From Photosystem I to Bio-Solar Cells


Scheme of the Design Strategy of Creating Bio-Solar Cells (adopted from Meshin et al., 2012).

A group of researchers from the Massachusetts Institute of Technology, the University of Tennessee and the Ecole Polytechnique Federale de Lausanne has presented an approach harvesting electrical energy from the integral membrane protein complex photosystem I. The strategy elaborated in the present study arrives at an energy conversion of 0,1 %. While this factor is way too small for any real-word application it means an improvement of a factor of 10.000 with respect to a previous work originating from the same principle investigator.
The paper by Mershin and collaborators which was published on the open access journal Science Reports oriented its principle idea on a previous work published by MIT researcher Shuguang Zhang and collaborators from 2004. The key idea is based on relying on the photosystem I (PS I) which is involved in a plant’s photosynthesis. The PS I permits to convert light energy into electron transfer. The original approach derived the PS I from plants and layered them on a glass substrate. However, the assembling and stabilizing needed the use of rather expensive chemicals and sophisticated lab equipment. Additionally, the resulting solar cell efficiency was orders of magnitude too weak to be of any practical use.
The refined preparation strategy allows a more straightforward assembly which should be easily repeatable and which may resolve one of the problems related to the field of biophotovoltaics, namely the extensive preparation and, hence, the poor repeatability. In the particular case one isolated the PSI from the thermophilic cyanobacteria Thermosynechococcus elongates which was then air-dried on nanostructured semiconducting substrates. Then the circuits were completed by liquid electrolyte and platinized glass as it is displayed in the figure. In contrast to the traditional flat electrode geometry the approach consisted in copying a strategy developed by nature, in the particular case of pine trees.
Basically, the development strategy was inspired by the observation that while most of the pines have bare trunks and a canopy of branches only at the very top, a few have small branches all the way down the length of the trunk, capturing any sunlight which travels through from the top to the bottom. Hence, Mershin and collaborators used nanocrystalline titanium dioxide or zinc oxide nanowires creating mesoscopic, high-surface area semiconducting electrodes. These provided an increased effective surface area for PS-I adsorption and light harvesting. The chosen setup reached at an energy conversion factor of about 0,1 %.
In nature PS I catalyzes light-driven electron transfer from reduced plastocyanin which is a copper-containing protein to ferredoxin which are iron-sulfur proteins. In the solar cell setup the plastocyanin was replaced by the Co(II)/Co(III) ion-containing electrolyte and the role of the ferredoxin was simulated by either the nanocrystalline titanium dioxide or the zinc oxide nanowires.

While the choice of both titanium dioxide or zinc oxide come along with the advantage that these  absorb UV light which generally tends to damage PS I they provide varying advantages and disadvantages. Both have larger effective surface areas with respect to a flat design with a significant higher figure of the nanocrystalline titanium dioxide structure. Zinc oxide, however, is less expensive and allows a one-hundred times faster charge carrier mobility. On the other side again the zinc oxide nanowire growth is simpler, requiring fewer steps, less energy and should be more easily adaptable to flexible conducting substrates. However, so far the performance of zinc oxide photoanodes have always underperformed when compared to identically sensitized titanium dioxide. This is ascribed to their to their lower roughness factor, poor dye loading, and the shunting of the photocurrent by the corrosion of ZnO. In their study the authors claimed that the short-circuit current densities differed by about a factor of ten in favor of the titanium dioxide solution which is mainly ascribed to the higher effective surface area.
Finally, the authors around the main author Mershin believe that since they have not performed any optimization “significant efficiency gains can be expected from increased loading, better oriented and more tightly coupled PS-I to photoanode, customization of stabilizing agents and better matching of bio-friendly electrolytes with photoanode/photocathode substrates.”
Further, the vision is that the design and methodology principles may stimulate other researchers to extend their research activities with respect to biophotovoltaics which is seen to be critical to get forward with these type of bio-solar cells. The long term wish is that encouraging others may lead to innovative optimizing strategies which one day may lead to bio-solar power that is truly ‘‘green’’.

Sources:
Mershin et al.
Self-assembled photosystem-I biophotovoltaics on nanostructured TiO2 and ZnO

Scientific Reports 2 : 234, 2012

Das et al.
Integration of Photosynthetic Protein Molecular Complexes in Solid-State Electronic Devices

Nano Letters 4: 1079-1083, 2004

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