synergistically resulted in abundant active sites to boost the electrocatalytic activity. CC/CNTs@CoS_0.74 Se_0.52 exhibited overpotentials for the HER and OER of 225 and 285 mV, respectively and attained a current density of 10 mA cm^-2 in alkaline solution. The as-prepared electrocatalysts could act as both cathode and anode in a water electrolyzer showing a cell voltage of 1.74 V and delivering 10 mA cm^-2 , comparable to those of noble-metal-based water electrolyzers.

Engineering Interface and Oxygen Vacancies of NixCo1-xSe2 to Boost Oxygen Catalysis for Flexible Zn-Air Batteries

Exploring efficient bifunctional oxygen electrocatalysts is a critical element for developing high-power-density metal-air batteries. Here, we propose an interface and oxygen vacancy engineering strategy to integrate subtle lattice distortions, oxygen vacancies, and nanopores on the surface of Ni_xCo_1-xSe₂-O interface nanocrystals, which exhibit efficient bifunctional catalytic performances for oxygen evolution and reduction. The results from X-ray absorption spectroscopy and electron spin resonance spectroscopy demonstrate that the defect structure can enlarge the number of active sites for electrocatalytic performances. Flexible Zn-air battery using Ni_xCo_1-xSe₂-O as a cathode displays large specific capacity and remarkable stability even after twisting at any angle, thus showing potential for wearable and portable electronic device application. The implementation of our method provides a powerful strategy for preparing advanced catalysts for energy utilization.

Constituent-tunable ternary CoM2xSe2(1-x) (M = Te, S) sandwich-like graphitized carbon-based composites as highly efficient electrocatalysts for water splitting

Herein, we fabricated two groups of component-tunable sandwich-like CoTe2xSe2(1-x) and CoS2ySe2(1-y) graphitized carbon-based composites. Due to the synergistic effect between the sandwich-like structures and anion-doping, CoTe2xSe2(1-x) and CoS2ySe2(1-y) demonstrated attractive catalytic performance towards the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER), respectively. After optimizing the composition, we found that sandwich-like Co(Te0.33Se0.67)2 possesses optimal OER properties, with a low Tafel slope (44 mV dec-1), a small η10 (272 mV) and eminent stability for 50 h; meanwhile, the Co(S0.72Se0.28)2 catalyst presents attractive HER performance, featuring a minimum Tafel slope (80 mV dec-1) and a low η10 (106 mV) in alkaline media. Additionally, density functional theory (DFT) calculations confirmed that ΔGH* of the Co(S0.72Se0.28)2 catalyst (0.182 eV) is closest to zero. Therefore, an alkaline electrolyzer was fabricated using Co(Te0.33Se0.67)2 as the anode and Co(S0.72Se0.28)2 as the cathode; it manifested striking catalytic activity and favorable stability for overall water splitting. This is the first time that the effects of different dopants (Te and S atoms) and doping contents on the electrocatalytic performance of CoSe2 were explored towards overall water splitting. Also, this research provides a new pathway for the fabrication of ternary dianion transition metal dichalcogenides as admirable electrocatalysts for water splitting.

Controllable synthesis of nickel sulfide nanocatalysts and their phase-dependent performance for overall water splitting

The exploitation of economical and highly efficient bifunctional electrocatalysts to promote oxygen evolution and hydrogen evolution reactions (OER and HER) for water splitting devices is urgently needed. Herein, a series of NiSx (i.e., NiS, Ni3S2, NiS2) nanocrystals with controllable phase and composition have been synthesized via a facile polyol solution process and the corresponding electrocatalytic properties towards OER and HER have been systematically investigated. Electrochemical results reveal that Ni3S2 exhibits superior OER and HER performance to NiS and NiS2, achieving 1.63 V to reach a current density of 10 mA cm-2 in the overall water splitting device, which is comparable to that of noble metal catalysts. Experimental and theoretical calculation investigations demonstrate that the remarkable catalytic properties of Ni3S2 could be attributed to the intrinsic metallic conductivity, abundant active sites and optimal Gibbs free-energy for catalyst-H* for HER. Moreover, a thicker layer of catalytically active species of NiOOH was generated on the surface of Ni3S2 due to the higher proportion of Ni, leading to a better OER performance. These results should shed light on the design and development of low cost and efficient transition metal chalcogenide electrocatalysts through phase and composition regulation for advanced electrochemical energy conversion devices.

Self-assembled Co0.85Se/carbon nanowires as a highly effective and stable electrocatalyst for the hydrogen evolution reaction

Self-assembled Co_0.85Se/carbon nanowires, constructed by Co_0.85Se nanoparticles homogenously embedded into carbon nanowires (Co_0.85Se@CNWs), have been synthesized through a facile solvothermal reaction and selenylation process. Compared to the bare Co_0.85Se NWs, the Co_0.85Se@CNW hybrid demonstrates high efficiency and stability for HER. It has a small Tafel slope of 43.4 mV dec⁻¹, a low onset potential of 138 mV vs. RHE, and a high cycling stability with more than 95% current retention after 1500 voltammetry cycles. The outstanding HER performance of Co_0.85Se@CNWs is attributed to its unique particle-in-nanowire architecture, which not only prevents the Co_0.85Se nanoparticles from aggregation, but also provides a highly conductive CNW matrix to promote the charge transfer in the electrocatalytic reaction, further enhancing the catalytic activity. This work provides a new strategy to rationally design transition metal-based selenide hybrids as highly effective and stable electrocatalysts for HER.

Self-assembled Co_0.85Se/carbon nanowires constructed from Co_0.85Se nanoparticles homogenously embedded into carbon nanowires (Co_0.85Se@CNWs).

In Situ Fabrication of Heterostructure on Nickel Foam with Tuned Composition for Enhancing Water-Splitting Performance

Exploiting economical and high-performance bifunctional electrocatalysts toward hydrogen and oxygen evolution reactions (HER/OER) is at the heart of overall water splitting in large-scale application. Herein, an in situ and stepwise strategy for synthesizing core-shell Ni₃ (S_1-x Se_x )₂ @NiOOH (0 ≤ x ≤ 1) nanoarray heterostructures on nickel foam with tailored compositions for enhancing water-splitting performance is reported. A series of Ni₃ (S_1-x Se_x )₂ nanostructures is firstly grown on nickel foam via an in situ reaction in a heated polyol solution system. Ni₃ (S_1-x Se_x )₂ @NiOOH nanocomposites are subsequently prepared via electrochemical oxidation and the oxidation degree is systematically investigated by varying the oxidation time. Benefitting from the vertical standing architecture, abundant exposed active sites, and synergetically interfacial enhancement, Ni₃ (S_0.25 Se_0.75 )₂ @NiOOH heterojunctions with electrochemical polarization for 8 h exhibit superior HER and OER behaviors, achieving a water-splitting current density of 10 mA cm^-2 at a small overpotential of 320 mV as well as boosted reaction kinetics and long-term stability. This work should shed light on the controllable synthesis of metal-based hybrid materials and provide a promising direction for developing the highest-performing electrocatalysts based on interfacial and heterostructural regulation for advanced electrochemical energy conversion technologies.

Self-Template Synthesis of Co-Se-S-O Hierarchical Nanotubes as Efficient Electrocatalysts for Oxygen Evolution under Alkaline and Neutral Conditions

We develop a facile self-template synthetic method to construct hierarchical Co-Se-S-O (CoSe _xS_{2- x}@Co(OH)₂) nanotubes on a carbon cloth as a self-standing electrode for electrocatalytic oxygen evolution reaction (OER). In the synthetic process, separate selenization and sulfurization on the Co(OH)F precursor in different solvents have played an important role in constructing CoSe _xS_{2- x} (Co-Se-S) hierarchical nanotubes, which was further transformed into the nanotube-like Co-Se-S-O via an in situ electrochemical oxidation process. The Co-Se-S-O obtained by the Kirkendall effect through two stepwise anion-exchange reactions represents the first quaternary Co-Se-S-O nanotube array, which dramatically enhances its surface area and conductivity. Further, it only requires low overpotentials of 230 and 480 mV to achieve a 10 mA cm^-2 current density. The OER performance of Co-Se-S-O is much more efficient than that of its monochalcogenide counterparts, as well as the commercial benchmark catalyst IrO₂.

Hydrogen evolution reactions boosted by bridge bonds between electrocatalysts and electrodes

The interfacial interactions between nanostructured electrode materials and electrodes play an important part in the performance enhancement of electrochemical energy devices. However, the mechanism of interfacial interactions, as well as its influence on device performance, still remains unclear and is rarely studied. In this work, a CoS₂ nanobelt catalyst assembled on Ti foil (CoS₂ nanobelts/Ti) is prepared through in situ chemical conversions and chosen as an example to probe the interfacial interactions between the CoS₂ catalyst and the Ti electrode, and the correlation between the interfacial interaction and the hydrogen evolution reaction (HER) performance. By a series of characterization studies and analyses, we propose that interfacial bridge bonds (Ti-S-Co and Ti-O-Co) in a covalent form may exist in the CoS₂ nanobelts/Ti as well as its precursor Co(OH)₃ nanobelts growing on Ti foil, which is further supported by density functional theory (DFT) calculations. Moreover, as a binder-free electrocatalytic electrode, the CoS₂ nanobelts/Ti shows boosted HER performance, including higher catalytic activity, and lower overpotential and Tafel slope, compared to its counterpart transformed from a solution-produced precursor. The HER performance enhancement is ascribed to the existence of interfacial bridge bonds that not only strengthen the electrode-catalyst mechanical integrity, but also serve as efficient charge transfer channels between the electrode and the catalyst, thus ensuring a stable and fluent electron transfer for the HER. Furthermore, the DFT calculations reveal that the CoS₂ nanobelts/Ti catalyst with interfacial covalent interactions can facilitate the adsorption of H⁺ ions/H₂ molecules and the desorption of H₂ molecules for an accelerated HER. This work provides a new insight into the interfacial interactions between electrodes and electrode materials in electrochemical devices, and paves the way for the rational design and construction of high-performance electrochemical devices for practical energy applications.

Role of non-metallic atoms in enhancing the catalytic activity of nickel-based compounds for hydrogen evolution reaction

The transition-metal compounds (MX) have gained wide attention as hydrogen evolution reaction (HER) electrocatalysts; however, the interaction between the non-metallic atom (X) and the metal atom (M) in MX, and the role of X in the enhanced catalytic activity of MX, are still ambiguous. In this work, we constructed a simple model [X/Ni(100)] to decipher the contribution of X towards enhancing the catalytic activity of NiX, which allows us to accurately predict the trend in HER catalytic activity of NiX based on the easily accessible physico-chemical characteristics of X. Theoretical calculations showed that the electronegativity (χX) and the principle quantum number (nX) of X are two important descriptors for evaluating and predicting the HER catalytic activity of NiX catalysts effectively. X atoms in the VIA group can enhance the HER activity of X/Ni(100) more significantly than those in the second period due to the large χX or nX. At a relatively low X coverage, the S/Ni(100) possesses the best HER activity among all of the discussed X/Ni(100) models, and the optimum surface S : Ni atomic ratio is about 22-33%. Further experiments demonstrated that the Ni-Ni3S2 catalyst with a surface S : Ni atomic ratio of 28.9% exhibits the best catalytic activity and lowest charge transfer resistance. The trend in catalytic activity of NiX with differing X offers a new possible strategy to exploit MX materials and design new active catalysts rationally.

Rational Construction of Hollow Core-Branch CoSe2 Nanoarrays for High-Performance Asymmetric Supercapacitor and Efficient Oxygen Evolution

Metal selenides have great potential for electrochemical energy storage, but are relatively scarce investigated. Herein, a novel hollow core-branch CoSe₂ nanoarray on carbon cloth is designed by a facile selenization reaction of predesigned CoO nanocones. And the electrochemical reaction mechanism of CoSe₂ in supercapacitor is studied in detail for the first time. Compared with CoO, the hollow core-branch CoSe₂ has both larger specific surface area and higher electrical conductivity. When tested as a supercapacitor positive electrode, the CoSe₂ delivers a high specific capacitance of 759.5 F g^-1 at 1 mA cm^-2 , which is much larger than that of CoO nanocones (319.5 F g^-1 ). In addition, the CoSe₂ electrode exhibits excellent cycling stability in that a capacitance retention of 94.5% can be maintained after 5000 charge-discharge cycles at 5 mA cm^-2 . An asymmetric supercapacitor using the CoSe₂ as cathode and an N-doped carbon nanowall as anode is further assembled, which show a high energy density of 32.2 Wh kg^-1 at a power density of 1914.7 W kg^-1 , and maintains 24.9 Wh kg^-1 when power density increased to 7354.8 W kg^-1 . Moreover, the CoSe₂ electrode also exhibits better oxygen evolution reaction activity than that of CoO.

Efficient alkaline hydrogen evolution electrocatalysis enabled by an amorphous Co–Mo–B film

An amorphous Co–Mo–B film on a Ti mesh (Co–Mo–B/Ti) acts as a durable hydrogen evolution reaction electrocatalyst with an overpotential of 110 mV to drive 20 mA cm⁻² in 1.0 M KOH.

Encapsulating CoS2–CoSe2 heterostructured nanocrystals in N-doped carbon nanocubes as highly efficient counter electrodes for dye-sensitized solar cells

The CoS₂–CoSe₂@N-doped carbon nanocubes were synthesized through simultaneous sulfurization and selenization of polydopamine coated Prussian blue analogs.

Enhanced electrocatalytic hydrogen generation from water via cobalt-doped Cu2ZnSnS4 nanoparticles

Herein, we adopted a novel noble metal-free Co-doped CZTS-based electrocatalyst for the hydrogen evolution reaction (HER), which was fabricated using a facile, effective, and scalable strategy by employing a sonochemical method. The optimized Co-doped CZTS electrocatalyst shows a superior HER performance with a small overpotential of 200 and 298 mV at 2 and 10 mA⁻¹, respectively, and Tafel slope of 73 mV dec⁻¹, and also exhibits excellent stability up to 700 cycles with negligible loss of the cathodic current. The ease of synthesis and high activity of the Co-doped CZTS-based cost-effective catalytic system appear to be promising for HER catalysis.

A novel noble metal-free Co-doped CZTS-based nano-electrocatalyst fabricated by employing a sonochemical method for the enhanced hydrogen evolution reaction (HER) and it shows a superior HER performance and exhibits excellent current stability.

A Ni(OH)2-CoS2 hybrid nanowire array: a superior non-noble-metal catalyst toward the hydrogen evolution reaction in alkaline media

The rising H₂ economy urgently demands active, durable and cost-effective catalysts for the electrochemical hydrogen evolution reaction (HER). However, improving the HER performance of electrocatalysts in alkaline media is still challenging. Herein, we report the development of a nickel hydroxide-cobalt disulfide nanowire array on a carbon cloth (Ni(OH)₂-CoS₂/CC) as a hybrid catalyst to significantly enhance the HER activity in alkaline solutions. Benefitting from heterogeneous interfaces in this 3D hybrid electrocatalyst, Ni(OH)₂-CoS₂/CC shows superior HER activity with only 99 mV overpotential to drive a current density of 20 mA cm^-2 in 1.0 M KOH, which is 100 mV less than that of CoS₂/CC. Moreover, Ni(OH)₂-CoS₂/CC exhibits long-term electrochemical durability with the maintenance of its catalytic activity for 30 h. Density functional theory calculations are performed to gain further insight into the effect of Ni(OH)₂-CoS₂ interfaces, revealing that Ni(OH)₂ plays a key role in water dissociation to hydrogen intermediates and CoS₂ facilitates the adsorption of hydrogen intermediates and H₂ generation. This work not only develops a promising electrocatalyst for the alkaline HER, but also paves a way to enhance the alkaline HER activity of CoS₂via the interface engineering strategy.

Metal-Organic Framework Template Derived Porous CoSe2 Nanosheet Arrays for Energy Conversion and Storage

Porous CoSe₂ on carbon cloth is prepared from a cobalt-based metal organic framework template with etching and selenization reaction, which has both a larger specific surface area and outstanding electrical conductivity. As the catalyst for oxygen evolution reaction, the porous CoSe₂ achieves a lower onset potential of 1.48 V versus the reversible hydrogen electrode (RHE) and a small potential of 1.52 V (vs RHE) at an anodic current density of 10 mA cm^-2. Especially, the linear sweep voltammogram curve of the porous CoSe₂ is in consist with the initial curve after durability test for 24 h. When tested as an electrode for supercapacitor, it can deliver a specific capacitance of 713.9 F g^-1 at current density of 1 mA cm^-2 and exhibit excellent cycling stability in that a capacitance retention of 92.4% can be maintained after 5000 charge-discharge cycles at 5 mA cm^-2. Our work presents a novel strategy for construction of electrochemical electrode.

Nanocrystalline Co0.85Se Anchored on Graphene Nanosheets as a Highly Efficient and Stable Electrocatalyst for Hydrogen Evolution Reaction

For the first time, a porous and conductive Co_0.85Se/graphene network (CSGN), constructed by Co_0.85Se nanocrystals being tightly connected with each other and homogeneously anchored on few-layered graphene nanosheets, has been synthesized by a facile one-pot solvothermal method. Compared to unhybridized Co_0.85Se, CSGN exhibits much faster kinetics and better electrocatalytic behavior for hydrogen evolution reaction (HER). The HER mechanism of CSGN is improved to Volmer-Tafel combination, instead of Volmer-Heyrovsky combination, for Co_0.85Se. CSGN has a very low Tafel slope of 34.4 mV/dec, which is much lower than that of unhybridized Co_0.85Se (41.8 mV/dec) and is the lowest ever reported for Co_0.85Se-based electrocatalysts. CSGN delivers a current density of 55 mA/cm² at 250 mV overpotential, much larger than that of Co_0.85Se (33 mA/cm²). Furthermore, CSGN shows superior electrocatalytic stability even after 1500 cycles. The excellent HER performance of CSGN is attributed to the unique porous and conductive network, which can not only guarantee interconnected conductive paths in the whole electrode but also provide abundant catalytic active sites, thereby facilitating charge transportation between the electrocatalyst and electrolyte. This work provides insight into rational design and low-cost synthesis of nonprecious transition-metal chalcogenide-based electrocatalysts with high efficiency and excellent stability for HER.

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 .

Dendritic growth of monolayer ternary WS2(1-x)Se2x flakes for enhanced hydrogen evolution reaction

Two-dimensional transition-metal dichalcogenides (TMDs) have attracted much research interest in the hydrogen evolution reaction (HER) due to their superior electrocatalytic properties. Beyond binary TMDs, ternary TMD alloys, as electrocatalysts, were also gradually acknowledged for their remarkable efficiency in HER. Herein, we successfully synthesized monolayer dendritic ternary WS_2(1-x)Se_2x flakes possessing abundant active edge sites on a single crystalline SrTiO₃ (STO(100)). And the obtained dendritic WS_2(1-x)Se_2x flakes could be transferred intact to arbitrary substrates, for example, SiO₂/Si and Au foils. Intriguingly, the transferred dendritic WS_2(1-x)Se_2x flakes on Au foil demonstrate a significant HER performance, reflected by a rather lower Tafel slope of ∼69 mV dec^-1 and a much higher exchange current density of ∼50.1 μA cm^-2 outshining other CVD-grown two-dimensional TMD flakes. Furthermore, our new material shows excellent stability in electro-catalyzing the HER, suggestive of its robustness for being an excellent electrocatalyst. We believe that this work broadens the outlook for the synthesis of two-dimensional TMDs toward satisfying the applications in electrocatalysis.

Efficient Catalysis of Hydrogen Evolution Reaction from WS2(1-x) P2x Nanoribbons

The rational design of Earth abundant electrocatalysts for efficiently catalyzing hydrogen evolution reaction (HER) is believed to lead to the generation of carbon neutral energy carrier. Owing to their fascinating chemical and physical properties, transition metal dichalcogenides (TMDs) are widely studied for this purpose. Of particular note is that doping by foreign atom can bring the advent of electronic perturbation, which affects the intrinsic catalytic property. Hence, through doping, the catalytic activity of such materials could be boosted. A rational synthesis approach that enables phosphorous atom to be doped into WS₂ without inducing phase impurity to form WS_2(1-x) P_2x nanoribbon (NRs) is herein reported. It is found that the WS_2(1-x) P_2x NRs exhibit considerably enhanced HER performance, requiring only -98 mV versus reversible hydrogen electrode to achieve a current density of -10 mA cm^-2 . Such a high performance can be attributed to the ease of H-atom adsorption and desorption due to intrinsically tuned WS₂ , and partial formation of NRs, a morphology wherein the exposure of active edges is more pronounced. This finding can provide a fertile ground for subsequent works aiming at tuning intrinsic catalytic activity of TMDs.

An efficient ternary CoP2xSe2(1-x) nanowire array for overall water splitting

Developing earth-abundant and efficient bifunctional electrocatalysts for realizing the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) under alkaline conditions is an intriguing challenge. Here, ternary necklace-like CoP_2xSe_2(1-x) nanowire arrays are synthesized via simultaneously phosphorizing and selenizing Co(OH)₂ nanowires. Owing to the substitution of the P atom in the ternary system, the optimal electronic structure of CoP_2xSe_2(1-x) can be obtained and the stability can also be enhanced for hydrogen evolution. Thus, the ternary CoP_2xSe_2(1-x) NWs are highly active for electrochemical hydrogen evolution in both acidic and alkaline media, achieving a current density of 10 mA cm ^-2 at overpotentials of 70 mV and 98 mV, respectively. To realize the overall water splitting, we further performed the experiment using the CoP_2xSe_2(1-x) NWs as a cathode and Co(OH)₂ NWs as an anode, which requires a cell voltage of 1.65 V to afford a water splitting current density of 10 mA cm ^-2 in strong alkaline media (1.0 M KOH).

Microwave-Initiated Facile Formation of Ni3Se4 Nanoassemblies for Enhanced and Stable Water Splitting in Neutral and Alkaline Media

Molecular hydrogen (H₂) generation through water splitting with minimum energy loss has become practically possible due to the recent evolution of high-performance electrocatalysts. In this study, we fabricated, evaluated, and presented such a high-performance catalyst which is the Ni₃Se₄ nanoassemblies that can efficiently catalyze water splitting in neutral and alkaline media. A hierarchical nanoassembly of Ni₃Se₄ was fabricated by functionalizing the surface-cleaned Ni foam using NaHSe solution as the Se source with the assistance of microwave irradiation (300 W) for 3 min followed by 5 h of aging at room temperature (RT). The fabricated Ni₃Se₄ nanoassemblies were subjected to catalyze water electrolysis in neutral and alkaline media. For a defined current density of 50 mA cm^-2, the Ni₃Se₄ nanoassemblies required very low overpotentials for the oxygen evolution reaction (OER), viz., 232, 244, and 321 mV at pH 14.5, 14.0, and 13.0 respectively. The associated lower Tafel slope values (33, 30, and 40 mV dec^-1) indicate the faster OER kinetics on Ni₃Se₄ surfaces in alkaline media. Similarly, in the hydrogen evolution reaction (HER), for a defined current density of 50 mA cm^-2, the Ni₃Se₄ nanoassemblies required low overpotentials of 211, 206, and 220 mV at pH 14.5, 14.0, and 13.0 respectively. The Tafel slopes for HER at pH 14.5, 14.0, and 13.0 are 165, 156, and 128 mV dec^-1, respectively. A comparative study on both OER and HER was carried out with the state-of-the-art RuO₂ and Pt under identical experimental conditions, the results of which revealed that our Ni₃Se₄ is a far better high-performance catalyst for water splitting. Besides, the efficiency of Ni₃Se₄ nanoassemblies in catalyzing water splitting in neutral solution was carried out, and the results are better than many previous reports. With these amazing advantages in fabrication method and in catalyzing water splitting at various pH, the Ni₃Se₄ nanoassemblies can be an efficient, cheaper, nonprecious, and high-performance electrode for water electrolysis with low overpotentials.

An amorphous dual action electrocatalyst based on oxygen doped cobalt sulfide for the hydrogen and oxygen evolution reactions

Here we electrodeposit an amorphous bifunctional electrocatalyst that is active for both the HER and OER under alkaline conditions which is based on oxygen doped cobalt sulfide.

Engineering the Electronic Structure of 2D WS2 Nanosheets Using Co Incorporation as Cox W(1- x ) S2 for Conspicuously Enhanced Hydrogen Generation

Transition metal dichalcogenides (TMDs), as one of potential electrocatalysts for hydrogen evolution reaction (HER), have been extensively studied. Such TMD-based ternary materials are believed to engender optimization of hydrogen adsorption free energy to thermoneutral value. Theoretically, cobalt is predicted to actively promote the catalytic activity of WS2 . However, experimentally it requires systematic approach to form Cox W(1- x ) S2 without any concomitant side phases that are detrimental for the intended purpose. This study reports a rational method to synthesize pure ternary Cox W(1- x ) S2 nanosheets for efficiently catalyzing HER. Benefiting from the modification in the electronic structure, the resultant material requires overpotential of 121 mV versus reversible hydrogen electrode (RHE) to achieve current density of 10 mA cm(-2) and shows Tafel slope of 67 mV dec(-1) . Furthermore, negligible loss of activity is observed over continues electrolysis of up to 2 h demonstrating its fair stability. The finding provides noticeable experimental support for other computational reports and paves the way for further works in the area of HER catalysis based on ternary materials.