Bioinspired Architecture of a Hybrid Bifunctional Enzymatic/Organic Electrocatalyst for Complete Ethanol Oxidation

Abstract

Enzymatic biofuel cells utilize enzymes to electrochemically catalyze the anodic oxidation of a fuel or the cathodic reduction of an oxidant due the high volumetric catalytic activity towards oxidation of biofuels, providing clean and renewable energy, besides presenting great potential as alternative power sources for low-power electronic devices. One way to amplify the electrocatalytic activity of the biosystem and also extend its service life is mix the proper enzyme with organic oxidation catalysts. This hybrid system enables improvement at the rate of electrocatalytic oxidation of several biofuels[1-3].Herein, we describe the development of a hybrid catalytic architecture combining OxOx with a recently developed TEMPO-modified linear poly(ethylenimine) (TEMPO-LPEI) coupled with carboxylated multi-walled carbon nanotubes (MWCNT-COOH) to transform ethanol into CO2, extracting 12 electrons per ethanol molecule at a single bioanode. We identify and quantify the products from electrochemical ethanol oxidation by high performance liquid chromatography (HPLC). The electrochemical experiments were carried out using an AUTOLAB potentiostat / galvanostat (Software NOVA 1.11) with a three-electrode standard cell. The cascade oxidation of ethanol was performed by the chronoamperometric technique (electrolysis) with applied potential of 0.600 V and analysis time of 43,200 s (12 hours). The products obtained after 12 hours of electrolysis were analyzed by HPLC-UV / RID in Full-Scan mode. On the basis of chromatographic results, the catalytic hybrid electrode system completely oxidized ethanol to CO2 after 12 h of electrolysis. The fact that the developed system can catalyze ethanol electrooxidation at a carbon electrode confirms that organic oxidation catalysts combined with enzymatic catalysts allow up to 12 electrons to be collected per fuel molecule. The Faradaic efficiency of the hybrid system investigated herein lies above 85%. Complete ethanol electrooxidation was achieved by using a hybrid bifunctional enzymatic/organic electrocatalyst, MWCNT-COOH/TEMPO-LPEI/OxOx, This new hybrid system represents a good alternative as compared to previous systems [1], not to mention its excellent selectivity, the high yields of CO2 as the final product, and the high electrochemical oxidation rates. This system also represents an example of a hybrid multifunctional catalyst that may be valuable and open up a number of exciting opportunities to fabricate new hybrid bioanode architectures as energy conversion/storage devices and also as part of multistep reaction cascade systems.