Confinement Effects on the Rate Performance of Redox Active Molecules for Pseudocapacitive Flowable Electrodes

Abstract

The pseudocapacitive flowable electrodes typically show high energy density because of the contribution of the faradaic charge of redox-active organic materials and the electric double layer charge of carbon materials. However, the redox reaction kinetics of organic molecules are slow due to poor diffusion kinetics. We recently reported that a pseudocapacitive flowable electrode exhibited bell-shaped cyclic voltammograms (peak separation (ΔE_p) = 0 mV); specifically, the molecules were confined within slit-shaped graphitic micropores of activated carbon (AC). Herein, we studied the relationship between charge storage and the reaction mechanism to tailor the electrochemical performance of a pseudocapacitive flowable electrode by half-cell study. The results show that the redox reaction of the confined molecules entailed a charge-transfer-controlled mechanism, while the unconfined molecules exhibited a mass-transfer-controlled system. This difference inhibited the fast charging and discharging of the pseudocapacitive flowable electrode. This study demonstrates that half-cell studies are crucial for clarifying the relationship between the charge storage and rate performances of pseudocapacitive flowable electrodes.

1. Introduction

Flow-assisted electrochemical systems, including redox flow batteries,¹ organic redox flow batteries,² and electrochemical flow capacitors,³ have been extensively studied to develop advanced grid-scale energy storage technologies. Electrochemical flow capacitors, which consist of carbon-based flowable electrodes, incorporate the advantageous characteristics of both flow batteries (high energy densities) and supercapacitors (high charge/discharge rates). Furthermore, they are promising candidates for high-rate grid applications, which are currently under development. Recently, pseudocapacitive flowable electrodes have been reported to achieve enhanced energy densities. Such electrodes are composed of redox-active materials (metal oxides⁴ or organic molecules⁵) and carbon materials. The energy density of pseudocapacitive flowable electrodes is typically higher than that of electrochemical flow capacitors because of the additional contribution of the faradaic charge of redox-active materials. Recently, the use of organic materials as redox-active materials for pseudocapacitive flowable electrodes has been widely studied.^6,7 However, the redox reaction kinetics of organic molecules are slow because of poor diffusion kinetics and slow electron transfer rates. These parameters critically impact the rate capability, and consequently, they must be maximized. Although the electron transfer rates of organic materials can be improved through molecular engineering,⁸ the diffusion kinetics remain inadequate owing to a technological barrier.

Various approaches for the suppression of the diffusion of redox-active organic materials have been reported based on their adsorption onto the electrode surface. Examples of such approaches include those based on the π–π stacking interactions between quinone-based molecules and a carbon surface,⁶ electrostatic interactions between the functional groups of molecules and the oxygen-containing functional groups on a carbon surface,⁹ and methods that employ covalent attachment.¹⁰ However, a small peak-potential separation (ΔE_p) is observed in these systems (ΔE_p > 10 mV). This could be attributed to the concentration overpotential arising from the weak adsorption of the molecules on the electrode.

We recently reported a pseudocapacitive flowable electrode composed of activated carbon and quinone-based molecules, which exhibited bell-shaped cyclic voltammograms (ΔE_p = 0 mV, an adsorption-controlled system).¹¹ We also proposed that an adsorption-controlled reaction system could be attributed to the confinement effects of aromatic compounds adsorbing on the micropores of <1 nm for carbon materials based on the hybrid reverse Monte Carlo simulations. The slit-shaped micropores caused the tight packing of molecules confined to the carbon walls by physisorption. Specifically, the quinone-based molecules were confined via strong π–π stacking interactions. The developed pseudocapacitive flowable electrode exhibited rapid charge/discharge characteristics, attributable to the adsorption-controlled reaction system of the micropore-confined molecules. Confinement effects can enhance the rate performance of quinone-based molecules; however, improving the energy storage is necessary to develop advanced grid-scale energy storage devices that require both energy and power densities.

In this study, we investigated the optimum condition of the flowable electrode to balance the energy storage and rate performance by focusing on the relationship between charge storage and the reaction mechanism of the aforementioned pseudocapacitive flowable electrode in order to tailor its electrochemical performance. To understand this relationship, the amount of the redox-active molecules was varied. 4,5-Dihydroxy-1,3-benzene disulfonic acid disodium salt monohydrate (BQDS), which is widely employed in redox flow batteries, was used as the redox-active material. The cyclic voltammograms associated with the confined (charge-transfer-controlled mechanism) and unconfined molecules (mass-transfer-controlled mechanism) were distinguished based on the b-value analysis.¹² Notably, the amount of charge stored in the pseudocapacitive flowable electrodes increased with the amount of BQDS, while the charge associated with the capacitive contribution decreased with increasing the BQDS concentration. The rate-determining steps of confined and unconfined BQDS were proton-coupled electron transfer and mass transfer, respectively. This paper provides insights into the redox reaction mechanism and serves as a design guide for pseudocapacitive flowable electrodes with a high energy density and rate performance.

2. Experimental

2.1 Reagents

BQDS (Sigma-Aldrich) was used as the electroactive molecule. Slit shaped graphitic micropore–rich carbon (AC; MSP-20; Kansai Coke and Chemicals Co.) and carbon black (CB; Vulcan XC-72, Cabot Corp.) were used as the carbon materials for the flowable electrode. H₂SO₄ was purchased from FUJIFILM Wako Pure Chemical. Milli-Q deionized water (>18.2 MΩ cm, Millipore) was used in all experiments. All materials were used as received, without further purification.

2.2 Compositional analysis

Adsorption–desorption isotherms were obtained using a volumetric apparatus (Quantachrome Autosorb–1), with N₂ gas as the adsorbate, at 77 K. The specific surface area, pore size distribution, and average micropore width of the powders were determined by non-local density functional theory (DFT) calculations. The amount of BQDS adsorbed on the AC was determined by ultraviolet–visible (UV-vis) spectroscopy (V-530, JASCO Corporation). The AC (400 mg) was dispersed in n mM BQDS (n = 1–10) + 0.5 M H₂SO₄ (200 mL). The suspension was stirred (15 min) and sonicated (30 min). Subsequently, it was centrifugated at 4000 rpm (2600 × g) for 30 min, and the supernatant was analyzed using UV-vis spectroscopy (190–600 nm, 400 nm min⁻¹).

2.3 Electrochemical measurements

Electrochemical measurements were conducted with a rotating disk electrode (SC–5, Nikko Keisoku) connected to an automatic polarization system (HSV–110, Hokuto Denko Corporation). A mirror-polished glassy carbon rod (ϕ = 6 mm), a Pt mesh, and a reversible hydrogen electrode (RHE) were used as the working, counter, and reference electrodes, respectively. A pseudocapacitive flowable electrode was prepared by dispersing the carbon materials (400 mg) in a mixture of 1 mmol dm⁻³ (mM) BQDS and 0.5 M H₂SO₄ (200 mL). Before the electrochemical measurements, potential cycling was conducted for 100 cycles at 298 K in a deaerated pseudocapacitive flowable electrode at scan and rotation rates of 200 mV s⁻¹ and 1600 rpm, respectively. The redox behaviors of the pseudocapacitive flowable electrodes were investigated by potential cycling at scan rates of 500–2 mV s⁻¹. The working electrode was rotated at 2500 rpm for 2 min before cyclic voltammetry, and measurements were initiated after 1 min to obtain reproducible data.

3. Results

The ratios of adsorbed and dissolved BQDS in the pseudocapacitive flowable electrode, determined by UV-vis spectroscopy, are summarized in Fig. 1. CB was used as the control sample, and the amount of BQDS adsorbed on the CB was negligible (∼0.03 mg/mg-carbon). Notably, the amount of adsorbed BQDS in the case of BQDS(1 mM)/AC was 6.6 times greater than that for BQDS(1 mM)/CB. This is attributed to the larger surface area of AC (2223 m² g⁻¹) than that of CB (235 m² g⁻¹).¹¹ Furthermore, the N₂ adsorption–desorption measurements showed a decrease in the micropore volume of the AC,¹¹ indicating that the BQDS adsorbed on the slit-shaped graphitic micropores of the AC. Based on these results, the amount of adsorbed BQDS could be increased by using AC with a large surface area and high micropore volume. The ratio of dissolved BQDS increased with the BQDS content in the pseudocapacitive flowable electrode. The similar result was obtained from other activated carbon materials (Fig. S1), indicating that the ratio change was not specific to this AC. The amount of BQDS adsorbed on the AC at equilibrium was 0.34 mg/mg-carbon. At a high BQDS concentration, BQDS adsorbed not only on the micropores but also on the meso- and macropores and outer surface of the AC.

Figure 1.

Ratio of adsorbed BQDS and dissolved BQDS for BQDS(1 mM)/CB and BQDS(n mM)/AC (n = 1, 3, 5, and 10).

The cyclic voltammograms of the pseudocapacitive flowable electrode are shown in Fig. 2. The cyclic voltammogram obtained for 1 mM BQDS + 0.5 M H₂SO₄ without any carbon material is shown in Fig. 2a. The reduction and oxidation peaks of BQDS are observed at 770 mV and 1100 mV vs. RHE, respectively. The peak-potential separation (ΔE_p) is 330 mV at a scan rate of 5 mV s⁻¹. Although i_pa was proportional to v^0.5 (Figs. S2 and S3), ΔE_p was larger than 59/n mV (n is the number of electrons involved in the reaction; n = 2), indicating that the charge transfer kinetics of dissolved BQDS was slow. Thus, the redox reaction of dissolved BQDS corresponded to an irreversible system. In the case of BQDS(1 mM)/CB, the ΔE_p value decreases to 30 mV (Fig. 2b), which could be attributed to the adsorption of BQDS onto the conductive carbon, thereby enhancing the proton-coupled electron transfer. In addition, BQDS(1 mM)/CB was a mass-transfer-controlled system owing to the weak adsorption of BQDS on the carbon surface. The ΔE_p value of BQDS(1 mM)/AC was 0 mV (Fig. 2c, adsorption-controlled system), in agreement with the literature.¹¹ In contrast, the current density was slightly different from that reported in a previous study.¹¹ This was attributed to the change in the amount of active materials of BQDS/AC due to flowable properties such as gravity and fluid nature. The total pore volume of AC was higher than that of CB, especially micropore volume.¹³ Our ongoing studies focus on investigating the origin of the adsorption-controlled reaction system of BQDS(1 mM)/AC by analyzing the micropore effect, and the results will be reported in the future. The ΔE_p value of BQDS(n mM)/AC (n ≥ 3) gradually increased with the BQDS content in the pseudocapacitive flowable electrode (Figs. 2d–2f), suggesting the presence of unconfined BQDS in the pseudocapacitive flowable electrode at high BQDS contents. The BQDS(10 mM)/AC reaction system entailed a mass-transfer-controlled mechanism (Figs. S2 and S3), whereas BQDS(n mM)/AC (3 ≤ n ≤ 5) was an intermediate reaction system between an adsorption-controlled and a mass-transfer-controlled system. This result could be attributed to the combined effects of confined and unconfined BQDS in the pseudocapacitive flowable electrode.

Figure 2.

Cyclic voltammograms of (a) 1 mM BQDS + 0.5 M H₂SO₄ (reproduce from Ref. 11), (b) BQDS(1 mM)/CB, (c) BQDS(1 mM)/AC, (d) BQDS(3 mM)/AC, (e) BQDS(5 mM)/AC, and (f) BQDS(10 mM)/AC dispersed in 0.5 M H₂SO₄ at a scan rate of 5 mV s⁻¹.

The currents associated with the mass-transfer- and adsorption-controlled processes were separated based on a b-value analysis.¹⁴ The currents obey the power law:   

\begin{equation} i = av^{b} \end{equation} (1)

where i is the measured current and v is the scan rate. Both a and b are adjustable parameters, and the b value can be determined from the slope of the plot of log(i) vs. log(v). If the b value is close to 0.5, the electrochemical behavior is diffusion controlled. By contrast, if the b value is close to 1.0, the electrochemical behavior entails a capacitive process (surface-controlled process). The b value of BQDS(1 mM)/AC at 0.9 V, the half-wave potential of the redox reaction of BQDS, was 0.94 (Fig. 3a), indicating that the observed current predominantly corresponds to a capacitive system (surface-controlled or capacitor-like kinetics). Furthermore, the b value decreased upon increasing the BQDS content in the pseudocapacitive flowable electrode; the b values of BQDS(3 mM)/AC, BQDS(5 mM)/AC, and BQDS(10 mM)/AC were 0.76, 0.70, and 0.60, respectively. This indicates that the redox reaction gradually changes to a diffusion-controlled reaction with an increase in the BQDS content in the pseudocapacitive flowable electrode.

Figure 3.

(a) b value of BQDS(n mM)/AC (n = 1–10) at 0.9 V vs. RHE and a scan rate of 5 mV s⁻¹, where b ≈ 1 indicates a capacitive process (surface-controlled or capacitor-like kinetics). (b) Stored charge of BQDS(n mM)/AC (n = 1–10) at a scan rate of 5 mV s⁻¹.

The total charge increased gradually with the BQDS content of the flowable electrode (Fig. 3b). Because the flowable electrode is composed of AC and BQDS, the surface oxidation charge and electric double layer charge of AC were also considered in the total charge. The cyclic voltammograms of BQDS(n mM)/AC (n = 1–10) for the electric double-layer charge of AC did not drastically change (Fig. S4), suggesting that the electrochemically active area of AC does not change substantially. Based on the b value analysis, the diffusion contribution was gradually increased with increasing BQDS content. In addition, based on Fig. 1, the ratio of dissolved BQDS increased with the BQDS content of the pseudocapacitive flowable electrode. Based on the results, the enhanced stored charge at high BQDS content could be attributed to the large amount of dissolved BQDS.

4. Discussion

The design guide of the flowable electrode for fast charge/discharge performance (capacitive contribution) and high stored charge is discussed. The results demonstrate that the relationship between the charge storage and the reaction mechanism of pseudocapacitive flowable electrodes is as follows: Charge storage can be enhanced by using large-surface-area carbon and by increasing the BQDS content in a pseudocapacitive flowable electrode. However, when the BQDS content is high, the reaction mechanism is dominated by a charge-transfer-controlled to a mass-transfer-controlled one. This change inhibits the fast charging and discharging of the flowable electrode. Therefore, comprehending the relationship between the charge storage and rate performance of pseudocapacitive flowable electrodes is crucial.

The availability of the concentration effects on the other aromatic compounds is discussed. This study used BQDS as a typical redox molecule widely studied as a positive electrode material for organic redox flow batteries. We previously demonstrated that the confinement effects were not only obtained from BQDS and other aromatic compounds.¹¹ Although the amount of adsorbed aromatic compounds is different owing to the molecular mass and structure, the concentration effects on the reaction mechanism could be obtained by other aromatic compounds.

Next, we discuss the aspect of the development of the carbon material to increase the amount of adsorbed quinone-based molecules. The capacitive contribution was obtained from the low BQDS content through b value analysis, which could be attributed to the adsorbed molecules on the micropores of AC. Based on the simulation results of a previous study,¹¹ we believe that the confinement effects are induced by the <1-nm-sized micropores of AC; however, the effective size of the confined effects cannot be experimentally shown at this point because AC has a wide pore width distribution. Thus, the investigation concerning the effective size to obtain confinement effects is necessary. Furthermore, the micropore volume (<1 nm) comprising the total pore volume of AC was 20–30 %.^15,16 This suggests that the adsorption site of confined molecules is small, which restricts the amount of adsorbed molecules. Therefore, the carbon material with a high <1-nm-sized micropore volume is necessary to enhance the total stored charge and rate performance.

5. Conclusions

We studied the relationship between the charge storage and reaction mechanism to tailor the electrochemical performance of pseudocapacitive flowable electrodes. The ratio of dissolved BQDS on AC in the flowable electrode decreased with increasing BQDS content of the flowable electrode. The cyclic voltammograms of BQDS(1 mM)/AC were indicative of a dominantly capacitive contribution, while the diffusion contribution increased with increasing BQDS content of the flowable electrode. These results suggest that the amount of dissolved BQDS affect the reaction mechanism, and the difference in the mechanisms inhibited the fast charging and discharging of the pseudocapacitive flowable electrode. This study demonstrates the importance of half-cell studies in clarifying the relationship between the charge storage and rate performance of pseudocapacitive flowable electrodes. Notably, the charge storage and rate performance can be improved by increasing the number of confined molecules, which can be achieved by synthesizing carbon materials with high-volume <1-nm-sized micropores.

Acknowledgment

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

Data Availability Statement

The data that support the findings of this study are openly available under the terms of the designated Creative Commons License in J-STAGE Data at https://doi.org/10.50892/data.electrochemistry.22344970.

• 1) The pseudocapacitive flowable electrodes typically show high energy density because of the contribution of the faradaic charge of redox-active organic materials and the electric double layer charge of carbon materials. However, the redox reaction kinetics of organic molecules are slow due to poor diffusion kinetics. We recently reported that a pseudocapacitive flowable electrode exhibited bell-shaped cyclic voltammograms (peak separation (ΔE_p) = 0 mV); specifically, the molecules were confined within slit-shaped graphitic micropores of activated carbon (AC). Herein, we studied the relationship between charge storage and the reaction mechanism to tailor the electrochemical performance of a pseudocapacitive flowable electrode by half-cell study. The results show that the redox reaction of the confined molecules entailed a charge-transfer-controlled mechanism, while the unconfined molecules exhibited a mass-transfer-controlled system. This difference inhibited the fast charging and discharging of the pseudocapacitive flowable electrode. This study demonstrates that half-cell studies are crucial for clarifying the relationship between the charge storage and rate performances of pseudocapacitive flowable electrodes.

CRediT Authorship Contribution Statement

Daisuke Takimoto: Conceptualization (Lead), Funding acquisition (Lead), Investigation (Lead), Supervision (Lead), Validation (Lead), Writing – original draft (Lead), Writing – review & editing (Lead)

Keisuke Suzuki: Data curation (Lead), Investigation (Lead), Writing – original draft (Supporting), Writing – review & editing (Supporting)

Sho Hideshima: Investigation (Supporting), Supervision (Supporting), Validation (Supporting), Writing – review & editing (Equal)

Wataru Sugimoto: Investigation (Supporting), Supervision (Supporting), Validation (Supporting), Writing – review & editing (Equal)

Conflict of Interest

The authors declare no conflict of interest in the manuscript.

Funding

Research Foundation for the Electrotechnology of Chubu

Kondo Memorial Foundation

Tokyo Ohka Foundation for The Promotion of Science and Technology

Footnotes

A part of this paper has been presented in the 89th ECSJ Meeting in 2022 (Presentation #1J08).

D. Takimoto: Present address: Department of Chemistry, Biology and Marine Science, Faculty of Science, University of the Ryukyus, 1-Senbaru, Nishihara, Nakagami, Okinawa 903-0213, Japan

S. Hideshima: Present address: Department of Applied Chemistry, Faculty of Science and Engineering, Tokyo City University, 1-28-1 Tamazutsumi, Setagaya, Tokyo 158-8557, Japan

D. Takimoto and S. Hideshima: ECSJ Active Members

W. Sugimoto: ECSJ Fellow

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