Wiki has a pretty good article on electron affinity: http://en.wikipedia.org/wiki/Electron_affinity
Here is the thought (I have not checked to see if this is a "re-invention" of someone else's idea) - take two electrodes and a working medium, and hydrogen is the only working medium that fits into this concept very well (73 kj/mol)... - such that one electrode has a much lower electron affinity than does the H2 (zinc works well ~0) and the other has a much higher (gold plated copper works here ~223). You need a source of energy to convert - like focused sunlight onto the back side of the zinc. The other electrode is finned and air-cooled. The zinc emitter can be a Zn plated bimetal, so that there is already a small thermoelectric effect. Query: will a flow of hydrogen between the heat source and a heat sink create an efficient flow of electrical energy on the electrodes? This flow can be accentuated by using a Stirling engine configuration, and especially the so-called lamina or thermoacoustic Stirling, where the hot and cold ends, respectively use the metals optimized for electron affinity and so have electrical leads. The current then adds to the mechanical energy, giving higher efficiency. Jones
