Recent progress in carbon-based materials boosting electrochemical water splitting

As environmental crises such as global warming become more and more serious due to the large amount of carbon dioxide emitted by the burning of fossil fuels, much attention has been paid to carbon neutrality. Hydrogen, with zero carbon content, is a clean and renewable energy carrier having a large energy density. It is considered as one of the most desirable alternatives to fossil fuels. Electrochemical water splitting, unlike the steam reforming process accelerating fossil fuels depletion and CO₂ emissions, can produce H₂ powered by renewable energy such as solar or wind. As a promising way to promote carbon neutralization, hydrogen production by electrolysis of water is meaningful both in terms of scientific research and practical application. In order to drive electrochemical water splitting with low power consumption, efficient, durable and affordable electrocatalysts with low overpotentials are in urgent need. Therefore, this mini-review briefly introduces the current development status and mainstream obstacles of carbon-based materials used in electrochemical water splitting.     

Emerging materials for plasmon-assisted photoelectrochemical water splitting

Energy production and environmental pollution are the two major problems the world is facing today. The depletion of fossil fuels and the emission of harmful gases into the atmosphere leads to the research on clean and renewable energy sources. In this context, hydrogen is considered an ideal fuel to meet global energy needs. Presently, hydrogen is produced from fossil fuels. However, the most desirable way is from clean and renewable energy sources, like water and sunlight. Sunlight is an abundant energy source for energy harvesting and utilization. Recent studies reveal that photoelectrochemical (PEC) water splitting has promise for solar to hydrogen (STH) conversion over the widely tested photocatalytic approach since hydrogen and oxygen gases can be quantified easily in PEC. For designing light-absorbing materials, semiconductors are the primary choice that undergoes excitation upon solar light irradiation to produce excitons (electron-hole pairs) to drive the electrolysis. Visible light active semiconductors are attractive to achieve high solar to chemical fuel conversion. However, pure semiconductor materials are far from practical applications because of charge carrier recombination, poor light-harvesting, and electrode degradation. Various heteronanostructures by the integration of metal plasmons overcome these issues. The incorporation of metal plasmons gained significance for improving the PEC water splitting performance. This review summarizes the possible main mechanisms such as plasmon-induced resonance energy transfer (PIRET), hot electron injection (HEI), and light scatting/trapping. It also deliberates the rational design of plasmonic structures for PEC water splitting. Furthermore, this review highlights the advantages of plasmonic metal-supported photoelectrodes for PEC water splitting.     

Solar Hydrogen Generation from Lignocellulose

Photocatalytic reforming of lignocellulosic biomass is an emerging approach to produce renewable H₂. This process combines photo‐oxidation of aqueous biomass with photocatalytic hydrogen evolution at ambient temperature and pressure. Biomass conversion is less energy demanding than water splitting and generates high‐purity H₂ without O₂ production. Direct photoreforming of raw, unprocessed biomass has the potential to provide affordable and clean energy from locally sourced materials and waste.     

铁基多相助催化剂光电化学水氧化研究进展

The use of fossil fuels has caused serious environmental problems such as air pollution and the greenhouse effect. Moreover, because fossil fuels are a non-renewable energy source, they cannot meet the continuously increasing demand for energy. Therefore, the development of clean and renewable energy sources is necessitated. Hydrogen energy is a clean, non-polluting renewable energy source that can ease the energy pressure of the whole society. The sunlight received by the Earth is 1.7× 10¹⁴ J in 1 s, which far exceeds the total energy consumption of humans in one year. Therefore, conversion of solar energy to valuable hydrogen energy is of significance for reducing the dependence on fossil fuels. Since Fujishima and Honda first reported on TiO₂ in 1972, it has been discovered that semiconductors can generate clean, pollution-free hydrogen through water splitting driven by electricity or light. Hydrogen generated through this approach can not only replace fossil fuels but also provide environmentally friendly renewable hydrogen energy, which has attracted considerable attention. Photoelectrochemical (PEC) water splitting can use solar energy to produce clean, sustainable hydrogen energy. Because the oxygen evolution reaction (OER) over a photoanode is sluggish, the overall energy conversion efficiency is considerably low, limiting the practical application of PEC water splitting. A cocatalyst is, thus, necessary to improve PEC water splitting performance. So far, the synthesis of first-row transition-metal-based (e.g., Fe, Co, Ni, and Mn) cocatalysts has been intensively studied. Iron is earth-abundant and less toxic than other transition metals, making it a good cocatalyst. In addition, iron-based compounds exhibit the properties of a semiconductor/metal and have unique electronic structures, which can improve electrical conductivity and water adsorption. Various iron-based catalysts with high activity have been designed to improve the efficiency of PEC water oxidation. This article briefly summarizes the research progress related to the structure, synthesis, and application of iron oxyhydroxides, iron-based layered double hydroxides, and iron-based perovskites and discusses the evaluation of the performance of these cocatalysts toward photoelectrochemical water oxidation.      

Biocatalytic Photosynthesis with Water as an Electron Donor

Efficient harvesting of unlimited solar energy and its conversion into valuable chemicals is one of the ultimate goals of scientists. With the ever‐increasing concerns about sustainable growth and environmental issues, numerous efforts have been made to develop artificial photosynthetic process for the production of fuels and fine chemicals, thus mimicking natural photosynthesis. Despite the research progress made over the decades, the technology is still in its infancy because of the difficulties in kinetic coupling of whole photocatalytic cycles. Herein, we report a new type of artificial photosynthesis system that can avoid such problems by integrally coupling biocatalytic redox reactions with photocatalytic water splitting. We found that photocatalytic water splitting can be efficiently coupled with biocatalytic redox reactions by using tetracobalt polyoxometalate and Rh‐based organometallic compound as hole and electron scavengers, respectively, for photoexcited [Ru(bpy)₃]²⁺. Based on these results, we could successfully photosynthesize a model chiral compound (L ‐glutamate) using a model redox enzyme (glutamate dehydrogenase) upon in situ photoregeneration of cofactors.     

Decoupled Artificial Photosynthesis

Natural photosynthesis (NP) generates oxygen and carbohydrates from water and CO₂ utilizing solar energy to nourish lives and balance CO₂ levels. Following nature, artificial photosynthesis (AP), typically, overall water or CO₂ splitting, produces fuels and chemicals from renewable energy. However, hydrogen evolution or CO₂ reduction is inherently coupled with kinetically sluggish water oxidation, lowering efficiencies and raising safety concerns. Decoupled systems have thus emerged. In this review, we elaborate how decoupled artificial photosynthesis (DAP) evolves from NP and AP and unveil their distinct photoelectrochemical mechanisms in energy capture, transduction and conversion. Advances of AP and DAP are summarized in terms of photochemical (PC), photoelectrochemical (PEC), and photovoltaic-electrochemical (PV-EC) catalysis based on material and device design. The energy transduction process of DAP is emphasized. Challenges and perspectives on future researches are also presented.     

Recent Progress on Nickel-Based Oxide/(Oxy)Hydroxide Electrocatalysts for the Oxygen Evolution Reaction

Developing clean and sustainable energies as alternatives to fossil fuels is in strong demand within modern society. The oxygen evolution reaction (OER) is the efficiency-limiting process in plenty of key renewable energy systems, such as electrochemical water splitting and rechargeable metal–air batteries. In this regard, ongoing efforts have been devoted to seeking high-performance electrocatalysts for enhanced energy conversion efficiency. Apart from traditional precious-metal-based catalysts, nickel-based compounds are the most promising earth-abundant OER catalysts, attracting ever-increasing interest due to high activity and stability. In this review, the recent progress on nickel-based oxide and (oxy)hydroxide composites for water oxidation catalysis in terms of materials design/synthesis and electrochemical performance is summarized. Some underlying mechanisms to profoundly understand the catalytic active sites are also highlighted. In addition, the future research trends and perspectives on the development of Ni-based OER electrocatalysts are discussed.     

Efficient solar generation of hydrogen fuel – a fundamental analysis

Solar water splitting can provide clean, renewable sources of hydrogen fuel, although prior models had indicated only low conversion efficiencies would be attainable. A novel model is derived for electrochemical solar water splitting processes by semiconductors, which is the first derivation of band edge restricted thermal enhanced solar water splitting efficiencies. A theoretical basis is developed for solar energy conversion efficiencies in the 50% range as determined with contemporary thermodynamic values. The theory combines photodriven charge transfer, with excess sub-bandgap insolation to lower the water potential, providing a process of highly efficient elevated temperature solar electrolysis of water to hydrogen fuel.     

Earth-Abundant Transition-Metal-Based Bifunctional Electrocatalysts for Overall Water Splitting in Alkaline Media

The depletion of fossil fuels has accelerated the search for clean, sustainable, scalable, and environmentally friendly alternative energy sources. Hydrogen is a potential energy carrier because of its advantageous properties, and the electrolysis of water is considered as an efficient method for its industrial production. However, the high-energy conversion efficiency of electrochemical water splitting requires cost-effective and highly active electrocatalysts. Therefore, researchers have aimed to develop high-performance electrode materials based on non-precious and abundant transition metals for conversion devices. Moreover, to further reduce the cost and complexity in real-world applications, bifunctional catalysts that can be simultaneously active on both the anodic (i.e., oxygen evolution reaction, OER) and cathodic (i.e., hydrogen evolution reaction, HER) sides are economically and technically desirable. This Minireview focuses on the recent progress in transition-metal-based materials as bifunctional electrocatalysts, including several promising strategies to promote electrocatalytic activities for overall water splitting in alkaline media, such as chemical doping, defect (vacancy) engineering, phase engineering, facet engineering, and structure engineering. Finally, the potential for further developments in rational electrode materials design is also discussed.     

Near infrared-driven photoelectrochemical water splitting: Review and future prospects

Photoelectrochemical (PEC) water splitting supplies an environmentally friendly, sustainable approach to generating renewable hydrogen fuels. Oxides semiconductors, e.g. TiO₂, BiVO₄, and Fe₂O₃, have been widely developed as photoelectrodes to demonstrate the utility in PEC systems. Even though significant effort has been made to increase the PEC efficiency, these materials are still far from practical applications. The main issue of metal oxides is the wide bandgap energy that hinders effective photons harvesting from sunlight. In solar spectrum, over 40% of the energy is located in the near-infrared (NIR) region. Developing sophisticated PEC systems that can be driven by NIR illumination is therefore essential. This review gives a concise overview on PEC systems based on the use of NIR-driven photoelectrodes. Promising candidates as efficient yet practical NIR-responsive photoelectrodes are suggested and discussed. Future outlooks on the advancement of PEC water splitting are also proposed.     

电池电极反应的新应用：分步法电解制氢气

可再生能源与电解水制氢技术的结合是实现可持续制氢的最佳途径. 然而,传统电解水技术中解决氢-氧同时、同步、同地产生的问题必须依赖于膜分离技术,大幅限制了氢-氧分离和氢气异地运输的灵活性,并阻碍了可再生能源(如风能、太阳能)与电解水技术的直接结合. 针对上述问题,作者课题组在近期提出了基于电池电极反应的分步法电解水制氢技术,即通过电池电极的可逆电化学反应将现有电解水过程拆分为制氢和制氧分立步骤,实现在无膜条件下氢气和氧气的分时、分地交替制备,提升了电解水制氢的灵活性,促进了可再生能源向氢能的直接转化. 本文将介绍这一新技术的研究进展,并分析这一技术的优点和面临的挑战.     

Rational Design and Application of Covalent Organic Frameworks for Solar Fuel Production

Harnessing solar energy and converting it into renewable fuels by chemical processes, such as water splitting and carbon dioxide (CO₂) reduction, is a highly promising yet challenging strategy to mitigate the effects arising from the global energy crisis and serious environmental concerns. In recent years, covalent organic framework (COF)-based materials have gained substantial research interest because of their diversified architecture, tunable composition, large surface area, and high thermal and chemical stability. Their tunable band structure and significant light absorption with higher charge separation efficiency of photoinduced carriers make them suitable candidates for photocatalytic applications in hydrogen (H₂) generation, CO₂ conversion, and various organic transformation reactions. In this article, we describe the recent progress in the topology design and synthesis method of COF-based nanomaterials by elucidating the structure-property correlations for photocatalytic hydrogen generation and CO₂ reduction applications. The effect of using various kinds of 2D and 3D COFs and strategies to control the morphology and enhance the photocatalytic activity is also summarized. Finally, the key challenges and perspectives in the field are highlighted for the future development of highly efficient COF-based photocatalysts.     

电化学合成纳米材料和小分子材料在电解制氢领域的应用

氢气是一种清洁、高效、可再生的新型能源,并且是未来碳中和能源供应中最具潜力的化石燃料替代品。因此,可持续氢能源制造具有极大的吸引力与迫切的需求,尤其是通过清洁、环保、零排放的电解水方法。然而,目前的电解水反应受到其缓慢的动力学以及低成本/能源效率的制约。在这些方面,电化学合成通过制造先进的电催化剂和提供更高效/增值的共电解替代品,为提高水电解的效率和效益提供了广阔的前景。它是一种环保、简单的通过电解或其他电化学操作,对从分子到纳米尺度的材料进行制造的方法。本文首先介绍了电化学合成的基本概念、设计方法以及常用方法。然后,总结了电化学合成技术在电解水领域的应用及进展。我们专注于电化学合成的纳米结构电催化剂以实现更高效的电解水制氢,以及小分子的电化学氧化以取代电解水制氢中的析氧共反应,实现更高效、 增值的共电解制氢。我们系统地讨论了电化学合成条件与产物的关系,以启发未来的探索。最后,本文讨论了电化学合成在先进电解水以及其他能量转换和储存应用方面的挑战和前景。     

Z‐Scheme Photocatalytic Water Splitting on a 2D Heterostructure of Black Phosphorus/Bismuth Vanadate Using Visible Light

Spontaneously solar‐driven water splitting to produce H₂ and O₂, that is, the conversion of solar energy to chemical energy is a dream of mankind. However, it is difficult to make overall water splitting feasible without using any sacrificial agents and external bias. Drawing inspiration from nature, a new artificial Z‐scheme photocatalytic system has been designed herein based on the two‐dimensional (2D) heterostructure of black phosphorus (BP)/bismuth vanadate (BiVO₄). An effective charge separation makes possible the reduction and oxidation of water on BP and BiVO₄, respectively. The optimum H₂ and O₂ production rates on BP/BiVO₄ were approximately 160 and 102 μmol g^?1 h^?1 under irradiation of light with a wavelength longer than 420 nm, without using any sacrificial agents or external bias.     

Molybdenum Carbide‐Embedded Multichannel Hollow Carbon Nanofibers as Bifunctional Catalysts for Water Splitting

With the environmental pollution and non‐renewable fossil fuels, it is imperative to develop eco‐friendly, renewable, and highly efficient electrocatalysts for sustainable energy. Herein, a simple electrospinning process used to synthesis Mo₂C‐embedded multichannel hollow carbon nanofibers (Mo₂C‐MCNFs) and followed by the pyrolysis process. As prepared lotus root‐like nanoarchitecture could offer rich porosity and facilitate the electrolyte infiltration, the Mo₂C‐MCNFs delivered favourable catalytic activity for HER and OER. The resultant catalysts exhibit low overpotentials of 114 mV and 320 mV at a current density of 10 mA cm^?2 for HER and OER, respectively. Furthermore, using the Mo₂C‐MCNFs catalysts as a bifunctional electrode toward overall water splitting, which only needs a small cell voltage of 1.68 V to afford a current density of 10 mA cm^?2 in the home‐made alkaline electrolyzer. This interesting work presents a simple and effective strategy to further fabricating tunable nanostructures for energy‐related applications.     

Adjustable Ternary FeCoNi Nanohybrids for Enhanced Oxygen Evolution Reaction

Water splitting as a greatly desired technology to produce clean renewable energy, but is hampered by the sluggish oxygen evolution reaction. So, the development of highly active and durable water oxidation electrocatalysts is of primarily significance for energy conversion. Here, a facial strategy to synthesize FeCoNi nanohybrids with adjustable morphological structures by using fluorine is introduced. The morphology and electrocatalytic activity of the sample is determined by the innovative introduction of fluorine. Among them, the overpotential at 10 mA cm⁻² of the best sample is approximately 97 mV lower than the commercial RuO₂ toward the oxygen evolution reaction in 1 m KOH. Additionally, the catalysts also have low Tafel slopes and remarkable stability.     

基于立方烷结构的分子催化剂在光催化水氧化中的研究进展

随着化石燃料大量使用带来的气候变化和环境污染问题日趋严重,寻找清洁高效的可再生能源用做传统化石燃料的替代品,已经成为当前的研究热点。光驱动的水分解反应被认为是太阳能制氢的可行途径。水的全分解包括两个半反应-水的氧化和质子还原。其中水的氧化反应是一个涉及四个电子和四个质子转移的复杂过程,需要很高的活化能,被认为是全分解水反应的瓶颈步骤。因此,开发高效、稳定、廉价丰产的水氧化催化剂是人工光合作用突破的关键因素。立方烷具有类似自然界光合作用酶光系统II(PSII)活性中心Mn_4CaO_5簇的结构,世界各国的科学家受自然界光合作用的启发,开发出了许多基于过渡金属的立方烷结构的催化剂,常见的有锰、钴和铜等立方烷催化剂。本文简要地综述了近年来立方烷分子催化剂在光催化水氧化中的研究进展。首先介绍了立方烷基光催化水氧化反应历程,继而详细介绍了基于有机配体的立方烷配合物和全无机的多金属氧酸盐立方烷水氧化催化剂,其次是半导体(BiVO4或聚合的氮化碳(PCN))为捕光材料复合立方烷分子催化剂的水氧化体系最新研究进展。最后总结并展望了该领域所面临的挑战及其前景。     

Chlorophyll sensitized BiVO4 as photoanode for solar water splitting and CO2 conversion

Converting solar energy into valuable hydrogen and hydrocarbon fuels through photoelectrocatalytic water splitting and CO_2 reduction is highly promising in addressing the growing demand for renewable and clean energy resources. However, the solar-to-fuel conversion efficiency is still very low due to limited light absorption and rapid bulk recombination of charge carriers. In this work, we present chlorophyll(Chl) and its derivative sodium copper chlorophyllin(ChlCuNa), as dye sensitizers, modified BiVO_4 to improve the photoelectrochemical(PEC) performance. The photocurrent of BiVO_4 is surprisingly decreased after a direct sensitization of Chl while the sensitization of ChlCuNa obviously enhances photocurrent of BiV04 electrodes by improved surface hydrophilicity and extended light absorption.ChlCuNa-sensitized BiV04 achieves an improved H_2 evolution rate of 5.43 μmol h~(-1) cm~(-2) in water splitting and an enhanced HCOOH production rate of 2.15 μmol h~(-1) cm~(-2) in CO_2 PEC reduction, which are1.9 times and 2.4 times higher than pristine BiVO_4, respectively. It is suggested that the derivative ChlCuNa is a more effective sensitizer for solar-to-fuel energy conversion and CO_2 utilization than Chl.     

2D Catalysts for CO2 Photoreduction: Discussing Structure Efficiency Strategies and Prospects for Scaled Production Based on Current Progress

Currently, the excessive consumption of fossil fuels is accompanied by massive emissions of CO₂, leading to severe energy shortages and intensified global warming. It is of great significance to develop and use renewable clean energy while reducing the concentration of CO₂ in the atmosphere. Photocatalytic technology is a promising strategy for carbon dioxide conversion. Clearly, the achievement of the above goals largely depends on the design and construction of catalysts. This review is mainly focused on the application of 2D materials for photocatalytic CO₂ reduction. The contribution of synthetic strategies to their structure and performance is emphasized. Finally, the current challenges, and prospects of 2D materials for photoreduction of CO₂ with high efficiency, even for practical applications are discussed. It is hoped that this review can provide some guidance for the rational design, controllable synthesis of 2D materials, and their application for efficient photocatalytic CO₂ reduction.     

Pigment–Acceptor–Catalyst Triads for Photochemical Hydrogen Evolution

In order to solve the problems of global warming and shortage of fossil fuels, researchers have been endeavoring to achieve artificial photosynthesis: splitting water into H₂ and O₂ under solar light illumination. Our group has recently invented a unique system that drives photoinduced water reduction through “Z‐scheme” photosynthetic pathways. Nevertheless, that system still suffered from a low turnover number (TON) of the photocatalytic cycle (TON=4.1). We have now found and describe herein a new methodology to make significant improvements in the TON, up to around TON=14–27. For the new model systems reported herein, the quantum efficiency of the second photoinduced step in the Z‐scheme photosynthesis is dramatically improved by introducing multiviologen tethers to temporarily collect the high‐energy electron generated in the first photoinduced step. These are unique examples of “pigment–acceptor–catalyst triads”, which demonstrate a new effective type of artificial photosynthesis.