Two pair of junctions between two dissimilar conductors, if held at different temperatures and connected in series, produce a voltage. Stacks of such junctions comprise a thermopile. Some thermopiles have been heated in satellites by nuclear decay heat. Thermocouple temperature transducers are thermo-electrics. Biological voltage generators.
Electric eels generate enough voltage to give a serious shock.
The human brain generates voltages and operates by the flow of current pulses between brain cells. It also send voltage signals to the heart and other muscles. Sensor cells generate voltages which send currents to the brain. Friction between two insulators produces electrostatic voltages on the insulators. Capacitors store electric charge provided by an external source of voltage and then discharge it when connected to a load.