A high temperature electrochemical system, such as a solid oxide fuel cell system, generates hydrogen and optionally electricity in a fuel cell mode. At least a part of the generated hydrogen is separated and stored or provided to a hydrogen using device. A solid oxide regenerative fuel cell system stores carbon dioxide in a fuel cell mode. The system generates a methane fuel in an electrolysis mode from the stored carbon dioxide and water by using a Sabatier subsystem. Alternatively, the system generates a hydrogen fuel in an electrolysis mode from water alone.