Facet Effects Enhance Formic Acid Oxidation by Regulating Subsurface Interstitial Nitrogen Occupation

Direct formic acid fuel cells (DFAFCs) are considered promising power sources for next-generation portable electronic devices due to their high theoretical energy density, excellent storage stability, and low fuel permeability. However, the multistep nature of the formic acid oxidation (FAO) pathway increases the complexity of designing efficient anode catalysts, posing a significant challenge to the commercialization of DFAFCs. Here, we present a unique example of significantly enhancing the catalytic performance of Pd-based catalysts for FAO through facet effect-regulated nitrogen occupation. Our approach involves the synthesis of nitrogen-doped Pd nanocubes (NCs) and cuboctahedrons (COs). Theoretical and experimental analyses reveal that the subsurface octahedral (O) site N occupation on the (100) facet reduces the energy barrier for the complete FAO process, thereby significantly enhancing the catalytic activity of PdN NCs. In contrast, the tetrahedral (T) site N occupation on the (111) facet increases the adsorption free energy difference between *HCOO and *COOH, which reduces the accumulation of CO and endows PdN COs with superior catalytic stability. This study provides valuable insights for the design of advanced anode catalysts in DFAFCs.