UBC Theses and Dissertations
Bifunctional oxygen reduction/evolution catalysts for rechargeable metal-air batteries and regenerative alkaline fuel cells Hosseini-Benhangi, Pooya
The electrocatalysis of oxygen reduction and evolution reactions (ORR and OER, respectively) on the same catalyst surface is among the long-standing challenges in electrochemistry with paramount significance for a variety of electrochemical systems including regenerative fuel cells and rechargeable metal-air batteries. Non-precious group metals (non-PGMs) and their oxides, such as manganese oxides, are the alternative cost-effective solutions for the next generation of high-performance bifunctional oxygen catalyst materials. Here, initial stage electrocatalytic activity and long-term durability of four non-PGM oxides and their combinations, i.e. MnO₂, perovskites (LaCoO₃ and LaNiO₃) and fluorite-type oxide (Nd₃IrO₇), were investigated for ORR and OER in alkaline media. The combination of structurally diverse oxides revealed synergistic catalytic effect by improved bifunctional activity compared to the individual oxide components. Next, the novel role of alkali-metal ion insertion and the mechanism involved for performance promotion of oxide catalysts were investigated. Potassium insertion in the oxide structures enhanced both ORR and OER performances, e.g. 110 and 75 mV decrease in the OER (5 mAcm-²) and ORR (-2 mAcm-²) overpotentials (in absolute values) of MnO₂-LaCoO₃, respectively, during galvanostatic polarization tests. In addition, the stability of K⁺ activated catalysts was improved compared to unactivated samples. Further, a factorial design study has been performed to find an active nanostructured manganese oxide for both ORR and OER, synthesized via a surfactant-assisted anodic electrodeposition method. Two-hour-long galvanostatic polarization at 5 mAcm-² showed the lowest OER degradation rate of 5 mVh-¹ for the electrodeposited MnOx with 270 mV lower OER overpotential compared to the commercial γ-MnO₂ electrode. Lastly, the effect of carbon addition to the catalyst layer, e.g. Vulcan XC-72, carbon nanotubes and graphene-based materials, was examined on the ORR/OER bifunctional activity and durability of MnO₂ LaCoO₃. The highest ORR and OER mass activities of -6.7 and 15.5 Ag-¹ at 850 and 1650 mVRHE, respectively, were achieved for MnO₂-LaCoO₃-multi_walled_carbon_nanotube-graphene, outperforming a commercial Pt electrode. The factors affecting the durability of mixed-oxide catalysts were discussed, mainly attributing the performance degradation to Mn valence changes during ORR/OER. A wide range of surface analyses were employed to support the presented electrochemical results as well as the proposed mechanisms.
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