This study investigates the electrochemical hydrogen oxidation and evolution reaction (HOR/HER) mechanisms on three active monometallic surfaces: Pt, Ir, and Pd, supported on carbon. The research addresses the effect of pH on HOR/HER rates in proton exchange membrane fuel cells (PEMFCs) and alkaline electrolytes. The findings indicate a significant decrease in activity on all three surfaces when moving from low to high pH. The reaction rate on Pt/C is controlled by the Volmer step, and the H-binding energy is identified as the sole descriptor for HOR/HER in alkaline electrolytes. The study compares the exchange current densities (i₀) of Pt/C, Pd/C, and Ir/C in both acidic and alkaline environments. The results show that Pt is more active than Ir and Pd in both environments. The i₀ values for Pt/C in alkaline conditions align with previous studies, while those for Pd/C and Ir/C are newly quantified. The study also reveals that the HOR/HER kinetics on Pt in alkaline electrolytes are limited by the Volmer step, and that the H-binding energy is the key factor influencing the reaction rate. The research challenges previous assumptions about the reaction mechanisms in alkaline conditions, suggesting that the HOR/HER on Pt is not governed by the interaction with H₂O/OH⁻ but rather by the Volmer step. The study also highlights the importance of H-binding energy in determining HOR/HER activity, and that current DFT models do not account for pH effects on H-binding energy. The findings suggest that future electrocatalysts for HOR/HER in alkaline environments should focus on tuning H-binding energy, improving computational models that include pH effects, and developing experimental methods to quantify H-binding energy. The study concludes that the HOR/HER activity on Pt in high pH environments is not enhanced by oxophilicity, and that the Volmer step is the rate-determining step in the reaction.This study investigates the electrochemical hydrogen oxidation and evolution reaction (HOR/HER) mechanisms on three active monometallic surfaces: Pt, Ir, and Pd, supported on carbon. The research addresses the effect of pH on HOR/HER rates in proton exchange membrane fuel cells (PEMFCs) and alkaline electrolytes. The findings indicate a significant decrease in activity on all three surfaces when moving from low to high pH. The reaction rate on Pt/C is controlled by the Volmer step, and the H-binding energy is identified as the sole descriptor for HOR/HER in alkaline electrolytes. The study compares the exchange current densities (i₀) of Pt/C, Pd/C, and Ir/C in both acidic and alkaline environments. The results show that Pt is more active than Ir and Pd in both environments. The i₀ values for Pt/C in alkaline conditions align with previous studies, while those for Pd/C and Ir/C are newly quantified. The study also reveals that the HOR/HER kinetics on Pt in alkaline electrolytes are limited by the Volmer step, and that the H-binding energy is the key factor influencing the reaction rate. The research challenges previous assumptions about the reaction mechanisms in alkaline conditions, suggesting that the HOR/HER on Pt is not governed by the interaction with H₂O/OH⁻ but rather by the Volmer step. The study also highlights the importance of H-binding energy in determining HOR/HER activity, and that current DFT models do not account for pH effects on H-binding energy. The findings suggest that future electrocatalysts for HOR/HER in alkaline environments should focus on tuning H-binding energy, improving computational models that include pH effects, and developing experimental methods to quantify H-binding energy. The study concludes that the HOR/HER activity on Pt in high pH environments is not enhanced by oxophilicity, and that the Volmer step is the rate-determining step in the reaction.