Method for Enhancing the Bifunctional Activity and Durability of Oxygen Electrodes with Mixed Oxide Electrocatalysts: Potential Driven Intercalation of Potassium

Abstract

The bifunctional oxygen reduction and evolution reaction (ORR and OER, respectively) electrocatalytic activity and durability of mixed oxides MnO₂-LaCoO₃ and MnO₂-Nd₃IrO₇, are investigated. The goal is to identify possible beneficial synergistic catalytic effects between the two oxides and to investigate the role of alkali-metal ions (Li⁺, Na⁺, K⁺ and Cs⁺) for promotion of electrocatalytic activity and durability. The combination of the two, structurally different, oxides, improves the bifunctional activity compared to the individual oxide components, as shown by either lower apparent Tafel slopes or higher exchange current densities for ORR and OER in 6 M KOH. Insertion of potassium ion in the oxide structure either by longer-term exposure to 6 M KOH or by an accelerated potential driven intercalation method, lowers further both the OER and ORR overpotentials. At constant current density of 5 mA cm⁻² (or 5 A g⁻¹ catalyst) for two hours, the OER overpotential is lowered by 110 mV and 152 mV due to potential driven potassium ion insertion in MnO₂-LaCoO₃ and MnO₂-Nd₃IrO₇, respectively. For ORR, at −2 mA cm⁻² (or −2 A g⁻¹ catalyst) the overpotential on MnO₂-LaCoO₃ is decreased by 75 mV. In addition, the stability of the potassium ion activated catalysts is also improved. The ORR activity promotion effect is specific to potassium compared to all other investigated alkali metal hydroxides (LiOH, NaOH, CsOH), whereas for OER, cesium ion also has a beneficial effect. The electrode kinetic results are supported by surface analysis showing the presence of potassium in the catalyst.