Covalently Functionalized Hydroxyl-Rich Few-Layer Graphene for Solid-State Proton Conduction and Supercapacitor Applications

Covalently functionalized hydroxyl-rich few-layer graphenes (HRFGs) have been synthesized from oxygen-functionalized few-layer graphene (OFG) via 1,3-cycloaddition reaction of azomethine ylide as an intermediate, and the best nanomaterial was obtained after 96 h of reaction (HRFG-96). HRFG-96 provided a superlative proton conductivity of 1.0 × 10–3 S cm–1 at room temperature and a low relative humidity of 40%. The material also displayed a weak humidity dependency for proton conduction. A remarkable proton conductivity of 3.94 × 10–2 S cm–1 was obtained at 95 °C, and 95% relative humidity along with exceptional thermal and long-term stability for seven days. This outstanding performance was attributed to a stable hydration layer formation through extensive hydrogen bonding between the excess hydroxyl groups and water molecules, which provided high proton conduction even at low-humidity conditions. Moreover, the shorter crystallite size of HRFG-96 led to substantial enhancement of grain boundaries, which consequently increased the space charge as well. As a supercapacitor electrode material, HRFG-96 provided specific capacitances of 389 F g–1 at 1 mV s–1 and 320 F g–1 at a current density of 1 A g–1 in a three-electrode configuration. The nearly 53% of specific capacitance was due to excess hydroxy functionalization, which resulted in pseudocapacitance because of proton-coupled electron-transfer reactions. The material revealed 121% cyclic stability after 25,000 cycles due to the enhanced wettability during cycling that lowered the charge transfer resistance at the electrode/electrolyte interface and improved site-to-site charge transport.