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Methanol and formic acid electrooxidation on nafion modified Pd/Pt{1 1 1}: The role of anion specific adsorption in electrocatalytic activity

Paper ID Volume ID Publish Year Pages File Format Full-Text
55005 47036 2013 7 PDF Available
Title
Methanol and formic acid electrooxidation on nafion modified Pd/Pt{1 1 1}: The role of anion specific adsorption in electrocatalytic activity
Abstract

The electrooxidation of formic acid and methanol on Pt{1 1 1} in aqueous sulphuric and perchloric acid as a function of both palladium coverage and Nafion adsorption is reported. Nafion is a proton conducting polymer commonly used as a membrane electrolyte in fuel cells. Understanding the interaction of Nafion with platinum and other bimetallic electrocatalytic surfaces is therefore of crucial importance if one is to optimise fuel cell performance. Here, we demonstrate that for formic acid electrooxidation, Nafion behaves as a weakly adsorbing electrolyte exhibiting similar properties to aqueous perchloric acid. Adsorbed palladium is observed to promote formic acid electrooxidation on Pt{1 1 1} and a maximum in electrooxidation rate is found when a single monolayer of palladium on Pt{1 1 1} is formed in agreement with previous studies. In contrast, aqueous sulphuric acid strongly inhibits formic acid electrooxidation due to extensive specific adsorption of (bi-)sulphate anions on the palladium adlayer which compete successfully with molecules of formic acid for adsorption sites. In all electrolytes studied, unique electrooxidation peaks ascribable to formic acid electrooxidation occurring on first and second/third layer palladium islands are observed. We ascribe this behaviour to a strong coupling of formic acid adsorption with changes in the local potential of zero total charge as a function of palladium coverage. Hence, it is deduced that Pd/Pt electrocatalysts utilising Nafion membranes should not prove deleterious to electrocatalytic activity. In contrast, similar measurements but using methanol instead of formic acid indicated that methanol electrooxidation is inhibited significantly when Nafion is adsorbed relative to activity recorded in aqueous perchloric acid. In addition, palladium adlayers on Pt{1 1 1} are found to inhibit methanol electrooxidation in all electrolytes studied, presumably because the key CH methanol dissociation step is less favoured on palladium compared to platinum. It is concluded that the role of specifically adsorbed anions is crucial in order to afford optimal fuel cell performance and that for methanol at least, proton conducting membranes other than Nafion should be considered although for formic acid, Nafion appears to be ideal.

Graphical abstractFigure optionsDownload full-size imageDownload high-quality image (125 K)Download as PowerPoint slideHighlights► The influence of Nafion on MeOH and HCOOH electrooxidation observed. ► Maximum electrooxidation activity at 1 monolayer of palladium for HCOOH. ► MeOH electrooxidation is inhibited on Pt{1 1 1} by Pd adsorption. ► For HCOOH electrooxidation, Nafion does not inhibit HCOOH adsorption. ► For MeOH electrooxidation, Nafion does inhibit MeOH adsorption.

Keywords
Platinum; Palladium; Nafion; Formic acid; Methanol; Electrooxidation
First Page Preview
Methanol and formic acid electrooxidation on nafion modified Pd/Pt{1 1 1}: The role of anion specific adsorption in electrocatalytic activity
Publisher
Database: Elsevier - ScienceDirect
Journal: Catalysis Today - Volume 202, 15 March 2013, Pages 128–134
Authors
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Subjects
Physical Sciences and Engineering Chemical Engineering Catalysis