The open-circuit potential of platinum electrodes coated on acid phosphoric-doped polybenzimidazole (PBI) was measured in dry air containing hydrogen. The study was carried out with three types of platinum electrode, including sputtered Pt thin film, a Pt grid, and a carbon-supported Pt gas diffusion electrode. The validity of the mixed potential model was demonstrated. The potentiometric response was analyzed in terms of sensitivity to hydrogen. The sputtered Pt film was not robust enough to be used as the sensitive electrode of a PBI-based hydrogen sensor. The best limit of detection and the greatest sensitivity near the explosive limit of hydrogen in dry air was obtained with the Pt grid hot pressed on PBI.