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

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) V_O (surface) >> V_O(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.