New Cell for DEMS Applicable to Different Electrode Sizes

A Versatile and Easy Method to Calibrate a Two-Compartment Flow Cell for Differential Electrochemical Mass Spectrometry Measurements

Online techniques for the quantitative analysis of reaction products have many advantages over offline methods. However, owing to the low product formation rates in electrochemical reactions, few of these techniques can be coupled to electrochemistry. An exception is differential electrochemical mass spectrometry (DEMS), which gains increasing popularity not least because of its high time resolution in the sub-second regime. DEMS is often combined with a dual thin-layer cell (a two-compartment flow cell), which helps to mitigate a number of problems that arise due to the existence of a vacuum|electrolyte interface. However, the efficiency with which this cell transfers volatile reaction products into the vacuum of the mass spectrometer is far below 100%. Therefore, a calibration constant that considers not only the sensitivity of the DEMS setup but also the transfer efficiency of the dual thin-layer cell is needed to translate the signals observed in the mass spectrometer into electrochemical product formation rates. However, it can be challenging or impossible to design an experiment that yields such a calibration constant. Here, we show that the transfer efficiency of the dual thin-layer cell depends on the diffusion coefficient of the analyte. Based on this observation, we suggest a two-point calibration method. That is, a plot of the logarithm of the transfer efficiencies determined for H2 and O2 versus the logarithm of their diffusion coefficients defines a straight line. Extrapolation of this line to the diffusion coefficient of another analyte yields a good estimate of its transfer efficiency. This is a versatile and easy calibration method, because the transfer efficiencies of H2 and O2 are readily accessible for a large range of electrode-electrolyte combinations.

The Effect of Water on the Quantification of Volatile Species by Differential Electrochemical Mass Spectrometry

Differential electrochemical mass spectrometry (DEMS) is one of the most powerful online techniques for quantitative determination of volatile species from electrochemical reactions. The products distribution as well as the respective production rate derived from DEMS measurements shed important light on the mechanisms and kinetics of complex reactions. In real measurements, the background mass signal of species to be detected changes with the reaction and the measurement conditions, which interferes the quantification of DEMS analysis. In this study, we analyzed systematically how the background mass signals of species change with the amount of water enters into the vacuum chamber from the electrolytic cell, since water is the dominant species in the cell with aqueous electrolyte. Our results reveal that during DEMS measurement, (1) there is a rather long time(>30 min) for the mass signals of volatile species to reach steady values after the filament for electronic ionization is turned on due to large sampling of water from the aqueous electrolyte; (2) the reaction of water with the hot filament changes the latter's surface state, it also produces H₂ and O₂, which can interfere the quantification of H₂ and O₂ produced by electrode reactions; (3) the ionization probabilities of other species are also affected by the change of the filament's surface state, the competition for ionization of water as well as the reaction between ionized water fragments with related species in the ionization chamber. Strategies on how to obtain reliable mass signals purely related to electrocatalytic reactions are provided.

Fast and sensitive method for detecting volatile species in liquids

This paper presents a novel apparatus for extracting volatile species from liquids using a "sniffer-chip." By ultrafast transfer of the volatile species through a perforated and hydrophobic membrane into an inert carrier gas stream, the sniffer-chip is able to transport the species directly to a mass spectrometer through a narrow capillary without the use of differential pumping. This method inherits features from differential electrochemical mass spectrometry (DEMS) and membrane inlet mass spectrometry (MIMS), but brings the best of both worlds, i.e., the fast time-response of a DEMS system and the high sensitivity of a MIMS system. In this paper, the concept of the sniffer-chip is thoroughly explained and it is shown how it can be used to quantify hydrogen and oxygen evolution on a polycrystalline platinum thin film in situ at absolute faradaic currents down to ∼30 nA. To benchmark the capabilities of this method, a CO-stripping experiment is performed on a polycrystalline platinum thin film, illustrating how the sniffer-chip system is capable of making a quantitative in situ measurement of <1% of a monolayer of surface adsorbed CO being electrochemically stripped off an electrode at a potential scan-rate of 50 mV s(-1).

Coupling of a scanning flow cell with online electrochemical mass spectrometry for screening of reaction selectivity

In this work the online coupling of a miniaturized electrochemical scanning flow cell (SFC) to a mass spectrometer is introduced. The system is designed for the determination of reaction products in dependence of the applied potential and/or current regime as well as fast and automated change of the sample. The reaction products evaporate through a hydrophobic PTFE membrane into a small vacuum probe, which is positioned only 50-100 μm away from the electrode surface. The probe is implemented into the SFC and directly connected to the mass spectrometer. This unique configuration enables fast parameter screening for complex electrochemical reactions, including investigation of operation conditions, composition of electrolyte, and material composition. The technical developments of the system are validated by initial measurements of hydrogen evolution during water electrolysis and electrochemical reduction of CO2 to various products, showcasing the high potential for systematic combinatorial screening by this approach.