I develop a new class of heat-to-electricity energy conversion technology, an electrochemical heat engine. It employs a working fluid undergoing symmetric redox reactions at the hot and cold terminals. Unlike conventional heat engines, electrochemical engines can deliver high power and efficiency at all scales, enabling both distributed and centralized heat recovery. Optimal efficiencies and power densities can be achieved by independently tuning the voltage, the thermal conductivity, and the electrical resistance through materials engineering. Such design flexibility overcomes intrinsic limitations in thermoelectric generators and mechanical heat engines.