Light-Harvesting Proteins and Biofilms on Iron Oxide Photoelectrodes

The so-called artificial photosynthesis has developed in at least two branches: semiconductor photoelectrochemistry with photoelectrochemical cells, and bio-molecular systems. Here we show bio-hybrid systems where metal oxide semiconductor photoelectrodes are fucntionalized at their surface with light harvesting proteins, specifically phycocyanin light antenna from blue-green algae. It has turned out in the early beginning of our studies that such funtionalization can yield a higher photocurrent and a higher hydrogen output. The actual underlying physico-chemical processes are not yet so well explored. We have therefore conducted electrochemical impedance studies on bio-hybride electrodes under physiological conditions such as to determine charge transfer processes. Moreover have we conducted x-ray photoemission studies on model systems in order to understand the valence band struc ture of the bio-electric interface and its charge transfer properties, particularly under dark conditions.

The first bio-hybrid electrodes were used by Melvin Calvin in the late 1950s where he sublimated chlorophyll on aquadag graphite interdigital electrodes. This was done merely to study the bio-organic material. Helmut Tributsch' dye-sensitized solar cells (DSSC) in the early 1970s were ZnO electrodes, i.e. semiconductors further functionalized with natural absorbing dye molecules. Since, DSSCs have developed as a field in photovoltaic solar energy conversion technology. Meanwhile, photoelectrodes for solar water splitting in photoelectrochemical cells (PEC) evolved as an independent field. This field has recently emerged again and is virtually taking center stage. Also here, functionalization of traditional semicondcutor electrodes with bio-organic motifs is gaining more and more interest. Hydrogenase is a frequently used protein for the promotion of hydrogen evolution on photocathodes. We have recently coated the light harvesting antenna protein C-phycocyanin on iron oxide photoelectrodes. This approach was quite promising. We measured an increrased photocurrent density at the photoanode and an increased hydrogen evolution at the counter electrode. During our studies we were pointed to the question of charge transfer across the bio-electronic interface which is formed by biomolecules and metal oxides. Because the expressioan and purification of C-phycocyanin, which is extracted from cyanobacteria, is a costly and laborious process, we explored also the use of complete cyanobacteria on the photoelectrodes. The thylakoid membrane and cell membrane may pose electrical barriers between the biocatalytic or light harvesting components in the photosynthetic apparatus and the semicondcutor electrode underneath. It is therefore necessary to quantify this barrier. For this, we are performing electroanalytical experiments and x-ray spectroscopy experiments operando and in situ, in the latter case with an actual anabaena biofilm with ambient pressure XPS on a bio-electrochemica cell.
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