I found an old platinum ring in a car park, cut it and beat a section into two strips of foil approx 100mmX2mmX0.3mm. Just holding these under the water tap connected to a multimeter shows one tenth of a volt. Still water is not so good, the voltage drops very quickly. Greater surface area greater voltage I guess. At a microscopic level I guess the catalyst is turning water to Oxygen and Hydrogen and giving off electrons.