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Letters to Analytical Chemistry New Double-Band-Electrode Channel Flow Differential Electrochemical Mass Spectrometry Cell: Application for Detecting Product Formation during Methanol Electrooxidation Hongsen Wang, Eric Rus, and He´ctor D. Abrun˜a* Department of Chemistry and Chemical Biology, Baker Laboratory, Cornell University, Ithaca, New York 14853-1301 We present a new double-band-electrode channel flow DEMS (differential electrochemical mass spectrometry) cell and demonstrate its application in mechanistic studies with particular relevance to fuel cells. The cell is composed of two band electrodes, which serve as working and detecting electrodes, respectively, separated by a porous Teflon membrane. The Teflon membrane serves as the interface between the aqueous solution and vacuum, through which gases and volatile species can be transported. The hydrodynamic electrochemical characteristics and mass spectrometric behavior have been characterized. With this DEMS cell, gaseous and volatile electrochemical products formed at the working electrode are monitored by mass spectrometry, while nonvolatile products can be selectively detected at the detecting (downstream) electrode. Thus, this system can be considered as the DEMS analogue of a rotating ring/disk electrode. As test cases, the electrooxidation of formaldehyde and methanol on carbon supported Pt nanoparticle catalysts have been studied using this new channel flow DEMS cell. In situ (or online) mass spectrometric detection of volatile electrochemical reaction products began in the early nineteen seventies, when Bruckenstein and Gadde employed a porous nonwetting electrode for the coupling of an electrochemical cell to a mass spectrometer.1 However, their system could only detect the integrated mass spectrometric current due to the closing-off of the vacuum system from the pump. The system was later improved by Wolter and Heitbaum,2,3 so that the time constant was sufficiently short so as to allow the real-time, online detection of volatile electrochemical reaction products during cyclic voltammetry and potential step chronoamperometry. With the appropriate design of the cell and vacuum systems, using turbomolecular pumps, product formation rates could be measured. To distinguish this technique from Bruckenstein’s integrating ap* Corresponding author. Tel: 1-607-255-4720. Fax: 1-607-255-9864. E-mail:
[email protected]. (1) Bruckenstein, S.; Gadde, R. R. J. Am. Chem. Soc. 1971, 93, 793–794. (2) Wolter, O.; Heitbaum, J. Ber. Bunsenges. Phys. Chem. 1984, 88, 2–6. (3) Wolter, O.; Heitbaum, J. Ber. Bunsenges. Phys. Chem. 1984, 88, 6–10. 10.1021/ac100320a 2010 American Chemical Society Published on Web 05/11/2010
proach, it was called “differential electrochemical mass spectrometry” (DEMS). In order to detect, via mass spectrometry, volatile species that are generated at an electrode surface, they have to be transported from the electrolyte phase to the vacuum system. To accomplish this, a membrane (e.g., a porous Teflon membrane), which separates the electrolyte from the vacuum, but is permeable to volatiles, is required. In addition, the electrogenerated species need to be transported from the electrode surface to this interface rapidly. To achieve this, either the electrode has to be placed very close to the membrane, so that the transport by diffusion will be sufficiently fast, or proper convection has to be employed.4,5 Several electrodes and cell types have been designed to achieve this goal. In the classical approach, the electrocatalyst layer, e.g., Pt, is sputter-deposited directly onto the porous Teflon membrane or a metallic lacquer, containing small metallic particles, is painted onto the Teflon membrane. In both of these cases, the membrane, which is mechanically supported by a stainless steel frit, serves as the working electrode and the interface between the electrolyte and vacuum. A typical cell for these electrodes has been described in ref 5. This type of DEMS cell has a very high collection efficiency, and the response time for electrochemical reactions is about 0.1 s. However, mass transport is not well-defined, and bulk electrodes cannot be employed with this cell. To enhance mass transport, Fujihira et al. placed a rotating rod above the stationary gas permeable porous electrode.6 Tegtmeyer et al. adopted a rotating porous electrode as an inlet system to the mass spectromer vacuum.7 However, this system is structurally complicated. Wasmus et al. positioned a rotating cylinder electrode near an inlet membrane window to the mass spectrometer.8 However, a high collection efficiency and a welldefined solution flow were difficult to establish. (4) Baltruschat, H. J. Am. Soc. Mass Spectrom. 2004, 15, 1693–1706. (5) Baltruschat, H. In Interfacial Electrochemistry: Theory, Experiment and Applications; Wieckowski, A., Ed.; Marcel Dekker, Inc.: New York, 1999. (6) Fujihira, M.; Noguchi, T. J. Electroanal. Chem. 1993, 347, 457–463. (7) Tegtmeyer, D.; Heindrichs, A.; Heitbaum, J. Ber. Bunsenges. Phys. Chem. 1989, 93, 201–206. (8) Wasmus, S.; Cattaneo, E.; Vielstich, W. Electrochim. Acta 1990, 35, 771– 775.
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In order to use bulk electrodes in DEMS experiments, several cells have been designed. A simple approach is that of a porous Teflon-membrane-covered pinhole, which serves as the gas inlet, located very close to the center of the electrode surface.9,10 Electrodes can be used in the hanging meniscus geometry. However, this configuration has the disadvantage that the volatile products are sampled not only from the small cylindrical volume between the pinhole and the electrode surface but also from the neighboring volume, leading to a complicated time dependence and longer response times. In order to quantitatively detect volatile products of electrochemical reactions on bulk electrodes, e.g., single crystal electrodes, Baltruschat and co-workers developed a thin layer DEMS cell.11,12 This cell can be used for both desorption experiments under stopped flow conditions and for the measurement of product formation rates under continuous flow. However, under conditions of continuous electrolyte flow, the collection efficiency is very low, as a considerable fraction of the products is transported out of the thin layer volume before it can diffuse through the Teflon membrane. To increase the collection efficiency under conditions of continuous electrolyte flow, Baltruschat and co-workers later designed a dual thin layer flow cell. The construction of this cell has been described in detail in refs 5, 13, and 14. To detect the oxidation products of small organic molecules under fuel cell operating conditions, several methods to interface a mass spectrometer with fuel cells have been developed. Stimming et al. developed a fuel cell DEMS setup in which the anode outlet liquid emission was led over a porous Teflon membrane that served as the interface between the liquid and the vacuum.15 Savinell et al. reported an approach allowing for the online DEMS detection in a technical fuel cell operating over the temperature range between 150 and 190 °C, in which a capillary inlet of the mass spectrometer is connected to the anode outlet of a gas-fed direct alcohol fuel cell.16 Nakagawa positioned a capillary probe of a mass spectrometer into the anode gas layer of a semipassive direct methanol fuel cell (DMFC) in order to evaluate the gas composition in contact with the anode.17 To study the oxidation of small organic molecules at elevated temperatures, a high temperature and high pressure DEMS cell has also been designed by Behm’s group.18 In general, DEMS is a very sensitive analytical technique with a detection limit of ∼0.1 nmol, capable of analyzing even submonolayers of organic adsorbates.4,19 By combining mass spectrometry with suitable electrochemical cells, one can quantify (9) Gao, Y.; Tsuji, H.; Hattori, H.; Kita, H. J. Electroanal. Chem. 1994, 372, 195–200. (10) Housmans, T. H. M.; Wonders, A. H.; Koper, M. T. M. J. Phys. Chem. B 2006, 110, 10021–10031. (11) Hartung, T.; Baltruschat, H. Langmuir 1990, 6, 953–957. (12) Baltruschat, H.; Schmiemann, U. Ber. Bunsenges. Phys. Chem. 1993, 97, 452–460. (13) Jusys, Z.; Massong, H.; Baltruschat, H. J. Electrochem. Soc. 1999, 146, 1093–1098. (14) Wang, H.; Lo ¨ffler, T.; Baltruschat, H. J. Appl. Electrochem. 2001, 31, 759– 765. (15) Seiler, T.; Savinova, E. R.; Friedrich, K. A.; Stimming, U. Electrochim. Acta 2004, 49, 3927–3936. (16) Wasmus, S.; Wang, J.-T.; Savinell, R. F. J. Electrochem. Soc. 1995, 142, 3825–3833. (17) Masdar, M. S.; Tsujiguchi, T.; Nakagawa, N. J. Power Sources 2009, 194, 618–624. (18) Jusys, Z.; Behm, R. J. ECS Trans. 2008, 16, 1243–1251.
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gaseous and volatile products of electrochemical reactions and organic adsorbates.14,20,21 Direct methanol fuel cells (DMFCs) hold great promise as power sources for portable electronic devices. Because of the enhanced level of research and development of new active anode electrocatalysts for DMFCs, (quantitative) DEMS has been extensively used to study the mechanism of methanol electrooxidation and kinetics of reaction pathways.14,22-29 Methanol electrooxidation on Pt-based catalysts generates CO2, formic acid, formaldehyde, and methyl formate. However, DEMS can only detect CO2 and methyl formate. Methanol interferes with the detection of formaldehyde due to the presence of fragments at m/z ) 28-30 and formic acid’s molecular ion at m/z ) 46 overlaps with that of isotopic CO2.25,30 In addition, at low concentration, formic acid (
