Effect of Carboxylic Acid-Doped Carbon Nanotube Catalyst on the

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Effect of carboxylic acid doped carbon nanotube catalyst on the performance of aqueous organic redox flow battery using the modified alloxazine and ferrocyanide redox couples Wonmi Lee, Byeong Wan Kwon, and Yongchai Kwon ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b10952 • Publication Date (Web): 09 Oct 2018 Downloaded from http://pubs.acs.org on October 11, 2018

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ACS Applied Materials & Interfaces

Effect of Carboxylic Acid Doped Carbon Nanotube Catalyst on the Performance of Aqueous Organic Redox Flow Battery Using the Modified Alloxazine and Ferrocyanide Redox Couples

Wonmi Lee, Byeong Wan Kwon and Yongchai Kwon*

Graduate school of Energy and Environment, Seoul National University of Science and Technology, 232 Gongneung-ro, Nowon-gu, Seoul, 01811, Republic of Korea.

* Corresponding authors. E-mail address: [email protected](Y. Kwon)

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Abstract Alloxazine and ferrocyanide are suggested as the redox couples for aqueous organic redox flow battery (AORFB). Alloxazine is further modified by carboxylic acid (COOH) groups (alloxazine-COOH) to increase the aqueous solubility and to pursue a desirable shift in the redox potential. For obtaining a better AORFB performance, the overall redox reactivity of AORFB should be improved by the enhancement of the rate determining reaction (RDR) of the redox couple. A carboxylic acid doped carbon nanotube (CA-CNT) catalyst is considered for increasing the reactivity. The utilization of CA-CNT allows for the induction of better redox reactivity of alloxazine-COOH due to the role of COOH within alloxazineCOOH as a proton donor, the fortified hydrophilic attribute of alloxazine-COOH, and the increased number of active sites. With the assistance of these attributes, the mass transfer of aqueous alloxazine-COOH molecules can be promoted. However, CA-CNT does not have an effect on the increase of redox reactivity of ferrocyanide because the redox reaction is not affected by the same influence of protons that the redox reactivity of alloxazine-COOH is affected by. Such behavior is proven by measuring the electron transfer rate constant and diffusivity. Regarding AORFB full cell testing, when CA-CNT is used as the catalyst for the negative electrode, the performance of the AORFB increases. Specifically, charge/discharge overpotential and IR drop potential are improved. As a result, voltage efficiency affected by the potentials increases to 64%. Furthermore, discharging capacity reaches 26.7Ahr·L-1, and the state of charge attains 83% even after 30 cycles.

Key words : aqueous organic redox flow battery; modified alloxazine; ferrocyanide; carboxylic acid doped carbon nanotube; rate determining reaction of redox couple


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1. Introduction Due to the growing concern about climate change from excessive use of fossil fuels and its effects on the environment, development towards renewable energy sources – such as wind and solar energies – has been in urgent demand.1,2 However, renewable energy sources characteristically thus far have had limitations due to the unpredictable amount of energy harvested.3,4 It is of import to mitigate these sorts of problems for the progression of renewable energy. One way to address the uncertainty of harvest is to develop large scale energy storage systems (ESS) which can store surplus energy. An example of a potential ESS is a redox flow battery (RFB) which uses metal-based active species, such as vanadium, iron, and chromium, and has received strong attention. It is known that the metal-based active species can offer outstanding benefits such as superior electron conductivity, good stability under acidic electrolytes, and high battery performance. Additionally, vanadium redox flow batteries (VRFB) can reduce the crossover of active species and show a long life span because of the use of the same active material in anolyte and catholyte. 5-9 The VRFB, however, has limitations regarding cost and operating temperatures. In terms of the cost, vanadium is very expensive because it is a rare metal. For example, to the cost in 2017 was $ 140 per Kwh, which was more expensive than other precious metals, such as Pd, Ag, and etc.10 In addition, its available temperature range is very narrow (10~40oC). If the operating temperature goes beyond this range, the V5+ ion is precipitated, resulting in a rapid degradation of the performance of VRFB.11 These factors negatively affect the balancing of plant (BOP) cost, which is required to be finely controlled. For example, precision designing for heat exchangers may be needed and an expensive BOP cost may become a big burden for easy market entry.



Thus, there have been many attempts to alleviate the problems of VRFB or replace it with a new type of flow battery, altogether. In this prospect, RFBs using organic materials dissolved in aqueous state as the active species is proposed as an alternative. For this purpose, different kinds of organic active species – such as quinones, aza aromatics (like alloxazine and quinoxaline), (2,2,6,6-Tetramethylpiperidin-1-yl) oxyl (TEMPO), violigen and polythiophene have been considered.12-19 RFBs consisting of organic active species can provide various benefits over conventional VRFB. Organic active species are relatively cheaper than their metal counterparts and can easily control the standard redox potential. They are usually considered less toxic and are also soluble under strong acidic/basic electrolyte conditions.20 Furthermore, the operating temperature range is not as crucial as it is for VRFB so, unlike vanadium ions, some of the organic active species candidates will not precipitate even at high temperatures. This advantage in acceptable temperature range enables the reduction in cost for BOP construction and the utilization of waste heat emitted from geothermal and combined heat and power (CHP).21 Although non-aqueous organic RFBs have been desirable for their high energy density due to high solubility in organic solvents and their wide potential window for operation, non-aqueous electrolytes have disadvantages such as lower ionic conductivity, limited current density, and higher cost than compared with aqueous electrolytes.22-26 Instead, when the organic species are used in an aqueous electrolyte, the problems of non-aqueous organic RFBs can be significantly alleviated and benefits such as high ionic conductivity, low cost, and high stability can be incorporated. Due to such merits, some research groups have studied RFBs using aqueous organic active species. For instance, Narayanan et al. introduced quinone-based materials as the active materials for RFB because the quinone could lead to a fast redox reaction due to its two-electron redox reaction mechanism, producing high coulombic efficiency (>95%). However, the open circuit voltage (OCV) (0.7V) was lower than that of VRFB (1.26V). In addition, its solubility in aqueous solution was low.27 In another case, Liu et al. used ferrocene modified by N-alkylation as the active species of the positive electrode. Although, initially, ferrocene has low solubility in aqueous solutions, solubility of the modified ferrocene significantly increased up to 4.0 M due to N-alkylation. When the modified


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ferrocene combined with methyl viologen was used as the redox couples, its cell potential reached 1.06V. However, due to the relatively complex N-alkylation process and the use of the toxic methyl viologen, this process needs improvement.28 In addition, Lin et al. reported an alkaline quinone flow battery using 2,6dihydroxyanthraquinone (DHAQ) and ferrocyanide/ferricyanide as the active species of the negative and positive electrode. Because ferrocyanide/ferricyanide has beneficial properties such as low toxicity and nonvolatility, cost-efficiency, nontoxicity, nonflammability, and safety, fabrication of AORFB using it is promising. According to their results, the current efficiency (CE) was above 99%, and the energy efficiency (EE) was 84% at 100mA∙cm-2.29 Aziz et al. first introduced alloxazine and ferrocyanide as the negative and positive active species for the AORFB. Because alloxazine does not solubilize well in water, it was modified by adding functional groups like COOH and OH to increase the solubility and shift the standard redox potential to a more desirable direction. Among them, in terms of aqueous solubility, the alloxazine modified by COOH (alloxazine-COOH) was the best as the active species for the negative electrode. Additionally, its synthetic procedure was simple. In this prospect, we conclude that the redox couple - alloxazine-COOH and ferrocyanide – is one of the mostly advanced redox couples for AORFB. In this study, alloxazine-COOH and ferrocyanide dissolved in KOH solution were also used as the active materials for RFB research. The redox reactions of the redox couple are shown in scheme 1.

Scheme 1. (a) redox reaction of alloxazine-COOH and (b) redox reaction of ferrocyanide.



In addition, we newly suggest the utilization of catalyst to enhance the redox reactivity of rate determining reaction (RDR) of the couple due to the lack of previously reported efforts. For doing that, we initially identify which one of the alloxazine-COOH and ferrocyanide-related redox reactions is the RDR then increase the reaction rate of the RDR by adopting a proper catalyst and evaluate how the catalyst progresses the reaction rate of RDR and the performance of AORFB. As the catalyst, carboxylic acid doped carbon nanotube (CA-CNT) is selected. The CA-CNT consists of a carbon-based material (carbon nanotube) and an oxygen functional group (carboxylic acid), which has good chemical stability against alkaline electrolyte and can be simply and easily applied to the electrode. In addition, the oxygen functional group of COOH can enhance hydrophilicity and catalytic activity of the electrode30,31.

2. Experimental 2.1 Synthesis of alloxazine and alloxazine-COOH CNT (Carbon nanotube, multi-walled, >98% carbon basis) and CA-CNT (Carbon nanotube, multiwalled, carboxylic acid functionalized, >8% carboxylic acid functionalized) powders that were utilized as


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catalysts were purchased from Sigma-Aldrich. For synthesizing alloxazine, o-phenlyenediamine and alloxan monohydrate were mixed with boric acid and acetic acid that were all purchased from Sigma-Aldrich. 20 mmol of o-phenylenediamine was initially dissolved into 170 mL of acetic acid with stirring, and 22 mmol of boric acid and 21 mmol of alloxan monohydrate were further added.32 The entire solution was then stirred. After the process, precipitated product was filtered, removed and washed with de-ionized (DI) water. For synthesizing the alloxazine-COOH, the 3,4-diaminobenzoic acid (purchased by Alfa Aesar) that was carboxylic acid functionalized o-phenylenediamine interacted with the alloxan monohydrate. The synthetic scheme of alloxazine and alloxazine-COOH is described in Fig. S1. The yield obtained during the synthesis of alloxazine and alloxazine-COOH was 90%. The synthesizing steps and conditions of alloxazine-COOH were the same to those of alloxazine. In addition, to determine the optimal mixing time for synthesizing the two materials (alloxazine and alloxazine-COOH), stirring time was controlled. As for the stirring time, a various time scale from 1 min to 6 h was designed and used. Furthermore, the following two different synthetic methods — identified as one-step and two-step methods – were used. In the one-step method, all of the materials (alloxan monohydrate, 3,4diaminobenzoic acid, boric acid, and acetic acid) were mixed simultaneously. In contrast, in the two-step method, 3,4-diaminobenzoic acid and acetic acid were initially mixed, then boric acid and alloxan monohydrate were added to the mixed solution with stirring for 3 h. The mixing time effect of the catalysts was also evaluated. Here, mixing indicates how long the alloxan monohydrate was held and reacted with 3,4-diaminobenzoic acid. As the mixing time, 2 and 24h were chosen.

2.2 Materials characterizations To identify whether carboxylic acid functional groups were well attached to the alloxazine, the



alloxazine sample consisting of carboxylic acid functional groups was analyzed by H NMR without further purification. The H NMR spectra was recorded using Varian Mercury 700 (700MHz) NMR spectrometers at 25 ℃. For the measurement, dimethyl sulfoxide (DMSO) was used as a solvent to dissolve the sample. To measure the solubility of all of the active species that were dissolved into 1 M KOH solution, UV-Vis spectrophotometry was used. Initially, the excessive quantities of active species (4g of each alloxazine and alloxazine-COOH) were dissolved in 4 mL of 1 M KOH. After that, the top of unprecipitated solution was collected and UV-Vis spectrum was measured by the UV-Vis spectrophotometry. A correlation of concentration versus absorbance peak was obtained by measuring the absorbance peak from the known concentration of each active species, and the five absorbance peaks that were extracted from different concentrations were used to form a linear calibration curve. The maximum solubility of each solution dissolved into 1M KOH was then estimated by the linear calibration curve. 2.3 Electrochemical measurements Cyclic voltammogram (CV) measurements were performed in the potential ranges of -1.2 ~ -0.4 V vs. Ag/AgCl for redox reaction of alloxazine-COOH and 0.2 ~ 0.8 V vs. Ag/AgCl for that of ferrocyanide which was obtained from the potassium ferrocyanide decahydrate purchased from Sigma-Aldrich using CHI 720D (CH Instruments, USA). For half-cell tests, Pt wire and Ag/AgCl were considered as the counter and reference electrodes, respectively, and a glass carbon disk (GCE) was used as the working electrode. As electrolytes, 0.1 M of both alloxazine-COOH and ferrocyanide was dissolved into 1.0 M KOH. In detailed explanation, 10 mL of each catalyst ink, which was prepared by mixing 10 mg catalyst powder with isopropanol, distilled water and 5% Nafion 117 solution, was loaded onto the GCE. To compare the redox reactivity of alloxazine and alloxazine-COOH, CV curves were measured at a scan rate of 10 and 100 mV∙s-1. These were also used to determine the (i) optimal condition of alloxazine-COOH, (ii) cell voltage obtained by the combination of alloxazine-COOH and ferrocyanide, (iii) reaction kinetics like diffusivity and electron transfer rate constant of alloxazine-COOH and ferrocyanide, and (iv) effect of CACNT catalyst on the redox reactivity of alloxazine-COOH.


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2.4 AORFB single cell measurements A scheme and actual cell kit of the AORFB single cell are depicted in Figure 1. For AORFB single cell tests, the solutions of alloxazine-COOH and ferrocyanide that were all dissolved into KOH were applied to negative and positive electrolytes. The catholyte contains 0.6 M alloxazine-COOH dissolved into 2.5 M KOH within a volume of 15 mL, while the anolyte contains 0.4 M potassium ferrocyanide and 0.04 M potassium ferricyanide dissolved into 1 M KOH within a volume of 56.3 mL. The active area of the AORFB single cell was 4 cm2 and a Nafion 117 membrane was used as the separator to reduce the crossover of active species and promote the proton conductivity.33 During charge and discharge steps, the current density was fixed at 60 mA·cm-2. Catalyst-coated carbon felts were used to evaluate effects of the catalysts on the AORFB performance. For catalyst coating, 40 mg of each catalyst was combined with a mixture consisting of 10.5 mL of ethyl alcohol and 0.5 mL of Nafion 117 solution. Then, the catalyst-coated carbon felts were dried at 80 °C. In addition, for investigating a correlation of open circuit voltage (OCV) and state of charge (SOC), the processing time was controlled for calculating the actual SOC of the AORFB single cell. Here, the actual SOC can be defined as the ratio of actual capacity to theoretical capacity, and this is calculated as a product of applied charging-time and current density.34 For doing that, the charging-time for obtaining the target SOC was applied to the corresponding AORFB full cell with an increase in voltage and, when the charging-time reached the predetermined time range, it was stopped for 30 s to measure OCV of the charging step.



Figure 1. The schematic of AORFB single cell

Results and discussion 3.1 Evaluation of redox reactivity of the organic redox couple species Regarding the redox couple, ferrocyanide was used as received, whereas alloxazine-COOH was modified from pure alloxazine as explained in the experimental section. For deciding the optimal alloxazine-COOH from pure alloxazine, the COOH mixing time, which can be considered as a dominant parameter to affect the modification of pure alloxazine, was performed at 5 min, 10 min, 1 h, 2 h, 3 h and 6 h. The redox reactivity of such synthesized alloxazine-COOHs was measured using CV curves that were scanned at the potential range of -1.2 ~ -0.4 V vs Ag/AgCl (Figure 2), while the chemical structure – which


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is measured to check whether COOH groups are well attached to alloxazine or not – were inspected by NMR (Figure S2). According to Figure 2, when the COOH mixing time was 2h, the peak current density (Ipa and Ipc), the ratio of peak current density (Ipa/Ipc) and the peak potential difference (ΔEp) – which are the main evidences of redox reactivity – were best in alloxazine-COOH and, thus, the alloxazine-COOH was selected as optimal and used for the subsequent processes. Even in the spectra obtained by NMR, the COOH groups were well attached to the alloxazine, forming proper alloxazine-COOH (Figure S2 a and b). According to the NMR analysis, three peaks of alloxazine-COOH were observed near 8.2-8.8 ppm, which means the COOH groups were well-formed.35 Conversely, there were not any peaks observed of pure-alloxazine in that region. In addition, the alloxazine-COOH samples prepared by both 2h one-step and 2h two-step processes (Figure S2 b) exhibited the same peak positions (Figure S2 c). On the other hand, the sample prepared by 24 h one-step process showed small peaks near 7.2-7.6 ppm. It is possible that these are from impurities or other by-products as previously reported (Figure S2 d).36 Ultimately, when the stirring time is too long, impurities or other by-products are probably produced during the formation of alloxazine-COOH.



Figure 2. (a) cyclic voltammograms for different mixing time effect on alloxazine-COOH redox reaction, (b) effect of stirring time on peak current densities representing redox reaction rate, (c) effect of stirring time on Ipa/Ipc, (d) effect of stirring time on peak potential separation. Another important parameter is the aqueous solubility of active species dissolved into KOH electrolyte (pH 14), because a high solubility induces the increases in energy density, power density, and AORFB performance. For the negative electrode, the attachment of COOH to pure alloxazine is supposed to increase its aqueous solubility in KOH electrolyte. To calibrate the aqueous solubility in purealloxazine and alloxazine-COOH samples, 2 g of each sample was dissolved into 2 mL of KOH electrolyte and the pure alloxazine-dissolved KOH and alloxazine-COOH-dissolved KOH were regularly extracted for solubility measurements and the results are represented in Figure 3. For the measurements, the absorbance peak detected at 432 nm was mainly recorded.37 According to the measurements, pure alloxazine was dissolved by ~0.15 M under 1M KOH, while that of alloxazine-COOH in 1M KOH was 2M. This shows that, with the addition of COOH, the solubility of alloxazine-COOH into 1 M KOH


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increases 13 times more than that of pure alloxazine. This is attributed to an increase in the hydrogen bonding between KOH and COOH by the addition of COOH and the improved hydrogen bonding subsequently increases the aqueous solubility of alloxazine-COOH dissolved into KOH.38 Furthermore, the OCV of alloxazine-COOH (anolyte) and ferrocyanide (catholyte) was shown to be 1.13 V, which is comparable with that of VRFB (1.26 V) (Figure S3).

Figure 3. The absorbance spectra of pure alloxazine-dissolved KOH and alloxazine-COOH-dissolved KOH measured using UV-Vis spectrophotometry. Next, it is important to determine which one of the alloxazine-COOH and ferrocyanide-related redox reactions is RDR since enhancing the redox reactivity of the RDR is a priority for increasing the performance of AORFB.39 To investigate the RDR, electron transfer rate constant (ks) and diffusion coefficient (D) are the major reaction kinetic parameters and are measured using Laviron’s and RandlesSevcik’s equations, respectively.40,41 For getting ks, multiple CV curves showing the redox reactivity of alloxazine-COOH and ferrocyanide are estimated under different scan rates and the correlations between scan rate and redox peak



currents (Ipa and Ipc vs. ν) and between logarithm of scan rate and difference in the redox peak potential and average peak potential (Epa –Eavg and Epc –Eavg vs. log ν) are represented in Figure 4 and S4. Laviron’s equation is used to calculate ks from the data. According to the calculation, ks of alloxazine-COOH and ferrocyanide is 0.002 and 0.044 s-1, respectively, indicating that alloxazine-COOH is the RDR. Besides the ks, D of the two active species is determined using the Randles-Sevcik’s equation: jp = 2.69×105z1.5D0.5v0.5c where jp is the peak current density, c is the concentration of active materials in the electrolyte, z is the number of electrons involved the electrode process, v is the potential sweep rate, and D is the diffusion coefficient of the active materials. To attain this value, a correlation between redox peak currents and logarithm of scan rate (Ipa and Ipc vs. ν1/2) is obtained. From the plot, D of alloxazine-COOH and ferrocyanide is determined as 5.9 x 10-6 and 5.6 x 10-6 cm2s-1, respectively. This means that the D of the two active species is similar to each other. Taken together, the redox reactivity of alloxazine-COOH was considered RDR and for increasing performance of AORFB, the redox reactivity of alloxazine-COOH should be improved.

Figure 4. Diagram showing the correlation of peak potential and logarithm of scan rate of (a) alloxazineCOOH and (b) ferrocyanide 3.2 Effect of catalyst on redox reactivity of the organic redox couple species


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To enhance the redox reactivity of alloxazine-COOH, new catalysts like carboxylic acid functionalized CNT (CA-CNT) and pure CNT are applied. The pure CNT is considered as a catalyst because the nanostructure of CNT may have a positive influence on the enhancement of the redox reactivity of alloxazine-COOH. In addition, with the utilization of CA-CNT – whose purpose is to increase the amount of hydrophilic groups, such as COOH and OH – the redox reactivity of alloxazine-COOH will be promoted further. To evaluate effects of the catalysts on the redox reactivity of alloxazine-COOH, CV curves are measured (Figure 5). According to Figure 5, when CA-CNT is used as the catalyst, the reaction rate of alloxazine-COOH (demonstrated as peak current density) increases more than that of alloxazine-COOH when tested without catalyst under similar experimental conditions. The theoretical mechanism explaining the effect of the CA-CNT catalyst on the redox reaction of alloxazine-COOH is presented in Figure 6. Two main reasons can be hypothesized. First, when the COOHs of the CA-CNT catalyst make contact with a strong base, such as KOH, they can act as proton donors and as a result, the metal carboxylate salts (COO- and K+) are produced, and the released proton easily interacts with the nitrogen of –C=N-C- in alloxazine-COOH, increasing the redox reactivity of alloxazine-COOH.42 Second, when it comes to hydrophilicity, the COOHs of CA-CNT make the catalyst more hydrophilic and this facilitates contact with the molecules of alloxazine-COOH due to improved mass transfer.43 In turn, this can lead to (i) an increase in the number of active sites put on the catalyst surface that can contact the aqueous anolyte and (ii) an increase in the reaction rate of alloxazine-COOH.



Figure 5. (a) Cyclic voltammograms , (b) anodic and cathodic peak current density, and (c) peak potential difference of alloxazine-COOH without catalyst and alloxazine-COOHs using CNT and CA-CNT catalysts


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Figure 6. The schematic showing how the CA-CNT catalyst affects the redox reactvity of alloxazineCOOH It is worth noting that unlike CA-CNT, the effect of CNT catalyst on the redox reactivity of alloxazine-COOH was not significant. This is because the CNT catalyst does not contain COOH groups promoting the redox reactivity of alloxazine-COOH. Despite this, due to its typical nanostructure that can produce a larger active surface area, the catalyst-sprayed carbon felt can lead to better electron transfer and redox reactivity than that without a catalyst.44 To more accurately quantify the effects of the corresponding catalysts on the redox reactivity of alloxazine-COOH, its kinetic parameters, such as ks and D, are again measured (Figures 7 and 8). In terms of ks, the result of alloxazine-COOH measured under the CA-CNT catalyst is 0.36 s-1, while its D is 9.4 x 10-4 cm2·s-1. In contrast, the results of alloxazine-COOH measured under the CNT catalyst and no catalyst



are 0.22 s-1 and 2.5 x 10-5 cm2·s-1 (CNT catalyst) and 0.002 s-1 and 5.9 x 10-6 cm2·s-1 (no catalyst). Indeed, with the adoption of CA-CNT catalyst, ks and D are 180 and 159 times more improved than measured without catalyst. Based on this, it is proven that the CA-CNT catalyst has a strong positive impact on enhancements in the kinetic parameters of alloxazine-COOH.

Figure 7. Cyclic voltammograms of (a) alloxazine-COOH without catalyst, (b) alloxazine-COOH using CA-CNT catalyst and (c) peak current density versus square root of scan rate of alloxazine-COOH without catalyst and alloxazine-COOHs using CNT and CA-CNT catalysts.


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Figure 8. Peak potential versus logarithm of scan rate of (a) alloxazine-COOH without catalyst and (b) alloxazine-COOH using CA-CNT catalyst. It should be noted that the CA-CNT catalyst was even applied to ferrocyanide for the positive electrode. However, this did not affect an increase in the redox reactivity of ferrocyanide. This is because the redox reaction of ferrocyanide is not influenced by the protons that mainly affect the redox reactivity of alloxazine-COOH. Such behavior was verified by measuring the CV curve (Figure S5). When the CV curves exhibiting redox reactivity of the three samples – ferrocyanide without catalyst, ferrocyanide with CNT and ferrocyanide with CA – were measured, there were no changes in redox reaction peak.

3.6 Performance evaluations of AORFB single cells using Alloxazine-COOH and Ferrocyanide redox couple According to the optimized catalytic activity and kinetic property data, the CA-CNT and CNT catalysts are useful as catalysts in the negative electrode of the AORFB single cell and it is necessary to investigate the effects of these catalysts on the performance of the AORFB single cell. For doing that, the AORFB single cells using CA-CNT and CNT catalysts were prepared and, as a control, the cell without catalyst was also fabricated. For the convenience of recognition, they were denoted as AORFBCA-CNT, AORFB-CNT and AORFB-No-Cat, respectively. Charge and discharge cycle tests of the AORFB single cells were initially performed and the result



is shown in Figure 9. According to Figure 9, AORFB-CA-CNT experiences the longest charge-discharge time during the entire cycles. For example, the total time of AORFB-CA-CNT for the initial four cycles was 14.1 h, while those of AORFB-CNT and AORFB-No-Cat were 13.4 and 12.2 h, respectively. In addition, in terms of the starting charge and discharge voltages, those of AORFB-CA-CNT, AORFBCNT and AORFB-No-Cat were 1.29 and 1.04 V (AORFB-CA-CNT), 1.30 and 1.03 V (AORFB-CNT), and 1.32 and 1.01 V (AORFB-No-Cat). There are several noticeable findings about the AORFB single cell test results. First, both charge/discharge overpotential and IR drop potential were improved with the use of catalysts, proving that the redox reaction of alloxazine-COOH – which was the RDS – was promoted by the catalysts. Second, in a comparison of CA-CNT and CNT catalysts, AORFB-CA-CNT resulted in a better performance than AORFB-CNT in terms of charge-discharge time and overpotential. As mentioned earlier, this is due to the proton donor role and the excellent hydrophilicity of CA-CNT. From Figure 9, it is expected that the VE and EE of AORFB-CA-CNT will be higher than of AORFB-CNT and AORFB-No-Cat. Thus, efficiencies of the AORFB single cells were measured and the result is represented in Figures S6 and S7. Although the CE was similar among them, the VE and EE of AORFB-CA-CNT were better than those of the others (the VE and EE of AORFB-CA-CNT, AORFBCNT and AORFB-No-Cat are 64% and 64%, 62% and 62%, and 60% and 60%, respectively). Similar CE indicates that the two active species (Alloxazine-COOH and Ferrocyanide) rarely cause crossover, and the crossover rate is even lower than that of VRFB using the same Nafion 117 membrane. This trend in efficiencies is well compatible with the trend in charge-discharge time and overpotential observed in Figure 9. Another parameter to support the effects of the catalysts on the performance of the AORFB single cell, discharging capacity, and SOC was considered (Figure 10). According to Figure 10, during the cycle test, those of AORFB-CA-CNT were best. Regarding the actual values, discharging capacity and SOC of AORFB-CA-CNT measured during 30 cycle were 26.7 Ahr·L-1 and 83%, respectively, while those of AORFB-CNT and AORFB-No-Cat were 25.2 Ahr·L-1 and 78% and 23.1 Ahr·L-1 and 72%, respectively.


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These results substantiate that the utilization of catalysts, especially CA-CNT, have a positive impact on the performance of AORFB-CA-CNT.

Figure 9. Charge-discharge curves of (a) AORFB-No-Cat, (b) AORFB-CNT, and (c) AORFB-CA-CNT measured for the initial four cycle and (d) charge-discharge curves at the first cycle of AORFB-No-Cat, AORFB-CNT, and AORFB-CA-CNT.



Figure 10. Discharging capacity and SOC of (a) AORFB-No-Cat, (b) AORFB-CNT and (c) AORFB-CACNT Figure S8 also shows the correlations between OCV and SOC of the AORFB single cells. All results demonstrated that OCV increased with an increase of SOC. This is because with an increase of SOC, the active species can be further consumed for the redox reaction and this reduces overpotential from playing a role in the degradation of OCV.45

Conclusion In this study, alloxazine and ferrocyanide were used as the active species for AORFB, with the additional modification of COOH to alloxazine (alloxazine-COOH). With the formation of alloxazineCOOH, both aqueous solubility and cell potential increased. For further improvement toward the overall redox reactivity and performance of AORFB, the RDR needed to be identified and an increase in its redox reactivity needed to be accomplished. To find out the RDR, kinetic parameters, such as electron transfer rate constant (ks) and diffusivity (D), were measured and, as a result, the redox reaction of alloxazine-COOH


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was determined to be the RDR. For increasing its reactivity, a CA-CNT catalyst was utilized. With the use of CA-CNT, a better redox reactivity of alloxazine-COOH was achieved. This was due to the proton donor role of COOH within alloxazine-COOH and the fortified hydrophilic attribute of alloxazine-COOH, as well as, the increased number of active sites by the adoption of CNT. In addition, the mass transfer of aqueous alloxazine-COOH molecules was also facilitated. The increase of redox reactivity of alloxazine-COOH by the use of CA-CNT was verified by measuring kinetic parameters (ks and D of alloxazine-COOH using CA-CNT catalyst were 0.36 s-1 and 9.4 x 10-4 cm2·s-1, respectively, while the values were 0.22 s-1 and 2.5 x 10-5 cm2·s-1 (alloxazine-COOH using CNT catalyst) and 0.002 s-1 and 5.9 x 10-6 cm2·s-1 (no catalyst)). Regarding the AORFB full cell tests, when the CA-CNT catalyst was adopted for the negative electrode, the performance of AORFB-CA-CNT increased. Charge/discharge overpotential and IR drop potential were improved, CE (99%) and EE (64%) also improved, and the discharging capacity and SOC of 26.7 Ahr·L-1 and 83%, respectively, were also observed even after the operation of 30 cycles. To summarize, an active species of AORFB, alloxazine-COOH was found to be the RDR and when a CA-CNT catalyst was newly adopted, both redox reactivity and mass transfer of the alloxazine-COOH were improved and these enhancements led to the further improvement in the performance of the AORFB full cell.

Acknowledgement This work was supported by the German−Korean joint SME R&D projects program of MOTIE/KIAT (No. 20151732) and by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) and the Ministry of Trade, Industry & Energy (MOTIE) of the Republic of Korea (20184030202230) and by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) and the Ministry of Trade, Industry & Energy(MOTIE) of the Republic of Korea (No. 20172420108550).

■ ASSOCIATED CONTENT Supporting Information



The Supporting Information is available free of charge on the ACS Publications website. It contains synthetic scheme, NMR, CV, efficiencies and OCV vs. SOC graph of Alloxazine and Alloxazine-COOH.

■ AUTHOR INFORMATION Corresponding Author *E-mail: kwony@ seoultech.ac.kr. ORCID Yongchai Kwon: 0000-0003-3118-401X

Notes The authors declare no competing financial interest.

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Table of Contents graphic




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Effect of Carboxylic Acid-Doped Carbon Nanotube Catalyst on the

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