Control of Hydration and Degree of p _z /π* Orbital Localization as Dual Keys for Durable Carbon Electrocatalysts in Acidic ORR

We highlight hydration control and the degree of p z /π* orbital localization as dual keys for designing durable carbon electrocatalysts for the oxygen reduction reaction in acidic media. While N-doped carbon catalysts exhibit adequate activity in alkaline conditions, their decreased activity in acidic environments remains a major barrier to practical fuel cell applications. We first discuss how pyridinic nitrogen sites serve as prototypical active centers, where protonation-electron transfer coupling promotes oxygen adsorption but simultaneously enhances hydration and counteranion crowding that deactivate the catalytic active site. Introducing hydrophobic domains provides a strategy to control hydration and recover accessibility to reactants. Beyond hydration effects, recent studies reveal that defect motifs such as five-membered rings can localize nonbonding orbitals near the Fermi level, complementing the extended π* states of pyridinic nitrogen and thereby stabilizing oxygenated intermediates. These insights suggest that mesoscale hydration control and the degree of p z /π* orbital localization at the atomic scale are synergistic design principles. Importantly, the improved CO tolerance and durability of metal-free carbon catalysts make them particularly advantageous when fuel flexibility is required, such as in direct methanol and formic acid fuel cells. Together, these principles provide a blueprint for constructing efficient, fuel-flexible, and durable carbon catalysts for future hydrogen and beyond-hydrogen energy systems.