Fuel cell and electrolysis depend critically on catalysis. Haldor Topsoe has been a leader in this field and initiated many innovations. The paper reviews the types of fuel cells, electrolyzers, and the thermodynamic aspects involved, and then discusses the technology from the viewpoint of available feedstocks. Important phenomena that determine the stability of the devices, such as carbon deposition and sulfur poisoning, are reviewed as well. Although fuel cells are still awaiting their commercial breakthrough, high-temperature electrolysis in combination with the catalytic synthesis undoubtedly has potential in sustainable energy scenarios. (C) 2015 Elsevier Inc. All rights reserved.