Electrochemical etching mediated enhanced supercapacitor performance of a binder-free Ni/Co/Mo carbonate hydroxide electrode material

Abstract

Transition metal carbonate hydroxides (TMCHs) possess impressive theoretical capacitance, rapid ion transfer ability, and minimal volume expansion during long charge–discharge cycles. Increasing the electrochemical active surface area/porosity of the material is expected to enhance the charge storage ability. This report highlights how the electrochemical etching of intercalated molybdate (MoO₄²⁻) anions can improve the charge storage ability of TMCHs via the reconstruction process. Herein, we have directly grown MoO₄²⁻ intercalated bimetallic cobalt/nickel carbonate hydroxide (C_XN_1−XMO) materials onto a redox inactive carbon cloth (CC) substrate via a hydrothermal route. The electrochemical activation of these materials results in the formation of bimetallic hydroxide/(oxy) hydroxide species. Meanwhile, the intercalated MoO₄²⁻ions have etched away from the material by leaving the pores inside, which is expected to increase the charge storage ability. Mainly, the material developed from C₇₅N₂₅MO (denoted as C₇₅N₂₅MO-A) exhibits an impressive specific capacitance (C_sp) of 2039 F g⁻¹ at 1 A g⁻¹ current density with 71% capacitive retention (∼99% coulombic efficiency) up to 5000 cycles. Furthermore, to unlock the importance of etching in the charge storage process, C₇₅N₂₅O was separately synthesized without a Mo source, showcasing inferior charge storage ability. The meticulous mechanistic investigations manifest that the etching of intercalated MoO₄²⁻ anions enhances the diffusion phenomenon within the C₇₅N₂₅MO-A material, while the charge storage is primarily attributed to the surface redox process in activated C₇₅N₂₅O-A. In addition, the etching process facilitates a good balance between diffusion and surface contribution, which is beneficial for better C_sp. Finally, a hybrid device C₇₅N₂₅MO-A (+)//AC (−) was fabricated, manifesting a maximum 1.5 V operational voltage window with a 17.30 W h kg⁻¹ energy density at a power density of 1510.55 W kg⁻¹. Furthermore, the device can retain 88% of its initial capacity up to 10 000 cycles, demonstrating the suitability of the material for real-world use.

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Introduction

The limited availability of non-renewable energy resources resulted in a surge in the energy crisis.^1–3 To mitigate this issue, various strategies such as energy harvesting, conversion, and storage using renewable feedstocks have been developed. For efficient energy accumulation and subsequent on-demand utilization, the development of proper energy storage devices is essential.⁴ Electrochemical supercapacitors (ESs) are the most promising candidates due to their high-power density and long cyclic stability.^5,6 ESs are principally of three types: (a) electrochemical double-layer capacitors (EDLCs), (b) pseudocapacitors, and (c) battery-type capacitors. EDLC materials follow non-faradic (non-redox) ion accumulation around the electrical double layer, whereas pseudocapacitors store charges via a faradaic electron transfer process.^7–9 Despite having sound power output, ESs still suffer from low specific energy.^9,10 Recently, battery-type supercapacitors have been designed to address this problem, as they provide high energy density, impressive rate capability, and charge storage via both faradaic and non-faradaic nature.^11,12

Among various materials, transition metal (Ni, Co, Mn, etc.) based Layered Double Hydroxides (LDHs) display efficient battery-type behaviour.^13–17 Recently, a new class of LDH materials called transition metal carbonate hydroxides (TMCHs) have emerged as potential candidates for supercapacitor application.^18,19 TMCH materials display promising cyclic stability, high energy density, reversible redox behavior, and enhanced hydrophilic properties due to CO₃²⁻ ions between the layers.^20–22 In addition, TMCH materials can be quickly grown on any conductive substrate, which minimizes the inherent contact resistance. Both monometallic^23,24 and bi-metallic carbonate^25,26 hydroxides showcased remarkable charge storage ability. Interestingly, this phenomenon can be further triggered by intercalating anions between the TMCH layers. For instance, Tian et al. recently studied how the intercalation of nitrate ions influences the charge storage ability of flower-like NiCo carbonate hydroxide (LDH) materials. The nitrate anion between the layers enhances the interlayer spacing and improves the diffusion behavior and specific capacity.²⁷ In another report, Zou and co-workers reported the enhanced electrochemical activity of NiFeCo-LDH by sodium dodecyl sulfonate (SDS) ion intercalation. SDS modulates the interlayer distance, provides an effective electrolyte migration channel, and increases the conductivity of the NiFeCo LDH material.²⁸

Furthermore, designing electrode materials with numerous metal centers significantly boosts the charge storage ability. Recently, in situ structural reconstruction via electrochemical activation of precursors has gained significant interest as it results in enhanced specific capacitance. This could be due to the facile accessibility of electrolyte ions to more metal centers.²⁹ The reconstruction generally occurs via the etching of functional moieties from precursors, thus providing more metal centers for electrolyte ions. In addition, the etching strategy alters the surface morphology, making it easier to transform metal precursors into their (oxy)hydroxides, ultimately improving the electrochemical performance.³⁰ Recently, Liu et al. studied how electrochemical activation can drive structural reconstruction via V etching, leading to enhanced electrochemical performance.³¹ In another report, Han et al. demonstrated excellent electrochemical activity of cobalt iron phosphide through surface reconstruction achieved via phosphorus etching.³² Our group has recently showcased an in situ structural reconstruction process involving etching of phosphate and vanadate ions, forming porous electroactive (oxy)hydroxide species.^33,34 However, there are limited reports on the use of this etching process for developing high-performing battery-type materials. Recently, our group has reported the formation of battery-type nickel-cobalt hydroxide-(oxy) hydroxide structures from nickel-cobalt pyrophosphate via the etching of pyrophosphate (P₂O₇⁴⁻) ions through in situ electrochemical activation.³⁵ Similarly, the etching of phosphate from bulk cobalt phosphate nanosheets has also been reported to boost specific capacitance.³⁶

In this context, metal molybdates (MMoO₄²⁻) (M = Ni, Co, Mn, etc.) are also considered potential candidates for supercapacitors due to various valence states and the ability to show structural reconstruction properties.^37,38 Our idea is to develop a bimetallic carbonate hydroxide material on a carbon cloth (CC) substrate by the direct growth technique through in situ incorporation of molybdate (MoO₄²⁻) ions into the interlayers. During electrochemical activation, the removal of molybdate anions is anticipated at a strong alkaline pH, which develops a porous structure and ultimately leads to improved specific capacitance. Furthermore, the CC offers excellent mechanical flexibility for SCs. Hence, the direct growth of metal carbonate hydroxides can facilitate the movement of the electrolyte ions and improve electrochemical activity.

Inspired by the abovementioned advantages, we have designed a binder-free direct synthesis of molybdate ion intercalated Ni, Co carbonate hydroxide materials. The resultant material, upon electrochemical activation, transforms into corresponding hydroxide-(oxy) hydroxide species. At first, a series of molybdate (MoO₄²⁻) ion intercalated bimetallic carbonate hydroxide C_xN_100−xMO (x = 0 to 100) materials were synthesized through a one-pot hydrothermal process on a CC substrate. The synthesis process included specific proportions of Co(NO₃)₂·6H₂O and Ni(NO₃)₂·6H₂O, a definite quantity of Na₂MoO₄·2H₂O, with water as a solvent, and urea as a hydrolyzing agent. Electrochemical activation transforms the as-prepared carbonate hydroxide materials to their corresponding Co, Ni-hydroxide-(oxy) hydroxide species via etching of intercalated MoO₄²⁻ ions. Moreover, such an etching process enhances the porosity of the active material, resulting in better migration of electrolyte ions and enhancing the charge storage ability. Primarily, the material derived after the reconstruction of C₇₅N₂₅MO (denoted as C₇₅N₂₅MO-A) displays a maximum specific capacitance (C_sp) of 2039 F g⁻¹ at 1 A g⁻¹ current density, along with 71% capacitive retention up to 5000 cycles. Furthermore, to unveil the role of MoO₄²⁻ ion in the charge storage mechanism, C₇₅N₂₅O was separately synthesized without the Mo precursor. Interestingly, the meticulous mechanistic investigation reveals that the etching of intercalated MoO₄²⁻ generates a more diffusion phenomenon in C₇₅N₂₅MO-A than activated C₇₅N₂₅O-A, leading to a good balance between the surface and intercalation redox process, thus improving the specific capacitance value. Finally, a hybrid device was fabricated using the activated carbon and electrochemically activated C₇₅N₂₅MO-A as negative and positive electrodes, respectively. The device (C₇₅N₂₅MO-A//AC) showcased an extended voltage range of 1.5 V and achieved a maximum energy density of 17.30 W h kg⁻¹ at the specific power density of 1510.55 W kg⁻¹. In addition, the C₇₅N₂₅MO-A//AC hybrid device manifested a promising capacitive retention rate of 88% even after 10 000 cycles, suggesting the suitability of the material for real-world use.

Results and discussion

All metal carbonate hydroxide materials were synthesized via a direct growth technique on a CC substrate using Co(NO₃)₂·6H₂O, Ni(NO₃)₂·6H₂O, Na₂MoO₄·2H₂O, and urea as precursors. Briefly, the aforementioned chemicals were dissolved in distilled water and subsequently reacted by a hydrothermal process, along with CC placed vertically on the sides of a Teflon vial (Scheme 1). The resulting bimetallic carbonate hydroxides (C_XN_100−XMO) (where X = 0, 75, 50, 25, 100) were deposited on the redox inactive CC substrate. However, in the absence of Ni and Co precursors, Na₂MoO₄ decomposes and stabilizes as MoO₄²⁻ ions in the alkaline environment (due to urea decomposition) (Fig. S1 and Note S1†). Thus, the formed MoO₄²⁻ ions intercalated within the as-prepared layered Ni/Co hydroxy carbonates during the growth process. Furthermore, to achieve the optimum composition, the Ni/Co ratio was altered (see the synthesis details in the experimental section in the ESI†). Afterward, the electrochemical activation transforms these CC-supported carbonate hydroxide electrode materials into active bimetallic Ni/Co hydroxide/(oxy) hydroxide materials via the etching of intercalated MoO₄²⁻ ions. The etching process creates a porous network with a rough flake-like microstructure that is anticipated to improve the charge storage ability (vide infra). The corresponding synthetic protocol and electrochemical conditions are elucidated in Scheme 1.

Scheme 1 Schematic illustration of the fabrication process for porous electroactive Co/Ni hydroxide/(oxy)hydroxide materials obtained from Co/Ni carbonate hydroxide through electrochemically driven MoO₄²⁻ etching.

Phase analysis

The phase purity of the as-synthesized materials was confirmed using powder X-ray diffraction (PXRD) analysis. The PXRD patterns of CMO (without nickel) closely matched the standard patterns of cobalt carbonate hydroxide hydrate (Co(CO₃)_0.5(OH)·0.11H₂O) (JCPDS: 048-0083). However, a limited cobalt hydroxide (Co(OH)₂) (JCPDS: 030-0443) phase was also detected. In contrast, NMO (without cobalt) corresponds to the formation of pure nickel carbonate hydroxide (Ni(CO₃)(OH)₂) (JCPDS: 035-0501) (Fig. S2†). Furthermore, to comprehend the effect of metal compositions on phase purity, the PXRD patterns of metal carbonate hydroxides with different Ni/Co ratios were also analyzed (Fig. S2†). It is noted that with increasing Ni concentrations, the peaks corresponding to Co(OH)₂ started disappearing. When Ni content exceeds 20%, the material comprises MoO₄²⁻ ions intercalated in the Co, Ni carbonate hydroxy hydrate material. Meanwhile, the materials with a higher Ni amount display a shift of the (100) and (221) of (Co(CO₃)_0.5(OH)·0.11H₂O), and (002) of (Ni(CO₃)(OH)₂) planes to higher angles. This observation indicates the lowering of the crystallite size and the replacement of Co with Ni centers. Furthermore, the C₇₅N₂₅MO material shows the presence of individual peaks of cobalt carbonate hydroxy hydrate (JCPDS: 048-0083) and nickel carbonate hydroxide (JCPDS: 035-0501), confirming the formation of a bimetallic carbonate hydroxide. Furthermore, to understand the impact of MoO₄²⁻ ions on the growth process, C₇₅N₂₅O was separately synthesized without any source of the Mo precursor. The diffraction patterns of the as-prepared C₇₅N₂₅O display broad peaks and the absence of the (100) plane of Co(CO₃)_0.5(OH)₂·0.11H₂O, manifesting poor stacking likely due to the absence of MoO₄²⁻ anions within the material (Fig. S3†). This outcome authenticates that the presence of MoO₄²⁻ ions increases the structural order in bimetallic carbonate hydroxide materials. In addition, the (002) plane for Ni(CO₃)(OH)₂ is observed at a lower 2θ value in C₇₅N₂₅MO compared to C₇₅N₂₅O, indicating that the probable intercalation of the MoO₄²⁻ anion into the lattice of C₇₅N₂₅MO occurred presumably during the synthesis.

Functional group analysis

The presence of the MoO₄²⁻ ions in the as-prepared carbonate hydroxide materials was further confirmed by functional group analysis. The FTIR spectra of all MoO₄²⁻ intercalated hydroxide materials show a broad absorption band between 3500–3590 cm⁻¹ region, corresponding to the O–H bond stretching.^39,40 In addition, a wide range of bands are also observed in the 600–1000 cm⁻¹ region, indicating the presence of MoO₄²⁻ ions (Fig. S4†).⁴¹ These results confirm the intercalation of MoO₄²⁻ units in the as-prepared Ni, Co-carbonate hydroxide materials.

Morphological and elemental analyses

Field Emission Scanning Electron Microscopy (FESEM) and High-Resolution Transmission Electron Microscopy (HR-TEM) studies were performed to examine the microstructural characteristics of the as-prepared carbonate hydroxide materials. The FESEM images of C₇₅N₂₅MO display a distinct nano-flake-like microstructure. The flakes are vertically aligned, grown uniformly throughout the one-dimensional CC, and composed of smooth edges (Fig. 1a–c). In addition, the TEM images support the formation of a distinct flake-like microstructure of C₇₅N₂₅MO (Fig. 1d). Interestingly, the sample without Mo, i.e., C₇₅N₂₅O, displays a vertically aligned nanowire-like microstructure (Fig. S5†). This observation reveals that the intercalated MoO₄²⁻ ion significantly controls the morphology, which could modulate the charge storage ability of the as-prepared carbonate hydroxide materials. In particular, the flake-like architecture is beneficial for the effective diffusion of electrolyte ions, leading to higher specific capacitance. Furthermore, the close examination of the different selected regions of the HRTEM images of C₇₅N₂₅MO reveals the presence of (221) and (340) planes of Co(CO₃)_0.5(OH)₂·0.11H₂O (JCPDS: 048-0083) and (520) planes of Ni(CO₃)(OH)₂ (JCPDS: 035-0501), confirming the formation of a bimetallic Ni, Co carbonate hydroxide (Fig. 1e–p).

Fig. 1 Morphological studies of the as-prepared C₇₅N₂₅MO material. (a–c) FESEM, (d) TEM, and deconvolution of the HR-TEM images. (e) P1 and (i) P2 regions of the HR-TEM images that are examined, the corresponding Fast Fourier Transform (FFT) pattern denotes the presence of (f) (221) planes & (j) (340) planes of Co(CO₃)_0.5(OH)·0.11H₂O (JCPDS: 48-0083), and inverse Fast Fourier Transform (IFFT) patterns obtained via mask-filtered analysis, displaying the lattice fringes of (g) (221), & (k) (340) planes. (inset: the mask-filtered FFT pattern). (h and l) Live profile analyses indicate interplanar spacing, (m) P2 region of the HR-TEM image that is analyzed, (n) FFT patterns showcase the presence of (520) planes of Ni(CO₃)(OH)₂ (JCPDS: 35-0501), (o) IFFT patterns (inset: the mask filtered FFT pattern), and (p) the live profile survey.

The mask-filtered study, inverse fast Fourier transformation (IFFT), and corresponding live pattern analyses demonstrate that the interplanar spacings are 0.264, 0.188, and 0.177 nm for the 221, 340, and 520 planes, respectively. In addition, the energy-dispersive X-ray spectroscopy (EDS) and the elemental mapping analyses confirm the presence of Co, Ni, Mo, and O elements and their uniform distribution in bimetallic C₇₅N₂₅MO (Fig. S6†). Furthermore, to quantify the precise atomic ratio of the metals in C₇₅N₂₅MO, the inductively coupled plasma atomic emission spectroscopy (ICP-AES) study was conducted. The ICP-AES analysis reveals that the Co : Mo : Ni atomic ratio is 1 : 0.04 : 0.12 in the precatalytic framework (Table S1†).

Textural properties of the materials

The importance of MoO₄²⁻ ion intercalation in the carbonate hydroxides was further verified by porosity analysis through the nitrogen adsorption–desorption experiment. As shown in Fig. S7,† the appearance of a type-IV isotherm reveals a mesoporous framework according to the IUPAC classification.⁴² Furthermore, the presence of an H-3 hysteresis loop at relatively high pressure manifests the existence of slit-like pore configurations both in C₇₅N₂₅MO and C₇₅N₂₅O. Nonetheless, the surface area and pore volume of the C₇₅N₂₅MO material are higher than those of C₇₅N₂₅O (see the details in Table S2, Fig. S7a and b†).

This result manifests that MoO₄²⁻ significantly regulates the porosity of the C₇₅N₂₅MO material.⁴³ Despite the similar kind of pore diameter, a higher hysteresis loop, as well as pore volume with ascending and descending isotherms at the periphery, indicates a tensile strength effect involving the ion-templating role of the molybdate anion in the as-prepared C₇₅N₂₅MO compared to C₇₅N₂₅O (Table S2†).^44,45 Furthermore, C₇₅N₂₅MO shows a narrower pore-size distribution plot obtained from the BJH pore distribution, suggesting a more uniform porosity distribution presumably due to molybdate incorporation (Fig. S7c and d†). We believe that the higher porosity, pore volume, and narrower pore-size distribution will facilitate the easier penetration of electrolytes into the mesoporous framework during the electrochemical activation of C₇₅N₂₅MO. This leads to the creation of abundant surface area and porosity via the etching of MoO₄²⁻ ions, thereby enhancing the capacitive performance of the molybdate-incorporated bimetallic C₇₅N₂₅MO carbonate hydroxide materials (vide infra).

Chemical state analysis

To probe the chemical state information of the elements present in the bimetallic C₇₅N₂₅MO carbonate hydroxide material, X-ray photoelectron spectroscopy (XPS) was performed. The XPS survey scan of the as-prepared C₇₅N₂₅MO materials clearly shows the presence of Co, Ni, Mo, C, and O elements (Fig. S8†). Furthermore, the deconvoluted high-resolution XPS spectrum of Co 2p shows two prominent peaks observed at 781.6 eV (2p_3/2) and 797.6 eV (2p_1/2) (Fig. 2a).⁴⁶ The noted binding energy (BE) difference between the two prominent peaks is ∼16 eV, confirming the existence of Co⁺².⁴⁷ Two shake-up satellite signals are also noted at 785.9 eV and 803.1 eV, supporting Co⁺² formation.⁴⁸ The deconvoluted Ni 2p spectrum shows two peaks at 856.5 (Ni 2p_3/2) and 874.1 eV (2p_1/2).^49,50 The noted BE difference is 17.6 eV, indicating the presence of Ni⁺² (Fig. 2b).^51,52 Furthermore, the characteristic satellite peaks are observed at 862 and 880.2 eV, confirming that nickel is present as Ni⁺².⁴⁹ The peak-fitted Mo 3d XPS scan displays two peaks at 232.5 eV and 235.5 eV, corresponding to Mo 3d_5/2 and 3d_3/2, respectively (Fig. 2c). The observed BE values validate the presence of Mo (+6) (or molybdate) in the as-prepared bimetallic C₇₅N₂₅MO carbonate hydroxide material.^53,54 Furthermore, the deconvoluted O 1s scan gives two peaks at 531 and 532.5 eV ascribed to the lattice oxygen bond and adsorbed water molecules, respectively (Fig. 2d).^55,56 These outcomes collectively prove the formation of a bimetallic MoO₄²⁻ intercalated Ni, Co carbonate hydroxide (C₇₅N₂₅MO) material.

Fig. 2 High-resolution XPS spectra of the as-prepared C₇₅N₂₅MO material. The narrow scan displays (a) Co 2p, (b) Ni 2p, (c) Mo 3d, and (d) O 1s regions.

Electrochemical activation

At first, the CC-supported directly grown electrode materials were subjected to the electrochemical activation process through the cyclic voltammetry (CV) test in a 2 M KOH electrolyte. CV was performed within the operational voltage window of 0–0.5 V (Vs. Ag/AgCl) with a 50 mV s⁻¹ scan rate (see the details in the ESI†). The CV study showcased a steady improvement in current density for the C₇₅N₂₅MO material up to 200 CV cycles. Specifically, the 200th cycle shows a significant enhancement in the current density compared to the 1st cycle (Fig. S9†). Such an increment in current density is likely associated with substantial structural changes (reconstruction) during the electrochemical activation. This activated electrode material is denoted as C₇₅N₂₅MO-A, and extensive physical characterization has been carried out to probe such changes during the CV cycles.

Characterization of the material obtained after electrochemical activation

The possible structural alteration of the activated C₇₅N₂₅MO-A electrode material was investigated on a carbon paper (CP) substrate. The PXRD analysis reveals that the initial metal carbonate hydroxide transformed into CoOOH/NiOOH (Fig. S10†). In addition, the FTIR spectrum of C₇₅N₂₅MO-A is devoid of MoO₄²⁻ vibrational modes (Fig. S11†). This observation suggests the removal of MoO₄²⁻ from the lattice of bimetallic Ni, Co carbonate hydroxide materials during the activation process. The etching process is expected to generate a porous microstructure, which may help improve the charge storage ability of the material. Moreover, the FESEM images confirm the anticipated formation of flake-like morphology with distinct porous networks. However, the surface of nanoflakes became rough and interconnected, which could be beneficial for better diffusion of electrolyte ions (Fig. 3a and b). In addition, the TEM images after the activation revealed a porous flake-like morphology with significant roughness on the surface (Fig. 3c and d). Furthermore, the deconvolution of the different selected regions of the HRTEM images of C₇₅N₂₅MO-A reveals the presence of (015), (110), and (100) planes of CoOOH (JCPDS: 07-0169), NiOOH (JCPDS: 27-0956), and Co(OH)₂ (JCPDS: 45-0031), respectively. These results collectively confirm the formation of bimetallic Ni, Co hydroxide(oxy) hydroxide species after preconditioning. The mask-filtered IFFTs and corresponding live pattern analyses demonstrate that the interplanar spacing is 0.184, 0.313, and 0.270 nm for the (015), (110), and (100) planes, respectively (Fig. 3e–p). Furthermore, the EDS and elemental mapping investigations of activated C₇₅N₂₅MO-A confirm the presence of Ni, Co, O, and K (due to KOH medium) elements, along with a minimal quantity of Mo, uniformly distributed throughout the structure (Fig. S12†). The notable decrease in Mo counts (measured in atomic percentage) in the EDS study further supports the etching of MoO₄²⁻ ions during the electrochemical activation process. In addition, the ICP-AES analysis was conducted to unveil the exact metal composition of C₇₅N₂₅MO after the electrochemical activation. The activated C₇₅N₂₅MO-A electrode material reveals that the Co : Mo : Ni ratio is 1 : 0.004 : 0.06 (Table S1†). This result confirms the leaching of Mo, likely as MoO₄²⁻ ions, from the bimetallic C₇₅N₂₅MO carbonate hydroxide lattice during the activation process. Furthermore, the XPS study was carried out to investigate the possible changes in the chemical state of the active material. The deconvoluted Co 2p spectrum shows two peaks at 781.3 eV (2p_3/2) and 796.9 eV (2p_1/2) (the BE separation ∼15.6 eV), indicating the presence of Co⁺².^57–59 Furthermore, two additional peaks are observed at 779.7 (2p_3/2) and 794.8 eV (2p_1/2) (BE separation ∼15.1 eV), corroborating the formation of Co⁺³ during activation (Fig. 4a).^54,55 The characteristics of shake-up satellite signals located at 785.8 eV and 803.7 eV also support the existence of Co⁺².⁶⁰ On the other hand, the Ni 2p spectrum showcased two peaks at 855.3 eV (2p_3/2) and 872.8 eV (2p_1/2) (BE separation 17.6 eV), suggesting Ni⁺² formation. Furthermore, two additional peaks are also observed at 856.4 and 874.5 eV (BE separation ∼18.1 eV), confirming the presence of Ni⁺³ (Fig. 4b).^61,62 In addition, two satellite signals are also noted at 862.7 (2p_3/2) and 881 eV (2p_1/2).⁶³ Hence, the presence of Co⁺²/Co⁺³ in Co 2p and Ni⁺²/Ni⁺³ in the Ni 2p spectra strongly manifests the formation of metal hydroxide (M(OH)₂)/(oxy)hydroxide (MOOH) species (M = Ni, Co). Interestingly, the narrow scan of the Mo 3d spectrum shows two peaks at 235.6 eV (3d_5/2) and 232.5 eV (3d_3/2), corroborating Mo⁶⁺ with reduced intensity (Fig. 4c).^53,54 These results further proved that Mo etching from the as-prepared material occurred during electrochemical activation. Furthermore, the peak-fitted O 1s spectrum displays three peaks, noted at 529.5, 531.1, and 532.3 eV, manifesting lattice oxygen (O), O from the hydroxyl group in an alkaline environment, and O from adsorbed water molecules, respectively (Fig. 4d).⁶⁴ The results above collectively prove the formation of the bimetallic hydroxide/(oxy)hydroxide material and the etching of Mo, likely as MoO₄²⁻, from the precursor during the electrochemical activation.

Fig. 3 Morphological studies after electrochemical activation (C₇₅N₂₅MO-A). (a and b) FESEM, (c and d) TEM, (e) P1 regions of the HR-TEM images that are examined, (f) the corresponding (FFT) pattern denotes the presence of (015) planes of CoOOH, (g) IFFT patterns (inset: mask-filtered analysis), (h) live profile analysis displaying the interplaner distance, (i) P2 regions of the HR-TEM images that are examined, (j) the corresponding (FFT) pattern denotes the presence of (110) planes of NiOOH, (k) IFFT patterns (inset: mask-filtered analysis), (l) live profile analysis, (m) P3 regions of the HR-TEM images that are examined, (n) the corresponding (FFT) pattern denotes the presence of (100) planes of Co(OH)₂, (o) IFFT patterns (inset: mask-filtered analysis), and (p) live profile analysis.

Fig. 4 High-resolution XPS study of electrochemically activated C₇₅N₂₅MO-A. The narrow scan XPS spectra of (a) Co 2p, (b) Ni 2p, (c) Mo 3d, and (d) O 1s regions.

Evaluation of supercapacitor performance

After the activation, the electrochemically reconstructed electrode materials were subjected to supercapacitor application. At first, the CV curves were recorded for all activated materials at a low scan rate of 2 mV s⁻¹ within the potential range of 0–0.5 V. The materials displayed distinct redox peaks and battery-like storage behavior (Fig. 5a). The CV curve of CMO-A exhibits a relatively larger area than that of NMO-A, indicating that the Co system has better charge storage (specific capacitance, C_sp) ability than the Ni system. Interestingly, mixing a requisite amount of Co and Ni significantly increases the area under the CV curve (Fig. 5a). Specifically, activated C₇₅N₂₅MO-A exhibits a higher area under the CV than individual CMO-A, NMO-A, C₇₅N₂₅O-A, and all other activated different bimetallic carbonate hydroxide materials (Fig. 5a and b). These results indicate that the efficient synergistic interaction between Ni and Co centers, along with the etching of intercalated MoO₄²⁻ anions, plays a vital role in modulating the charge storage ability of carbonate hydroxide materials.

Fig. 5 (a) CV curves of all activated as-prepared carbonate hydroxide materials, (b) comparison of the CV curves between C₇₅N₂₅MO-A and C₇₅N₂₅O-A, (c) CV curves of activated C₇₅N₂₅MO-A recorded at varying scan rates, (d) estimation of the b value of activated C₇₅N₂₅MO-A, (e) C_s (specific capacity) vs. ν^−1/2 plot, and (f) the % of capacitive and diffusion contribution (at varying scan rates) is represented in a bar diagram of activated C₇₅N₂₅MO-A.

Investigation of the charge storage mechanism

To understand the mechanism of charge storage, CVs of C₇₅N₂₅MO-A (Fig. 5c) and C₇₅N₂₅O-A (Fig. S13†) were recorded at different scan rates (2 to 100 mV s⁻¹). The increasing scan rate shifts cathodic and anodic peaks toward more negative and positive potential regions, respectively. These peak-to-peak separations at high scan rates delineate the charge storage, which primarily occurs through the diffusion phenomenon.³⁵ This encouraged us to investigate the nature of charge storage in C₇₅N₂₅MO-A and C₇₅N₂₅O-A materials. To probe the mechanism, the b value was calculated by analyzing the relationship between the log(peak current) vs. log(scan rate) plot using the power law (eqn (1) and (2)).⁶⁵

i = aν^b (1)

log(i) = log(a) + b log(ν) (2)

where ν represents the scan rate (mV s⁻¹), i refers to the peak currents at the cathode (i_cathode) and anode (i_anode). The b value determines the behavior of the charge storage. A b value of ∼1 signifies that the material follows the surface redox charge storage process (pseudo-capacitor behavior) whereas, a b value of ∼0.5 indicates diffusion-controlled behavior (intercalation or a battery-type material). The electrochemically activated C₇₅N₂₅MO-A electrode displays a b value of ∼0.72 (Fig. 5d), suggesting that the material stores the charge by utilizing both the surface and intercalation redox process (diffusion phenomenon).⁶⁶ On the other hand, activated C₇₅N₂₅O-A displays a b value of ∼1, indicating the occurrence of predominantly surface-redox behavior in the charge storage (Fig. S14†). This finding highlights the impact of MoO₄²⁻ ion etching in enhancing the diffusion phenomenon for C₇₅N₂₅MO-A compared to C₇₅N₂₅O-A, thus significantly modulating the charge storage ability (specific capacitance) of bimetallic C₇₅N₂₅MO carbonate hydroxides. Moreover, Trasatti's method was utilized to quantify the diffusion and capacitive contributions to the charge storage. Principally, the Trassati plot correlates with the diffusion (q_diff) and surface (q_sur) charges, which vary with scan rates. The total charge (q_total) restored under the CV curve is the sum of q_sur and q_diff (eqn (3) and (4)).^8,67

q_t = q_sur + q_diff (3)

q_total = q_sur + const ν^−1/2 (4)

Generally, the electrode kinetics occurs faster at the surface compared to the bulk. Hence, the q_sur hardly varies with the scan rate (ν). In contrast, the q_diff is inverse to the square root of the scan rate (ν^−1/2). Therefore, the q_sur is estimated from the y-axis intercept of q_totalvs. ν^−1/2 plot (Fig. 5e). Finally, the diffusion component (q_diff) was calculated by subtracting q_sur from q_total. Activated C₇₅N₂₅MO-A exhibits 67.8 (∼68) % diffusion contribution at a 2 mV s⁻¹ scan rate, which drops to 51.1% at a 25 mV s⁻¹ scan rate. These results highlight the dominant impact of the surface contribution at high scan rates (Fig. 5f).

GCD analysis of the as-prepared carbonate hydroxide materials

Galvanostatic charge–discharge (GCD) studies were conducted with all the reconstructed materials within the operational voltage window of 0–0.5 V at 1 A g⁻¹ current density (Fig. 6a). Subsequently, the specific capacitance (C_sp) values of these activated materials were determined by recording the discharge times from the GCD curves. The activated C₇₅N₂₅MO-A carbonate hydroxide displays significantly higher discharge time than individual CMO-A and NMO-A (Fig. 6b). This result further manifests better charge storage ability of bimetallic compounds over the monometallic systems. The calculated C_sp values of the materials are 275 F g⁻¹ (NMO-A), 428 F g⁻¹ (CMO-A), and 2039 F g⁻¹ (C₇₅N₂₅MO-A) at 1 A g⁻¹ current density. Furthermore, CMO-A and NMO-A show distinct quasi-triangular and plateau-like GCD profiles, authenticating the pseudo-capacitor and battery-like charge storage nature, respectively.⁵ Interestingly, the bimetallic system consists of both plateau and quasi-triangular characteristics in the charge–discharge curves, suggesting a battery-type pseudo-capacitance nature. In addition, among all the bimetallic carbonate hydroxides, activated C₇₅N₂₅MO-A displays the highest C_sp value (Fig. 6a). The estimated C_sp values of all the activated materials are shown in the bar diagram (Fig. 6b). Furthermore, the capacitive performance of activated C₇₅N₂₅MO-A is comparable with that of other LDH-based materials that have been reported recently (Table 1). Subsequently, the GCD graphs of C₇₅N₂₅MO-A were recorded at different current densities (Fig. 6c). The materials display 76.7% C_sp retention at 20 A g⁻¹ current density, revealing good rate capability at high current density (Fig. 6d). In addition, activated C₇₅N₂₅MO-A provides a higher discharge time in the GCD curve than C₇₅N₂₅O-A (C_sp = 121 F g⁻¹), indicating a higher C_sp value of the former (Fig. 6e).

Fig. 6 Electrochemical performance of the as-prepared materials. (a) GCD curves (recorded at 1 A g⁻¹⁻¹), (b) bar diagram representing the C_sp of all electrochemically activated materials. (c) GCD curves of activated C₇₅N₂₅MO-A measured at different current densities 1–20 A g⁻¹, (d) specific capacitance (F g⁻¹) vs. current density (A g⁻¹) plot indicating the retention rate at high current density, (e) comparison of GCD curves of C₇₅N₂₅MO-A and C₇₅N₂₅O-A recorded at 1 A g⁻¹, and (f) cycling stability test of activated C₇₅N₂₅MO-A performed up to 5000 cycles.

Table 1 Comparison table for the specific capacitance value of activated C₇₅N₂₅MO and the literature reports (three electrode system) (*NF = nickel foam substrate; CC = carbon cloth substrate)

Sl. No	Electrode material (electrode substrate)	Specific capacitance (F g^−1)/specific capacity (C g^−1)	Electrolyte	Voltage window	Stability	References
1	Ni Co–Mo oxide (NF*)	1837.7 F g^−1 (at 1 A g^−1)	6 M KOH	0–0.5 V	81.2% (6000 cycles)	68
2	NiCo LDH nanocage (NF)	1671 F g^−1 (at 1 A g^−1)	1 M KOH	0–0.45 V	67.9% (10 000 cycles)	69
3	Ni-MOF-derived NiCo LDH (NF)	1272C g^−1 (at 2 A g^−1)	3 M KOH	0–0.38 V	—	70
4	ZIF 67/NiCo-LDH composite (CC*)	2210.6 F g^−1 (at 1 A g^−1)	2 M KOH	0–0.45 V	86.3% (10 000 cycles)	71
5	NiMoO4/CoMoO4 nanorods (NF)	1445 F g^−1 (at 1 A g^−1)	1 M KOH	0–0.5 V	79% (3000 cycles)	72
6	NiCo(CO3)(OH)2 (NF)	2576.4 F g^−1 (3 A g^−1)	1 M KOH	0–0.5 V	—	73
	NiCo(CO3)(OH)2 (without NF)	1460.2 (3 A g^−1)
7	MoO4^2− intercalated activated C75N25MO-A (CC)	2039 F g^−1 (at 1 A g^−1)	2 M KOH	0–0.5 V	71% (5000 cycles)	This work

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This observation unlocks the impact of MoO₄²⁻ etching on modulating the redox phenomenon and the charge storage ability of C₇₅N₂₅MO-A due to facile accessibility to the electrolytes towards several Ni and Co centers present in the material.

EIS and cycling stability studies

Electrochemical impedance spectroscopy was carried out at open-circuit potential (OCP) to probe the charge-transfer resistance (R_ct). The Nyquist plot displays a semicircle in the high-frequency region and delineates the required R_ct value. In addition, the angle of the vertical line observed in the low-frequency zone implies the inherent diffusion phenomenon of the electrode materials. All the impedance graphs were fitted with the corresponding equivalent circuits, demonstrating that activated NMO-A exhibits inferior diffusion with a higher R_ct (∼32.6 ohms) than CMO-A (∼10.4 ohms). This result manifested better charge storage ability of the Co centers compared to Ni centers (Fig. S15†).

Interestingly, all the activated bimetallic Ni, Co carbonate hydroxide materials display comparatively lower R_ct than individual CMO-A and NMO-A, indicating better charge storage ability. Specifically, activated C₇₅N₂₅MO-A exhibits a notably low R_ct (∼3.6 ohm) value, suggesting facile electron migration across the electrode–electrolyte interface. The details of all circuit fitting parameters of all the materials are presented in Table S3.† Moreover, a steeper vertical line in the low-frequency region shown by activated C₇₅N₂₅MO-A as compared to C₇₅N₂₅O-A validates the occurrence of an improved diffusion phenomenon in the former, which is enabled by the etching of intercalated MoO₄²⁻ ions (Fig. S15b†). The etching process develops porous networks within C₇₅N₂₅MO-A, forming new channels for the electrolyte ions to diffuse throughout the material, thus enhancing charge storage ability. Finally, the durability of the activated C₇₅N₂₅MO-A material was tested to determine its viability for practical use. The material exhibits good stability, displaying 71% capacitive retention and excellent coulombic efficiency ∼99%, up to 5000 GCD cycles (recorded at 5 A g⁻¹ current density) (Fig. 6f). These findings suggest that the material displays superior electrochemical robustness during numerous charge–discharge cycles under harsh alkaline conditions. However, the observed mild decline in the retention rate could be due to the inherent battery-like (intercalation redox behavior) charge storage nature of Ni, Co-based materials. During the long charge–discharge process, the electrolyte ions intercalate-deintercalate into the battery-type electrode materials, resulting in a continuous volume expansion and contraction of the material. This caused significant mechanical stress in the material, leading to the collapse of a few ion transport channels, which resulted in capacity decay during the cycling stability studies.

Post-stability physical characterization

To examine the possible changes in the electrode material after the electrochemical stability test (C₇₅N₂₅MO-S), comprehensive physical characterization was performed. The XPS survey scan of the post-cycling stability electrode material displays the presence of Ni, Co, O, and C (Fig. S16†). However, the survey scan lacks any Mo signals, indicating the complete removal of Mo after the cycling stability test. Furthermore, the high-resolution deconvoluted XPS scan of the Co and Ni 2p spectra shows the presence of Co⁺²/Co⁺³ and Ni⁺²/Ni⁺³, respectively, even after a prolonged cycling stability test (Fig. S17a–c†).

The exact peak positions are listed in Table S4.† Furthermore, fitting of Ni, Co 2p, and O 1s XPS spectra collectively proves the formation of the M(OH)₂/MOOH species (M = Ni, Co) similar to that noted after the activation process (Fig. S17†). These results indicate that the electrochemically active material possesses excellent structural stability. Interestingly, the high-resolution Mo 3d scan displays no distinct signals, further confirming the complete etching of Mo, likely MoO₄²⁻, after the extended stability test (Fig. S17d†). The post-stability FESEM and TEM images show retention of the porous flake-like microstructure as acquired after the activation process (Fig. 7a–d). However, the surface of the flakes became much rougher, presumably caused by the effective etching of Mo, likely as MoO₄²⁻, which is beneficial for better diffusion of electrolytes. Moreover, the deconvolution of the various selected regions of the HRTEM images of C₇₅N₂₅MO-S reveals the presence of (101), (110), and (100) planes of CoOOH (JCPDS: 007-0169), NiOOH (JCPDS: 027-0956), and Co(OH)₂ (JCPDS: 045-0031), respectively. These results collectively confirm the formation of bimetallic Ni, Co hydroxide(oxy) hydroxide species after preconditioning (Fig. 7e–p). Furthermore, the EDS study and elemental mapping analysis confirmed the presence of Co, Ni, O, and C (due to the carbon cloth substrate) and significant loss of Mo in the material after the extended durability test (Fig. S18†). The etching mechanism of MoO₄²⁻ is put in a nutshell and described in the ESI (Note S2†). The aforementioned evidence collectively manifests the good electrochemical stability of active M(OH)₂/MOOH (M = Ni, Co) species even after a prolonged electrochemical cycling stability study. Furthermore, to comprehend the reaction mechanism involved in charge storage, we have conducted the PXRD analysis of the electrodes at both charged and discharged states (charging voltage: 0.5 V vs. Ag/AgCl and discharged voltage: 0 V vs. Ag/AgCl) on the CP substrate. As shown in Fig. S19,† during the charging, Ni/CoOOH species are formed through the oxidation of Co^2+/Ni²⁺ ions into Co^3+/Ni³⁺. During the discharge, the same Ni/CoOOH is converted into Ni/Co(OH)₂via the reduction of Co^3+/Ni^3+. Thus, the probable charge–discharge reaction of C₇₅N₂₅MO-A could be demonstrated as follows (eqn (5) and (6)):

Ni, Co (OOH) + H₂O + e⁻ → Ni, Co (OH)₂ + OH⁻ (discharging) (5)

Ni, Co (OH)₂ + OH⁻ → Ni, Co (OOH) + H₂O + e⁻ (charging) (6)

Fig. 7 Morphological studies after the electrochemical stability test (C₇₅N₂₅MO-S). (a and b) FESEM, (c and d) TEM, (e) P1 regions of the HR-TEM images that are examined, (f) the corresponding (FFT) pattern denotes the presence of (101) planes of CoOOH, (g) IFFT patterns (inset: mask-filtered analysis), (h) live profile analysis displaying the interplaner spacing, (i) P2 regions of the HR-TEM images that are examined, (j) the corresponding (FFT) pattern denotes the presence of (110) planes of NiOOH, (k) IFFT patterns (inset: mask-filtered analysis), (l) live profile analysis, (m) P3 regions of the HR-TEM images that are examined, (n) the corresponding (FFT) pattern denotes the presence of (100) planes of Co(OH)₂, (o) IFFT patterns (inset: mask-filtered analysis), and (p) live profile analysis.

Fabrication of a hybrid device (two electrode configuration)

The observed high charge storage ability of C₇₅N₂₅MO-A encouraged us to develop a hybrid device to further enhance the specific energy. The hybrid device was fabricated using electrochemically active C₇₅N₂₅MO-A and activated charcoal (AC) as the positive (+ve) and negative (−ve) electrodes, respectively. At first, the CVs of individual C₇₅N₂₅MO and AC were recorded in three-electrode configurations (Fig. 8a). The AC and electrochemically activated C₇₅N₂₅MO-A electrodes displayed distinct electrochemical double layer (EDLC) and battery-like charge storage characteristics within their operational voltage windows of −1 to 0 V and 0–0.5 V, respectively. Subsequently, the mass balance was compiled, and the above two electrodes were connected in series. The operational voltage window of the C₇₅N₂₅MO-A//AC hybrid device was optimized by recording the CVs within the 1.2 to 1.8 V range with a fixed scan rate of 10 mV s⁻¹ (Fig. S20a†). The CV data manifest that the highest operational voltage window achieved using the C₇₅N₂₅MO-A//AC hybrid device is 0–1.5 V (Fig. S20a†). After that, the CVs of the hybrid device were recorded at different scan rates, revealing both the faradaic and EDLC behavior (Fig. 8b). Furthermore, the GCD study was employed to verify the operational voltage of the hybrid device. The GCD curves recorded at different voltages (1.2 to 1.8 V) at a fixed current density of 2 A g⁻¹ displayed a maximum operational window of 1.5 V (Fig. S20b†). Subsequently, the GCD curves were measured at various current densities ranging from 2 to 20 A g⁻¹ (Fig. 8c). The C₇₅N₂₅MO-A//AC hybrid device delivered a high specific capacitance (C_sp) of 55.37 F g⁻¹ (23.07 mA h g⁻¹) at 2 A g⁻¹, and retained 13.2 F g⁻¹ (5.5 mA h g⁻¹) at a high current density of 20 A g⁻¹, demonstrating a decent rate capability (Fig. 8d). Nevertheless, when current densities are elevated, both the specific capacity and specific current density decrease. This is associated with reduced diffusion time when undergoing rapid charge–discharge.⁷⁴ Furthermore, the Ragone plot estimates the power and energy density of C₇₅N₂₅MO-A//AC hybrid devices. The C₇₅N₂₅MO-A//AC hybrid device showcases a maximum energy density of 17.30 W h kg⁻¹ at a power density of 1510.55 W kg⁻¹, which could be retained at 4.12 W h kg⁻¹ at a very high-power density of 14 981.81 W kg⁻¹ (Fig. 8e). Finally, the electrochemical robustness of the hybrid device was investigated by the cycling stability test. The device shows 88% capacitive retention up to 10 000 GCD cycles (recorded at a fixed current density of 10 A g⁻¹) with ∼99% coulombic efficiency (Fig. 8f). In addition, the CV and GCD curves before and after the stability test demonstrated a minimal drop in capacitive performance even after 10 000 cycles (Fig. S21a and b†). The impedance study of the hybrid device showcases good capacitive behavior (Fig. S21c†), and the corresponding circuit fitting parameters are listed in Table S5.† These outcomes collectively imply excellent electrochemical stability and good chemical reversibility of the hybrid device. We believe that the formation of such a porous Ni/Co hydroxide-(oxy) hydroxide material and suitable specific capacitance in a stable 1.5 V operational range is crucial for achieving a satisfactory energy density and enduring stability. A comparison of the electrochemical performance of the asymmetric hybrid device with that of recently reported literature is listed in Table S6.†

Fig. 8 (a) CVs of C₇₅N₂₅MO-A and AC recorded in different voltage regions at a fixed scan rate of 5 mV s⁻¹ (using a three electrode system), (b) CVs of the C₇₅N₂₅MO-A//AC hybrid device obtained by varying the scan rates (10 to 100 mV s⁻¹), (c) GCD curves of the C₇₅N₂₅MO-A//AC hybrid device measured at different current densities (2 to 20 A g⁻¹), (d) variation of C_s (mA h g⁻¹) (C_sp) (F g⁻¹) vs. current density(A g⁻¹), (e) Ragone plot to establish the energy density and power density relationship, and (f) cycling stability test of the C₇₅N₂₅MO-A//AC hybrid device recorded at a fixed current density of 10 A g⁻¹.

Conclusions

This work highlights an in situ electrochemical reconstruction strategy to boost the specific capacitance of layered TMCH materials. A binder-free direct growth process is used to prepare a molybdate (MoO₄²⁻) ion intercalated bimetallic TMCH on a redox inactive carbon cloth (CC) substrate employing nickel, cobalt, and molybdenum salts via a hydrothermal synthesis approach. The electrochemical activation of these metal carbonate hydroxides results in the conversion of active metal hydroxide/(oxy)hydroxide materials through the reconstruction process. Meanwhile, the intercalated MoO₄²⁻ ions have etched away, creating pores within the material. Such a porous network is expected to have enhanced charge storage ability due to facile electrolyte migration. Specifically, the material originated from C₇₅N₂₅MO (named C₇₅N₂₅MO-A) exhibits a promising specific capacitance of 2039 F g⁻¹ at 1 A g⁻¹ current density, along with 71% capacitive retention up to 5000 GCD cycles. Furthermore, to unveil the role of MoO₄²⁻ etching towards charge storage, C₇₅N₂₅O was separately prepared without any Mo source. The in-depth mechanistic investigation reveals that the etching of intercalated MoO₄²⁻ improves the diffusion contribution in activated C₇₅N₂₅MO-A compared to C₇₅N₂₅O-A, leading to a higher specific capacitance. Furthermore, a hybrid device (C₇₅N₂₅MO-A//AC) was demonstrated using C₇₅N₂₅MO and AC, achieving a 1.5 V operating voltage window with an energy density of 17.30 W h kg⁻¹ at 1510.55 W kg⁻¹ power density. Moreover, C₇₅N₂₅MO-A//AC exhibits an 88% capacitive retention rate after 10 000 GCD cycles, manifesting that the materials are valuable for practical use. To our knowledge, this is the first report on electrochemical etching of intercalated molybdate (MoO₄²⁻) anions from TMCH materials in supercapacitor applications. We believe that this work will unleash the potential of molybdate etching in TMCH materials, which could be further extended with other TMCHs for energy storage purposes.

Data availability

The data that support the findings of this study are available from the corresponding author upon reasonable request.

Author contributions

Mohua, Harish, and V. M. designed and conceived the project. Mohua and Harish conducted material synthesis, physical characterization, and all the electrochemical studies. Mohua, Harish, and Avishek prepared the manuscript. Avishek and Heramba contributed to conducting two-electrode device fabrication, electrochemical studies, and manuscript editing. Sourav contributed to various characterization studies and BET analyses. Dhrubojyoti and Nitik contributed to XRD, FTIR analyses, and figure editing. Soumalya upgraded the schematic. V. M. supervised the entire work with valuable input to improve the quality of the work. Prior to the submission, all authors approved the ultimate version of the manuscript.

Conflicts of interest

The authors declare no financial interest.

Acknowledgements

The author, Mohua, would like to acknowledge the Department of Science and Technology (DST), India, for providing a National Postdoctoral Fellowship (NPDF) (project no. PDF/2017/001629/PMS) and CSIR-EMRII-ASPIRE for the project (03WS (001)/2023-24/EMRII/ASPIRE). For fellowships, Harish, Soumalya, and Sourav thank UGC and NPDF (PDF/2017/001728/ES). Avishek and K. H. V. S. R. sincerely acknowledge the DST-Inspire Fellowship. Mohua also thanks Dr Sagar Ganguli for helpful suggestions. Avishek thanks Dr Khushboo Paliwal for continuous encouragement during the revision work and manuscript writing. The author, V. M., thanks DST, India (DST/TMD/MES/2K17/70), for providing project funding. V. M. acknowledges IISER Kolkata for providing the DST-FIST facility (TEM), IACS, Kolkata, for the XPS, and IIT Bombay for the ICP-AES facility. Nitik acknowledges Pratyay and Snigdha for their suggestions. The authors thank Viplove Mishra, Diya Raveendran, and Biplob Jyoti Hazarika for their valuable inputs during revision work.

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5se00627a

‡ These authors equally contributed to this work.

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