Artificial photosynthesis: Advanced nanomaterials and use of biocatalysts for novel photoelectrochemical cells

Abstract

The capture and storage of solar energy in chemical form by a photoelectrochemical (PEC) cell is an elegant and simple solution to the increasing energy demands and needs for chemicals and fuels from non-fossil sources. We first developed a facile fabrication method of a monolithic solid-state PEC (SSPEC) cell, which utilizes a solid-state polymer electrolyte and minimizes the distance between the two electrodes. The recent progress in understanding of surface proton conduction in porous ceramic-based electrolytes led to the idea of using the SSPEC cell for hydrogen production through water vapor splitting. The traditionally Pt-based cathode was replaced by an earth-abundant photocathode, g-C3N4, which increased the intrinsic photo-induced electrical field by a factor of three. Such a monolithic SSPEC cell working fully in the gas phase was demonstrated first time, and can play an important role for clean energy production in rural or other areas where grid infrastructure and clean water sources are limited or absent. CO2 utilization is considered a key player in a carbon-free energy economy, emphasising the importance of CO2 capture and conversion. We have successfully demonstrated how a bio-catalyst (enzyme) can be coupled with traditional chemical engineering for this purpose. To approach this, a stable, highly photo-active material - Ta3N5 nanotubes - has been synthesized and tested in a photoelectrochemical cell. It reaches close to its theoretical performance in photo-assisted water splitting. We then coupled the Ta3N5 nanotubes as the photoabsorber together with an optimised enzyme - formate dehydrogenase (FDH) - in a photoelectrochemical cell. The aim is to mimic natural photosynthesis to reduce CO2 into a valuable chemical form – formic acid. A solar-driven reduction of CO2 and water to formic acid at close to 100% faradaic efficiency has been reached.