Identifying Upper d-Band Edge as Activity Descriptor for Ammonia Oxidation on PtCo Alloys in Low-Temperature Direct Ammonia Fuel Cells

Low-temperature direct ammonia fuel cell (DAFC) stands out as a more secure technology than the hydrogen fuel cell system, while there is still a lack of elegant bottom-up synthesis procedures for efficient ammonia oxidation reaction (AOR) electrocatalysts. The widely accepted d-band center, even with consideration of the d-band width, usually fails to describe variations in AOR reactivity in many practical conditions, and a more accurate activity descriptor is necessary for a less empirical synthesis path. Herein, the upper d-band edge, ε_u, derived from the d-band model, is identified as an effective descriptor for accurately establishing the descriptor-activity relationship. Using the PtCo alloy with varying atomic composition as an example, the ε_u value succeeds in reflecting the corresponding trends of AOR activity, showing striking linear correlation with a coefficient of determination (R²) as high as 0.90. The effectiveness of the established descriptor-activity relationship is verified experimentally. The optimum electrocatalyst delivers an excellent peak current density of 74.04 A g^-1 at 5 mV s^-1, and the assembled DAFC generates a high power density, outperforming the majority of the extensively reported systems. This work brings fundamental insights into the relationship between chemical reactivity and electronic structure and benefits rational optimization of AOR electrocatalyst for next-generation low-temperature DAFC.