Supporting Figure Voltammetry of [IrCl 6 ] 3-Oxidation on Graphite. A steady-state voltammogram of an ultramicroelectrode under a radial diffusion regime is independent of the scan rate. The differences between the voltammograms recorded at different scan rates in this figure arise from the fact that, for scan rates slower than 100 mV s -1 , the diffusion layer thickness become comparable to the dimensions of the microdroplet electrochemical cell, which leads to the thin-layer cell diffusional effects. Conversely, faster scans result in a deviation from the steadystate due to the increasing contributing from the linear diffusion. Scanning at 100 mV s -1 yields >95 % steady-state microelectrode response, considering the electrode radius (2.5 µm) and the diffusion coefficient of [IrCl 6 ] 3-(2.1 × 10 -6 cm 2 s -1 ), and was therefore chosen as a good compromise to achieve the steady-state response, while avoiding the undesirable thin-layer cell effects. The limiting current (I lim ) obtained on graphite at 100 mV -1 was generally found to be in agreement with the theoretical value for a disk ultramicroelectrode (I lim = 4nFDcr, where n is the number of electron transferred, F is the Faraday constant, D is the diffusion coefficient of the mediator, c is the mediator concentration, and r is the radius of the microelectrode). Supporting Figure Raman Spectroscopy and XPS of hBN and Graphite. a, Raman spectra of hBN on graphite. The presence of the E 2g mode at ~1368 cm -1 confirms the hexagonal phase of hBN, 5 but its intensity in 1L and few-layer hBN is negligible in comparison to the G and 2D modes in graphite/graphene and therefore prevent detailed analysis. In order to avoid laserinduced damage, the Raman spectra were acquired after the electrochemical measurements. Topleft inset: an optical image of the different hBN layers showing the collection spots. b, Survey XPS spectra of bulk hBN (top) and graphite (bottom). c-e, High-resolution XPS spectra of the N 1s, B 1s, and C 1s regions, showing single Gaussians without shoulders for N and B at expected binding energies and FWHM ~1 eV, indicating low concentration of impurities in hBN. The C 1s binding energy corresponds to the sp 2 carbon hybridization and contains a shoulder at the high-energy tails, indicating partial surface oxidation common in graphite. The XPS quantification was not considered reliable due to the low XPS sensitivity for light elements.