THE key point that you fail to consider and the real limiting problem with vehicular fuel cell technology has been that the amount of platinum catalyst needed to build the cells was simply too high (those NASA fuel cells are OLD OLD technology and use LOTS of platinum). That problem has been whipped, and the needed amounts of catalyst reduced by at least one order of magnitude. Another was that the efficiency dropped off drastically when using air as the oxidant (the NASA cells used pure oxygen). Ballard has whipped that problem on their PEM fuel cells, and current cells are as efficient on air as older cells were on pure oxygen.