Logan hopes that further modifying the chemistry of the brush will improve the results even more. "We now already know more about which types of stainless steel work best," he says. "And we will also want to minimize hydrogen bubbles being trapped between the bristles because this can make recovery of the gas less efficient."

He also emphasizes that high surface area is not everything. A brush made from carbon with an even higher surface area did 14 times worse than the naked steel-brush core, and when the researchers cut the steel brush in half to allow closer spacing of the two electrodes, they got even better results than with the full brush, even though they lost half of the surface area.

Lars Angenent, an associate professor of biological and environmental engineering at Cornell University, says that big challenges remain, and he argues that the effect of electrode spacing is going to be one of the biggest limitations of MEC technology. "I think this work is great, but the next question is, can you scale it up so it's economical?" he says. "In a larger system, moving ions through liquid between cathode and anode is more difficult, so you will produce less hydrogen per unit volume."

Patrick Hallenbeck, a professor of bacteriology at the University of Montreal, in Canada, agrees with Angenent that scaling will be a challenge. However, he is optimistic that with the platinum limitation gone, the outlook for MECs is good: "By showing that platinum can be effectively replaced by stainless steel, Logan's group have removed a critical barrier. These devices were first described only four years ago, and there has been tremendous progress since then. Further developments may very well move MEC devices into the realm of practical application."