(Digital Presentation) High-Resolution Imaging of Active Sites Under Reaction Conditions for Carbon-Based Electrocatalysis

Due to the abundance and the electrochemical versatility of carbon, it is becoming an increasingly popular material for electrocatalytic applications. Whether it is used as a support material, as a catalyst, or even as a bifunctional catalyst, insights into the reaction processes are of fundamental value. In this light, we employ electrochemical scanning tunneling microscopy (EC-STM) to in-situ evaluate electrode surfaces' behavior and identify the nature of the active sites.[1] The experimental distinction between inactive and active sites of a catalytic system can be achieved by comparing the noise level of surface sites in the EC-STM signal while a reaction is ‘Off’ or ‘On’, respectively. The tunneling current will be stable under both conditions if the scanning tip is positioned over an inactive site. Over an active site, reactions occurring within the tunneling gap will influence the EC-STM signal, which can be observed as locally confined noise features superimposed on the surface morphology. Here, we examine highly ordered pyrolytic graphite (HOPG) in alkaline and acidic media as a model system for carbon-based structures. In an alkaline medium, we compare the activity of specific surface sites under oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) conditions (Figure 1a).[2] In both cases, predominantly steps and defects are active. However, in the case of the OER, the terraces also play a role. For the hydrogen evolution reaction (HER) in acidic media, it was possible to identify individual active sites on the ‘honeycomb’ structured surface with down to atomic resolution (Figure 1b).[3] Apart from HOPG, the technique will be demonstrated for metal-organic frameworks, another class of promising catalysts for ORR and OER. References: [1] Pfisterer, J. H.; Liang, Y.; Schneider, O.; Bandarenka, A. S. Nature 2017, 549, 74. [2] Haid, R. W.; Kluge, R. M.; Schmidt, T. O.; Bandarenka, A. S. Electrochim. Acta 2021, 382, 138285. [3] Kluge, R. M.; Haid, R. W.; Stephens, I. E. L.; Calle-Vallejo, F.; Bandarenka, A. S. Phys. Chem. Figure Caption: Figure 1: a) n-EC-STM measurement of a step under ORR (red) and OER (green) conditions, as well as during reaction ‘Off’ (blue). The most active sites in both cases can be found at steps, indicated by the distinct noise spikes at these positions compared to reaction ‘Off’. Terraces are inactive towards the ORR. However, they do play a role in the OER.[2] © 2021 Elsevier Ltd. b) Atomically resolved active sites on HOPG under HER conditions. Bright spots in the measurements indicate active centers. They can be observed predominantly at step edges and defects, while the basal plane (terrace) remains inactive.[3] © 2021 The Royal Society of Chemistry Figure 1