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-MnOx) 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) similar to 0.2 mu g(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 MnOx geometries. Together, electrochemical measurements, DFT calculations, X-ray absorption spectroscopy, and valence-band X-ray photoelectron spectroscopy suggest synergistic electronic interactions between Ag and MnOx yield enhanced oxygen adsorption, and thus ORR activity, with DFT highlighting the Ag-MnOx 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.

Automated summary

Understanding material-property relationships in mixed-element catalyst systems is crucial for renewable electrochemical energy technologies. In this study, the nature and dynamics of highly active Ag-MnOx catalyst surfaces for ORR were investigated using an experimental-theoretical approach. Well-mixed Ag-Mn co-deposited thin films were synthesized and showed enhanced specific activity compared to pure Ag. The enhancement was attributed to the tuned d-band of the material surfaces resulting from the optimal hybridization of electronic structures in specific Ag and MnOx geometries.