Hydrogen extraction efficiency of newish nano particle AA battery The internet is ablaze with the new nano particle based extraction method of converting water to hydrogen. In 2002/2003 when I was 16 there was a similar craze about a lawn mower motor which supposedly ran on water. Indeed there were even conversion kits on the internet for vehicles. They didn't work because of a fundamental inefficiency of the conversion. Will this new nanotechnology improve the process so that cars can run on converted water or does there still exist a limit?
 A: The article you refer to is about the electrolytic splitting of water.
A 100% efficient electrolytic cell would require a voltage of about 1.23V to split water, but for various reasons a simple electrolytic cell requires about 1.48V. The difference between the voltages is called the overpotential, and it increases the amount of power needed to split the water because the power required per unit of hydrogen produced is proportional to the cell voltage. The excess power goes into heating the hydrogen and oxygen produced, and in this case it means that simple cells are about 83% efficient at converting electricity into hydrogen.
Catalysts can be used to increase the efficiency, and indeed platinum based catalysts can be used to reduce the overpotential and make cells with near 100% efficiency. The problem is that platinum is expensive. The result from the Stanford team is that a much cheaper nickel based catalyst can achieve the same efficiency as platinum. The paper is here, but note that it is behind a paywall.
If the catalyst proves to be stable enough then it will be useful for electrolytic production of hydrogen, but the improvement in efficiency isn't going to change the world overnight. It still takes a lot of power to electrolyse water so it's only feasible when cheap electricity is available.
