Here in sunny Orlando the convention center is all abustle with sneaker-and-suit-wearing scientists including a few who know some things about alternative energy research. Yesterday, in a session entitled “New Frontiers in Electrochemical Energy Conversion and Storage” several up and coming new faculty presented their latest research. Highlights from two of the presentations are described below:
David E Cliffel, Vanderbilt University
Title: Plant Power: Electrochemical Energy Conversion Using Photosystem I
Content: Although, as we have learned in class, photosynthesis in plants is highly inefficient at converting sunlight into usable energy, individual components of photosynthesis are actually very efficient. One of these, photosystem I, has a 97% conversion of incident light into electron/hole pairs. Dr. Cliffel has isolated this protein complex and immobilized many complexes onto an electronically conductive array. Importantly, the immobilization process appears not to denature the protein to the degree that the proteins are still electronically active. A set of cells powered by photosystem I has been used to operate a calculator, as shown in this youtube:
Although the process has yet to be optimized, the high efficiency of photosystem I gives it great potential as a future source of solar energy.
Broader impact/connection to policy: The development of new sources of photo-induced energy is important to society, and though this is a very underdeveloped system it diversifies our ability to harness the power of light. Dr. Cliffel’s research was largely funded by the NSF division of materials research, whose mission is (in part) “to make new discoveries about the behavior of matter and materials; to create new materials and new knowledge about materials phenomena.” Without this government support his research would not be possible.
“Photosystem I-based biohybrid photoelectrochemical cells.” Ciesielski, P.N.; Hijazi, F.M.; Scott, A.M.; Faulkner, C.J.; Beard, L.; Emmett, K; Rosenthal, S.J.; Cliffel, D.; Jennings, G.K. Bioresource Technology 101 (2010) 3047-3053.
Shannon W Boettcher, California Institute of Technology
Materials for Solar Energy Conversion: Photoelectrochemical Power and Hydrogen Production from Silicon Rod Arrays
Content: Dr. Boettcher has spent his postdoc years at CIT developing a new silicon-based photochemical cell. Instead of a 2D planar surface of silicon, the cell consists of a forest of silicon rods grown from a spot-deposited copper catalyst.
The geometry used here can “tolerate the use of low-purity Si with a short minority carrier diffusion length, while allowing high solar-energy conversion efficiencies, by providing a short minority carrier diffusion length equal to the wire radius.” (1) While conventional ultrapure 2D Si cells have the best solar efficiency to date their cost is extremely prohibitive to mass production. This technique offers a way to use cheaper Si (and less of it) with a higher conversion due to shorter diffusion length.
The take-home message from Dr. Boettcher’s talk is that the Si rod arrays have been optimized to be competitive with 2D polycrystalline Si cells in the arena of absorption. In a cell volume containing less than 5% Si, the rod arrays can absorb up to 85% of above-the-bandgap direct sunlight and may increase photovoltaic efficiency up to 20x due to effective optical concentration. For comparison, a commercial polycrystalline Si solar cell absorbs 87% of above-the-bandgap sunlight.
Broader impact/connection to policy: The struggle in the photovoltaic world has been one between cost and efficiency. While ultrapure Si-based cells have achieved a conversion efficiency of up to ~15%, their high cost makes them an unsuitable solution for mass production. The Si rod array system presented here has not been optimized for conversion efficiency, but shows potential because of the high optical concentration achieved. Scientists around the world are trying to find low-cost, highly efficient materials for the conversion of sunlight to electricity. They are funded largely by government sources. Dr. Boettcher was funded by BP as well as the DOE Energy Frontiers Research Centers (EFRC). The purpose of the EFRC is to “accelerate the scientific breakthroughs needed to build a new 21st century energy economy” which Dr. Boettcher’s work achieves through the optimization of optical concentration.
Most recent publication:
(1) “Enhanced absorption and carrier collection in Si wire arrays for photovoltaic applications.” Kelzenberg, M.D.; Boettcher, S.W.; Petykiewicz, J.A.; Turner-Evans, D.B.; Putnam, M.C.; Warren, E.L.; Spurgeon, J.M.; Briggs, R.M.; Lewis, N.S.; Atwater, H.A. Nature Materials. Published Online: February 14, 2010. DOI: 10.1038/NMAT2635.