On the pH dependence of electrochemical proton transfer barriers

TY - JOUR

T1 - On the pH dependence of electrochemical proton transfer barriers

AU - Rossmeisl, Jan

AU - Chan, Karen

AU - Skúlason, Egill

AU - Björketun, Mårten E.

AU - Tripkovic, Vladimir

PY - 2016/3/15

Y1 - 2016/3/15

N2 - The pH dependence of rate of the hydrogen evolution/oxidation reaction HER/HOR is investigated. Based on thermodynamic considerations, a possible explanation to the low exchange current for hydrogen reactions in alkaline is put forward. We propose this effect to be a consequence of the change in configurational entropy of the proton as it approaches the surface. As a proton crosses the outer Helmholtz plane, it will lose a fraction of its entropy before it can interact with the electrode surface, which gives rise to an entropic barrier. The size of this barrier will depend on the electrostatic environment in the double layer region. The entropic barrier can be rate determining only when the surface catalysis is fast. Therefore the effect of pH is most pronounced on good catalysts and for fast reactions. This entropic barrier is also in a good agreement with the unusually low prefactor measured in experiments of good catalysts such as Pt. In such catalysts, the enthalpy barrier of 0.1-0.2 eV of the rate-determining step does not come from any of the surface reactions (Volmer, Tafel or Heyrovsky) but instead from the proton transfer into the outer Helmholtz layer.

AB - The pH dependence of rate of the hydrogen evolution/oxidation reaction HER/HOR is investigated. Based on thermodynamic considerations, a possible explanation to the low exchange current for hydrogen reactions in alkaline is put forward. We propose this effect to be a consequence of the change in configurational entropy of the proton as it approaches the surface. As a proton crosses the outer Helmholtz plane, it will lose a fraction of its entropy before it can interact with the electrode surface, which gives rise to an entropic barrier. The size of this barrier will depend on the electrostatic environment in the double layer region. The entropic barrier can be rate determining only when the surface catalysis is fast. Therefore the effect of pH is most pronounced on good catalysts and for fast reactions. This entropic barrier is also in a good agreement with the unusually low prefactor measured in experiments of good catalysts such as Pt. In such catalysts, the enthalpy barrier of 0.1-0.2 eV of the rate-determining step does not come from any of the surface reactions (Volmer, Tafel or Heyrovsky) but instead from the proton transfer into the outer Helmholtz layer.

KW - Entropic barrier

U2 - 10.1016/j.cattod.2015.08.016

DO - 10.1016/j.cattod.2015.08.016

M3 - Journal article

SN - 0920-5861

VL - 262

SP - 36

EP - 40

JO - Catalysis Today

JF - Catalysis Today

ER -