Electrochemical Noise At Electrode-Electrolyte Interfaces In the Absence of Externally Added Redox Active Species

Observables in traditional electrochemical experiments that are utilized for the quantification of electrode-electrolyte interface properties include current, voltage and charge.  Measurements of these observables tend to be contaminated by spontaneous fluctuations (i.e. “noise”) that limit overall accuracy, and despite the apparent importance of the effect of noise on electrochemical experimentation, the topic has received limited attention to date.  The coupling of chemical and electrical effects that is characteristic of electrochemical phenomena would indicate that the origin of the noise present at the electrode-electrolyte interface lies in the chemistry underlying the process of charge transfer occurring at the interface.  However, most analyses of noise phenomena in electrochemical systems rely on empirical analogies between the nonlinear electrode-electrolyte interface and linear solid-state circuits, thus reducing the complexity of noise generation by the charge transfer reaction to noise generation by elementary resistive impedances, in accordance with the traditional Nyquist analysis.  In this paper, a mechanistic analysis of the equilibrium noise generation processes at a metallic electrode-aqueous electrolyte interface is presented for the specific case when the electrochemical system does not contain any externally added redox active moieties.  The analysis draws on recent experimental and theoretical investigations of the quantum mechanical, non-adiabatic electronic transitions that characterize the charge transfer process between protons in the electrolyte and the metallic electrode, and which are responsible for what is colloquially referred to as a “leakage” current.  Experimental evidence in support of the noise analysis is presented as well, wherein the design of low noise instrumentation for the acquisition of noise spectra is described.  The effect of solution pH, electrode surface functionalization and electrolyte composition on the measured noise is analyzed by considering the mechanistic origin of the electrochemical noise.  Finally, the suppression of the spontaneous fluctuations by electronic feedback action is proposed, and the effect of the applied feedback on the local interface chemistry is examined.