Recycling Biowaste to Synthesize Nitrogen-Doped Highly Porous Activated Carbon Scaffolds for Selenium Stuffing with Superior Electrochemical Properties

The sodium–selenium batteries (Na–Se) have attracted excessive research attention owing to their high theoretical energy density and low cost. However, their pragmatic realization is still interrupted by low electrical conductivity of the selenium and polyselenide shuttle effect. In this study, nitrogen-doped highly porous activated carbons with different morphologies are prepared from red-onion waste via thermal pyrolysis. The interconnected 3D structure of carbons was used as the template to synthesize the selenium–carbon composites using the melt-infusion method, followed by selenium vapor deposition for Na–Se batteries. The composite electrodes with physiochemically trapped selenium (loadings between 50.5 and 70.1%) exhibited significant capability to confine the polyselenide shuttle effect. The nitrogen-doped highly porous activated carbon delivered a maximum reversible capacity of 394 mA h g–1 (at 50 mA g–1) with a good cycling performance (capacity retention of 91%) against Na+/Na. Similarly, the selenium–carbon composite electrode exhibited a maximum reversible capacity of 1105 mA h g–1 at 50 mA g–1 (equivalent to an energy density of 1768 W h/kg at 78 W/kg) with a good cycling performance (capacity retention of 86%) for Na–Se batteries. The electrochemical mechanisms are characterized using ex situ X-ray diffraction, cyclic voltammetry, and electrochemical impedance spectroscopy techniques, and it was found that the superior capacity is attributed to the unique design of composites, which ensures the fast kinetics of Na+ ions through interconnected 3D channels and high restrain against dissolution of polyselenides into an electrolyte. Such strategies, understandings, and recycling methods could shed light on synthesis of metal-selenium batteries in near future.