Understanding the effects of electrolyte pH and spectator ions on electrocatalysis

Restricted (Penn State Only)
Author:
Mccrum, Ian Taylor
Graduate Program:
Chemical Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
May 26, 2017
Committee Members:
  • Michael John Janik, Dissertation Advisor
  • Michael John Janik, Committee Chair
  • Michael Anthony Hickner, Committee Member
  • Robert Martin Rioux Jr., Committee Member
  • Chao-Yang Wang, Outside Member
Keywords:
  • electrocatalysis
  • density functional theory
  • hydrogen oxidation
  • alkaline fuel cells
  • specific adsorption
  • ion effects
Abstract:
Recent experimental evidence suggests that the rate and mechanisms of many electrocatalytic reactions depend on electrolyte pH and the identity of the alkali metal cation present in an alkaline electrolyte. In particular, the rate of the hydrogen oxidation reaction, important in hydrogen fuel cells, is 2-3 orders of magnitude slower in an alkaline electrolyte than in an acid electrolyte, even on the most active platinum catalyst. While it is well known that many anions effect the rates of electrocatalytic reactions, through their specific adsorption and blocking of active sites on the electrode surface, the mechanism by which alkali metal cations exert their effects is unknown. Both experiment and density functional theory modeling of the electrode-electrolyte interface are used in this dissertation to better understand how pH and alkali metal cations effect electrocatalytic reactions. Density functional theory calculations show that alkali metal cation specific adsorption is favorable at low potentials to many electrode surfaces, including platinum, in an alkaline electrolyte. Once on the surface, these alkali metal cations show only a small interaction with adsorbed hydrogen, but a significant weakening of adsorbed hydroxide. These results explain an experimentally observed anomalous shift in the low potential features of cyclic voltammograms measured on Pt(110), Pt(100), and stepped Pt surfaces with increasing pH, which correlate with the pH dependence of the rate of the hydrogen oxidation reaction. The rate of the hydrogen oxidation reaction is experimentally measured in alkaline electrolytes, and is found to depend on the alkali metal cation present, following the trend Li > Na > K > Cs. The density functional theory calculated trend in the effect of these cations on hydroxide adsorption matches the trend in rate, supporting that adsorbed hydroxide may be an intermediate in the hydrogen oxidation reaction. To design highly active hydrogen oxidation/evolution catalysts, both hydrogen adsorption strength and hydroxide adsorption strength must be considered.