(Invited) Confinement Effects on the Redox Reaction of Aromatic Molecules
Authors: Takimoto Daisuke 
Published: December 2024

Introduction

Aromatic compounds have been studied as electrode materials for redox flow batteries and electrochemical flow capacitors. Electrochemical flow capacitors, consisting of carbon-based flowable electrodes, incorporate the advantageous characteristics of flow batteries and supercapacitors. Recently, pseudocapacitive flowable electrodes have been reported to achieve enhanced energy densities. The graphitic walls of the slit-shaped micropores in carbon materials have a unique interaction potential energy. Herein, we demonstrate the anomalous redox behavior of aromatic compounds physisorbed in the slit-shaped micropores of activated carbon (AC).1 In addition, we introduce the insight of the confinement effects.2

Experimental

4,5-Dihydroxy-1,3-benzene disulfonic acid disodium salt monohydrate (BQDS) was used as a redox active molecule. A flowable electrode was prepared by simply dispersing carbon material in 1 mM BQDS + 0.5 M H2SO4. Electrochemical measurements of the flowable electrodes were conducted using a rotating disk electrode connected to an automatic polarization system. A mirror-polished glassy carbon rod (diameter, 6 mm) was used as the working electrode, a Pt mesh as the counter electrode, and a reversible hydrogen electrode (RHE) as the reference electrode. Before the electrochemical measurements, break-in potential cycling between 0 and 1200 mV vs. RHE was conducted using a de-aerated flowable electrode at a scan rate of 200 mV s−1 for 100 cycles while rotating the working electrode (1600 rpm). The redox behavior of the flowable electrode was investigated by potential cycling at scan rates of 500–2 mV s−1. Before cyclic voltammetry, the working electrode dispersion was stirred at 2500 rpm for 2 min, and the measurement was commenced after 1 min to obtain reproducible data. In addition, the effect of the micropore geometry of AC on the adsorption-controlled redox reaction was investigated.2

Results and Discussion

The electrochemical behavior of various aromatic compounds was studied to demonstrate the proof-of-concept of a micropore-confined electrode reaction. The peak-potential separation (DE
p)of BQDSH2/BQDS redox reaction was 330 mV at 5 mV s-1. The value of DE
p decreased to 30 mV when carbon black (CB) with negligible microporosity was dispersed in the electrolyte. Remarkably, DE
p = 0 mV was observed when micropore-rich AC is dispersed in the electrolyte. Based on the X-ray scattering analysis and HRMC simulations, we found that the micropore-confined aromatic compounds behave as adsorbed species in an adsorption-based system. The charge/discharge performance of a two-electrode cell was investigated. AQDS/AC and BQDS/AC were employed as negative and positive electrodes, respectively. The features of the charge/discharge curves of the cell changed from battery-type to pseudocapacitor-type when these electrodes were used. This behavior is attributed to the rapid charge-transfer step of micropore-confined molecules compared with that of unconfined molecules.

In conclusion, we found that the adsorption-controlled process of the redox reaction for BQDS could be achieved using slit-shaped graphitic micropores with widths less than 1 nm in AC.

Acknowledgments

This work was partially supported by the Electrotechnology of Chubu, Kondo Memorial Foundation, and Tokyo Ohka Foundation for The Promotion of Science and Technology.

References

D. Takimoto, K. Suzuki, R. Futamura, T. Iiyama, S. Hideshima, and W. Sugimoto, ACS Appl. Mater. Interfaces, 14, 31131 (2022).

D. Takimoto, K. Suzuki, S. Hideshima, and W. Sugimoto, Electrochemistry, 91, 077006 (2023).