Bacterial Biofilm As Active Matrix for Catalytic Centers: Electroreduction of Carbon Dioxide
Authors: Seta Ewelina , Rutkowska Iwona Agnieszka , Kulesza Pawel J 
Published: February 2020

There is constantly growing interest in the electrochemical reduction of CO2 and applying the process of binding the carbon to synthetic fuels production, as well as alcohols, hydrocarbons, aldehydes or carboxylic acids. Because of the stability of the CO2 molecule, its electrocatalytic reduction is characterized by high over-potentials: consequently selective and efficient electrocatalysts are needed.
A typical approach to efficient electroreduction of carbon dioxide is the use of metallic catalysts (e.g. Cu, Pd, Pt, Ru), either in heterogeneous and homogeneous reactions. Recently, the alternative for efficient CO2 electroreduction is also utilization of biological systems, including microbial electrocatalysts and whole cell biocatalysts. These systems often utilize microorganisms in a form of bacterial biofilms at the electrode surfaces, to enhance rates of chemical reactions. Biofilms are very stable layers composed of microbes well-adhering to various surfaces. Differentiation in microbial metabolic pathways, associated with the occurrence of different types of enzymes in microbial cells, creates opportunity to catalyze redox processes at ambient conditions, also carbon dioxide electroreduction. Methods for improvements of electrical connection between microbial cells and electrode surfaces are needed, and one of the ideas is to utilize highly porous conductive polymers, which provide good electrical coupling and facilitate immobilization of the cells. Practical bioelectrochemical systems often use also carbon nanotubes together with biological layers due to their capabilities of penetrating microbial structures and improving the overall conductivity. These properties are particularly important in developing biosensors and microbial fuel cells.
There has been proposed a hybrid multilayered matrix, composed of polyaniline-supported Yersinia enterocolitica biofilm, multi-walled carbon nanotubes (MWCNTs) and dispersed Pt nanoparticles (nPt) on the top. The underlayer of polymer, biofilm and MWCNTs has been demonstrated to work as highly active assistance for nPt during carbon dioxide electroreduction in neutral electrolyte (phosphate buffer at pH=6.1). Application of the hybrid system allows clear distinction of the CO2-reduction currents from those originating from the hydrogen evolution, unlike the bare nPt dispersed at glassy carbon surface. The result complies with the enhancement effect toward process of carbon dioxide reduction rather than hydrogen evolution. Bacterial biofilm itself is not catalytically active toward reduction of CO2, however its hydrated structure allows water molecules to easily move through it, and permits aqueous electrolyte to flow undisrupted at the electrocatalytic interface. Application of polyaniline can be commentated in terms of improvement and stabilization of the biofilm adherence and utilization of MWCNTs improve charge distribution within the hybrid film. Assessing based on the results of the diagnostic stripping-type voltammetric experiments, carbon monoxide seems to be the main product of CO2-reduction process at the proposed hybrid layer-supported nPt catalyst. However formation of some quantities of formic acid and methanol cannot be excluded.