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)via1,3-cycloaddition reaction of azomethine ylide as an intermediate, and the bestnanomaterial was obtained after 96 h of reaction (HRFG-96). HRFG-96 provided a superlative proton conductivity of 1.0x10-3Scm-1at room temperature and a low relative humidity of 40%. The material also displayed a weak humidity dependency for protonconduction. A remarkable proton conductivity of 3.94x10-2Scm-1was obtained at 95 degrees C, and 95% relative humidity along withexceptional thermal and long-term stability for seven days. This outstanding performance was attributed to a stable hydration layerformation through extensive hydrogen bonding between the excess hydroxyl groups and water molecules, which provided highproton conduction even at low-humidity conditions. Moreover, the shorter crystallite size of HRFG-96 led to substantialenhancement 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-1at 1 mV s-1and 320 F g-1at a current density of 1 A g-1in a three-electrodeconfiguration. The nearly 53% of specific capacitance was due to excess hydroxy functionalization, which resulted inpseudocapacitance because of proton-coupled electron-transfer reactions. The material revealed 121% cyclic stability after 25,000cycles due to the enhanced wettability during cycling that lowered the charge transfer resistance at the electrode/electrolyte interfaceand improved site-to-site charge transport.

Automated summary

Covalently functionalized hydroxyl-rich few-layer graphenes (HRFGs) were synthesized from oxygen-functionalized few-layer graphene (OFG) via 1,3-cycloaddition reaction, resulting in HRFG-96 after 96 h of reaction. HRFG-96 exhibited outstanding proton conductivity and thermal stability at both room temperature and high humidity conditions. The material also showed enhanced capacitance and cyclic stability as a supercapacitor electrode material, attributed to excess hydroxy functionalization and improved charge transport properties.