Emerging Materials for Hydrogen and Oxygen Electrochemical Devices: From Synthesis to Application Prospects

Hydrogen is a kind of clean energy with high calorific value and non-pollution. There are many methods for hydrogen production. Fuel processing technologies transform a hydrogen containing material such as coal, petroleum, or natural gas into a hydrogen rich stream. However, these processes need an external heat source for the reactor and produce large amounts of carbon dioxide. Hydrogen production by electrolysis of water is regarded as an advanced technology to make effective use of renewable resources, such as wind power, solar power, etc., to achieve energy storage and conversion. Water electrolysis includes hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). These reactions are normally catalyzed by precious metals, such as platinum (Pt) and iridium (Ir)-based catalysts, which limits the large-scale application of electrolysis of water. Thus, it is necessary to develop alternative catalysts with low cost and high performance. Two-dimensional (2D) materials have considerable application prospect in electrocatalysis of H2O because of their unique structural and electronic properties. In addition, 2D materials with a reduced dimension compared with the bulk material exhibits several distinctive properties, such as high specifc surface area, high thermal and electric conductivity and more catalytic active sites. In this review, the key scientific issues and the latest advances in the two half-reactions (HER and OER) of electrocatalytic water splitting with 2D materials are systematically summarized. The mechanisms of HER and OER are discussed briefly. The involved 2D materials for HER in this work include graphene, graphene encapsulated transition-metal catalysts, g-C3N4 and 2D transition-metal dichalcogenides, while for OER contain layered double hydroxide (LDH) and graphene encapsulated transition-metal catalysts materials. For graphene, g-C3N4 and 2D transition-metal dichalcogenides, there are various techniques to enhance the catalytic activity of the materials, such as the introduction of defects, heteroatom-doped (N, B, P, S or metal atoms) and functional groups. For graphene encapsulating earth-abundant transition metal nanoparticles, the activity of electrocatalytic water splitting can be improved by the electron transfer from the metal core. Furthermore, the utilization of strong coupling between various 2D materials is another facile approach to optimize the catalytic activity. This review enumerates several typical 2D materials and recent applications for the two half-reactions of electrocatalytic water splitting respectively. The future challenges and opportunities in this field are also discussed. The strategy for designing novel HER electrocatalysts with high performance mainly focuses on the electronic structure engineering of 2D materials to modify electronic properties and optimize the adsorption and desorption behavior of H atoms. The design of high-performance and long-term durability OER electrocatalysts in acidic medium still remains a major challenge. Although the obtained electrocatalysts for water splitting still suffer from some serious problems when it comes to large-scale applications, the unique electronic structure of 2D materials and possibility of modifcations on the surface offer opportunities to synthesize novel electrocatalysts with low cost, high catalytic activity and high stability. Thus, it is possible to adopt 2D materials as catalysts in electrocatalytic water splitting reactions. It is expected to give guidance for the comprehension of 2D materials and their applications in electrocatalytic water splitting.

Non-metal doping regulation in transition metal and their compounds for electrocatalytic water splitting

Layered double hydroxides derived from waste for highly efficient electrocatalytic water splitting: Challenges and implications towards circular economy driven green energy

Layered double hydroxides (LDHs) exhibit great potential in electrocatalytic water splitting due to the unique 2D feature and an adjustable structure composed of different metal centers. In addition, LDHs have the advantage of being inherently inexpensive compared to other catalysts and have good stability in electrocatalytic water splitting. Up to now, numerous methods have been put forward to improve the activity of LDHs in electrocatalytic water splitting, a comprehensive introduction and comb to the fabrication methods and modification strategies is helpful for the followers to get a clear vein to carry out efficient manipulation to the development of high promising LDHs catalysts. In this review, the basic principles of water electrolysis, and the evaluation indexes are introduced first, and then the basic properties and commonly utilized methods in the fabrication of LDHs are introduced. After that, the oxygen evolution reaction (OER), hydrogen evolution reaction (HER), and overall water splitting (OWS) performance of different LDHs-based catalysts and analyze the merits and shortcomings of LDHs in electrocatalytic water splitting is compared. Based on this, the advanced strategies for improving the performance of LDHs is introduced and give a brief prospect for the development of LDHs-based materials in electrocatalysis.

Increased environmental pollution due to vast fossil-fuel consumption has necessitated efficient use of energy while exploring clean and non-carbonaceous fuel to address the increasing global energy demand.1 Hydrogen, as the most lightweight fuel, has the ability to provide a clean, reliable and affordable energy supply without any greenhouse gas emission.1 However, efficient and economic production of hydrogen, along with cost effective storage and distribution are the major bottlenecks for the commercial deployment of hydrogen as a fuel. There is therefore a critical need to address these issues.2-6 Electrolytic water splitting is considered as a promising approach for the economic and efficient production of hydrogen, as it does not involve greenhouse gas emissions and toxic byproducts.7 However, the high capital cost, mainly due to use of expensive noble metal electro-catalysts (e.g. Pt, IrO2, RuO2) is a major impetus for the development of non-PGM catalyst based PEM water electrolysis system. Engineering of non-noble metal electro-catalysts with high electrochemical activity for hydrogen evolution reaction (HER) will offer some reduction in the overall capital cost of PEM based water electrolysis systems. It is hence, important to identify electro-catalysts for HER (for use as the cathode of PEM water electrolyzers) with similar/lower overpotential, similar/superior cathodic current density (i.e., electrochemical activity) and similar/superior long term stability as that of state of the art Pt/C.In the present study, nanostructured DTMS based electro-catalysts have been studied for HER using first principles calculations. The results of the theoretical studies are experimentally verified by synthesizing the DTMS electro-catalyst nanoparticles. The TEM image of synthesized nanoparticles (NPs) is shown in Fig. 1. The electrochemical characterization of DTMS as cathode electro-catalysts for PEM water electrolysis system has been carried out using 0.5 M sulfuric acid (H2SO4) electrolyte (pH~0), 1 M potassium phosphate buffer (pH 7), 1 M KOH (pH 14), Pt wire counter electrode and Hg/Hg2SO4 reference electrode (+0.65V vs NHE) at a scan rate of 10 mV/sec and temperature of 26oC. The DTMS catalysts exhibit onset overpotential for HER similar to Pt/C (~10 mV vs RHE). The overpotential required to obtain current density of ~100 mA/cm2is similar to that of Pt/C in acidic, neutral and basic media, indicating the excellent electrochemical activity for HER. Moreover, the DTMS catalyst exhibits electrochemical stability in acidic media similar to that of Pt/C reflecting its potential as an alternative low cost non-PGM electro-catalyst system. Additionally, these novel electro-catalysts with unique composition and electronic structure also show promising response as the cathode electro-catalysts in PEC water splitting cell. The results of the synthesis, microstructural characterization, theoretical first principles study and electrochemical activity of these novel electro-catalysts will thus be presented and discussed.

Advance in rare earth element modified nanomaterials for enhanced electrocatalytic water splitting

As green and sustainable methods to produce hydrogen energy, photocatalytic and electrochemical water splitting have been widely studied. In order to find efficient photocatalysts and electrocatalysts, materials with various composition, size, and surface/interface are investigated. In recent years, constructing suitable nanoscale hetero-interfaces can not only overcome the disadvantages of the single-phase material, but also possibly provide new functionalities. In this review, we systematically introduce the fundamental understanding and experimental progress in nanoscale hetero-interface engineering to design and fabricate photocatalytic and electrocatalytic materials for water splitting. The basic principles of photo-/electro-catalytic water splitting and the fundamentals of nanoscale hetero-interfaces are briefly introduced. The intrinsic behaviors of nanoscale hetero-interfaces on electrocatalysts and photocatalysts are summarized, which are the electronic structure modulation, space charge separation, charge/electron/mass transfer, support effect, defect effect, and synergistic effect. By highlighting the main characteristics of hetero-interfaces, the main roles of hetero-interfaces for electrocatalytic and photocatalytic water splitting are discussed, including excellent electronic structure, efficient charge separation, lower reaction energy barriers, faster charge/electron/mass transfer, more active sites, higher conductivity, and higher stability on hetero-interfaces. Following above analysis, the developments of electrocatalysts and photocatalysts with hetero-structures are systematically reviewed.

A review on cobalt phosphate-based materials as emerging catalysts for water splitting

Due to the rapidly increasing demand for energy and environmental sustainability, stable and economical hydrogen production has received increasing attention in the past decades. In this regard, hydrogen production through photo- or electrocatalytic water splitting has continued to gain ever-growing interest. However, the existing catalysts are still unable to fulfill the demands of high-efficiency, low-cost, and sustainable hydrogen production. Layered metal trichalcogenidophosphate (MPQ3) is a newly developed two-dimensional material with tunable composition and electronic structure. Recently, MPQ3 has been considered a promising candidate for clean energy generation and related water splitting applications. In this minireview, we firstly introduce the structure and methods for the synthesis of MPQ3 materials. In the following sections, recent developments of MPQ3 materials for photo- and electrocatalytic water splitting are briefly summarized. The roles of MPQ3 materials in different reaction systems are also discussed. Finally, the challenges related to and prospects of MPQ3 materials are presented on the basis of the current developments.

Electrochemical water splitting to produce green hydrogen offers a promising technology for renewable energy conversion and storage, as well as realizing carbon neutrality. The efficiency, stability, and cost of electrocatalysts toward hydrogen evolution reaction (HER) and electrocatalytic overall water splitting (EOWS) at large current densities are essential for practical application. In this review, the key factors that determine the catalytic performance of electrocatalysts at large current densities are summarized from the angel of thermodynamic and kinetic correlation. The corresponding design strategies are presented. The electronic structure and density of active sites that affect the adsorption/desorption of intermediates are considered as the thermodynamic aspects, while charge transfer and mass transport capabilities closely associated with electrode resistance and intermediate diffusion are assigned as kinetic effects. Recent development of bifunctional and integrated electrocatalysts toward EOWS is also discussed in detail. Finally, the perspective and direction on the electrocatalytic water splitting under large current density are proposed. This comprehensive overview will offer profound insights and guidance for the continued advancement of this field.

Prussian blue analogues and their derived nanomaterials for electrocatalytic water splitting

Hydrogen production by electrocatalytic water splitting is considered to be an effective and environmental method, and the design of an electrocatalyst with high efficiency, low cost, and multifunction is of great importance. Herein, we developed a amorphous Co-FeOOH/crystalline CoCe-MOF heterostructure (defined as Co-FeOOH/CoCe-MOF/NF) though a convenient cathodic electrodeposition strategy as a high-efficiency bifunctional electrocatalyst for water electrolysis. The Co-FeOOH/CoCe-MOF/NF nanocrystals provide remarkable electronic conductivity and plenty of active sites, and the crystalline/amorphous heterostructure with generates synergistic effects, providing plentiful active sites and efficient charge/mass transfer. Benefiting from this, the designed Co-FeOOH/CoCe-MOF/NF displays ultralow overpotentials of 226 and 74 mV to achieve 10 mA cm-2 for oxygen evolution reaction and hydrogen evolution reaction, and also shows the superior performance for overall water splitting with a low voltage of 1.55 V at 10 mA cm-2 in 1 M KOH. The work reveals a design of superior activity, cost-effective and multifunctional electrocatalysts for water splitting.

Anchoring platinum clusters in CoP@CoNi layered double hydroxide to prepare high-performance and stable electrodes for efficient water splitting at high current density.

Hydrogen gas is widely regarded as a key energy carrier for achieving net-zero carbon emissions by 2050, with green hydrogen, produced via electrochemical water splitting, being central to this goal. However, the production of green hydrogen from renewable sources remains limited, accounting for ~4% of global hydrogen production in 2021 [1]. Water electrolysis systems, while theoretically operating at 1.23 V, require higher voltages in practice due to membrane gas crossover, which reduces efficiency and raises costs [2]. This study explores a novel approach to supplement green hydrogen generation through indirect water splitting, where soluble redox mediators, primarily used for energy storage, extend their capability to perform Redox-Mediated Hydrogen Evolution Reactions (RM-HER) in external catalytic reactors [3,4].Building on prior work that established a stable manganese-vanadium flow battery for energy storage, the current research shifts focus to the anolyte (V3+/V2+) redox mediator for RM-HER [5]. The study aims to refine electrolyte composition to enhance hydrogen gas yields and efficiencies in flow-assisted redox-mediated electrolysis systems. To assess the relationship between faradaic gas efficiency and coulombic efficiency for cycled electrolytes, we utilize ultramicroelectrodes to uncover key factors—such as the state of charge (SOC) of the anolyte and manganese crossover—that affect the onset potential of RM-HER. Furthermore, the study examines the role of different electrocatalysts to identify ideal materials that can withstand the effects of manganese while maximizing hydrogen generation.We believe that the electrochemical environment and improved electrocatalytic surfaces are central to understanding the efficiency of hydrogen gas production. The electrochemical characterization techniques (e.g., cyclic voltammetry and electrochemical impedance spectroscopy) utilized in this study help evaluate the reusability of the electrolyte, providing insights into the system’s potential for a seamless switch between these two complementary use-case scenarios: energy storage and RM-HER. This approach could lead to a more efficient, cost-effective, and scalable process for hydrogen production, contributing to the broader adoption of green hydrogen and sustainable solutions. References Emanuele Taibi, et al., Green Hydrogen Cost Reduction: Scaling up Electrolysers to Meet the 1.5⁰C Climate Goal, International Renewable Energy Agency - IRENA, 2020.Symes, M.D. and L. Cronin, Decoupling hydrogen and oxygen evolution during electrolytic water splitting using an electron-coupled-proton buffer. Nature Chemistry, 2013. 5(5): p. 403-409.Reynard, D. and H. Girault, Combined hydrogen production and electricity storage using a vanadium manganese redox dual-flow battery. Cell Reports Physical Science, 2021. 2(9): p. 16.Reynard, D., et al., Vanadium-Manganese Redox Flow Battery: Study of MnIII Disproportionation in the Presence of Other Metallic Ions. Chemistry – A European Journal, 2020. 26(32): p. 7250-7257.Chaurasia, S., et al., Investigating Manganese–Vanadium Redox Flow Batteries for Energy Storage and Subsequent Hydrogen Generation. ACS Applied Energy Materials, 2024.

Membraneless Electrolyzers for Low-Cost Hydrogen Production in a Renewable Energy Future