Highly Porous N-Doped Graphitic Carbon from Bio-Waste As Bifunctional Elactrocatalyst for Hybrid Na-Air Battery

Abstract

Rechargeable metal-air batteries have emerged as one of the most promising candidates for sustainable energy storage in the near future due to its high energy density [1,2]. In such a context, Lithium-air battery with a very high theoretical energy density (5200 Wh kg-1) received some very first attentions, but researchers soon realized that low Li abundance on Earth, poor cyclic stability and high operation overpotential make them not only high costs but also unrealistic in practical applications. More recently, Sodium-air batteries (Na-air) have risen as an effective alternate to Li-air batteries, given the facts that Na-air batteries are much safer and low cost. Na-air battery has several other advantages including low overpotential and high power density, arising from much higher ionic conductivity of Na-ion based solid electrolyte (e.g. Na3Zr2Si2PO12) and high solubility of discharge generated product (i.e. NaOH rather than LiOH) [3,4]. Based on cell design and electrolyte type, Na-air batteries can be classified into two major types: Non-aqueous and Aqueous (or) hybrid batteries [5]. Recent studies have shown that aqueous Na-air batteries hold some edges over non-aqueous systems, including low operation overpotential, higher rate capability, improved energy efficiency, and higher power density. Bifunctional catalysts are prominent to attain high capacity, maximum energy efficiency and long cycle-life for aqueous rechargeable Na-air batteries [6,7]. In this work, we report the synthesis of bifunctional noble metal free, highly porous N-doped graphitic carbon from bio-waste. The highly porous carbon showed higher specific surface area of 994 m2 g-1. Electrocatalytic property of the carbon based material was evaluated using linear sweep voltammetry with rotating ring disk electrode (RRDE). The N-doped graphitic carbon nanostructures showed better oxygen evolution activity compared to Pt/C catalyst. Highly porous N-doped graphitic carbon was investigated as a potential electrocatalyst for rechargeable hybrid Na-air battery for the first time. The fabricated hybrid Na-air battery with the N-doped graphitic carbon catalyst as air-cathode delivered low overpotential and its round trip energy efficiency reached above ~85 %. The hybrid Na-air battery exhibited stable cycle performance up to 20 cycles. Acknowledgement Author B.S. gratefully acknowledges the DST (SERB), New Delhi, India (PDF/2015/00217) for providing Fellowship.