Supercapacitors are devices capable of absorbing and delivering large amounts of energy over short time-scales. They are ideal for high-power/low-energy applications such as automotive acceleration and braking. In an attempt to reduce costs and improve safety we are developing a new generation of aqueous supercapacitors made from polymer-derived carbons loaded with N and S and porosity optimised to improve performance. One major challenge is to decouple pseudocapacitance associated with the heteroatoms from double layer capacitance from electrolyte in the porosity. To optimise performance we need to understand the ingress of electrolyte into the porosity: Contrast-Matching SANS can measure this and we will use D2O to decouple open from inaccessible porosity our new carbons. This will help us develop a new class of supercapacitor electrode material.