Photoelectrochemical Hydrogen Production

Direct conversion of sunlight into chemical fuels is an attractive prospect for a future energy infrastructure based on sustainable sources. This chapter motivates the choice for photoelectrochemical water splitting as a promising route toward solar fuels. It starts by describing some of the challenges associated with the use of fossil fuels, and gives a brief overview of alternative energy sources. It illustrates the advantages of using chemical fuels as a large-scale storage solution, and describes the role of hydrogen as one of the key components of these fuels. Finally, some benchmarks for photoelectrochemical water splitting are given, and various approaches and materials demands for practical devices are discussed.

In this chapter, the basic principles of photoelectrochemical water splitting are reviewed. After a brief introduction of the photoelectrochemical cell and the electrochemical reactions involved, the electronic structure and properties of semiconductors are discussed. The emphasis is on metal oxide semiconductors, and special attention is given to the presence of ionic point defects in these materials. This is followed by a closer look at the semiconductor/electrolyte interface. The energy conversion efficiency and different definitions of the quantum efficiency are treated next. The chapter concludes with a brief outline of the material’s requirements and challenges facing the development of highly efficient photoelectrodes.

This chapter presents an overview of several photoelectrochemical characterization techniques and the equipment needed to carry out these measurements. It starts with a detailed description of the photoelectrochemical cell and its components. A few selected cell designs are shown and discussed, and several considerations for choosing suitable photoelectrode substrates, electrolyte solutions, and counter and reference electrodes are given. This is followed by a description of two experimental setups for photocurrent measurements, one for measurements under simulated sunlight and one for wavelength-dependent (monochromatic) measurements. The components of these setups are described, with special emphasis given to the inner workings of the potentiostat and the various types and specifications of solar simulators. The information that can be obtained from photocurrent measurements, such as photocurrent onset potentials, performance limiting factors, and quantum efficiencies is described next. The final section reviews the principles, equipment, and practical considerations for Mott–Schottky measurements. Common pitfalls of impedance measurements are outlined, and several strategies and precautions to avoid or minimize measurement errors are given.

Due to its abundance, stability, and ability to absorb solar irradiation, Hematite, α-Fe₂O₃, has been investigated for its application in solar hydrogen production via water splitting for more than three decades. However, the recent application of nanostructuring techniques has provided significant advances in the performance of hematite photoanodes. Here, the basic material properties, the attractive aspects, and the challenges presented by hematite for photoelectrochemical (PEC) water splitting are reviewed. Various methods of enhancing performance by nanometer morphology control are detailed and the resulting PEC performances are compared. These techniques, including solution-based routes for porous thin films and nanowire arrays, potentiostatic anodization for nanotube arrays, electrodeposition, ultrasonic spray pyrolysis, and atmospheric pressure chemical vapor deposition, have increased the understanding of the material parameters critical to the performance of hematite, and resulted in an increase of quantum efficiency to over 20% with 450 nm light (compared to 6% with optimized single crystals under similar conditions) and an overall solar-to-hydrogen (STH) conversion efficiency of 3.3% when used in a tandem device. In addition, the remaining limitations of morphology, carrier recombination, slow oxidation kinetics, and flatband potential are presented with the recent advances and approaches in overcoming them and realizing the full potential of hematite for solar hydrogen production.

An efficient solar energy conversion system for hydrogen production from water is most important for genuinely sustainable development. Photoelectrode and photocatalyst powder systems using mixed oxide semiconductors have been widely investigated. Their water-splitting reaction mechanisms are very similar and, therefore, interaction and exchange of information between both fields are highly desirable. In this chapter, some mixed metal oxide semiconductors to utilize visible light in both fields are introduced. Moreover, the screening of oxide semiconductor materials with high efficiency under visible light irradiation is a main issue for both fields, and high-throughput screening systems for new semiconductors are also introduced.

The “Achilles Heel” of solar energy is the need for storage to allow for energy utilization for transportation and nighttime use. Hydrogen is an ideal energy carrier that can be stored and transported. Therefore, cost-effective generation of hydrogen with sunlight via water photoelectrolysis is the critical breakthrough needed for transition to a renewable energy-based hydrogen economy. A semiconductor-based photoelectrolysis system may have cost advantages over using either a photovoltaic cell coupled to an electrolyzer or solar thermochemical cycles for water splitting. Unfortunately, there is no known semiconducting material or combination of materials with the electronic properties and stability required to efficiently and economically photoelectrolyze water. Semiconducting oxides can have the required stability but present theoretical methods are insufficient to a priori identify materials with the required properties. Most likely, any useful material will be a complex oxide containing many elements whereby each contributes to the required material properties such as light absorption across the solar spectrum, stability, and electrocatalytic activity. The large number of possible multicomponent metal oxides, even if only ternary or quaternary materials are considered, points to the use of high-throughput combinatorial methods to discover and optimize candidate materials. In this chapter, we will review some techniques for the combinatorial production and screening of metal oxides for their ability to efficiently split water with sunlight.

The key to successful deployment of photoelectrochemical (PEC) water-splitting for commercial renewable hydrogen production will be in the identification and development of innovative semiconductor materials systems and devices, likely involving multijunction configurations. Multijunction approaches offer some of the best hope for achieving practical PEC hydrogen production in the near term, but complex materials and interface issues still need to be addressed by the scientific community. This chapter explores the challenges and benefits of large-scale solar water splitting for renewable hydrogen production, with specific focus on the multijunction PEC production pathways. The technical motivation and approach in the R&D of multijunction PEC devices and systems are considered, and examples of progress in laboratory scale prototypes are presented.

Photoelectrochemical water splitting could become an important contributor to the production of hydrogen and so provide a route to the storage of solar energy, but is not yet commercially viable. Improved materials are needed. To produce hydrogen for less than $3/kg, so as to be able to compete with existing energy sources, system costs of $160/m², for a 10% efficient material requiring less than 0.8 V bias with 15 years durability would be needed.

The prospect of future progress in water photoelectrolysis critically depends upon the discovery and application of new materials, structures, and device architectures. Developments in closely related areas, such as solar cells, provide ample guidance for the application of new concepts in nanomaterials and nanophotonics to the challenges confronting electrochemical energy conversion devices. This review examines opportunities that have emerged as a consequence of new synthetic routes for nanostructured semiconductors and metals. Design criteria for building efficient devices are considered for semiconductors with low mobility and short carrier lifetimes. It is then shown how these design criteria can be modified by exploiting the plasmon resonance of metallic nanostructures.