Importance of Specific Interactions between Support, Metal Oxide Nanostructures and Noble Metal or Organometallic Sites in Efficient Electrocatalytic and Bioelectrocatalytic Reduction of Oxygen

Systems utilizing carbon nanostructures (e.g. nanotubes) and noble metal nanoparticles can form advanced electrocatalytic materials with well-defined composition, structure and thickness. We explore here the ability of inorganic structures to stabilize and derivatize metal (Au, Pt, Pd) and carbon nanostructures. Among inorganic systems, polyoxometallates of tungsten are attractive since they can not only adsorb irreversibly on solid surfaces but also exhibit reversible stepwise multi-electron transfer reactions. The concept of the layer-by-layer formation of hybrid inorganic assemblies composed of anionic polyoxometallate-protected carbon nanotubes (or metal nanoparticles) and ultra-thin nanostructures of positively charged metal oxide (vanadia, titania, zirconia) nanostructures (alone or in combination with tungsten oxide) will be described and discussed here. The resulting novel composite materials are characterized by fast dynamics of charge propagation. When it comes to the electrocatalytic reduction of oxygen,  polyoxometallates can also be applied to stabilize and link Pt, Pd and various alloyed Pt-based catalytic nanoparticles.

To produce bioelectrocatalytic materials for oxygen reduction, 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 metalloporphyrin redox centers), whereas the undesirable hydrogen peroxide intermediate are further decomposed at the cytochrome sites.

Acknowledgement

We appreciate collaboration with Krzysztof Miecznikowski, Adam Lewera, Beata Dembinska, Jakub Sek, and Anna Dobrzeniecka from Faculty of Chemistry, University of Warsaw.