Insight into the diversity of dimensionality in silver-based coordination polymers for enhanced supercapacitor performance

Abstract

Novel silver-based coordination polymers, [(2Ag)_0.5·(4,4′-bipy)_0.5·(DNB)]_n and {[Ag(4,4′-bipy)]·(NDC)_0.5·2H₂O}_n (where DNB = 2,4-dinitrobenzoate and NDC = 2,6-naphthalene dicarboxylate), denoted as DNB-CP and NDC-CP, respectively, have been synthesized by a one-pot simple hydrothermal process to be used as high-performance electrode materials for supercapacitor applications. Structural, microstructural, and compositional analyses of the synthesized coordination polymers have been performed using various techniques. The electrochemical behavior of the fabricated coordination polymer-based electrode materials is examined using cyclic voltammetry (CV) and galvanostatic charge/discharge (GCD). The silver-based coordination polymers act as adequate electrode materials, and their supercapacitor performance depends upon the structure dimensionality. The thermal stability of the coordination polymers is affected by the dimensionality, which is confirmed by thermogravimetric analysis (TGA). The coordination polymer DNB-CP forms a 1D polymeric chain, whereas NDC-CP shows combined ionic and coordinate bonding interactions. As the dimensionality increased in the CPs, the rate of ion transport and efficient charge transfer processes increased, resulting in enhanced electrochemical properties. The DNB-CP electrode delivers a good specific capacitance of 971.08 mF cm⁻² at a 10 mV s⁻¹ scan rate after 500 cycles, which is much better than the NDC-CP showing a specific capacitance of 78.81 mF cm⁻² at 50 mV s⁻¹. Furthermore, the electrodes fabricated from DNB-CP can be used in electrochemical energy storage due to their exceptional electrochemical properties.

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1. Introduction

Due to the shortage of non-renewable energy resources, there is a requirement for rechargeable energy storage devices, such as batteries and supercapacitors.¹ Supercapacitors have attracted interest because of their excellent cycling stability, large charge discharge current, and ability to operate at high temperatures.^2–4 For the efficient operation of supercapacitors in a three-electrode arrangement, the working electrode should have high thermal stability, corrosion resistivity, high electrical conductivity, appropriate chemical and electrochemical stability under a wide range of pH conditions, and suitable surface wettability.^5–10 The materials should have the ability to transport faradaic charge for the enhancement of capacitance performance. Additionally, they should exhibit a symmetrical GCD profile and good rectangular shape of cyclic voltammetry curves. For redox active electrodes, fast and reversible redox processes lead to better specific capacitance and extended operation cycles.^11,12 The better energy and power density due to their high specific capacitance, higher conductivity, and lower resistance compared to carbon materials makes transition metal oxides suitable substitutes for supercapacitors (SCs).^13,14 Generally, the charge storage mechanism in metal oxides (MOs) is based on the faradaic behavior. There are numerous examples of various mixed-MOs which, by developing a high surface area and involving versatile functionalities in metal organic frameworks, can be designed as electrode materials for SCs to deliver enhanced performance.^15,16

In the literature, solid-state supercapacitors are reported to have issues of low conductivity, and the same problem is present in pristine MOFs.^17,18 To resolve these issues, silver metal ions have been used to synthesize coordination polymers. Coordination polymers (CPs) are known as the best example materials for energy storage applications.^19,20 For the synthesis of CPs, metal ions act as nodes and organic ligands as linkers and these are joined together through coordinate bonds to form infinite arrangements. Aromatic carboxylates can play a useful role in the fabrication of novel CPs because of their adaptability, structure directionality and easy deprotonation for bonding with metal nodes.^21–23 It has been mentioned in the literature that silver (Ag)-based CPs exhibit the highest electrical conductivity, thermal conductivity, and reflectivity compared to other transition metals. The reason for their high electrical and thermal conductivity is that their last s subshell holds a single electron that is loosely attached to the nucleus. This makes it very movable and conductive for effective energy transfer. Because Ag has the lowest contact resistance, low chemical reactivity, and high thermal conductivity, it is appropriate to be utilized as an electrode material for SCs. The coordination around Ag ions in CPs can be linear, trigonal, and tetrahedral. They have better affinity for hard donor atoms (e.g., nitrogen or oxygen), and soft donor atoms (e.g., sulfur), giving promising conditions and an adaptable building unit for the fabrication of CPs. Silver ions are appropriate for the building of Ag⋯Ag contacts or argentophilic interactions, which may be ligand supported or unsupported interactions along with coordinate bonding, and this is important for the fabrication of complexes with special properties. Methodical studies have been performed by engaging an aromatic carboxylic acid with different arrangements of carboxylate groups, and also changing the reaction conditions. Among the conductive metal ions, silver-based electrodes could be the ideal material due to their bonding interactions, better electrical conductivity, and low internal resistance,^24–26 and be utilized in photocatalysis and battery applications.

Initially, silver nano-dendrite and cellulose acetate based flexible thin films^27,28 and silver nanowire²⁹ networks with nickel hydroxide core–shell electrodes were fabricated for supercapacitor applications.³⁰ The 3D Ag-MOF (Ag₆Mo₇O₂₄)²⁴ and 2D high nucleus silver nano-cluster based electrodes³¹ have been reported as green energy sources for high-performance supercapacitors.³² Similar to the nanostructures prepared from 0D MOFs, the fabrication of 1D MOFs is also advantageous for energy conversion and storage processes by virtue of the dominance of the fast electron passage, short pathways for ion diffusion, and large specific surface area.³³

1D classified structures can significantly enable mass transport and expose the catalytically active sites of inner spaces.^34,35 Silver-based CPs have attracted interest as probable contenders for SC applications due to their stable metallic and high conductive nature, and that is the reason there is no requirement for any functionalization or modification, such as pyrolysis, calcination, or doping.² The electrochemical redox sites are present around the silver ion, and chains of CPs provide a necessary route for the electron transfer within the active material. They have high specific capacitance, and high tolerance for low to high scan rates. They show rectangular CV shapes, long cyclic life, and a high percentage of up to 90% capacity retention.¹⁶ The rational design of the synthesis of 1D CPs plays a vital role because the charge transfer properties and electrical conductivity of CPs can be tailored to make them effective candidates for high-performance energy applications. Keawploy et al.³⁶ described eco-friendly and conductive cotton-based electrodes with superior bending ability by incorporating conductive silver and carbon-coated textiles, designed for innovative and flexible supercapacitors. These electrodes display excellent electrochemical activity in an alkaline (6 M KOH) electrolyte, achieving remarkable values of areal specific capacitance and ultrahigh capacitance retention (∼130%) after ten thousand cycles. In another study, by the same research group, textile electrodes were fabricated via screen-printing with graphene, carbon nanotubes, and silver, and aimed at enhancing the performance of flexible supercapacitors.³⁷ These electrodes exhibited an ultrahigh specific areal capacitance (677.12 mF cm⁻²) at a current density of 0.0125 mA cm⁻². Herein, we attempt to design two CPs, DNB-CP, and NDC-CP, by a facile hydrothermal method. The single crystal X-ray diffraction (SCXRD) revealed that both CPs have different coordination environments (Fig. 1). DNB-CP is purely a 1D coordination polymer, whereas NDC-CP is a combination of 1D polymeric and ionic bonding interactions. The effectual ion transport and charge transfer courses are enabled by extended structure dimensionality, resulting in improved electrochemical properties in DNB-CP compared with NDC-CP. The DNB-CP exhibited excellent cycling stability at 10 mV s⁻¹ after 500 cycles, and the GCD capacitive retention is 101% at 4 mA cm⁻² after 10 000 cycles. The maximum specific capacitance exhibited by DNB-CP was 971.08 mF cm⁻² at a 10 mV s⁻¹ scan rate and 660 mF cm⁻² at a 4 mA cm⁻² current density in a 1 M H₂SO₄ aqueous electrolyte. The electrode prepared from DNB-CP exhibited adequate electrical double layer capacitor (EDLC) behavior, and NDC-CP exhibited pseudocapacitive (PC) behaviour. Thus, this research work explains the role of dimensionality and bonding interactions in the crystal systems of silver-based CPs in their supercapacitor performance.

Fig. 1 The coordination modes of DNB and NDC anions with Ag ions in CPs, DNB-CP and NDC-CP.

2. Experimental

2.1. Materials and physical measurements

All the chemicals were commercially available and used as received. Silver nitrate (AgNO₃), 2,4-dinitrobenzic acid (DNBA), and 2,6-naphthalenedicarboxylic acid (NDCA) as precursors were purchased from Sigma-Aldrich Co., Ltd. N-Methyl-2-pyrrolidinone (NMP) as a solvent and polyvinylidene fluoride (PVDF) as a binder were also procured from Sigma-Aldrich Co., Ltd. Acetonitrile and ethanol were obtained from Finar Limited. Carbon fibre cloth as a substrate was purchased from Sainergy Fuel Cell India Private Limited. Throughout the experiment, ultrapure doubly deionized water (resistivity ∼18.2 MΩ cm), obtained from the Sartorius-mini plus UV instrument, was used. Conc. sulfuric acid was purchased from Merck and used for the electrolyte as received after dilution.

Elemental analyses for carbon, hydrogen, and nitrogen were conducted with a CHNS-O analyzer Flash-EA-1112 series. IR spectra of coordination polymers were measured on a Perkin ELMER FTIR spectrometer in the range 4000–400 cm⁻¹. Raman spectra were recorded on a HORIBA LabRAM Odyssey Raman spectrometer equipped with a confocal microscope, using a 532 nm excitation laser line. Powder X-ray diffraction (PXRD) measurements were made on a Rigaku Miniflex X-ray diffractometer with Cu Kα (λ = 1.5406 Å) radiation. Thermogravimetric analysis (TGA) data were collected on a NetzschTG-209 instrument with a heating rate of 10 °C per minute. UV-vis spectra were recorded using a UV-vis spectrophotometer. For this study, the samples were prepared in dimethyl sulfoxide (DMSO) at a concentration of 10⁻⁵ M and measured in a quartz cuvette with a 1.0 cm path length. The oxidation states of CPs, DNB-CP and NDC-CP were recorded using a PHI 5000 Versa Probe III X-ray photoelectron spectrometer (XPS). Cyclic voltammetry (CV) curves and galvanostatic charge–discharge (GCD) of DNB-CP and NDC-CP were recorded with a Biologic EC Lab electrochemical workstation using a three-electrode system at ambient temperature. The data were plotted using the software EC-Lab. Hirshfeld surface analysis was performed using Crystal Explorer 3.1. This is a beneficial system to discover short contacts, such as H⋯H, H⋯O, N⋯H, C–H⋯π and π⋯π contacts.

2.2. General procedure (Scheme 1)

2.2.1 Synthesis of [(2Ag)_0.5·(4,4′-bipy)_0.5·(DNB)]_n (DNB-CP). A mixture of AgNO₃ (0.169 g, 1.0 mmol), DNBA (0.212 g, 1.0 mmol), 4,4′-bipy (0.156 g, 1.0 mmol), acetonitrile (6 mL), and H₂O (6 mL) was put in a steel-jacketed Teflon-lined vessel and heated at 120 °C for 24 hours, and then cooled to room temperature. Colorless needle shaped crystals were obtained (yield: 76%). IR (KBr pellet, cm⁻¹): 3097 (w) (Ar–H), 1591 (sh) (COO_asym), 1532 (sh) (N–O_asym), 1387 (sh) (COO_sym), 1349 (sh) (N–O_sym), 1058 (w) (C–O), 831 (w)–807 (w) (Ag–O), 732 (w) (Ag–N). Elem. anal. calcd for C₁₂H₇N₃O₆Ag (%), (M_r = 397.08 g): C, 36.30; H, 1.78; N, 10.58; found: C, 36.44; H, 1.72; N, 10.43.

2.2.2 Synthesis of {[Ag(4,4′-bipy)]·(NDC)_0.5·2H₂O}_n (NDC-CP). A mixture of AgNO₃ (0.169 g, 1.0 mmol), NDCA (0.216 g, 1.0 mmol), 4,4′-bipy (0.156 g, 1.0 mmol), methanol (6 mL), and H₂O (6 mL) was put in a steel-jacketed Teflon-lined vessel and heated at 120 °C for 24 hours, and then cooled to room temperature. Colorless needle-shaped crystals were obtained (yield: 78%). IR (KBr pellet, cm⁻¹): 3389 (b) (O–H), 3039 (w) (Ar–H), 1562 (sh) (COO_asym), 1410 (sh) (COO_sym), 1065 (w) (C–O), 625 (sh) (Ag–N). Elem. anal. calcd for C₁₆H₁₅N₂O₄Ag (%), (M_r = 407.17 g): C, 47.09; H, 3.68; N, 6.79; found: C, 47.30; H, 3.62; N, 6.63.

2.3. X-ray crystallography

The crystals were grown by heating at 120 °C in a Teflon-lined vessel (hydrothermal process) using a mixture of different solvents, like a distilled water–acetonitrile mixture for DNB-CP and a distilled water–methanol mixture for NDC-CP (Fig. S1, ESI†). The crystal data was collected using a Bruker's Apex-III CCD diffractometer using Mo Kα (λ = 0.71069 Å) at room temperature. The crystal data was processed using the SAINT, which included corrections for Lorentz and polarization effects.³⁸ The structures were determined by direct methods, using SIR-92,³⁹ and refined through the full-matrix least squares refinement methods⁴⁰ based on F², using SHELX-2016. An empirical absorption correction was applied using SADABS from Bruker.⁴¹ All non-hydrogen atoms (C, N, and O) were refined anisotropically. The hydrogen atoms of water molecules were identified from the difference Fourier synthesis and were refined isotropically with U_iso values 1.2 times that of associated oxygen atoms with the distances fixed at 0.82(2) Å. All other hydrogen atoms were fixed geometrically with their U_iso values 1.2 times those of associated atoms for phenylene carbons. All calculations were performed using the Wingx package.⁴² The geometries of the coordination polymers and key hydrogen bonding interactions were calculated using the PARST program.⁴³ All the coordination polymers were illustrated using ORTEP, DIAMOND, and MERCURY programs. The structure refinement data for DNB-CP and NDC-CP are presented in Table 1. The selected bond lengths (Å) and angles (°) around the Ag⁺ ions are listed in Table TS1 (ESI†), and significant H-bonding interactions are given in Table TS2 (ESI†).

Table 1 Crystal data and structure refinement for DNB-CP and NDC-CP

Identification code	DNB-CP	NDC-CP
Empirical formula	C12H7AgN3O6	C16H15AgN2O4
Formula weight	397.08	407.17
Temperature	296(2) K	296(2) K
Wavelength	0.71073 Å	0.71073 Å
Crystal system	Monoclinic	Triclinic
Space group	C2/c	P1̄
Unit cell dimensions	a = 25.474(14) Å, α = 90°	a = 7.1387(3) Å, α = 90.441(17)°
	b = 14.648(8) Å, β = 94.997(17)°	b = 9.6061(4) Å, β = 94.919(15)°
	c = 6.969(4) Å, γ = 90°	c = 11.4125(5) Å, γ = 108.82(16)°
Volume	2591(2) Å^3	737.52(5) Å^3
Z	8	2
Density (calculated)	2.036 Mg m^−3	1.834 Mg m^−3
Absorption coefficient	1.592 mm^−1	1.389 mm^−1
F(000)	1560	408
Crystal size	0.15 × 0.09 × 0.05 mm^3	0.14 × 0.10 × 0.08 mm^3
Theta range for data collection	1.60 to 31.15°.	1.79 to 29.36°.
Index ranges	−29 ≤ h ≤ 37, −20 ≤ k ≤ 21, −7 ≤ l ≤ 10	−9 ≤ h ≤ 9, −12 ≤ k ≤ 13, −15 ≤ l ≤ 15
Reflections collected	10 702	14 602
Independent reflections	4129 [R(int) = 0.0955]	4026 [R(int) = 0.0185]
Completeness to theta = 26.80°	99.3%	99.4%
Absorption correction	Semi-empirical from equivalents	Semi-empirical from equivalents
Max. and min. transmission	0.7462 and 0.5436	0.7459 and 0.6469
Refinement method	Full-matrix least-squares on F^2	Full-matrix least-squares on F^2
Data/restraints/parameters	4129/0/202	4026/4/220
Goodness-of-fit on F^2	0.883	1.057
Final R indices [I > 2sigma(I)]	R 1 = 0.0594, wR2 = 0.1436	R 1 = 0.0279, wR2 = 0.0706
R indices (all data)	R 1 = 0.1263, wR2 = 0.1781	R 1 = 0.0341, wR2 = 0.0753
Largest diff. peak and hole	1.052 and −1.680 e Å^−3	0.698 and −0.666 e Å^−3
CCDC number	2354671	2354689

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Scheme 1 Synthetic procedure and structural arrangement in crystal lattice of (a) DNB-CP, and (b) NDC-CP.

2.4. Electrochemical measurements

These coordination polymers have been used as electrode materials in three-electrode systems for supercapacitor applications. Ag/AgCl and platinum (Pt) electrodes were applied as the reference and counter electrodes, respectively. Cyclic voltammetry tests were performed at different scan rates (10, 50, and 100 mV s⁻¹) in the potential window between 0.0 and +1.0 V. GCD studies were performed at different current densities (2, 4, 6, 8, and 10 mA cm⁻²). 1.0 M H₂SO₄ aqueous solution was used as the electrolyte in the electrochemical measurements. Before starting electrochemical measurements, stability testing of DNB-CP and NDC-CP was carried out in 1.0 M H₂SO₄ electrolyte. Both coordination polymers were dipped in the electrolyte for 12 hours, washed several times with water to eliminate the traces of acid content, and desiccated well. A comparison of PXRD data before and after the acid treatment was carried out for both DNB-CP and NDC-CP. The powder XRD data before and after acid treatment show identical peaks, and only a small difference in the peak intensities was observed.

For electrode preparation, there is the formation of the homogeneous powder of CPs (i.e., DNB-CP or NDC-CP), PVDF (polyvinylidene difluoride) as a binder^44,45 and activated carbon fibre cloth in an 8 : 1 : 1 ratio. The main role of the binder is to increase the binding capacity of the polymeric material (DNB-CP or NDC-CP) with the surface of the substrate (carbon fibre cloth). PVDF generally decreases the conductivity, so to compensate for this effect, activated carbon was used in an equal ratio to the binder so that the overall conductivity of the sample could be maintained. Initially, 8 mg of CP (DNB-CP or NDC-CP) were measured, 1.0 mg of PVDF, and 1.0 mg of activated carbon. These were all mixed, and the mixture was ground for 30 minutes vigorously until a fine powder was formed that was approximately ∼9 mg. The slurry was prepared by the addition of 200 μL of NMP and the solution was stirred for 24 hours for the proper dispersion of the powder mixture in the solvent to make a homogeneous mixture. NMP is known as a good solvent for electrochemical measurements because of its high boiling temperature (∼170 °C) and low viscosity.^44,46 NMP provides the best electrochemical performance because it can minimize the resistance of electron transfer at the interface of the electrode and electrolyte. The low viscosity of NMP reduces the chances of agglomeration of the active material because of the proper diffusion and high solubility of the binder in the solvent. Carbon fibre cloth was used as a substrate for electrode preparation. Before using it for drop-casting of the active material, the carbon fibre cloth was cut to size 2.0 × 1.0 cm² and sonicated in deionized water, ethanol, and acetone separately for 10 min. Then, the prepared carbon fibre cloth was dried in a vacuum oven for 30 minutes. 100 μL of the homogenous mixture was drop-cast on the carbon fibre cloth and the electrode was dried in a vacuum oven at 80 °C for 24 hours.

The cyclic voltammetry (CV) technique was used for assessment of the oxidation and reduction of species and the analysis of electron-transfer initiated chemical reactions. The specific capacitance (mF cm⁻²) from the CV curves can be obtained using the following equation:

where is the area enclosed by the CV curve; A is the dipped area of the working electrode; ΔV is the potential window; and ϑs is the scan rate in mV s⁻¹.

Galvanostatic charge–discharge (GCD) is a conventional technique that is used to test the cycle life and performance of electrochemical devices. Charge and discharge are performed at a constant current until the potential reaches a specific value. The specific capacitance can be calculated by using the following equation:

C = IΔt/(mΔV)

where I (A) is the discharge current; Δt (s) is the discharge time in the potential range of V; m (g) is the mass of active material; and ΔV (V) is the potential window.

The energy density (E) (mW h cm⁻²) and power density (P) (mW cm⁻²) were calculated using the following equations:

E = C(V)²/2 × 3600 and P = E × 3600/t,

respectively, where C is the specific capacitance of the active material; and V is the potential window of discharge.

The coulombic efficiency (CE) can be calculated from the following equation:

CE = Q_output/Q_input (ratio of charge removed from SC to charge used to restore the original capacity).

3. Results and discussion

3.1. Crystal structure description of DNB-CP and NDC-CP

The DNB-CP is solved in the monoclinic centrosymmetric space group C2/c. Its asymmetric unit consists of two Ag⁺ ions (Ag1 and Ag2) with 0.5 site occupancy, one DNB anion, and half a 4,4′-bipy (Fig. 2a). Both Ag⁺ ions are T-shaped, connecting with one nitrogen atom of 4,4′-bipy (N1 with Ag1 and N2 with Ag2) and two carboxylate oxygen atoms (O1 and O2) of the DNB anion. The Ag–N and Ag–O distances are in the range of 2.331(6)–2.346(7) and 2.193(4)–2.228(4) Å, respectively, whereas the bond angles of O1–Ag1–N1, O1–Ag1–O1^#1, N2–Ag2–O2, and O2–Ag2–O2^#1 (#1: −x, y, −z + 1/2) are ca. 99.20(1)°, 161.58°, 98.46°, and 163.09°, respectively (Table TS1, ESI†). The DNB anion shows bidentate bridging with two Ag⁺ ions with a Ag1⋯Ag2 distance of ca 2.876(2) Å.⁴⁷ The coordination modes of this complex are (μ₂-κ²,η¹:η¹).^48,49 Two Ag⁺ ions and two DNB anions form a dimeric unit in which both DNB anions are perpendicular to each other, having a dihedral angle of ca 73.94°. The dihedral angle of the planes of the DNB anions (red and green) and 4,4′-bipy (pink) is 53.41°, as shown in Fig. S2 (ESI†). Two dimeric units are interconnected by coordination of nitrogen atoms of 4,4′-bipy with different Ag⁺ ions, forming a 1D polymeric chain along the b axis (Fig. 2b). The dihedral angles (°) of –COO and –NO₂ w.r.t. the phenyl ring and dihedral angles (°) between the two aromatic rings of 4,4′-bipy in DNB-CP are given in Table TS3 (ESI†). The π⋯π interactions are absent, and weak H-bonding interactions are present in this coordination polymer. The oxygen atoms of the o-NO₂ and p-NO₂ groups of the DNB anion show H-bonding contacts with protons of the phenyl rings of the DNB anion and 4,4′-bipy, forming a 2D H-bonded network in the ac plane (Fig. S3, ESI†).

Fig. 2 (a) ORTEP showing the asymmetric unit of DNB-CP at 30% probability (half a unit of 4,4′-bipy is shown), (b) 1D coordination polymer of DNB-CP along the b axis, (c) ORTEP showing the asymmetric unit of NDC-CP at 30% probability, (half a unit of the NDC anion is shown), (d) the H-bonded network in NDC-CP in the ac plane.

The coordination polymer NDC-CP is similar to the reported complexes,^50–52 but this CP was synthesized again and characterized by single crystal X-ray diffraction and other spectroscopic techniques. The NDC-CP is solved in the triclinic centrosymmetric space group P1̄. Its asymmetric unit contains one Ag⁺ ion, one 4,4′-bipy, half an NDC anion, and two lattice water molecules (Fig. 2c). The geometry around the Ag⁺ ion is linearly coordinated with nitrogen atoms N1 and N2 of two 4,4′-bipy rings. The average Ag–N bond distance is 2.158(2) Å, and the N–Ag–N bond angle is 176.11(6)°. This CP is an example of a 1D and H-bonded complex in which the Ag⁺ ion coordinates with 4,4′-bipy to form a 1D polymeric chain of the [Ag(4,4′-bipy)₂]⁺ cationic unit along the c axis. The carboxylate oxygen atoms O1 and O2 of the NDC anion are H-bonded to protons of O1W with O⋯O distances in the range 2.740(3)–3.556(2) Å. Two lattice waters O1W (blue color balls), two O1 and O2 of two NDC anions are H-bonded to form six membered H-bonded rings with supramolecular synthon R⁴₄(12) (Fig. S4, ESI†) and further extended to form a 1D H-bonded chain along the c axis. These two H-bonded chains are further linked to each other via strong H-bonding interactions through lattice water O2W (green color balls) to form a 2D H-bonded sheet in the ac plane (Fig. 2d). Both the 1D polymeric chain and H-bonded chain are alternately present and connected to each other via H-bonding interactions, showing a 2D sheet in the bc plane (Fig. S5, ESI†). There are also H-bonding interactions between two lattice water molecules, O1W and O2W, with an O⋯O distance of ca 3.186(3) Å and ∠O2W–H21W⋯O1W = 142° (Table TS2, ESI†). The lattice water molecules are H-bonded to the carboxylate oxygen atoms of the NDC anion and protons of both the NDC anion and 4,4′-bipy, thus forming a 3D H-bonded network. Additionally, two 1D polymeric chains of [Ag(4,4′-bipy)₂]⁺ cationic units show π⋯π interactions of centroid (C1–C5, N1)⋯centroid (C6–C10, N2) = 3.755 Å of 4,4′-bipy to extend the network along the c axis (Fig. S6, ESI†).

3.2. Powder X-ray diffraction patterns (PXRDs)

By comparing the powder X-ray diffraction patterns of DNB-CP and NDC-CP, it has been concluded that the experimental and generated (or simulated) powder patterns are similar with respect to the positions and intensities of the diffraction peaks (Fig. 3a and b), confirming the phase purity of the CPs. The crystalline nature and the formation of different phases in the CPs have also been studied by PXRD. In case of DNB-CP, the Miller indices (hkl) for peaks at 2θ values of 6.96, 12.06, 13.92, 14.27, 14.78, 17.61, 21.01, 22.29, 22.74, 25.24, 30.37, 32.36, 39.54, and 40.51 are (200), (310), (400), (111), (111), (021), (330), (13−1), (131), (716), (402), (82−1), (26−1) and (64−2), respectively. These outcomes are in good agreement with CCDC 2354671, and this confirms the establishment of a monoclinic unit cell structure with the C2/c space group. In the case of NDC-CP, the Miller indices (hkl) for peaks at 2θ values of 7.79, 9.73, 12.22, 12.73, 15.94, 18.09, 18.73, 19.52, 23.45, 25.02, 26.48, 27.38, 28.68, 30.27, 30.83, 32.98, and 33.62 are (001), (010), (01−1), (011), (101), (01−2), (012), (020), (003), (2−10), (200), (2−20), (121), (12−2), (031), (03−2), and (1−1−4), respectively. These outcomes are also in good agreement with CCDC 2354689, and this confirms the development of a triclinic unit cell structure with the P1̄ space group.

Fig. 3 Comparison of generated or simulated (black) and experimental (red) powder patterns of DNB-CP (a), and NDC-CP (b); IR spectra of DNB-CP (c), and NDC-CP (d); Raman spectra of DNB-CP (e), and NDC-CP (f); TGA spectra of DNB-CP (g), and NDC-CP (h).

3.3. FTIR and Raman spectroscopy

The IR spectra of DNB-CP and NDC-CP are given in Fig. 3c and d. There is no absorption band present close to 3400 cm⁻¹ due to the absence of any water molecule in the crystal lattice of DNB-CP. The wide absorption band that appears at 3389 cm⁻¹ is a result of the stretching vibrations of the –OH group from lattice water molecules. Strong characteristic bands of the vibrations of asymmetric and symmetric –COO groups are observed in the regions of 1562–1599 and 1387–1410 cm⁻¹, respectively. In the case of DNB-CP, intense bands corresponding to the vibrations of asymmetric and symmetric N–O groups are detected at 1532 and 1349 cm⁻¹, respectively. Characteristic bands of M–O/M–N groups are observed in the range 625–831 cm⁻¹.

The Raman spectra of DNB-CP and NDC-CP are given in Fig. 3e and f. The Raman peak values for the functional groups can vary depending on the specific coordination environment and bonding characteristics of various interactions in the CPs. The main peaks in the Raman spectra of DNB-CP and NDC-CP include C=O, C–C, Ag–O and Ag–N stretching vibrations, which typically fall within the ranges of 1600–1700, 1200–1400, 500–700, and 200–400 cm⁻¹, respectively. The evaluation of characteristic bands in the IR and Raman spectra of DNB-CP and NDC-CP is given in Table TS4 (ESI†). In DNB-CP, the intense bands of C=O and N–O are detected at 1613 and 1291 cm⁻¹, respectively. The weak bands for Ag–O are observed at 655 and 573 cm⁻¹, whereas for Ag–N they are observed at 379 cm⁻¹. In NDC-CP, the medium peak located at 1590 cm⁻¹ is assigned to the vibrations of the C=O group, and the peak at 309 cm⁻¹ to the Ag–N group.

3.4. Thermogravimetric analysis of DNB-CP and NDC-CP

Thermogravimetric analysis (TGA) was executed under a nitrogen environment. The TGA graphs of DNB-CP and NDC-CP are shown in Fig. 3g and h. The graph for DNB-CP is constant up to 193 °C, and further rapid weight loss is detected in two steps, starting from 193 °C to 239 °C and then from 240 °C to 588 °C, with loss of 63.35% of the weight (calcd, 65.02%) of 4,4′-bipy and the DNB anion. 36.65% (calcd, 34.98%) of the weight of AgO₂ remained at the end. An exothermic peak (9.23 W g⁻¹) was detected at 237 °C due to the initial disintegration of 4,4′-bipy and the DNB anion. In NDC-CP, a loss of 8.91% (calcd, 8.86%) was observed at 171 °C due to two lattice water molecules. After that, loss of 64.84% (calcd, 63.89%) of the weight corresponds to the loss of 4,4′-bipy and the NDC anion. At the end, up to 400 °C, 26.25% (calcd, 25.20%) of the weight is left due to silver oxide. A sharp exothermic peak of 394 W g⁻¹ was detected at 371 °C due to the disintegration of 4,4′-bipy and the NDC anion.

3.5. UV-vis absorption studies

The liquid-state UV absorption spectra of DNB-CP and NDC-CP were recorded at room temperature (Fig. S7 and S8, ESI†). Band assignments for DNB-CP and NDC-CP were made and compared with earlier reports in the literature.^53,54 The absorption peak at about 338 nm in DNB-CP is assigned to the intraligand charge transfer transitions. An absorption peak in NDC-CP was observed at 327 nm, along with one shoulder peak at 342 nm due to the intraligand charge transfer transitions.

3.6. Morphological studies of the electrodes prepared from DNB-CP and NDC-CP

Scanning electron microscopy (SEM) images revealed the morphologies of the DNB-CP and NDC-CP based electrodes, and were used to check the supercapacitor behavior in 1 M H₂SO₄ electrolyte media. The morphology of the active material drop-cast on the conductive substrate (e.g., conductive carbon fibre cloth) plays an important role in the advanced electrochemical properties. The reason for this is that the electrode materials require many active locations for intercalation or deintercalation with the ions of the electrolyte during the charging or discharging process. SEM images of electrodes prepared from DNB-CP and NDC-CP are shown in Fig. 4. All the SEM images were recorded before the cycling process. The images of DNB-CP show the agglomeration of nonuniform spheres on the fibres of the conductive carbon fibre cloth. In NDC-CP, small crystalline materials are fused together and covering the conductive surface. The variation in the grain sizes of the CPs can significantly influence their electronic properties. Smaller grain sizes are often associated with increased grain boundary density, which can enhance the scattering mechanisms for charge carriers. This scattering can lead to a decrease in electrical conductivity. On the contrary, larger grains may facilitate better charge transport due to fewer grain boundaries, suggesting a potential pathway for optimizing the electronic performance through controlled grain growth.

Fig. 4 SEM images of DNB-CP (a)–(c), and NDC-CP (d)–(f). The representative SEM images recorded at high and low magnifications resolve the microstructures of the electrodes before electrochemical charging and discharging.

3.7. XPS investigation

The surface composition, functional groups, and oxidation states in DNB-CP and NDC-CP were studied using XPS. For a thorough review of elemental analysis, the binding energies of DNB-CP and NDC-CP and their fundamental elements were assessed using an XPS study. Fig. 5 and 6 display the XPS spectra of C, N, O, and Ag elements present in DNB-CP and NDC-CP. The survey scans for DNB-CP (Fig. 5a) and NDC-CP (Fig. 6a) showed the existence of C (1s), O (1s), N (1s), Ag (3d_3/2), and Ag (3d_5/2) without impurities. In DNB-CP, the binding energy spectrum of C (1s) displayed values of 288.85, 285.66, and 284.68 eV corresponding to O–C=O, C–O, and C–H groups, respectively (Fig. 5e). For element N (1s), Fig. 5d shows binding energy values of 399.47 and 406.34 eV, while for element O (1s), the binding energy was found to be 533.22 eV. For Ag (3d), two distinct peaks are shown at 368.78 and 374.75 eV, assigned to Ag (3d_5/2) and Ag (3d_3/2), respectively. Similarly, in NDC-CP, a pair of peaks corresponding to Ag (3d_5/2) and Ag (3d_3/2) are present at 368.38 and 374.33 eV, respectively. There is a 6.0 eV difference between the binding energies for the peaks of Ag (3d_5/2) and Ag (3d_3/2), and as established in the literature,² this indicates the successful reduction of the starting silver precursor (AgNO₃ salt) to silver during the synthesis of DNB-CP and NDC-CP under facile hydrothermal conditions. The Ag (3d) peaks transfer to the lower binding energy; this alteration in binding energy occurs because the Ag metal exhibits inconsistent properties when it is oxidized. The C (1s) spectrum of NDC-CP can be deconvoluted into two peaks at 289.03 and 284.85 eV, assigned to O–C=O and C–H groups, respectively (Fig. 6e). The broad N (1s) spectra for DNB-CP and NDC-CP from 397.85 to 401.61 eV are a result of the conductive nature of 4,4′-bipy (Fig. 5d and 6d). All the studies, including single crystal X-ray diffraction, FT-IR, Raman, PXRD, SEM and XPS analysis are in good agreement for the synthesis and conformation of both coordination polymers.

Fig. 5 XPS: (a) survey spectrum of DNB-CP; (b) Ag 3d, (c) O 1s, (d) N 1s, and (e) C 1s spectra.

Fig. 6 XPS: (a) survey spectrum of NDC-CP; (b) Ag 3d, (c) O 1s, (d) N 1s, and (e) C 1s spectra.

3.8. Hirshfeld surface analysis

Hirshfeld surface analysis was executed on both DNB-CP and NDC-CP to visualize the effect of the intermolecular non-bonding interactions (strong, moderate, and weak) on the surface molecules and examine the percentage of various strong and weak contacts from the 2D fingerprinting plot. The d_norm (d_i and d_e are normalized functions) was recorded on the Hirshfeld surface (Fig. 7a) to understand the contact areas with different colors (red, blue, and grey in order of increasing distance with respect to the sum of the van der Waals radii)⁵⁵ in DNB-CP and NDC-CP. The formula to calculate d_norm is given below:

d_norm = (d_i − r^vdW_i)/r^vdW_i + (d_e − r^vdW_e)/r^vdW_e

Here, d_i = the distance from Hirshfeld surface to the nearest nucleus inside the surface.

Fig. 7 (a) Molecular Hirshfeld d_norm surfaces, shape index, and curvedness in DNB-CP and NDC-CP. (b) 2D fingerprint plots in DNB-CP and NDC-CP, where the areas of various intermolecular contacts are revealed. (c) The percentage of Hirshfeld surface areas for various intermolecular contacts in DNB-CP and NDC-CP.

d _e = the distance from Hirshfeld surface to the nearest nucleus outside the surface.

d _norm = a normalized contact distance, which is defined in terms of d_i, d_e and van der Waals (vdW) radii (r^vdW) of the atoms.

In the shape index surface, the adjacent red and blue triangles show the existence of π⋯π stacking contacts in the crystal structure of NDC-CP, which are lower in number in the case of DNB-CP. The convex regions due to the carbon atoms of the aromatic ring inside the surface are denoted by blue triangles. The red triangles signify the concave areas due to the carbon atoms of the π-stacked molecule above it. The large flat region with a blue border across the structure gives proof of the existence of π⋯π interactions, which is clearly shown on the curved surface of NDC-CP. The DNB-CP is a polymeric compound, but it does not have any water molecules in the crystal structure, so there are fewer regions of red spots compared to the NDC-CP having two lattice water molecules. As revealed in the fingerprinting plot (Fig. 7b), the O⋯H/H⋯H (40.3%) contacts in DNB-CP are comparatively lower in percentage as compared to the O⋯H/H⋯H (51.9%) contacts in the NDC-CP (Fig. 7c). NDC-CP is an example of a combined ionic and polymeric compound in which NDC anions are present in the lattice of the crystal system, alternately with the 1D polymeric chain of [Ag(4,4′-bipy)]_n. The largest part of the 2D fingerprinting plot is taken by O⋯H/H⋯O contacts in both cases (DNB-CP and NDC-CP), but NDC-CP covers more of the area than DNB-CP. According to the crystal structure, there are no π⋯π and C–H⋯π contacts in DNB-CP, so the C⋯C contacts are fewer than in NDC-CP. The O⋯O (5.9%) and C⋯O (5.8%) contacts largely correspond to the DNB anion in DNB-CP, and are almost negligible in the case of NDC-CP. The Ag⋯O (5.9%) contacts are more in DNB-CP compared to Ag⋯O (2.8%) contacts in NDC-CP, whereas Ag⋯N (3.6%) contacts are fewer in DNB-CP as compared to Ag⋯N (5.3%) contacts in NDC-CP. The reason for the difference in the percentages of Ag⋯O and Ag⋯N is that in DNB-CP, oxygen atoms of the DNB anion and nitrogen atoms of 4,4′-bipy are coordinating with Ag⁺ ions, whereas in NDC-CP, only the nitrogen atoms of 4,4′-bipy are coordinating with Ag⁺ ions. The separate 2D fingerprint plots for each intermolecular interaction in DNB-CP and NDC-CP are illustrated in Fig. S9 and S10 (ESI†). It has been concluded that both Hirshfeld surfaces and fingerprint plots enable the evaluation of various intermolecular interactions in the construction of supramolecular motifs in the crystal structure.

3.9. Electrochemical performance

It has been observed in the literature that the dimensionality of the CPs that are used as active materials for electrodes affects the electrochemical behavior for supercapacitor performance.^56–60 The electrochemical behavior of the silver-based CPs is analyzed using electrochemical methods such as CV and GCD. An experimental set-up for slurry and electrode preparation and instrumental set-up of a three-electrode system are used to investigate the supercapacitor performance of DNB-CP and NDC-CP based electrodes (Fig. S11, ESI†). Detailed information related to the weight of the DNB-CP and NDC-CP used for the preparation of electrodes is given in Table TS5 (ESI†). To check the cyclic stability of the DNB-CP and NDC-CP electrodes, they were immersed in 1.0 M H₂SO₄ (aq.) electrolyte up to 1.0 cm, and the CV cycles were carried out at different scan rates of 10, 50 and 100 mV s⁻¹. In DNB-CP, with the increase in scan rate ranging from 10, 50, to 100 mV s⁻¹, the current density increased from 3.12 mA cm⁻² to 14.77 mA cm⁻² and then to 20.23 mA cm⁻², respectively (Fig. 8a). The DNB-CP showed cyclic stability upon increasing the current density from 3.12 mA cm⁻² to 5.14 mA cm⁻² at a scan rate of 10 mV s⁻¹ after 500 cycles (Fig. 8b). The DNB-CP electrode shows constant values of current densities at high scan rates of 50 mV s⁻¹ and 100 mV s⁻¹, and starts to degrade with the cycling process. This is because at high scan rates, the active material starts to shed from the conductive carbon substrate during the cycling test. In DNB-CP, the charge under the CV curve is the electrical double layer capacitance (EDLC) that is shown in Fig. 8a and b. The redox process happened at the electrode surface, but the redox potential range is not shown in the graph, and is above 1.0 V. In NDC-CP, at a scan rate of 10 mV s⁻¹, silver redox peaks are observed in the potential range 0.33 to 0.62 V (Fig. 8c). These silver redox peaks are diminished after going towards high applied scan rates (50 mV s⁻¹ and 100 mV s⁻¹) because these faradaic peaks are buried under the large double layer currents of the active electrode material. There is a similar trend of increase in current density with DNB-CP after applying high scan rates; as the scan rate goes from 10 to 50 and 100 mV s⁻¹, the maximum current densities are 0.28, 1.96, and 2.83 mA cm⁻², respectively. The presence of redox peaks in the CV curve of NDC-CP at 10 mV s⁻¹ confirms the pseudocapacitive nature of the active material that is diminished at high scan rates. The strong redox peak display in the CV curve is due to the transition in the oxidation state between Ag⁺ and Ag²⁺. The process of ionic intercalation and deintercalation in the electrode material is facilitated by the applied potential during the electrochemical activity. When a specific potential is applied, the electrolyte ions move inside and outside of the electrode material. This results in the material transitioning to the Ag²⁺ state during oxidation and reverting to the Ag⁺ state during the reduction process. Upon comparison of the CV graphs of DNB-CP and NDC-CP, it was observed that the CV curve of the DNB-CP electrode shows good EDLC behavior, which indicates that the passage of ions at the electrode–electrolyte interface makes it a good electrical double-layer supercapacitor (Fig. 8d).

Fig. 8 (a) CV cycles of DNB-CP at different scan rates (10, 50, and 100 mV s⁻¹). (b) CV stability of DNB-CP at 10 mV s⁻¹ up to 500 cycles. (c) CV cycles of NDC-CP at different scan rates. (d) Comparison of the CV curves of DNB-CP and NDC-CP at different scan rates. (e) Comparison of the 2nd and 100th CV curves of NDC-CP, showing the conversion of pseudocapacitive (faradaic) to EDLC (non-faradaic) behavior. (f) Comparison of specific capacitance vs. cycle number for DNB-CP and NDC-CP at different scan rates.

The charge under the CV curve corresponds to EDLC in the case of DNB-CP and the redox process in the case of NDC-CP at the electrode surface. With cycling at 10 mV s⁻¹, the redox peaks in the CV graph of NDC-CP remain analogous up to 50 cycles, indicating the superior rate capability of the electrode with active material (Fig. S12, ESI†). The oxidation peak at the anode moderately shifts towards lower potential, and the reduction peak at the cathode slightly shifts towards the higher potential range. There is a transition of the CV curves of NDC-CP at 10 mV s⁻¹ after 100 cycles from pseudocapacitive to EDLC behavior with increasing enclosed area under the curve (Fig. 8e) and a current density from 0.28 to 0.68 mA cm⁻². As the number of CV cycles increases, the number of active sites for the adsorption of ions increases. Therefore, the area under the curve also increases, which signifies a higher rate of adsorption of ions (Fig. 8b and e). The DNB-CP shows a current density of 3.12 mA cm⁻² at a 10 mV s⁻¹ scan rate, which is higher than NDC-CP showing a current density of 0.30 mA cm⁻² (Fig. S13, ESI†).

From the area enclosed under the CV curve at different scan rates, the specific capacitance (mF cm⁻²) has been calculated for DNB-CP and NDC-CP (Fig. 8f). There is no regular trend for specific capacitance values with increasing scan rates from 10 to 100 mV s⁻¹. The DNB-CP at 10 mV s⁻¹ shows a high specific capacitance value of 971.08 mF cm⁻², and the NDC-CP at 50 mV s⁻¹ shows a high specific capacitance value of 78.81 mF cm⁻². The trend of specific capacitance (mF cm⁻²) for DNB-CP and NDC-CP is as follows: 971.08 (DNB-CP@10 mV s⁻¹) > 199.52 (DNB-CP@100 mV s⁻¹) > 78.81 (NDC-CP@50 mV s⁻¹) > 48.42 (NDC-CP@10 mV s⁻¹) > 46.36 (NDC-CP@100 mV s⁻¹) > 41.76 (DNB-CP@50 mV s⁻¹). The energy densities of DNB-CP and NDC-CP follow a similar trend to the specific capacitances (Fig. S14, ESI†). The DNB-CP at 10 mV s⁻¹ shows a higher energy density of 0.135 mW h cm⁻², and NDC-CP shows a higher energy density of 0.011 mW h cm⁻² at a 50 mV s⁻¹ scan rate. For DNB-CP, the total capacitance results from the electric double layer at all scan rates due to the movement of charges. In contrast to this, for NDC-CP, the total capacitance results from the pseudocapacitance at low scan rate. Upon increasing the scan rate, the total capacitance is due to the combination of double-layer and pseudocapacitance in silver CPs. Upon increasing the potential, Ag ions are oxidized to the higher state without a current drop. This is a well-known mechanism in electrically conducting polymers, and transition metal oxides.^24,25,28

To investigate the galvanostatic charge discharge (GCD) in both electrodes, they were immersed in 1.0 M H₂SO₄ electrolyte up to 1.0 cm and checked at a broad range of current densities (2, 4, 6, 8, and 10 mA cm⁻²). The GCD graphs for DNB-CP and NDC-CP as active materials on two different electrodes at 2 mA cm⁻² current density showed large charging and discharging compared to at other current densities like 4, 6, 8, and 10 mA cm⁻² (Fig. S15 and S16, ESI†). It can be observed from the GCD graph for DNB-CP and NDC-CP at different current densities that DNB-CP at 2 mA cm⁻² shows greater charging and discharging than NDC-CP at 2 mA cm⁻². The masses of the active materials (DNB-CP and NDC-CP) deposited on the conductive carbon fibre cloth are almost equal (Table TS5, ESI†), but the activities shown by the electrodes are different, because the supercapacitor activity is affected by the dimensionality of the CPs. For the charge storage system, the electrode charge storage mechanism consists of two parts: first is the capacitance of the electric double layer and second is the diffusion-controlled involvement of the redox reaction of the electrochemically active substance. The charging and discharging for DNB-CP and NDC-CP at different current densities are given in Fig. 9a, and the areal specific capacitances for DNB-CP and NDC-CP at different current densities (2, 4, 6, and 10 mA cm⁻²) after 100 cycles are almost constant (Fig. 9b). The comparison of mass specific capacitance (F g⁻¹) for DNB-CP and NDC-CP is given in Fig. S17 (ESI†). At 4 mA cm⁻² current density, DNB-CP shows maximum specific capacitance, and even the specific capacitance of NDC-CP at 4 mA cm⁻² is much less than that of DNB-CP. The DNB-CP (Fig. 9c and d) at 4 mA cm⁻² shows better results due to high energy density and better power density (Table TS6, ESI†). The DNB-CP electrode shows stability up to 10 000 s GCD cycles at 4 mA cm⁻² (Fig. 9e). This shows that the DNB-CP based electrodes are more efficient for energy storage devices. In DNB-CP, all the CV curves show a semi-rectangular shape at different potential windows, which establishes the electric double layer behaviour. The high symmetry in the applied potential window shows the substantial electrochemical reversibility in the CPs. The coulombic efficiency has been calculated by using the formula CE = Q_output/Q_input. It has been shown that for DNB-CP, the CE improved upon increasing the current density in the range 2 to 10 mA cm⁻² (Fig. S18, ESI†). A comparison of the coulombic efficiency (%) of DNB-CP at different current densities up to 100 GCD cycles is given in Fig. 9f. The GCD capacitive retentions are 106%, 101%, 101% and 100% for DNB-CP at 2, 4, 6, and 10 mA cm⁻², respectively, after 100 cycles. There are identical charge and discharge times in each curve, indicating strong capacitance activity and a reversible redox process of the CPs. During the discharge process, a distinct potential plateau at 0.33 V is detected. This plateau is attributed to the electrochemical adsorption–desorption process and the faradaic redox reactions occurring at the electrode–electrolyte interface, consistent with the observations from the CV curves. A comparative study of the earlier reported compounds that have been explored for supercapacitor applications is shown in Table 2. The specific capacitance values achieved in our research work are comparable to those in the reported literature, and even superior to some earlier reports. However, our research mainly focused on a mechanistic approach, which provides a comparison of supercapacitor performance across different systems based on the dimensionality, morphology of the electrode material, its polarization, and the selected electrolyte.

Fig. 9 (a) GCD graph for DNB-CP and NDC-CP at different current densities, (b) comparison of specific capacitance (mF cm⁻²) vs. cycle number, (c) comparison of energy density (mW h cm⁻²) vs. power density (mW cm⁻²), (d) comparison of specific capacitance vs. energy density, (e) GCD cycles for 10 000 s in DNB-CP at 4 mA cm⁻², and (f) graph of coulombic efficiency (%) vs. cycle number at a range of current densities in DNB-CP.

Table 2 Comparative study of previously reported silver-based compounds for supercapacitor applications

Name of electrode	Electrolyte	Capacitance	Energy density	Power density	Ref.
WO3	PVA/H2SO4 gel electrolyte	22.6 mF cm^−2	1.13 × 10^−3 mW h cm^−2@0.05 mA cm^−2	—	6
Ag-based 3D nanostructure	5.0 M KOH	0.5 F cm^−2@6.4 mA cm^−2	385.87 μW h cm^−2	3.82 μW cm^−2	24
Ag/CNTs	6.0 M KOH	30.6 mF cm^−2	61.0 mW h m^−2	1.2 W m^−2	25
Ag/Au/polypyrrole	PVA–H3PO4 gel electrolyte	0.58 mF cm^−2	—	—	26
Ag NWs/WO3	1.0 M H2SO4	13.6 mF cm^−2	—	—	29
Ag & CNTs composite	Solid gel electrolyte (PVA–KOH)	677.12 mF cm^−2@0.0125 mA cm^−2	—	—	37
		282.75 mF cm^−2@0.05 mA cm^−2
		298.89 mF cm^−2@0.0625 mA cm^−2
		288.75 mF cm^−2@0.125 mA cm^−2
		319.17 mF cm^−2@0.1875 mA cm^−2
0.25Ag-AC-0.1	6.0 M KOH	632 mF cm^−2@5 mA cm^−2	—	—	36
		627 mF cm^−2@10 mA cm^−2
		595 mF cm^−2@15 mA cm^−2
Activated carbon–silver–cotton (AC–Ag–CT)	6 M KOH	950 mF cm^−2@0.05 mA cm^−2	—	—	36
Ag–AC–CT flexible supercapacitor	PVA–KOH	589 mF cm^−2@0.05 mA cm^−2	11.23 mW h cm^−2	24.86 mW cm^−2	36
			3.26 mW h cm^−2	46.55 mW cm^−2
			1.66 mW h cm^−2	84.92 mW cm^−2
			1.06 mW h cm^−2	115.83 mW cm^−2
			0.51 mW h cm^−2	147.6 mW cm^−2
DNB-CP	1 M H2SO4	971.08 mF cm^−2@10 mV s^−1	0.135 mW h cm^−2@10 mV s^−1	—	This work
		660.01 mF cm^−2@4 mA cm^−2	0.092 mW h cm^−2@4 mA cm^−2	2.02 mW cm^−2
NDC-CP	1 M H2SO4	78.81 mF cm^−2@50 mV s^−1	0.012 mW h cm^−2@50 mV s^−1	—	This work
		32.01 mF cm^−2@2 mA cm^−2	0.004 mW h cm^−2@2 mA cm^−2	0.4 × 10^−2 mW cm^−2

---

4. Conclusion

In conclusion, silver coordination polymers (DNB-CP and NDC-CP) have been effectively fabricated by a facile one-pot hydrothermal approach. Electrochemical studies of DNB-CP and NDC-CP based electrodes have been carried out to determine their electrochemical behavior with three-electrode systems. Single crystal X-ray diffraction studies agree with Hirshfeld surface calculations and other spectroscopic characterizations. Both DNB-CP and NDC-CP form 1-D polymeric chains, but NDC-CP has additional ionic bonding interactions, which decrease the charge transfer, ion transport, and charge storage capacity of NDC-CP as compared to DNB-CP. From Hirshfeld surface analysis, it is evident that the strong hydrogen bonding interactions are greater in DNB-CP, so the wettability of DNB-CP is higher than that of NDC-CP in aqueous electrolytes (1.0 M H₂SO₄). The DNB-CP electrode shows better electrochemical behavior than the NDC-CP electrode. Therefore, it is considered an effective material for the preparation of high-performance electrode materials for supercapacitors. Upon comparing the CV and GCD of both electrodes, the specific capacitance of DNB-CP is higher than that of NDC-CP at similar scan rates and current densities, even though both are prepared using similar methods. With the GCD graph, it has been confirmed that the charge–discharge time of DNB-CP is higher than that of NDC-CP. The DNB-CP delivers a maximum specific capacitance of 970.08 mF cm⁻² at 10 mV s⁻¹ in an aqueous electrolyte. In DNB-CP, the charge under the CV curve is due to the double-layer capacitance, and in the case of NDC-CP, the redox process at the electrode surface. Upon increasing the current density, the calculated capacitance decreases due to shedding of the active material on the conductive substrate. From FESEM images, the DNB-CP sample has agglomerated spherical particles, which contributes to greater ion adsorption, whereas NDC-CP has small crystalline particles fused together, which hinders ion adsorption. In conclusion, the DNB-CP electrode at a 10 mV s⁻¹ scan rate shows excellent cyclic stability, and a current density of 4 mA cm⁻² results in better GCD ability with higher specific capacitance, energy density, and power density.

Author contributions

AKJ: literature survey, methodology, single crystal X-ray analysis, investigation, writing of original draft, and visualization. RK: literature survey, and conducted experiments, data compilation. LG: data collection of SEM, XPS studies, conceptualization, resources, writing, review, and editing, supervision, and project administration, conceptualization, supervision, and project administration.

Data availability

All the supporting data has been given in the ESI.† This manuscript includes the crystal structure data. The related files, like CIF and checkcifs, have been provided. The information related to the software has also been given.

Conflicts of interest

The authors declare no competing financial interest.

Acknowledgements

Dr Amanpreet Kaur Jassal, Assistant Professor at Guru Nanak Dev University Amritsar, Punjab, India, is thankful to the university and specifically the Department of Chemistry for providing superior infrastructure and leading-edge facilities. AKJ acknowledges the financial support from the Science and Engineering Research Board (SERB) project no: EEQ/2023/000610. Dr Viswanath Balakrishnan, Associate Professor, School of Mechanical and Materials Engineering, Indian Institute of Technology Mandi, H. P. India, is acknowledged for providing the laboratory facilities at IIT Mandi.

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Footnote

† Electronic supplementary information (ESI) available: CCDC 2354671 and 2354689. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4nj04451g

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