Seawater battery performance enhancement enabled by a defect/edge-rich, oxygen self-doped porous carbon electrocatalyst

Low-cost oxygen evolution/reduction reaction (OER/ORR) catalysts are critically important for energy conversion/storage systems. Here, a porous carbon (PC) catalyst is prepared as a low-cost catalyst from grapefruit peel biowaste using a facile hydrothermal carbonization combined with a chemical activation process. When examined as an OER/ORR catalyst in seawater, the PC exhibits an unexpected, highly efficient catalytic activity. From density functional theory (DFT) calculations, we found that sp³-bonded carbon atoms, zigzag edges with or without -COOH and -OH, and armchair edges with -O- are responsible for its superior OER/ORR activity. The suitability of the PC as a catalyst is tested in the half-cell of a Na/seawater battery, which exhibits a decreased voltage gap of ∼0.47 V between charge and discharge voltage curves. This value is even lower than those of Pt/C (∼0.68 V), IrO₂ (∼0.66 V) and MnO₂ (∼0.73). Also, a full cell of the metal-free seawater battery is assembled using hard carbon and PC as an anode and a catalyst, respectively. The full cell shows a lower voltage gap (∼0.65 V) with the voltage efficiency of ∼83-84% and excellent cycle life over 100 cycles. Our results confirm PC derived from grapefruit peels as an alternative to expensive Pt/C and IrO₂ catalysts for the OER/ORR activities in seawater batteries.