Micro- and nanostructured pyrolytic carbon electrodes for biophotovoltaic applications

The growing interest in renewable energy sources to address climate change and reduce dependency on fossil fuels underscores the importance of the development of new alternative energy technologies like biophotovoltaics (BPVs). BPVs offer a relatively novel approach by utilizing the capabilities of photosynthetic organisms to convert solar energy into electrical energy. The light is absorbed in the thylakoid membranes of the photosynthetic organism, resulting in the photocatalytic splitting of water molecules into protons, oxygen, and electrons. These electrons then continue in further reactions of photosynthesis or can be attracted by the external electrode (anode) passing through the electrical circuit to the cathode, resulting in the electric current generation. Moreover, BPVs are unlike other methods of solar energy conversion, capable of electricity production even in lack of light due to other metabolic processes, which also generate electrons. The photocatalytic split of water during photosynthesis is a remarkably efficient reaction with close to 100% quantum efficiency under optimal conditions. However, electron transfer efficiency from these organisms to external electrodes remains a significant limitation.
This thesis explores the potential of pyrolytic carbon (PC) electrodes as biophotoanodes for BPV systems, focusing on how different surface structures affect the photocurrent generation with the cyanobacteria Synechocystis sp. PCC 6803. This multidisciplinary research investigates using 3D micro-structured and nano-structured PC electrodes to enhance this electron transfer process. The project employed a variety of microfabrication techniques to create electrodes with increased surface roughness and area to promote bacterial attachment and improve the interface for electron transfer. Moreover, different electron transfer regimes were tested: direct electron transfer (DET), investigating direct interaction between the electrode surface and the attached cyanobacteria; mediated electron transfer (MET), employing an exogenous mediator for leveraging the electrons from the surface of the bacteria to the electrode.
Three main categories of electrodes with different surface structures were tested with the cyanobacteria utilizing a custom testing setup and developed procedure: nanostructured, microstructured, and hierarchically structured electrodes, combining both micro- and nanostructures. The research demonstrated that all electrode configurations enhanced photocurrent generation via DET compared to conventional 2D electrodes, with hierarchically structured electrodes showing the most notable increases. Nanostructured electrodes were tested for MET, which showed a further increase compared to DET. However, higher applied potentials were necessary for MET measurement.
Overall, this project developed a suitable testing system for biophotoanodes for BPV application, demonstrated the suitability of PC as a material for this purpose, and the possibility of performance enhancement by introducing various surface structures and exogenous mediator. Importantly, the biophotoanodes exhibited remarkable stability, standing out in the current state-of-the-art and bringing interesting insights into the research of BPVs.