(84bf) Imidazole-Based Concentrated Hydrogen-Bonded Electrolytes for Energy Storage Applications

Recently, Concentrated Hydrogen Bonded Electrolytes (CoHBEs) have emerged as promising alternatives to conventional electrolytes, owing to their wide electrochemical stability windows, high energy density, low volatility and high proton conductivity.¹ Similarly, imidazole-based systems have been recognized as promising CoHBEs because of their intrinsic hydrogen-bonding network, which enhances proton conductivity by supporting Grotthuss proton diffusion. However, their practical application is currently limited by their high viscosity and solid-state behavior at room temperature (RT). In this study, we explored the potential use of imidazole derivatives and redox-active acids to improve proton conduction in CoHBE systems by leveraging the H-bonding network and proton conduction for Proton-Coupled Electron Transfer (PCET) reactions. The addition of acidic species to imidazole-based systems increases the conductivity by promoting the dissociation of the antiparallel supramolecular structure of the imidazole chains, enhancing the charge transfer capacity of the electrolyte, as described by Cosby et al.^2,3. Furthermore, the redox-active nature of the acid species allows for reversible oxidation and reduction reactions, enabling them to act as electron shuttles and promoting enhanced PCET capacity within the electrolyte.

In preliminary studies, hydroquinone sulfonic acid (HQSA) and 3-amino-4-hydroxybenzoic acid (AHBA) were selected as quinone and azaquinone acid derivatives, respectively, based on their reported redox behavior in acidic electrolytes.^4,5 Several molar compositions (8:1, 4:1, 1:1) of the imidazole derivatives and the redox-active species were tested to form liquid mixtures at RT and the electrochemical behavior of the resulting liquid mixtures was evaluated by cyclic voltammetry. Liquid mixtures are more likely to form at RT when the molar concentration of imidazole is significantly greater than that of the acid (8:1). Liquid imidazole-based supporting electrolyte was necessary to improve the solubility of the mixtures required for the electrochemical characterization by cyclic voltammetry (CV). Electrochemical studies revealed that the redox activity of the acidic species was maintained in some samples at RT. However, it is important to consider the variation in the redox potentials (E_1/2) towards negative values in comparison with the E_1/2 values reported for the aforementioned aqueous acidic electrolytes.^4,5 The observed negative potential values are attributed to the basic nature of imidazole when mixed with acidic species, where imidazole acts as a proton acceptor, neutralizing the acid and forming a salt. This can change the pH and affect the redox behavior of the components in the mixture. Therefore, when designing and optimizing these mixtures for use as electrolytes, careful consideration of the concentration and selection of acid species is necessary.

References:

1. Ghahremani, R., Savinell, R. F. & Gurkan, B. Perspective—Hydrogen Bonded Concentrated Electrolytes for Redox Flow Batteries: Limitations and Prospects. J. Electrochem. Soc. 169, 030520 (2022).
2. Cosby, T., Holt, A., Griffin, P. J., Wang, Y. & Sangoro, J. Proton Transport in Imidazoles: Unraveling the Role of Supramolecular Structure. J. Phys. Chem. Lett. 6, 3961–3965 (2015).
3. Cosby, T., Vicars, Z., Heres, M. & Sangoro, J. Associating Imidazoles: Elucidating the Correlation between the Static Dielectric Permittivity and Proton Conductivity. Phys. Rev. Lett. 120, 136001 (2018).
4. Beiginejad, H., Nematollahi, D. & Varmaghani, F. Electrochemical Oxidation of Some Aminophenols in Various pHs. J. Electrochem. Soc. 160, H41 (2012).
5. Zhang, Z. J. & Chen, X. Y. Illustrating the effect of electron withdrawing and electron donating groups adherent to p-hydroquinone on supercapacitor performance: The cases of sulfonic acid and methoxyl groups. Electrochimica Acta 282, 563–574 (2018).