Interface Modulation of MoS2 /Metal Oxide Heterostructures for Efficient Hydrogen Evolution Electrocatalysis

Insights into the pH effect on hydrogen electrocatalysis

Hydrogen electrocatalytic reactions, including the hydrogen evolution reaction (HER) and the hydrogen oxidation reaction (HOR), play a crucial role in a wide range of energy conversion and storage technologies. However, the HER and HOR display anomalous non-Nernstian pH dependent kinetics, showing two to three orders of magnitude sluggish kinetics in alkaline media compared to that in acidic media. Fundamental understanding of the origins of the intrinsic pH effect has attracted substantial interest from the electrocatalysis community. More critically, a fundamental molecular level understanding of this effect is still debatable, but is essential for developing active, stable, and affordable fuel cells and water electrolysis technologies. Against this backdrop, in this review, we provide a comprehensive overview of the intrinsic pH effect on hydrogen electrocatalysis, covering the experimental observations, underlying principles, and strategies for catalyst design. We discuss the strengths and shortcomings of various activity descriptors, including hydrogen binding energy (HBE) theory, bifunctional theory, potential of zero free charge (pzfc) theory, 2B theory and other theories, across different electrolytes and catalyst surfaces, and outline their interrelations where possible. Additionally, we highlight the design principles and research progress in improving the alkaline HER/HOR kinetics by catalyst design and electrolyte optimization employing the aforementioned theories. Finally, the remaining controversies about the pH effects on HER/HOR kinetics as well as the challenges and possible research directions in this field are also put forward. This review aims to provide researchers with a comprehensive understanding of the intrinsic pH effect and inspire the development of more cost-effective and durable alkaline water electrolyzers (AWEs) and anion exchange membrane fuel cells (AMFCs) for a sustainable energy future.

Interfacial electronic modulation of NiCo decorated nano-flowered MoS2 on carbonized wood as a remarkable bifunctional electrocatalyst for boosting overall water splitting

The development of a cost-effective and efficient bifunctional electrode for overall water splitting holds significant importance in accelerating the sustainable advancement of hydrogen energy. The present study involved a bifunctional catalytic electrode was prepared by loading NiCo-modified 1T/2H MoS2 onto carbonized wood (NiCo-MoS2-CW) using the hydrothermal and electrodeposition techniques. The XPS analysis revealed that NiCo-modified MoS2 exhibited a weak electron characteristic, which facilitated the ionization of H2O and significantly enhanced the Volmer step. The XPS analysis unveiled that NiCo-modified MoS2 displayed a weak electron characteristic, thereby promoting the ionization of H2O and substantially augmenting the Volmer step. The electrocatalytic performance of the NiCo-MoS2-CW in 1.0 M KOH is remarkably impressive, exhibiting minimal overpotentials of only 64 mV (10 mA cm-2) and 216 mV (50 mA cm-2) for the hydrogen evolution reaction and oxygen evolution reaction, respectively. The NiCo-MoS2-CW || NiCo-MoS2-CW electrolytic cell can achieve a cell voltage of only 1.69 V to achieve a current density of 50 mA cm-2. Overall, this study proposes a potential approach to improve the catalytic efficiency of overall water splitting by modulating the interfacial electronic properties of MoS2.

Dynamic restructuring of nickel sulfides for electrocatalytic hydrogen evolution reaction

Transition metal chalcogenides have been identified as low-cost and efficient electrocatalysts to promote the hydrogen evolution reaction in alkaline media. However, the identification of active sites and the underlying catalytic mechanism remain elusive. In this work, we employ operando X-ray absorption spectroscopy and near-ambient pressure X-ray photoelectron spectroscopy to elucidate that NiS undergoes an in-situ phase transition to an intimately mixed phase of Ni3S2 and NiO, generating highly active synergistic dual sites at the Ni3S2/NiO interface. The interfacial Ni is the active site for water dissociation and OH* adsorption while the interfacial S acts as the active site for H* adsorption and H2 evolution. Accordingly, the in-situ formation of Ni3S2/NiO interfaces enables NiS electrocatalysts to achieve an overpotential of only 95 ± 8 mV at a current density of 10 mA cm-2. Our work highlighted that the chemistry of transition metal chalcogenides is highly dynamic, and a careful control of the working conditions may lead to the in-situ formation of catalytic species that boost their catalytic performance.

Nickel and Molybdenum Composites Decorated Nitrogen-Doped Carbon Catalysts from Spent Coffee Grounds for Alkaline Hydrogen Evolution

Biomass-derived materials can help develop efficient, environmentally friendly and cost-effective catalysts, thereby improving the sustainability of hydrogen production. Herein, we propose a simple method to produce nickel and molybdenum composites decorated spent coffee grounds (SCG) as an efficient catalyst, SCG(200)@NiMo, for electrocatalytic hydrogen production. The porous carbon supporter derived form SCG provided a larger surface, prevented aggregation during the high temperature pyrolysis, optimized the electronic structure by N and provided a reducing atmosphere for the oxides reduction to form heterojunctions. The sieved SCG showed obvious improvement of HER performance and enhanced conductivity and long-term durability. The obtained SCG(200)@NiMo exhibits the highest electrochemical performance for the hydrogen evolution reaction process, as evidenced by the overpotential of only 127 mV at a current density of ɳ10 and 97.7 % catalytic activity retention even after 12 h of operation. This work may stimulate further exploration of efficient electrocatalysts derived from biomass.

Directed Dual Charge Pumping Tunes the d-Orbital Configuration of Pt Cluster Boosting Hydrogen Evolution Kinetic

Achieving high catalytic activity with a minimum amount of platinum (Pt) is crucial for accelerating the cathodic hydrogen evolution reaction (HER) in proton exchange membrane (PEM) water electrolysis, yet it remains a significant challenge. Herein, a directed dual-charge pumping strategy to tune the d-orbital electronic distribution of Pt nanoclusters for efficient HER catalysis is proposed. Theoretical analysis reveals that the ligand effect and electronic metal-support interactions (EMSI) create an effective directional electron transfer channel for the d-orbital electrons of Pt, which in turn optimizes the binding strength to H*, thereby significantly enhancing HER efficiency of the Pt site. Experimentally, this directed dual-charge pumping strategy is validated by elaborating Sb-doped SnO2 (ATO) supported Fe-doped PtSn heterostructure catalysts (Fe-PtSn/ATO). The synthesized 3%Fe-PtSn/ATO catalysts exhibit lower overpotential (requiring only 10.5 mV to reach a current density of 10 mA cm- 2), higher mass activity (28.6 times higher than commercial 20 wt.% Pt/C), and stability in the HER process in acidic media. This innovative strategy presents a promising pathway for the development of highly efficient HER catalysts with low Pt loading.

Two-Dimensional Nanoarchitectonics for Two-Dimensional Materials: Interfacial Engineering of Transition-Metal Dichalcogenides

Transition-metal dichalcogenides (TMDs) have attracted increasing attention in fundamental studies and technological applications owing to their atomically thin thickness, expanded interlayer distance, motif band gap, and phase-transition ability. Even though TMDs have a wide variety of material assets from semiconductor to semimetallic to metallic, the materials with fixed features may not show excellence for precise application. As a result of exclusive crystalline polymorphs, physical and chemical assets of TMDs can be efficiently modified via various approaches of interface nanoarchitectonics, including heteroatom doping, heterostructure, phase engineering, reducing size, alloying, and hybridization. With modified properties, TMDs become interesting materials in diverse fields, including catalysis, energy, electronics, transistors, and optoelectronics.

Coupling MoS2 nanosheets with CeO2 for efficient electrocatalytic hydrogen evolution at large current densities

We developed an efficient MoS2 nanosheet electrode coupled with CeO2via a hydrothermal process to facilitate water adsorption and dissociation, which displayed good HER activity and stability at a large current density of 500 mA cm-2. In situ Raman spectroscopy confirmed the formation of hydroxide ions based on the strengthening of the Ce-O bond during the HER.

Three-dimensional ordered macroporous molybdenum doped NiCoP honeycomb electrode for two-step water electrolysis

Two-step alkaline water electrolysis is considered a safe and efficient method for producing hydrogen from renewable energy. Reversal of the current polarity in a bifunctional electrocatalyst used as a gas evolution electrode (GEE) in two-step water electrolysis can generate H₂/O₂ at different times and in different spaces. The design of a bifunctional electrocatalyst with high durability and excellent activity is imperative to achieving continuous, safe, and pure H₂ generation via two-step alkaline water electrolysis. Here, we present for the first time a novel 3D Mo-doped NiCo phosphide honeycomb electrocatalyst that was grown on nickel foam (3D Mo-NiCoP/NF) and fabricated using polystyrene as a template. The electrocatalyst exhibited extremely low overpotentials in both the hydrogen evolution reaction (HER; 117 mV at 10 mA/cm²) and the oxygen evolution reaction (OER; 344 mV at 100 mA/cm²). As a bifunctional electrocatalyst for two-step alkaline water electrolysis, the device had a 1.784 V cell voltage at 10 mA/cm², 95% decoupling efficiency, and ∼83% energy conversion efficiency. Taken together, the use of 3D Mo-NiCoP/NF as a GEE reduced the complexity and lowered the cost of the electrolyzer. The latter could be used to construct highly competitive water-splitting systems for continuous H₂ production and green energy harvesting.

CoP@Ni core-shell heterostructure nanowire array: A highly efficient electrocatalyst for hydrogen evolution

The inferior performance of non-precious metals on electrocatalytic hydrogen evolution can be mainly attributed to the inappropriate adsorption strength of intermediates. A core-shell heterostructure of CoP@Ni is developed by hydrothermal reaction, thermal phosphorization and subsequent electrodeposition, with metallic Ni supported by CoP nanowire array. The as-prepared CoP@Ni core-shell heterostructure nanowire array has a superior activity on hydrogen evolution in alkaline electrolyte, with an overpotential of 71 mV to drive 10 mA cm^-2 and a Tafel slope of 66 mV dec^-1. The electronic structure of CoP@Ni is tuned for modulating the adsorption strength of intermediates. The theoretical calculations reveal that CoP@Ni has metallic characteristics with a zero-bandgap, which leads to the promoted charge transfer. More importantly, the intrinsic activity of CoP@Ni is greatly increased, with a lower energy barrier in the reaction pathway. This work points out the importance of constructing the heterostructure for improving the intrinsic activity, which can pave the way to the exploration of high-performance and cost-effective electrocatalysts for hydrogen production.

Construction of Nitrogen-Doped Biphasic Transition-Metal Sulfide Nanosheet Electrode for Energy-Efficient Hydrogen Production via Urea Electrolysis

Urea-assisted hybrid water splitting is a promising technology for hydrogen (H₂ ) production, but the lack of cost-effective electrocatalysts hinders its extensive application. Herein, it is reported that Nitrogen-doped Co₉ S₈ /Ni₃ S₂ hybrid nanosheet arrays on nickel foam (N-Co₉ S₈ /Ni₃ S₂ /NF) can act as an active and robust bifunctional catalyst for both urea oxidation reaction (UOR) and hydrogen evolution reaction (HER), which could drive an ultrahigh current density of 400 mA cm^-2 at a low working potential of 1.47 V versus RHE for UOR, and gives a low overpotential of 111 mV to reach 10 mA cm^-2 toward HER. Further, a hybrid water electrolysis cell utilizing the synthesized N-Co₉ S₈ /Ni₃ S₂ /NF electrode as both the cathode and anode displays a low cell voltage of 1.40 V to reach 10 mA cm^-2 , which can be powered by an AA battery with a nominal voltage of 1.5 V. The density functional theory (DFT) calculations decipher that N-doped heterointerfaces can synergistically optimize Gibbs free energy of hydrogen and urea, thus accelerating the catalytic kinetics of HER and UOR. This work significantly advances the development of the promising cobalt-nickel-based sulfide as a bifunctional electrocatalyst for energy-saving electrolytic H₂ production and urea-rich innocent wastewater treatment.

Combining Highly Dispersed Amorphous MoS3 with Pt Nanodendrites as Robust Electrocatalysts for Hydrogen Evolution Reaction

Surface modification of electrocatalysts to obtain new or improved electrocatalytic performance is currently the main strategy for designing advanced nanocatalysts. In this work, highly dispersed amorphous molybdenum trisulfide-anchored Platinum nanodendrites (denoted as Pt-a-MoS₃  NDs) are developed as efficient hydrogen evolution electrocatalysts. The formation mechanism of spontaneous in situ polymerization MoS₄ ^2- into a-MoS₃ on Pt surface is discussed in detail. It is verified that the highly dispersed a-MoS₃ enhances the electrocatalytic activity of Pt catalysts under both acidic and alkaline conditions. The potentials at the current density of 10 mA cm^-2 (η₁₀ ) in 0.5 m sulfuric acid (H₂ SO₄ ) and 1 m potassium hydroxide (KOH) electrolyte are -11.5 and -16.3 mV, respectively, which is significantly lower than that of commercial Pt/C (-20.2 mV and -30.7 mV). This study demonstrates that such high activity benefits from the interface between highly dispersed a-MoS₃ and Pt sites, which act as the preferred adsorption sites for the efficient conversion of hydrion (H⁺ ) to hydrogen (H₂ ). Additionally, the anchoring of highly dispersed clusters to Pt substrate greatly enhances the corresponding electrocatalytic stability.

Space-Confined One-Step Growth of 2D MoO2 /MoS2 Vertical Heterostructures for Superior Hydrogen Evolution in Alkaline Electrolytes

2D material-based heterostructures are constructed by stacking or spicing individual 2D layers to create an interface between them, which have exotic properties. Here, a new strategy for the in situ growth of large numbers of 2D heterostructures on the centimeter-scale substrate is developed. In the method, large numbers of 2D MoS₂ , MoO₂ , or their heterostructures of MoO₂ /MoS₂ are controllably grown in the same setup by simply tuning the gap distance between metal precursor and growth substrate, which changes the concentration of metal precursors feed. A lateral force microscope is used first to identify the locations of each material in the heterostructures, which have MoO₂ on the top of MoS₂ . Noteworthy, the creation of a clean interface between atomic thin MoO₂ (metallic) and MoS₂ (semiconducting) results in a different electronic structure compared with pure MoO₂ and MoS₂ . Theoretical calculations show that the charge redistribution at such an interface results in an improved HER performance on the MoO₂ /MoS₂ heterostructures, showing an overpotential of 60 mV at 10 mA cm^-2 and a Tafel slope of 47 mV dec^-1 . This work reports a new strategy for the in situ growth of heterostructures on large-scale substrates and provides platforms to exploit their applications.

Temperature-Induced Structure Transformation from Co0.85Se to Orthorhombic Phase CoSe2 Realizing Enhanced Hydrogen Evolution Catalysis

Transition-metal chalcogenides (TMC) have been widely studied as active electrocatalysts toward the hydrogen evolution reaction due to their suitable d-electron configuration and relatively high electrical conductivity. Herein, we develop a feasible method to synthesize an orthorhombic phase of CoSe₂ (o-CoSe₂) from the regeneration of Co_0.85Se, where the temperature plays a key role in controlling the structure transformation. To the best of our knowledge, this is the first report about this synthetic route for o-CoSe₂. The resulting o-CoSe₂ catalysts exhibit enhanced hydrogen evolution reaction performance with an overpotential of 220 mV to reach 10 mA cm^-2 in 1.0 M KOH. Density functional theory calculations further reveal that the change in the Gibbs free energy of hydrogen, water adsorption energy, and the downshifted d-band center make o-CoSe₂ more suitable for accelerating the HER process.

Surface engineering strategies for MoS2 towards electrochemical hydrogen evolution

Water splitting driven by renewable energy sources is an environmentally friendly and sustainable way to produce hydrogen as an ideal energy source in the future. Electrocatalysts can promote the water splitting performance at the both ends. Therefore, the development of cost-effective, high-performance electrocatalysis is a key factor in promoting water decomposition and renewable energy conversion. Among candidates, layered molybdenum disulfide (MoS 2 ) is considered as a most promising electrocatalyst to replace Pt for hydrogen evolution reaction (HER). Surface atomic engineering and interface engineering can induce new physicochemical properties for MoS 2 to greatly enhance HER activity. In this report, we summarize the latest improvement strategies and research progress to improve the catalytic activity of MoS 2 -based material catalysts through the surface and interface atomic and molecular engineering, thus effectively improving HER process. In addition, some unsolved problems in the large-scale application of modified MoS 2 catalyst are also discussed.

The Effect of the 3D Nanoarchitecture and Ni-Promotion on the Hydrogen Evolution Reaction in MoS2 /Reduced GO Aerogel Hybrid Microspheres Produced by a Simple One-Pot Electrospraying Procedure

The transition toward renewable energy sources requires low-cost, efficient, and durable electrocatalysts for green H₂ production. Herein, an easy and highly scalable method to prepare MoS₂ nanoparticles embedded in 3D partially reduced (pr) graphene oxide (GO) aerogel microspheres (MoS₂ /prGOAMs) with controlled morphology and composition is described. Given their peculiar center-diverging mesoporous structure, which allows easy access to the active sites and optimal mass transport, and their efficient electron transfer facilitated by the intimate contact between the MoS₂ and the 3D connected highly conductive pr-GO sheets, these materials exhibit a remarkable electrocatalytic activity in the hydrogen evolution reaction (HER). Ni atoms, either as single Ni atoms or NiO aggregates are then introduced in the MoS₂ /prGOAMs hybrids, to facilitate water dissociation, which is the slowest step in alkaline HER, producing a bifunctional catalyst. After optimization, Ni-promoted MoS₂ /prGOAMs obtained at 500 °C reach a remarkable η₁₀ (overpotential at 10 mA cm^-2 ) of 160 mV in 1 m KOH and 174 mV in 0.5 m H₂ SO₄ . Moreover, after chronopotentiometry tests (15 h) at a current density of 10 mA cm^-2 , the η₁₀ value improves to 147 mV in alkaline conditions, indicating an exceptional stability.

Oxygen Vacancy and Core-Shell Heterojunction Engineering of Anemone-Like CoP@CoOOH Bifunctional Electrocatalyst for Efficient Overall Water Splitting

Constructing cost-efficient and robust bifunctional electrocatalysts for both neutral and alkaline water splitting is highly desired, but still affords a great challenge, due to sluggish hydrogen/oxygen evolution reaction (HER/OER) kinetics. Herein, an in situ integration engineering strategy of oxygen-vacancy and core-shell heterojunction to fabricate an anemone-like CoP@CoOOH core-shell heterojunction with rich oxygen-vacancies supported on carbon paper (CoP@CoOOH/CP), is described. Benefiting from the synergy of CoP core and oxygen-vacancy-rich CoOOH shell, the as-obtained CoP@CoOOH/CP catalyst displays low overpotentials at 10 mA cm^-2 for HER (89.6 mV/81.7 mV) and OER (318 mV/200 mV) in neutral and alkaline media, respectively. Notably, a two-electrode electrolyzer, using CoP@CoOOH/CP as bifunctional catalyst to achieve 10 mA cm^-2 , only needs low-cell voltages in neutral (1.65 V) and alkaline (1.52 V) electrolyte. Besides, systematically experimental and theoretical results reveal that the core-shell heterojunction efficiently accelerates the catalytic kinetics and strengthens the structural stability, while rich oxygen-vacancies efficiently decrease the kinetic barrier and activation energy, and reduce the energy barrier of the rate-determining-step for OER intermediates, thus intrinsically boosting OER performance. This work clearly demonstrates that oxygen-vacancy and core-shell heterojunction engineering provide an effective strategy to design highly-efficient non-precious, bi-functional electrocatalysts for pH-universal water splitting.

Valence Engineering of Polyvalent Cobalt Encapsulated in a Carbon Nanofiber as an Efficient Trifunctional Electrocatalyst for the Zn-Air Battery and Overall Water Splitting

The rapid development of electrochemical power systems has prompted high demand for nonprecious trifunctional electrocatalysts with superior performance, prolonged stability, and low cost for oxygen reduction reaction (ORR), oxygen evolution reaction (OER), and hydrogen evolution reaction (HER). Here, a valence engineering strategy is devised to construct a morphology with polyvalent cobalt encapsulated in nitrogen-doped carbon nanofibers (Co/N-CNFs). The diverse cobalt valence states of the Co/N-CNF catalysts contribute to their excellent catalytic effect and high durability in multiple electrochemical processes. The optimal Co/N-CNF catalyst fabricated exhibits a high half-wave potential of ORR (0.86 V) and low overpotentials of OER (380 mV) and HER (241 mV) at 10 mA cm^-2. The Co/N-CNF-based Zn-air battery possesses a high charge-discharge efficiency as well as a good cycle stability (50 h at 10 mA cm^-2 and 120 h at 20 mA cm^-2), much superior to the Pt/C-based batteries. Furthermore, the Co/N-CNF catalyst could perform efficient overall water splitting.

Recent advances in MoS2-based materials for electrocatalysis

The increasing energy demand and related environmental issues have drawn great attention of the world, thus necessitating the development of sustainable technologies to preserve the ecosystems for future generations. Electrocatalysts...

Directionally In Situ Self-Assembled, High-Density, Macropore-Oriented, CoP-Impregnated, 3D Hierarchical Porous Carbon Sheet Nanostructure for Superior Electrocatalysis in the Hydrogen Evolution Reaction

3D ZIF-67-particles-impregnated cellulose-nanofiber nanosheets with oriented macropores are synthesized via directional-freezing-assisted in situ self-assembly, and converted to 3D CoP-nanoparticle (NP)-embedded hierarchical, but macropores-oriented, N-doped carbon nanosheets via calcination and phosphidation. The obtained nanoarchitecture delivers overpotentials at 10 and 50 mA cm^-2 and Tafel slope of 82.1 and 113.4 mV and 40.8 mV dec^-1 in 0.5 M H₂ SO₄ , and of 97.1 and 136.6 mV and 51.2 mV dec^-1 in 1 M KOH, all of which are superior to those of the most reported non-noble-metal-based hydrogen evolution reaction (HER) catalysts. This catalyst even surpasses commercial Pt/C for a much lower overpotential at high current densities, which is essential for large-scale hydrogen production. Its catalytic activity can be further optimized to become one of the best in both 0.5 M H₂ SO₄ and 1 M KOH. The outstanding catalytic activity is ascribed to the uniformly-dispersed small CoP NPs in the 3D carbon sheets and the hierarchical nanostructure with rich oriented pores. This work develops a facile, economical, and universal self-assembly strategy to fabricate uniquely nanostructured hybrids to simultaneously promote charge transfer and mass transport, and also offers an inexpensive and high-performance HER catalyst toward industry-scale water splitting.

Enhanced hydrogen evolution reaction in alkaline solution by constructing strong metal-support interaction on Pd-CeO2-x-NC hybrids

Diminishing the size of metal nanostructures can significantly improve the performance of catalysts. However, the self-aggregation of small particles is still an insurmountable obstacle, resulting in the unfavorable stability and recyclability. Herein, we designed and fabricated the Pd-CeO_2-x-NC catalyst though an accurate deposition strategy to downsize the Pd particle to sub-nanometer level and enhance its running stability. The CeO_2-x nanoclusters were firstly dispersed on the nitrogen-doped carbon nanosheets. Further, the active Pd sub-nanoclusters were accurately scattered on the surface of CeO_2-x ascribing to the strong metal-support interaction (SMSI) between Pd and CeO_2-x, which was beneficial to promote the catalytic activity. Subsequently, the high oxidation state Pdⁿ⁺ species were formed due to the electron transfer from Pd to CeO_2-x caused by the SMSI effect. Strikingly, the HER performance of Pd-CeO_2-x-NC was surprisingly correlated with the ratio of Pdⁿ⁺, suggesting Pdⁿ⁺ acted as the dominant active species. Besides, the SMSI effect stabilized the valence state of active Pdⁿ⁺ species and prevented the sub-nanometer Pd clusters from aggregation, which played a vital role for the enhanced stability of the hybrid catalyst. This synthetic process described here is contributed to prepare various nanostructured catalysts with satisfactory stability through the direct targeting strategy.

Three-dimensional cross-linked Co-MoS2 catalyst on carbon cloth for efficient hydrogen evolution reaction

MoS₂-Based materials are promising hydrogen evolution reaction (HER) electrocatalysts. However, their HER activities are restrained by the poor population of HER activated edge centers, the large area exposed HER inert basal planes, and low conductivity. Fixing these problems on one system is an effective strategy, but it remains a challenge due to the harsh synthetic conditions. Herein, cobalt carbonate hydroxide (CoCH) nanosheets were used as the substrate for preparing a three-dimensional self-supported cross-linked (3DSC) Co-MoS₂ nanostructured HER catalyst, which possesses abundant active centers and fast electronic transfer ability. In addition, Co activates the basal-plane sulfur atom in MoS₂ to be the HER reactive center effectively. Benefiting from these advantages, 3DSC Co-MoS₂ electrode integrated on carbon cloth (CC) shows that it can drive the current density of 10 and 100 mA cm^-2 with only 40 and 119 mV overpotentials, respectively, which is superior to other MoS₂-based HER catalysts reported recently. This research provides a facile strategy for the improvement of efficient HER electrocatalysts.

MoS2/graphene van der Waals heterojunctions combined with two-layered Au NP for SERS and catalysis analyse

MoS₂-plasmonic hybrid platforms have attracted significant interest in surface-enhanced Raman scattering (SERS) and plasmon-driven photocatalysis. However, direct contact between the metal and MoS₂ creates strain that deteriorates the electron transport across the metal/ MoS₂ interfaces, which would affect the SERS effect and the catalytic performance. Here, the MoS₂/graphene van der Waals heterojunctions (vdWHs) were fabricated and combined with two-layered gold nanoparticles (Au NP) for SERS and plasmon-driven photocatalysis analyse. The graphene film is introduced to provide an effective buffer layer between Au NP and MoS₂, which not only eliminates the inhomogeneous contact on MoS₂ but also benefits the electron transfer. The substrate exhibits excellent SERS capability realizing ultra-sensitive detection for 4-pyridinethiol molecules. Also, the surface catalytic reaction of p-nitrothiophenol (PNTP) to p,p-dimercaptobenzene (DMAB) conversion was in situ monitored, demonstrating that the vdWHs-plasmonic hybrid could effectively accelerate reaction process. The mechanism of the SERS and catalytic behaviors are investigated via experiments combined with theoretical simulations (finite element method and quantum chemical calculations).

Nitrogen-Doped Cobalt Diselenide with Cubic Phase Maintained for Enhanced Alkaline Hydrogen Evolution

The introduction of heteroatoms is one of the most important ways to modulate the intrinsic electronic structure of electrocatalysts to improve their catalytic activity. However, for transition metal chalcogenides with highly symmetric crystal structure (HS-TMC), the introduction of heteroatoms, especially those with large atomic radius, often induces large lattice distortion and vacancy defects, which may lead to structural phase transition of doped materials or structural phase reconstruction during the catalytic reaction. Such unpredictable situations will make it difficult to explore the connection between the intrinsic electronic structure of doped catalysts and catalytic activity. Herein, taking thermodynamically stable cubic CoSe₂ phase as an example, we demonstrate that nitrogen incorporation can effectively regulate the intrinsic electronic structure of HS-TMC with structural phase stability and thus promote its electrocatalytic activity for the hydrogen evolution activity (HER). In contrast, the introduction of phosphorus can lead to structural phase transition from cubic CoSe₂ to orthorhombic phase, and the structural phase of phosphorus-doped CoSe₂ is unstable for HER.

Roadmap and Direction toward High-Performance MoS2 Hydrogen Evolution Catalysts

MoS₂ intrinsically show Pt-like hydrogen evolution reaction (HER) performance. Pristine MoS₂ displayed low HER activity, which was caused by low quantities of catalytic sites and unsatisfactory conductivity. Then, phase engineering and S vacancy were developed as effective strategies to elevate the intrinsic HER performance. Heterojunctions and dopants were successful strategies to improve HER performance significantly. A couple of state-of-the-art MoS₂ catalysts showed HER performance comparable to Pt. Applying multiple strategies in the same electrocatalyst was the key to furnish Pt-like HER performance. In this review, we summarize the available strategies to fabricate superior MoS₂ HER catalysts and tag the important works. We analyze the well-defined strategies for fabricating a superior MoS₂ electrocatalyst, propose complementary strategies which could help meet practical requirements, and help people design highly efficient MoS₂ electrocatalysts. We also provide a brief perspective on assembling practical electrochemical systems by high-performance MoS₂ electrocatalysts, apply MoS₂ in other important electrocatalysis reactions, and develop high-performance two-dimensional (2D) dichalcogenide HER catalysts not limited to MoS₂. This review will help researchers to obtain a better understanding of development of superior MoS₂ HER electrocatalysts, providing directions for next-generation catalyst development.

Scalable Synthesis of Tungsten Disulfide Nanosheets for Alkali-Acid Electrocatalytic Sulfion Recycling and H2 Generation

WS₂ nanosheets hold great promise for a variety of applications yet faces a grand challenge in terms of large-scale synthesis. We report a reliable, scalable, and high-yield (>93 %) synthetic strategy to fabricate WS₂ nanosheets, which exhibit highly desirable electrocatalytic properties toward both the alkaline sulfion (S^2- ) oxidation reaction (SOR) and the acidic hydrogen evolution reaction (HER). The findings prompted us to develop a hybrid alkali-acid electrochemical cell with the WS₂ nanosheets as bifunctional electrode catalysts of alkaline anodic SOR and acidic cathodic HER. The proof-of-concept device holds promise for self-power or low-electricity electrolytic H₂ generation and environmentally friendly recycling of sulfion with enhanced electron utilization efficiency.

Hydrodynamic synthesis of Fe2O3@MoS2 0D/2D-nanocomposite material and its application as a catalyst in the glycolysis of polyethylene terephthalate

We report a fast and simple synthesis of Fe₂O₃@MoS₂ 0D/2D-nanocomposite material using a Taylor–Couette flow reactor. A Taylor–Couette flow with high shear stress and mixing characteristics was used for fluid dynamic exfoliation of MoS₂ and deposition of uniform Fe₂O₃ nanoparticles, resulting in a Fe₂O₃@MoS₂ in the form of 0D/2D-nanocomposite material. Using Taylor–Couette flow reactor, we could synthesize Fe₂O₃@MoS₂ 0D/2D-nanocomposite material at a rate higher than 1000 mg h⁻¹ which is much higher than previously reported production rate of 0.2–116.7 mg h⁻¹. The synthesis of Fe₂O₃@MoS₂ nanocomposite was achieved in an aqueous solution without thermal or organic solvent treatment. Exfoliated MoS₂ nanosheets show an average thickness of 2.6 ± 2.3 nm (<6 layers) and a lateral size of 490 ± 494 nm. Fe₂O₃ nanoparticles have an average size of 7.4 ± 3.0 nm. Fe₂O₃ nanoparticles on chemically and thermally stable MoS₂ nanosheets show catalytic activity in the glycolysis of polyethylene terephthalate (PET). High conversion of PET (97%) and a high yield (90%) for bis(hydroxyethyl) terephthalate (BHET) were achieved in a reaction time of 3 h at the reaction temperature of 225 °C.

Fe₂O₃@MoS₂ 0D/2D-nanocomposite material was synthesized in an aqueous solution using a Taylor–Couette flow reactor.

Sulfur defect rich Mo-Ni3S2 QDs assisted by O-C[double bond, length as m-dash]O chemical bonding for an efficient electrocatalytic overall water splitting

Developing earth-abundant and highly efficient electrocatalysts is critical for further development of a system. The metal (M) doping strategy and inorganic/organic composite are two common strategies to improve the performance of electrocatalysts for overall water splitting (OWS). In this paper, two strategies are subtly used to prepare Mo-Ni₃S₂ quantum dots (QDs) with rich sulfur defects through Moⁿ⁺ doping Ni₃S₂ and introduction of trisodium citrate by a two-step hydrothermal reaction. Results show that high sulfur defects can be controllably prepared as the lattice mismatch and active sites can be efficiently increased via Moⁿ⁺ doping. Moreover, the introduction of trisodium citrate with carboxyl functional groups not only enhances the degree of sulfur defects around the metal center, changes the morphology of sulfide to distribute the active centers evenly, but also endow the metal center with strong valence changing ability with organic characteristics. The in situ Raman study reveals that O-C[double bond, length as m-dash]O promotes the formation of the real active site M-OOH by the way of self-sacrifice during the OER process. Mo-Ni₃S₂ QDelectrocatalyst shows excellent performance in the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), achieving a current density of 10 mA cm^-2 at the overpotentials of 115 mV and 222 mV with very good chemical stability, superior than that of most of the reported materials. The OWS reaction can provide a current density of 10 mA cm^-2 and 50 mA cm^-2, which only needs 1.53 V and 1.74 V with excellent industrial application prospects.

Hierarchically Ordinated Two-Dimensional MoS2 Nanosheets on Three-Dimensional Reduced Graphene Oxide Aerogels as Highly Active and Stable Catalysts for Hydrogen Evolution Reaction

Hydrogen gas (H2) is being intensively proposed as a next-generation clean energy owing to the depletion of fossil fuels. Electrochemical water splitting is one of the most promising processes for hydrogen production. Furthermore, many efforts focusing on electrochemical water splitting have been made to develop low-cost, electrochemically active, and stable catalysts for efficient hydrogen production. MoS2 has emerged as an attractive material for developing catalysts for the hydrogen evolution reaction (HER). Hence, in this study, we design hierarchically ordinated two-dimensional (2D) MoS2 nanosheets on three-dimensional (3D) reduced graphene oxide (rGO) (H-2D/3D-MoS2-rGO) aerogel structures as a new class of electrocatalysts for the HER. We use the one-pot hydrothermal synthesis route for developing high-performance electroactive materials for the HER. The as-prepared H-2D/3D-MoS2-rGO contains a unique 3D hierarchical structure providing large surface areas owing to the 3D porous networks of rGO and more active sites owing to the many edge sites in the MoS2 nanosheets. In addition, the H-2D/3D-MoS2-rGO structure exhibits remarkable electrochemical properties during the HER. It shows a lower overpotential than pure MoS2 and excellent electrochemical stability owing to the large number of active sites (highly exposed edge sites) and high electrical conductivity from the rGO structure.

A new phosphonium-based ionic liquid to synthesize nickel metaphosphate for hydrogen evolution reaction

The electrochemical hydrogen evolution reaction (HER) is a promising strategy for production of hydrogen; however, it is still restricted by appropriate efficient and low-cost electrocatalysts. Herein, for the first time, the n-octylammonium hypophosphite, a kind of protic ionic liquid (IL), was used as a new phosphorus source for the manufacture of nickel metaphosphate (Ni₂P₄O₁₂) electrocatalysts. In contrast to traditional multi-step fabrication processes, the n-octylammonium hypophosphite acted as both reactant and solvent to synthesize Ni₂P₄O₁₂ by a one-step calcination approach. The obtained Ni₂P₄O₁₂ as an alkaline HER catalyst required a low overpotential of 116 mV at -10 mA cm^-2 and a small Tafel slope of 97 mV dec^-1, comparable to the majority of reported Ni-based materials and other phosphate catalysts. Furthermore, this catalyst exhibited robust stability with no distinct attenuation of current density after a long-term durability test in 1 M KOH. Therefore, this task-specific IL strategy with a simple reaction system, reducing the occurrence of side reactions, provided a new perspective on design of high-efficiency metaphosphate electrocatalysts.

Modulation of electronic structures in two-dimensional electrocatalysts for the hydrogen evolution reaction

The electrocatalytic hydrogen evolution reaction (HER) has attracted substantial attention owing to its important role in realizing economic and sustainable hydrogen production via water electrolysis. Designing two-dimensional (2D) materials with large surface area, highly exposed surface sites and facile charge transport pathways is highly attractive for promoting the HER activity of the earth-abundant catalysts, and conducting rational modulations in the electronic structures is considered to be promising in further optimizing the intrinsic HER activity and thus realizing promoted HER performance. In this Feature Article, we systematically summarize recent progress in the modulation of the electronic structures of 2D HER electrocatalysts via multiple strategies including elemental doping, formation of alloyed structures, defect engineering, facet engineering, phase regulation, interface engineering and hybridization of the nanocatalysts with 2D substrates, and discuss the role of electronic structures in optimizing the intrinsic HER activity of 2D HER catalysts. We anticipate that this Feature Article will offer helpful guidance for oriented design and optimization of efficient electrocatalysts for scalable and economic hydrogen production.

Sequential Electrodeposition of Bifunctional Catalytically Active Structures in MoO3 /Ni-NiO Composite Electrocatalysts for Selective Hydrogen and Oxygen Evolution

Exploring earth-abundant and highly efficient electrocatalysts is critical for further development of water electrolyzer systems. Integrating bifunctional catalytically active sites into one multi-component might greatly improve the overall water-splitting performance. In this work, amorphous NiO nanosheets coupled with ultrafine Ni and MoO₃ nanoparticles (MoO₃ /Ni-NiO), which contains two heterostructures (i.e., Ni-NiO and MoO₃ -NiO), is fabricated via a novel sequential electrodeposition strategy. The as-synthesized MoO₃ /Ni-NiO composite exhibits superior electrocatalytic properties, affording low overpotentials of 62 mV at 10 mA cm^-2 and 347 mV at 100 mA cm^-2 for catalyzing the hydrogen and the oxygen evolution reaction (HER/OER), respectively. Moreover, the MoO₃ /Ni-NiO hybrid enables the overall alkaline water-splitting at a low cell voltage of 1.55 V to achieve 10 mA cm^-2 with outstanding catalytic durability, significantly outperforming the noble-metal catalysts and many materials previously reported. Experimental and theoretical investigations collectively demonstrate the generated Ni-NiO and MoO₃ -NiO heterostructures significantly reduce the energetic barrier and act as catalytically active centers for selective HER and OER, synergistically accelerating the overall water-splitting process. This work helps to fundamentally understand the heterostructure-dependent mechanism, providing guidance for the rational design and oriented construction of hybrid nanomaterials for diverse catalytic processes.