Advanced electrode materials: The role of double-linker Ni and Co metal-organic frameworks in electrochemical energy storage

In recent years, metal-organic frameworks (MOFs) have garnered significant interest as novel electrode materials for energy storage devices. However, many MOFs' limited conductivity and capacity have hindered their widespread application. This study presents a practical approach to enhancing MOF conductivity by incorporating two organic linkers, ethylenediaminetetraacetic acid (EDTA) and 2,6-pyridine dicarboxylic acid (PDC), into a pristine MOF structure to form a high-dimensional framework. To the best of our knowledge, this is the first comprehensive evaluation of a double-linker MOF featuring these two linkers for supercapacitor applications. A simple solvothermal method was used to incorporate EDTA and PDC linkers, promoting hydrogen bonding, multiple coordination modes, and π-stacking interactions, which contribute to the stabilization and formation of high-dimensional Ni and Co frameworks. Structural analysis using X-ray diffraction (XRD) and Fourier-transform infrared spectroscopy (FTIR) confirmed the presence of functional groups from the linkers. The morphology and surface roughness of the synthesized materials were analyzed using scanning electron microscopy (SEM) and a surface profilometer, respectively. High-resolution transmission electron microscopy (HRTEM) images confirmed the polycrystalline nature of the MOFs, while the TEM image at 50 nm magnification revealed a layered structure consisting of thin, transparent sheets. This observation highlights a lightweight, porous framework characterized by uniform thickness and smooth edges, indicating the successful synthesis of MOFs with minimal defects. Furthermore, nuclear magnetic resonance (NMR) and X-ray photoelectron spectroscopy (XPS) results corroborate the formation of the framework. The high porosity of the double-linker MOFs enabled enhanced ion transport from the electrolyte during faradaic reactions, providing favorable pathways for charge transfer and leading to excellent electrochemical performance. The supercapacitive behavior of the synthesized mono-linker and double-linker MOFs was thoroughly investigated using galvanostatic charge/discharge (GCD) experiments in 3 M KOH electrolyte, cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS). Notably, the double-linker Ni- and Co-MOFs exhibited superior performance, with specific capacitances of 984 Fg⁻¹ and 950 Fg⁻¹, respectively, at a current density of 1 A g⁻¹. This significantly improved their mono-linker counterparts (mono-metallic Ni: 379 Fg⁻¹, mono-metallic Co: 452 Fg⁻¹). The mono-linker MOFs were converted to their oxides upon annealing at 400 °C. Interestingly, annealing the double-linker MOFs at the same temperature altered their phase, particularly in the case of Co, which transitioned from hybrid to pseudocapacitive behavior. The molarity of the aqueous electrolyte was also optimized. A two-electrode device based on the double-linker MOFs demonstrated remarkable stability, with an energy density of 76 Wh kg⁻¹ and a power density of 800 W kg⁻¹. These findings indicate that double-linker MOFs are promising candidates for high-efficiency energy storage devices. Future research could further optimize these double-linker MOFs' synthesis and structural properties to fully harness their potential for addressing the growing global energy demand.