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)
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.
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
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.
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
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)