Engineering the Electrical Conductivity of Lamellar Silver-Doped Cobalt(II) Selenide Nanobelts for Enhanced Oxygen Evolution

Tailoring atomically local electric field of NiFe layered double hydroxides with Ag dopants to boost oxygen evolution kinetics

The demand for clean energy sources has driven focus towards advanced electrochemical systems. However, the sluggish kinetics of the oxygen evolution reaction (OER) constrain the energy conversion efficiency of relevant devices. Herein, a one-step method is reported to grow oxygen vacancies (Vo) rich NiFeAg layered double hydroxides nanoclusters on carbon cloth (Vo-NiFeAg-LDH/CC) for serving as the self-supporting electrode to catalyze OER. The OER performance of Vo-NiFeAg-LDH/CC has been remarkably enhanced through Ag and Vo co-modification compared with pristine NiFe-LDH, achieving a low Tafel slope of 49.7 mV dec-1 in 1 m KOH solution. Additionally, the current density of Vo-NiFeAg-LDH/CC is 3.23 times higher than that of the state-of-art IrO2 at 2 V under an alkaline flow electrolyzer setup. Theoretical calculations and experimental results collectively demonstrate that Ag dopant and Vo strengthen the O* adsorption with active sites, further promoting the deprotonation step from OH* to O* and accelerating the catalytic reaction. In a word, this work clarifies the structural correlation and synergistic mechanism of Ag dopant and Vo, providing valuable insights for the rational design of catalyst for renewable energy applications.

Coupling Ni Single Atomic Sites with Metallic Aggregates at Adjacent Geometry on Carbon Support for Efficient Hydrogen Peroxide Electrosynthesis

Single atomic catalysts have shown great potential in efficiently electro-converting O2 to H2O2 with high selectivity. However, the impact of coordination environment and introduction of extra metallic aggregates on catalytic performance still remains unclear. Herein, first a series of carbon-based catalysts with embedded coupling Ni single atomic sites and corresponding metallic nanoparticles at adjacent geometry is synthesized. Careful performance evaluation reveals NiSA/NiNP-NSCNT catalyst with precisely controlled active centers of synergetic adjacent Ni-N4S single sites and crystalline Ni nanoparticles exhibits a high H2O2 selectivity over 92.7% within a wide potential range (maximum selectivity can reach 98.4%). Theoretical studies uncover that spatially coupling single atomic NiN4S sites with metallic Ni aggregates in close proximity can optimize the adsorption behavior of key intermediates *OOH to achieve a nearly ideal binding strength, which thus affording a kinetically favorable pathway for H2O2 production. This strategy of manipulating the interaction between single atoms and metallic aggregates offers a promising direction to design new high-performance catalysts for practical H2O2 electrosynthesis.

Reinforcement of Electrocatalytic Oxygen Evolution Activity Enabled by Constructing Silver-Incorporated NiCo-PBA@NiFe-LDH Hierarchical Nanoboxes

Complicated oxygen evolution reaction (OER) poses the bottleneck in improving the efficiency of hydrogen production through water electrolysis. Herein, an integrated strategy to modulate the electronic structure of NiFe layered double hydroxide (NiFe-LDH) is reported by constructing Ag-incorporated NiCo-PBA@NiFe-LDH heterojunction with a hierarchical hollow structure. This "double heterojunction" facilitates local charge polarization at the interface, thereby promoting electron transfer and reducing the adsorption energy of intermediates, ultimately enhancing the intrinsic activity of the catalyst. It is noteworthy that an exchange bias field is observed between NiCo-PBA and NiFe-LDH, which will be conducive to regulating the electron spin states of metals and facilitating the production of triplet oxygen. Additionally, the unique hierarchical nanoboxes provide a large specific surface area that ensures adequate exposure to adsorption sites and active sites. Profiting from the synergistic advantages, the overpotential is as low as 190 mV at a current density of 10 mA cm-2, with a low Tafel slope of 21 mV dec-1. Moreover, density functional theory (DFT) calculation further substantiated that the incorporation of Ag in the heterojunction can effectively reduce the adsorption energy of reactant intermediates and enhance the conductivity.

Lattice Oxygen Activation through Deep Oxidation of Co4N by Jahn-Teller-Active Dopants for Improved Electrocatalytic Oxygen Evolution

Triggering the lattice oxygen oxidation mechanism is crucial for improving oxygen evolution reaction (OER) performance, because it could bypass the scaling relation limitation associated with the conventional adsorbate evolution mechanism through the direct formation of oxygen-oxygen bond. High-valence transition metal sites are favorable for activating the lattice oxygen, but the deep oxidation of pre-catalysts suffers from a high thermodynamic barrier. Here, taking advantage of the Jahn-Teller (J-T) distortion induced structural instability, we incorporate high-spin Mn3+ (t 2 g 3 e g 1 ${{t}_{2g}^{3}{e}_{g}^{1}}$ ) dopant into Co4N. Mn dopants enable a surface structural transformation from Co4N to CoOOH, and finally to CoO2, as observed by various in situ spectroscopic investigations. Furthermore, the reconstructed surface on Mn-doped Co4N triggers the lattice oxygen activation, as evidenced experimentally by pH-dependent OER, tetramethylammonium cation adsorption and online electrochemical mass spectrometry measurements of 18O-labelled catalysts. In general, this work not only offers the introducing J-T effect approach to regulate the structural transition, but also provides an understanding about the influence of the catalyst's electronic configuration on determining the reaction route, which may inspire the design of more efficient catalysts with activated lattice oxygen.

Defect-engineered Sulfur Vacancy Modified NiCo2 S4-x Nanosheet Anchoring Polysulfide for Improved Lithium Sulfur Batteries

The low conductivity of sulfur and the shuttle effect of lithium polysulfides (LiPSs) are the two intrinsic obstacles that limit the application of lithium-sulfur batteries (LSBs). Herein, a sulfur vacancy introduced NiCo2 S4 nanosheet array grown on carbon nanofiber (CNF) membrane (NiCo2 S4-x /CNF) is proposed to serve as a self-supporting and binder-free interlayer in LSBs. The conductive CNF skeleton with a non-woven structure can effectively reduce the resistance of the cathode and accommodate volume expansion during charge-discharge process. The bonding between CNF matrix and NiCo2 S4 nanosheet is enhanced by in situ growth, ensuring fast electron transfer. Besides, the sulfur vacancies in NiCo2 S4 enhance the chemisorption of LiPSs, and the highly active sites at vacancies can accelerate the LiPSs conversion kinetics. LSB paired with NiCo2 S4-x /CNF interlayer achieved improved stability in 500 cycles at 0.2 C and long life of 3000 cycles at 3 C. More importantly, a high areal capacity of 9.69 mAh cm-2 is achieved with a sulfur loading of 10.8 mg cm-2 and a low electrolyte to sulfur (E/S) ratio of 4.8. This work provides insight into the sulfur vacancy in catalysis design for LiPSs conversion and demonstrates a promising direction for electronic defect engineering in material design for LSBs.

In Situ Anodic Oxidation Tuning of NiFeV Diselenide to the Core-Shell Heterojunction for Boosting Oxygen Evolution

Developing non-noble metal-based core-shell heterojunction electrocatalysts with high catalytic activity and long-lasting stability is crucial for the oxygen evolution reaction (OER). Here, we prepared novel core-shell Fe,V-NiSe₂@NiFe(OH)_x heterostructured nanoparticles on hydrophilic-treated carbon paper with high electronic transport and large surface area for accelerating the oxygen evolution rate via high-temperature selenization and electrochemical anodic oxidation procedures. Performance testing shows that Fe,V-NiSe₂@NiFe(OH)_x possesses the highest performance for OER compared to as-prepared diselenide core-derived heterojunctions, which only require an overpotential of 243 mV at 10 mA cm^-2 and a low Tafel slope of 91.6 mV decade^-1 under basic conditions. Furthermore, X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) confirm the morphology and elementary stabilities of Fe,V-NiSe₂@NiFe(OH)_x after long-term chronopotentiometric testing. These advantages are largely because of the strong synergistic effect between the Fe,V-NiSe₂ core with high conductivity and the amorphous NiFe(OH)_x shell with enriched defects and vacancies. This study also presents a general approach to designing and synthesizing more active core-shell heterojunction electrocatalysts for OER.

Niobium-Incorporated CoSe2 Nanothorns with Electronic Structural Alterations for Efficient Alkaline Oxygen Evolution Reaction at High Current Density

Developing cost-effective, highly active, and robust electrocatalysts for oxygen evolution reaction (OER) at high current density is a critical challenge in water electrolysis since the sluggish kinetics of the OER significantly impedes the energy conversion efficiency of overall water splitting. Here, a 1D nanothorn-like Nb-CoSe₂ /CC (CC=carbon cloth) structure was developed as an efficient OER catalyst. The optimized Nb-CoSe₂ /CC catalyst exhibited remarkable OER performance with the low overpotentials of 220 mV at 10 mA cm^-2 and 297 mV 200 mA cm^-2 and a small Tafel slope (54.1 mV dec^-1 ) in 1.0 m KOH electrolyte. More importantly, the Nb-CoSe₂ /CC electrode displayed superior stability after 60 h of continuous operation. In addition, cell voltages of 1.52 and 1.93 V were required to achieve 10 and 500 mA cm^-2 for the electrolyzer made of Nb-CoSe₂ /CC (anode) and the Pt/C (cathode). Density functional theory (DFT) calculations combined with experimental results revealed that incorporating niobium into the CoSe₂ could optimize the adsorption free energy of the reaction intermediates and enhance the conductivity to improve the catalytic activity further. Additionally, the super-hydrophilicity of Nb-CoSe₂ /CC resulting from the surface defects increased the surface wettability and facilitated reaction kinetics. These results indicate that Nb-CoSe₂ /CC intrinsically enhances OER performance and possesses potential practical water electrolysis applications.

Implanting an Electron Donor to Enlarge the d-p Hybridization of High-Entropy (Oxy)hydroxide: A Novel Design to Boost Oxygen Evolution

High-entropy (HE) electrocatalysts are becoming a research hotspot due to their interesting "cocktail effect" and have great potential for tailored catalytic properties. However, it is still a great challenge to illustrate their inherent catalytic mechanism for the "cocktail effect", and there is also a paucity of quantitative descriptors to characterize the specific catalytic activity and give logical design strategies for HE systems. Herein, the unexpected activation of all metal sites in HE Cu-Co-Fe-Ag-Mo (oxy)hydroxides for the oxygen evolution reaction (OER) is reported, and it is found that metal-oxygen d-p hybridization, as an effective descriptor, can indicate the intrinsic activity of each metal site. According to the quantitative hybridization, introducing an electron donor (e.g., Ag) is raised and verified to reinforce the electrocatalytic activity of the HE system. Consequently, Ag-decorated Co-Cu-Fe-Ag-Mo (oxy)hydroxide (Ag@CoCuFeAgMoOOH) electrocatalysts are constructed by an electrochemical reconstruction method, and their OER performances are thoroughly characterized. The Ag@CoCuFeAgMoOOH is verified with a low overpotential (270 mV at 100 mA cm^-2 ) and a small Tafel slope (35.3 mV dec^-1 ), as well as good electrochemical stability. The favorable activity of the electron donor and underlying synergistic "cocktail effect" are demonstrated and disclosed. This work opens up a new strategy to guide the design/fabrication of advanced HE electrocatalysts.

Engineering the oxygen vacancies of rocksalt-type high-entropy oxides for enhanced electrocatalysis

High-entropy oxides (HEOs), a class of compounds that include five or more elemental species, have gained increasing attraction for their capability of optimizing the target properties. To date, even though some high-entropy oxides have been successfully prepared, their applications still need to be explored. In the present study, a lithium-manipulation strategy for constructing transition metal oxides (LTM) via a modified solid-state method was investigated. The as-synthesized LTM contained six highly dispersed metal species (Li, Fe, Co, Ni, Cu, Zn) and demonstrated a rocksalt-type structure. Besides, with the introduction of Li, more oxygen vacancies were produced which was also accompanied by shrinking of the lattice constant. When the molar ratio of Li was equal to the other TM cations (LTM16.7), the electrical conductivity was greatly enhanced by a factor of 10 times. Moreover, LTM16.7 achieved the best HER (η = 207 mV at 10 mA cm^-2) and OER performances (η = 347 mV at 10 mA cm^-2) with elevated electrical conductivity. To facilitate further design of this new kind of materials, we also conducted DFT calculations and elemental alternation experiments, which showed that Fe acted as electrocatalytic sites in this HEOs system. This Li-incorporation strategy opens a new way to understand and modify defect-related HEOs.

Phosphorous Doped Two-dimensional CoFe2O4 Nanobelt Decorated with Ru Nanoclusters and Co-Fe Hydroxide as Efficient Electrocatalysts Toward Hydrogen Generation

Developing efficient and durable hydrogen evolution reaction (HER) electrocatalysts has attracted considerable concerns for large-scale hydrogen generation. In this work, phosphorous doped two-dimensional (2D) CoFe2O4 nanobelt decorated with Ru and...

Structural Transformation of Heterogeneous Materials for Electrocatalytic Oxygen Evolution Reaction

Electrochemical water splitting for hydrogen generation is a promising pathway for renewable energy conversion and storage. One of the most important issues for efficient water splitting is to develop cost-effective and highly efficient electrocatalysts to drive sluggish oxygen-evolution reaction (OER) at the anode side. Notably, structural transformation such as surface oxidation of metals or metal nonoxide compounds and surface amorphization of some metal oxides during OER have attracted growing attention in recent years. The investigation of structural transformation in OER will contribute to the in-depth understanding of accurate catalytic mechanisms and will finally benefit the rational design of catalytic materials with high activity. In this Review, we provide an overview of heterogeneous materials with obvious structural transformation during OER electrocatalysis. To gain insight into the essence of structural transformation, we summarize the driving forces and critical factors that affect the transformation process. In addition, advanced techniques that are used to probe chemical states and atomic structures of transformed surfaces are also introduced. We then discuss the structure of active species and the relationship between catalytic performance and structural properties of transformed materials. Finally, the challenges and prospects of heterogeneous OER electrocatalysis are presented.

Hydrothermal synthesis, characterization and photocatalytic activity of Mg doped MoS2

In this research work nanoparticles of Mg (0, 1, 2 and 3%) doped MoS2 are prepared by Hydrothermal method at 200 °C for 9 h. Scanning Electron Microscope (SEM) for surface morphology, Fourier Transform Infrared Spectroscopy (FTIR) for structural and chemical bonding and UV-visible spectroscopy for optical properties are used. SEM showed that sheet-like structure has changed into stone-like shaped when Mg has doped into MoS2. From FTIR, Mo–O, Mo=S, and H–O bond peaks are becoming dim and new chemical bonds S=O, Mo=O, Mg–O, CH and OH are forming with the increase of Mg doping. UV-visible spectroscopy showed that MoS2 has an indirect bandgap 2.21 eV. Band gap decreased from 1.84 to 1.82 eV when the Mg doping was increased from 1 to 2%, respectively. As Mg concentration was increased i.e. 3% then band gap increased to 1.88 eV. Photocatalytic activity (PCA) of undoped and Mg doped MoS2 is appraised by degrading rhodamine blue (RhB) and methylene blue (MB) dyes. The results showed that PCA (in presence of visible light) Mg doped MoS2 is greater than pure MoS2 which significantly increased the photocatalytic properties.

Constructing highly active Co sites in Prussian blue analogues for boosting electrocatalytic water oxidation

High-valence cobalt sites are considered as highly active centers for the oxygen evolution reaction (OER) and their corresponding construction is thus of primary importance in the pursuit of outstanding performance. Herein, we report the design and facile synthesis of abundant high-valence cobalt sites by introducing Zn²⁺ into CoFe Prussian blue analogues (PBAs). The modification results in the drastic morphological transformation from a pure phase (CoFe-PBA) to a three-phase composite (CoFeZn-PBA), with a significant increase not only the amount of highly oxidized Co sites but the specific surface area (by up to 4 times). Moreover, the obtained sample also exhibits outstanding electric conductivity. Consequently, an excellent OER performance with an overpotential of 343 mV@10 mA cm^-2 and a Tafel slope of 75 mV dec^-1 was achieved in CoFeZn-PBA, which outperforms the commercial IrO₂ catalyst. Further analysis reveals that CoFeZn-PBA becomes (oxyhydr)oxides after the OER.

Clean and Affordable Hydrogen Fuel from Alkaline Water Splitting: Past, Recent Progress, and Future Prospects

Hydrogen economy has emerged as a very promising alternative to the current hydrocarbon economy, which involves the process of harvesting renewable energy to split water into hydrogen and oxygen and then further utilization of clean hydrogen fuel. The production of hydrogen by water electrolysis is an essential prerequisite of the hydrogen economy with zero carbon emission. Among various water electrolysis technologies, alkaline water splitting has been commercialized for more than 100 years, representing the most mature and economic technology. Here, the historic development of water electrolysis is overviewed, and several critical electrochemical parameters are discussed. After that, advanced nonprecious metal electrocatalysts that emerged recently for negotiating the alkaline oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) are discussed, including transition metal oxides, (oxy)hydroxides, chalcogenides, phosphides, and nitrides for the OER, as well as transition metal alloys, chalcogenides, phosphides, and carbides for the HER. In this section, particular attention is paid to the catalyst synthesis, activity and stability challenges, performance improvement, and industry-relevant developments. Some recent works about scaled-up catalyst synthesis, novel electrode designs, and alkaline seawater electrolysis are also spotlighted. Finally, an outlook on future challenges and opportunities for alkaline water splitting is offered, and potential future directions are speculated.

Doping regulation in transition metal compounds for electrocatalysis

In electrocatalysis, doping regulation has been considered as an effective method to modulate the active sites of catalysts, providing a powerful means for creating a large variety of highly efficient catalysts for various reactions. Of particular interest, there has been growing research concerning the doping of two-dimensional transition-metal compounds (TMCs) to optimize their electrocatalytic performance. Despite the previous achievements, mechanistic insights of doping regulation in TMCs for electrocatalysis are still lacking. Herein, we provide a systematic overview of doping regulation in TMCs in terms of background, preparation, impacts on physicochemical properties, and typical applications including the hydrogen evolution reaction, oxygen evolution reaction, oxygen reduction reaction, CO₂ reduction reaction, and N₂ reduction reaction. Notably, we bridge the understanding between the doping regulation of catalysts and their catalytic activities via focusing on the physicochemical properties of catalysts from the aspects of vacancy concentrations, phase transformation, surface wettability, electrical conductivity, electronic band structure, local charge distribution, tunable adsorption strength, and multiple adsorption configurations. We also discuss the existing challenges and future perspectives in this promising field.

1 T-MoSe2 monolayer supported single Pd atom as a highly-efficient bifunctional catalyst for ORR/OER

The development of highly-efficient catalysts for oxygen reduction reaction (ORR) or oxygen evolution reaction (OER) is highly crucial for the commercial applications of some novel energy-related devices. Herein, using comprehensive first-principles computations, the potential of a variety of single metal-based catalysts supported by MoSe₂ nanosheet to boost the ORR or OER process was evaluated. The computations revealed that these considered metal atoms can be more stably anchored on 1 T-MoSe₂ than those of on 2H-MoSe₂. In particular, the supported Ni and Pd catalysts on 1 T-MoSe₂ exhibit high OER activity due to their quite low overpotential (0.47 and 0.49 V). Meanwhile, the anchored Pd atom on 1 T-MoSe₂ also displays excellent ORR performance with an ultra-low overpotential of 0.32 V, thus implying its superior bifunctional activity for ORR/OER. Our results provide a quite promising avenue to design a new class of MoSe₂-based single atom catalysts for fuel cells, which also further enriches the application fields of MoSe₂ nanosheets in advanced catalysis.

Engineering High-Spin State Cobalt Cations in Spinel Zinc Cobalt Oxide for Spin Channel Propagation and Active Site Enhancement in Water Oxidation

Spinel zinc cobalt oxide (ZnCo₂ O₄ ) is not considered as a superior catalyst for the electrochemical oxygen evolution reaction (OER), which is the bottleneck reaction in water-electrolysis. Herein, taking advantage of density functional theory (DFT) calculations, we find that the existence of low-spin (LS) state cobalt cations hinders the OER activity of spinel zinc cobalt oxide, as the t_2g ⁶ e_g ⁰ configuration gives rise to purely localized electronic structure and exhibits poor binding affinity to the key reaction intermediate. Increasing the spin state of cobalt cations in spinel ZnCo₂ O₄ is found to propagate a spin channel to promote spin-selected charge transport during OER and generate better active sites for intermediates adsorption. The experiments find increasing the calcination temperature a facile approach to engineer high-spin (HS) state cobalt cations in ZnCo₂ O₄ , while not working for Co₃ O₄ . The activity of the best spin-state-engineered ZnCo₂ O₄ outperforms other typical Co-based oxides.

Defect-engineered three-dimensional vanadium diselenide microflowers/nanosheets on carbon cloth by chemical vapor deposition for high-performance hydrogen evolution reaction

In the past decades, defect engineering has become an effective strategy to significantly improve the hydrogen evolution reaction (HER) efficiency of electrocatalysts. In this work, a facile chemical vapor deposition (CVD) method is firstly adopted to demonstrate defect engineering in high-efficiency HER electrocatalysts of vanadium diselenide nanostructures. For practical applications, the conductive substrate of carbon cloth (CC) is selected as the growth substrate. By using a four-time CVD method, uniform three-dimensional microflowers with defect-rich small nanosheets on the surface are prepared directly on the CC substrate, displaying a stable HER performance with a low Tafel slope value of 125 mV dec^-1and low overpotential voltage of 295 mV at a current density of 10 mA cm^-2in alkaline electrolyte. Based on the results of x-ray photoelectron spectra and density functional theory calculations, the impressive HER performance originates from the Se vacancy-related active sites of small nanosheets, while the microflower/nanosheet homoepitaxy structure facilitates the carrier flow between the active sites and conductive substrate. All the results present a new route to achieve defect engineering using the facile CVD technique, and pave a novel way to prepare high-activity layered electrocatalysts directly on a conductive substrate.

The rational design of Cu2-xSe@(Co,Cu)Se2 core-shell structures as bifunctional electrocatalysts for neutral-pH overall water splitting

Highly active and stable bifunctional electrocatalysts for H2 generation from neutral-pH water are desired, but difficult to achieve. The modification of the electronic and crystal structure of a material by element doping, morphology design and constructing a complex is a valid strategy for obtaining high-performance catalysts toward overall water splitting. In this study, a novel Cu2-xSe@(Co,Cu)Se2 core-shell structure with ultrathin (Co,Cu)Se2 nanosheets anchored as a shell on an internal Cu2-xSe core was fabricated, for the first time, by integrating the three above-mentioned modification methods. Benefiting from the synergistic effect between components and the unique structure, the Cu2-xSe@(Co,Cu)Se2 core-shell structure can serve as an efficient bifunctional electrocatalyst for both HERs and OERs in neutral-pH electrolytes with a current density of 10 mA cm-2 at the overpotentials of 106 mV and 396 mV, respectively. Additionally, the material just requires a cell voltage of 1.73 V to afford a current density of 10 mA cm-2 in a neutral two-electrode electrolyzer. Such performances significantly outperform control catalysts and analogues. Even more importantly, the original concept of coordinated regulation presented in this work can broaden our horizons in the design of new and highly efficient catalysts for neutral water splitting.

Dual Role of Silver Moieties Coupled with Ordered Mesoporous Cobalt Oxide towards Electrocatalytic Oxygen Evolution Reaction

Herein, we show that the performance of mesostructured cobalt oxide electrocatalyst for oxygen evolution reaction (OER) can be significantly enhanced by coupling of silver species. Various analysis techniques including pair distribution function and Rietveld refinement, X-ray absorption spectroscopy at synchrotron as well as advanced electron microscopy revealed that silver exists as metallic Ag particles and well-dispersed Ag2 O nanoclusters within the mesostructure. The benefits of this synergy are twofold for OER: highly conductive metallic Ag improves the charge transfer ability of the electrocatalysts while ultra-small Ag2 O clusters provide the centers that can uptake Fe impurities from KOH electrolyte and boost the catalytic efficiency of Co-Ag oxides. The current density of mesostructured Co3 O4 at 1.7 VRHE is increased from 102 to 211 mA cm-2 with incorporation of silver spices. This work presents the dual role of silver moieties and demonstrates a simple method to increase the OER activity of Co3 O4 .

Recent progress in transition metal selenide electrocatalysts for water splitting

The urgent demand of scalable hydrogen production has motivated substantial research on low cost, efficient and robust catalysts for water electrolysis. In order to replace noble metals and their derivatives, transition metal (Fe, Co, Ni, Mo, Cu, etc.) selenides have demonstrated promising catalysis on both hydrogen and oxygen evolutions. Very recently, a number of reports have presented a variety of approaches to enhance their electrocatalytic activity. This review summarizes the most recent progress in transition metal selenide electrocatalysts for HER, OER, and overall water splitting. The merits and limitations of metal selenides are also discussed in the aspects of structure and composition. Moreover, we highlight new strategies and future challenges for design and synthesis of high performance electrocatalysts.

Hierarchical 0D-2D Co/Mo Selenides as Superior Bifunctional Electrocatalysts for Overall Water Splitting

Development of efficient electrocatalysts combining the features of low cost and high performance for both the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) still remains a critical challenge. Here, we proposed a facile strategy to construct in situ a novel hierarchical heterostructure composed of 0D−2D CoSe₂/MoSe₂ by the selenization of CoMoO₄ nanosheets grafted on a carbon cloth (CC). In such integrated structure, CoSe₂ nanoparticles dispersed well and tightly bonded with MoSe₂ nanosheets, which can not only enhance kinetics due to the synergetic effects, thus promoting the electrocatalytic activity, but also effectively improve the structural stability. Benefiting from its unique architecture, the designed CoSe₂/MoSe₂ catalyst exhibits superior OER and HER performance. Specifically, a small overpotential of 280 mV is acquired at a current density of 10 mA·cm⁻² for OER with a small Tafel slope of 86.8 mV·dec⁻¹, and the overpotential is 90 mV at a current density of 10 mA·cm⁻² for HER with a Tafel slope of 84.8 mV·dec⁻¹ in 1 M KOH. Furthermore, the symmetrical electrolyzer assembled with the CoSe₂/MoSe₂ catalysts depicts a small cell voltage of 1.63 V at 10 mA·cm⁻² toward overall water splitting.

Synergistic tuning of oxygen vacancies and d-band centers of ultrathin cobaltous dihydroxycarbonate nanowires for enhanced electrocatalytic oxygen evolution

Simple and controllable synthesis of efficient and robust non-noble metal electrocatalysts towards the oxygen evolution reaction (OER) is highly desired and challenging in the development of sustainable energy conversion technologies. Herein, we report a facile one-step solvothermal synthesis of cobaltous dihydroxycarbonate nanowires (Co-OCH NWs) with a tunable diameter ranging from 8.7 to 16.7 nm, which were able to exhibit an interesting diameter-dependent catalytic activity towards the OER. It should be highlighted that the thinnest nanowires (8.7 nm) demonstrated the best OER catalytic activity among the as-prepared nanowires, showing an overpotential of only 307 mV at 10 mA cm-2 and a Tafel slope of 75 mV dec-1 in 1.0 M KOH solution. Based on comprehensive analysis, the excellent electrocatalytic activity of Co-OCH NWs was ascribed to the simultaneous achievement of an enlarged specific surface area, increased oxygen vacancy concentration and favorable position of the 3d-band center for the Co-OCH NWs with the continuous decrease of their diameters. More importantly, this work has emphasized that synergistic tuning of the oxygen vacancy concentration and d-band center position of nanomaterials via facile size control enables boosting their electrocatalytic performance substantially, thereby opening a simple route to design and prepare Earth-abundant electrocatalysts with higher efficiency and lower cost.

Rational phase transformation and morphology design to optimize oxygen evolution property of cobalt tungstate

In this work, a facile and feasible soft template method with the aid of buffer solution is successfully applied to synthesize high-order mesoporous cobalt tungstate for the first time. Attributing to the regulation of reaction solution's pH value and the existence of template, the phenomenon of phase transformation occurs, and high-order mesoporous structure is formed. Because of the variation of phase and morphology, only 448 mV can deliver a current density of 10 mA cm^-2 with a small Tafel slope (61 mV dec^-1) for mesoporous cobalt tungsten oxide hydroxide, while the cobalt tungstate nanoparticles cannot satisfy the basic demand of electrocatalysts. Herein, rational phase transformation and morphology design can significantly affect the property of oxygen evolution, which can provide vast opportunities to turn into candidates for the novel oxygen evolution catalyst.

Approaching the activity limit of CoSe2 for oxygen evolution via Fe doping and Co vacancy

Electronic structure engineering lies at the heart of efficient catalyst design. Most previous studies, however, utilize only one technique to modulate the electronic structure, and therefore optimal electronic states are hard to be achieved. In this work, we incorporate both Fe dopants and Co vacancies into atomically thin CoSe₂ nanobelts for /coxygen evolution catalysis, and the resulted CoSe₂-D_Fe–V_Co exhibits much higher catalytic activity than other defect-activated CoSe₂ and previously reported FeCo compounds. Deep characterizations and theoretical calculations identify the most active center of Co₂ site that is adjacent to the V_Co-nearest surface Fe site. Fe doping and Co vacancy synergistically tune the electronic states of Co₂ to a near-optimal value, resulting in greatly decreased binding energy of OH* (ΔE_OH) without changing ΔE_O, and consequently lowering the catalytic overpotential. The proper combination of multiple defect structures is promising to unlock the catalytic power of different catalysts for various electrochemical reactions.

While electronic-structure engineering lies at the heart of catalyst design, most previous studies utilize only one technique to tune the electronic states. Here, authors demonstrate that Fe doping and Co vacancy work synergistically to approach the activity limit of CoSe₂ for oxygen evolution reaction.

Electro-synthesized Co(OH)2@CoSe with Co-OH active sites for overall water splitting electrocatalysis

Constructing noble metal-free electrocatalytically active sites for the simultaneous hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in alkaline solution is key to realizing electricity-driven water splitting in practical applications. Here, we rationally designed Co(OH)₂@CoSe nanorods (NRs) as an excellent bifunctional electrocatalyst by an in situ electrochemical transformation strategy, where the Co-based nanorod template was converted into Co(OH)₂@CoSe at the cathode. The obtained electrode exhibits superior electrocatalytic activity for both the HER (overpotential of 208 mV at 20 mA cm^-2) and the OER (268 mV at 20 mA cm^-2) at high current density in a 1 M KOH solution. The theoretical calculations and experimental evidence indicate that the chemical coupling Co-OH active site between Co(OH)₂ and CoSe regulates the hydrogen adsorption and desorption energy and fast electron transfer capability, which is responsible for the improved HER. Moreover, the Co(OH)₂@CoSe NRs can be further converted into CoOOH nanosheets which serve as OER active sites. Toward practical electrolytic cell applications, the Co(OH)₂@CoSe nanorods as both the cathode and anode achieved a current density of 100 mA cm^-2 at 1.94 V for overall water splitting, better than that of noble metal-based electrocatalysts.

ZnO-Templated Selenized and Phosphorized Cobalt-Nickel Oxide Microcubes as Rapid Alkaline Water Oxidation Electrocatalysts

Oxygen electrocatalysis is of remarkable significance for electrochemical energy storage and conversion technologies, together with fuel cells, metal-air batteries, and water splitting devices. Substituting noble metal-based electrocatalysts by decidedly effective and low-cost metal-based oxygen electrocatalysts is imperative for the commercial application of these technologies. Herein, a novel strategy is presented to fabricate selenized and phosphorized porous cobalt-nickel oxide microcubes by using a sacrificial ZnO spherical template and the resulting microcubes are employed as an oxygen evolution reaction (OER) electrocatalyst. The selenized samples manifest desirable and robust OER performance, with comparable overpotential at 10 mA cm^-2 (312 mV) as RuO₂ (308 mV) and better activity when the current reaches 13.7 mA cm^-2 . The phosphorized samples exhibit core-shell structure with low-crystalline oxides inside amorphous phosphides, which ensures superior activity than RuO₂ with the same overpotential (at 10 mA cm^-2 ) yet lower Tafel slope. Such a surface doping method possibly will provide inspiration for engineering electrocatalysts applied in water oxidation.

General approach to construct hierarchical-structured porous Co–Ni bimetallic oxides for efficient oxygen evolution

A novel, general and controllable approach was developed to produce a series of hierarchical-structured porous Co–Ni bimetallic oxides via topochemical transformation of bimetallic coordination polymers for boosted oxygen evolution catalysis.

CoSe2 nanobelt coupled with CoMoO4 nanosheet as efficient electrocatalysts for hydrogen and oxygen evolution reaction

Designing non-noble electrocatalysts with low-cost and efficient is crucial to the development of sustainable and clean energy resources. Here, we synthesized a novel S–CoSe2/CoMoO4 and O–CoSe2/CoMoO4hybrid electrocatalysts for the HER and OER, respectively. Possibly due to the synergetic chemical coupling effects between CoSe2/DETA and CoMoO4, and the introduction of S heteroatom and oxygen vacancy, this hybrid could expose enough active edges, and then promoted the constructed hybrid displayed superior hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) catalytic performance. The S–CoSe2/CoMoO4 sample afforded a current density of 10 ​mAcm-2 at a small overpotential of 177 ​mV and a small Tafel slope of 54 ​mV dec−1. Moreover, the oxidized CoSe2/CoMoO4 (O–CoSe2/CoMoO4) also displayed a remarkable catalytic property for OER with a small Tafel slope of 43 ​mV dec−1, as well as excellent stability in 1.0 ​M KOH. Therefore, this noble-metal-free and highly efficient catalyst enables prospective applications for electrochemical applications.

•The novel S-CoSe2/CoMoO4 and O-CoSe2/CoMoO4 hybrid electrocatalysts are reported. •The oxygen vacancy resulted in O-CoSe2/CoMoO4 displayed excellent stability for OER. •This non-noble based electrocatalysts are valuable for the future clean energy.

Amorphization activated ruthenium-tellurium nanorods for efficient water splitting

Pursuing active and durable water splitting electrocatalysts is of vital significance for solving the sluggish kinetics of the oxygen evolution reaction (OER) process in energy supply. Herein, theoretical calculations identify that the local distortion-strain effect in amorphous RuTe₂ system abnormally sensitizes the Te-pπ coupling capability and enhances the electron-transfer of Ru-sites, in which the excellent inter-orbital p-d transfers determine strong electronic activities for boosting OER performance. Thus, a robust electrocatalyst based on amorphous RuTe₂ porous nanorods (PNRs) is successfully fabricated. In the acidic water splitting, a-RuTe₂ PNRs exhibit a superior performance, which only require a cell voltage of 1.52 V to reach a current density of 10 mA cm⁻². Detailed investigations show that the high density of defects combine with oxygen atoms to form RuO_xH_y species, which are conducive to the OER. This work offers valuable insights for constructing robust electrocatalysts based on theoretical calculations guided by rational design and amorphous materials.

Elctrochemical water splitting is of vital significance for energy conversion and storage. Here the authors show an electrocatalyst based on amorphous ruthenium-tellurium porous nanorods which exhibit significantly improved OER performance than its crystalline counterparts