Electrodeposition of ultrathin NiBDC lamellar arrays as a novel binder-free electrode for flexible all-solid-state supercapacitors

Abstract

The Ni-based metal–organic framework (Ni-MOF) shows significant potential for energy storage due to its high specific capacity and active sites for electrochemical reactions. This study investigates the microstructural control of NiBDC on indium tin oxide/polyethylene terephthalate (ITO/PET) by varying the electrodeposition time (NiBDC/ITO/PET). The resulting microstructure exhibits a well-ordered and uniform porous array, with NiBDC nanosheets growing into ultra-thin layers of 50 nm thickness. As an electrode material for flexible, binder-free all-solid-state supercapacitors, the NiBDC/ITO/PET electrode demonstrates optimal electrochemical performance at an electrodeposition time of 30 minutes. In a three-electrode configuration, the NiBDC/ITO/PET electrode material achieves an areal capacitance of 72.2 mF cm⁻² at a discharge current density of 50 μA cm⁻². Even after 2000 cycles, it retains over 90% of its initial capacity at a current density of 500 μA cm⁻². The resultant symmetric supercapacitor device exhibits remarkable mechanical flexibility and robust cycling stability, maintaining over 87.8% of its initial specific capacitance after 5000 cycles. Moreover, three such devices connected in series can power a light-emitting diode (LED), demonstrating practical energy storage applications.

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1. Introduction

Supercapacitors (SCs) are highly sought-after energy storage devices for green energy applications, offering several advantages over traditional batteries.¹ These include their lightweight design, excellent performance, long cycle life, and high power density. SCs can be classified into two categories based on their energy storage mechanisms.²

As a critical energy storage device, traditional supercapacitors have faced limitations in application due to the rigid nature of their electrodes, typically composed of powdered materials. This constraint has rendered them unsuitable for integration into wearable electronic devices. Recently, there has been growing interest among researchers in flexible supercapacitors (FSCs) due to their outstanding flexibility, lightweight nature, and capacity to endure compression.^3,4 The selection of electrode materials significantly influences the electrochemical performance of FSCs. Hence, developing an efficient method to produce flexible electrode materials is paramount.⁵ Given the constraints of device volume, faradaic capacitive electrode materials exhibiting high areal-capacitance are anticipated to enhance energy density and better fulfill practical requirements. Among the diverse Faraday capacitive electrode materials, metal–organic frameworks (MOFs), consisting of organic ligands and metal nodes (metal ions or clusters), represent a novel class of crystalline porous materials that have garnered significant interest and have been extensively researched recently.^6–9 MOFs serve as a promising bridge between nanomaterials and devices owing to their tunable porous structures, variable metal nodes, ligands, and abundant active sites.¹⁰ Due to their intrinsic advantages, MOFs also present promising prospects in gas adsorption and separation, catalysis, and energy storage applications.¹¹ However, their limited conductivity has historically constrained their application as electrode materials.^12,13 Many MOFs are brittle and prone to agglomeration due to conventional synthetic methods. The microstructure of electrode materials profoundly impacts the electrochemical performance of supercapacitors. Yue et al.¹⁴ introduced a one-step solvothermal method for the fabrication of hollow nanospheres of NiCoMn-MOF. The resulting NiCoMn-MOF exhibits remarkable cycling stability, retaining 84.1% of its capacity after 5000 cycles. Sun et al.¹⁵ mitigated severe agglomeration in Zn/Ni-MOF composites by regulating the feed mass of rGO sheets. Recently, ordered and uniformly structured nanosheet arrays have emerged as a focal point amidst diverse microstructures. The porous array structure effectively mitigates volume expansion and contraction during prolonged cycling tests when utilized as electrode materials, thereby significantly influencing electrochemical performance.^16,17 Moreover, compared to bulk MOFs, this structure provides a larger reactive surface area and facilitates rapid diffusion of electrons and ions.^14,18 Nevertheless, the controlled design of MOFs with specific array morphology and alignment remains a considerable challenge, particularly in achieving this through gentle and straightforward synthesis methods without requiring high temperatures and pressures. A variety of synthesis methods for producing MOFs have been widely reported, encompassing solvothermal, mechanochemical, ultrasonic exfoliation, and interface-assisted methods.¹⁹ Many researchers have explored the utilization of conductive MOFs as electrode materials in supercapacitors to enhance energy efficiency.²⁰ Directly depositing MOFs onto conductive substrates is a viable method to ensure a high electrochemically active surface area while facilitating rapid electron transport.²¹ Moreover, it efficiently eliminates the requirement for binders and conductive additives. The electrodeposition process allows for the direct fabrication of MOFs onto conductive substrates, which can be achieved at ambient temperature, within a short time, and without the need for expensive or complex equipment.^22,23

Due to the merits of electrochemical deposition, numerous studies have been documented. For instance, Aghazadeh et al.²³ fabricated a TM-MOF@NF (Ni Foam) electrode using 2-methylimidazole as the organic ligand, which exhibited a sheet-like structure and achieved an excellent specific capacitance of 412 C g⁻¹ at a current density of 1 A g⁻¹, along with outstanding long-term cycling stability. Gu et al.²⁴ deposited 2D NiPc-MOF on NF and utilized the electrode in supercapacitors, demonstrating ultra-long cycling properties with 90% initial capacitance retention even after 10 000 cycles. NF faces limitations due to its lack of flexibility and structural stability in the field of FSCs. The inflexibility of the electrodeposited substrate imposes stricter requirements on flexible electrode materials. Carbon cloth (CC) and ITO/PET have been extensively investigated as alternative electrodeposited substrates. For example, Wang et al.²⁵ synthesized Co-MOF on CC via the electrodeposition method, using 1,3,5-benzene tricarboxylic acid as the organic ligand. The assembled device exhibited a high areal-capacitance of 1784 mF cm⁻² at a current density of 1 mA cm⁻². However, the specific capacitance of the electrode is influenced by the occurrence of a double-layer adsorption mechanism on the surface of CC. ITO/PET refers to poly(ethylene terephthalate) (PET) coated with a layer of indium tin oxide (ITO), providing excellent flexibility and transparency properties. ITO/PET serves not only as a substrate but also as a directly usable material for device packaging. Nguyen et al.²⁶ utilized electrophoretic deposition and galvanostatic deposition techniques to deposit multi-walled carbon nanotube paper (MWCNP) directly onto ITO/PET. There is limited documentation on the direct deposition of monometallic MOFs onto ITO/PET and their subsequent use as SC electrodes.

In this work, the electrodeposition method was employed to fabricate NiBDC onto the ITO/PET substrate. NiBDC features a porous array structure that effectively mitigates volume changes during electrochemical processes, thereby enhancing the cycling performance of the NiBDC/ITO/PET electrode. By controlling the deposition time, various morphologies and electrochemical properties were observed in the NiBDC/ITO/PET system with a three-electrode configuration. Specifically, at a deposition time of 30 minutes, the synthesized NiBDC/ITO/PET-30 electrode exhibited hierarchical and porous structures, which expose numerous active sites for electrochemical reactions and promote rapid ion diffusion. This electrode exhibited excellent electrochemical performance, achieving a maximum areal-capacitance of 72.2 mF cm⁻² at a discharge current density of 50 μA cm⁻² and retaining 90.7% of its initial capacity at a current density of 500 μA cm⁻². Moreover, when integrated into a symmetrical device, it demonstrated an ultrahigh areal capacitance of 21.5 mF cm⁻² at a current density of 500 μA cm⁻² and exhibited satisfactory mechanical flexibility. In addition, the fabricated device demonstrates good cycling stability and is capable of powering LEDs. This simple, environmentally friendly, and cost-effective fabrication method provides a practical approach for realizing flexible electrodes.

2. Experimental section

2.1. Chemicals

Nickel nitrate hexahydrate [Ni(NO₃)₂·6H₂O] (99%, 290.79 g mol⁻¹) and 1,4-benzenedicarboxylic acid [H₂BDC] (166.13 g mol⁻¹) were of analytical grade and purchased from Aladdin. ITO/PET was purchased from South China Science & Technology Co., Ltd (Guangdong, China). N-Dimethylformamide [DMF] (73.09 g mol⁻¹, viscosity (25 °C): 0.92 cP), anhydrous sodium sulfate [Na₂SO₄] (142.04 g mol⁻¹), acetone (58.08 g mol⁻¹, viscosity (25 °C): 0.32 cP), lithium trichloride [LiCl] (42.39 g mol⁻¹), and polyvinyl alcohol [PVA] (variable, typically in the range of 30 000–200 000 g mol⁻¹) were purchased from Sinopharm Chemical Reagent Co. Ltd. All aqueous solutions were precisely prepared using standard deionized water.

2.2. Synthesis of NiBDC/ITO/PET

Firstly, commercial ITO/PET substrates (1 cm × 2 cm) were sequentially cleaned by immersion in ethanol and deionized water, respectively. Then the cleaned ITO/PET was dried at 50 °C for 2 h. The synthesis scheme of NiBDC/ITO/PET is shown in Scheme S1 of the ESI.† Metal sources Ni(NO₃)₂·6H₂O (2 mmol) were dissolved in 30 mL of DMF, followed by the addition of 0.1 mM of H₂BDC to the above solution to prepare the electrolyte solution. NiBDC was electrodeposited on the clean ITO/PET at a constant potential of −1.0 V for 5 minutes using a standard three-electrode system (with platinum wire and silver chloride serving as the counter and reference electrodes, respectively).²⁷ The obtained NiBDC/ITO/PET-5 was rinsed with DI water and then dried at 50 °C for 2 h. The final product was denoted as NiBDC/ITO/PET-5. NiBDC/ITO/PET samples with extended electrodeposition times (10 min, 20 min, and 30 min) were prepared using the same procedure as described above, designated as NiBDC/ITO/PET-10, NiBDC/ITO/PET-20, and NiBDC/ITO/PET-30, respectively.

2.3. Material characterization

A scanning electron microscope (SEM, Hitachi, S-4800) was employed to determine morphological characteristics and elemental mapping of the fabricated NiBDC/ITO/PET. Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HR-TEM, FEI Talos F200X) were employed to characterize NiBDC/ITO/PET-30. The elemental state and element distribution were analyzed by X-ray photoelectron spectroscopy (XPS, PHI-5702, Physical Electronics), Fourier transform infrared spectroscopy (FT-IR, Spectrum Two, PerkinElmer), Raman spectroscopy (Renishaw inVia, LabRAM HR Evolution) and ultraviolet-visible (UV-vis) spectrophotometry (PerkinElmer LAMBDA 35) were conducted using a Bruker Vector 22.

2.4. Electrochemical measurements

Electrochemical measurements were carried out on a CHI660E electrochemical workstation (CH Instruments, Shanghai). The electrochemical tests were performed using a standard three-electrode system at room temperature in 0.5 M Na₂SO₄. The counter electrode was a platinum wire (∅0.5 × 37 mm), and the reference electrode was a calomel electrode. The as-prepared sample was used as the working electrode. Cyclic voltammetry (CV) and galvanostatic charge/discharge (GCD) tests were performed in Na₂SO₄ electrolyte over the potential window from −0.4 to 0.4 V, correspondingly. Electrochemical impedance spectroscopy analysis was also done by applying a perturbation sinusoidal voltage 5 mV over the frequency range of 100 kHz to 10 MHz.

To investigate the performance of NiBDC/ITO/PET-30 electrodes in devices, a sandwiched-type flexible symmetrical supercapacitor was assembled. Two pieces of NiBDC/ITO/PET-30 are used as the electrodes, the PVA/LiCl gel (2 g of PVA and 3 g of LiCl was added to 25 mL of deionized water, then heated to 85 °C for 2 h under stirring to obtain a clear and transparent gel) was used as a solid electrolyte.²⁸

Galvanostatic charge and discharge measurements were conducted at various current densities. The resulting data were subsequently used to calculate the specific capacitance, energy density, and power density using the following equations:²⁹

I, ΔV, S, and ν represent the current, the potential window, the active area of a single electrode, and the scanning speed, respectively.I, t, S, and ΔV denote the current, the discharge time (s), the active area of a single electrode, and the potential window, respectively.C, ΔU, V, and t refer to the areal capacitance, the operating voltage window, the volume of a single device, and the constant current discharge time, respectively.i, v, k₁, and k₂ represent the current, the scan rate, the capacitance control constant, and the diffusion control constant, respectively.

3. Results and discussion

3.1. Characterization

SEM analysis was performed to examine the microscopic morphologies of the samples deposited with NiBDC for different deposition times of NiBDC, as shown in Fig. 1. Typically, alveolate-structured MOF flakes were observed to form perpendicular alignments on the surface of ITO/PET.³⁰ Short deposition times resulted in incomplete growth of NiBDC nanosheets, with an indistinct porous array structure. This suggests significant agglomeration.^31,32 Following a deposition time of 30 minutes, the porous array structure of NiBDC gradually exhibited more orderly and well-arranged characteristics. NiBDC developed into thinner lamellae with a thickness of 50 nm, as depicted in Fig. 1g and h. The SEM results indicate that deposition time influences the extent of growth of the porous array structures of NiBDC.³³ The porous array structure of NiBDC effectively mitigates volume expansion of the electrode during pseudocapacitive electrochemical reactions.³² As shown in Fig. 1i, energy dispersive spectroscopy (EDS) analysis revealed a homogeneous distribution of C, O, and Ni elements throughout the NiBDC/ITO/PET structure. The TEM image of pure NiBDC/ITO/PET-30 (Fig. S1, ESI†) confirms its 2D nanosheet structure, which displays a lamellar morphology, as observed in the SEM images. The selected area electron diffraction (SAED) pattern of NiBDC/ITO/PET-30 reveals distinct diffraction spots, clearly confirming its polycrystalline structure.^34–36

Fig. 1 SEM images of NiBDC/ITO/PET-5 (a) and (b), NiBDC/ITO/PET-10 (c) and (d), NiBDC/ITO/PET-20 (e) and (f), NiBDC/ITO/PET-30 (g) and (h), and SEM image and elemental mapping of C, O, and Ni of NiBDC/ITO/PET-30 (i).

To further investigate the chemical composition of the samples, NiBDC/ITO/PET was characterized using Fourier transform infrared spectroscopy (FT-IR). The results, depicted in Fig. 2, revealed consistent spectral features of all samples. The broad band at approximately 3615 cm⁻¹ corresponds to the benzene dicarboxylic acid linkers, in line with the presence of metal ions.²⁰ It is assumed that the absorption peak of the hydroxyl group occurs at 3423 cm⁻¹. The prominent peaks at 1571 and 1379 cm⁻¹ are attributed to the asymmetric and symmetric stretching vibrations of carboxyl groups, respectively.²³ The weak band present at 1653 cm⁻¹ confirms the presence of the C–O group, indicating the successful production of the NiBDC sample. Furthermore, the strong characteristic peak observed at approximately 1505 cm⁻¹ is associated with the bending vibration of the C–H bonds in the benzene rings.¹⁶ In addition, the weak peaks located at 802 cm⁻¹ may correspond to the stretching vibration of the Ni–O bond.³⁷

Fig. 2 (a) FT-IR spectra, (b) Raman spectroscopy and (c) UV-Vis spectra of NiBDC/ITO/PET-5, NiBDC/ITO/PET-10, NiBDC/ITO/PET-20, and NiBDC/ITO/PET-30.

In Fig. 2(b), the Raman spectra of all NiBDC/ITO/PET samples consistently exhibit prominent peaks in the range of 400–800 cm⁻¹, corresponding to Ni–O vibrations. Additionally, peaks observed at 1611, 1396, 1098, and 870 cm⁻¹ are attributed to the carboxylate groups and C–H bonds of the benzene rings within the BDC ligands of NiBDC.^36,38 As the deposition time increases (from NiBDC/ITO/PET-5 to NiBDC/ITO/PET-30), the intensity of these peaks progressively enhances, indicating a greater accumulation of NiBDC material. This trend may also reflect improved crystallinity, which is associated with superior electrochemical performance. Furthermore, minor peaks are observed across the spectra, likely attributed to residual MOF structures or secondary phases. Notably, longer deposition times yield sharper and more defined peaks, signifying enhanced crystallinity. This improvement typically correlates with increased conductivity and stability, reinforcing the material's potential for energy storage applications. The UV-Vis absorption spectra of NiBDC/ITO/PET films with varying deposition times are shown in Fig. 2(c). As observed, all samples exhibit distinct absorbance in the visible region (400–700 nm), with variations that depend on the film thickness. The absorbance intensity increases with longer deposition times, indicating that thicker films result in stronger absorption, likely due to a higher material content available for interaction with incident light. These results suggest that both the film thickness and material composition of the NiBDC/ITO/PET films significantly influence their optical absorption properties.³⁹

The elemental composition and chemical state of the NiBDC/ITO/PET sample were analyzed using X-ray photoelectron spectroscopy (XPS). Fig. 3 illustrates the presence of elements such as C, O, and Ni in the sample.^15,40 Additionally, high-resolution spectra of O 1s, C 1s, and Ni 2p were acquired to comprehensively analyze the chemical characteristics of these elements. In Fig. 3(a), the O 1s spectra exhibit two distinct photoelectron peaks at 530.7 and 533.5 eV, corresponding to C=O and Ni–O bonds, respectively.⁴¹ The C 1s XPS spectrum (Fig. 3(c)) can be resolved into three main peaks located at 284.5, 285.7, and 288.3 eV, assigned to C–C, C–O, and O=C–O bonds, respectively, associated with BDC.^42,43Fig. 3(d) shows the Ni 2p XPS spectrum with peaks at 854.8 and 873.3 eV, corresponding to Ni 2p_3/2 and Ni 2p_1/2, along with satellite peaks at 861.1 and 879.4 eV.⁴⁰ These XPS analysis results further verify the composition of NiBDC/ITO/PET.

Fig. 3 (a) XPS spectrum of NiBDC/ITO/PET-30; XPS spectra of (b) O 1s, (c) C 1s, and (d) Ni 2p of NiBDC/ITO/PET-30.

3.2. Electrochemical measurements

As previously mentioned, varying deposition times of NiBDC/ITO/PET are expected to result in different morphologies of the samples. Furthermore, the impact of these morphological differences on the electrochemical properties is also explored.²³ To elucidate the electrochemical behavior of all NiBDC/ITO/PET samples, these electrodes were evaluated using a three-electrode setup with a 0.5 M Na₂SO₄ electrolyte.⁴⁴ The results from cyclic voltammetry (CV), galvanostatic charge/discharge (GCD), and electrochemical impedance spectroscopy (EIS) are presented in Fig. 4. CV curves of NiBDC/ITO/PET are shown in Fig. 4(a) at a scan rate of 10 mV s⁻¹. The NiBDC/ITO/PET-30 electrode exhibits a significantly larger integrated area, indicating the highest storage capacitance among the samples. Furthermore, Fig. 4(b) illustrates GCD curves for the four samples tested at a current density of 50 μA cm⁻², consistent with the CV test results. Among the electrodes, NiBDC/ITO/PET-30 exhibited the longest discharge time. The areal-capacitances of NiBDC/ITO/PET-5, NiBDC/ITO/PET-10, NiBDC/ITO/PET-20, and NiBDC/ITO/PET-30 were calculated to be 1.17, 10.8, 41.04, and 72.2 mF cm⁻², based on the discharge time. The unique porous framework array structure helps alleviate the volume expansion during the cycling process, thereby improving electrochemical performance. The lamellar structure of NiBDC/ITO/PET-30 allows for enhanced ion transport and diffusion, which are critical for achieving high capacitance. As illustrated in Fig. 4(c) and Fig. S2, Table S1 (ESI†), the NiBDC/ITO/PET-30 showed lower equivalent series resistance (R_s = 7.58 Ω) and charge-transfer resistance (R_ct = 8.94 Ω), indicating faster charge transfer and higher ion diffusion rates, thereby enhancing rate capability.^45,46 There is little difference between the fitted and experimental Nyquist plots. Based on the aforementioned analysis, the NiBDC/ITO/PET-30 electrode demonstrates the best electrochemical performance and was subsequently selected for further testing.

Fig. 4 (a) CV curves at a scan rate of 10 mV s⁻¹ of NiBDC/ITO/PET-5, NiBDC/ITO/PET-10, NiBDC/ITO/PET-20, and NiBDC/ITO/PET-30, (b) GCD curves at a current density of 50 μA cm⁻² of these samples, and (c) Nyquist plots of these samples.

The CV curves of the NiBDC/ITO/PET-30 electrode at various scan rates from 10 mV s⁻¹ to 150 mV s⁻¹ are shown in Fig. 5(a). The CV loops show symmetric shapes without remarkable redox peaks, implying the reversible electrochemical reaction.⁴⁷ The quasi-rectangular shape suggests excellent EDLC behavior of the NiBDC/ITO/PET-30 electrode, facilitated by efficient ion accessibility within the porous structure.⁴⁸ Table S2 (ESI†) provides the areal capacitance values for the NiBDC/ITO/PET-30 electrode, determined using eqn (1) across various scan rates. The results indicate a declining trend in areal capacitance with increasing scan rates, likely due to restricted ion access to the electrode's active sites at higher scan rates. At a low scan rate of 10 mV s⁻¹, the areal capacitance reaches its peak value of 74.1 mF cm⁻². Furthermore, it is noteworthy that the CV curves exhibit consistent behavior even at relatively high scan rates. The GCD profile of the NiBDC/ITO/PET-30 electrode was obtained at current densities ranging from 50 to 500 μA cm⁻² within the potential window of −0.4 to 0.4 V. As shown in Fig. 5(b), the plots generally displayed a triangular shape, strong reversibility, and negligible voltage drops.^47,49 The electrode achieved an areal-capacitance of 72.2 mF cm⁻² at the current density of 50 μA cm⁻², as determined from the GCD curves shown in Fig. 5(c). Table S3 (ESI†) presents a detailed summary of the areal capacitance values for the NiBDC/ITO/PET-30 electrodes, calculated using eqn (2) at varying current densities. The relatively low specific capacitance observed at 0.5 mA cm⁻² may be attributed to limitations in Ni²⁺ ion diffusion or incomplete activation of the active material. To gain further insight into the charge transfer and storage mechanisms, we analyzed the CV curves of the NiBDC/ITO/PET-30 electrode at scan rates ranging from 5 to 30 mV s⁻¹. Fig. 5(d) depicts the CV curve of the NiBDC/ITO/PET-30 electrode at a scan rate of 30 mV s⁻¹, where the capacitance contribution accounts for 88.6% of the total capacitance calculated using eqn (5). Fig. 5(e) compares the capacitance contributions across different scan rates, showing that as the scan rate increases from 5 to 30 mV s⁻¹, the diffusion-controlled contribution decreases, while the capacitance-controlled contribution progressively increases.⁵⁰Fig. 5(f) demonstrates the cycle stability of the NiBDC/ITO/PET-30 electrode, revealing that after 2000 charge/discharge cycles, the specific capacitance decreased to 90.7% and the Coulombic efficiency remained above 97% at 500 μA cm⁻², demonstrating the excellent cycling stability of the electrode.⁵¹

Fig. 5 CV curves (a) of NiBDC/ITO/PET-30 at different scan rates, GCD curves (b), and specific capacitance (c) of the NiBDC/ITO/PET-30 at various current densities (50, 100, 150, 200, 300, and 500 μA cm⁻²); (d) capacitive and diffusion contribution at 30 mV s⁻¹ for the NiBDC/ITO/PET-30 electrode; (e) bar graph of the capacitive and diffusion contribution at different scan rates; (f) long-term cycling performance and Coulombic efficiency of the NiBDC/ITO/PET-30 electrode at 0.5 mA cm⁻².

To illustrate practical applications of the NiBDC/ITO/PET-30 electrode, two pieces of NiBDC/ITO/PET-30 electrodes with a PVA/LiCl hydrogel were employed to fabricate a flexible all-solid-state symmetric supercapacitor device.^41,52Fig. 6(a) presents the CV curves obtained at various scan rates within the potential range of −0.4 to 0.4 V. The areal capacitance values derived from the CV curves are summarized in Table S4 (ESI†). At a low scan rate of 10 mV s⁻¹, the areal capacitance reaches its peak value of 39.5 mF cm⁻². As the scan rate increases to 20, 30, 40, 50, 100, and 150 mV s⁻¹, the corresponding areal capacitance values decrease to 36.8, 34.1, 25.3, 23.8, 22.5, and 20.8 mF cm⁻², respectively. The device exhibits excellent reversibility, as evidenced by the CV profiles maintaining their initial shape even at higher scan rates.^5,22 As expected, the GCD plots depict electrochemical behavior consistent with the CV curves within the potential range of −0.4 to 0.4 V. Fig. 6(b) illustrates that all GCD curves of the devices exhibit nearly triangular shapes, indicating low resistance and rapid ion/electron transport kinetics in the symmetric device.⁵³ As illustrated in Fig. 6(c), the micro-device showed equivalent series resistance (R_s) of 16.4 Ω and charge-transfer resistance (R_ct) of 12.8 Ω, indicating faster charge transfer and higher ion diffusion rate. The corresponding areal-capacitance values of the device are presented in Fig. 6(d) and Table S5 (ESI†); these areal-capacitances were calculated from the GCD curve and measure 40.8, 35.4, 33.5, 27.1, 24.6, and 22.2 mF cm⁻².

Fig. 6 The NiBDC/ITO/PET-30//NiBDC/ITO/PET-30 symmetric supercapacitor. CV curves at different scan rates (a), GCD curves (b), Nyquist plots (c) and the specific capacitance (d) at different current densities.

The flexibility of the all-solid-state devices was assessed through CV tests.⁵⁴ The device underwent testing at 180° under different bending times (Fig. 7(a)), demonstrating well-maintained plot shapes without significant changes. Additionally, CV performances were conducted at various bending angles (0°, 30°, 90°, 120°, and 180°) at a scan rate of 50 mV s⁻¹. The electrochemical characteristics of the all-solid-state device exhibited no obvious variations across different angles (Fig. 7(b)). Both assessments underscore the exceptional mechanical flexibility of the all-solid-state device.⁵⁵ GCD curves were acquired for a single device and two devices connected in series and parallel, at a current density of 300 μA cm⁻², as depicted in Fig. 7(c) and (d), respectively. The GCD curves display ideal symmetric and triangular shapes, consistent with those of the individual device. Furthermore, under identical test conditions, the operating voltage window and capacitance double, illustrating the adherence of the flexible all-solid-state supercapacitor to the principles governing series and parallel connections.⁵⁶ The flexible device retains over 82.8% of its initial performance after 2000 cycles at a current density of 500 μA cm⁻² under a bending angle of 180° (Fig. S3, ESI†), demonstrating exceptional cycling stability.

Fig. 7 (a) CV curves of the supercapacitor at bending times of 0, 50, 100, 200, and 400 at a scan rate of 50 mV s⁻¹. (b) CV curves of a symmetric supercapacitor at bending angles of 0°, 30°, 90°, 120°, and 180° at a scan rate of 50 mV s⁻¹. Series GCD curves (c) and parallel GCD curves (d) of supercapacitors at 0.3 mA cm⁻².

The flexible all-solid-state supercapacitor exhibits an impressive areal-capacitance retention ratio, maintaining over 87.8% retention and the Coulombic efficiency remained above 95%, even after 5000 cycles at a current density of 500 μA cm⁻² (Fig. 8a), indicating excellent cycle stability. As shown in the inset of Fig. 8, the successful illumination of a red LED by three devices connected in series serves as a proof-of-concept for the technology.¹⁷ The results demonstrate the outstanding cycling stability and practical applications of the devices prepared. Particularly noteworthy is the energy density of 3.62 mW h cm⁻³ achieved by the NiBDC/ITO/PET-30//NiBDC/ITO/PET-30 symmetric device at a power density of 89.53 mW cm⁻³, surpassing the performance of certain other MOF-based devices reported in earlier literature (Fig. 8(b)^{11,20,24,54,57} and Table S6 (ESI†)).^58–65

Fig. 8 (a) Long-term cycling performance and Coulombic efficiency of the device at a current density of 0.5 mA cm⁻² for 5000 cycles. The insets display the picture of assembled symmetric supercapacitors for powering a red LED light. (b) Ragone plot (power density vs. energy density) of the device.

4. Conclusion

In conclusion, the electrodeposition method has successfully anchored the NiBDC material onto ITO/PET substrates with ultrathin lamellar arrays. The electrochemical performance of the NiBDC/ITO/PET-30 electrode is excellent. Specifically, the as-prepared NiBDC/ITO/PET-30 exhibits a high areal-capacitance of 72.2 mF cm⁻² at 50 μA cm⁻² and demonstrates outstanding cycling stability, retaining 90.7% of its initial capacity after 2000 cycles at 500 μA cm⁻². The device, composed of two NiBDC/ITO/PET-30 electrodes without any polymer binder or conductive agent, exhibits an impressive specific capacitance of 22.2 mF cm⁻² at a current density of 500 μA cm⁻². Moreover, it demonstrates an energy density of 3.62 mW h cm⁻³ at a power density of 89.53 mW cm⁻³. After 5000 cycles, the areal-capacitance retention remains above 87.8%. Additionally, it is noteworthy that three micro-devices could power a red LED. For NiBDC-based supercapacitors, the scalability of the electrode fabrication process, particularly through the electrodeposition method, plays a pivotal role in its potential integration into various applications. This method can also be applied to fabricate other MOF-based electrode materials, thereby enhancing the commercial viability and practical applicability of this technology.

Data availability

The authors confirm that the data supporting the findings of this study are available within the article.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work is jointly supported by NSFC (62174087, 22179064), the Priority Academic Program Development of Jiangsu Higher Education Institutions (YX03001), the Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), and the Synergistic Innovation Center for Organic Electronics and Information Displays.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4nj04206a

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