"Botte recognized that urine contains two compounds that could be a source of hydrogen: ammonia and urea. Place an electrode in wastewater, apply a gentle current, and voila: hydrogen gas that can be used to power a fuel cell. Her system operates similarly to the electrolysis of water, a process that can be used to produce hydrogen for fuel cells - except that ammonia and urea hold their hydrogen atoms less tightly than water does, so less energy is required to split them off." (Quoted from article)