Oxygen Reduction in Si-Air Battery Employing MnO2 Catalyst - a Phenomenological Thermodynamic Analysis

Abstract

The steps involved in Oxygen Reduction Reaction (ORR) mechanism are as follows: Step (i) : Diffusion of molecular oxygen on MnO2 catalyst Step (ii): Chemisorption of molecular oxygen at MnO2 surface Step (iii): Dissociation of molecular oxygen on MnO2 surface Step (iv): Reduction of molecular oxygen on MnO2 surface and reduction of MnO2 (Mn(IV)) to Mn2O3 (Mn(III)) Step (v): Diffusion of oxygen anion on the MnO2 surface Step (vi): Incorporation of oxygen in MnO2 lattice at the oxygen vacant site Assumptions : (i) maintenance of electroneutrality (ii) VO (surface) >> VO(bulk) of MnO2 lattice (iii) electrons for oxygen reduction and reduction of Mn4+ to Mn3+ comes from anodic reaction (iv) effects due to formation of MnF2 during discharge of Si-air battery is neglected. (v) wherever partial bonds are formed or dissociated, the bond energy is averaged over the coordination number or total number of vacant sites as the case may be (vi) the pressure of oxygen gas is written using the van der Waals equation of state Derived Equation can now be utilized to estimate the free energy of activation for ORR mechanism in Si – air battery and engineer catalysts suitable for fast ORR in these batteries.