The effective transport of proton and reactant within the electrode is heavily influenced by the pore structure of the carbon support hosting both Pt particles and ionomers. To this end, the impact of carbon structure on the Pt position, ionomer distribution, and voltage losses under various relative humidity (RH) conditions has been systematically investigated. The findings reveal that low-loading solid carbon-based catalysts display impressive performance at low RH, while high-loading porous carbon-based catalysts stand out at high RH. Furthermore, porous carbon-based catalysts exhibit high mass activity at low current density due to their reduced susceptibility to the poisonous effects of sulfonate groups. Meanwhile, solid carbon catalysts facilitate a more uniform ionomer thin-film and create a more active three-phase interface area, resulting in a satisfactory performance at the high current density region and low local-O₂ transport resistance.