Electrosynthesis of Co-ZIF Using Bio-Derived Solvents: Electrochemical Evaluation of Synthesised MOFs as a Binder-Free Supercapacitor Electrode in Alkaline Electrolyte

Abstract

Supercapacitors hold promise for energy storage due to their exceptional power density and fast charge/discharge cycles. However, their performance hinges on the electrode material. Zeolitic imidazolate frameworks (ZIFs) are attractive options due to their tailorable structure and high surface area. But traditional ZIF synthesis relies on toxic solvents derived from fossil fuels, hindering their envisioned environmental benefit. This study explores using bio-derived solvents for a greener and potentially superior approach. The researchers employed anodic electrodeposition to synthesise cobalt-based ZIFs (Co-ZIFs) as supercapacitor electrode materials. Two linkers (2-methylimidazole and benzimidazole) and two bio-derived solvents (CyreneTM and γ-valerolactone (GVL)) were investigated. X-ray diffraction analysis revealed that bio-derived solvents enhanced the crystallinity of Co-ZIFs compared to traditional solvents. Notably, CyreneTM promoted better crystallinity for Co-bIM/Co-mIM structures. The Full Width at Half Maximum (FWHM) analysis suggests CyreneTM promotes Co-bIM/Co-mIM crystallinity (lower FWHM). Co-mIM in CyreneTM exhibits the best crystallinity (FWHM = 0.233) compared to other ZIF samples. Scanning electron microscopy confirmed these findings, showing larger and well-defined crystals for bio-derived solvent-synthesised ZIFs. The choice of solvent significantly impacted the final ZIF structure. While 2-methylimidazole consistently formed ZIF-67 regardless of the solvent, benzimidazole exhibited solvent-dependent behaviour. GVL yielded the highly porous Co-ZIF-12 structure, whereas DMF (N,N-dimethylformamide) and CyreneTM produced the less porous ZIF-9. This work reports the first-ever instance of ZIF-12 synthesis via an electrochemical method, highlighting the crucial interplay between solvent and precursor molecule in determining the final ZIF product. The synthesised binder-free Co-ZIF electrodes were evaluated for supercapacitor performance. The capacitance data revealed GVL as the most effective solvent, followed by DMF and then CyreneTM. This suggests GVL is the preferred choice for this reaction due to its superior performance. The ZIF-12-based electrode exhibits an impressive specific capacitance (Csp) of 44 F g⁻1, significantly higher than those achieved by ZIF-9-Cyrene (1.2 F g⁻1), ZIF-9-DMF (2.5 F g⁻1), ZIF-67-GVL (35 F g⁻1), ZIF-67-Cyrene (6 F g⁻1), and ZIF-67-DMF (16 F g⁻1) at 1 A g−1. This surpasses the Csp of all other ZIFs studied, including high-performing ZIF-67(GVL). ZIF-12(GVL) maintained superior Csp even at higher current densities, demonstrating exceptional rate capability. Among the bio-derived solvents, GVL outperformed CyreneTM. Notably, the Co-bIM in the GVL sample exhibited a ZIF-12-like structure, offering potential advantages due to its larger pores and potentially higher surface area compared to traditional ZIF-67 and ZIF-9 structures. This work presents a significant advancement in Co-ZIF synthesis. By utilising bio-derived solvents, it offers a more sustainable and potentially superior alternative. This paves the way for the eco-friendly production of Co-ZIFs with improved properties for supercapacitors, gas separation, catalysis, and other applications.