Operando X-ray mapping of ion transport and arrangement in a carbon-based supercapacitor electrode.

Position- and time-resolved X-ray transmission (XRT) and small-angle X-ray scattering (SAXS) are used to map out ion transport and nanoscale ion arrangement in an in-plane carbon-based supercapacitor working electrode with 1 M RbBr aqueous electrolyte. XRT reveals that the charge balancing mechanism depends on the electrode geometry, shifting from ion exchange to predominantly Rb+ transport as the distance between working electrode and counter electrode is increased. SAXS shows that, despite these differences in ion mobilisation, the nanoscale ion arrangement within the smallest carbon nanopores remains essentially unchanged across the electrode and independent of the charging protocol. Small, but systematic differences are observed between sub-nanometre micropores (strong confinement) and larger micropores/mesopores (weaker confinement), the latter showing a distinct time dependence of ion concentration changes attributed to diffusive equilibration processes in these pores. Overall, these findings show that ion transport varies with position on the electrode, whereas the nanoscale ion arrangement does not. The smallest micropores behave uniformly across the electrode, and larger micropores and mesopores likewise show identical local arrangement at all positions, although their additional reservoir-character leads to a slight position dependent ion transport, similar to the overall bulk ion diffusion.