By Alex Rettie, The University of Texas at Austin, Materials Chemistry for Energy Group and MRS Student Chapter
Big Picture - Photoelectrochemistry for solar fuels
Photoelectrochemical (PEC) cells convert solar energy to chemical fuels such as hydrogen or methanol, potentially generating clean energy from a renewable source.1,2 At the heart of these cells are semiconducting photoelectrodes that absorb sunlight and convert it to charge, which, for example, can be used to split water into hydrogen and oxygen. Doing this process efficiently and stably has been a lofty goal of the research community for nearly 40 years.
Overview – Bismuth vanadate photoanodes
A relatively new contender for the photoanode material in PEC cells is bismuth vanadate (BiVO4), a bright yellow powder that is used as a pigment in paint. It absorbs an acceptable amount of sunlight to be efficient, as well as having well-positioned energy levels for use in a “tandem PEC cell” with two light-absorbing electrodes to split water.3 Improvements in doping, morphology, and electrocatalysts have recently led to record efficiencies. Abdi et al.4 optimized a system consisting of a compact thin film of tungsten-doped BiVO4 and an amorphous silicon solar cell, achieving a laboratory solar-to-hydrogen efficiency of 4.9% – the highest reported to date. Comparable photocurrents were demonstrated by Kim and Choi5 using an undoped, nanoporous BiVO4 electrode via electrodeposition, coupled with a nickel and iron oxyhydroxide electrocatalyst. This is very promising considering their system is not fully optimized.
Although impressive device performance is upon us, charge transport and electrocatalysts are limiting factors in efficiency and are poorly understood. However, as our mechanistic understanding is bolstered by fundamental property measurements and new catalysts are discovered, expect efficiencies of BiVO4-based PEC cells to rise.
Allen Bard’s seminal overview of the field of photoelectrochemistry
1. Bard, A. J., “Photoelectrochemistry,” Science 207, 4427 (1980)
Harry Gray’s look at the rise and his vision for the promise of photoelectrochemisty and solar fuels
2. Gray, H. B., “Powering the planet with solar fuel”, Nature Chem. 1, 7 (2009)
Very recent paper demonstrating water splitting from a BiVO4-Cu2O cell, with operation limited by electrocatalyst detachment
3. Bornoz P., et al., “A bismuth vanadate-cuprous oxide tandem cell for overall solar water splitting,” J. Phys. Chem. C, (2014)
Paper reporting record efficiency from a system using all low-cost materials and gradient doping of tungsten to enhance charge separation
4. Abdi, F. F., et al., “Efficient solar water splitting by enhanced charge separation in a bismuth vanadate-silicon tandem photoelectrode,” Nature Comm. 4, 2195 (2013)
Paper detailing alternative approach to maintaining charge separation in BiVO4 through a nanostructured morphology
5. Kim,T. W. and Choi K.-S., “Nanoporous BiVO4 Photoanodes with Dual-Layer Oxygen Evolution Catalysts for Solar Water Splitting,” Science 343, 990 (2014)