Real-time detection of sub-monolayer desorption phenomena during electrochemical reactions: Instrument development and applications

This thesis presents the development of a novel analysis instrument for performing highly sensitive electrochemistry mass spectrometry (EC-MS) measurements in real-time. The instrument is based on a micofabricated membrane chip which is used to establish a direct loss-free coupling between wet electrochemistry and the vacuum of a mass spectrometer, thereby enabling a high detection probability of volatile analyte species by the mass spectrometer. The instrument exhibits a higher sensitivity than conventional differential electrochemical mass spectrometry (DEMS), while maintaining a fast response time, making it highly suitable for electrochemistry studies.
Incorporating the membrane chip into a stagnant thin-layer electrochemistry cell, 100% collection efficiency is ensured, which makes it possible to utilize the full dynamic range of a standard MS, and thereby analyze desorption phenomena during electrochemical measurements with sub-monolayer sensitivity. The membrane chip furthermore makes it possible to directly control the dissolved gas atmosphere experienced by the working electrode by dosing gases through the membrane. Thereby reactant gases can be introduced, either for stead-state electrocatalytic measurements, or by transiently perturbing the system with single pulses of reactant gas injections.
The capabilities of the instrumentation are demonstrated by studying electrochemical hydrogen evolution (HER), oxygen evolution (OER), CO oxidation
and CO stripping on polycrystalline platinum. The latter two are made possible by the transient and steady-state introduction CO reactant gas, respectively, through the membrane. A mass transport model is used to describe the analytetransport from the surface of an electrode, through the stagnant thin-layer cell, through the membrane chip and into the mass spectrometer. By applying this model to HER, OER, CO oxidation and CO stripping experiments, it is demonstrated possible to match anticipated mass spectrometer signals based on current measurements on the electrode, to the actual measured mass spectrometer signal. This mapping function thereby directly couples mass spectrometer data with faradaic currents on the electrode surface.
The EC-MS instrumentation presented herein is utilized to discover a new electrocatalytic phenomenon during electrochemical CO reduction: By exposing a copper catalyst to dioxygen prior to constant-potential electrolysis, a new reaction pathway towards methane production is temporarily established. The phenomenon is shown only to affect the formation of methane, leaving ethylene and hydrogen formation unaffected. Using density functional theory (DFT) it is demonstrated that adsorbed oxygen on surface sites adjacent to undercoordinated kink sites destabilizes the binding energy of CO, while stabilizing the binding energy of CHO due to a geometric tilting effect. This causes the initial protonation towards methane, which is otherwise known to be rate-limiting on kink sites, to become energetically favorable. The phenomenon is short-lived, as the adsorbed oxygen reduces away within ∼1 s during which only a few turn-overs of methane occurs.
Furthermore, a preliminary EC-MS study is presented, which reveals a new desorption phenomenon on polycrystalline copper: By initially priming a copper electrode at cathodic potentials close to hydrogen evolution potentials, the desorption of ∼ 50 pmol of gaseous hydrogen, corresponding to ∼ 10% of a
monolayer, is measured when scanning to potential anodic of the reversible hydrogen potential (RHE). The phenomenon is proven to be electrochemically triggered, unrelated to copper oxidation and invariant to pH on the RHE scale. The amount of desorbed hydrogen is likewise proven to be unaffected by priming conditions and potential scan rates. By variation of the upper and lower potential limits the ad- and desorbtion potentials for hydrogen on copper are measured to be -0:125 and +0:05 V vs RHE, respectively. The proposed mechanism is that hydrogen adsorbs to the surface at cathodic potentials prior to HER, and remain surface bound until it becomes energetically favorable to adsorb OH at more cathodic potentials, at which hydrogen is expelled through a surface replacement reaction. As only ∼10% of a monolayer, is observed, the hydrogen is believed only to remain adsorbed on stronger binding step sites. The proposed mechanism is not yet verified with any theoretical predictions. The presented electrochemical studies exemplifies some of the unique capabilities enabled by the presented EC-MS instrumentation. The potential for future application of the technique could be wide-spread and enable unique insight into electrochemical reaction mechanism through careful study of sub-monolayer desorbtion phenomena similar to the ones presented herein.