Electrochemical Characterization of Catalytic Activities of Manganese Oxides to Oxygen Reduction in Alkaline Aqueous Solution

The catalytic function and activity of manganese oxides (MnOx: Mn 2 O 3 , Mn 3 O 4 , Mn 5 O 8 , and MnOOH) to the electrochemical reduction of O 2 in 0.10 M KOH aqueous solution have been investigated by cyclic voltammetry at MnOx/Nafion-modified gold electrodes. Two successive reduction current peaks were observed at Nafion-modified electrodes in the cyclic voltammograms, i p,1 for a two-electron reduction of O 2 to hydrogen peroxide (HO 2 ) and i p,2 for a two-electron reduction of HO 2 to OH - . The peak current heights of i p,1 and i p,2 changed greatly depending on the kind of MnOx species incorporated into the MnOx/Nafion-modified gold electrodes; i p,1 increased and i p,2 decreased. On the assumption that HO 2 produced in the first reduction step is chemically decomposed into O 2 and OH with a catalytic action of MnOx and that this regenerated O 2 is reduced again in the same first reduction step, we evaluated the catalytic activity of MnOx using the values of i p,1 and i p,2 . MnOOH provided the highest catalytic activity to the electrochemical reduction of O 2 . This result was supported by another experiment by using a chemical method where catalytic decomposition of HO 2 with MnOx was estimated by measuring the O 2 concentration directly with a commercial oxygen sensor.

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