how to build hydrogen fuel cells
There isn't one as such. Even the experts can't build one thats cost effective and probably never will for cars so not likely an ammature could. I know as I've tried to get one for yrs for my EV to turn it into a hybrid. Besides their high cost they use H2 which is a very ineff fuel taking twice the energy to make it as you get out of it. So even if a fuel cell was 50% eff, with the H2 being 50% eff only gives 25% eff which a car engine can do!! Small ones, 1-5 kw, for combined home heating, electric power where the waste heat is used to make close to 100% eff is about the only practical use for them maybe.
