Quaternary Ammonium Cation Specific Adsorption on Platinum Electrodes: A Combined Experimental and Density Functional Theory Study

Abstract

Quaternary ammonium cations provide anionic conductivity in polymer ionomers and membranes. While study has been devoted to their alkaline stability and to improving hydroxide conductivity, there are limited studies on the electrode/cationic polymer interface. We use density functional theory (DFT) and cyclic voltammetry to examine the adsorption of tetramethyl-, tetraethyl-, tetrapropyl-, and benzyltrimethylammonium cations to platinum electrode surfaces. DFT results demonstrate that adsorption to Pt(111), Pt(100), and Pt(110) is favorable at low potentials in an alkaline electrolyte and that van der Waals interactions contribute significantly. Near-surface solvation weakens the adsorption of the long alkyl chain cations, and promotes the adsorption of the shorter chain cations, suggesting adsorption of shorter-chain cations may be possible at low potentials even in acidic electrolytes. The cations retain most of their charge on adsorption, which contributes to repulsive interactions between adsorbates. Steric hindrance further contributes to coverage dependence, with only low coverage adsorption (∼1/9–1/4 monolayer) being stable within the electrochemical window of an aqueous electrolyte. Our experimental results show that the quaternary ammonium cations blocked surface sites, hindering the adsorption of hydrogen and hydroxide. This observation is qualitatively consistent with DFT results, showing that these organic cations favorably adsorb at low potentials in alkaline electrolytes.