Exploration of Charge Storage Behavior of Binder-Free EDL Capacitors in Aqueous Electrolytes

Bhupender Pal; Kalyan Jyoti Sarkar; Bing Wu; Lukáš Děkanovský; Vlastimil Mazánek; Rajan Jose; Zdeněk Sofer

doi:10.1021/acsomega.2c07143

Exploration of Charge Storage Behavior of Binder-Free EDL Capacitors in Aqueous Electrolytes

ACS Omega. 2023 Jan 5;8(2):2629-2638. doi: 10.1021/acsomega.2c07143. eCollection 2023 Jan 17.

Authors

Bhupender Pal¹, Kalyan Jyoti Sarkar¹, Bing Wu¹, Lukáš Děkanovský¹, Vlastimil Mazánek¹, Rajan Jose^{2

3}, Zdeněk Sofer¹

Affiliations

¹ Department of Inorganic Chemistry, University of Chemistry and Technology Prague, Technicka 5, 166 28 Prague 6, Czech Republic.
² Center for Advanced Intelligent Materials Technology, Universiti Malaysia Pahang, Kuantan 26300, Malaysia.
³ Faculty of Industrial Sciences & Technology, Universiti Malaysia Pahang, Kuantan 26300, Malaysia.

Abstract

Charge storage in electrochemical double-layer capacitors (EDLCs) is via the adsorption of electrolyte counterions in their positive and negative electrodes under an applied potential. This study investigates the EDLC-type charge storage in carbon nanotubes (CNT) electrodes in aqueous acidic (NaHSO₄), basic (NaOH), and neutral (Na₂SO₄) electrolytes of similar cations but different anions as well as similar anions but different cations (Na₂SO₄ and Li₂SO₄) in a two-electrode Swagelok-type cell configuration. The physicochemical properties of ions, such as mobility/diffusion and solvation, are correlated with the charge storage parameters. The neutral electrolytes offer superior charge storage over the acidic and basic counterparts. Among the studied ions, SO₄ ^2- and Li⁺ showed the most significant capacitance owing to their larger solvated ion size. The charge stored by the anions and cations follows the order SO₄ ^2- > HSO₄ ^- > OH^- and Li⁺ > Na⁺, respectively. Consequently, the CNT//Li₂SO₄//CNT cell displayed outstanding charge storage indicators (operating voltage ∼0-2 V, specific capacitance ∼122 F·g^-1, specific energy ∼67 W h·kg^-1, and specific power ∼541 W·kg^-1 at 0.5 A·g^-1) than the other cells, which could light a red light-emitting diode (2.1 V) for several minutes. Besides, the CNT//Li₂SO₄//CNT device showed exceptional rate performance with a capacitance retention of ∼95% at various current densities (0.5-2.5 A·g^-1) after 6500 cycles. The insights from this work could be used to design safer electrochemical capacitors of high energy density and power density.