The device bears no resemblance to Mother Nature’s counterparts on oaks, maples and other green plants, which scientists have used as the model for their efforts to develop this new genre of solar cells. About the shape of a poker card but thinner, the device is fashioned from silicon, electronics and catalysts, substances that accelerate chemical reactions that otherwise would not occur, or would run slowly. Placed in a single gallon of water in a bright sunlight, the device could produce enough electricity to supply a house in a developing country with electricity for a day, Nocera said. It does so by splitting water into its two components, hydrogen and oxygen.Note that he says "supply a house in a developing country", and does not give a figure as to the actual amount of energy produced per unit area.
I was going to hammer him on the energy content of one gallon of water, but it turns on gallon of water has 0.42 kg of hydrogen which will yield 16.7 kWh when recombined with oxygen (either by burning or in a fuel cell). That's a little less than my daily average and some (or a lot) will be lost to inefficiency, but at least it is in the right ballpark. I still disagree that a credit card sized solar cell will receive enough sunlight to do that conversion in a day unless a lot more light is focused on it.