Elucidating the role of cobalt nanoparticles and Mn-phosphate in etched ZIF-67/phthalimide-NC and phthalimene oxide for supercapacitor and electrochemical oxygen evolution reaction applications

Abstract

Electrochemical supercapacitors and the electrochemical oxidation of biomass-derived oxygenates have great significance for long-term high-performance devices. However, appropriate sites with redox features remain a bottleneck for electrochemical oxidation and capacitance retention. Herein, N-doped carbon sheets with Mn-phosphate-doping and Co-metal nanoparticles were synthesized via a facile one-pot activation and calcination of the layered potassium phthalimide salt without inclusion of any additional activators or template. The unique 2D-structure of the obtained microporous carbon flakes with a layered structure provides a sturdy N-C matrix for prolonged charging/discharging with abundant active adsorption sites and an effective route for rapid electrolyte ion transport with a shorter diffusion distance for the adsorption/desorption of ions. Through these merits, K-Ph-NC offers high capacitance and outstanding rate performance with an incredible energy density in capacitor devices, and the specific capacitance of the as-prepared K-Ph-NC is proportional to the number of micropores. K-Ph-NC was further transformed to a K-Ph-Oxide, a graphene oxide version of K-phthalimide, by using an improved Hummer's method by using Mn-salt and phosphoric acid, which resulted in a phthalimene oxide doped with Mn-phosphate. In addition, a composite of K-Ph-NC with ZIF-67 was thermally calcined at 700 °C under an Ar atmosphere, which resulted in e-ZIF-67/K-Ph-NC with an etched surface. A comparative electronic and structural analysis followed by a capacitance retention and electrochemical oxygen evolution reaction study revealed the role of Co-nanoparticles as compared to the Mn-phosphate doping in the resulting materials. A symmetric supercapacitor device exhibited a maximum SE value of 22.7 W h kg⁻¹ with a maximum SP of 10 416.7 W kg⁻¹, which is mainly due to the favorable microporous pore architecture in e-ZIF-67/K-Ph-NC as compared to K-Ph-NC and K-Ph-Oxide. This highlights the role of cobalt nanoparticles in e-ZIF-67/K-Ph-NC with an etched outer surface. A promising overpotential of 450 mV at 10 mA cm⁻² in the OER by e-ZIF-67/K-Ph-NC can be correlated to the charge transfer resistance across the electrode–electrolyte interface.

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Introduction

Heterogeneous nucleation and diffusion-controlled growth processes are major techniques for utilizing the oxygen functional groups on the graphene oxide (GO) surface as preferential active sites, which leads to high-density loading of heteroatomic nanoparticles.¹ A pyrolysis-leaching treatment can transform a suitable molecule into holey activated carbon sheets (HACS) with large surface areas, hierarchical pores with electroactive nitrogen and oxygen dopants with abundant surface layers and redox-active surfaces from hetero-atom doped porous nanosheets.² HACs have the potential to achieve unlimited lifespan and ultrafast charging capability via the surface-adsorption mechanism, along with a faradaic surface reaction at the hetero-atom dopants (O, N).^3,4 Emerging 2D materials can be nanomodified via heteroatom doping, defect engineering, template-etching, interlayer engineering, and 2D-heterostructure engineering, which allows for the customization of the inherent structure of 2D materials to significantly enhance performance.^5,6 The high packing density of potassium phthalimide-derived carbon is responsible for its tailored porosity and functionality with retention of the textural properties.⁷ Holey graphenic materials with abundant micro-/mesopores provide dense, porous, and high-loading functional metal particles and metal active sites in electrodes with enhanced surface area and capacitive performance due to the high packing density and efficient ion transport pathway.⁸ Thermal and chemical-oxidative etching can cause in-plane nanoholes in the graphene sheets, wherein defect sites are converted to nanoholes across exfoliated GO nanosheets when using oxidation etchant KMnO₄.^9,10 A step beyond the investigation of 2D-holey graphenic materials involves composite materials of 2D-graphene and ZIF-67 using partially or fully hollow particle morphology control by etching for tailored properties.¹¹ Hollow structures with hierarchical pores provide exposure to the active sites, which offers great advantages in electrochemical performance by creating dual active sites most often.¹² Electrochemical water-oxidation with high-energy loss of the oxygen evolution reaction (OER) can be accelerated by increased adsorption of intermediates via d–p orbital hybridization within a high entropy system, wherein Co nanoparticles and atoms streamlined strategies by potentially integrating efficient functional materials.¹³ Cobalt nanoparticles exsolved from perovskites exhibit mass activity with low charge transfer resistance and high kinetic OER activity.¹⁴ A well-dispersed Co species (metallic and nanoparticles) in a carbon C-matrix offers very low overpotential toward a self-powered water-splitting system. High loading of metal particles and good dispersion in the C-matrix explained the high electrocatalytic performance.¹⁵ Zeolitic imidazole frameworks (ZIFs), a type of porous metal–organic framework (MOF), exhibit great potential towards charge storage and redox reactions due to several characteristics, such as a large pore volume and regulated pore size and structure.^16–22 Apart from their direct application, ZIFs have demonstrated exceptional capabilities as templates to generate distinctive structures due to their adaptable structures, particularly hollow architectures, i.e. etched surfaces. In addition to their large specific surface area, hollow structures can help alleviate volume expansion during electrochemical processes and allow fast faradaic redox reactions during charging/discharging.²³

Herein, we demonstrate a series of N-doped carbon sheets with Mn-phosphate and Co-metal nanoparticle (CoNP) doping via a facile one-pot activation and calcination of the layered potassium phthalimide salt without inclusion of any additional activators or templates (Fig. 1). The optimal K-Ph-NC sample produces a large 2D nanosheet with a uniform sub-nanometer microporous distribution of roughly 1.06 nm and an immense specific surface area of 1060 m² g⁻¹. A robust N-C matrix for extended charging/discharging with a large number of active adsorption sites and a successful pathway for quick electrolyte ion transport with a reduced diffusion distance for ion adsorption/desorption are provided by the special microporous carbon flakes with layered architectures. Through these merits, K-Ph-NC offers high capacitance and outstanding rate performance with incredible energy density in capacitor devices with the specific capacitance proportional to the number of micropores at 0.80 nm. This established a relation between the capacitive performance and critical pore size below 1 nm. The high surface area K-Ph-NC was further transformed to a K-Ph-Oxide, a graphene version of K-phthalimide, by using an improved Hummer's method with Mn-salt and phosphoric acid, which resulted in a phthalimene oxide (K-Ph-Oxide) doped with Mn-phosphate. To explore the ability of K-Ph-NC to be functionalized with transition metal nanoparticles, a composite with ZIF-67 was made by calcination at 700 °C under an Ar atmosphere to give an etched e-ZIF-67/K-Ph-NC (Fig. 1). The corresponding symmetric supercapacitor device exhibited a maximum SE value of 22.7 W h kg⁻¹ with a maximum SP of 10 416.7 W kg⁻¹, which is mainly due to the favorable microporous pore architecture in e-ZIF-67/K-Ph-NC as compared to K-Ph-NC and K-Ph-Oxide. This highlights the role of cobalt nanoparticles in e-ZIF-67/K-Ph-NC with an etched outer surface. A promising overpotential of 450 mV at 10 mA cm⁻² in the OER by e-ZIF-67/K-Ph-NC can be correlated to the charge transfer resistance across the electrode–electrolyte interface.

Fig. 1 Preparation of potassium phthalimide-derived K-Ph-NC followed by oxidation by the improved Hummer's method to give K-Ph-Oxide. The formation of a composite of ZIF-67 and K-phthalimide followed by carbonization at 700 °C gave etched-ZIF-67/K-Ph-NC.

Results and discussion

Oxidation of K-Ph-NC to K-Ph-Oxide and e-ZIF-67/K-Ph-NC

The surface oxidation of K-Ph-NC to form K-Ph-Oxide is a novel strategy to convert the carbon surface to an oxidized surface with oxygen functionality. As is well known, graphene can be converted into graphene oxide using the improved Hummer's method.²⁴ However, this surface oxidation strategy is limited to converting graphitic to graphenic samples only. As shown in Fig. 1, the potassium phthalimide salt was pyrolyzed under argon at 700 °C, followed by an acid wash to result in K-Ph-NC, which exhibits a bulk sheet structure of graphitic N-doped carbon. The oxidation of K-Ph-NC by the improved Hummer's method with a mixture of H₃PO₄ and H₂SO₄ followed by KMnO₄ ensures surface engineering of the porous carbon flakes. This oxidation of the carbon-framework of K-Ph-NC results in K-Ph-Oxide consisting of doped-Mn-phosphate in the carbon framework (Fig. 1). The oxidation of K-Ph-NC resulted in the incorporation of a large percentage (57%) of oxygen on the carbon atoms of the graphenic framework (Fig. S1†). K-Ph-Oxide also contains heteroatoms such as P, S, and N. The N sites are evident due to the parent K-phthalimide salt, which is retained in K-Ph-NC and K-Ph-Oxide. The P and S-atom sites come from the reaction of K-Ph-NC with a mixture of H₃PO₄ and H₂SO₄. The formation of Mn sites in K-Ph-Oxide is due to the reaction of KMnO₄ with K-Ph-NC during the oxidation by the improved Hummer's method (Fig. 1). The elemental composition in Fig. S1c† describes the decoration of Mn centers along with heteroatoms including P, S, and N, confirming the high functionality of K-Ph-Oxide. In contrast, when the pyrolysis of K-phthalimide was performed by mixing with ZIF-67 at 700 °C for 2 h, unlike K-phthalimide as a pure salt, a composite e-ZIF-67/K-Ph-NC was produced (Fig. 1). Specifically, the improved Hummer's method enables the formation of a derivative of graphene oxide, namely K-Ph-Oxide-NC, with significant Mn-phosphate doping.

The X-ray diffraction (XRD) pattern of K-Ph-NC showed graphitic planes at 2θ 26.7° and 44.1° corresponding to the (001) and (100) planes, respectively. The XRD pattern reveals that the K-Ph-Oxide consists of manganese phosphate planes of (002), (221), (302), (003), (402), and (512) at 2θ of 18.14°, 19.01°, 25.31°, 27.16°, 29.95°, and 35.61°, respectively (JCPDS No. 029-0893) (Fig. 2b). The presence of Co metallic particles is confirmed from the (200), (220), (400), (420), (422), (440), (600) and (620) planes at 2θ of 17.35°, 24.58°, 35.04°, 39.34°, 44.54°, 50.51°, 53.59°, 56.74°, respectively (JCPDS No. 024-0327) (Fig. 2c). The graphitic carbon planes for K-Ph NC are confirmed at 23.44° and 44.14° (Fig. 2c). However, the nature of the XRD plots for K-Ph NC and e-ZIF-67/K-Ph-NC are similar. The peaks of the Co nanoparticles in e-ZIF-67/K-Ph-NC can be distinguished from those of K-Ph-NC. N₂ adsorption–desorption analysis of K-Ph-NC revealed a high BET surface area of 1060 m² g⁻¹ resulting from the K-phthalimide pyrolysis, which forms an extended 2D-surface of N-doped C (Fig. 2d) with a significant microporous network as revealed from the pore width versus pore volume profile (Fig. 2e). The pore analysis suggests the presence of significant micropores in K-Ph-NC with an average micropore size of 1.06 nm. Therefore, metal center functionalization on both of the above materials would give a microporous network that facilitates many electrochemical processes. Moreover, ex situ SEM and Raman spectral investigation after 250 cycles of galvanometric charge–discharge reveals the extent of the intensity drop of the e-ZIF-67/K-Ph-NC. But there is no significant change in the d-band and g-band shifts. Even though the material starts degrading after 250 cycles, there are no morphological changes as revealed from the scanning electron microscopy (SEM) analysis (Fig. S4a–f†) of e-ZIF-67/K-Ph-NC before cycling and after 250 cycles of CV, and the ex situ SEM images of e-ZIF-67/K-Ph-NC after 250 cycles (Fig. S4g–k†).

Fig. 2 XRD of (a) K-Ph-NC, (b) K-Ph-Oxide, and (c) e-ZIF-67/K-Ph-NC. N₂-adsorption–desorption profile of (d) K-Ph-NC with the pore size distribution plot (inset) and (e) K-Ph-Oxide with the pore size distribution plot (inset). (f) Soft X-ray absorption Mn-L_2,3 edge spectra of K-Ph-Oxide containing Mn-sites.

Mn³⁺ centers are decorated in K-Ph-Oxide as confirmed by soft X-ray absorption spectroscopy measurement. The peak at the Mn-L₃ edge confirmed the presence of the Mn³⁺ oxidation state in K-Ph-Oxide, which represents Mn-phosphate (Fig. 2f).^25,26 The incorporation of Mn-phosphate in the carbon framework is supported by the XRD pattern (Fig. 2b). Survey spectra of K-Ph-Oxide are shown in Fig. 3a. The C 1s XPS deconvoluted spectrum reveals the presence of three kinds of C centers, two of which are oxygen-containing C on the surface of K-Ph-Oxide (Fig. 3b). The N 1s XPS spectral pattern suggests that both pyrrolic and pyridinic Ns are available in K-Ph-NC in a 60 : 40 ratio approximately (Fig. 3c). The comparison of the elemental atom% composition suggests a difference in the C% of K-Ph-Oxide compared to K-Ph-NC. Moreover, the 532 eV peak in the deconvoluted O K-edge XPS spectrum confirms the presence of a graphene-like oxide structure (Fig. 3d).²⁶

Fig. 3 (a) XPS survey peaks for the K-Ph-Oxide. Deconvoluted XPS spectral peaks of (b) C 1s, (c), N 1s, and (d) O 1s peaks for K-Ph-Oxide. (e) XPS survey spectra for e-ZIF-67/K-Ph-NC. Deconvoluted peaks of the high-resolution XPS spectrum of (f) C 1s, (g) Co 2p, (h) N 1s, and (i) O 1s for e-ZIF-67/K-Ph-NC.

XPS was employed to further investigate the elemental composition and electronic structure of the e-ZIF-67/K-Ph-NC. The survey spectrum (Fig. 3e) of e-ZIF-67/K-Ph-NC revealed the coexistence of C, Co, N, and O elements. The deconvoluted C 1s spectra (Fig. 3f) support the presence of sp² and sp³ carbon atoms with the detection of Co²⁺ 2p, proving the successful formation of Co NPs (Fig. 3g). N 1s spectrum deconvoluted curve fitting shows the pyrrolic and pyridinic N in the framework of e-ZIF-67/K-Ph-NC (Fig. 3h). The O 1s spectrum (Fig. 3i) showed overlaps of C–O and C=O groups.

The high-resolution transmission electron microscopy (HR-TEM) image of K-Ph-Oxide shows a CNT-type multiwalled architecture that originates from K⁺-ion based etching at 700 °C, producing a 2-D network of oxide functionality resembling graphene oxide. The process of K-Ph-Oxide formation from K-Ph (Fig. 1) by the improved Hummer's method resembles graphene oxide formation from graphite.²⁴ It is envisaged that the intra-molecular chemical interaction between the oxygen sp² lone pair (lp) and carbon π orbitals is the structural origin for the stabilization of K-Ph-Oxide.²⁷ The high-resolution TEM image (Fig. 4a) for K-Ph-Oxide shows multiwall-type channels as in multiwalled carbon nanotubes, which suggests a tubular multiwalled structure. The unique 2D-structure of K-Ph-Oxide is clear from the TEM images (Fig. 4b–d) at various resolutions. A certain variation of the density of dark spots corresponding to the carbon nanotube-like architecture was found at all resolutions, confirming a non-uniform distribution of this architecture throughout the entire matrix of K-Ph-Oxide. The TEM images of e-ZIF-67/K-Ph-NC (Fig. 4e–h) show a box-type shape of ZIF-67-NC particles originating from ZIF-67, wherein 2D-sheet-like K-Ph-NC is clear. At slightly higher resolution at the 100 nm scale, partially occupied K-Ph-NC was seen with e-ZIF-67 particles (Fig. 4g). The dark-field TEM image (Fig. 4h) confirms the presence of e-ZIF-67-NC in the composite, wherein the ZIF-67-NC surface is clear as compared to the bright field images (Fig. 4e–g).

Fig. 4 (a) HR-TEM image and TEM images at (b) 50 nm, (c) 100 nm and (d) 200 nm of K-Ph-Oxide. TEM images of e-ZIF-67/K-Ph-NC at (e) 0.5 μm, (f) 200 nm and (g and h) 200 nm, and (i) dark-field image at 200 nm.

Supercapacitor studies

All three synthesized materials shown are potential candidates for capacitance applications, and comparison of their capacitance enhancement with underlying factors depending on the Mn 2p and Co 2p centers and their electronics was subjected to detailed investigation in a supercapacitor model. The electrochemical performance of all samples K-Ph-NC, K-Ph-Oxide, and e-ZIF-67/K-Ph-NC was investigated systematically via cyclic voltammetry (CV), galvanostatic charge–discharge (GCD), and impedance spectroscopy. The coated electrodes were immersed in 3 M KOH electrolyte and further tested for potential window stabilization. The voltage window was finalized from −1 to 0 V for e-ZIF-67/K-Ph-NC and K-Ph-NC.

The comparative CV profiles for a set of samples at a scan rate of 100 mV s⁻¹ are shown in Fig. 5a. As evident from the comparative CV shapes, the e-ZIF-67/K-Ph-NC exhibited a more enclosed area under the CV curve, explaining its improved electrochemical activity.²⁸ The CV curves of e-ZIF-67/K-Ph-NC at varying scan rates from 5 to 200 mV s⁻¹ are shown in Fig. 5b, while for samples K-Ph-NC and K-Ph-Oxide they are shown in Fig. S4a and c.† e-ZIF-67/K-Ph-NC exhibited a very high capacitance of 385.4 F g⁻¹ at a scan rate of 5 mV s⁻¹, while K-Ph-NC and K-Ph-Oxide exhibited a capacitance of 242 and 185.9 F g⁻¹, respectively. The capacitance values for these three samples at different scan rates are displayed in Fig. 5c. The e-ZIF-67/K-Ph-NC and K-Ph-NC samples exhibited good retention at higher scan rates as well. In contrast, the performance off the K-Ph-Oxide sample degraded drastically at higher scan rates, owing to its low surface area. Interestingly, the e-ZIF-67/K-Ph-NC sample showed high performance compared to different carbon-based materials, as tabulated in Table S1.† Moreover, this improved performance for e-ZIF-67/K-Ph-NC can also be explained by its very low charge transfer resistance, as studied by impedance spectroscopy (Fig. S5†). The e-ZIF-67/K-Ph-NC sample exhibited a small bulk resistance of 0.59 Ω and a negligible charge transfer resistance of 0.07 Ω compared to 0.6 Ω (bulk) and 0.24 Ω (charge transfer) for K-Ph-NC. The highly reduced charge transfer resistance further supports the improved supercapacitive performance of the e-ZIF-67/K-Ph-NC sample. Further insights into the charge storage mechanism were acquired by utilizing the power law and Dunn's method for the e-ZIF-67/K-Ph-NC sample, as displayed in Fig. S6.† The “b” value calculated using the power law (i = av^b) exhibited a value of 0.82, which indicates that both electric double layer capacitance (EDLC) and pseudocapacitance (Fig. S6a†) processes are involved. Moreover, the contribution of these two processes at different scan rates is plotted in Fig. S6b.† The EDLC capacitance increases on increasing the scan rate, which is consistent with earlier reports.^29,30

Fig. 5 (a) Comparative CV profiles for different samples e-ZIF-67/K-Ph-NC and K-Ph-NC at a scan rate of 100 mV s⁻¹. (b) CV profiles at scan rates varying from 5 to 200 mV s⁻¹ for e-ZIF-67/K-Ph-NC. (c) The comparative three-electrode capacitance performance for e-ZIF-67/K-Ph-NC, K-Ph-NC, and K-Ph-Oxide at different scan rates.

Following the assessment of the three-electrode performance, the e-ZIF-67/K-Ph-NC substance was extended to examine the packed two-electrode performance. The active material was coated over carbon cloth, and then packed in a symmetric configuration using filter paper as the separator and 2 M tetraethylammonium tetrafluoroborate in propylene carbonate (PC) as the electrolyte.³¹ The voltage window for the device was finalized by performing CV curves at 100 mV s⁻¹ at different voltages varying from 1 V to 3.4 V, as displayed in Fig. 6a. The voltage window was fixed at 3 V, and further CV and GCD performances were recorded. The CV curves at different scan rates for the assembled symmetric supercapacitor device (SSD) are shown in Fig. 6b. As anticipated, the device exhibited a nearly rectangular CV shape, indicating that most of the charge is being stored by the electrostatic adsorption–desorption of electrolyte ions.

Fig. 6 (a) Voltage window variation for the packed device of e-ZIF-67/K-Ph-NC at a scan rate of 100 mV s⁻¹. (b) CV curves at different scan rates, and (c) GCD curves at different current densities, namely 0.033, 0.05, 0.1, 0.2, 0.3, 0.4, 0.5, 1, 3, and 5 A g⁻¹. (d) Ragone plot comparing the SSD performance of e-ZIF-67/K-Ph-NC with different literature-reported carbon-based SC devices.

Further, the GCD curves at different current densities, namely 0.033, 0.05, 0.1, 0.2, 0.3, 0.5, 1, 3, and 5 A g⁻¹, are displayed in Fig. 6c. The material exhibited triangular charge–discharge curves without any visible IR drop. Based on the capacitance values calculated from the GCD curves, the specific energy (SE) and specific power (SP) values were calculated and plotted in the Ragone plot. A comparison of different two-dimensional carbon materials for SSDs with our device is shown in the Ragone plot in Fig. 6d. These reported materials include carbon sheets,³² two-dimensional graphitic carbon,³³ carbon nanosheets,³⁴ and sponge-like graphene sheets.³⁵ Interestingly, our device exhibited a maximum specific energy (SE) value of 22.7 W h kg⁻¹ with a maximum specific power (SP) of 10 416.7 W kg⁻¹ (Fig. 6d). A possible reason for the excellent performance of e-ZIF-67/K-Ph-NC can be attributed to the 2D morphology and etched outer surface, along with high surface area. Moreover, the microporous pore architecture in e-ZIF-67/K-Ph-NC plays a major role in enhancing efficiency as compared to other ZIF-67 and K-phthalimide-derived composites. The symmetric supercapacitor device was connected in series, and could provide power to a 1.5 V red color light emitting diode (LED) with high brightness. This supports the experimental data of energy storage. The images of the LED glowing when connecting two SSDs in series (Fig. S8†) suggest its real field application towards supercapacitor devices.

Electrochemical oxygen evolution activity

The electrocatalytic activity of the as-prepared e-ZIF-67/K-Ph-NC sample for the OER was investigated in a typical three-electrode system with 1.0 M KOH as the electrolyte. The active material coated on glassy carbon (GC) was used as the electrode material for a 3-electrode OER to determine the predominant active sites. Before examining the electrochemical OER performance, a few cyclic voltammetry (CV) scans were recorded with the e-ZIF-67/K-Ph-NC and K-Ph-NC materials (deposited on GC) within a potential window of 0.5–1.5 V (vs. RHE) at a scan rate of 30 mV s⁻¹ to activate the catalytic sites. The initial observation of LSV for K-Ph-NC failed to reach a current density of 10 mA cm⁻². In contrast, e-ZIF-67/K-Ph-NC, containing N-containing Co active sites, displayed an improved OER activity with an overpotential (η₁₀) of 450 mV. The absence of active metal sites limits the adsorption of water molecules at the catalytic active sites in K-Ph-NC, compromising its OER activity. We designed and incorporated Co nanoparticles with an etched framework in a potassium phthalimide C-matrix and investigated its OER activity.

A strong redox feature with an E_1/2 value of 1.28 V (vs. RHE) was observed in the CV cycles for e-ZIF-67/K-Ph-NC, which can be ascribed to the oxidation of Co embedded on the e-ZIF-67/K-Ph-NC framework. There was an increment of 0.2 mA current in the highest current density with the progress of the CV cycles from the 1st to 30th cycle, accompanied by a constant peak position of the redox couple after cycle 1. The cathodic shift of the redox feature after the 1st cycle indicates the cooperative activation of the catalyst. Linear sweep voltammetry (LSV) was recorded within a potential range of 1.0–2.2 V (vs. RHE) with e-ZIF-67/K-Ph-NC after the activation of the electrode (by 10 CV cycles at 5 mV s⁻¹). The redox peak within the 1.2 to 1.4 V range in the LSV curves indicates the in situ formation of Co(III)-species, followed by a steady increase in the catalytic current after 1.5 V (vs. RHE), which reached up to 80 mA cm⁻² at 2.2 V (vs. RHE) indicating a catalytic OER (Fig. 7a). The best activity was obtained with 1 mg of mass loading, which implied monolayer formation on the electrode surface to achieve enhanced OER performance. The electrode shows a promising and better overpotential of 450 mV compared to K-Ph-NC at 10 mA cm⁻² towards an effective electrocatalyst. The improved OER activity of e-ZIF-67/K-Ph-NC can be correlated to the charge transfer resistance across the electrode–electrolyte interface. The lower Tafel value in e-ZIF-67/K-Ph-NC reflects its better charge transfer kinetics during reaction and efficient electrocatalyst than K-Ph-NC (Fig. 7b). An electrochemical impedance spectroscopic (EIS) study was performed at a constant potential of 1.54 V (vs. RHE) and the semicircular Nyquist plot and corresponding equivalent circuit fitting provided the charge-transfer resistance (R_ct) of the electrode and indicated that the electrode–electrolyte junction follows a double layer formation (Fig. 7c). The R_ct value obtained for e-ZIF-67/K-Ph-NC was 41.48 Ω, indicating the rapid charge transfer kinetics across the electrode–electrolyte junction, which greatly influences the electrocatalytic activity. The better catalytic performance of e-ZIF-67/K-Ph-NC for the OER and supercapacitor activity is mainly attributed to the large specific surface area and good electrical conductivity. Besides faster kinetics, another crucial factor that greatly influences the catalyst performance is the number of active sites participating in the reaction, which together constitute the electrochemically active surface area (ECSA). The ECSA is directly related to the double-layer capacitance (C_dl) value of the material. The C_dl can be calculated from the non-faradaic current response of the electrode (Fig. S8†). The ECSA can be determined from the C_dl value and the specific capacitance (C_s) of the electrode being investigated. The obtained C_dl value for e-ZIF-67/K-Ph-NC is 0.414 mF, which corresponds to an ECSA value of 20.7 cm² (Fig. S8a†). The ECSA normalized activity was also in good agreement with the intrinsic OER activity of the catalyst.

Fig. 7 (a) Polarization curves obtained from the OER-LSV study with e-ZIF-67/K-Ph-NC and K-Ph-NC. (b) Tafel slope obtained from the LSV curve for e-ZIF-67/K-Ph-NC and K-Ph-NC. (c) Nyquist plots for e-ZIF-67/K-Ph-NC and K-Ph-NC from the data obtained for the EIS studies. (d) CV cycling stability of e-ZIF-67/K-Ph-NC.

Conclusion

In conclusion, N-doped large-sized carbon sheets with Mn-phosphate-doping and with Co-NPs via a facile one-pot activation and calcination of the layered potassium phthalimide salt without any additional activators or template were prepared. K-Ph-NC consists of large-sized 2D nanosheets with a lateral size of about 5 μm with a large specific surface area of 1060 m² g⁻¹ and a uniform sub-nanometer microporous distribution of ∼0.60 nm dimension. The unique layered structure provides a sturdy N-C matrix for prolonged charging/discharging with abundant active adsorption sites and an effective route for rapid electrolyte ion transport with a shorter diffusion distance for adsorption/desorption of ions. Through these merits, K-Ph-NC offers high capacitance and outstanding rate performance with incredible energy density in capacitor devices with proportional specific capacitance. The high surface area K-Ph-NC was further transformed to K-Ph-Oxide, a K-phthalimide-derived surface oxidized graphenic material analogous to graphene oxide, by using an improved Hummer's method (Mn-salt and phosphoric acid), which resulted in a phthalimene oxide doped with Mn-phosphate. A composite of K-phthalimide-derived N-C functionalized with Co nanoparticles and ZIF-67 was thermally calcined at 700 °C under an Ar atmosphere, which resulted in a composite with an etched surface denoted as e-ZIF-67/K-Ph-NC. A comparative electronic and structural analysis followed by a capacitance retention and electrochemical OER performance study of e-ZIF-67/K-Ph-NC with K-Ph-Oxide as compared to K-Ph-NC reveals the significance of Co-NPs as compared to Mn-phosphate sites. Moreover, this system is found to exhibit better electrocatalytic activity when compared to other cobalt-based systems (Table S1†). A symmetric supercapacitor device exhibited a maximum SE value of 22.7 W h kg⁻¹ with a maximum SP of 10 416.7 W kg⁻¹, which is mainly due to the favorable microporous pore architecture in e-ZIF-67/K-Ph-NC as compared to K-Ph-NC and K-Ph-Oxide. This highlights the role of cobalt nanoparticles in e-ZIF-67/K-Ph-NC with an etched outer surface. A promising overpotential of 450 mV at 10 mA cm⁻² in the OER by e-ZIF-67/K-Ph-NC can be correlated to the charge transfer resistance across the electrode–electrolyte interface.

Data availability

Data are available in the ESI.†

Conflicts of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

SD wishes to acknowledge research funding support by the Science & Engineering Research Board (SERB), Department of Science & Technology, Government of India through a core research grant number CRG/2023/000044 (2024–2027). SD also wishes to thank the Department of Biotechnology, Ministry of Science & Technology, Government of India for a DBT-Energy Bioscience-Biofuels research grant number BT/PR38594/PBD/26/795/2020 (2023–2026). SD gratefully thanks the UGC-DAE Consortium for Scientific Research for a project under the Collaborative Research Scheme (CRS) project grant number CRS/2022-23/01/680 (2023–2026) and Raja Ramanna Centre for Advanced Technology, Indore Beam Line 3 and 9 for soft and hard X-ray absorption spectroscopy.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4se00979g

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