The Howe Group, and collaborators from the Knowles Group in the Department of Chemistry, have published a new paper in Nature Energy.
The world's energy demand is at an all-time high, and is only set to increase with the rising global population, putting substantial strain on our current energy resources. With fossil fuels depleting and the threat of climate change from their use, alternative renewable energy sources are urgently required. Solar power represents a particularly attractive alternative, as the Earth receives around 10,000 times more energy from the sun than is currently required to meet the demands of the entire human population.1-2 Although photovoltaic devices are widely used to harvest solar energy, they have disadvantages of not being able to store energy and in requiring expensive materials for technology manufacture.3-8
In the last few years, the use of biological solar cells, known as biophotovoltaics (BPVs), has developed as a low-cost and environmentally friendly method for harvesting solar energy and converting it into an electric current.9-12 Within BPV devices photosynthetic microorganisms absorb light, transferring its energy to some of their electrons (electron excitation). These excited electrons then travel through a series of protein complexes (electron carriers), with some ultimately being exported outside of the cell. BPVs exploit these secreted electrons by directing them into an electrical circuit to generate current for powering devices.13-17 BPVs can also generate current in the dark, using compounds synthesised in the light.
To date, all demonstrated BPVs have consisted of a single compartment, with charging (light harvesting and electron excitation) taking place alongside power delivery (electron transfer to the electrical circuit).9 This has prevented individual optimisation of these two separate processes, which could account for the present low efficiency of BPVs compared with their synthetic analogues.
In their new paper, Saar et al. present a two chamber system where charging and power delivery are physically separated. Constructing BPVs in this way allows the two compartments to be designed independently for optimal performance. As fluids behave differently on minute scales (convective mixing is suppressed18), the Howe Group and collaborators miniaturised the power delivery unit, enabling them to omit a semipermeable membrane, required in single chamber BPVs to separate the device into positively and negatively charged compartments, from their design. Not only does this decrease the internal resistance of their two chamber BPV to reduce electrical losses, it also reduces the cost of the system and removes reported problems associated with membrane use (drying out, degradation, fouling and clogging19-21). By then replacing the Synechocystis algae in their BPV with a mutant strain genetically-modified to minimise non-productive loss of electric charge during photosynthesis, the researchers were able to create a BPV with a power density of 0.5 W/m2, five-times higher than that of previously described BPV devices.22-23
When asked about the next steps for this research, lead author Kadi Saar from the Department of Chemistry commented that "Independent optimisation of the core processes involved in the operation of BPVs has enabled us to achieve manifold advancement in BPV power outputs compared to previously demonstrated systems. To increase the output further we are looking to optimise the system design but also to evolve algal cells that would act as even more effective catalysts. At the same time we are looking to develop as good insight as possible for which specific scenarios such devices are the most useful. Semiconductor-based synthetic photovoltaics are usually produced in dedicated facilities away from where they are used. It is not impossible that in the areas where both water and photosynthetic organisms are abundant, the production of BPVs could be carried out directly by the local community."
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