EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

Solid-liquid interfaces play a central role in a variety of chemical process including electrochemistry, electrolysis, and catalysis. They are at the heart of modern devices such as batteries, fuel cells, supercapacitors, and electrocatalytic reactors. The electrical double layer (EDL) which forms at the solid-electrolyte interface, plays a key role in moderating these processes. Due to the difficulty in probing this thin layer, very few experimental mechanistic studies under realistic reaction conditions have been reported. Free-standing graphene is largely transparent to both electrons and photons. When used as the solid interface in an electrochemical environment, a variety of microscopic and surface-sensitive spectroscopic techniques can be used to probe that interface from the upper side. Single layer graphene (SLG) has the three desired traits: it is electrically conductive, mechanically robust, and atomically thin, which makes it ideal for this job.
Our study examines some of the fundamental issues in such a set-up. We have built an electrochemical micro-reactor cell enclosed by a SLG membrane, which also acts as the working electrode. The double layer thickness is varied controllably by changing the pH of the aqueous NaOH solution. Since our SLG is not doped via any support or contaminants, the changes occur only due to doping from applied electric field. Kelvin probe force microscopy (KPFM) measurements are used to monitor the shift of Fermi-energy. Due to cleanliness of our technique, our result can be described fairly well by a model considering the applied electrochemical potential, modified by the capacitative drop in the EDL. Differences between experimental values and those predicted by the model can be explained by electrochemical doping during the evolution of oxygen and hydrogen.