Biomass-Derived P,N Self-Doped Hard Carbon as Bifunctional Oxygen Electrocatalyst and Anode Material for Seawater Batteries

Owing to the demand for low-cost batteries with safety, Na-seawater batteries (SWBs) have received considerable attention as a new energy storage system (ESS). In SWB, it is necessary to use an advanced oxygen evolution/reduction reaction (OER/ORR) catalyst for high energy efficiency (EE) in the cathode and a good sodium storage material for a highly reversible capacity in the anode part. In this study, nanostructured and N and P dual-doped hard carbon is fabricated by simply carbonizing abundant biomass, pine pollen. The oxygen electrocatalytic performance of pine pollen carbon (PPC) for a cathode is confirmed by a seawater half-cell test, which exhibits the lowest overpotential reported among Pt/C (20 wt%) and commercial hard carbon (HC) electrodes. The sodium-storage performance of PPC as an anode active material is tested using a coin-type Na half-cell, which exhibits a higher reversible capacity than that of the HC electrode. To reduce the manufacturing cost, this SWB, comprising both PPC electrodes at the anode and cathode, are fabricated and shown an EE of 74% and a coulombic efficiency (CE) of 98%. This study proposes a low-cost and safe ESS system by utilizing seawater as catholyte, bifunctional (OER/ORR, sodiation/desodiation) electrode configuration, and abundantly available biomass carbon.