The QCM combined with voltage modulation at the front electrode: Applications in electrochemistry and beyond

A novel QCM-based instrument is described, which combines multifrequency lockin amplification with a periodic modulation of the electrical potential at the front electrode. The instrument amounts to a fast electrochemical quartz crystal microbalance (EQCM), which exploits accumulation and averaging. The time resolution is a few milliseconds. The frequency resolution after accumulation overnight is a few mHz, which allows to detect fractional coverage at a level of below 1%. The instrument determines frequency and bandwidth on a few different overtones. Being a QCM-D, it supplies information beyond gravimetry. A change in the bulk viscosity, which leads to Gordon-Kanazawa-type behavior, has a signature characteristically different from surface effects. Surface effects lead to Sauerbrey-type behavior. The QCM-D per se cannot distinguish between adsorption of rigid layers and slight changes in the Newtonian viscosity of the diffuse double layer. This distinction is possible based on the response time after sudden jumps in the electrode potential because the response from the diffuse double layer is faster than adsorption and desorption, the latter being linked to the Helmholtz layer. Electrogravimetric effects are seen at almost all charged interfaces, regardless of whether or not there is charge transfer in the Faraday sense. Application examples include the charge reversal in the diffuse double layers formed by inert, monovalent electrolytes, the underpotential deposition of copper on gold, and square wave electrogravimetry on a reversible redox pair.