Differential mass spectrometry DEMS

The mechanisms of the oxidation of solvents such as THF and PC were studied by several groups, utilizing FTIR and XPS spectroscopy [107-109] and on-line mass spectrometry (DEMS-differential, electrochemical mass spectroscopy [110-112]). For example, using ex situ FTIR spectroscopy, Lacaze et al. [46] showed that THF in FiC104 solutions are polymerized on electrodes biased to high potentials. The proposed mechanism involves oxidation of C104 as an initial step, as shown in Scheme 7 [46,102], ESR measurements also support such a mechanism. However, there are also suggestions for possible direct oxidation... 

We will first describe briefly the main experimental techniques coupled with electrochemical methods Infrared Reflectance Spectroscopy (IRS), Electrochemical Quartz Crystal Microbalance (EQCM), Differential Electrochemical Mass Spectrometry (DEMS), Chemical Radiotracers and High Performance Liquid Chromatography (HPLC). 

Whether the oxidative or single nonoxidative desorption of the carbon monoxide occurs from the polycrystalline nickel surface was studied by differential electrochemical mass spectrometry (DEMS) [172] ... 

As a result, Wolter and Heitbaum obtained a mass signal proportional to the rate of the electrochemical reaction, i.e. to the current flowing through the porous electrode. Because of the applied pumping technique and because of the proportionality between electrochemical current (i.e. derivative of consumed charge) and mass signal, the method was called differential electrochemical mass spectrometry (DEMS)." ... 

From the above, it can be seen that most of the degradation factors in ESs are related to the electrolyte. Hahn et al. [91] used online mass spectrometry (differential eleetrochemical mass spectrometry, DEMS) to analyze the gas products evolved in an EDLC with AC electrodes in 1 M TEABF4/PC during the charging/ overcharging process. CO2 (m/z = 44), propene (m/z = 41), and H2(m/z = 2) were... 

In one of the most relevant papers in this field, Dima et al. [13] studied the electrocatalytic behavior of different polycrystalline metals such as Ru, Rh, Ir, Pd, Pt, Cu, Ag and Au for nitrate (100 mM) reduction in 0.5 M H2SO4. On the basis of the peak current density related to nitrate reduction on cyclic voltammograms, the activities of each electrode were compared. It was determined that rhodium is the most active catalyst among the noble metals for the reduction of nitrate, with the activity decreasing in the order Rh, Ru, Ir, Pt, Pd and Cu, Ag, Au for transition metals. The high electrocatalytic performance of Rh for nitrate reduction was also observed by Brylev et al. [19]. By using Differential Electrochemical Mass Spectrometry (DEMS), a reduction mechanism for nitrate reduction has been determined for transition metals (Fig. 2). 

Electrochemical measurements can also be coupled with mass spectrometry. Figure 2.17 shows a schematic diagram of the apparatus for differential electrochemical mass spectrometry (DEMS). Here the chamber connected directly to the electrochemical cell and the mass spectrometer (MS) is pumped differentially by turbo pumps PA and PB. Electrolysis products are passed into the ionization chamber (i), analyzed in the quadrapole mass filter (ii), and detected with either a Faraday cup (iii) or electron multiplier (iv). Such DEMS measurements can be used in situ to identily electrolysis products. This may lead to an understanding of the electron-transfer reaction mechanism and optimization of the reaction process. 

Lanz and Novak" studied gas evolution at thick graphite electrodes in y-butyrolactone EC DMC electrolyte by Differential Electrochemical Mass Spectrometry (DEMS). TIMREX SPG 6, SPG 15 and SPG 44 carbons were tested. They found that SEI formation on these thick electrodes was not yet complete after the first charge/discharge cycle. The amount of ethylene and hydrogen gas evolved decreases with increasing percentages of GBL in an EC/DMC electrolyte, indicating that the SEI layer is built up from GBL rather than from EC decomposition products. 

Abstract In this chapter, we present new insights in direct alcohol fuel cell-related anode electrocatalysis based on quantitative differential electrochemical mass spectrometry (DEMS) studies. First, we review the history and development of the DEMS technique, as well as the calibration method for quantification. We then discuss some contributions of quantitative DEMS to the study of the mechanism of methanol electrooxidation on Pt and PtRu model catalysts. We also discuss quantitative DEMS studies of the mechanism of dissociative adsorption and electrooxidation of ethanol and acetaldehyde at Pt, Pt3Sn, PtRu, and PtRh nanoparticle catalysts. Finally, the mechanism of dissociative adsorption and electrooxidation of ethylene glycol and its oxidative derivatives on carbon-supported Pt, Pt3Sn, and PtRu nanoparticle catalysts are discussed, based on quantitative DEMS results. 

KMC simulations have also been combined with differential electrochemical mass spectrometry (DEMS) to give new insights into the mechanisms and kinetics of adsorbed CO electro-oxidation on a platinum electrode. On the basis of DEMS experimental observations, the authors proposed a... 

Recently the proposed Oz evolution mechanism was supported by the results of a DEMS (Differential Electrochemical Mass Spectrometry) study performed by Wohlfahrt-Mehrens and Heitbaum [71] on Ru electrodes. Using this mass spectroscopic technique and lsO labeling for the determination of reaction products during 02 evolution, it could be verified that the oxygen of the oxide formed on Ru takes part in the 02 evolution process. The same observation was made for Ru02 electrodes when using labeled H2lsO. 

Mass spectrometry (MS) is an extremely powerful method of chemical analysis and the possibility of measuring electrochemical reaction products via MS was first suggested by Grambow and Bruckenstein (1977). The technique of differential electrochemical mass spectroscopy (DEMS) was later perfected and pioneered by Wolter and Heitbaum (1984). 

A quantitative study of the electro-oxidation of ethanol on carbon-supported Pt, PtRu and PtsSn catalysts was made by Behm and co-woikers " nsing combined voltarmnetric and on-line Differential Electrochemical Mass Spectrometry measnrements (DEMS). 

---