Chemist Daniel Nocera of Harvard University and his team joined forces with synthetic biologist Pamela Silver of Harvard Medical School and her team to craft a kind of living battery, which they call a bionic leaf for its melding of biology and technology. The device uses solar electricity from a photovoltaic panel to power the chemistry that splits water into oxygen and hydrogen, then adds pre-starved microbes to feed on the hydrogen and convert CO2 in the air into alcohol fuels. The team’s first artificial photosynthesis device appeared in 2015—pumping out 216 milligrams of alcohol fuel per liter of water—but the nickel-molybdenum-zinc catalyst that made its water-splitting chemistry possible had the unfortunate side effect of poisoning the microbes.
So the team set out in search of a better catalyst, one that would play well with living organisms while effectively splitting water. As the teamreports in Science on June 2, they found it in an alloy of cobalt and phosphorus, an amalgam already in use as an anticorrosion coating for plastic and metal parts found in everything from faucets to circuit boards. With a little charge, this new catalyst can assemble itself out of a solution of regular water, cobalt and phosphate—and phosphate in water actually is good for living things like the Ralstonia eutrophabacteria that make up the back half of the bionic leaf. Run an electric current from a photovoltaic device through this solution at a high enough voltage and it splits water. That voltage is also higher than what is needed to induce the cobalt to precipitate out of the solution and form the cobalt phosphide catalyst, which means when the bionic leaf is running there are always enough electrons around to induce the catalyst’s formation—and therefore no excess metal left to poison the microbes or bring the bionic leaf’s water-splitting to a halt. “The catalyst can never die as it’s functioning,” Nocera says, noting that the new artificial leaf has been able to run for up to 16 days at a stretch.