The in situ SAXS data fits apply the concept of plurigaussian random fields , as explained below. We assume the carbon nanopore structure and the corresponding GRF ( ) to be constant during the in situ SAXS experiment. The GRF parameters are taken from a fit to the experimental ex situ SAXS intensity of the empty nanoporous carbon electrode (Supplementary Fig. and Supplementary Table ). Therefore the experimental SAXS curve was normalized by its integrated intensity