Examples of
EC-MS data

In the experiment, Pt is used to electrolyze water to hydrogen and oxygen using electrochemistry. Sending a carbon monoxide (CO) pulse through the membrane microchip, the Pt electrode is poisoned. On the right, a representation of the mass spectrometry (MS) data versus common potential is shown. This type of plot is commonly used in differential electrochemical mass spectrometry (DEMS), online electrochemical mass spectrometry (OLEMS), or membrane inlet mass spectrometry (MIMS) graphs. In the plot on the left, time resolution and dependency is preserved, whereas on the right, time information is lost.

Figure 1. H2 evolution and CO stripping experiments using a 5 mm polycrystalline Pt electrode in 1M HClO4. Potential sweeps are carried out at 20 mV/s. a) Different signals from the mass spectrometer corresponding to H2 (m/z=2), He (m/z=4), CO (m/z=28) and CO2 (m/z=44) along with the potential and current density as a function of time. b) MS signal as a function of applied voltage and CVs corresponding to the colored sweeps in a).(1)

Figure 1 shows a CO stripping experiment performed with the Spectro Inlets EC-MS system. Starting from the left of Figure 1, one complete potential cycle is undertaken between 0.060 and 1.150 V vs RHE with a scan rate of 20 mV/s starting with a cathodic sweep from 0.45 V vs RHE to characterize the state of the electrode surface. Following the cycle, the potential is held at 0.45 V vs RHE while CO is injected through the chip from t = 180 s to t = 240 s (for further details on the gas exchange functionality, see section). A cathodic CO displacement current is observed, centered around t = 185 s. Following CO exposure, two cycles identical to the first one are carried out. The first cycle shows HER poisoning followed by the oxidation of the adsorbed CO at t = 340 s, and the CO2 released is immediately observed as a signal at m/z = 44, which decays slowly thereafter. The last cycle is identical to the cycle before CO exposure, confirming that the electrode surface and setup have returned to their original state free of CO. The integrated CO stripping peak from the cyclic voltammagram corresponds to 370 pmol CO2, or about 75% of a monolayer assuming a flat surface, in agreement with literature values. The integrated calibrated mass spectrometer signal for CO2, after subtracting the background from oxidation of residual adventitious carbon, is 345 pmol, or about 70% of a monolayer assuming a flat surface, in good agreement with the electrochemistry data. In comparison, the amount of hydrogen produced at t = 420 s in Figure 1 is 24 pmol, corresponding to 5% of a monolayer, i.e., 5% of a catalytic turnover assuming all surface atoms are active. In addition to the remarkable signal-to-noise ratio for detection of a sub-monolayer of desorbed product, this is, to the best of our knowledge, the fastest full execution of a CO stripping experiment ever reported. By this, we mean the shortest total time required for surface area measurement by CO stripping including:
  • The proof, by HER poisoning, that the surface has been completely covered by CO.
  • The proof, by prior and subsequent cyclic voltammetry, that the surface has returned to its initial, completely uncovered, state.
In this electrochemical experiment, chronoamperometry on a copper electrode is performed. Different reactant gases are used to saturate the electrolyte. If an Argon (Ar) pulse is sent, there is no observed effect on the CO reduction reaction products. Instead, if oxygen is sent, a transient boost of methane formation on the copper electrode is observed, whereas ethylene production is unaffected. Thanks to the unique patented membrane microchip inlet technology the system is fully quantifiable, thus the axes can be labelled with picomoles per second (pmol/s). Quantification is not possible in differential electrochemical mass spectrometry (DEMS), online electrochemical mass spectrometry (OLEMS), or membrane inlet mass spectrometry (MIMS)

Figure 2. The effect of oxygen demonstrated by two consecutive constant-potential CO electroreduction experiments performed at -0.9 V vs RHE. Gaseous Ar (a) and O2 (b) are injected as 90 s pulse injections into the carrier gas stream of the membrane chip, while holding the potential at 0.0 V vs RHE. This demonstrates that only gaseous O2 can activate the transient production of CH4.

Several key features of the Spectro Inlets system are highlighted in this experiment

Gas dosing with reactant gases. The system allows to control the gas saturation of the electrolyte and study the effect of different gases on electrodes. Electrolyte saturation is obtained within a second, thus avoiding the need of bubbling gas in external reservoirs for long time. See section for further details on the gas exchange feature. Single turnover sensitivity. Each molecule produced on the electrode is collected into the MS, so the total production can be easily quantified. Notably, while the EC signal is drowned in capacitance immediately after the change of potential, the MS signal provides unique insight into the transient behavior at the electrode surface. See Section 6 for further details on the high sensitivity of our instrument. Product detection. The instrument can easily measure CO reduction products, including multi-C products such as ethylene, ethanol, propene, etc.
In this electrochemical experiment, electrochemistry mass spectrometry (EC-MS) data on a CO reduction is shown. Cyclic voltammetry and chronoamperometry on a polycrystalline copper (Cu) electrode are shown. On the left, data is displayed against common synchronized time, whereas in b) the data is shown against applied working electrode potential vs reversible hydrogen electrode (RHE). The view in b) is commonly used in conventional DEMS (differential electrochemical mass spectrometry), online electrochemical mass spectrometry (OLEMS), or membrane inlet mass spectrometry (MIMS). However, in such graph the time resolution is lost. Furthermore, in DEMS, OLEMS, and MIMS, quantification is not possible.

Figure 3. a) EC-MS plot demonstrating the electrochemical desorption of gaseous H2 both at cathodic potential during HER and at potential anodic of the reversible hydrogen potential. The phenomenon shows during potential cycling from -0.3 to 0.45V vs RHE at a scan rate of 50 mV/s. b) EC-MS measurements plotted as a function of potential, where the anodic potential limit is set to 0.45, 0.60 and 0.85 V vs RHE, plotted in blue, green and red, respectively. Arrows indicate the direction of the potential scan during MS data acquisition. c) MS measurement of the anodic H2 desorption feature at different scan rates, indicating a strong potential dependence. d) Isolation of the anodic desorption feature by resting the electrode at an intermediate potential of -0.05 V vs RHE for 60 s and 120 s, respectively, in between HER and anodic desorption. HER is performed at -0.25 V vs RHE and anodic desorption is performed by sweeping the potential to 0.45 V vs RHE with 50 mV/s.

This experiment was made possible by the unique features of the Spectro Inlet system

Submonolayer sensitivity. The amount of hydrogen desorbed at anodic potential is 10% of a monolayer. Quantification. The amount of anodically desorbed hydrogen (~50 pmol) could be quantified because of the well-defined electrolyte volume, electrode-membrane distance, and molecular flow from the electrode to the vacuum chamber. Besides, the 100% collection into the MS chamber is key to obtain quantitative data. Real-time measurement. The anodic hydrogen release lasts only a few seconds. The extraordinary time resolution together with the high sensitivity of the system allow to measure fast transient phenomena in a fully quantitative fashion.

D. B. Trimarco, thesis, Department of Physics, Technical University of Denmark (2017)