As it turns out, the polymer called polypyrrole has been used before, but resulted in too weak a response to generate much electricity at all. The new approach interlocks another, more rigid polymer, called polyol-borate, into the polypyrrole. Interweaving both materials gives the film a much larger displacement motion as the water vapor is absorbed and then evaporated. It is this displacement motion that is converted to electrical energy.
I wouldn't look for this new stuff to be lifting cars for chassis repair anytime soon, but the developers think the power will be sufficient to run a whole host of micro-electrical devices. Just goes to show you, there is more power in water than just the hydrogen and oxygen.