[derexitus]

I've been reading about hydrogen fuel cells and saw that they consist of an anode and cathode (filled with hydrogen and oxygen in that order) and a Proton Exchange Membrane or Polymer Electrolyte Membrane (PEM).
   My main question is what makes an element appropriate to be used as an electrode? Some examples of common electrodes were Copper, Zinc, Gold, Platinum, Rhodium, etc. Some of these elements are inert/reactive, but I'm still a bit confused on as to what makes an element a good electrode. Is it because they have 4 or less valence electrons? Or is it simply because of their property of conductivity?

I just need clarification on the subject. If there's a simple answer to this that I'm missing, please tell me and I'll do more research on them.

Thanks!

[Borek]

There is no simple answer.

Material for electrodes is selected taking into account its conductivity, mechanical properties and - most important - how easily the reaction proceeds on its surface. In general this is some mix of brute force experimenting (testing everything you can think off) and educated guessing (we know Pt works nicely, what about similar metals?).