For reasons explained before, we'll likely all be driving electric cars long before we ever see mass-market vehicles powered by hydrogen fuel cells, which was once the great clean-car hope. Still, the fuel-cell approach is obviously worth researching, and now researchers have lit upon a particularly promising fuel source. Oh yes, urine:

Using hydrogen to power cars has become an increasingly attractive transportation fuel, as the only emission produced is water - but a major stumbling block is the lack of a cheap, renewable source of the fuel. Gerardine Botte of Ohio University may now have found the answer, using an electrolytic approach to produce hydrogen from urine—the most abundant waste on Earth—at a fraction of the cost of producing hydrogen from water.

Urine's major constituent is urea, which incorporates four hydrogen atoms per molecule—importantly, less tightly bonded than the hydrogen atoms in water molecules. Botte used electrolysis to break the molecule apart, developing an inexpensive new nickel-based electrode to selectively and efficiently oxidise the urea. To break the molecule down, a voltage of 0.37V needs to be applied across the cell—much less than the 1.23V needed to split water.

Good to know! Meanwhile, there's an opposing school of thought that, while piss-powered cars are awfully promising, we should really be conserving our urine for other, more important ecological purposes:

However, Logan does feel that it would be a good idea to start saving up our urine—although not for the hydrogen. 'You have to remember about the P [phosphorus] in pee—globally we need to start thinking about conserving phosphorus for fertiliser, because, just like oil, one day the deposits are all going to run out and we need to start building phosphorus recycling into our infrastructure,' he says.

More on peak phosphorous here.