Solid-State NMR. Solid State NMR experiments were performed at 9.4 T (400 MHz 1 H Larmor frequency) using a Bruker Avance Neo. The samples were spun at a MAS frequency of 20kHz in a Bruker 3.2 mm HXY triple resonance probe and direct polarisation C MAS NMR experiments were acquired using a triple-pulse excitation experiment to efficiently suppress probe background signals (92.6 kHz radiofrequency field pulse strength). NMR experiments were performed on samples doped with Cr(acac)3 to shorten the T 1 ( 13 C) relaxation time and allow a quantitative recycling delay of 10s, which was used in our experiments. 50 mg of carbon powder was mixed with 1 ml 0.5 wt/vl% Cr(acac)3 in EtOH to create a slurry. The solvent was allowed to evaporate overnight, affording a solid. The spectra were referenced on adamantane setting the CH resonance at 28.8 ppm.