Free-floating Reaction Centers (RCs) versus Attached Monolayer of RCs in Bio-photoelectrochemical Cells

Abstract

The high quantum efficiency (~100%) in the bacterial photosynthetic reaction center (RC) has inspired research on the application of RCs to build protein based solar cells. Conventionally, applying RCs as the photosensitive layer on the surface of a carbon electrode has shown poor photocurrents in the cells. The low photocurrent is partly due to the weak absorption of light in the monolayer of RCs. Also, an Atomic Force Microscopy image of the electrode shows lots of defects on the immobilized RCs at the electrode surface. In this work, we have built a biophotoelectrochemical cell in which the RCs are floating in the electrolyte instead of being attached to the surface of an electrode. Despite the simple structure of the cell, the photocurrent is significantly higher in the new cell compared to when RCs are attached to an electrode. The amplitude of current reached to ~40 nA for free floating RCs, about five times larger than that in the cell with attached RCs. The aging effect was studied in both cells in a course of a week. The lifetime of attached RCs on electrode surface was slightly better than solubilized RCs in the electrolyte. Also, it is found that the mechanism which governs the charge transfer from RCs to the electrodes is the same in both bio-photoelectrochemical cells.

Hybrid Wiring of the Rhodobacter sphaeroides Reaction Center for Applications in Bio-photoelectrochemical Solar Cells

“…The increasing demand for the production of energy without a direct link to combustion of a fossil fuel and the accompanying production of CO 2 has brought attention to the deployment of biomolecules for fabrication of biophotoelectrochemical cells. The biophotoelectrochemical cell uses technologies that exploit biomimetic means of energy conversion by utilizing plant-derived photosystems. , Different types of protein complexes may be employed to fabricate a biophotoelectrochemical cell, including reaction centers (RCs) from the Rhodobacter (R.) sphaeroides bacterium, plant photosystems, and bacteriorhodopsin proteins. − Several studies of the R. sphaeroides RC have shown promise for the utilization of this RC in biophotoelectrochemical cells. ,,− ,− The RC is a transmembrane protein that has nearly 100% quantum yield of primary charge separationi.e., the formation of charged primary donor (P + ) and final acceptor (Q B – )and an efficient stabilization of separated charges. − Most RC-integrated photoelectrochemical cells fabricated to date have been comprised of a cell with isolated RCs or RCs surrounded with a light harvesting (LH) pigment–protein antenna attached to a working electrode, immersed in an electrolyte with one or more redox mediators. ,,,,,,,, The use of RC–LH pigment–protein by several groups has shown improved photocurrent densities over those obtained with the RC alone. ,, Although the RC’s internal quantum efficiency is very high and the use of LH ring around the RC was shown to enhance the photon absorption, , the charge transfer between RCs and electrodes is another feature that influences biomolecule-based solar energy conversion.…”

Large Photocurrent Response and External Quantum Efficiency in Biophotoelectrochemical Cells Incorporating Reaction Center Plus Light Harvesting Complexes

“…The long recombination time and the expected greater than 20% power conversion efficiency of potential devices based on the Rb. sphaeroides RC, have inspired several research groups to utilize RCs in photoelectrochemical cells for harvesting solar energy. ,− In such devices, diffusible mediators were used to transfer light-induced charges from the RCs to the electrodes through reversible redox reactions . However, the limited optical absorption spectrum and extinction coefficient of the RC complex is a concern for achieving high photocurrent in a cell.…”