**Preface to "Advanced Materials and Technologies for Fuel Cells"**

The urgent need for a transition toward a sustainable energy system is boosting research on clean and carbon net-zero processes, suitable for replacing the combustion of fossil fuels. Importantly, fuel cells (FCs) are a cornerstone that are able to sustain both the hydrogen and renewable-energy-based technologies. On one end, the power-to-fuel approach will be needed to fully exploit the intermittent and off-peak power availability of wind and solar energy, while on the other end the efficient conversion of H2 into electricity will be necessary, especially for non-stationary applications such as transportation. Fuel cells are available at different ranges of temperature and at different scales of power density; they operate with varying fuel types and also provide reversible operation as electrolyzers in order to obtain fuels with the highest efficiency. Not all types of fuel cells and electrolyzers share the same technology readiness level, and there is much available room still for new concepts and approaches. This Special Issue showcases compelling examples of the present developments in FC science and technology, with original and high-quality contributions from all over the world. Out of the 13 papers collected, eight are dedicated to different aspects of solid oxide fuel cells (SOFCs), from materials of single cells (electrolyte and electrodes) and stacks (glass sealing), to the simulation of CH4 internal reforming and to field tests in off-shore conditions. A detailed description of non-conventional operating modes in polymer electrolyte membrane (PEM) fuel cells is given in two brilliant papers, while hydrogen production by alkaline electrolyzers is the subject of the paper by Le Bideau et al. Furthermore, this Special Issue is enriched by two very interesting contributions from You et al. about microbial fuel cells and from Hernandez Rivera et al. about soft ´ drink–fueled paper cells. Editing this volume was an exciting professional and personal experience and, therefore, we are deeply grateful to all of the authors and reviewers that contributed to making it such a success. Finally, we would like to thank the *Energies* Editorial Office and especially the MDPI Managing Editor, Ms. Adele Min, for her constant support.

#### **Massimo Viviani, Antonio Barbucci, Maria Paola Carpanese, Sabrina Presto** *Editors*
