Engineering metal-metal oxide surfaces for high-performance oxygen reduction on Ag-Mn electrocatalysts

Understanding fundamental material-property relationships in mixed-element catalyst systems is crucial to advancing the viability of renewable electrochemical energy technologies, an important part of creating a more sustainable future. Herein, we report our insight on the nature and dynamics of highly active silver-manganese oxide (Ag-MnO_x) catalyst surfaces for the oxygen reduction reaction (ORR) via a combined experimental-theoretical approach. Experimentally, we synthesize well-mixed Ag-Mn co-deposited thin films that are measurably flat and smooth, despite Mn surface migration and oxidation upon air exposure and electrochemical measurements. Cyclic voltammetry in 0.1 M KOH demonstrates up to 10-fold specific activity enhancements over pure Ag at 0.8 V vs. RHE for Ag-rich films (70-95% Ag in bulk). To further understand the Ag-Mn system, separate samples were synthesized with small amounts of Mn sequentially deposited onto the surface of a pure Ag thin film (Mn@Ag), ranging from partial to full surface coverage (down to 0.3 nm_Mn cm⁻²_geo ∼ 0.2 μg_Mn cm⁻²_geo). These sequentially deposited Mn@Ag films show analogous performance to their co-deposited counterparts indicating similar enhanced active sites. With density functional theory (DFT), we calculate that this enhancement arises from the tuned d-band of these material surfaces owing to the optimal hybridization of the electronic structures in specific Ag and MnO_x geometries. Together, electrochemical measurements, DFT calculations, X-ray absorption spectroscopy, and valence-band X-ray photoelectron spectroscopy suggest synergistic electronic interactions between Ag and MnO_x yield enhanced oxygen adsorption, and thus ORR activity, with DFT highlighting the Ag-MnO_x interface sites as the most enhanced. This work demonstrates how combined experimental-theoretical methods can help design electrocatalysts with enhanced electrocatalytic properties and understand the nature of complex mixed metal-metal oxide surfaces.