pH Dependence of Bifunctional OER/ORR Electrocatalysis

For many important electrocatalytic reactions, activity is known to depend on electrolyte pH in a non-Nernstian manner. Possible explanations for this phenomenon include electric-field effects,¹ decoupled proton-electron transfer,² and competing reaction pathways.³ In this talk, I will discuss our hybrid experimental-computational investigation of manganese oxide’s unique bifunctional OER/ORR activity, as well as its anomalous pH dependence. Our model system, -K_0.1MnO₂, is among the highest-performing Mn oxide catalysts for both the OER and the ORR, with ORR activity rivaling that of Pt in basic electrolytes.⁴ Using a novel grand-canonical DFT approach, we show that a pH-dependent interfacial electric field weakens the adsorption of key OER/ORR intermediates, leading to a non-Nernstian shift in onset potential for both reactions. The material’s pH and cation-dependent activity is characterized via rotating ring disk electrochemistry (RRDE), HRTEM, and <i>operando</i> scanning transmission x-ray microscopy (STXM). Understanding the atomic-scale origins of this unique bifunctional activity may help enable higher performing, lower cost regenerative fuel cells and metal-air batteries. <br/><br/><br/>[1] Kelly, S.R. <i>et al.</i> <i>J. Phys. Chem. C.</i> <b>2020</b>, 124, 27, 14581–14591.<br/>[2] Giordano, L. <i>et al.</i> <i>Cat. Tod.</i> <b>2016</b>, <i>262</i>, 2–10.<br/>[3] Liu, X. <i>et al.</i> <i>Nat. Commun.</i> <b>2019</b>, <i>10</i> (32).<br/>[4] Meng, Y. <i>et al.</i> <i>J. Am. Chem. Soc.</i> <b>2014</b>, <i>136</i> (32), 11452–11464.