Facet Impact of CuMn2O4 Spinel Nanocatalysts on Enhancement of the Oxygen Reduction Reaction in Alkaline Media

Surface-structure engineering represents an attractive strategy to optimize the energy conversion performance of nanocatalysts using their deliberately controlled exposed facets. To further exploit the potential of non-Pt-group metal-based spinel catalysts for the alkaline oxygen reduction reaction (ORR), a cathodic fuel cell reaction, we hereby report a strategy of ORR improvement by controlling the crystallographic facets of ultra-small CuMn2O4 spinel nanocatalysts through a developed colloidal synthesis approach. The synthesis of CuMn2O4 nanocrystals with morphological control relies on the design and selection of the Cu/Mn precursors with striking discrepancies in reaction kinetics. Following carbon loading and an annealing post-treatment of the as-synthesized nanocatalysts, the exclusively {101} facet-exposed CuMn2O4 spinel nano-octahedra exhibit improved electrocatalytic activity toward the ORR in 1 M KOH, when compared to their spherical counterparts, exhibiting a mass activity (MA) of 37.6 A/g at 0.85 V. After 10,000 cycles of the ORR durability test, the nano-octahedra still retain an MA of 24.5 A/g, which is twice that of the CuMn2O4 spinel nanospheres. Structural characterizations after durability testing indicate that the MA decay is likely associated with a decrease in the Mn3+ fraction and the emergence of Cu+ on CuMn2O4 nano-octahedral surfaces. As a paradigm, this synthesis approach could be extended to other Mn-based spinel nanocatalysts with precise shape control, enabling us to understand and establish the relationship between the surface lattice/valence state and electrocatalytic properties.