Typical Synthesis

Cu3(HHTP)2 was synthesised by modifying a recently published literature procedure1. A solution of Cu(NO3)2.3H2O (0.127 g, 0.526 mmol, 1.65 eq) and aqueous ammonia (35 %) solution (0.829 mL, 15.0 mmol, 47 eq) in distilled water (2 mL) was prepared. The resulting dark blue solution was added dropwise to a dispersion of 2,3,6,7,10,11-hexahydroxytriphenylene hydrate, H6HHTP.xH2O, (0.103 g, 0.318 mmol, 1.00 eq) in distilled water (8.4 mL). The resulting mixture was heated in a mini tube furnace (Xiamen Tmaxcn Inc.) in a 40 mL screw vial(Thermo Scientific; B7999-6), closed with a screw cap fitted with a septum as a safety precaution in the event of over pressurisation, at 80 oC for 24 h. The dark blue precipitate formed was separated by centrifugation, and the supernatant layer was discarded. The dark blue precipitate was then washed successively with water (3 × 30 mL), ethanol (4 × 30 mL), and acetone (4 × 30 mL). Washing was performed by centrifuging the precipitate with the desired washing solvent for 15 – 30 minutes before removing the supernatant layer and replacing with fresh washing solvent. No soaking of the precipitate was performed. The resulting dark blue powder was dried at 75 oC under dynamic vacuum for 72 h and then stored in a N2-filled glovebox until used.

We found that rapid washing (completed in ca. 5 h) and activation of the synthesised Cu3(HHTP)2 to minimise its exposure to air was required to ensure a high porosity and a wider

stable double-layer voltage window.

Electric Double-Layer Capacitor Assembly

Symmetric electric double-layer capacitors (EDLCs) with Cu3(HHTP)2 composite and acetylene black film electrodes were prepared in Swagelok PFA-820-6 union tube fittings withhomemade stainless steel plugs as current collectors. Electrodes were cut from freestandingfilms in a N2-filled glovebox using a ¼’’ coin cell disc punching machine (Xiamen Tmaxcn Inc.), 7with areal mass loadings ranging between 10 – 35 mg cm-2. An excess of 1 M tetraethylammonium tetrafluoroborate (NEt4BF4) in anhydrous acetonitrile was used as an electrolyte. This solution was prepared in a N2-filled glovebox. Whatman glass microfibre filter (GF/A), cut with a ⅜’’ stainless steel manual punching cutter, was used as separator. This was dried in vacuo at 100 oC for 24 h prior to use. EDLCs were hand-sealed until air-tight before being removed from the glovebox for electrochemical testing.Symmetric electric double-layer capacitors (EDLCs) with YP50F film electrodes were prepared as coin cells in coin cell case (Xiamen Tmaxcn Inc.).Electrodes were cut from freestanding YP50F films with areal mass loadings ranging between 10 – 15 mg cm-2. The electrodes were dried in vacuo at 100 oC for at least 24 h prior to assembling the cell in a N2-filled glovebox. A 1 M solution of NEt4BF4 in anhydrous acetonitrile was used as an electrolyte. This solution was prepared in a N2-filled glovebox. Whatman glass microfibre filter (GF/A) was used as separator. This was dried in vacuo at 100 oC for 24 h prior to use. Each coin cell contained two SS316 separator disks and one SS316 spring to ensure sufficient pressure in the cell. The coin cells were sealed in the glovebox using a coin cell crimper (Xiamen Tmaxcn Inc.).

Cu3(HHTP)2 composite cells were assembled in Swagelok PFA-820-6 union tube fittings as opposed to in CR2032 SS316 coin cell cases (Cambridge Energy Solutions) as the removal of the electrodes for subsequent analysis without discharging the cell was easier from the fittings.

Three-Electrode Cell Assembly

Three-electrode cells were prepared in Swagelok PFA-820-3 union tube fittings with homemade stainless steel plugs as current collectors. Cu3(HHTP)2 composite electrodes with areal mass loadings ranging between 12-20 mg cm-2 were used as working electrodes. Overcapacitive YP50F activated carbon film electrodes with areal mass loadings of 35 – 40 mg cm-2 were used as counter electrodes. Ag wire was used as a pseudo-reference electrode.A 1 M solution of tetraethylammonium tetrafluoroborate (NEt4BF4) in anhydrous acetonitrile was used as an electrolyte. All measurements were performed under dry and oxygen-freeconditions in a vacuum glove box (Xiamen Tmaxcn Inc.). Under these conditions, the ferrocene–ferricenium (Fc/Fc+) redox couple was measured at 0.63 ± 0.01 V versus Ag. All potentials discussed for the threeelectrode cell are referenced to Ag.