Modification of high surface area electrodes for the electrocatalytic reduction of CO2

Abstract

Vulcan XC72 was modified with 9,10-anthraquinone (AQ) and polypyridyl ruthenium complexes using three approaches. Techniques employed include spontaneous diazonium coupling reaction, benzimidazole forming reaction, and formation of benzimidazole-based polymers on the carbon surface. -- Following the spontaneous diazonium coupling reaction for immobilization of AQ, a surface coverage limit far below that expected for a close-packed monolayer was observed. Varying reaction conditions and pretreatments to the carbon surface failed to increase the surface coverage beyond this limit. The limit was found to include both covalently bound and physisorbed AQ. It was found that washing the modified carbon with benzene was necessary to remove all physisorbed AQ. Upon removal of physisorbed material, differences in surface coverage values for covalently bound AQ were observed to vary, depending on reaction conditions and surface pretreatments. An azo-linkage between the immobilized species and the carbon surface was observed using cyclic voltammetry. X-ray photoelectron spectroscopy (XPS) and cyclic voltammetry results indicate approximately 10% of immobilized molecules are bound through an azo-linkage. -- A novel approach that coupled aryl diazonium molecules to surface bound carboxylic acid functionality, producing a benzimidazole linkage, was developed and explored using AQ. Two unique forms of AQ were observed following the reaction procedure. XPS results indicated that one was the desired, benzimidazole-linked form, while the other was likely bound through an amine linkage. The ability to immobilize polymeric systems on carbon surfaces using this technique was examined through sequential reactions. Following successful sequential immobilizations, a one-pot reaction was then employed to immobilize benzimidazole-based polymers containing bidentate coordination sites on the carbon surface. -- The immobilization of polypyridyl ruthenium complexes was performed using all three approaches. Poor voltammetric behavior was observed for carbon powders modified using any of the three techniques. However, thermogravimetric analysis, XPS and elemental analysis indicated the presence of significant amounts of ruthenium in modified samples. Evidence of ruthenium ligands in the modified material was observed, suggesting polymerization of ruthenium complexes is responsible for the poor electrochemistry. -- The polybenzimidazole-based polymers used in this study were found to be active redox catalysts for the reduction of CO₂. Product analysis indicated that none of the common CO₂ reduction products are formed.