Acid-directed preparation of micro/mesoporous heteroatom doped defective graphitic carbon as bifunctional electroactive material: Evaluation of trace metal impurity

Abstract

Extensive research to explore cost-effective carbon materials as electrocatalysts and electrode materials for energy conversion and storage has been conducted in the recent literature. This raised a crucial question regarding the origin of this electrocatalytic activity from the heteroatom doping/ hierarchical porous defect-rich architecture and/ or the trace metal impurities introduced during synthesis/ inherent to the precursor. In this work, an insight into this issue is provided by considering a lignocellulosic biowaste, Euryale Ferox (foxnut) shells as a precursor to derive micro/ mesoporous defective graphitic carbon sheets by the phosphoric acid (H3PO4) dictated in-situ carbonization for oxygen reduction reaction (ORR) and supercapacitor applications. The sample synthesized at 900 °C (FP900) shows an onset potential of 0.98 V vs. reversible hydrogen electrode (RHE), ORR current density of 5.5 mA cm−2, and current stability of 93% (in 10 h measurement) in 0.1 M KOH. In addition, a symmetric supercapacitor device is assembled using the prepared material and the specific capacitance of 207.5 F g−1 at 1 A g−1 is obtained. An attempt to explain the origin of the electrochemical performance is made by establishing parallels with the physicochemical characterizations. The inherently doped heteroatoms give rise to electroactive functionalities and the wide enough pore size distribution improves the active sites utilization efficiency by enhancing the accessibility to electrolytic ions resulting in better electrochemical performance. Furthermore, the contribution from the intrinsic trace metal impurities is evaluated using X-ray fluorescence (XRF) spectroscopy. The presented research clarifies the non-contributing nature of trace metal species owing to the inaccessibility of active sites and lower abundance in F900 and FP900, respectively.