Effect of Local Charge Distribution on Graphite Surface on Nafion Polymer Adsorption as Visualized at the Molecular Level

Abstract

Since the perfluorosulfonic acid location distribution is not currently controlled in the proton-exchange membrane fuel cell electrode coating process, an improvement in electrode performance and durability is likely possible by manipulating the polymer structure in solution and its interaction with the electrode surfaces in the ink formulation. This paper used in-situ liquid atomic force microscopy (AFM) to directly image the local charge distribution on a model highly ordered pyrolytic graphite (HOPG) wafer surface in H2O and ethanol:water (EtOH:H2O) = 1:1 w/w solvent at varying solution pH(e). The zeta potential for HOPG graphite was measured against pH(e) in EtOH–H2O solvent blends, while its actual charge location distribution is also mapped in water and EtOH:H2O = 1:1 w/w solvent by in-situ AFM using an amine-grafted tip. Significant charge density was found at HOPG step sites with a high negative band at the edge and a partially compensating positive band at the adjacent lower terrace. The anionic charge is assigned to grafted carboxylic acid groups which then release hydronium ion either to the diffuse counterion cloud in solution above the surface or to direct adsorption on the lower terrace within an electrostatic screening distance. At sufficiently low pH(e), the charge density at the step edge fades as the carboxylic acid pKa is reached, while a random location distribution of positive charge develops on the open HOPG terrace that is assigned to further hydronium ion adsorption away from the step edge. The equilibrium adsorption of Nafion polymer on HOPG from EtOH:H2O = 1:1 w/w was determined to be electrostatically controlled using zeta-potential and in-situ liquid AFM imaging. The adsorption begins below the HOPG isoelectric point and is preferentially located at the step edge.