Bridging Electrochemistry and Ultrahigh Vacuum: “Unburying” the Electrode–Electrolyte Interface

Abstract

Metrics & MoreArticle Recommendations * sı Supporting InformationCONSPECTUS: Electrochemistry has a central role in addressing the societal issues of our time, including the United Nations' Sustainable Development Goals (SDGs) and beyond. At a more basic level, however, elucidating the nature of electrode−electrolyte interfaces is an ongoing challenge due to many reasons, but one obvious reason is the fact that the electrode−electrolyte interface is buried by a thick liquid electrolyte layer. This fact would seem to preclude, by default, the use of many traditional characterization techniques in ultrahigh vacuum surface science due to their incompatibility with liquids. However, combined UHV-EC (ultrahigh vacuum-electrochemistry) approaches are an active area of research and provide a means of bridging the liquid environment of electrochemistry to UHV-based techniques. In short, UHV-EC approaches are able to remove the bulk electrolyte layer by performing electrochemistry in the liquid environment of electrochemistry followed by sample removal (referred to as emersion), evacuation, and then transfer into vacuum for analysis. Through this Account, we highlight our group's activities using UHV-EC to bridge electrochemistry with UHV-based X-ray and ultraviolet photoelectron spectroscopy (XPS/UPS) and scanning tunneling microscopy (STM). We provide a background and overview of the UHV-EC setup, and through illustrative examples, we convey what sorts of insights and information can be obtained. One notable advance is the use of ferrocene-terminated self-assembled monolayers as a spectroscopic molecular probe, allowing the electrochemical response to be correlated with the potential-dependent electronic and chemical state of the electrode−monolayer− electrolyte interfacial region. With XPS/UPS, we have been able to probe changes in the oxidation state, valence structure, and also the so-called potential drop across the interfacial region. In related work, we have also spectroscopically probed changes in the surface composition and screening of the surface charge of oxygen-terminated boron-doped diamond electrodes emersed from high-pH solutions. Finally, we will give readers a glimpse into our recent progress regarding real-space visualizations of electrodes following electrochemistry and emersion using UHV-based STM. We begin by demonstrating the ability to visualize large-scale morphology changes, including electrochemically induced graphite exfoliation and the surface reconstruction of Au surfaces. Taking this further, we show that in certain instances atomically resolved specifically adsorbed anions on metal electrodes can be imaged. In all, we anticipate that this Account will stimulate readers to advance UHV-EC approaches further, as there is a need to improve our understanding concerning the guidelines that determine applicable electrochemical systems and how to exploit promising extensions to other UHV methods.

In situ and ex situ approaches for molecular scale understanding of electrochemical interfaces

“…[58][59][60] In addition, emersion of the Au(111) electrode while the potential is applied in a KI aqueous solution has enabled studies of atomic-level superstructures using UHV-STM. 21) Thus, it is possible to obtain complementary information to in situ measurements, but it is currently challenging to apply this technique to cases where the affinity between the electrolyte solution and electrode is high. 57) For example, if a hydrophilic electrode is emersed from an aqueous electrolyte solution, the exposed aqueous solution remains on the electrode surface, and the electrolyte precipitates on the surface during vacuum evacuation, making it impossible to obtain any desired information.…”

“…1(f)]. 20,21) Specifically, the electrode is emersed from solution with the potential applied to the electrode in an electrochemical environment, and the state of the electric double layer is maintained as much as possible for precise measurement in UHV (ex situ measurement). Although many attempts at such ex situ measurements have been reported over the past half century, 22) we have succeeded in extracting various useful information by using electrochemically active species as probes.…”