the field stalled. But it has been reborn in several labs thanks to
a renewed focus on the energy problem and climate change—and
because of the emergence of new technologies.
Yang’s lab is improving on a basic design that was developed
in the 1970s at the National Renewable Energy Laboratory. It
has two light-sensitive electrodes coated with a catalyst—Yang
is using nickel, which is inexpensive—that together split water
into oxygen and hydrogen. In the original setup, the electrodes
were flat, but Yang instead uses arrays of nanowires made from
silicon and other semiconductors. Because the nanowires have
100 times the surface area of flat electrodes that could fit into the
same space, they can hold more of the catalyst, greatly boosting
the efficiency of the reaction.
However, splitting water is the easy half of photosynthesis.
Plants go further, using the hydrogen from water in reactions that
turn carbon from the air into complex molecules. Yang wants to
do this too. After all, our planes and cars don’t run on hydrogen;
they need gasoline and other chemically complex fuels.
To catalyze that part of the process, Yang relies on another
technology that wasn’t around in the ’70s. He and colleagues
have shown that genetically engineered bacteria nestled amid
the nanowires function as “living catalysts.” They take up the
hydrogen split from the water and combine it with carbon dioxide to make methane and other hydrocarbons that are needed
for fuels or plastics. The bugs do this with natural enzymes that
carry out a series of reactions chemists have not yet been able to
master with synthetic catalysts.
Yang’s system currently matches the efficiency of photosynthesis, storing under 1 percent of the energy captured from
sunlight in the form of chemical bonds. That’s not bad for a
proof-of-concept demonstration, but making it more efficient
and thus cost-effective will be essential.
Yang hopes to eventually switch to synthetic catalysts instead
of bacteria, which are tricky to keep alive. But fully eliminating
the bugs might not be necessary, given the urgent need for clean
fuels. “If it has to be a hybrid approach, that’s okay,” he says.
6 and 7 Some bacteria in the
system produce methane,
which can be used directly as
a fuel; others make acetate,
which is fed to other genetically
engineered bacteria to make
fuels and plastics. Here,
engineered E. coli feed on
8 Analytical tools including mass
spectrometers are used to
verify that the bacteria made
the desired chemical. So far, the
system is as efficient as natural