Enhanced energy efficiency of aqueous organic redox flow batteries: carbon-based heterostructure electrodes guided by an interface engineering strategy†

Xiaohui Yang^a, Xiong Li^a, Tongle Xu^a, Hongyun Cai^bc, Can Zhao^c, Na Song^a and Peng Ding*^a
aResearch Center of Nanoscience and Nanotechnology, College of Sciences, Shanghai University, 99 Shangda Road, Shanghai, 200444, PR China. E-mail: dingpeng@shu.edu.cn
^bSuqian Unitechem Co., Ltd, 88 Yangzi Road, Suqian, 223800, PR China
^cSuqian Time Energy Storage Technology Co., Ltd, 67 Huashan Road, Suqian, 223800, PR China

First published on 6th August 2024

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1. Introduction

With the aim of meeting the sustainable development of mankind and achieving the large-scale utilization of renewable energy, aqueous organic redox flow batteries (AORFBs) have made a notable distinction among various energy storage technologies with their cost-effectiveness, high energy efficiency, and long service life.^1–4 However, the application of TEMPO/Methyl Viologen (TEMPTMA/MV) based AORFBs is severely hindered by low electrocatalytic activity and poor kinetic reversibility.^5–7 As a key component of the AORFB, the performance of the electrode directly affects the diffusion of active substances, the rate of electrochemical reactions, and the internal resistance of the battery, and then determines the overall performance of the battery.^8–11 Therefore, it is crucial to design the electrode systematically to fulfill the significant electrocatalysis activity requirements of the TEMPTMA/MV AORFB.

Carbon-based materials represented by carbon felt (CF) are considered to be the most feasible electrode materials in TEMPTMA/MV AORFBs due to their excellent strength, wide operating voltage range, three-dimensional interconnecting conductive network, and electrochemical stability.^12–15 However, the direct use of the original materials as electrodes failed to achieve ideal results, leading to the exploration of a variety of approaches to enhance battery performance by focusing on enhancing the electroactivity of the prepared electrodes. The decoration of CF electrodes primarily involves heteroatom doping,^16–21 activation of the electrode body,²² and the loading of carbon nanomaterials to construct multidimensional composite electrodes.^23,24 As a result, the treatments significantly enhance the electrocatalytic activity, expand the specific surface areas, and optimize the interfaces of the electrode–electrolyte. Such improvements are designed to promote the charge transfer rate on electrodes and accelerate the kinetics of the electrochemical reactions, accordingly increasing the overall energy efficiency, power output, and cycle life of the battery.^25–27 For example, Li et al. fabricated hollow porous carbon spheres from a Ni-MOF template, with subsequent application to graphite felt. The electrodes showed excellent electrocatalytic performance due to the hollow spheres providing multiple reaction sites for vanadium ion.²⁸ Wu et al. proposed a composite electrode by dispersing wrinkle-like carbon (WLC) sourced from Aspergillus niger onto graphite felt. The introduction of heteroatoms enhanced the charge transfer capability, leading to the improved electrocatalysis activity of the electrode.²⁹ He et al. used the electrodeposition method to coat graphite felt with polyaniline nanofibers to obtain a nitrogen-doped carbon electrode. The collaboration between carbon nanonetworks and nitrogen atoms increased the reaction sites, ensuring a steady path for efficient charge transfer.³⁰ These studies demonstrate that the improved electrochemical activity of the fabricated electrodes can be attributed to the balanced dispersion of doped heteroatoms and the construction of multiple structures, ultimately resulting in superior overall performance in batteries.

Herein we propose a novel reduced graphene oxide-carbon felt (rGOCF) constructed from a heterostructure through an interface engineering strategy. The heterostructures of rGOCF electrodes were primarily composed of one-dimensional fibers and two-dimensional nanosheets, doped with oxygen atoms. The interface engineering strategy was introduced to prevent the agglomeration of GO sheets on the electrode, which effectively increased the number of interfaces within the heterostructure, resulting in an enlarged specific surface area of the electrodes. The uniform distribution of heteroatoms and the construction of carbon-based heterostructures in rGOCF effectively enhanced charge transfer within the heterointerface network. Additionally, the atomic rearrangement and abundant lattice defects at the interfaces could accelerate ion migration in TEMPTMA/MV AORFBs. As a result, the rGOCF electrodes with heterostructures presented an energy efficiency of 80.05% for 200 cycles at a current density of 50 mA cm⁻². The performance of rGOCF not only demonstrates the feasibility of being an electrode material to promote the widespread application of AORFBs, but also highlights the universality of using heterostructure electrodes in various novel broad-scale energy storage systems.

2. Experimental section

2.1 Materials and equipment

Carbon felt (CF) was provided by Suqian Liansheng Technology Co., Ltd (Suqian, China). Graphene oxide (GO) was provided by Hangzhou Gaoxi Technology Co., Ltd (Hangzhou, China). Absolute ethanol was provided by Sinopharm Chemical Reagent Co., Ltd. The ultrasonic bath (JP-030S) was provided by Skymen Technology Corporation Limited (Shenzhen, China). The vacuum oven (DZG-6050D) was provided by Shanghai Sen Xing Experimental Instrument Co., Ltd (Shanghai, China). The tubular furnace (XD-1400S) was provided by Zhengzhou Brother Furnace Co., Ltd (Zhengzhou, China).

2.2 Characterization

The morphology and crystallographic features of both CF and rGOCF were observed by scanning electron microscopy (JSM-7500F, JEOL, Japan) and transmission electron microscopy (JEM-F200). The internal structure of the material was further observed using a confocal Raman spectrometer (INVIA) at a laser wavelength of 633 nm. The BET surface area of the materials was determined using an Quantachrome Autosorb-iQ2 after degassing at 300 °C for 8 h. The elemental composition was characterized using an X-ray photoelectron spectrometer (AXIS ULTRFDLD) with Al Kα as the radiation source. The crystal lattice structure was studied using an XRD system (D/max-2200/PC, Rigaku, Japan) with the rate of 20° min⁻¹ from 10° to 90°. A four-point probe meter (CXT2665) was employed to determine the electrical conductivity of materials.

2.3 Electrochemical measurements

Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) tests were carried out with a CHI604E electrochemical workstation (Shanghai Chenhua, China). The three-electrode system was operated at room temperature, carbon felt with the size of 1 cm × 2 cm was employed as the working electrode, while the saturated calomel electrode (SCE) functioned as the reference electrode and platinum gauze electrode as the counter electrodes. The supporting electrolyte was 0.5 M NaCl. The CV tests involved setting the voltage windows at −0.8 to −0.5 V for the anode and 0.4 to 1.1 V for the cathode, with a scanning speed of 1 mV s⁻¹. During the EIS testing, reactions were analyzed from 10⁵ Hz to 10⁻² Hz with a 5 mV amplitude for both positive and negative parts.

The electrochemical performance of TEMPTMA/MV AORFB with a carbon felt area of 5 cm² (22 mm × 22 mm) as both cathode and anode was explored. AMVN was chosen to serve as the ion exchange membrane. The positive electrolyte contained 1 M TEMPTMA + 0.5 M NaCl and the negative electrolyte contained 1 M MV + 0.5 M NaCl. Each of the positive and negative electrolytes was stored separately with a capacity of 20 ml. The battery operated with a flow rate of 100 ml min⁻¹. The charging and discharging tests were conducted on an NEWARE Battery Test System (CT-4008Tn-5V12A-S1, Shenzhen, China) with the potential from 0.90 V to 1.50 V.

2.4 Preparation of graphene oxide-modified carbon fiber felt

The CF was sectioned into small squares of 22 mm × 22 mm, followed by an ethanol soak and 90 minutes of ultrasound to eliminate organic impurities. The prepared CF was subjected to a thermal drying process within an oven set at 70 °C for 8 h and then ready for subsequent utilization. The GO sheets were introduced on the CF via liquid impregnation and thermal reduction methods. Different amounts of GO dispersions were diluted and subjected to ultrasound irradiation for 30 min to obtain uniform dispersions of 1 mg ml⁻¹, 3 mg ml⁻¹, and 5 mg ml⁻¹. Due to the hydrophobic properties of the CF, the prepared CF was immersed in the GO dispersions for vacuum treatment to ensure complete saturation. Subsequently, the GO sheets were uniformly deposited on the CF after ultrasonic treatment for 30 min. Excess solution was then removed and dehydrated in a 70 °C oven for 12 h. The dried CF was subjected to high-temperature thermal reduction in a muffle furnace, heated at a rate of 10 °C min⁻¹ to 500 °C, sustained for 3 h, subsequently cooled naturally. The pristine carbon felt is labeled CF, and the carbon felt samples modified with various concentrations of rGO sheets were named rGOCF-1, rGOCF-3, and rGOCF-5 respectively. The detailed preparation processes of rGOCF electrodes are shown in Fig. 1 below.

3. Results and discussion

The morphology of rGO sheets on CF was observed by the SEM characterization. Fig. 2(a) and (d) show that the carbon fibers within pristine CF exhibited clean surfaces without impurities. Through liquid phase impregnation and thermal reduction process, the rGO sheets were effectively coated on the CF to form rGO layers, with the deposited thickness increasing as the concentration of the GO dispersion increased from 1 to 5 mg ml⁻¹ (Fig. S1†). Based on the regulation of the interface engineering strategy, the rGOCF-3 electrode was uniformly coated with rGO layers. As shown in Fig. 2(b) and (e), the rGO layers formed a tightly interconnected layered structure on the carbon fibers, enriching the reactive sites at the heterointerface and enhancing the adsorption capacity of active substances on the heterostructure electrode. Through the thermal reduction method, the reduced graphene oxide and carbon fibers were bonded via covalent chemical bonds and physical adsorption.^31,32 The detailed heterostructure of rGOCF was characterized by high-resolution transmission electron microscopy (HRTEM). Fig. 3(c) presents the presence of additional layers on the CF surface, confirming the deposition of the rGO layers. The HRTEM image in Fig. 3(f) displays lattice fringes that demonstrate an orderly transition between rGO and CF, with a lattice spacing of 0.352 nm, which corresponds to the lattice spacing of the (002) plane of graphene. The consistency between TEM images and SEM images showed that the surface of CF was wrapped with a thin layer of reduced graphene oxide to form heterostructure electrodes. Additionally, Fig. 2(g) displays the EDS mappings of the rGOCF-3 electrode. The surface of the rGOCF-3 heterostructure electrode was composed of carbon and oxygen, indicating that the introduction of rGO sheets could effectively increase the oxygen content of the heterostructure electrode. Besides, it could be inferred that the formation of the heterostructure might be related to the interactions between oxygen or hydrogen containing functional groups at defect sites on the CF and rGO sheets.

XPS was used to detect the elemental components, O and C atoms, within the electrodes. As depicted in Fig. 3(a) and (d), the rGOCF electrode exhibited a higher O/C ratio compared to the pristine CF, demonstrating the effective wrapping of CF by rGO sheets and the formation of the rGOCF heterostructure electrode. The results of the rGOCF-3 electrode showed a distinct O 1s peak and an O/C ratio of 17.89%, which was higher than that of rGOCF-1 (16.5%) and rGOCF-5 (14.53%), reflecting a rich distribution of oxygen groups on its surface. Fig. 3(b) and (c) illustrate that the C 1s peak at 284.8 eV was deconvoluted to reveal four separate peaks: OC–OH (288.5 eV), CO (285.8 eV), C–O (284.8 eV), and C–C (284.1 eV).^33–35 The ratios of peak areas in Fig. 3(e) were calculated to quantify the various functional groups on the surface of electrodes. With the rise in GO dispersion concentration, variations in the areas of the four peaks indicated the formation of various carbon bond configurations in the heterostructure electrode. The areas of four peaks changed with an increasing concentration of the GO dispersions, demonstrating that different amounts of carbon bonding configurations were formed on the surfaces of the heterostructure electrodes. The surface of the rGOCF-3 heterostructure electrode possessed an abundance of oxygen-containing groups through interfacial regulation, distinguished by its elevated concentration of CO functional groups. Studies have found that the oxygen functional groups act as catalysts, which promote the absorption of active substances on the electrode surface and accelerate the redox reactions.^31,36 Fig. 3(f) displays the conductivity of various electrodes. Due to the restoration of the π–π stacked structure and electron configuration following the reduction of GO, the conductivity of all electrodes was notably increased.^37,38 The rise in conductivity effectively diminished the resistance at the interfaces of electrodes and bipolar plates.

To investigate the variation in the structure of pristine CF and rGOCF electrodes, XRD and Raman spectroscopy measurements were taken. As shown in Fig. 4(a), all samples, CF, rGOCF-1, rGOCF-3, and rGOCF-5, revealed two distinct peaks at 1334 and 1598 cm⁻¹ in the Raman spectra, corresponding to the defect carbon (D peak) and graphitic carbon (G peak) respectively.^39,40 The relative intensity of the D peak to the G peak (I_D/I_G) quantifies the disorder and order of materials. A higher I_D/I_G ratio indicates more defects present on the electrode surface. The rGOCF-3 heterostructure electrode showed an increased I_D/I_G ratio than CF, suggesting more lattice defects and an increase in edge-functionalized oxygen atoms, which was consistent with the results of XPS. The XRD spectrum in Fig. 4(b) shows two prominent characteristic peaks at approximately 25.4° and 42.7°, corresponding to the graphite's (002) and (100) planes.^41,42 The stability of the characteristic diffraction peaks in both position and intensity suggested that the rGO sheets had no effect on the primary structure of the graphite phase in the CF substrate.

N₂ adsorption–desorption isotherms were used to quantitatively determine the changes in porosity and BET surface area of samples. From Fig. 4(c), the electrodes exhibited type IV isotherm characteristics at relative pressure, with a steep H2-type hysteresis loop present, indicating the coexistence of mesopores and macropores. Fig. 4(d) demonstrates that through the regulation of the interface engineering strategy, the coating of rGO sheets in the form of a thin layer greatly increased the BET surface areas of various electrodes. The BET surface area of the electrodes followed the order of rGOCF-3 > rGOCF-5 > rGOCF-1 > CF. This result indicated that the rGOCF-3 heterostructure electrode possessed more active sites on its surface compared to CF. In addition, Fig. 4(e) illustrates the contact angle measurements for each electrode. Since the electrolyte was an aqueous solution, the test was conducted using water instead of the electrolyte. The readings were obtained following a 4 s period with the drop on the samples. The CF electrode exhibited strong hydrophobic properties, with the contact angle reaching 141°. Conversely, rGOCF-1, rGOCF-3, and rGOCF-5 exhibited better lyophilic properties due to the heterostructure formed by the introduction of rGO nanosheets. Enhanced wettability enlarged the interface area of the electrode–electrolyte, resulting in better efficiency of the TEMPTMA/MV AORFB.

The high catalytic activity of the heterostructure electrodes towards both cathode and anode electrolytes was confirmed by CV curves. The CF impregnated with different concentrations of GO dispersions showed varying electrochemical activities. Fig. 5(a) and (d) show that the redox peak currents of all rGOCF electrodes were higher than those of CF exhibiting excellent reversibility. In particular, the CV curves of the rGOCF-3 heterostructure electrode displayed the best redox reversibility with the highest peak current intensity for both the cathode and anode sides. As shown in Table S1,† the redox reactions of positive on rGOCF-3 (I_pa = 15.86 mA, I_pc = 13.57 mA) exhibited larger peak currents than pristine CF (I_pa = 11.19 mA, I_pc = 8.86 mA). The difference in peak potential (ΔE_p = E_pa − E_pc) between the anode and cathode of rGOCF-3 was 0.082 V for the negative sides, which was lower than that of the CF (0.087 V). It was evident from the results that rGOCF-3 showed enhanced electrocatalytic activity and increased conductivity. Moreover, a consistent trend of rGOCF-3 > rGOCF-5 > rGOCF-1 > CF was observed in both the O/C ratios and electrochemical activity of the rGOCF heterostructure electrodes, suggesting that the introduction of oxygen atoms might be linked to faster redox reactions at the electrode in AORFBs.

Consequently, rGOCF-3 was regarded as the best electrode for more detailed research. To obtain kinetic parameters of the redox reactions for both the cathode and anode sides, CV curves for both the pristine CF and rGOCF-3 were recorded at different scan rates, respectively. The electrochemical parameters for both CF and rGOCF-3 heterostructure electrodes are shown in Fig. 5(b) and (e). Based on the Randles–Sevcik equation,^43,44 the CF and rGOCF-3 electrodes exhibited a strong linear relationship between peak currents and the square root of the scan rate, confirming that the reactions were governed by diffusion.^45,46 Fig. 5(c) and (f) show that rGOCF-3 presented larger slopes compared to the pristine CF. Thus, it was suggested that faster mass transfer processes occurred on the rGOCF-3 heterostructure electrode. In Table S2,† the diffusion coefficients (D values) for the positive part on the rGOCF-3 electrode were recorded as 6.13 × 10⁻⁴ cm² s⁻¹ and 4.7 × 10⁻⁴ cm² s⁻¹, while those on the CF were 2.9 × 10⁻⁴ cm² s⁻¹ and 1.94 × 10⁻⁴ cm² s⁻¹, respectively. For the negative part, the D value of rGOCF-3 also showed a larger trend than the CF electrode. Moreover, the well-known Nicholson method enabled the determination of the electron transfer rate constants (k) for CF and rGOGF-3 electrodes.⁴⁵ For the rGOCF-3 electrode, the k values were recorded at 2.77 × 10⁻² and 5.3 × 10⁻² cm s⁻¹ for oxidation and reduction parts, exceeding the CF electrode's values of 2.37 × 10⁻² and 2.98 × 10⁻² cm s⁻¹.

Furthermore, EIS tests were performed to quantify the resistance of the electrodes. As shown in Fig. 6(a) and (b), a semicircle part and a linear part could be observed in the Nyquist diagram of both CF and rGOCF electrodes. The semicircle represents a regime governed by kinetic controls and the linear part represents a diffusion controlled regime.⁴⁷ The corresponding fitting results are presented in Fig. 6(c), R_s is the electrode resistance and the R_ct is the charge transfer resistance. The rGOCF-3 heterostructure electrode showed the lowest R_s and R_ct values compared with other electrodes, which proved that rGOCF-3 had better electrochemical activity in redox reactions. The consistency between the EIS and CV results indicated that the rGOCF-3 heterostructure electrode possessed improved catalytic activity.

Fig. 6(d) illustrates that the catalytic performance of rGOCF heterostructure electrodes for organic reactions in the TEMPTMA/MV AORFB could be attributed to the following: exploiting the dual characteristics of rGO sheets with a large specific surface area and rich oxygen vacancy, the graphene sheets were introduced into three-dimensional carbon felt to form a carbon heterostructure through an interface engineering strategy. The precise regulation of the interfaces between carbon fibers and graphene sheets increased the interface area of the electrode–electrolyte, thereby improving the solid–liquid affinity. During the reaction, the construction of the carbon-based heterostructure facilitated the formation of rich networks of heterogeneous interfaces to enhance charge transfer. Additionally, the carbon-based heterostructure effectively utilized the intrinsic hydroxyl groups to form hydrogen bonds with electrolyte molecules, thereby promoting the capture of active substances on the heterostructure electrode surfaces and accelerating the reaction kinetics.

The TEMPTMA/MV AORFB was assembled to thoroughly investigate the charging and discharging characteristics of the CF and rGOCF-3 heterostructure electrodes. Fig. 7(a) shows the charge–discharge curves of the rGOCF-3 and CF electrodes at a current density of 50 mA cm⁻². In particular, the rGOCF-3 heterostructure electrode exhibited superior charge–discharge performance, with a higher discharge voltage plateau and a lower charging voltage plateau compared to the CF electrode. Fig. 7(b)–(d) illustrate the battery performance analysis by calculating the voltage efficiency (VE), energy efficiency (EE), and discharge capacity at different current densities (30, 50, 70 mA cm⁻²). It has been observed that the battery equipped with the rGOCF-3 electrode exhibited ultra-high EE and VE, with the disparity in efficiency between the CF and rGOCF-3 heterostructure electrodes notably enlarging as current densities increase. Furthermore, the discharge capacity of the battery with the rGOCF-3 heterostructure electrode also displayed enhanced performance at higher current densities. The improvements were primarily ascribed to the rapid electron transfer at the electrolyte–electrode interface and the highly effective catalytic performance of rGO sheets. Surprisingly, the battery with rGOCF-3 demonstrated an EE of 80.05% at a high current density of 50 mA cm⁻², while the CF was 76.41% (Fig. 7(e)).

The stability of CF and rGOCF-3 heterostructure electrodes during prolonged cycling was assessed by analyzing the CE, VE, EE, and discharge capacities, as presented in Fig. 7(f)–(h). Fig. 7(f) and (h) show that the CF electrode exhibited lower EE and discharge capacity, which decreased with increasing number of cycles due to the lower electrocatalytic activity toward the active substance. In comparison, the rGOCF-3 heterostructure electrode maintained a stable energy efficiency after operating for 200 cycles at a current density of 50 mA cm⁻². Moreover, the rGOCF-3 heterostructure electrode sustained a consistent discharge capacity through 200 cycles, with a decrease of 0.322 mA h for each cycle. As shown in Fig. S5,† the electrode structure remained intact after 200 cycles, and the rGO sheets maintained without falling off. This demonstrated the strong adhesion of rGO to carbon fibers, thus providing the basis for the outstanding long-term performance of rGOCF-3. The performances of AORFBs with electrodes prepared in this study were compared with other carbon electrodes, and the comparison results are summarized in Table S3.† Overall, the interface engineering strategy enabled the uniform distribution and stable bonding of rGO sheets on CF, along with enhanced catalytic activity, which collectively facilitated rapid electron transfer at heterointerfaces. As a result, the rGOCF-3 heterostructure electrode exhibited outstanding durability and high energy efficiency.

4. Conclusion

In summary, the rGOCF heterostructure electrode was successfully prepared by liquid phase impregnation and a thermal reduction method through an interface engineering strategy. The interface engineering strategy was employed to adjust the electrode surface structure to ensure that the rGO sheets were uniformly coated on the electrode in the form of a thin layer. Compared to the pristine CF electrode, the heterostructure electrodes exhibited significant enhancements in electrocatalytic activity in TEMPTMA/MV AORFBs. The expansion of the specific surface area caused by the construction of the heterogeneous interfaces could increase the number of active sites. The introduction of oxygen functional groups led to improved wettability, which effectively shortened the diffusion path for active substances and accelerated the redox reaction kinetics. The TEMPTMA/MV AORFB employing the optimized heterostructure electrode, rGOCF-3, demonstrated remarkable improvements in energy efficiency of 80.05% and voltage efficiency of 80.46%, at the current density of 50 mA cm⁻² over 200 cycles. The heterostructure electrode constructed by the interface engineering strategy proposed in this work demonstrates promise for use as a high-performance electrode in AORFBs.

Data availability

The data supporting this article have been included as part of the ESI.†

Author contributions

Xiaohui Yang: conceptualization, formal analysis, writing (original draft), data curation, methodology; Xiong Li: formal analysis, writing (review and editing), Tongle Xu: methodology, writing (review and editing); Hongyun Cai: formal analysis; Can Zhao: methodology; Na Song: data curation, methodology; Peng Ding: formal analysis, writing (review and editing), funding acquisition.

Conflicts of interest

There are no conflicts of interest to declare.

Acknowledgements

This work was financially supported by the following funds: the National Key Research and Development Program of China (2022YFB3707800), National Natural Science Foundation of China (No. 52073168), and Key Program of Science and Technology of Yunnan Province (No. 202302AB080022).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4ta04723k

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