Hello internet! Please excuse any silly questions as I am not (obviously) a professionally educated physicist (yet!). I was doing a school project on alternative energy sources and became rather interested in hydrogen fuel cells, specifically to replace batteries in small scale, high energy requirement situations such as drones. I know that we can make them very small (see https://www.newscientist.com/article/dn16370-worlds-smallest-fuel-cell-promises-greener-gadgets/) however these micro-cells run at very low voltages and currents. Is there any way of calculating/roughly working out how big a fuel cell has to be to produce a certain voltage and/or current? (of course the size of the hydrogen fuel tank would not be taken into consideration here. Assume that there is an external hydrogen supply.) Also does anybody know of a way to calculate how much energy you can get from a certain volume of hydrogen, or how long a certain volume would last if it were being used to make a constant electrical current of x amps at y volts? Thanks in advance for any insight you can offer!