Recent Advances and Emerging Trends in Photo‐Electrochemical Solar Energy Conversion

Abstract

Solar energy is a renewable and carbonfree energy source, and it is by far the most abundant, comprising greater than 99% of the total amount of all renewable energies available on Earth. [3] However, in order to displace fossil fuels, large-scale energy storage solutions will be required to address the intermittency of solar irradiation (and other renewable energies). Although new battery technologies will likely meet the need for cost-effective shorter time scale energy storage (1-3 days), fuels are the only effective option for longer term and seasonal storage. [4] The field of solar water splitting takes inspiration from Nature in photosynthesis by using light energy to extract electrons from water, and to store those electrons in high-energy chemical bonds (i.e., fuels). For the simple water splitting reaction, this generates hydrogen fuel and oxygen as a by-product. The energy of the solar light is stored in the hydrogen molecule, which can then participate directly in a hydrogen-based economy, [5] or be reacted with CO 2 in Fischer-Tropsch-type processes to generate carbonbased fuels, which are compatible with our current energy infrastructure. If the CO 2 used in the reaction with solar hydrogen is derived from CO 2 in the air, then the carbon-based fuels would be overall carbon neutral. However, using the CO 2 stream from a coal-fired power plant does not make sense as a strategy for carbon abatement, as it would be much more efficient from a life cycle perspective to use the solar energy directly and leave the coal unburned. [6,7] In any case, renewable hydrogen will play an important role in future energy systems and the chemical industry. In addition to being a carbon-free energy carrier (suitable for use in fuel cells, for example), renewable hydrogen will be needed to supply all of the current processes that rely on fossil fuelderived hydrogen, such as the large-scale synthesis of ammonia through the Haber-Bosch process, which is used as fertilizer for feeding the world population. The question then is, what is the most cost-effective method to generate solar hydrogen?Presently, the most effective solar energy conversion devices are based on photovoltaics (PV). For semiconductor-based water splitting to generate solar hydrogen, there are two conceptual options: a system that uses separated devices to harvest the light and to electrolyze water (PV-coupled electrolysis), and an integrated system that combines the light capture and catalytic water splitting interface in the same material (photoelectrochemistry) (Figure 1a). There are also varying degrees of Photo-electrochemical (PEC) solar energy conversion offers the promise of low-cost renewable fuel generation from abundant sunlight and water. In this Review, recent developments in photo-electrochemical water splitting are discussed with respect to this promise. State-of-the-art photo-electrochemical device performance is put in context with the current understanding of the necessary requirements for cost-effective solar hydrogen generation (in ter...

In situ characterizations of photoelectrochemical cells for solar fuels and chemicals

“…Over the years, much of the research eventually shifted to development of multinary metal oxides for solar fuel production. 32,38 Among these, bismuth vanadate (BiVO 4 ) is perhaps the most successful photoanode material developed to date. Efficiencies nearing 90% of the theoretical maximum have been achieved with BiVO 4 photoanodes, leading to devices displaying 8.1% solar-to-hydrogen (STH) efficiency for unassisted solar water splitting when coupled with a PV cell.…”

Operandoelectrochemical study of charge carrier processes in water splitting photoanodes protected by atomic layer deposited TiO2

“…Therefore, it is highly desirable to develop an operando methodology that can provide high spatial and temporal charge dynamics of the underlying charge transfer processes during PEC operation. 18,19 Photoelectrochemical impedance spectroscopy (PEIS) is an established method for characterizing charge carrier phenomena under operational conditions. It has been commonly employed to detect the origin of limitations and improve the efficiency of these solar energy conversion devices.…”

Directly Photoexcited Oxides for Photoelectrochemical Water Splitting

“…In this flourishing field, numerous reviews have been published on differenta spects, such as materials, [5][6][7][8] protection strategies, [9][10][11] and practical applications. [12,13] Reviews on oxide photoelectrodes (PEs) are among them,w hich focus on photocathodes, materials, and cocatalysts, [6,7,14,15] whereas this review intends to sort the most recent progress in oxide PEs by cell configurations, which are defined by charge-separationt ypes. An abbreviated terminology for the facilitated description of various PEC types is introduced herein.…”