Enhanced active sites using LDH nanosheets@rGO as bifunctional cathode catalyst for high-capacity rechargeable Li-O2 battery

The exfoliated nickel-cobalt layered double hydroxide nanosheets (LDHNS) and LDHNS@reduced grapheneoxide (LDHNS@rGO) composites were synthesized by exfoliating the bulk NiCoLDH using a facile sonication route. The crystal structure, morphology, and chemical environments of the synthesized samples were examined using various characterization techniques. Oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) kinetics on pristine LDHNS, LDHNS@rGO composites, and rGO catalysts were studied utilizing a rotating ring disc electrode (RRDE). It is observed that the LDHNS@rGO80 catalyst demonstrated superior ORR/OER kinetics among the pristine and other composite electrocatalysts examined. The LDHNS@rGO80 showed a more positive onset potential of 0.93 V, a high limiting current density of 8.39 mA cm-2 with a half wave potential (E1/ 2) of 0.86 V, and showed a low bifunctionality index of 0.96 V as well as, and with a low OER overpotential (eta onset) of 395 mV. The LDHNS grains with enhanced active sites acted as the reaction centers, while the rGO provided conductive network, resulting in reduced overpotential and increased current density for the ORR/ OER. Consequently, the pristine LDHNS or the LDHNS@rGO80 composite as cathode catalyst used in coin-type Li-air battery exhibited 3.0 V stable open circuit voltage (OCV). Interestingly, the fabricated Li-O2 battery (LOB) with the LDHNS@rGO80 (LDHNS with 80 wt.% rGO) air-cathode delivered a high discharge capacity of 9550 mA h g-1 at 100 mA g-1 and lit the green LED bulb for more than 40 h continuously on a single charge.