W2C nanodot-decorated CNT networks as a highly efficient and stable electrocatalyst for hydrogen evolution in acidic and alkaline media

Highly Active W2C-Based Composites for the HER in Alkaline Solution: the Role of Surface Oxide Species

The hydrogen evolution reaction (HER) is a crucial electrochemical process for the proposed hydrogen economy since it has the potential to provide pure hydrogen for fuel cells. Nowadays, hydrogen electroproduction is considerably expensive, so promoting the development of new non-noble catalysts for the cathode of alkaline electrolyzers appears as a suitable way to reduce the costs of this technology. In this sense, a series of tungsten-based carbide materials have been synthesized by the urea-glass route as candidates to improve the HER in alkaline media. Moreover, two different pyridinium-based ionic liquids were employed to modify the surface of the carbide grains and control the amount and nature of their surface species. The main results indicate that the catalyst surface composition is modified in the hybrid materials, which are then distinguished by the appearance of tungsten suboxide structures. This implies the action of ionic liquids as reducing agents. Consequently, differential electrochemical mass spectrometry (DEMS) is used to precisely determine the onset potentials and rate-determining steps (RDS) for the HER in alkaline media. Remarkably, the modified surfaces show high catalytic performance (overpotentials between 45 and 60 mV) and RDS changes from Heyrovsky-Volmer to Heyrovsky as the surface oxide structures get reduced. H2O molecule reduction is then faster at tungsten suboxide, which allows the formation of the adsorbed hydrogen at the surface, boosting the catalytic activity and the kinetics of the alkaline HER.

Polyoxometalate-Structured Materials: Molecular Fundamentals and Electrocatalytic Roles in Energy Conversion

Polyoxometalates (POMs), a kind of molecular metal oxide cluster with unique physical-chemical properties, have made essential contributions to creating efficient and robust electrocatalysts in renewable energy systems. Due to the fundamental advantages of POMs, such as the diversity of molecular structures and large numbers of redox active sites, numerous efforts have been devoted to extending their application areas. Up to now, various strategies of assembling POM molecules into superstructures, supporting POMs on heterogeneous substrates, and POMs-derived metal compounds have been developed for synthesizing electrocatalysts. From a multidisciplinary perspective, the latest advances in creating POM-structured materials with a unique focus on their molecular fundamentals, electrocatalytic roles, and the recent breakthroughs of POMs and POM-derived electrocatalysts, are systematically summarized. Notably, this paper focuses on exposing the current states, essences, and mechanisms of how POM-structured materials influence their electrocatalytic activities and discloses the critical requirements for future developments. The future challenges, objectives, comparisons, and perspectives for creating POM-structured materials are also systematically discussed. It is anticipated that this review will offer a substantial impact on stimulating interdisciplinary efforts for the prosperities and widespread utilizations of POM-structured materials in electrocatalysis.

Structure and oxygen vacancy engineered CuCo-layered double oxide nanotube arrays as advanced bifunctional electrocatalysts for overall water splitting

In recent years, as a green renewable energy production technology, electrochemical water splitting has demonstrated high development potential. Many materials have been reported as successful catalysts in the water-splitting field. However, it is still a huge challenge to produce bifunctional electrocatalysts for the efficient and sustainable generation of hydrogen and oxygen simultaneously. Herein, we successfully developed oxygen vacancies abundant CuCo layered double oxide (O_v-CuCo-LDO) hollow nanotube arrays (HNTAs) loaded on nickel foam as advanced electrocatalysts for total water splitting. When the current density was 10 mA cm^-2, the O_v-CuCo-LDO HNTAs exhibited outstanding onset overpotentials of 53.9 and 72.5 mV for the hydrogen evolution and oxygen evolution reactions (HER and OER) in alkaline medium, respectively, because of the bimetallic synergistic effect between the cobalt and copper and the unique hollow porous structure. In addition, an as-assembled O_v-CuCo-LDO||O_v-CuCo-LDO electrolytic cell showed a small potential of 1.55 V to deliver a current density of 10 mA cm^-2. Moreover, it also showed remarkable durability after long-term overall water splitting for more than 20 h. The research results in this paper are of great interest to practical applications of the water decomposition process, providing clear and in-depth insights into preliminary robust and efficient multifunctional electrocatalysts for overall water splitting.

Multi-interface MoS2/Ni3S4/Mo2S3 composite as an efficient electrocatalyst for hydrogen evolution reaction over a wide pH range

The exploitation of cost-efficiently electrocatalysts for hydrogen evolution reaction (HER) over a wide pH range remains a challenge. Herein, we prepared a novel multi-interface MoS₂/Ni₃S₄/Mo₂S₃ composite on carbon cloth (CC) that acts as an efficient electrocatalyst over a wide pH range through a facile one-pot strategy, where (NH₄)₄[NiH₆Mo₆O₂₄]·5H₂O (abbreviated to NiMo₆) as a bimetallic precursor and Ni(NO₃)₂·6H₂O as one of the raw materials and salt are used together with thiourea (TU) for converting them into the MoS₂/Ni₃S₄/Mo₂S₃ load on CC (abbreviated as MoS₂/Ni₃S₄/Mo₂S₃/CC). MoS₂/Ni₃S₄/Mo₂S₃/CC-24 h shows a distinguished electrocatalytic performance towards HER with long-term stability in acid and alkaline media. It presents low overpotentials of 38 mV and 51 mV in 0.5 M H₂SO₄ and 1.0 M KOH at 10 mA cm^-2, respectively. This work can deliver a new idea to fabricate cost-efficient and long-term durability HER electrocatalysts over a broad pH range.

Effect of cobalt phosphide (CoP) vacancies on its hydrogen evolution activity via water splitting: a theoretical study

Defect engineering plays an important role in improving the performance of catalysts. To clarify the roles of Co and P vacancies in CoP for water splitting, a theoretical study based on density functional theory was carried out in this paper. The geometric and electronic structures, activity and stability of the CoP (101)B surface, CoP (101)B with the Co vacancy (Co_vac) and the P vacancy (P_vac) are investigated. The results indicate that the CoP (101)B surface with P_vac and Co_vac can enhance the electron transfer to the surface. The P_vac will upward shift the Co d-band center near the vacancy site, which promotes the adsorption of H on the Co atom. As a result, the bridge Co-Co sites near the vacancy become the active sites for the hydrogen evolution reaction (HER) (ΔG_H* = 0.01 eV). The loss of the Co atom also results in an upward shift of its d-band center, which will enhance the H adsorption on the adjacent Co sites. The unevenly distributed electrons due to the presence of vacancies on the surface cause spontaneous dissociation of H₂O molecules. Furthermore, the thermodynamic analysis and surface energy find that the CoP (101)B and (101)B facets with Co_vac and P_vac present good stability. The current work has shed light onto the mechanism of water splitting on the surface of phosphide with vacancies. Our study suggests that engineering vacancies on CoP is a feasible route to improve its catalytic activity.

Construction of an N-Decorated Carbon-Encapsulated W2C/WP Heterostructure as an Efficient Electrocatalyst for Hydrogen Evolution in Both Alkaline and Acidic Media

Tungsten carbide (W₂C) has emerged as a potential alternative to noble-metal catalysts toward hydrogen evolution reaction (HER) owing to its Pt-like electronic configuration. However, unsatisfactory activity, dilatory electron transfer, and inefficient synthesizing methods, especially for nanoscale particles, have severely hindered its large-scale applications. Herein, a novel heterostructure composed of W₂C and tungsten phosphide (WP) embedded in nitrogen-decorated carbon (W₂C/WP@NC) was constructed as an efficient HER electrocatalyst. The as-prepared W₂C/WP@NC catalyst exhibits remarkable electrocatalytic activity and robust durability toward HER both in acids and bases. More notably, the W₂C/WP@NC catalyst demonstrates low overpotentials of 116.37 and 196.2 mV to afford a current density of 10 mA cm^-2 and reveals slight potential decays of about 6.4 and 7.64% over 12 h continuous operation in bases and acids, respectively. The overall water-splitting performance was further evaluated using the W₂C/WP@NC catalyst as the cathode and commercial RuO₂ as the anode in an electrolyzer, which can realize an overall current density of 10 mA cm^-2 and maintain long durability of more than 12 h with a small cell voltage of 1.723 V. This work opens up new opportunities for exploring cost-efficient electrocatalysts in sustainable energy conversion.

Constructing a WC/NCN Schottky Junction for Rapid Electron Transfer and Enrichment for Highly Efficient Photocatalytic Hydrogen Evolution

The low charge-transfer efficiency and slow surface reaction kinetics are the main factors affecting the performance of carbon nitride photocatalysts. Here, a Schottky heterostructure (WCN) was constructed by combining WC with porous carbon nitride nanosheets with a cyanide group (NCN). The Schottky junction provides a convenient way for photoinduced electrons to transfer and promotes the effective separation of photoinduced carriers. Furthermore, due to the good conductivity of WC and an electronic structure similar to Pt, the W atom in WC as the active site of hydrogen production can realize efficient reaction kinetics. In this way, the WCN Schottky heterostructure showed a 2.0- and 5.0-fold enhancement in photocatalytic H₂ evolution as compared to the single NCN component under visible-light and near-infrared light irradiation. By combining with theoretical simulations, as an electron acceptor in the WCN heterostructure, WC can effectively improve the charge-transfer efficiency and also act as an active site for hydrogen production.

Structure-controlled tungsten carbide nanoplates for enhanced hydrogen evolution reaction

Developing a low-cost and durable non-noble metal eletrocatalyst for hydrogen evolution reaction (HER) is critical in efficient hydrogen production. Herein, tungsten carbide nanoplates (WC NPs) with typical mesoporous structure were prepared by a controlled hydrothermal reaction followed by a gas-solid carburization process. The crystal phases, microstructure and chemical components of the nanoplates were characterized, and their electrochemical properties were measured. The results show that the as-prepared WC NPs expose active sites upmost, and exhibit enhanced conductivity and superior HER performance in acid solution in terms of a small η 10 (overpotential to obtain a current density of 10 mA cm−2) of 120 mV, a Tafel slope of 58 mV dec−1 and outstanding long-term cycling stability. These indicate that the HER properties of WC NPs are dramatically enhanced compared to that of all phase pure WC materials reported in recent years. This enhancement can be attributed to their unique structural and electronic properties, which can be exploited to improve the electrochemical properties of traditional non-noble metal material.

N-Doped-carbon/cobalt-nanoparticle/N-doped-carbon multi-layer sandwich nanohybrids derived from cobalt MOFs having 3D molecular structures as bifunctional electrocatalysts for on-chip solid-state Zn-air batteries

The unsatisfactory energy efficiency and leakable liquid electrolytes in conventional Zn-air batteries with intrinsic semi-open structures greatly limit their opportunity to be a safe micropower source for flexible/wearable electronics. Herein, N-doped-carbon/cobalt-nanoparticle/N-doped-carbon (NdC-CoNP-NdC) multi-layer sandwich nanohybrids were first synthesized via the pyrolysis of a well-designed Co-MOF precursor with a 3D molecular framework. Profiting from the synergistic effect enabled by the interlayer-confined growth of monodispersed cobalt nanoparticles having high activity/stability and a thousand-layer-cake porous N-self-doped carbon skeleton of high conductivity and additional active sites as well as the reasonable design of a multi-layer sandwich interface structure between them that acts as an interconnected nanoreactor, the as-obtained NdC-CoNP-NdC multi-layer sandwich nanohybrids exhibit excellent bifunctional catalytic activity of a small ORR/OER gap (0.83 V). We followed a planar electrode configuration design with an interdigital carbon cloth coated with NdC-CoNP-NdC as the air cathode and an interdigital Zn foil as the metal anode as well as the introduction of a polyacrylamide-co-polyacrylic/6 M KOH alkaline gel as an incombustible solid-state electrolyte. Thus, on-chip all-solid-state rechargeable Zn-air batteries (OAR-ZABs) are further developed, achieving a cycle life up to 150 cycles per 50 h, high power density/specific capacity as much as 57.0 mW cm^-2/771 mA h g^-1, respectively, and excellent coplanar integration capability and mechanical flexibility for working steadily under bending deformation. Eventually, as an additional advancement, an autonomous smartwatch powered by the coplanar integrated OAR-ZABs is demonstrated, which possesses excellent integrity and flexibility and is comfortably wearable for timing and step counting dynamically; this demonstrates the successful application of assembling OAR-ZABs into highly integrated wearable electronics as a compatible micropower source.

Fabrication of a 3D self-supporting Ni–P/Ni2P/CC composite and its robust hydrogen evolution reaction properties in alkaline solution

The Ni–P/Ni₂P/CC composite exhibits excellent alkaline HER properties derived from Ni–P filling the sites on the CC not occupied by Ni₂P nanosheets.

3D hollow Co-Fe-P nanoframes immobilized on N,P-doped CNT as an efficient electrocatalyst for overall water splitting

The rational design of nonprecious and high-efficiency bifunctional electrocatalysts with advanced structural and compositional preponderance for water electrolysis is of paramount importance for the generation of sustainable and clean energy. Herein, for the first time, a novel three-dimensional (3D) hollow hybrid electrocatalyst, Co-Fe-P nanoframe immobilized on N,P-doped carbon nanotubes (CoFeP NFs/NPCNT), was synthesized by selectively etching a CNT-composited Co,Fe-based Prussian blue analogue and subsequent phosphorization. Benefiting from its unique 3D hollow nanoarchitecture, which offers rich porosity and abundant catalytically active sites and guarantees excellent conductivity and structural stability, the hollow CoFeP NFs/NPCNT hybrid delivered pronounced catalytic activity for oxygen evolution (or hydrogen evolution) in alkaline electrolyte, with a low overpotential of 278 (or 132) mV at 10 mA cm-2, small Tafel slope of 39.5 (or 62.9) mV dec-1 and prominent long-term stability. Therefore, when CoFeP NFs/NPCNT was employed as the cathode and anode toward overall water-splitting, it required a quite small cell voltage of only 1.56 V to afford a current density of 10 mA cm-2, and displayed outstanding electrocatalytic stability over 60 h, greatly approaching the performance of the commercial Pt/C(-)//RuO2(+) electrolyzer and outperforming most other non-noble-based electrolyzers.

Hierarchically Porous W-Doped CoP Nanoflake Arrays as Highly Efficient and Stable Electrocatalyst for pH-Universal Hydrogen Evolution

It is still challenging to develop high-efficiency and low-cost non-noble metal-based electrocatalysts for hydrogen evolution reaction (HER) in pH-universal electrolytes. Herein, hierarchically porous W-doped CoP nanoflake arrays on carbon cloth (W-CoP NAs/CC) are synthesized via facile liquid-phase reactions and a subsequent phosphorization process. The W-CoP NAs/CC hybrid can be directly employed as a binder-free electrocatalyst and delivers superior HER performance in pH-universal electrolytes. Especially, it delivers very low overpotentials of 89, 94, and 102 mV to reach a current density of 10 mA cm^-2 in acidic, alkaline, and neutral electrolytes, respectively. Furthermore, it shows a nearly 100% Faradaic efficiency as well as superior long-term stability with no decreasing up to 36 h in pH-universal electrolytes. The outstanding electrocatalytic performance of W-CoP NAs/CC can be mainly attributed to the porous W-doped nanoflake arrays, which not only afford rich exposed active sites, but also accelerate the access of electrolytes and the diffusion of H₂ bubbles, thus efficiently promoting the HER performance. This work provides a new horizon to rationally design and synthesize highly effective and stable non-noble metal phosphide-based pH-universal electrocatalysts for HER.

Three-dimensional macroporous W2C inverse opal arrays for the efficient hydrogen evolution reaction

The development of a low-cost and durable non-precious metal-based electrocatalyst for the hydrogen evolution reaction (HER) is important to realize highly efficient overall water splitting. Here, we report the design and fabrication of a binder-free electrocatalyst of three-dimensional macroporous ditungsten carbide (W2C) inverse opal (W2C IO) arrays by a facile thermal carburization process with WO3 IO as a template. The as-fabricated W2C IO exhibits superior electrocatalytic performance in 0.5 M H2SO4 solution in terms of a low overpotential of 146 mV to reach a current density of 10 mA cm-2, a low Tafel slope of 78 mV dec-1 and excellent long-term stability. The superior performance can be attributed to the favorable electronic structure and hydrogen adsorption Gibbs free energy of W2C evidenced by theory calculations and the enhancement of the charge/mass transfer process by the 3D macroporous arrayed electrode design.

High-Performance Ru@C4N Electrocatalyst for Hydrogen Evolution Reaction in Both Acidic and Alkaline Solutions

We report a high-performance Ru@C₄N electrocatalyst for the hydrogen evolution reaction (HER) in both acidic and alkaline solutions. This catalyst is synthesized by annealing a complex of a covalent organic framework compound coordinated with ruthenium synthesized by a "one-pot" solvothermal method. This Ru@C₄N catalyst shows excellent electrocatalytic activity toward the hydrogen evolution reaction (HER) in both acidic and alkaline solutions with very low overpotentials at 10 mA/cm² (6 mV in 0.5 M H₂SO₄ solution; 7 mV in 1.0 M KOH solution), which outperforms the commercial catalyst Pt/C. The Ru@C₄N electrocatalyst also exhibits high HER turnover frequencies of 0.93 H₂ per s in 0.5 M H₂SO₄ and 0.65 H₂ per s in 1.0 M KOH solutions at 25 mV as well as superior performance stability.

Low-cost high-performance hydrogen evolution electrocatalysts based on Pt-CoP polyhedra with low Pt loading in both alkaline and neutral media

Low-cost Pt-CoP hollow polyhedra exhibited prominent performance for the HER in both basic and neutral solutions.

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).

Carbon impurity-free, novel Mn,N co-doped porous Mo2C nanorods for an efficient and stable hydrogen evolution reaction

Carbon impurity-free, novel Mn,N co-doped porous Mo₂C nanorods reduce the hydrogen adsorption energy, functioning as efficient HER electrocatalysts.