Found this reference on a search of the chemical composition of urine:
"Urine contains 95% water and 5% solids. More than 1000 different
mineral salts and compounds are estimated to be in urine. So far, our
scientific community knows of about 200 elements. Some substances are:
vitamins, amino acids, antibodies, enzymes, hormones, antigens, interleukins,
proteins, immunoglobulins, gastric secretory depressants, tolergens,
immunogens, uric acid, urea, proteoses, directin, H-11 (a growth inhibitory
factor in human cancer), and urokinase. Believe it or not, scientists have
know for years that urine is antibacterial, anti-protozoal, anti-fungal, anti-
viral, and anti-tuberculostatic! "
So what is required for a good electrolyte material in a fuel cell? I have no idea; but if you can find a cheap material to go inside of a type of fuel cell, then you will have solved the biggest barrier to using a fuel cell: COST!!