Photoelectrochemical Water Splitting

The world energy demand has been steadily growing in the past decades as the world population increases and more nations develop to higher standards of living. Traditional solutions such as fossil fuels and nuclear energy have not been able to arrive to a sustainable plan on how to supply this energy the next few decades, and many armed conflicts have been started due to the limited access of such a scarce resource. Moreover, they are responsible for toxic waste and greenhouse gas emissions, which are causing one of the most important environmental crises in the history of the planet. Thus, alternative solutions must be considered in the energy transition that would be able to supply the needed energy in the future. Renewable energies, including wind, solar, biomass, wave and geothermal among others, are the main hope to cover the energy needs of society in the future, since it is a more sustainable way of harvesting energy and these resources are virtually infinite in terms of time scalability. In particular, solar energy is the most readily abundant energy source in most areas of the world, since the amount of solar energy received by the earth every year is thousands of times higher than the energy demand. In addition, it is considered one of the sources with the least impact in the surrounding environment among all the renewable energy sources, since it does not produce sound, and the most common techniques do not produce toxic waste. For these reasons, solar energy has experimented a steep growth in production and implementation recently. However, if solar energy sources are to play a crucial role in the necessary energy transition, they must be able to supply a constant amount of power throughout the year. One of the main problems that solar energy faces is its daily and seasonal fluctuations due to the nature of this source, which threaten to destabilize the electricity network if solar energy is to be installed at very large scale. Thus, reliable systems for energy storage must be installed to assure that the fluctuations in the energy source do not affect the energy supply chain. So far, batteries have been used as the main energy storage system. However, they are rather bulky and expensive, with toxic and rare materials at their core, and thus ineffective for long-term energy storage. One of the most promising approaches to this issue, especially to long term storage, is the use of hydrogen as an energy storage material for solar energy. Hydrogen has a high energy density and can be stored as a pressurized gas, a liquid, a metal hydride, or further converted in more common hydrocarbons such as methane or ethanol. An interesting way to achieve hydrogen using solar energy is to drive a photoelectrochemical (PEC) reaction, in which a semiconductor material is excited, producing an electron-hole pare that would be directly used to drive the electrochemical reaction of water electrolysis, also called water splitting. This book gives an account of the main physical principles governing this process, identifying important barriers and areas of potential improvements. In particular, there seems to be three major steps that may limit the performance of these devices: the charge carrier separation in the semiconductor material used as photoelectrode; the interface between the semiconductor and the electrolyte, including the charge injection from one to the other, the catalytic activity at the surface and the possible stability issues that can occur; and the ion transfer and optimum pH within the electrolyte itself. All these issues have been further explored here. The main strategies applied so far to achieve a good charge carrier generation, separation and injection are reviewed within this book, with the most important materials investigated in the field to date. There seems to be a special focus historically in TiO2 and Fe2O3, as they are among the first materials to be investigated and developed. Here, the main reaso...