Chemists turn carbon dioxide into fuel with GaP
A research group at the University of California, San Diego, has shown how compound semiconductor solar cells can convert atmospheric carbon dioxide into carbon monoxide - a useful gas for industrial chemists and, potentially, a fuel.
Cliff Kubiak and graduate student Aaron Sathrum used p-type GaP and GaAsP to harness enough energy from sunlight to split carbon dioxide molecules via an electrochemical reaction.
The semiconductor material acts as an electrode, and in conjunction with two thin layers of chemical catalysts, it produces both carbon monoxide and oxygen through electrolysis.
Although the work is at a very early stage, it could one day result in a simple way to simultaneously reduce the atmospheric concentration of carbon dioxide, and produce a gas used in detergent and plastics manufacturing.
Sathrum explained why the group chose GaP for its experiment: "First, p-GaP has a large bandgap of 2.26 eV, which can generate a sufficiently large photovoltage for splitting carbon dioxide."
"Second - and a critical point for conversion efficiency - the energetic positions of the valence and conduction bands [in GaP] are very close to the electrochemical potentials for the two half-reactions required for splitting carbon dioxide."
The team used commercially-available 2-inch GaP wafer material to fabricate the electrode. Sathrum says that although multi-junction solar cells could provide higher photovoltaic conversion efficiency, it is of greater importance that the energy levels in the semiconductor are well-matched to the energy required to split carbon dioxide molecules.
"The bandgap of the materials used in multi-junction solar cells are smaller, and may not be able to provide a high enough internal potential to drive the splitting reactions," Sathrum explained.
While the increasing level of atmospheric carbon dioxide has become the cause célèbre of the current enviro-political agenda, one problem for the UCSD team is that the concentration of the greenhouse gas remains vanishingly low at under 0.3 percent.
This means that converting large quantities of carbon dioxide into the more useful chemicals will be challenging. On top of that, the cost of p-type GaP remains quite high, and the fabrication of reliable electrical contacts is something of a black art, says Sathrum.
• In similar research, a team at the University of Tokyo in Japan has developed a way to split water molecules and create hydrogen gas, which could be used to power fuel cells.
According to a report at Nikkei.net, Kazunari Domen's team used GaN and ZnO powders to harness 400-500 nm light for the photolysis reaction.
