Defective Ceria as a Structural and Electronic Modifier of Nitrogen-Doped Carbon and Cobalt-Nitrogen-Carbon Electrocatalysts for Oxygen Reduction in Alkaline Electrolyte

Abstract

The oxygen reduction reaction (ORR) is central to many sustainable energy technologies such as fuel cells, metal-air batteries, and peroxide electrosynthesis, but the scalability of ORR-based technologies requires cost-effective, active, and selective electrocatalysts free from the costly and scarce platinum-group-metals (PGMs). Despite considerable advances, state-of-the-art PGM-free N-doped carbon (N-C) and metal-nitrogen-carbon (M-N_x/C) electrocatalysts face performance bottlenecks. Defect engineering in ceria (CeO₂), a promising electrocatalyst modifier, leveraging its Ce³⁺/Ce⁴⁺ redox and oxygen-vacancy (V_o) capacity, offers a promising route to modulate the electronic structure of N-C and M-N_x/C electrocatalysts. However, how V_o in ceria modulate the electronic structure and the 2e^-/4e^- ORR selectivity of N-C and Co/N-C electrocatalysts remains underexplored. Here, we engineer V_o-rich ceria via plasma and thermal reduction, followed by embedding into zeolitic imidazolate frameworks (Zn-ZIF8 and Co/Zn-ZIF) and pyrolysis (900 ^oC) to derive ceria@N-C and ceria@Co/N-C electrocatalysts. The structural and physicochemical properties of the ceria variants, ceria@ZIF precursors, and developed electrocatalyst were investigated. The electrochemical properties and ORR performance were evaluated in alkaline media (0.1 M KOH). V_o-rich ceria samples exhibited enhanced onset and half-wave potentials relative to as-synthesized ceria, while modulating the 2e^- and 4e^- ORR selectivity differently depending on the host matrix and surrounding type of active site structure(s). In Co-free systems, Ce³⁺-V_o centers led to mixed 2e^-/4e^- pathways, while in Co-containing systems, V_o-rich ceria partially suppressed the dominant 4e^- ORR of Co nanoparticles. ECSA-normalized activity confirmed that V_o-rich ceria modulates selectivity but dilutes intrinsic site activity, revealing a trade-off between activity and selectivity. This work establishes V_o defect engineering in ceria as a lever in modulating ZIF-derived ORR electrocatalysts as a strategy toward the rational design of application-ready PGM-free electrocatalysts.