Photoelectrochemical Hydrogen Production

Abstract

The solar energy coming down onto the earth's surface amounts to ca. 3 X 10 J per year, which is approximately 10 times the world-wide yearly energy consumption. Search for the efficient conversion of solar energy into other useful forms is, in view of the increasing anxiety about the exhaustion of energy resources, one of the most important challenges of future research and technology. In systems designed for the purpose of converting solar energy into electricity and/or chemical energy, two principal criteria must be fulfilled. The first is the absorption, by some chemical substance, of solar irradiation, leading to the creation of electron (a reduced chemical moiety)-hole (an oxidized chemical moiety) pairs. The second is an effective separation of these electron-hole pairs with little energetic loss, before they lose the input energy through recombination. Such a photoinduced charge separation can proceed effectively provided an electric field (potential gradient) has been established at the position where the primary photoexcitation takes place. In general, a potential gradient can be produced at an interface between two different substances (or different phases). For example, a very thin (ca. 50 A) lipid membrane separating two aqueous solutions inside the chloroplast of green plants is believed to play the essential role for the process of photosynthesis, which is the cheapest, and probably most successful solar conversion system available so far. Another well-known example is a photocell or a solar cell, in which the photogenerated electron hole pairs are driven efficiently in opposite directions by an electric field existing at the boundary (junction) of nand p-type semiconductors (or at metal/semiconductor junctions). A potential gradient can also be created, by a process described later in more detail, at an interface of a semiconducting material and a liquid phase. Hence, if a semiconductor is used as an electrode which is connected to another (counter) electrode, photoexcitation of the semiconductor can generate an electric work through an external load and, simultaneously, make proceed chemical (redox) reaction on the surface of each electrode. On the other hand, in a system where semiconductor particles are suspended in a solution, excitation of the semiconductor can lead to redox processes at the interface region. These systems have recently drawn the attention of a large number of investigators primarily in connection with solar energy conversion. The present chapter deals with the principles and recent advances in the investigation of light energy conversion systems based on the semiconductor/liquid junctions, focusing on fuel production as well as electrical energy generation. Biophotoelectrochemical processes (photoinduced charge generation and separation using compounds or organisms of biological origin) are also reviewed.