Abstract
Altering electrochemical interfaces by using electrolyte effects or so-called “electrolyte engineering” provides a versatile means to modulate the electrochemical response. However, the long-standing challenge is going “beyond cyclic voltammetry” where electrolyte effects are interrogated from the standpoint of the interfacial properties of the electrode/electrolyte interface. Here, we employ ferrocene-terminated self-assembled monolayers as a molecular probe and investigate how the anion-dictated electrochemical responses are translated in terms of the electronic and structural properties of the electrode/monolayer/electrolyte interface. We utilise a photoelectron-based spectroelectrochemical approach that is capable of capturing “snapshots” into (1) anion dependencies of the ferrocene/ferrocenium (Fc/Fc+) redox process including ion-pairing with counter anions (Fc+–anion) caused by differences in Fc+–anion interactions and steric constraints, and (2) interfacial energetics concerning the electrostatic potential across the electrode/monolayer/electrolyte interface. Our work can be extended to provide electrolyte-related structure-property relationships in redox-active polymers and functionalised electrodes for pseudocapacitive energy storage.
Highlights
Altering electrochemical interfaces by using electrolyte effects or so-called “electrolyte engineering” provides a versatile means to modulate the electrochemical response
Cyclic voltammograms (CVs) of Fc SAM on Au(111) in 0.1 M NaTFSI, NaPF6 and NaClO4 (Fig. 2b) show that in addition to the apparent formal potentials (Eo′), the nature of the anion influences the degree of conversion to Fc+
It is important to reiterate the role of TFSI− in creating an environment that inhibits the full conversion to Fc+ leading to conditions where at XPS/UPS can readily probe the influence of the electrostatic potential across the electrode/monolayer/electrolyte interface
Summary
Altering electrochemical interfaces by using electrolyte effects or so-called “electrolyte engineering” provides a versatile means to modulate the electrochemical response. The motivation stems from the desire to augment the functionality of devices by controllably altering key parameters such as the redox potential, reaction pathway, reversibility and stability Achieving this requires an intimate knowledge of the interfacial properties as this affects the charge transfer process and the overall electrochemical response. This fuels the desire to obtain structure–property relationships that sufficiently encompasses the intricacies of the electrochemical environment at the electrode/electrolyte interface[1,2,3,4]. The UHV-EC approach used in the present study involves electrode immersion (Fig. 2a and Supplementary Fig. 1) where electrochemical measurements are performed in a dedicated chamber under Ar atmosphere This is followed by removal under potential control and transfer to vacuum to enable a “snapshot-like” analysis into the electrochemical-induced changes. Notable work using UHV-EC by Kolb and Hansen attributed the observation of systematic XPS binding energy shifts with electrode potential as an indicator that the electrochemically-induced changes (i.e. double layer) can be conserved following electrode immersion[1,2,26,37,38]
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