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 reasons behind these choices were investigated, especially based on the physical principles previously explained. In addition, it is also interesting to look at possible catalysts for these reactions, both in the areas of precious metals and earth abundant materials, and to further explore the strategy of including protective layers to avoid corrosion of the photoelectrodes. Moreover, some emerging trends such as new more complex materials, nanostructures of such semiconductors, and the application of multijunctions and membranes are reviewed.

In addition, the fabrication techniques and measuring methods are listed, identifying possible sources of practical challenges. Practical issues regarding the fabrication techniques seem to have been one of the main limits for the performance of more earth-abundant materials, and thus further understanding on how these techniques affect the material properties of the semiconductors fabricated up to date. Moreover, there has been several instances of irregular or uninformed reporting of performances within this field, thus, understanding the different measurement techniques and how to relate those to the final expected performance and calculated solar-to-hydrogen efficiencies is crucial to raise the reporting standards of the field.Finally, the economic feasibility of such approach into a reactor design and a hydrogen production plant are discussed, allowing to draw some general conclusions and indicating future approaches that must be thoroughly investigated and improve to arrive to an economic and efficient PEC system. This is especially relevant since, so far, most of the PEC devices reported are in the scale of millimeters to centimeters. Thus, looking forward to the implementation of such devices at large scale, possible bottlenecks and additional equipment needed is of vital importance for a reliable economic analysis.

In summary, this book tries to give an overview of the field of photoelectrochemical water splitting, by looking at the physics, the state-of-the-art devices, emerging trends and fabrication and measurement techniques. Moreover, the economic feasibility based on these reported performances and trends has been investigated. This analysis allows drawing some conclusions in the feasibility of the methods presented, and their role on the energy transition for future societies.