Live Cell and Hybrid Material Based Bio-Photoelectrochemical Cells for Clean Solar Energy Conversion

Abstract

Photosynthetic organisms and complexes are attractive starting materials for solar energy conversion (SEC). I will describe here how the remarkable photocatalytic activity of the photosynthetic apparatus can provide overall water splitting with oxygen and hydrogen production in Bio-Photo-Electro-Chemical (BPEC) cells (Fig. 1). Use of isolated photosynthetic systems tightly associated with different electrodes has the benefit of potentially high reaction center density. However, the cross-section for light absorption is poor, leading to sub-optimal activity. Disconnection from biological repair mechanisms leads to limited lifetime. On the other hand, use of intact, live systems have the benefit of evolutionarily optimized light harvesting antennas for improved energy absorption and transfer, and all of these organisms have excellent repair systems leading to essentially endless activity. However, how these intact systems can be functionally connected to the BPEC depends on the morphology, permeability and flux of electron carriers. We will show here that we can obtain significant electrical current using a variety of live photosynthetic organisms, each with benefits and drawbacks. Cyanobacteria1,2 and microalgae3 are applied directly to the BPEC anode, providing continuous current densities in the 10’s of μA/cm2. Use of photosynthetic tissues from macroalgae (seaweeds)4 or higher plants5 require tight association with metallic anodes (stainless steel, aluminum, etc.) leading to current densities of >50mA/cm2, with high ionic strength electrolyte solutions supporting these increased current densities. Succulents can serve as their own BPEC6. In all cases, the major electron carrier is NADPH which is released by the cells and tissues into the electrolyte. A different strategy is to use isolated photosynthetic complexes and chemically connect them to the electrochemical cell. To overcome the problem of lower number of light absorbing chromophores connected to each reaction center we can use either isolated biological light harvesting (LH) complexes or nanoparticles (NPs). As Photosystem II (PSII) is the only enzyme that catalyzes light-induced water oxidation, we focus on its use in these systems. PSII can be isolated from a wide variety of organisms, each with relative benefits or disadvantages: isolation ease and cost, quantity per cell, stability to heat or other external stress, etc. PSII contains chlorophyll a which absorbs well in the 400-500nm and 600-700nm region, limiting the quantum efficiency in the gap in the green absorption region (500 - 600 nm). To overcome this limitation, we have used two strategies: stabilizing the interaction between PSII and isolated thermophilic cyanobacterial Phycobilisomes LH complex within an Os-complex-modified hydrogel on macro-porous indium tin oxide electrodes (MP-ITO). This results in notably improved, wavelength dependent, incident photon-to-electron conversion efficiencies7. Another strategy was to isolate market-purchased spinach PSII (cheap and abundant) and associate them via a unique binding shell to gold-nanoparticles (Au-NPs)8. These Au-NP were shown to serve as both light harvesting modules in the green gap, as well as conduits of electrons from the bound PSII to the BPEC, leading to record current densities were obtained. Figure 1