(33a) Anode Characteristics of MDH-Catalyzed Bio-Fuel Cell: A Multi-Scale Modeling Study

Proton Exchange Membrane (PEM) fuel cells are one of the promising fuel cells being studied these days where platinum is needed as catalyst, which is expensive, operates at low temperatures and easily gets poisoned with carbon monoxide which is a byproduct of oxidation reactions. A possible alternative is using enzymes as bio-catalysts [1]. Methanol dehydrogenase (MDH) is one such enzyme which has been used as an anodic catalyst for a methanol-fed biofuel cell producing enough power for small electronic device applications [2]. In practice, however there are power output limitations associated with this MDH fuel cell, which may potentially be eliminated or reduced if the reactivity of this enzyme during the oxidation of methanol at the molecular level is clearly understood and also the cell i.e the polarization characteristics will help understand the power density and stability of the fuel cell.

Methanol Dehydrogenase (MDH) is a water-soluble quinoprotein that oxidizes methanol and other primary alcohols to their corresponding aldehydes. From various studies the crystal structure of MDH has been characterized and it has been determined that the enzyme active center contains a Ca²⁺ ion, pyrroloquinoline quinone (PQQ) and amino acids. Ca²⁺ ion in the active site holds the PQQ in place and also acts as a Lewis acid during the oxidation mechanism [3,4]. From previous studies, it was observed that the energy barrier for the oxidation of methanol was influenced by the atomic size of the ion in the active site of MDH. Thus, the various studies on ion-modified enzymes indicate that ions play a vital and diverse role in enzyme catalysis [5]. From the possible mechanisms for methanol oxidation by MDH proposed in the literature, the Hydride Transfer (H-T) mechanism seems, to the best of our knowledge, to be the preferred one under normal conditions.

Here, a multiscale approach has been employed to study the cell characteristics of the methanol fed bio-fuel cell. Based on the exposed residues methanol molecules face upon approaching the MDH active site through the enzyme binding pocket, a MDH active site model was considered to test the H-T methanol oxidation mechanism. Density Functional Theory calculations are performed to investigate reaction pathways. Information regarding energy barriers and pre-exponential factors thus obtained determine the reaction rates involved in each step of the methanol oxidation mechanism by MDH. These microscopic details are then provided as inputs in a Kinetic Monte Carlo (kMC) program, and the methanol oxidation process is modeled. These simulations help evaluate the kinetics and their dependence on various factors like obstacle density, substrate and active site concentrations, temperature and time, and the nature of the ion in the MDH active site. To study the anode characteristics of the fuel cell the potential dependence of the reaction rate is studied using Kinetic Monte Carlo program and the voltammetric scans are presented and compared with experimental results.

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