Polymer-Modified 3D Electrode Interfaces for Efficient Charge Extraction from Photosynthetic Microorganisms

Cyanobacteria are abundant photosynthetic microorganisms that can be interfaced with electrodes in semi-artificial photosynthetic systems for solar-electricity or solar-fuel generation. [1] This approach relies on the ability of the living organisms to exchange electrons efficiently with the electrode. Redox-active and conductive polymers may be introduced at the cell-electrode interface to improve charge transfer efficiency by providing a direct route for electrons to reach the electrode thus overcoming sluggish diffusional kinetics. At present, the polymer properties that are key for efficient wiring of cyanobacteria to electrodes are poorly understood.<br/>In this study, we systematically tested and compared the capacity of a common osmium-based redox polymer (P_Os) and the conductive polymer poly(3,4-ethylenedioxythiophene) (PEDOT) to act as a wiring tool for <i>Synechocystis</i> sp. PCC 6803 on hierarchically-structured inverse opal indium tin oxide (IO-ITO) electrodes. By using stepped chronoamperometry and normalising photocurrents against cell loading, we were able to identify the conditions under which each polymer served only as an immobilisation matrix, enhancing the photocurrent by means of increasing the cell loading rather than by mediation. The contribution of various parameters including polymer type (redox vs conductive), deposition method (dropcasting vs electropolymerisation), deposition geometry (layered vs mixed), polymer morphology and polymer loading towards optimal photocurrent outputs were deconvoluted. Increasing the polymer loading in both cases led to an enhanced mediation effect up until a to a certain point when photocurrents were reduced presumably by a shading effect. This problem was overcome by changing the electrode geometry to micropillar ITO electrodes in which the light path can penetrate through the structure unimpeded. We also investigated the interaction of the polymer with a Synechocystis mutant lacking extracellular polymeric substances (EPS). The PEDOT-modified electrodes produced higher photocurrents overall vs the P_Os-modified electrodes (20-fold vs 5-fold enhancement respectively) relative to unmodified electrodes at a low light intensity of 1 mW cm^-2. However, the larger photocurrents extracted using PEDOT come at a thermodynamic cost, as the onset potential for mediation is 200 mV higher for PEDOT than for P_Os. Overall, this study identifies polymer properties that are essential for successful cyanobacteria-electrode wiring and informs future interface design strategies for photosynthetic organisms on complex 3D structures.<br/><br/><b>References</b><br/>[1] J. Z. Zhang, P. Bombelli, K. P. Sokol, A. Fantuzzi, A. W. Rutherford, C. J. Howe, E. Reisner, <i>Journal of the American Chemical Society</i> <b>2018</b>, 140, 6–9