Differential Electrochemical Mass Spectroscopy (DEMS)
DEMS and in situ FTIR
Direct alcohol fuel cells are those which utilize small organic molecules, such as methanol or ethanol, as fuels without first reforming them to hydrogen gas. These devices promise to be an efficient means of converting chemical energy to electrical energy, but to become practical, there are materials related obstacles which must first be overcome. Improving the sluggish kinetics of fuel electrooxidation at the anode is a major hurdle. In order to gain mechanistic insights into this reaction and guide the search for new catalysts, we employ two techniques which couple spectroscopy with electrochemistry: Differential Electrochemical Mass Spectroscopy (DEMS) and in situ Fourier Transform Infrared Spectroscopy (FTIR).
Using DEMS, volatile chemical species generated at a catalyst can be detected mass spectrometrically with very little time delay. As an example, figure 1 shows the electrooxidation of methanol at a PtPb catalyst, and the simultaneously recorded signals for CO2 and methyl formate produced by the oxidation. With proper calibration, DEMS can even be used to determine the current efficiency for CO2 production, which indicates what fraction of the oxidation current arises from complete oxidation of the fuel.
In the in situ FTIR apparatus, an infrared beam is reflected from an electrode surface in an electrochemical environment. Species in a thin layer of solution between the electrode and the CaF2 window of the cell, or adsorbed on the electrode surface are probed by the IR beam. In this respect, in situ FTIR provides a good complement to DEMS, which cannot directly detect adsorbed species. Figure 2 shows a set of IR spectra collected during formic acid oxidation at a Pt and at a PtPb electrode. While the formation of CO2 is readily apparent in both spectra, the formation of adsorbed carbon monoxide (COads) is only visible on Pt. The lack of COads formation on PtPb may help to explain its superior catalytic activity for formic acid. At low potentials, the oxidation of COads is very slow, so it acts a poison by blocking the catalyst surface.
Being able to identify and quantify the products of electrochemical reactions is invaluable in devising new catalysts and characterizing their activity. We are currently using these techniques to analyze new catalysts developed both in our and our collaborators' groups.