Hydrogen is an ideal candidate for the replacement of fossil fuels in the future due to zero emission of carbonaceous species during its utilization. Water electrolysis is a dependable link of primary renewable energy and stable hydrogen energy. In this work, the fundamentals of water electrolysis, current popular electrocatalysts developed for cathodic hydrogen evolution reaction (HER) and anodic oxygen evolution reaction (OER) in liquid electrolyte water electrolysis are reviewed. The main HER catalysts include noble metals, non-noble metals and composites, noble metal-free alloys, metal carbides, chalcogenides, phosphides and metal-free materials while the OER catalysts are focused on efficient Co-based, Ni-based materials and layered double hydroxide (LDH) materials. The strategies to improve catalytic activity, long-term durability and endurance to electrochemical erosion are introduced. The main challenges and future prospects for the further development of electrodes for water electrolysis are discussed. It is expected to give guidance for the development of novel low-cost nanostructured electrocatalysts for electrochemical water splitting.

Nanostructured catalysts for electrochemical water splitting: current state and prospects

Hydrogen evolution on platinum is a key reaction for electrocatalysis and sustainable energy storage, yet its pH-dependent kinetics are not fully understood. Here we present a detailed kinetic study of hydrogen adsorption and evolution on Pt(111) in a wide pH range. Electrochemical measurements show that hydrogen adsorption and hydrogen evolution are both slow in alkaline media, consistent with the observation of a shift in the rate-determining step for hydrogen evolution. Adding nickel to the Pt(111) surface lowers the barrier for hydrogen adsorption in alkaline solutions and thereby enhances the hydrogen evolution rate. We explain these observations with a model that highlights the role of the reorganization of interfacial water to accommodate charge transfer through the electric double layer, the energetics of which are controlled by how strongly water interacts with the interfacial field. The model is supported by laser-induced temperature-jump measurements. Our model sheds light on the origin of the slow kinetics for the hydrogen evolution reaction in alkaline media. Despite its role in electrocatalysis and hydrogen generation, a complete understanding of the hydrogen evolution reaction on platinum remains elusive. Here, a detailed kinetic study of hydrogen adsorption and evolution on Pt(111) highlights the role of interfacial water reorganization in the hydrogen adsorption step.

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Interfacial water reorganization as a pH-dependent descriptor of the hydrogen evolution rate on platinum electrodes

In recent years, nanocrystals of metal sulfide materials have attracted scientific research interest for renewable energy applications due to the abundant choice of materials with easily tunable electronic, optical, physical and chemical properties. Metal sulfides are semiconducting compounds where sulfur is an anion associated with a metal cation; and the metal ions may be in mono-, bi- or multi-form. The diverse range of available metal sulfide materials offers a unique platform to construct a large number of potential materials that demonstrate exotic chemical, physical and electronic phenomena and novel functional properties and applications. To fully exploit the potential of these fascinating materials, scalable methods for the preparation of low-cost metal sulfides, heterostructures, and hybrids of high quality must be developed. This comprehensive review indicates approaches for the controlled fabrication of metal sulfides and subsequently delivers an overview of recent progress in tuning the chemical, physical, optical and nano- and micro-structural properties of metal sulfide nanocrystals using a range of material fabrication methods. For hydrogen energy production, three major approaches are discussed in detail: electrocatalytic hydrogen generation, powder photocatalytic hydrogen generation and photoelectrochemical water splitting. A variety of strategies such as structural tuning, composition control, doping, hybrid structures, heterostructures, defect control, temperature effects and porosity effects on metal sulfide nanocrystals are discussed and how they are exploited to enhance performance and develop future energy materials. From this literature survey, energy conversion currently relies on a limited range of metal sulfides and their composites, and several metal sulfides are immature in terms of their dissolution, photocorrosion and long-term durability in electrolytes during water splitting. Future research directions for innovative metal sulfides should be closely allied to energy and environmental issues, along with their advanced characterization, and developing new classes of metal sulfide materials with well-defined fabrication methods.

Recent advances in metal sulfides: from controlled fabrication to electrocatalytic, photocatalytic and photoelectrochemical water splitting and beyond

This work is a review of the recent trends in the photoelectrocatalytic conversion of solar energy into electricity or hydrogen. It focuses on photocatalytic fuel cells and photoelectrocatalytic water splitting systems and presents both the basic principles and the design of devices. It includes a broad review of materials employed for the construction of photoanodes, photocathodes and tandem cells and highlights the related research fields which are expected to be of interest in the near future. The review is intended to become a basic manual for new adepts to the field and at the same time a handy reference to experienced researchers.

Review of recent trends in photoelectrocatalytic conversion of solar energy to electricity and hydrogen

Developing photoanodes with efficient sunlight harvesting, excellent charge separation and transfer, and fast surface reaction kinetics remains a key challenge in photoelectrochemical water splitting devices. Here we report a new strongly coupled ternary hybrid aerogel that is designed and constructed by in situ assembly of N-deficient porous carbon nitride nanosheets and NiFe-layered double hydroxide into a 3D N-doped graphene framework architecture using a facile hydrothermal method. Such a 3D hierarchical structure combines several advantageous features, including effective light-trapping, multidimensional electron transport pathways, short charge transport time and distance, strong coupling effect, and improved surface reaction kinetics. Benefiting from the desirable nanostructure, the ternary hybrid aerogels exhibited remarkable photoelectrochemical performance for water oxidation. Results included a record-high photocurrent density that reached 162.3 μA cm–2 at 1.4 V versus the reversible hydrogen e...

Strongly Coupled Ternary Hybrid Aerogels of N-deficient Porous Graphitic-C3N4 Nanosheets/N-Doped Graphene/NiFe-Layered Double Hydroxide for Solar-Driven Photoelectrochemical Water Oxidation.

Solar water splitting, which has been a topic of intensive research interest for several decades, is one of the promising approaches to utilize renewable energy to maintain the sustainable prosperity of our society. However, until now no mature photoelectrochemical cell has been used in practical large-scale applications because of the difficulties to satisfy all the harsh requirements, including high energy conversion efficiency, high stability and low cost. This feature article reviews the recent progress in developing photocathodes for photoelectrochemical cells for solar hydrogen production. Both the development of the p-type semiconductor light absorbers and the efforts to develop synergistic approaches to improve the overall performance of the photocathode are discussed.

Recent progress in photocathodes for hydrogen evolution

Recently, kesterite Cu2ZnSnS4 (CZTS) has been regarded as a promising candidate for future light absorbers due to its excellent photovoltaic potential. To further improve the efficiency of associated solar cells, both the bulk and interface issues, which give rise to detrimental carrier collection loss, are the key problems that need to be resolved. Herein, by using a thermal co-evaporation/annealing method, we have systematically investigated the Rb incorporation effects on the quality of Cu2(Cd,Zn)SnS4 (CCZTS) thin films and related solar devices. The results show that the Rb incorporation could engineer both the bulk and interface significantly, specifically improving bulk quality with enhanced grains and suppressed defects, passivating the rear interface with less secondary phase accumulation, and forming a favorable front band alignment. Benefiting from these multifunctional benefits, carrier collection efficiency is greatly improved with the enhancement of short-circuit current density (JSC) and open-circuit voltage (VOC). As a result, a champion efficiency of 10.64% is achieved, which is the highest value for pure sulfide CZTS solar cells made by the thermal evaporation method.

Zi Ye

Papers

Recent progress in photocathodes for hydrogen evolution

Fabrication of high-efficiency Cu2(Zn,Cd)SnS4 solar cells by a rubidium fluoride assisted co-evaporation/annealing method