Electrochemical performance of a Li⁺-enriched metallohydrogel as an electrolyte and electrode material for supercapacitors

Abstract

Li⁺-enriched metallohydrogel (MG-Co) has been synthesized via in situ LiOH deprotonation of the pre-gelator (H₉SAL) followed by coordination with Co²⁺ to develop a pure gel electrolyte and electrode material without adding any external dopant to the gel matrix. The electrochemical performance of MG-Co as an electrolyte and electrode material has shown promising features to be utilized in supercapacitor applications.

---

The advancement in energy storage devices has initiated a tremendous requirement for new smart materials to meet functionalities needed for the development of these devices.^1–5 Among the various types of energy storage devices, supercapacitors are efficient and effective energy storage systems due to their excellent power density, stability and reasonable fabrication cost.^6–9 In batteries and supercapacitors, liquid electrolytes were widely used for several decades due to their good ionic conductivities (10⁻³ to 10⁻² S cm⁻¹) and better interfacial contacts with electrodes. However, liquid electrolyte features are restricted when used for portable and flexible electronic devices due to leakage, internal corrosion and volatilization.^10–12 Considering above-mentioned facts, the selection of electrolytes as well as electrode materials becomes crucial to achieve a supercapacitor for advance applications such as flexible and portable electronics.¹⁰ Therefore, there is a persistent demand for the development of ionic conductive gel-phase materials to address problems associated with liquid electrolytes. In addition, there is an urgent demand for novel electrode materials to propel the evolution of flexible supercapacitors. Gel polymer electrolytes (GPE) have been widely used in batteries and supercapacitors due to their ability to effectively overcome the limitations of liquid electrolytes, such as leakage, internal corrosion and volatilization.^11–13 In this context, metallogels, a well-known class of supramolecular soft materials, could be a suitable choice for the development of electrolytes in supercapacitor applications due to their outstanding electrochemical properties.^14–16 We have explored in our previous research how the incorporation of metal ions not only triggers gel formation but also intrinsically induces ionic conductivity in the metallogels.^17,18 Moreover, the dried form of metallogel (xerogel) could be used as an electrode material in supercapacitors due to their good porosity and capability to accommodate metal cations in redox active sites.¹⁹ There are no reports of a single metallogel material that serves the dual roles of an electrolyte and electrode material.

Typically, gel polymer electrolytes are prepared by impregnating a polymer membrane matrix with ionic liquid, thereby achieving the desired conductivity and mechanical strength within a specific range.^20,21 However, in adapting this methodology, the process becomes complex, tedious, and cost-ineffective; moreover, it involves toxic substances.²² Furthermore, it is worth mentioning that the incorporation of any external dopant to the gel phase system leads to degradation of gel-associated properties, for example, morphology and rheology.^15,23 Hence, there continues to be a need for novel gel polymer electrolytes (GPEs) that offer straightforward processing, cost-effective methods, and non-toxicity to address safety concerns. Importantly, these GPEs should simultaneously preserve essential parameters such as ionic conductivity and mechanical robustness to utilize them for the fabrication of supercapacitors. The conductive metallogels could be one of the promising candidates to address above-mentioned problems, since these metallogels facilitate one-step synthesis of Li⁺ enriched mechanically strong conductive gel phase system.

Considering the emerging demand for electrolytes and electrode materials in the fabrication of advanced flexible supercapacitors, in the present work, we developed a Li⁺-assisted Co²⁺-induced metallohydrogel through deprotonation of the synthesized gelator molecule (H₉SAL) with LiOH in H₂O followed by treatment with aqueous Co(OAc)₂. Further, the developed metallohydrogel was utilized as an electrolyte and electrode material for the fabrication of a supercapacitor (Scheme 1).

Scheme 1 Schematic depiction of the synthesis of metallohydrogel and its application as an electrolyte and electrode material for supercapacitors.

To develop a gel-based electrolyte and electrode material, we first synthesized a novel low-molecular-weight gelator (H₉SAL) via condensation reaction between 5-aminosalicylic acid and 2,4,6-trihydroxybenzene-1,3,5-tricarbaldehyde (yield ∼80%). Labile protons available in the H₉SAL were deprotonated using LiOH in H₂O. Further, aliquot addition of aqueous Co(OAc)₂·4H₂O to the above deprotonated solution provided a dark-red metallohydrogel (MG-Co). The inverted vial test method was used to preliminarily confirm the gel formation, as shown in Scheme 1. The chemical structure of the H₉SAL gelator and its complexation with Co²⁺ were comprehensively examined and characterized through various instrumental techniques (Scheme S1 and Fig. S1–S6, ESI†).

UV-visible titration study was carried out to confirm the binding of Co²⁺ to the chelating sites of the H₉SAL gelator (Fig. S4, ESI†). A clear red solution of LiOH-deprotonated H₉SAL (1 × 10⁻⁵ M, H₂O, 298 K, ε = 54 800) showed an absorption peak at 396 nm. Further, aliquot addition of aqueous Co(OAc)₂ (1 × 10⁻³ M, H₂O) to the deprotonated solution of H₉SAL leads to the concomitant significant decrease in the absorbance band at 396 nm through an isosbestic point at 300 nm, suggesting the possibility of Co²⁺ binding to the chelating sites of the H₉SAL gelator. Moreover, an absorption peak is observed in the near infrared (NIR) region at 975 nm (1 × 10⁻³ M, ε = 301), which corresponds to the d–d transition band for Co(II) complex.²⁴ FT-IR spectral analysis was performed to get clear insights on the interaction between Co²⁺ and the H₉SAL gelator. Upon analysing the FT-IR spectra, metallation effect was observed over the vibrational bands of H₉SAL (Fig. S5, ESI†).²³ Electrospray ionization (ESI) mass study was carried out to confirm the coordination complex as proposed, with the help of UV-vis and FT-IR analysis. ESI-mass for diluted MG-Co showed molecular ion peaks at 703.05 (calcd. 703.06), 771.91 (calcd. 771.98) and 906.94 (calcd. 906.95) corresponding to molecular ion species [(C₃₀H₁₇N₃O₁₂)(H₂O)CoLi₂ + H]⁺, [(C₃₀H₁₃N₃O₁₂)Co₂·H₂O Li₂ + H]⁺ and [(C₃₀H₁₃N₃O₁₂)(H₂O)₆Co₃Li₂ + H]⁺, respectively, confirming the coordination complexation between deprotonated H₉SAL and Co²⁺ (Fig. S6, ESI†). Furthermore, the observed isotopic abundance patterns of molecular ion peaks closely align with the simulated pattern (Fig. S6, ESI†). The morphological features of MG-Co xerogel were explored using FE-SEM analysis. The FE-SEM image illustrates the existence of a connected fibrous network within MG-Co, which likely plays a role in establishing a gel matrix (Fig. 1). This matrix facilitates the entrapment of solvent molecules, which leads to the formation of a gel. In addition, it is evident from the FE-SEM image that MG-Co has good porosity; thus, it could be a suitable choice as an electrode material with enhanced electrochemical behaviour.²⁵ Moreover, energy-dispersive X-ray and elemental mapping analysis jointly confirm the presence of uniformly distributed Co elements in MG-Co gel samples (Fig. 1 and S7, ESI†). PXRD patterns obtained for the H₉SAL gelator and MG-Co describe their crystalline and amorphous natures, respectively (Fig. S8, ESI†).

Fig. 1 (A) FE-SEM image of MG-Co xerogel; (B–D) elemental mapping images of MG-Co showing the presence of C, O and Co elements, respectively.

As the mechanical properties of the gel are one of the deciding factors for their use in targeted applications, we performed extensive rheological study over the as-synthesized MG-Co (2% w/v) gel sample to claim its suitability for the fabrication of a supercapacitor. First, stress and strain sweep experiments were carried out, in which storage (G′) and loss (G′′) moduli were recorded against applied stress and strain. It is evident from Fig. S9† that the magnitude of G′ is approximately one order greater than that of G′′ within the viscoelastic linear region, thus disclosing the true gel phase nature of synthesized MG-Co (Fig. S9, ESI†).²⁶ Furthermore, obtained results from dynamic frequency sweep experiments well support the true gel-phase behaviour of MG-Co. Thus, with help of overall rheological results, it can be ascertained that MG-Co has the potential to be utilized for the fabrication of supercapacitors.

To utilize MG-Co as the gel electrolyte, transport number evaluation was carried out by DC polarization method (Experimental section, ESI†). MG-Co provided the transport number t_Li+ of 0.15, which may be accredited to the availability of Li⁺ within the metallogel matrix contributing to ionic conduction (Fig. S10, ESI†). However, this transport number value is not adequate to utilize MG-Co as an gel electrolyte for a supercapacitor. Therefore, we prepared the metallohydrogel by incorporating additional 1 M LiOH following the same synthesis procedure as MG-Co. The gel obtained by incorporating excess LiOH can be termed as MG-Co-Li. Interestingly, MG-Co-Li provided the transport number of 0.68, which is significant for its utilization as a gel electrolyte for supercapacitor applications (Fig. S10, ESI†).²⁷MG-Co, 1 M LiOH aqueous solution and MG-Co-Li showed ionic conductivity of 14.73 × 10⁻³, 26.63 × 10⁻² and 40.93 × 10⁻² S cm⁻¹, respectively (Fig. S11, ESI†). To monitor the stability of MG-Co and MG-Co-Li against the influence of variable electrical signals, impedance studies with respect to time at higher and lower frequencies were carried out over MG-Co and MG-Co-Li (Fig. S12, ESI†). The magnitude and phase of impedance were almost unchanged at lower frequency (0.1 Hz) within a 3 hours time; however, there was slight variation in magnitude as well as phase of impedance upon applying a higher frequency of 10⁶ Hz for both gels. The invariable nature of impedance against time indicates the electrically stable property of prepared metallohydrogels.^16,23

Inspired by the obtained suitable transport number values of MG-Co-Li, we further investigated its electrochemical performance by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and galvanostatic charge–discharge (GCD) analysis using a 3-electrode system. MG-Co-Li gel was used as an electrolyte, and MG-Co xerogel was employed as active material to build the electrode with activated carbon and PVDF as binder in the w/w ratio of 75 : 20 : 5. Cyclic voltammetry curves for MG-Co xerogel electrode in 1 M LiOH aqueous solution and MG-Co-Li gel are shown in Fig. 2 at various scan rates (100 to 10 mV s⁻¹). MG-Co xerogel electrode in 1 M LiOH liquid electrolyte showed a broad cyclic voltammogram (CV) curve at 100 mV s⁻¹ scan rate, thus indicating the possibility of a double-layer charge storage mechanism; moreover, the CV curve remained unaffected under varying scan rate from 100 to 10 mV s⁻¹.¹² However, liquid electrolyte is not suitable for flexible electronic device applications due to leakage, internal corrosion and volatilization. Considering the drawbacks of liquid electrolyte, we used MG-Co-Li gel as the electrolyte; interestingly, the CV curve for MG-Co xerogel in the prepared MG-Co-Li gel electrolyte retained the broad loop at 100 mV s⁻¹ scan rate, in comparison with liquid electrolyte (1 M LiOH). In addition, the CV curve at 25 mV s⁻¹ in the case of gel electrolyte shows anodic and cathodic peaks at 0.26 and −0.1 V, respectively, thus confirming the existence of the pseudocapacitive mechanism along with the double-layer charge storage mechanism.²⁸ Furthermore, the GCD charge–discharge characteristics of MG-Co xerogel in 1 M LiOH liquid and MG-Co-Li gel electrolytes provided the specific capacitance of 65.5 and 86 F g⁻¹, respectively, at 1 A g⁻¹ (Tables S1 and S2, ESI†). The specific capacitance is maintained almost constantly up to 5 cycles in the 1 M LiOH liquid and MG-Co gel electrolytes. It is evident that the specific capacitance of the MG-Co xerogel electrode is improved when MG-Co-Li is used as gel electrolyte. To further investigate the improvement in specific capacitance when MG-Co-Li is used as the gel electrolyte, we conducted EIS study over the same 3-electrode system. Nyquist impedance plots shown in Fig. 3 present one semicircle followed by a linear evolution in the 1 M LiOH and MG-Co-Li gel electrolytes. To quantitatively analyse the Nyquist plots, we fitted the plots with a suitable electrical circuit model. The semicircle was fitted with a parallel combination of charge-transfer resistance (R_ct) and capacitance; the R_ct arises due to the electrode–electrolyte interface.¹⁸ A series resistance (R_s) has been connected to the parallel combination, accounting for the resistance of electrode material (MG-Co xerogel, activated carbon and PVDF) as well as contact resistance between the electrode and current collector (stainless steel foil).²⁸ Further, considering the non-ideal behaviour of the electrode interface, a constant phase element has been added to the parallel circuit to fit the linear evolution appearing in the lower frequency region. As evident, in the MG-Co-Li electrolyte, the charge-transfer resistance of the cell is appreciably lower than that of the cell consisting of the 1 M LiOH liquid electrolyte, indicating that using MG-Co-Li as an electrolyte favours smooth charge transport by facilitating an ion migration channel between the anode and cathode. This phenomenon has already been explored in our previous research work, as systems with a higher degree of organization or ordered systems, typically exhibit greater conductance compared to systems with random arrangements.^18,29 Therefore, the better conductance properties of MG-Co-Li electrolyte, along with the existence of the pseudocapacitive mechanism in the case of MG-Co-Li electrolyte and the MG-Co electrode system, are plausible reasons behind the improvement in the specific capacitance of the overall cell, as depicted in Fig. 4.^12,30 Moreover, the connected fibrous network within MG-Co upholds the wettability of the gel by effectively capturing an ample quantity of water. It is important to highlight that the issue of solvent evaporation poses a substantial challenge in gel electrolytes, thus making MG-Co-Li extremely advantageous as a gel electrolyte with exceptional water retention capabilities. Therefore, the combination of MG-Co-Li as an gel electrolyte and MG-Co xerogel as an electrode material is promising for supercapacitor applications.

Fig. 2 (A and B) CV curves of MG-Co xerogel in 1 M LiOH aqueous solution and MG-Co-Li gel electrolyte, respectively. (C and D) GCD curves of MG-Co xerogel in 1 M LiOH aqueous solution and MG-Co-Li gel electrolyte, respectively.

Fig. 3 (A) Specific capacitance vs. cycle number plot; (B) Nyquist impedance plot along with fitted circuit model for 3-electrode cell.

Fig. 4 Schematic representation for utilization of the metallohydrogel as an electrolyte and electrode material for supercapacitors.

In summary, through this work, we developed a Li⁺-enriched metallohydrogel (MG-Co) without using any external dopant and analysed its electrochemical performance for supercapacitor applications. MG-Co was synthesized via deprotonation of the pre-gelator (H₉SAL) with LiOH followed by treatment with Co(OAc)₂. The gel formation was well established through UV-vis, NMR and FT-IR spectroscopic techniques. FE-SEM study disclosed the nanofibrous morphology of the MG-Co gel. The electrochemical performance of MG-Co-Li gel and MG-Co xerogel was evaluated by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and galvanostatic charge–discharge (GCD) analysis. They showed MG-Co-Li has improved electrochemical properties compared to the liquid system while overcoming the drawbacks of liquid electrolyte. Thus, it is anticipated that using MG-Co-Li as an gel electrolyte and MG-Co xerogel as an electrode material could be suitable for supercapacitor applications.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

MD and MKD are thankful to SERB, New Delhi, for financial support through NPDF (PDF/2020/001935). We are also thankful to CRDT, IIT Indore, for financial support through project number IITI/CRDT/2022-23/06. YK is thankful to IIT Indore for the Fellowship. The authors are thankful to the Dean of Scientific Research, King Khalid University, for financial support via grant number RGP2/345/45. We also thank SIC, IIT Indore, for extending instrumental facilities.

Notes and references

1. Y. Tong, J. Yang, J. Li, Z. Cong, L. Wei, M. Liu, S. Zhai, K. Wang and Q. An, J. Mater. Chem. A, 2023, 11, 1061.
2. K. Nasrin, V. Sudharshan, K. Subramani and M. Sathish, Adv. Funct. Mater., 2022, 32, 2110267.
3. Y. Bao, L. Han, W. Peng, Z. Yang, J. Yang, Z. Ji, M. Xu and L. Pan, Sustainable Energy Fuels, 2023, 7, 2094.
4. L. Lv, B. Hui, X. Zhang, Y. Zou and D. Yang, Chem. Eng. J., 2023, 452, 139443.
5. D. Liu, Z. Wang, Q. Qian, J. Wang, J. Ren, H. Chen, W. Xing and N. Zhou, Adv. Funct. Mater., 2023, 33, 2214301.
6. Y. Zhou, et al. , Energy Environ. Sci., 2021, 14, 1854.
7. W. Liu, et al. , J. Energy Chem., 2023, 80, 68.
8. C. Xiong, Y. Zhang and Y. Ni, J. Power Sources, 2023, 560, 232698.
9. H. H. Xu, L. Yue, Y. Tang, F. Liu, H.-J. Zhu and S.-J. Bao, Rare Met., 2023, 42, 430.
10. M. Khalid, B. S. De, A. Singh, S. P. Thota and S. Shahgaldi, ACS Appl. Energy Mater., 2023, 6(15), 7857.
11. S. Davino, D. Callegari, D. Pasini, M. Thomas, I. Nicotera, S. Bonizzoni, P. Mustarelli and E. Quartarone, ACS Appl. Mater. Interfaces, 2022, 14(46), 51941.
12. C. K. Karan, S. Mallick, C. R. Raj and M. Bhattacharjee, Chem.–Eur. J., 2019, 25, 14775.
13. F. Elizalde, J. Amici, S. Trano, G. Vozzolo, R. Aguirresarobe, D. Versaci, S. Bodoardo, D. Mecerreyes, H. Sardon and F. Bella, J. Mater. Chem. A, 2022, 10, 12588.
14. Y. Kumar and M. Dubey, ACS Appl. Mater. Interfaces, 2023, 15, 11970.
15. C. Mahendar, M. K. Dixit, Y. Kumar and M. Dubey, J. Mater. Chem. C, 2020, 8, 11008.
16. Y. Kumar and M. Dubey, Chem. Commun., 2022, 58, 549.
17. P. Verma, A. Singh and T. K. Maji, Chem. Sci., 2021, 12, 2674.
18. V. K. Pandey, M. K. Dixit, S. Manneville, C. Bucher and M. Dubey, J. Mater. Chem. A, 2017, 5, 6211.
19. X. Liang, J. Wu, Z. Hua and G. Liu, Chem. Commun., 2023, 59, 2811.
20. X. Cheng, J. Pan, Y. Zhao, M. Liao and H. Peng, Adv. Energy Mater., 2018, 8, 1702184.
21. C. St-Antoine, D. Lepage, G. Foran, A. Prébé, D. Aymé-Perrot, D. Rochefort and M. Dollé, J. Mater. Chem. A, 2023, 11, 10984.
22. D. Lee, Y. Song, Y. Song, S. J. Oh, U. H. Choi and J. Kim, Adv. Funct. Mater., 2022, 32, 2109907.
23. Y. Kumar, C. Mahendar, A. Kalam and M. Dubey, Sustainable Energy Fuels, 2021, 5, 1708.
24. S. Pandey, P. P. Das, A. K. Singh and R. Mukherjee, Dalton Trans., 2011, 40, 10758.
25. E. Thirugnanasambandam, G. Shanmugam and A. M. S. Hameed, Inorg. Chem., 2022, 61, 17873.
26. C. Mahendar, Y. Kumar, M. K. Dixit and M. Dubey, Soft Matter, 2020, 16, 3436.
27. Y. Xu, L. Gao, X. Wu, S. Zhang, X. Wang, C. Gu, X. Xia, X. Kong and J. Tu, ACS Appl. Mater. Interfaces, 2021, 13, 23743.
28. C. Shi, H. Cao, S. Li, L. Guo, Y. Wang and J. Yang, J. Energy Storage, 2022, 54, 105270.
29. C. Mahendar, Y. Kumar, M. K. Dixit, M. Mukherjee and M. Dubey, Mol. Syst. Des. Eng., 2021, 6, 654.
30. X. Peng, H. Liu, Q. Yin, J. Wu, P. Chen, G. Zhang, G. Liu, C. Wu and Y. Xie, Nat. Commun., 2016, 7, 11782.

---

Footnote

† Electronic supplementary information (ESI) available: Experimental procedures, synthetic scheme, UV-vis, NMR, FT-IR, ESI-MS, EDS and rheological data. See DOI: https://doi.org/10.1039/d4se00096j

---