Effect of Lithium and Potassium Cations on the Electrocatalytic Properties of Carbon and Manganese Oxide Electrocatalysts Towards the Oxygen Reduction Reaction in Concentrated Alkaline Electrolyte

摘要

The impact of alkaline cations (Li~+ and K~+) on the electrocatalytic properties of high-surface-area carbon (HSAC) and nanometric manganese oxides (MnOx) deposited onto HSAC (MnOx/C) used as oxygen reduction reaction (ORR) electrodes has been studied in concentrated LiOH or KOH media. The electrochemical characterizations firstly reveal that HSAC have good ORR electrocatalytic properties in these strong alkaline electrolytes, in agreement with the literature. However, although MnOx/C exhibits high ORR activity in NaOH and KOH media below 1 M (Roche et al., J Phys Chem C 111:1434, 2007), the present study reveals their deactivation in concentrated LiOH or KOH electrolytes because of an insufficient activity of water. The latter is indeed not compatible with a sufficient presence of protons in solution, thereby limiting the necessary proton insertion into the MnOx lattice, a prerequisite for ORR activity (Roche et al., J Phys Chem C 111:1434, 2007). In addition, when LiOH electrolyte is used, another effect penalizes the electrode performances; Li~+ ions may insert into the MnOx lattice and stabilize both the Mn atoms at the oxidation state 3 and the oxygen groups at the carbon surface, which prevents their role of redox mediating species and further blocks the catalytic process, eventually yielding increased ORR overpotential.