The company describes this generator as a solid state device, but the diagrams show the reliance on fluid/flow of hydrogen between the hot side and the cold side for moving some protons around. That seems to be something in between the semiconductor-based solid state thermoelectric generators that are already commonly understood and some kind of generator with moving solid parts.

It still seems like a low maintenance solution to have a closed loop of hydrogen, but that seems like a potential maintenance/failure point, as well, to rely on the chamber to remain filled with hydrogen gas.