Building a Differential Electrochemical Mass Spectrometry (DEMS): A Powerful Toll for Investigation of (photo)Electrochemical Processes

Abstract

The development of electrochemical devices more efficient for energy conversion and storage are a challenge for modern society. Among this class of devices, fuel cells, batteries, and photovoltaic cells stand out. Despite their importance and practicality, the electrochemical processes that take place in these devices are not always easily understood. In addition, the detection and identification of the products and intermediaries, as well as the reaction paths usually are extremely complex, due to the different species produced and also several reaction steps that take place simultaneously in this type of system. In this context, analytical techniques have been combined to electrochemistry systems to investigate the electrochemical processes in real-time. FTIR (infrared spectroscopy with Fourier transform), MS (mass spectrometry) are some of examples that coupled techniques. Therefore, these setups provide great opportunities for the electrochemistry field, which also includes catalysts design improvement and reaction investigations under different experimental conditions.1-3 The electrochemical mass spectrometry (EMS) technique has stood out as an important tool for the identification and analyses of qualitatively/quantitatively gaseous, volatile intermediates, and products generated during electrochemical reactions. The interface between the mass spectrometer’s high-vacuum system and the electrochemical cell’s room pressure was composed of a hydrophobic porous membrane connected to the inlet system. The ionic current intensity associated with the products was monitored and correlated to the faradaic current3. In 1984 Wolter and Heitbaum4 refined the Electrochemical Mass Spectroscopy technique, employing a new vacuum system design consisting of two pumping stages, giving high sensitivity to the technique and allowing the on-line detection of gaseous with response-time ≤ 0.5 s, four times faster than the first setup developed. The Wolter and Heitbaum’s method was called Differential Electrochemical Mass Spectroscopy (DEMS). Here, differential pumping is related to a two-stage pressure reduction, where the gas flow is initially expanded to a low vacuum and later to a high vacuum, where the mass spectrometer is located4. This work presents the instrumental design used to construct a DEMS to investigate and elucidate (photo)electrochemical reactions. The DEMS design selected was composed of a two-stage differentially pumped vacuum system (Figure 1(I)). The instrument is composed of three main sections: electrochemical cell; hydrophobic microporous membrane, and mass spectrometer, including the vacuum system (turbomolecular and pre-vacuum pumps). The electrochemical cell used, was employed initially by Baltruschat5, this cell use a modified Polytetrafluoroethylene (PTFE) porous membrane interface as the working electrode, which is prepared by metal sputtering onto a microporous membrane.An example of the relation between cyclic voltammogram (CV) and mass-spectrum cyclic voltammograms (MSCV) using the DEMS constructed in this work is shown in Fig. 1((II)a-b), respectively (CVs only in H2SO4 is presented as an insert in Fig. 1(a)). The working electrode was sputtered of platinum (50 nm thickness) onto the PTFE microporous membrane interface. The counter electrode was a platinum mesh with a high surface area, the reference electrode was a reversible hydrogen electrode (RHE). The electrochemical cell was directly attached to the pre-vacuum chamber of the mass spectrometer High purity N2 was used to de-aerate the solution.The cyclic voltammograms were recorded at a sweep rate of 10 mV s-1. While the faradaic current and the ion mass current for the generated product, CO2 (m/z = 44 (CO2 +), were monitored at the same time. Fig. 1(c-e) shows the faradaic (d) and ionic currents (e) profiles vs. time obtained for 6 CVs, the potential is also presented. As can be observed, the faradaic current has an excellent correlation with the ionic current, conducting to a great synchronization between the voltammetric scans and the ionic signal profile. Besides, the results show a significant deactivation of the electrocatalyst, which is followed by the ionic signal (m/z = 44). As reported in the literature6, the formic acid electrooxidation produces a poison intermediate, which is formed from the dissociative adsorption of formic acid on the electrode surface, this poison specie was posteriorly identified as CO. Therefore, the DEMS technique can provide important information to determine the reaction mechanisms. Bashyam, R.; Zelenay, P., Nature 2006, 443 (7107), 63-66. Bruckenstein, S.; Gadde, R. R., Journal of the American Chemical Society 1971, 93 (3), 793-794. Chen, A.; Lipkowski, J., The Journal of Physical Chemistry B 1999, 103 (4), 682-691. Wolter, O.; Heitbaum, J., 1984, 88 (1), 2-6. Baltruschat, H., Journal of the American Society for Mass Spectrometry 2004, 15 (12), 1693-1706. Herrero, E.; et al.., Journal of Electroanalytical Chemistry 1993, 350 (1), 73-88. Figure 1