Measuring electrocatalytic activity on a local scale with scanning differential electrochemical mass spectrometry

This paper describes a new technique entitled scanning differential electrochemical mass spectrometry (SDEMS) that combines a quadrupole mass spectrometer with a membrane-covered capillary inlet and a high resolution positioning system that is designed to perform spatial mapping in solution near an electrode interface. Potential applications of this technique include the local characterization of anode catalysts for fuel cells as well as a range of analytical measurements and combinatorial screening studies. The capabilities of this technique are demonstrated by monitoring product evolution in several model electrocatalytic reactions, including the hydrogen evolution reaction, carbon monoxide oxidation, and the direct oxidation of methanol on platinum and platinum-ruthenium electrodes. The inlet of the SDEMS is based upon a small diameter capillary tube to which a nanoporous, hydrophobic membrane is attached. The capillary inlet is positioned near a substrate electrode using a three-dimensional positioning system. The effect of capillary substrate separation and substrate current on the sensitivity and time response of mass spectrometer's ion current are illustrated during hydrogen evolution at a platinum substrate. The sensitivity is demonstrated further by detection of carbon dioxide evolution during the oxidation of a monolayer of carbon monoxide adsorbed on platinum. The ability to address more complex reactions involving complete and partial oxidation products is illustrated with methanol oxidation. In order to demonstrate the ability of this technique to perform spatial mapping, an eight-element band electrode was interrogated for hydrogen evolution and methanol oxidation. Detection of ion currents associated with complete and partial oxidation products of methanol on a set of platinum-ruthenium band electrodes illustrates the ability of this method to spatially discriminate between various reactive sites on a surface, which has potential utility in analytical characterization as well as application as a screening tool in combinatorial catalysis studies. (C) 2003 The Electrochemical Society.