Unveiling Complete Ethanol Oxidation through a Hybrid Enzymatic and Organic Catalyst Cascade in an Electrochemical Micro-Reactor Device

Abstract

Biological fuel cells have been extensively reported as a efficient and sustainable device to convert chemical energy to electrical energy [1]. Biofuel cells (BFCs) are alternative energy sources which use enzymes (enzymatic biofuel cell) or microorganisms (microbial fuel cell) as the electrocatalysts instead of the traditional noble metal catalysts. Ethanol is a fuel that can be used in combined enzymatic fuel cell due to its high energy density (8.03 kW h kg -1), low emission, and storage security [2]. The advantage of enzymes is that they exhibit high specificity and turnover rate, however, mimicking the natural metabolism from ethanol through electrometabolic pathways is enormously difficult. To overcome the limitations of using multiple enzymes to achieve the complete ethanol oxidation, a small molecule organic catalyst can be employed. TEMPO (2,2,6,6-tetramethylpiperidin-1-yl) oxyl) is a small organic catalyst that does not present limited substrate specificity such as enzymes face, due to its ability to oxidize oxygen, nitrogen and sulfur-containing functional groups. Thus, hybrid system enables improvement at the rate of electrocatalytic oxidation of several biofuels [2 - 4]. Herein, we described the development of a hybrid catalytic architecture combining an organic oxidation catalyst, 4-amino-TEMPO (TEMPO-NH2), and a recombinant enzyme, oxalate decarboxylase (OxDc) to catalyze complete ethanol electrooxidation. The catalyst system was coupled with a novel small scale electrolysis cell utilizing polycaprolactone (PCL) and carbon composite electrodes for bulk electrolysis. Electrochemical meansurements and product detection by nuclear magnetic resonance (NMR) after 12 hours of electrolysis have shown for the first time that an organic catalyst and decarboxylase enzyme enhances the overall steps in the cascade operating at acidic pH enabling enhanced electrochemical oxidation of ethanol to CO2. To validate the reaction sequence, a novel micro-reactor device enabled low-cost product analysis and allows for high analyte sensitivity. Overall, this work demonstrated that bi-cascade of TEMPO-NH2 and OxDc can be combined in an electrochemical cell to synergistically catalyze the oxidation of substrates that make up the steps of the ethanol oxidation cascade and is promising for a wide array of applications, including biosensors, environmental monitoring, and biofuel cells.