Spearhead Metal Ultramicroelectrodes Based on Carbon Nanoelectrodes as Local Voltammetric pH Sensors

We suggest a new method for preparing metal electrodes based on carbon nanoelectrodes (CNEs), which are suitable for application in shearforce scanning electrochemical microscopy (SECM). The as-prepared CNEs were focused ion beam (FIB)-processed, subsequently thermally recessed, and finally, Pt or Au were electrodeposited into the cavity of the recessed CNEs leading to insulator-free metal electrodes with sizes of around 2 µm in diameter. For their application as local voltammetric pH sensors, the electrodes were calibrated in a concentration range of 11.6 M H⁺ to 9 M OH⁻. As a case study, we determined the local pH in situ using the developed Pt microelectrode positioned in close proximity to an Ag-based gas diffusion electrode (GDE) which was operated for the oxygen reduction reaction (ORR). We could observe a shift of the Pt_ox reduction peak of ΔE = −700 mV, which is exceeding previous observations at Ag-based GDEs by far. This novel type of electrode will pave the way to perform shearforce SECM with an even higher spatial resolution due to their spearhead shape.