Open Collections

Pei, Yu

University of British Columbia

Efforts to develop cost-efficient, non-precious metal electrodes for reversible oxygen reduction and evolution reactions (ORR/OER) face challenges due to slow kinetics and rapid degradation. To accelerate the development of rechargeable metal-air batteries and regenerative fuel cells, this thesis systematically advances the understanding of nanostructure influence on MnOx electrochemical behaviours and develops innovative approaches for enhancing their stability and activity, positioning the MnOₓ electrodes as promising and sustainable alternatives to precious metal-based electrodes for ORR/OER catalysis. First, this work studies the electrochemical behavior and ORR/OER activity of core-porous shell Mn/Mn₃O₄ nanoparticles in comparison with MnO₂. The core-shell MnOₓ demonstrates higher ORR/OER activities during cycling in O₂-saturated KOH, outperforming exclusive ORR catalysis (influence of potential range) or OER catalysis in a N₂-purged environment (effect of O₂ presence). The unique behaviour of core-shell MnOₓ is attributed to the charge transfer between the low-valent Mn core and the high-valent Mn₃O₄ shell, thereby regenerating Mn(III) active sites through comproportionation. Next, the optimization of gas diffusion electrode (GDE) preparation and conditioning is explored. HNO₃-pretreatment on polytetrafluoroethylene (PTFE)-coated carbon papers was found to reduce overpotentials and eliminate the influence of PTFE loading on voltammograms. The Vulcan XC-72 and graphene mixture showed improved durability by stabilizing Mn sites and restraining Mn₂ oxidation. Lastly, a set of activation protocols was designed to generate and stabilize highly active MnOₓ phases. After cyclic voltammetry conditioning, the MnOₓ/Vulcan/graphene electrode demonstrated extended galvanostatic cyclic stability and a smaller potential gap between the anodic and cathodic reactions. Finally, in-situ electrochemical metallic cation-incorporation on MnOₓ GDE is investigated. In comparison to electrodes prepared through the chronopotentiometry and chronoamperometry approaches, the Co-MnOₓ electrodes prepared using the potentiodynamic method exhibited a significant increase in ORR catalytic current for up to 4.5 times higher and 2.1 times higher at 0.75 VRHE, respectively. Among the explored metal cations, the Ni²⁺ incorporation showed the most significant improvement in terms of activity and cyclic stability. The Ni-MnOₓ GDE demonstrated excellent stability for over 120 cycles in O₂-saturated KOH without losing OER activity, surpassing the Pt/C-IrO₂ benchmark.

Text

2024-01-10

Attribution-NonCommercial-NoDerivatives 4.0 International

http://hdl.handle.net/2429/87204

Chemical and Biological Engineering

University of British Columbia

UBCV

http://creativecommons.org/licenses/by-nc-nd/4.0/