Optimized Metal Chalcogenides for Boosting Water Splitting

Energy-Efficient Co-production of Benzoquinone and H2 Using Waste Phenol in a Hybrid Alkali/Acid Flow Cell

In both the manufacturing and chemical industries, benzoquinone is a crucial chemical product. A perfect and economical method for making benzoquinone is the electrochemical oxidation of phenol, thanks to the traditional thermal catalytic oxidation of phenol process requires high cost, serious pollution and harsh reaction conditions. Here, a unique heterostructure electrocatalyst on nickel foam (NF) consisting of nickel sulfide and nickel oxide (Ni9S8-Ni15O16/NF) was produced, and this catalyst exhibited a low overpotential (1.35 V vs. RHE) and prominent selectivity (99 %) for electrochemical phenol oxidation reaction (EOP). Ni9S8-Ni15O16/NF is beneficial for lowering the reaction energy barrier and boosting reactivity in the EOP process according to density functional theory (DFT) calculations. Additionally, an alkali/acid hybrid flow cell was successfully established by connecting Ni9S8-Ni15O16/NF and commercial RuIr/Ti in series to catalyze phenol oxidation in an alkaline medium and hydrogen evolution in an acid medium, respectively. A cell voltage of only 0.60 V was applied to produce a current density of 10 mA cm-2. Meanwhile, the system continued to operate at 0.90 V for 12 days, showing remarkable long-term stability. The unique configuration of the acid-base hybrid flow cell electrolyzer provides valuable guidance for the efficient and environmentally friendly electrooxidation of phenol to benzoquinone.

Next-Generation Green Hydrogen: Progress and Perspective from Electricity, Catalyst to Electrolyte in Electrocatalytic Water Splitting

Green hydrogen from electrolysis of water has attracted widespread attention as a renewable power source. Among several hydrogen production methods, it has become the most promising technology. However, there is no large-scale renewable hydrogen production system currently that can compete with conventional fossil fuel hydrogen production. Renewable energy electrocatalytic water splitting is an ideal production technology with environmental cleanliness protection and good hydrogen purity, which meet the requirements of future development. This review summarizes and introduces the current status of hydrogen production by water splitting from three aspects: electricity, catalyst and electrolyte. In particular, the present situation and the latest progress of the key sources of power, catalytic materials and electrolyzers for electrocatalytic water splitting are introduced. Finally, the problems of hydrogen generation from electrolytic water splitting and directions of next-generation green hydrogen in the future are discussed and outlooked. It is expected that this review will have an important impact on the field of hydrogen production from water.

A facile N-doped NiFe(B) (Oxy)hydroxide monolithic electrode for enhanced water oxidation

N-doped NiFe(B) (oxy)hydroxide can promote the catalytic activity for an alkaline oxygen evolution reaction (OER) significantly, but fabrication is difficult. Herein, we introduced a B-induction route to the N-NiFe(B) (oxy)hydroxide monolithic electrode under a relatively low temperature. We observed an excellent catalytic performance benefiting from an optimal electronic structure, enlarged surface area and improved hydrophilicity. Moreover, this mild protocol could be extended to fabricate an S-doped NiFe-based catalyst. This research could aid large-scale manufacture.

Synthesis and Oxygen Evolution Reaction Application of a Co-Cd Based Bimetallic Metal-Organic Framework

In the realm of renewable energy technologies, the development of efficient and durable electrocatalysts is paramount, especially for applications like electrochemical water splitting. This research focuses on synthesizing a novel bimetallic metal-organic framework (BMMOF11) using earth-abundant elements, cobalt (Co) and cadmium (Cd). BMMOF11 showcases a distinctive structure with distorted octahedral chains of CoO and CdO, linked by benzene tricarboxylic acid (BTC). Our study primarily investigates the electrocatalytic efficiency of BMMOF11, particularly in water oxidation reactions. For practical analysis, BMMOF11 was anchored onto nickel foam, forming BMMOF11/NF, to evaluate its electrocatalytic properties. Electrochemical testing revealed that BMMOF11/NF begins water oxidation at an onset potential of 1.62 V versus RHE, demonstrating high activity with a lower overpotential of 0.4 V to achieve a current density of 10 mA/cm2 . Moreover, BMMOF11/NF maintained stable water splitting performance, sustaining a current density of approximately 70 mA/cm2 under a voltage of 1.9 V relative to RHE. These findings indicate that BMMOF11/NF is a promising candidate for large-scale electrochemical water splitting, offering a blend of high activity and stability.

Synchronous Surface-Interface and Crystal-Phase Engineered Multifaceted Hybrid Nanostructure of Fe-(1T)-VSe2 Nanosheet and Fe-CoSe2 Nanorods Doped with P for Rapid HER and OER, Kinetics

Two different nanostructures of two dissimilar highly-potent active electrocatalysts, P-dopped metallic-(1T)-Fe-VSe2 (P,Fe-1T-VSe2 ) nanosheet and P-dopped Fe-CoSe2 (P,Fe-CoSe2 ) nanorods are hybridized and integrated into a single heterostructure (P,Fe-(VCo)Se2 ) on Ni-foam for high-performance water splitting (WS). The catalytic efficiency of VSe2 nanosheets is first enhanced by enriching metallic (1T)-phase, then forming bimetallic Fe-V selenide, and finally by P-doping. Similarly, the catalytic efficiency of CoSe2 nanorods is boosted by first fabricating Fe-Co bimetallic selenide and then P-doping. To develop super-efficient electrocatalysts for WS, two individual electrocatalysts P,Fe-1T-VSe2 nanosheet and P,Fe-CoSe2 are hybridized and integrated to form a heterostructure (P,Fe-(VCo)Se2 ). Metallic (1T)-phase of transition metal dichalcogenides has much higher conductivity than the 2H-phase, while bimetallization and P-doping activate basal planes, develop various active components, and form heterostructures that develop a synergistic interfacial effect, all of which, significantly boost the catalytic efficacy of the P,Fe-(VCo)Se2 . P,Fe-(VCo)Se2 shows excellent performance requiring very low overpotential (ηHER = 50 mV@10 mAcm-2 and ηOER = 230 mV@20 mAcm-2 ). P,Fe-(VCo)Se2 (+, -) device requires a cell potential of 1.48 V to reach 10 mA cm-2 for overall WS.

Oxalic Acid-Assisted Vacancy Engineering Promotes Iron-Copper Sulfide Nanosheets for High-Current Density Water Oxidation

The effective defect and interface coupling are pivotal for the promotion of the catalytic activity for the oxygen evolution reaction. Herein, we report novel hybrid nanosheets with sulfur vacancies composed of FeS2 and Cu39S28 grown on Cu foam (Vs-FeS2/Cu39S28). The optimal Vs-FeS2/Cu39S28 exhibits a high current output of 500 mA cm-2 at a low overpotential of 370 mV and robust stability for 60 h at 100 mA cm-2, surpassing the values of most previously reported Cu-based catalysts. Furthermore, a two-electrode electrolyzer made by pairing the prepared catalyst with commercial Pt/C requires a low cell voltage of 1.75 V at 100 mA cm-2 and is retained over 80 h. Key to its excellent performance is the synergism between intertwined FeS2 and Cu39S28 domains, enriched by the deliberate introduction of sulfur vacancies, thus optimizing the electronic structure and causing the proliferation of catalytic active sites. This work presents a potent Cu-based electrocatalyst and emphasizes the leveraging of non-precious metals for efficient water oxidation.

From VIB- to VB-Group Transition Metal Disulfides: Structure Engineering Modulation for Superior Electromagnetic Wave Absorption

The laminated transition metal disulfides (TMDs), which are well known as typical two-dimensional (2D) semiconductive materials, possess a unique layered structure, leading to their wide-spread applications in various fields, such as catalysis, energy storage, sensing, etc. In recent years, a lot of research work on TMDs based functional materials in the fields of electromagnetic wave absorption (EMA) has been carried out. Therefore, it is of great significance to elaborate the influence of TMDs on EMA in time to speed up the application. In this review, recent advances in the development of electromagnetic wave (EMW) absorbers based on TMDs, ranging from the VIB group to the VB group are summarized. Their compositions, microstructures, electronic properties, and synthesis methods are presented in detail. Particularly, the modulation of structure engineering from the aspects of heterostructures, defects, morphologies and phases are systematically summarized, focusing on optimizing impedance matching and increasing dielectric and magnetic losses in the EMA materials with tunable EMW absorption performance. Milestones as well as the challenges are also identified to guide the design of new TMDs based dielectric EMA materials with high performance.

Metal-ion doping in metal-organic-frameworks: modulating the electronic structure and local coordination for enhanced oxygen evolution reaction activity

The doping of metal-organic frameworks (MOFs) with metal-ions has emerged as a powerful strategy for enhancing their catalytic performance. Doping allows for the tailoring of the electronic structure and local coordination environment of MOFs, thus imparting on them unique properties and enhanced functionalities. This frontier article discusses the impact of metal-ion doping on the electronic structure and local coordination of MOFs, highlighting the effects on their electrocatalytic properties in relation to the oxygen evolution reaction (OER). The fundamental mechanisms underlying these modifications are explored, while recent advances, challenges, and prospects in the field are discussed. In addition, experimental techniques that can be applied to tackle the realization of effective metal-ion doping of MOFs are also noted briefly.

Chiral Inorganic Nanomaterials for Photo(electro)catalytic Conversion

Chiral inorganic nanomaterials due to their unique asymmetric nanostructures have gradually demonstrated intriguing chirality-dependent performance in photo(electro)catalytic conversion, such as water splitting. However, understanding the correlation between chiral inorganic characteristics and the photo(electro)catalytic process remains challenging. In this perspective, we first highlight the chirality source of inorganic nanomaterials and briefly introduce photo(electro)catalysis systems. Then, we delve into an in-depth discussion of chiral effects exerted by chiral nanostructures and their photo-electrochemistry properties, while emphasizing the emerging chiral inorganic nanomaterials for photo(electro)catalytic conversion. Finally, the challenges and opportunities of chiral inorganic nanomaterials for photo(electro)catalytic conversion are prospected. This perspective provides a comprehensive overview of chiral inorganic nanomaterials and their potential in photo(electro)catalytic conversion, which is beneficial for further research in this area.

Colloidal semiconductor nanocrystals: from bottom-up nanoarchitectonics to energy harvesting applications

Colloidal semiconductor nanocrystals (NCs) have been extensively investigated owing to their unique properties induced by the quantum confinement effect. The advent of colloidal synthesis routes led to the design of stable colloidal NCs with uniform size, shape, and composition. Metal oxides, phosphides, and chalcogenides (ZnE, CdE, PbE, where E = S, Se, or Te) are few of the most important monocomponent semiconductor NCs, which show excellent optoelectronic properties. The ability to build quantum confined heterostructures comprising two or more semiconductor NCs offer greater customization and tunability of properties compared to their monocomponent counterparts. More recently, the halide perovskite NCs showed exceptional optoelectronic properties for energy generation and harvesting applications. Numerous applications including photovoltaic, photodetectors, light emitting devices, catalysis, photochemical devices, and solar driven fuel cells have demonstrated using these NCs in the recent past. Overall, semiconductor NCs prepared via the colloidal synthesis route offer immense potential to become an alternative to the presently available device applications. This feature article will explore the progress of NCs syntheses with outstanding potential to control the shape and spatial dimensionality required for photovoltaic, light emitting diode, and photocatalytic applications. We also attempt to address the challenges associated with achieving high efficiency devices with the NCs and possible solutions including interface engineering, packing control, encapsulation chemistry, and device architecture engineering.

Surface Reconstruction of Covellite CuS Nanocrystals for Enhanced OER Catalytic Performance in Alkaline Solution

Oxygen evolution reaction (OER) is one of the important half-reactions in energy conversion equipment such as water-spitting devices, rechargeable metal-air batteries, and so on. It is beneficial to develop efficient and low-cost catalysts that understand the reaction mechanism of OER and analyze the reconstruction phenomenon of transition metal sulfide. Interestingly, copper sulfide and cuprous sulfide with the same components possess different reconstruction behaviors due to their different metal ion valence states and different atomic arrangement modes. Because of a unique atomic arrangement sequence and certain cationic defects, the reconstruction phenomenon of CuS nanomaterials are that S2- is firstly oxidized to SO4 2- and then Cux + is converted into CuO via Cu(OH)2 . In addition, the specific "modified hourglass structure" of CuS with excellent conductivity is easier to produce intermediates. Compared with Cu2 S, CuS exhibits excellent OER activity with a lower overpotential of 192 mV at 10 mA cm-2 and remarkable electrochemical stability in 1.0 m KOH for 120 h. Herein, this study elucidates the reconstruction modes of CuS and Cu2 S in the OER process and reveals that CuS has a stronger CuS bond and a faster electronic transmission efficiency due to "modified hourglass structure," resulting in faster reconstruction of CuS than Cu2 S.

Lattice-Strain Engineering for Heterogenous Electrocatalytic Oxygen Evolution Reaction

The energy efficiency of metal-air batteries and water-splitting techniques is severely constrained by multiple electronic transfers in the heterogenous oxygen evolution reaction (OER), and the high overpotential induced by the sluggish kinetics has become an uppermost scientific challenge. Numerous attempts are devoted to enabling high activity, selectivity, and stability via tailoring the surface physicochemical properties of nanocatalysts. Lattice-strain engineering as a cutting-edge method for tuning the electronic and geometric configuration of metal sites plays a pivotal role in regulating the interaction of catalytic surfaces with adsorbate molecules. By defining the d-band center as a descriptor of the structure-activity relationship, the individual contribution of strain effects within state-of-the-art electrocatalysts can be systematically elucidated in the OER optimization mechanism. In this review, the fundamentals of the OER and the advancements of strain-catalysts are showcased and the innovative trigger strategies are enumerated, with particular emphasis on the feedback mechanism between the precise regulation of lattice-strain and optimal activity. Subsequently, the modulation of electrocatalysts with various attributes is categorized and the impediments encountered in the practicalization of strained effect are discussed, ending with an outlook on future research directions for this burgeoning field.

Research Progress of Hydrogen Production Technology and Related Catalysts by Electrolysis of Water

As a clean and renewable energy source for sustainable development, hydrogen energy has gained a lot of attention from the general public and researchers. Hydrogen production by electrolysis of water is the most important approach to producing hydrogen, and it is also the main way to realize carbon neutrality. In this paper, the main technologies of hydrogen production by electrolysis of water are discussed in detail; their characteristics, advantages, and disadvantages are analyzed; and the selection criteria and design criteria of catalysts are presented. The catalysts used in various hydrogen production technologies and their characteristics are emphatically expounded, aiming at optimizing the existing catalyst system and developing new high-performance, high-stability, and low-cost catalysts. Finally, the problems and solutions in the practical design of catalysts are discussed and explored.

Melamine-Based Hydrogen-bonded Systems as Organoelectrocatalysts for Water Oxidation Reaction

Applications of small organic molecules and hydrogen-bonded aggregates, instead of traditional transition-metal-based electrocatalysts, are gaining momentum for addressing the issue of low-cost generation of H₂ to power a sustainable environment. Such systems offer the possibility to integrate desired functional moieties with predictive structural repetition for modulating their properties. Despite these advantages, hydrogen-bonded organic systems have largely remained unexplored, especially as electrocatalysts. Melamine and adipic acid-based hydrogen-bonded organic ionic (BMA) and co-crystal systems developed under varying temperatures are explored as electrocatalysts for water oxidation reaction (WOR). These systems are easily modifiable with precisely designed molecular architecture and judiciously positioned nitrogen atoms. Combined effect of charge-assisted hydrogen bonding stabilizes the ionic BMA system under corrosive alkaline conditions and augments its remarkable electrocatalytic WOR activity, achieving a current density of 10 mA cm^-2 at an overpotential of 387 mV and Faradaic efficiency ∼94.5 %. The enhanced electrocatalytic ability of BMA is attributed to its hydrophilic nature, unique molecular composition with complementary hydrogen-bonded motifs and a high density of positively charged nitrogen atoms on the surface, that facilitates electrostatic interactions and accelerate charge and mass transport processes culminating in a turnover frequency of ∼0.024 s^-1 . This work validates the potential of hydrogen-bonded molecular organo-electrocatalysts towards WOR.

IrPd Nanoalloy-Structured Bifunctional Electrocatalyst for Efficient and pH-Universal Water Splitting

The lack of high efficiency and pH-universal bifunctional electrocatalysts for water splitting to hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) hinders the large-scale production of green hydrogen. Here, an IrPd electrocatalyst supported on ketjenblack that exhibits outstanding bifunctional performance for both HER and OER at wide pH conditions is presented. The optimized IrPd catalyst exhibits a specific activity of 4.46 and 3.98 A mg_Ir ^-1 in the overpotential of 100 and 370 mV for HER and OER, respectively, in alkaline conditions. When applied to the anion exchange membrane electrolyzer, the Ir₄₄ Pd₅₆ /KB catalyst shows a stability of >20 h at a current of 250 mA cm^-2 for water decomposition, indicating promising prospects for practical applications. Beyond offering an advanced electrocatalyst, this work also guides the rational design of desirable bifunctional electrocatalysts for HER and OER by regulating the microenvironments and electronic structures of metal catalytic sites for diverse catalysis.

Oxygen Evolution/Reduction Reaction Catalysts: From In Situ Monitoring and Reaction Mechanisms to Rational Design

The oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) are core steps of various energy conversion and storage systems. However, their sluggish reaction kinetics, i.e., the demanding multielectron transfer processes, still render OER/ORR catalysts less efficient for practical applications. Moreover, the complexity of the catalyst-electrolyte interface makes a comprehensive understanding of the intrinsic OER/ORR mechanisms challenging. Fortunately, recent advances of in situ/operando characterization techniques have facilitated the kinetic monitoring of catalysts under reaction conditions. Here we provide selected highlights of recent in situ/operando mechanistic studies of OER/ORR catalysts with the main emphasis placed on heterogeneous systems (primarily discussing first-row transition metals which operate under basic conditions), followed by a brief outlook on molecular catalysts. Key sections in this review are focused on determination of the true active species, identification of the active sites, and monitoring of the reactive intermediates. For in-depth insights into the above factors, a short overview of the metrics for accurate characterizations of OER/ORR catalysts is provided. A combination of the obtained time-resolved reaction information and reliable activity data will then guide the rational design of new catalysts. Strategies such as optimizing the restructuring process as well as overcoming the adsorption-energy scaling relations will be discussed. Finally, pending current challenges and prospects toward the understanding and development of efficient heterogeneous catalysts and selected homogeneous catalysts are presented.

Facet Engineering of Advanced Electrocatalysts Toward Hydrogen/Oxygen Evolution Reactions

The crystal facets featured with facet-dependent physical and chemical properties can exhibit varied electrocatalytic activity toward hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) attributed to their anisotropy. The highly active exposed crystal facets enable increased mass activity of active sites, lower reaction energy barriers, and enhanced catalytic reaction rates for HER and OER. The formation mechanism and control strategy of the crystal facet, significant contributions as well as challenges and perspectives of facet-engineered catalysts for HER and OER are provided.

The electrocatalytic water splitting technology can generate high-purity hydrogen without emitting carbon dioxide, which is in favor of relieving environmental pollution and energy crisis and achieving carbon neutrality. Electrocatalysts can effectively reduce the reaction energy barrier and increase the reaction efficiency. Facet engineering is considered as a promising strategy in controlling the ratio of desired crystal planes on the surface. Owing to the anisotropy, crystal planes with different orientations usually feature facet-dependent physical and chemical properties, leading to differences in the adsorption energies of oxygen or hydrogen intermediates, and thus exhibit varied electrocatalytic activity toward hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). In this review, a brief introduction of the basic concepts, fundamental understanding of the reaction mechanisms as well as key evaluating parameters for both HER and OER are provided. The formation mechanisms of the crystal facets are comprehensively overviewed aiming to give scientific theory guides to realize dominant crystal planes. Subsequently, three strategies of selective capping agent, selective etching agent, and coordination modulation to tune crystal planes are comprehensively summarized. Then, we present an overview of significant contributions of facet-engineered catalysts toward HER, OER, and overall water splitting. In particular, we highlight that density functional theory calculations play an indispensable role in unveiling the structure–activity correlation between the crystal plane and catalytic activity. Finally, the remaining challenges in facet-engineered catalysts for HER and OER are provided and future prospects for designing advanced facet-engineered electrocatalysts are discussed.

Synergistic Effect in a Metal-Organic Framework Boosting the Electrochemical CO2 Overall Splitting

It is a very important but still challenging task to develop bifunctional electrocatalysts for highly efficient CO₂ overall splitting. Herein, we report a stable metal-organic framework (denoted as PcNi-Co-O), composed of (2,3,9,10,16,17,23,24-octahydroxyphthalocyaninato)nickel(II) (PcNi-(O^-)₈) ligands and the planar CoO₄ nodes, for CO₂ overall splitting. When working as both cathode and anode catalysts (i.e., PcNi-Co-O||PcNi-Co-O), PcNi-Co-O achieved a commercial-scale current density of 123 mA cm^-2 (much higher than the reported values (0.2-12 mA cm^-2)) with a Faradic efficiency (CO) of 98% at a low cell voltage of 4.4 V. Mechanism studies suggested the synergistic effects between two active sites, namely, (i) electron transfer from CoO₄ to PcNi sites under electric fields, resulting in the raised oxidizability/reducibility of CoO₄/PcNi sites, respectively; (ii) the energy-level matching of cathode and anode catalysts can reduce the energy barrier of electron transfer between them and improve the performance of CO₂ overall splitting.

Layer-Structured Anisotropic Metal Chalcogenides: Recent Advances in Synthesis, Modulation, and Applications

The unique electronic and catalytic properties emerging from low symmetry anisotropic (1D and 2D) metal chalcogenides (MCs) have generated tremendous interest for use in next generation electronics, optoelectronics, electrochemical energy storage devices, and chemical sensing devices. Despite many proof-of-concept demonstrations so far, the full potential of anisotropic chalcogenides has yet to be investigated. This article provides a comprehensive overview of the recent progress made in the synthesis, mechanistic understanding, property modulation strategies, and applications of the anisotropic chalcogenides. It begins with an introduction to the basic crystal structures, and then the unique physical and chemical properties of 1D and 2D MCs. Controlled synthetic routes for anisotropic MC crystals are summarized with example advances in the solution-phase synthesis, vapor-phase synthesis, and exfoliation. Several important approaches to modulate dimensions, phases, compositions, defects, and heterostructures of anisotropic MCs are discussed. Recent significant advances in applications are highlighted for electronics, optoelectronic devices, catalysts, batteries, supercapacitors, sensing platforms, and thermoelectric devices. The article ends with prospects for future opportunities and challenges to be addressed in the academic research and practical engineering of anisotropic MCs.

Sulfate-Decorated Amorphous-Crystalline Cobalt-Iron Oxide Nanosheets to Enhance O-O Coupling in the Oxygen Evolution Reaction

The electrochemical oxygen evolution reaction (OER) plays a fundamental role in several energy technologies, which performance and cost-effectiveness are in large part related to the used OER electrocatalyst. Herein, we detail the synthesis of cobalt-iron oxide nanosheets containing controlled amounts of well-anchored SO₄^2- anionic groups (CoFe_xO_y-SO₄). We use a cobalt-based zeolitic imidazolate framework (ZIF-67) as the structural template and a cobalt source and Mohr's salt ((NH₄)₂Fe(SO₄)₂·6H₂O) as the source of iron and sulfate. When combining the ZIF-67 with ammonium iron sulfate, the protons produced by the ammonium ion hydrolysis (NH₄⁺ + H₂O = NH₃·H₂O + H⁺) etch the ZIF-67, dissociating its polyhedron structure, and form porous assemblies of two-dimensional nanostructures through a diffusion-controlled process. At the same time, iron ions partially replace cobalt within the structure, and SO₄^2- ions are anchored on the material surface by exchange with organic ligands. As a result, ultrathin CoFe_xO_y-SO₄ nanosheets are obtained. The proposed synthetic procedure enables controlling the amount of Fe and SO₄ ions and analyzing the effect of each element on the electrocatalytic activity. The optimized CoFe_xO_y-SO₄ material displays outstanding OER activity with a 10 mA cm^-2 overpotential of 268 mV, a Tafel slope of 46.5 mV dec^-1, and excellent stability during 62 h. This excellent performance is correlated to the material's structural and chemical parameters. The assembled nanosheet structure is characterized by a large electrochemically active surface area, a high density of reaction sites, and fast electron transportation. Meanwhile, the introduction of iron increases the electrical conductivity of the catalysts and provides fast reaction sites with optimum bond energy and spin state for the adsorption of OER intermediates. The presence of sulfate ions at the catalyst surface modifies the electronic energy level of active sites, regulates the adsorption of intermediates to reduce the OER overpotential, and promotes the surface charge transfer, which accelerates the formation of oxygenated intermediates. Overall, the present work details the synthesis of a high-efficiency OER electrocatalyst and demonstrates the introduction of nonmetallic anionic groups as an excellent strategy to promote electrocatalytic activity in energy conversion technologies.

Electrochemical Reduction of CO2 to C1 and C2 Liquid Products on Copper-Decorated Nitrogen-Doped Carbon Nanosheets

Due to the significant rise in atmospheric carbon dioxide (CO₂) concentration and its detrimental environmental effects, the electrochemical CO₂ conversion to valuable liquid products has received great interest. In this work, the copper-melamine complex was used to synthesize copper-based electrocatalysts comprising copper nanoparticles decorating thin layers of nitrogen-doped carbon nanosheets (Cu/NC). The as-prepared electrocatalysts were characterized by XRD, SEM, EDX, and TEM and investigated in the electrochemical CO₂ reduction reaction (ECO₂RR) to useful liquid products. The electrochemical CO₂ reduction reaction was carried out in two compartments of an electrochemical H-Cell, using 0.5 M potassium bicarbonate (KHCO₃) as an electrolyte; nuclear magnetic resonance (¹H NMR) was used to analyze and quantify the liquid products. The electrode prepared at 700 °C (Cu/NC-700) exhibited the best dispersion for the copper nanoparticles on the carbon nanosheets (compared to Cu/NC-600 & Cu/NC-800), highest current density, highest electrochemical surface area, highest electrical conductivity, and excellent stability and faradic efficiency (FE) towards overall liquid products of 56.9% for formate and acetate at the potential of -0.8V vs. Reversible Hydrogen Electrode (RHE).

Atomically Interfacial Engineering on Molybdenum Nitride Quantum Dots Decorated N-doped Graphene for High-Rate and Stable Alkaline Hydrogen Production

The development of low-cost, high-efficiency, and stable electrocatalysts for hydrogen evolution reaction (HER) under alkaline conditions is a key challenge in water electrolysis. Here, an interfacial engineering strategy that is capable of simultaneously regulating nanoscale structure, electronic structure, and interfacial structure of Mo₂ N quantum dots decorated on conductive N-doped graphene via codoping single-atom Al and O (denoted as AlO@Mo₂ N-NrGO) is reported. The conversion of Anderson polyoxometalates anion cluster ([AlMo₆ O₂₄ H₆ ]^3- , denoted as AlMo6) to Mo₂ N quantum dots not only result in the generation of more exposed active sites but also in situ codoping atomically dispersed Al and O, that can fine-tune the electronic structure of Mo₂ N. It is also identified that the surface reconstruction of AlOH hydrates in AlO@Mo₂ N quantum dots plays an essential role in enhancing hydrophilicity and lowering the energy barriers for water dissociation and hydrogen desorption, resulting in a remarkable alkaline HER performance, even better than the commercial 20% Pt/C. Moreover, the strong interfacial interaction (MoN bonds) between AlO@Mo₂ N and N-doped graphene can significantly improve electron transfer efficiency and interfacial stability. As a result, outstanding stability over 300 h at a current density higher than 100 mA cm^-2 is achieved, demonstrating great potential for the practical application of this catalyst.

Diffusion-Mediated Morphological Transformation in Bifunctional Mn2O3/CuO-(VO)3(PO4)2·6H2O for Enhanced Electrochemical Water Splitting

A strategical approach for morphological transformation and heterojunction formation was utilized to suppress the shortcomings of uni-metal oxide electrocatalysts and enhance their bifunctionality. In situ generation of copper oxide (CuO) over the surface of manganese oxide (Mn₂O₃) resulted in a morphological transformation from solid spheres to hollow spherical structures due to the ion-exchange diffusion (Kirkendall effect) of Cu ions into Mn₂O₃ particles. This hollowness resulted in the advancement of the bifunctional electrocatalytic behavior of Mn₂O₃/CuO (overpotential (η₁₀) of 280 mV for an OER and 310 mV for an HER at a current density of 10 mA/cm²) by virtue of increased exposed surface active sites aiding the adsorption of water molecules on the surface. The increased electrochemical active surface area (ECSA/C_dl = 34 mF/cm²) and reduced charge transfer resistance resulted in the formation of Mn₂O₃/CuO hollow spheres to achieve an approximately threefold enhancement in the turnover frequency (TOF) compared to the bare Mn₂O₃. The electrocatalytic efficiency of Mn₂O₃/CuO was further enhanced by virtue of the faster charge transfer coefficient of two-dimensional (2D) vanadyl phosphate hexahydrate (VOP) sheets deposited over its surface. This boosted the overall water splitting with attained overpotential (η₁₀) values of 190 and 220 mV with Tafel slopes of 60 and 105 mV/decade for an OER and HER, respectively. The morphological transformation and formation of an n-p heterojunction between Mn₂O₃ and CuO based on their work function (φ) values evaluated from the density functional theory (DFT) calculation and the effect of the VOP overlayer for faster reaction kinetics at the electrolyte interface resulted in an ∼10-fold increment in TOF values compared to the bare counterpart.

Bimetal Modulation Stabilizing a Metallic Heterostructure for Efficient Overall Water Splitting at Large Current Density

Large current-driven alkaline water splitting for large-scale hydrogen production generally suffers from the sluggish charge transfer kinetics. Commercial noble-metal catalysts are unstable in large-current operation, while most non-noble metal catalysts can only achieve high activity at low current densities <200 mA cm^-2 , far lower than industrially-required current densities (>500 mA cm^-2 ). Herein, a sulfide-based metallic heterostructure is designed to meet the industrial demand by regulating the electronic structure of phase transition coupling with interfacial defects from Mo and Ni incorporation. The modulation of metallic Mo₂ S₃ and in situ epitaxial growth of bifunctional Ni-based catalyst to construct metallic heterostructure can facilitate the charge transfer for fast Volmer H and Heyrovsky H₂ generation. The Mo₂ S₃ @NiMo₃ S₄ electrolyzer requires an ultralow voltage of 1.672 V at a large current density of 1000 mA cm^-2 , with ≈100% retention over 100 h, outperforming the commercial RuO₂ ||Pt/C, owing to the synergistic effect of the phase and interface electronic modulation. This work sheds light on the design of metallic heterostructure with an optimized interfacial electronic structure and abundant active sites for industrial water splitting.

Recent advances in solution assisted synthesis of transition metal chalcogenides for photo-electrocatalytic hydrogen evolution

Hydrogen evolution from water splitting is considered to be an important renewable clean energy source and alternative to fossil fuels for future energy sustainability. Photocatalytic and electrocatalytic water splitting is considered to be an effective method for the sustainable production of clean energy, H₂. This perspective especially emphasizes research advances in the solution-assisted synthesis of transition metal chalcogenides for both photo and electrocatalytic hydrogen evolution applications. Transition metal chalcogenides (CdS, MoS₂, WS₂, TiS₂, TaS₂, ReS₂, MoSe₂, and WSe₂) have received intensified research interest over the past two decades on account of their unique properties and great potential across a wide range of applications. The photocatalytic activity of transition metal chalcogenides can further be improved by elemental doping, heterojunction formation with noble metals (Au, Pt, etc.), non-chalcogenides (MoS₂, In₂S₃, NiS_1-X), morphological tuning, through various solution-assisted synthesis processes, including liquid-phase exfoliation, heat-up, hot-injection methods, hydrothermal/solvothermal routes and template-mediated synthesis processes. In this review we will discuss recent developments in transition metal chalcogenides (TMCs), the role of TMCs for hydrogen production and various strategies for surface functionalization to increase their activity, different synthesis methods, and prospects of TMCs for hydrogen evolution. We have included a brief discussion on the effect of surface hydrogen binding energy and Gibbs free energy change for HER in electrocatalytic hydrogen evolution.

Surface Activation and Ni-S Stabilization in NiO/NiS2 for Efficient Oxygen Evolution Reaction

Manipulating the active species and improving the structural stabilization of sulfur-containing catalysts during the OER process remain a tremendous challenge. Herein, we constructed NiO/NiS₂ and Fe-NiO/NiS₂ as catalyst models to study the effect of Fe doping. As expected, Fe-NiO/NiS₂ exhibits a low overpotential of 270 mV at 10 mA cm^-2 . The accumulation of hydroxyl groups on the surface of materials after Fe doping can promote the formation of highly active NiOOH at a lower OER potential. Moreover, we investigated the level of corrosion of M-S bonds and compared the stability variation of M-S bonds with Fe at different locations. Interestingly, Fe bonded with S in the bulk as the sacrificial agent can alleviate the oxidation corrosion of partial Ni-S bonds and thus endow Fe-NiO/NiS₂ long-term durability. This work could motivate the community to focus more on resolving the corrosion of sulfur-containing materials.

Constructing electrospun spinel NiFe2O4 nanofibers decorated with palladium ions as nanosheets heterostructure: boosting electrocatalytic activity of HER in alkaline water electrolysis

The development of efficient electrocatalysts for the water splitting process and understanding their fundamental catalytic mechanisms are highly essential to achieving high performance in energy conversion technologies. Herein, we have synthesised spinel nickel ferrite nanofibers (NiFe₂O₄-NFs) via an electrospinning (ES) method followed by a carbonization process. The resultant fiber was subjected to electrocatalytic water splitting reactions in alkaline medium. The catalytic efficiency of the NiFe₂O₄-NFs in OER was highly satisfactory. But it is not high enough to catalyse the HER process. Hence, palladium ions were decorated as nanosheets on NiFe₂O₄-NFs as a heterostructure to improve the catalytic efficiency for HER. Density functional theory (DFT) confirms that the addition of palladium to NiFe₂O₄-NFs helps to reduce the effect of catalyst poisoning and improve the efficiency of the catalyst. In an alkaline hybrid electrolyser, the required cell voltage was observed as 1.51 V at a fixed current density of 10 mA cm^-2.

Fabrication and Characterization of Nanostructured Rock Wool as a Novel Material for Efficient Water-Splitting Application

Rock wool (RW) nanostructures of various sizes and morphologies were prepared using a combination of ball-mill and hydrothermal techniques, followed by an annealing process. Different tools were used to explore the morphologies, structures, chemical compositions and optical characteristics of the samples. The effect of initial particle size on the characteristics and photoelectrochemical performance of RW samples generated hydrothermally was investigated. As the starting particle size of ball-milled natural RW rises, the crystallite size of hydrothermally formed samples drops from 70.1 to 31.7 nm. Starting with larger ball-milled particle sizes, the nanoparticles consolidate and seamlessly combine to form a continuous surface with scattered spherical nanopores. Water splitting was used to generate photoelectrochemical hydrogen using the samples as photocatalysts. The number of hydrogen moles and conversion efficiencies were determined using amperometry and voltammetry experiments. When the monochromatic wavelength of light was increased from 307 to 460 nm for the manufactured RW>0.3 photocatalyst, the photocurrent density values decreased from 0.25 to 0.20 mA/mg. At 307 nm and +1 V, the value of the incoming photon-to-current efficiency was ~9.77%. Due to the stimulation of the H+ ion rate under the temperature impact, the Jph value increased by a factor of 5 when the temperature rose from 40 to 75 °C. As a result of this research, for the first time, a low-cost photoelectrochemical catalytic material is highlighted for effective hydrogen production from water splitting.

Robust Porous WC-Based Self-Supported Ceramic Electrodes for High Current Density Hydrogen Evolution Reaction

Developing an economical, durable, and efficient electrode that performs well at high current densities and is capable of satisfying large-scale electrochemical hydrogen production is highly demanded. A self-supported electrocatalytic "Pt-like" WC porous electrode with open finger-like holes is produced through industrial processes, and a tightly bonded nitrogen-doped WC/W (WC-N/W) heterostructure is formed in situ on the WC grains. The obtained WC-N/W electrode manifests excellent durability and stability under multi-step current density in the range of 30-1000 mA cm-2 for more than 220 h in both acidic and alkaline media. Although WC is three orders of magnitude cheaper than Pt, the produced electrode demonstrates comparable hydrogen evolution reaction performance to the Pt electrode at high current density. Density functional theory calculations attribute its superior performance to the electrode structure and the modulated electronic structure at the WC-N/W interface.

The Pivotal Role of s-, p-, and f-Block Metals in Water Electrolysis: Status Quo and Perspectives

Transition metals, in particular noble metals, are the most common species in metal-mediated water electrolysis because they serve as highly active catalytic sites. In many cases, the presence of nontransition metals, that is, s-, p-, and f-block metals with high natural abundance in the earth-crust in the catalytic material is indispensable to boost efficiency and durability in water electrolysis. This is why alkali metals, alkaline-earth metals, rare-earth metals, lean metals, and metalloids receive growing interest in this research area. In spite of the pivotal role of these nontransition metals in tuning efficiency of water electrolysis, there is far more room for developments toward a knowledge-based catalyst design. In this review, five classes of nontransition metals species which are successfully utilized in water electrolysis, with special emphasis on electronic structure-catalytic activity relationships and phase stability, are discussed. Moreover, specific fundamental aspects on electrocatalysts for water electrolysis as well as a perspective on this research field are also addressed in this account. It is anticipated that this review can trigger a broader interest in using s-, p-, and f-block metals species toward the discovery of advanced polymetal-containing electrocatalysts for practical water splitting.

Super-Hydrophilic Leaflike Sn4P3 on the Porous Seamless Graphene-Carbon Nanotube Heterostructure as an Efficient Electrocatalyst for Solar-Driven Overall Water Splitting

Water splitting using renewable energy resources is an economic and green approach that is immensely enviable for the production of high-purity hydrogen fuel to resolve the currently alarming energy and environmental crisis. One of the effective routes to produce green fuel with the help of an integrated solar system is to develop a cost-effective, robust, and bifunctional electrocatalyst by complete water splitting. Herein, we report a superhydrophilic layered leaflike Sn₄P₃ on a graphene-carbon nanotube matrix which shows outstanding electrochemical performance in terms of low overpotential (hydrogen evolution reaction (HER), 62 mV@10 mA/cm², and oxygen evolution reaction (OER), 169 mV@20 mA/cm²). The outstanding stability of HER at least for 15 days at a high applied current density of 400 mA/cm² with a minimum loss of potential (1%) in acid medium infers its potential compatibility toward the industrial sector. Theoretical calculations indicate that the decoration of Sn₄P₃ on carbon nanotubes modulates the electronic structure by creating a higher density of state near Fermi energy. The catalyst also reveals an admirable overall water splitting performance by generating a low cell voltage of 1.482 V@10 mA/cm² with a stability of at least 65 h without obvious degradation of potential in 1 M KOH. It exhibited unassisted solar energy-driven water splitting when coupled with a silicon solar cell by extracting a high stable photocurrent density of 8.89 mA/cm² at least for 90 h with 100% retention that demonstrates a high solar-to-hydrogen conversion efficiency of ∼10.82%. The catalyst unveils a footprint for pure renewable fuel production toward carbon-free future green energy innovation.

Heteroatom-Doped Amorphous Cobalt-Molybdenum Oxides as a Promising Catalyst for Robust Hydrogen Evolution

The simultaneous manipulation of the catalytic activity and intrinsic electrical conductivity in a unified system is difficult yet meaningful to unravel the possible strategy that can enhance the hydrogen evolution reaction (HER) performance. Therefore, we propose a simple strategy to enhance the HER performance based on low-temperature redox reaction with ZIF-67@ZIF-8 as a sacrificial template to prepare zinc-doped amorphous CoMo₈O_x (denoted as Zn/aCMO). Benefiting from the excellent compositional- and amorphous-based structural advantages of more exposure active sites, optimized electron transfer as well as a stable frame structure, the as-prepared electrode can drive hydrogen evolution at current densities of 10, 50, and 100 mA cm^-2, which need ultralow overpotentials of 59, 138, and 189 mV, respectively, and the Tafel slope of the electrode was 66.2 mV dec^-1 (1 M KOH). Meanwhile, the intrinsic activity of the prepared low-cost electrocatalyst was also determined, and the turnover frequency was up to 1.49 s^-1 at an overpotential of 100 mV. In addition, after continuous testing for 160 h, there was a slight decay at the overpotential of 130 mV.

Anderson-Type Polyoxometalate-Assisted Synthesis of Defect-Rich Doped 1T/2H-MoSe2 Nanosheets for Efficient Seawater Splitting and Mg/Seawater Batteries

Designing high-performance hydrogen evolution reaction (HER) catalysts is crucial for seawater splitting. Herein, we demonstrate a facile Anderson-type polyoxometalate-assisted synthesis route to prepare defect-rich doped 1T/2H-MoSe₂ nanosheets. As demonstrated, the optimized defect-rich doped 1T/2H-MoSe₂ nanosheets display low overpotentials of 116 and 274 mV to gain 10 mA cm^-2 in acidic and simulated seawater for the HER, respectively. A magnesium (Mg)/seawater battery was fabricated with the defect-rich doped 1T/2H-MoSe₂ nanosheet cathode, displaying the highest power density of up to 7.69 mW cm^-2 and stable galvanostatic discharging over 24 h. The theoretical and experimental investigations show that the superior HER and battery performances of the heteroatom-doped MoSe₂ nanosheets are attributed to both the improved intrinsic catalytic activity (effective activation of water and favorable subsequent hydrogen desorption) and the abundant active sites, benefiting from the favorable catalytic factors of the doped heteroatom, 1T phase, and defects. Our work presents an intriguing structural modulation strategy to design high-performance catalysts toward both HER and Mg/seawater batteries.

Plasma Engineering of Basal Sulfur Sites on MoS2 @Ni3 S2 Nanorods for the Alkaline Hydrogen Evolution Reaction

Inexpensive and efficient catalysts are crucial to industrial adoption of the electrochemical hydrogen evolution reaction (HER) to produce hydrogen. Although two-dimensional (2D) MoS₂ materials have large specific surface areas, the catalytic efficiency is normally low. In this work, Ag and other dopants are plasma-implanted into MoS₂ to tailor the surface and interface to enhance the HER activity. The HER activty increases initially and then decreases with increasing dopant concentrations and implantation of Ag is observed to produce better results than Ti, Zr, Cr, N, and C. At a current density of 400 mA cm^-2 , the overpotential of Ag500-MoS₂ @Ni₃ S₂ /NF is 150 mV and the Tafel slope is 41.7 mV dec^-1 . First-principles calculation and experimental results reveal that Ag has higher hydrogen adsorption activity than the other dopants and the recovered S sites on the basal plane caused by plasma doping facilitate water splitting. In the two-electrode overall water splitting system with Ag500-MoS₂ @Ni₃ S₂ /NF, a small cell voltage of 1.47 V yields 10 mA cm^-2 and very little degradation is observed after operation for 70 hours. The results reveal a flexible and controllable strategy to optimize the surface and interface of MoS₂ boding well for hydrogen production by commercial water splitting.

A Fe-doped Co-oxide Electrocatalyst Synthesized Through Post-Modification Method Toward Advanced Water Oxidation

In the context of the ever-increasing energy crisis, electrocatalytic water splitting has attracted widespread attention as an effective means to provide clean energy. However, the oxygen evolution reaction (OER), which...

A sulfur-doped Ni2P electrocatalyst for the hydrogen evolution reaction

The porous structure of a sulfur-doped Ni2P (S-Ni2P) electrocatalyst is used for the electrolysis of water for hydrogen evolution.

Polyoxometalate-based metal–organic complexes and their derivatives as electrocatalysts for energy conversion in aqueous systems

The research progress on polyoxometalate-based metal–organic complexes and their derivatives as electrocatalysts in sustainable and clean energy conversion applications in aqueous systems is summarized.

Engineering Lattice Oxygen Activation of Iridium Clusters Stabilized on Amorphous Bimetal Borides Array for Oxygen Evolution Reaction

Developing robust oxygen evolution reaction (OER) catalysts requires significant advances in material design and in-depth understanding for water electrolysis. Herein, we report iridium clusters stabilized surface reconstructed oxyhydroxides on amorphous metal borides array, achieving an ultralow overpotential of 178 mV at 10 mA cm^-2 for OER in alkaline medium. The coupling of iridium clusters induced the formation of high valence cobalt species and Ir-O-Co bridge between iridium and oxyhydroxides at the atomic scale, engineering lattice oxygen activation and non-concerted proton-electron transfer to trigger multiple active sites for intrinsic pH-dependent OER activity. The lattice oxygen oxidation mechanism (LOM) was confirmed by in situ ¹⁸ O isotope labeling mass spectrometry and chemical recognition of negative peroxo-like species. Theoretical simulations reveal that the OER performance on this catalyst is intrinsically dominated by LOM pathway, facilitating the reaction kinetics. This work not only paves an avenue for the rational design of electrocatalysts, but also serves the fundamental insights into the lattice oxygen participation for promising OER application.