Perspective—New Perspectives from Classical Transition State Theory: The Hydrogen Evolution Reaction on Metal Electrodes

Abstract

New perspectives derive from classical transition state theory applied to hydrogen evolution reactions (HER). Trasatti found exchange current density j 0 increases exponentially with metal electrode work function Φ. The elementary electron transfer step <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>M</mml:mi> <mml:mo stretchy="false">(</mml:mo> <mml:mi>e</mml:mi> <mml:mo stretchy="false">)</mml:mo> <mml:mo>+</mml:mo> <mml:msubsup> <mml:mrow> <mml:mi>H</mml:mi> </mml:mrow> <mml:mrow> <mml:mi mathvariant="italic">ads</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> </mml:mrow> </mml:msubsup> <mml:mo>⇄</mml:mo> <mml:mi>M</mml:mi> <mml:mo stretchy="false">(</mml:mo> <mml:mn>0</mml:mn> <mml:mo stretchy="false">)</mml:mo> <mml:mo>+</mml:mo> <mml:msubsup> <mml:mrow> <mml:mi>H</mml:mi> </mml:mrow> <mml:mrow> <mml:mi mathvariant="italic">ads</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>•</mml:mo> </mml:mrow> </mml:msubsup> </mml:math> specifies the metal in the transition state and derives rate equations where metal specific physiochemical property Φ is explicit. Linearity of log j 0 with Φ is quantified. Φ raises j 0 because Φ lowers the activation energy for electrocatalysis. Rate expressions that embed material specific physiochemical properties provide opportunities for a priori electrocatalyst design and fundamental insights on electron transfer kinetics. New perspectives derive from revisit of established HER data with classical transition state theory.