I've been collecting old robot magazines and looking through them occasionally. (According to Google Translate) Honda planned on putting a Hydrogen fuel cell in Asimo! Now I'm gonna have to calculate how bad of an idea that is . . .

Honda’s FCX hydrogen fuel cell car used a PEFC (proton exchange membrane) fuel cell manufactured by Ballard Power Systems. That fuel cell had a power output of 62 kW.

The 2000, 2001, and 2002 versions of Asimo used a 38.4V/10Ah nickel metal hydride battery that weighed 7.7 kilograms. With a run time of ~30 minutes, that gives Asimo an average power draw of 768 watts (from my experience this number is what I would expect)

In 1995, Ballard had achieved a power density of 700 watts per kilogram for their PEFC fuel cell stack. However that is just the weight of the stack; looking at modern fuel cell generators, they fall between 3 and 4 kg for the total system minus the tank.

To generate 700 watts, the fuel cell would be consuming between 9 and 14 liters of hydrogen per minute. This is at a pressure of 0.05 - 0.06 MPa. That is half a gram per minute of hydrogen.

Our original battery solution weighed in at 7.7 kg. If we assume we don’t need any capacitors and ignore the mass of a DC-DC converter, we can subtract the weight of the fuel cell (4 kg) to get a maximum tank mass of 3.7 kg.

I found a 3.5 kg cylindrical tank made of carbon fiber with an aluminum liner. It can hold 12 liters at 35 MPa. This is 347 grams of hydrogen allowing for a runtime of an amazing 11 and a half hours! The size (54 cm by 19 cm) and limited lifetime (500 cycles) would be an issue.