Apparently, it's an insulated membrane that only lets protons through. A proton can move through the membrane more easily than an electron can. It has more to do with other properties of the material and the ions than the effective size of electrons vs protons.
Assuming my reference to proton exchange fuel cells is in the ballpark, the mechanism is to use the membrane plus pressure to separate the hydrogen into electrons and protons. The electrons are routed through the load, i.e. extracted electrical power, and the protons pass through the membrane. The inactive (non-dissociated) hydrogen molecules are blocked from the "cold" side by the semi-permeable membrane.
The electrons and protons recombine on the cold side to reform hydrogen. That hydrogen has to be compressed sufficiently to move it back to the "hot" side of the system. This is probably a pretty substantial compression and will probably limit the efficiency of the system - it also raises the "perpetual motion machine" red flag in my mind, except the sun is adding lots of thermal energy to make it go for real.
I'm picturing the system working at a molecular level like reverse osmosis of seawater: http://en.wikipedia.org/wiki/Reverse_osmosis. Reverse osmosis requires a substantial amount of pressure (~1000 psi) to force the water through the semi-permeable membrane while the salt cannot pass through the membrane and thus stays on the salty side.
