Electrochemical wiring of cyanobacteria to anodes using polymers towards biohybrid devices for solar-chemical production

Abstract

To accelerate the phasing out of fossil fuels, the innovation of sustainable technologies that convert solar energy to chemical energy will be crucial. Bio-photoelectrochemical devices using living microorganisms as catalysts for solar-fuel conversion could fill this gap. As a material, living microorganisms are abundant, capable of self-repair and reproduction, and possess excellent catalytic capabilities. The integration of photosynthetic microorganisms which perform solar-driven water oxidation into such devices is hindered by poor electron exchange efficiencies with the electrode. Polymeric mediators may be introduced at the cell-electrode interface to improve charge transfer by providing a direct route for electrons to reach the electrode, overcoming sluggish diffusional kinetics. At present, the polymer properties that are key for efficient wiring of cyanobacteria to electrodes are poorly understood. Bridging this knowledge gap will be essential for guiding rational design of next-generation polymers specifically tailored to achieve maximal output. In this work, we systematically tested two common polymers — the conjugated polymer poly(3,4-ethylenedioxythiophene) (PEDOT) and an osmium-based redox polymer — in terms of their ability to act as wiring tools for the model cyanobacterium Synechocystis sp. PCC 6803 on three-dimensional electrode structures. By using tailored analytical photoelectrochemistry methods, 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), immobilisation geometry (layered vs mixed), light management properties, polymer morphology and polymer loading towards optimal photocurrent outputs were deconvoluted. Under the conditions tested here, the osmium-modified electrodes produced higher photocurrents vs the PEDOT-modified electrodes (62-fold vs 27-fold enhancement respectively) relative to unmodified electrodes at a low light intensity. The success of the osmium polymer system was partially attributed to the electrostatic interaction between the polymer chains and the extracellular polymeric substances produced by the cells, enabling it to adopt a configuration conducive to efficient wiring. Longevity tests were conducted to assess the capacity of cyanobacteria to sustain enhanced outputs in the presence of the osmium-based polymer over an extended period. Although the polymer-mediated system exhibited superior stability compared to that obtained using a state-of-the-art diffusional mediator, further improvements in overall photocurrent densities and longevity are needed for practical applications. In the final section, we attempted to demonstrate the first iteration of a bio-photoelectrochemical device that uses solely microbial catalysts to drive solar-to-chemical energy conversion. This proof-of-concept system connected the osmium polymer-modified cyanobacteria-based anode, which performs light-driven water oxidation, to a Geobacter-based cathode, which reduces fumarate to succinate during respiration. Despite our efforts, we were unable to confirm electrons derived from water oxidation were ultimately used to produce succinate, due to complex and overlapping metabolic pathways in Geobacter, which made it challenging to precisely track electrons originating from the anode. While this presented a setback, we established an adaptable, general framework for wiring bacteria in a bioelectrochemical system, identified critical bottlenecks, and devised strategies to address them. The fundamental insights gained here provide a valuable roadmap for future work in developing sustainable, standalone energy storage technologies that use living catalysts.