The differences in the catalytic activities of metals towards HER in an acidic medium (0.5M H 2 SO 4 ) is assumed to be explainable via the Sabatier Principle. According to the Sabatier principle, HER activity depends on metal-hydrogen (E M-H ) binding strength, which leads to a volcano type plot when using E M-H as a descriptor of the activity of metals for HER. In this scenario, metals like Pt or Pt-group metals are excellent electrocatalysts for HER due to their thermoneutrality for hydrogen adsorption (optimum metal-hydrogen binding energy leads to ΔE M-H ~0 eV). But this reasoning cannot be extended to the alkaline HER, particularly it fails to explain the sluggish reaction kinetics of HER on Pt in alkaline media.