With the sun and water being our planet's most abundant resources, it's important to know how best to exploit them in today's energy transition. One strategy is to store energy from the sun in chemical form to produceH2 and O2, and then convert it into electrical energy via the fuel cell. This requires a mastery of photocatalysis/electrolysis reactions to take advantage of the properties of semiconductor junctions - liquid to dissociate water intoH2 and O2. The physical laws of photocatalysis and photoelectrocatalysis are well known, but we are slow to develop high-performance photoelectrolyzers. Here again, it's mainly a question of materials: which photoanodes and photocathodes ? How can their band gaps be adjusted? Which catalysts, which electrocatalysts ? These are just some of the questions that remain unanswered.
A combination of chemical composition and elaboration (size-morphology-microstructure) at the material level has made it possible to increase photocurrent, while the judicious choice of catalysts has reduced overvoltage. Currently, the most efficient material for photoelectrolysis (~6%) is a nano-textured Fe203 containing IrO2 as catalyst. To overcome this barrier, current work is focused on the fabrication of electrocatalysts obtained either by surface-grafting chemistry or by in situ electrodeposition; the most promising to date being Co-Pi, a material obtained by electrodeposition from a Co(NO3)2 and K3PO4 electrolyte bath. Progress on the device front is also significant, with the development of tandem and hybrid cells taking advantage of nanostructuring to mimic the " Z pattern " of photosynthesis. The dream is to make an artificial leaf of simple design from abundant materials and with a minimum of engineering (D. Nocera, MIT). Since the sun is an unlimited source of energy, no matter what the yield, the only real challenge remains the realization of large-scale, reliable and low-cost wireless electrolyzers. There's still a long way to go.
