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hat is the specific energy for hydrogen and how can I calculate the mass of hydrogen required to produce electricity for a day at 90% effficiiency?
To produce one electron (e-) per day in a 90% efficient fuel cell via the overall reaction
0.5 H2 + 0.25 O2 --> 0.5 H2O + e-
requires 1.111… molecule of H2 per day, or 3.7 x 10-24 grams/day.
Multiply if your electrical needs are greater.