Conducting-Polymer-Supported Bacterial Biofilms As Active Matrices for Noble Metal and Molecular Catalytic Centers: Enhancement of Electroreduction of Oxygen and Carbon Dioxide
Authors: Kulesza Pawel J , Seta Ewelina , Lotowska Weronika , Rutkowska Iwona Agnieszka , Wadas Anna 
Published: February 2020

Hybrid matrices composed of porous conducting polymer - polyaniline (PANI) or poly(3,4- ethylenedioxythiophene)  (PEDOT) - underlayer, robust bacterial biofilm and the multi-walled carbon nanotubes have been demonstarted to function as highly active supports for dispersed catalztic centers (both noble metal nanoparticles and molecular ligand complexes). We explore unique properties of biofilms, i.e. polymeric aggregates of microorganisms, in which cells adhere to each other on the electrode surfaces. Such systems are characterized by extracellular electron transfers involving c-type cytochromes (heme-containing proteins). Biofilms grown on inert carbon electrode substrates tend to exhibit electrocatalytic properties towards oxygen and hydrogen peroxide reductions in neutral media. The processes have been found to be further enhanced by introduction of multi-walled carbon nanotubes (MCNTs) that are modified with ultra-thin layers of organic (e.g. 4-(pyrrole-l-yl) benzoic acid. We expect here attractive electrostatic interactions between carboxyl-group containing anionic adsorbates and positively charged domains of the biofilm with c-type cytochrome enzymatic sites. Coexistence of the above components leads to synergistic effect that is evident from positive shift of the oxygen reduction voltammetric potentials and significant increase of voltammetric currents. Most likely, the reduction of oxygen has been initiated at the molecular (e.g. intentionally added cobalt porphyrin redox centers), whereas the undesirable hydrogen peroxide intermediate are further decomposed at the cytochrome sites.

Among other important observations is the significant enhancement of activity of Pt nanoparticles toward electroreduction of carbon dioxide in the presence of such a robust bacterial biofilm as  Y. enterocolitica. Although interactions between platinum and such a complex ecosystem as biofilm are difficult to describe precisely, it is reasonable to postulate some inhibition of the hydrogen evolution (competitive reaction) at platinum nanoparticles in presence of bacterial microcolonies in favor of enhancement of the CO2-reduction. It is also noteworthy that biofilms are highly hydrated, and existence of open water channels between the bacterial microcolonies facilitates flow of supporting electrolyte. Combination of the hydrophobic polymer (PANI) structures with hydrophilic bacterial aggregates seems to produce attractive hybrid supports for electrocatalysts operating in aqueous media. Based on numerous repetitive diagnostic experiments in both argon and carbon dioxide saturated electrolytes, it can be stated that, by growing and supporting the biofilm onto the PANI polymer underlayer, the overall stability of the biofilm-based system has been improved. Indeed, we have not effectively observed any degradation of the hybrid catalytic films (both in terms of electrocatalytic activity and physicochemical stability) during our prolonged diagnostic experiments.  Because measurements have been performed in neutral (phosphate buffer at pH=6.1) rather than acid medium, the PANI films (regardless of the presence of some HSO4
- anions) are not expected to be very well conducting. Consequently, we have also introduced MWCNTs to the electrocatalytic interface to facilitate charge distribution there.