Charge storage is critical for numerous applications, including energy storage devices and catalysis. Metal-organic frameworks (MOFs), with their three-dimensional, high surface area structures, are promising candidates. MOFs are formed by coordinating metal ions and organic linkers, but creating effective MOF nanostructures with minimal random crystal formation remains challenging. Traditional charge storage materials like carbon and metal oxides suffer from limited surface area and poor conductivity, resulting in low energy density and slow charging rates. While carbon-based materials offer high electrochemical performance, thermal stability, and conductivity, achieving high energy density with chemical stability is difficult. MOFs, with their designable, flexible structures and high porosity, address these limitations. Their tunable pore size and chemical diversity make them ideal for charge storage. However, traditional MOF synthesis methods often involve harsh conditions, lengthy processes, low yields, and poor reproducibility, hindering their practical applications. This research explores chronoamperometry electrodeposition as a novel technique to overcome these limitations, aiming to fabricate MOFs with high electroactive sites and good conductivity for efficient charge storage.

The literature extensively covers the use of MOFs for energy storage and conversion applications. Studies highlight the potential of MOFs in lithium-ion batteries and supercapacitors due to their high surface area and tunable properties (Zhao et al., 2016; Ke et al., 2015). However, challenges remain in synthesizing MOFs with desirable properties at scale. Previous work has explored various methods to improve MOF synthesis, including hydrothermal and solvothermal approaches (Wu et al., 2018; Qihang Zhou et al., 2022). These methods often lack the control over film thickness and functionality offered by electrodeposition. Research has also focused on improving the performance of supercapacitors using various carbon-based materials (Miao et al., 2020), metal oxides (Mustaqeem et al., 2022), and MOF-based electrodes (Zhao et al., 2018; Zhao et al., 2023). The current research builds upon this foundation by introducing a novel electrodeposition method for controlled MOF synthesis, addressing the limitations of previous approaches.

This study employed chronoamperometry electrodeposition to synthesize Co-MOFs on a nickel foam substrate. The precursor solution contained Co(NO3)2·6H2O (15 mM) and 1,2-benzenedicarboxylic acid (BDC, 10 mM) in a 1:1 DMF:DDH2O mixture. Prior to electrodeposition, the nickel foam was rigorously cleaned using mechanical and chemical methods (ultrasonication in 0.1 M HCl). Electrodeposition was performed using a three-electrode system: the nickel foam as the working electrode, a platinum plate as the counter electrode, and an Ag/AgCl electrode as the reference electrode. A constant potential of -1.57 V vs. Ag/AgCl was applied for 300 s. The nucleation and growth kinetics of the Co-MOF film were analyzed using current-time transients and fitted to the Scharifker-Hills (SH) model to determine the nucleation mechanism (instantaneous or progressive). Material characterization involved X-ray diffraction (XRD), energy-dispersive spectroscopy (EDS), and Fourier transform infrared spectroscopy (FTIR) to confirm the formation of Co-MOF and analyze its composition and structure. Electrochemical characterization was conducted in a three-electrode cell using 1 M KOH electrolyte. Techniques included cyclic voltammetry (CV) at various scan rates (10-100 mV/s), galvanostatic charge-discharge (GCD) at different current densities (4-20 A/g), and electrochemical impedance spectroscopy (EIS) (0.01 Hz to 10000 Hz) to determine the specific capacitance, energy density, power density, and cyclic stability. The charge storage mechanism was investigated using Dunn's method to analyze the capacitive and diffusion-controlled contributions at different scan rates. Finally, a two-electrode asymmetric supercapacitor cell was assembled using the Co-MOF/NF as the positive electrode and activated carbon cloth (ACC) as the negative electrode, with a Celgard 3501 separator. The asymmetric device was also subjected to CV, GCD, and EIS to evaluate its performance and long-term cyclic stability (up to 50,000 cycles).

Chronoamperometry analysis revealed that Co-MOF deposition followed an instantaneous nucleation mechanism. XRD, EDS, and FTIR confirmed the successful synthesis of Co-MOF with the expected elemental composition and functional groups. SEM imaging showed the formation of agglomerated Co-MOF structures on the nickel foam, creating a high surface area. In the three-electrode configuration, the Co-MOF/NF electrode exhibited a maximum specific capacitance of 1618.56 F/g at 4 A/g, high energy density (56.2 Wh/kg at 1000 W/kg), and remarkable cyclic stability (97% capacitance retention after 5000 cycles). Analysis of the charge storage mechanism showed a significant capacitive contribution (82% at 100 mV/s), suggesting suitability for high-power applications. EIS analysis indicated low equivalent series resistance (ESR) and charge transfer resistance (Rct), confirming high conductivity. In the two-electrode asymmetric supercapacitor, the Co-MOF/NF/ACC device showed a specific capacitance of 315.33 F/g at 0.2 A/g and an energy density of 63.06 Wh/kg at 479.94 W/kg. The asymmetric device maintained 99.6% capacitance retention over 50,000 cycles. Importantly, three of these devices connected in series successfully powered 37 yellow LEDs, demonstrating practical applicability.

The results demonstrate the effectiveness of electrodeposition for creating high-performance Co-MOF electrodes for supercapacitors. The superior electrochemical performance, particularly the high capacitance, energy density, power density, and exceptional cyclic stability (97% after 5000 cycles in the three-electrode system and 99.6% after 50000 cycles in the two-electrode asymmetric device), surpass many previously reported MOF-based supercapacitors. The predominantly capacitive behavior at higher scan rates points to its suitability for high-power applications. The successful demonstration of powering multiple LEDs highlights its practical potential. The electrodeposition method offers advantages over conventional MOF synthesis techniques, providing precise control over the MOF morphology and thickness on the conductive substrate, enhancing conductivity and reducing aggregation issues. This work contributes significantly to the advancement of high-performance energy storage materials.

This study successfully demonstrates the synthesis of a high-performance Co-MOF electrode for supercapacitors via electrodeposition. The resulting electrode exhibits excellent electrochemical performance and remarkable cyclic stability, exceeding those of many previously reported MOF-based supercapacitors. The findings showcase the potential of electrodeposition as a scalable and efficient method for producing MOF-based energy storage materials. Future research could explore different MOF compositions and electrodeposition parameters to further optimize performance and investigate the scalability of this method for industrial applications.

While this study demonstrates impressive results, some limitations exist. The study focuses solely on Co-MOF; exploring other MOF compositions could reveal further performance improvements. The long-term stability tests were conducted under specific conditions; testing under broader conditions would strengthen the generalizability of the findings. A more comprehensive economic analysis comparing the cost-effectiveness of this electrodeposition method with other MOF synthesis techniques would be valuable.