Water electrolysis: from textbook knowledge to the latest scientific strategies and industrial developments.

Prussian blue analogues and their derived nanomaterials for electrocatalytic water splitting

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.

Chapter 15 - Role of hydrogen generation technologies for renewable hydrogen production

Electrocatalytic water splitting has appeared to be a sustainable green technology for hydrogen and oxygen production, and noble metal-based electrocatalysts, like Pt for hydrogen evolution reaction (HER) and RuO2/IrO2 for oxygen evolution reaction (OER) have been proved to be state-of-the-art in water electrolyzers. However, high cost and scarcity of noble metals hinder large-scale applications of these electrocatalysts in practical commercial water electrolyzers. As an alternative, transition metal based electrocatalysts have attracted great attention because of the exciting catalytic performance, cost-effectiveness and abundant availability. However, their long-term stability in water splitting devices is unsatisfactory because of agglomeration and dissolution in the harsh operating environment. A possible solution to this issue is encapsulating transition metal (TM) based materials in stable and highly conductive carbon nanomaterials (CNMs) to make a hybrid of TM/CNMs, and the performance of TM/CNMs could be further enhanced by heteroatom (N-, B-, and dual N,B-) doping to carbon network in CNMs to break the carbon electroneutrality due to the different electronegativity, modulate the electronic structure to facilitate the adsorption of reaction intermediates, and promotion of efficient electron transfer to enhance the number of catalytically active sites for water splitting operation. In this review article, the recent progress of TM-based materials hybridizing with CNMs, N-CNMs, B-CNMs, and N,B-CNMs as electrocatalysts towards HER, OER as well as overall water splitting have been summarized, and the challenges and future prospects are also discussed.

Boosting electrocatalytic water splitting by magnetic fields

Metal-organic and porous organic framework in electrocatalytic water splitting

Insights over the in-situ grown copper sulfide/NiFe-LDH composites for alkaline and urea water electrolysis

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.

Two-dimensional (2D) nanostructures have enabled noble-metal-based nanomaterials to be promising electrocatalysts toward overall water splitting due to their inherent structural advantages, including a high specific surface active area, numerous low-coordinated atoms, and a high density of defects and edges. Moreover, it is also disclosed that the electronic effect and strain effect within 2D nanostructures also benefit the further promotion of the electrocatalytic performance. In this review, we have focused on the recent progress in the fabrication of advanced electrocatalysts based on 2D noble-metal-based nanomaterials toward water splitting electrocatalysis. First, fundamental descriptions about water-splitting mechanisms, some promising engineering strategies, and major challenges in electrochemical water splitting are given. Then, the structural merits of 2D nanostructures for water splitting electrocatalysis are also highlighted, including abundant surface active sites, lattice distortion, abundant surface defects, electronic effects, and strain effects. Additionally, some representative water-splitting electrocatalysts have been discussed in detail to highlight the superiorities of 2D noble-metal-based nanomaterials for electrochemical water splitting. Finally, the underlying challenges and future opportunities for the fabrication of more advanced electrocatalysts for water splitting are also highlighted. We hope that this review article provides guidance for the fabrication of more efficient electrocatalysts for boosting industrial hydrogen production via water splitting.

Hydrogen is regarded as an ideal energy carrier for the hydrogen economy that could replace the current hydrocarbon economy in order to achieve global energy security and mitigate climate change. For this purpose, H2 has to be produced from renewable sources (e.g., solar and wind) without producing global-warming CO2. (Photo)electrolysis of water into H2 and O2 is one of the most promising technologies for the production of renewable H2, which requires (photo)electrocatalysts of high efficiency, chemical robustness, and scalability. An essential attribute required for high-efficiency (photo)electrodes is high crystallinity with few defects to facilitate charge transfer without recombination. To this end, fabrication of photoelectrodes is usually completed with high temperature thermal annealing in a furnace. However, conventional thermal annealing (CTA) always results in undesirable crystal sintering, which reduces the surface area, and damage to the transparent conducting oxide (TCO) substrate. An emerging alternative method, hybrid microwave annealing (HMA), offers the beneficial effect of the high-temperature annealing (crystallinity) while minimizing its negative effects of sintering and TCO damage, enabling the fabrication of efficient (photo)electrodes for water splitting. HMA combines direct microwave heating with additional heating from an effective microwave absorber (called a susceptor), thereby avoiding a nonuniform temperature distribution between the interior and exterior of the synthesized material. More importantly, an extremely high temperature of the entire sample can be reached in only a few minutes. Compared with CTA, HMA has several advantages in the preparation of (photo)electrodes: (i) formation of a high-purity phase; (ii) high crystallinity with fewer defects; (iii) preservation of the original nanostructure; (iv) less damage to the TCO substrate for photoelectrodes; (v) smaller nanocrystals and uniform dispersion of catalyst particles. Overall, HMA is a convenient, ultrafast, and energy-economical technology for the synthesis of efficient (photo)electrodes. In this Account, we discuss recent progress made in our laboratory on HMA for preparing photoanodes (Fe2O3, BiVO4, ZnFe2O4, and Fe2TiO5), photocathodes (Cu2O and CuFeO2), and a graphene-based electrocatalyst (MoS2/graphene composite), which exhibit distinctive behavior and efficient performance in (photo)electrocatalytic water splitting. In particular, we have advanced the HMA technique further to synthesize hematite-based photoanodes with core-shell heterojunction nanorods (Nb,Sn:Fe2O3@FeNbO4 and Ta,Sn:Fe2O3@FeTaO4) by solid-solid interface reaction, which simultaneously achieves multiple doping effects (Nb or Ta, Sn) to improve the photoelectrocatalysis of water splitting. Thus, this Account focuses on the synthetic aspects of HMA, which may offer new research opportunities for the synthesis of other metal oxide (photo)electrode materials and hybrid electrocatalysts in the fields of solar energy conversion and storage, secondary batteries, and H2 fuel production.

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

Electrocatalytic water splitting is a crucial area in sustainable energy development, and the development of highly efficient bifunctional catalysts that exhibit activity toward both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is of paramount importance. Co3O4 is a promising candidate catalyst, owing to the variable valence of Co, which can be exploited to enhance the bifunctional catalytic activity of HER and OER through rational adjustments of the electronic structure of Co atoms. In this study, we employed a plasma-etching strategy in combination with an in situ filling of heteroatoms to etch the surface of Co3O4, creating abundant oxygen vacancies, while simultaneously filling them with nitrogen and sulfur heteroatoms. The resulting N/S-VO-Co3O4 exhibited favorable bifunctional activity for alkaline electrocatalytic water splitting, with significantly enhanced HER and OER catalytic activity compared to pristine Co3O4. In an alkaline overall water-splitting simulated electrolytic cell, N/S-VO-Co3O4 || N/S-VO-Co3O4 showed excellent overall water splitting catalytic activity, comparable to noble metal benchmark catalysts Pt/C || IrO2, and demonstrated superior long-term catalytic stability. Additionally, the combination of in situ Raman spectroscopy with other ex situ characterizations provided further insight into the reasons behind the enhanced catalyst performance achieved through the in situ incorporation of N and S heteroatoms. This study presents a facile strategy for fabricating highly efficient cobalt-based spinel electrocatalysts incorporated with double heteroatoms for alkaline electrocatalytic monolithic water splitting.

With the rapid increase of global temperature and depletion of fossil fuels, developing sustainable energy resources is crucial nowadays. Among the various renewable energy source generation approaches, water splitting has attracted increasing attention for clean energy generation and efficient energy storage. Electrochemical production of hydrogen from solar electricity is also an attractive option for generating energy in the form of a hydrogen which could be used at a later stage for electricity. Over the past decades, despite of significant achievements have been obtained, the solar-to-hydrogen (STH) efficiency is still too low for practical applications. Low solar to hydrogen (STH) conversion efficiency is due to the suppressed water splitting reactions by high overpotential, especially oxygen evolution reaction. Due to relatively high overpotential than hydrogen evolution reaction(HER), the oxygen evolution reaction(OER) is a key reaction in water splitting. To overcome this problem, current studies are focused on development of efficient, abundant and inexpensive OER catalyst. The implementation of efficient electrocatalyst leads to decreased overpotentials, thereby making the whole process more energy-efficiently. Currently, the most efficient catalysts for water splitting are noble-metal catalysts such as Pt-group metals, Ru and Ir-based compounds. Unfortunately, the scarcity and high cost of noble metals seriously impede its large-scale applications in electrocatalytic water splitting. It is therefore highly attractive to explore earth-abundant materials to overcome this obstacle. In past decades, Ni has emerged as an important non-noble metal due to its catalytic power for water splitting and Ni-based compounds have been intensively studied as efficient OER and HER catalysts. Among the various type of Ni based materials, layered transition-metal alloy with Fe have attracted much attention of researchers because of their special redox character and good accessibility for the reaction species. Bimetallic electrocatalysts also have attracted increasing attention as a reliable approach to enhanced electrocatalytic activity for the HER. In the meantime, inspired by the abundant element Ni used in nature, various Ni-based catalysts have been designed to catalyze the conversion of H2O to H2 in commercial alkaline electrolyzers. Among these, Ni–Mo alloys are well-known non-precious-metal electrocatalysts for hydrogen production in alkaline electrolytes because of the increased intrinsic electrocatalytic caused by appropriate binding energy to hydrogen activity compared to pure Ni.Herein we report an approach to improve efficiency of water splitting electrodes based on flexible NiFe-based foil. Anodic oxidation method is applied to enhance the oxygen evolution activity of NiFe alloy foil. The anodized NiFe alloy foil exhibit significant higher activity than the corresponding Ni foam in base conditions. The anodic oxidation method generate NiFe oxide and hydroxide layers on NiFe alloy surface, act as electrocatalyst. Spontaneously, the anodic oxidation method widen the specific surface area of water splitting electrodes. Increased reaction sites and catalytic behavior of NiFe hydroxide is the reason of improved water splitting property. Because Ni based alloy like NiMo is remarkable hydrogen evolution catalyst, similar to noble metal, electrodeposition method is applied to enhance the hydrogen evolution activity of NiFe alloy foil. NiMo electrodeposited NiFe alloy foil exhibit highly enhanced activity than pure NiFe foil. Combining with 23% Si based solar cell using anodized NiFe alloy foil as anode and electrodeposited NiMo as cathode, PV-EC cell shows excellent STH properties around 18%.

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

With increasing global demand for energy, rapid depletion of fossil fuels and intensification of environmental concerns, exploring clean and sustainable energy carriers to replace fossil fuel is becoming critical. Among the various alternatives, hydrogen has been intensively regarded as a promising energy carrier to fulfill the increasing energy demand due to its large energy density per unit mass and eco-friendly production possibilities. However, hydrogen does not exist in molecular structure in nature, and it is essential to obtain efficient and sustainable H2 production technologies. Alkaline water electrolysis is an effective, clean and sustainable process to produce high-quality hydrogen. In this process, highly active electrocatalysts for the hydrogen evolution reaction (HER) are required to accelerate the sluggish kinetics and lower the overpotentials (η) for efficient hydrogen evolution. To date, a noble metal, platinum (Pt), is the state-of-art electrocatalyst for HER. However, exploration of alternative electrocatalysts with low cost and excellent electrocatalytic activity is of vital importance to realize large-scale hydrogen production through water electrolysis. Generally, an electrochemically active catalyst should have an optimal hydrogen adsorption free energy to allow efficient catalytic hydrogen adsorption/desorption. In alkaline solution, dissociation of water onto the electrocatalyst determines the overall HER efficiency. This thesis focuses on rational design and synthesis of different earth-abundant electrocatalysts for electrocatalytic HER in alkaline media. Through facile anion or cation doping strategies, electrocatalysts with abundant accessible active sites, enhanced electronic conductivity and accelerated HER kinetics have been systematically fabricated, characterized and evaluated. First, an efficient HER electrocatalyst in alkaline media was fabricated by incorporating sulfur atoms into a cobalt (hydro)oxide crystal structure. The resultant catalyst exhibits a remarkably enhanced HER activity with a low-overpotential of 119 mV at 10 mA/cm2 and an excellent durability. The results suggest that cobalt hydroxide benefits water adsorption and cleavage, while the negatively charged sulfur ligands facilitate hydrogen adsorption and desorption on the surface of electrocatalysts, leading to significantly promoted Volmer and Heyrovsky steps for HER in alkaline media. Second, exploring bifunctional electrocatalysts which can simultaneously accelerate the HER and oxygen evolution reaction (OER) activities plays a key role in alkaline water splitting. Here, sulfur atoms were incorporated into the mixed transition metal hydroxide with high OER performance to render excellent HER activity. The enhanced catalytic activity towards HER was confirmed by a synergistic effect between the retained metal hydroxide host and the incorporated sulfur atoms. In addition, the full water splitting electrolyzer equipped with fabricated bifunctional electrocatalysts as anode and cathode materials exhibited remarkable overall water splitting performance comparable to that with benchmark Pt and RuO2 electrocatalysts. The S/Se co-doped Co3O4 nanosheets on carbon cloth were fabricated by a facile room temperature chalcogen atom incorporation methodology and were applied as the electrocatalyst for HER in alkaline media. The sulfur and selenium atoms were homogeneously distributed on the surface by forming Co-S or Co-Se bonds which play a key role in the structural change in electrochemical activation. The obtained electrocatalysts demonstrated remarkably improved HER activity compared to that of the original Co3O4. Finally, molybdenum doped cobalt hydroxide was fabricated with significantly accelerated HER kinetics. The introduced Mo sites not only effectively facilitate water dissociation process and desorption of the OHads intermediates, but also simultaneously optimize the hydrogen adsorption free energy. Therefore, the in situ-generated Mo-doped amorphous cobalt hydroxide exhibited a remarkable HER performance in alkaline media with an overpotential of only -80 mV at a current density of 10 mA/cm2. This thesis innovatively explores strategies to improve the catalytic activity towards HER of metal (hydro)oxide in alkaline media. The surface foreign atom doping was demonstrated to manipulate the surface structure of catalysts, thus not only improving the water dissociation processes, but also facilitating the hydrogen adsorption/desorption on the catalysts. The demonstrated facile and effective strategies could be adopted for the fabrication of cost-effective and highly active catalysts for other important chemical reactions for energy conversion applications.