Foreword: materials for energy harvesting, conversion and storage (ICOME 2017) Jean-Michel Nunzi, Rachid Bennacer and Mohammed El Ganaoui Eur. Phys. J. Appl. Phys., 85 2 (2019) 20901 Published online: 08 March 2019 DOI: 10.1051/epjap/2018180320