A Voltage-Modulation QCM: Application to Kinetic Studies on Electrode Processes

• Applicant: Professor Dr. Diethelm Johannsmann
• Subject Area: Physical Chemistry of Molecules, Liquids and Interfaces, Biophysical Chemistry ; Physical Chemistry of Solids and Surfaces, Material Characterisation
• Term: since 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 500106119

A new, fast electrochemical quartz crystal microbalance (EQCM) shall be coupled with fast modulation of the potential of the working electrode in order to access the kinetics of processes at the electrode. The time resolution is 100 µs. The modulation makes this instrument largely independent of instrumental drifts. Because the processes to be studied are periodic, accumulation and averaging can bring the precision of the frequency readings into the sub-mHz range. Applications can be from the field of electrochemistry, where processes without electron transfer will also be studied. In the latter case, the electric potential affects the charge in the electrical double layer and the pH near the electrode.Initially, instrumental improvements are to be implemented. Noise shall be further reduced and measurements in the frequency domain (sinusoidal excitation with variation of frequency) and the time domain (pulse or step excitation) shall be compared with respect to their performance. Procedures will be developed, that can be used to distinguish between adsorption/desorption, changes in the viscoelasticity of the diffuse double layer, and changes in the viscoelasticity of the bulk. A first set of phenomena to be studied is the electroresponsivity of thin organic films. For weak polyelectrolytes, electroresponsivity is often related to pH-induced swelling and deswelling. The kinetics of swelling and deswelling often are different. Surface-attached chains of poly[2(dimethylamino)ethyl methacrylate] (pDMAEMA) will be a model system. Furthermore, the adsorption and desorption of polymers with a constitution similar to pDMAEMA will be investigated. These materials are related to electrodeposition paints, where adsorption and desorption are reversible, here. A second application will be electron transfer to redox-active molecules in solution. These electrogravimetric experiments will complement cyclic voltammetry. From the time derivative of the frequency shift, an apparent mass transfer rate will be calculated, which can be directly compared to the electric current. The differences in the shape of the curves (current and apparent mass transfer) shall be understood quantitatively. Pulsed methods (analogous to square-wave voltammetry) shall allow to separate the different contributions to the apparent mass transfer rate. These experiments shall be extended to simple reactions from organic electrosynthesis (i.e., irreversible reactions).A long-term perspective of this work is the application of voltage modulation to ordinary adsorption processes, which can improve the depth of information obtained with the QCM, exploiting the adsorbate’s response to voltage modulation.

• DFG Programme: Research Grants