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Multimodal porous and nitrogen-functionalized electrode based on graphite felt modified with carbonized porous polymer skin layer for all-vanadium redox flow battery
Materials Today Energy 11 (2019) 159e165
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Multimodal porous and nitrogen-functionalized electrode based on graphite felt modified with carbonized porous polymer skin layer for all-vanadium redox flow battery Dae-Soo Yang a, Jae Hee Han a, b, Jun Woo Jeon a, Jang Yong Lee a, Dong-Gyun Kim a, Dong Hack Seo b, Byoung Gak Kim a, Tae-Ho Kim a, *, Young Taik Hong a, ** a b
Advanced Materials Division, Korea Research Institute of Chemical Technology (KRICT) 141 Gajeong-ro, Yuseong-gu, Daejeon 305-600, Republic of Korea Department of Chemical Engineering, Hanyang University, 17 Haengdang-dong, Seongdong-gu, Seoul 133-791, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 13 August 2018 Received in revised form 13 November 2018 Accepted 15 November 2018
Novel nitrogen-functionalized multimodal porous graphite felt (GF) electrodes for all-vanadium redox flow batteries (VRFB) have been developed using a simple binder-free fabrication method. We synergistically combined a polymer of intrinsic microporosity (PIM), as a carbon and nitrogen precursor, with vapor-induced phase separation. The GF with a carbonized PIM skin layer showed a high surface area and multimodal pore architecture featuring interconnected micro-, meso-, and macropores. The enhanced electrochemical reactivity and wettability, and excellent electronic conductivity of the prepared electrodes successfully improved the overall kinetics of the redox reactions of the vanadium ion species by providing highly active catalytic sites and efficient ion and electron transport pathways. This resulted in the outstanding performance of VRFB single cells using this material as electrodes. © 2018 Elsevier Ltd. All rights reserved.
Keywords: Vanadium redox flow battery Porous carbon High performance electrode Redox chemistry
1. Introductions Due to the growing integration of renewable energy sources into the existing electricity grid, optimization of the storage and subsequent release of electrical energy plays a significant role in improving the whole power system [1e4]. Vanadium redox flow batteries (VRFBs) have attracted considerable attention due to their high capacity, fast response time, long cycle life, flexible cell configuration, and low maintenance, making them an ideal energy storage solution for load shifting in power grids, as well as in wind and solar energy conversion systems [5e10]. VRFBs exploit the fact that vanadium can exist in solution with four different oxidation states and store electrical energy via the redox reactions of the vanadium ion species contained in two separate electrolyte solutions. As a key component, the electrodes in each half-cell provide active sites for the redox reactions involving the vanadium ions, which directly affect the cell
* Corresponding author. ** Corresponding author. E-mail address: [email protected] (T.-H. Kim). https://doi.org/10.1016/j.mtener.2018.11.003 2468-6069/© 2018 Elsevier Ltd. All rights reserved.
performance of the VRFB system [11e14]. According to previous studies of the redox reaction mechanism for VRFBs, the electrochemical processes during charging/discharging comprise multiple electron transfer and ion exchange steps between vanadium ions in the electrolyte and the protons from functional groups on the electrode surface. This highlights the significance of developing electrode materials with both high electronic conductivity and a large specific surface area and surface functionality [15e18]. Among the many carbon-based materials evaluated as VRFB electrode materials, carbon felt (CF)- and graphite felt (GF)-based electrodes are preferable due to their high electronic conductivity, stability under acidic conditions, and corrosion resistance. In addition, their three-dimensional macroporous structure lowers the electrolyte flow resistance and provides mechanical stability at a relatively low cost [8,19,20]. Nevertheless, pristine CF and GF have porous structures with low specific surface areas and poor surface functionality, limiting their electrochemical activity in vanadium electrolytes [21]. Various techniques have been used to increase the activity of CF and GF electrodes, including thermal and chemical treatments, electrochemical activation, plasma or microwave treatments, and nitrogen doping; these methods focus on the development of oxygenated and/or nitrogenated functional groups on the surface of
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CF and GF, which act as reactive sites towards vanadium redox reactions [9,18,22e27]. However, these approaches face the persistent challenge of providing a well-balanced pore architecture combining micro-, meso-, and macropores [28,29]. For example, the use of strong bases as activating agents creates microporous structures with high oxygen functionality, but is not effective in the formation of mesopores [30]. Considering the mass transport through micropores, the reaction kinetics also depend on the mass transfer rate of the diffusion of vanadium ions at the electrolyte and electrode interface [31]. During charging (discharging), fast diffusion of V3þ(V2þ) and VO2þ(VOþ 2 ) ions from the bulk anolyte and catholyte, respectively, into the active sites on each electrode surface is beneficial to improve the kinetic reversibility of the redox reactions. In this regard, optimal pore networks should allow efficient vanadium ion transport while hosting sufficient active sites, which can be realized in multimodal porous structures featuring interconnected micro-, meso-, and macropores. Although microporous and mesoporous materials such as Ketjenblack carbon, porous graphenes, mesoporous carbon, and metal-organic frameworks (MOFs) have been investigated for various electrochemical applications [32e35], the development of carbon electrodes with multimodal porous architectures, particularly for VRFB applications, remains relatively immature. Polymers of intrinsic microporosity (PIMs) feature a continuous microporous network as a consequence of their fused-ring and ladder-like structures integrated with contortion sites. This structure provides uniform and inter-connected micropores (<2 nm) and a high surface area (300e1000 m2 g1) [36]. Typically the chemical structure of PIM-1 is shown in Fig. S1. In contrast to other intrinsically microporous materials, such as mesoporous silica, zeolite, and MOFs, PIMs have advantageous form-factor flexibility, as well as good solubility/processability in organic solvents, facilitating the use of methods such as spin-coating, solution casting, and electrospinning [37e40]. Recently, it was found that these materials can maintain their microporosity and structural integrity even after being carbonized (due to their intrinsic bonding characteristics), which broadens their use as precursors for microporous carbon materials [41]. Carbonized PIM-1 shows uniformly sized micropores with an average diameter <1 nm, while the specific surface area of carbonized PIM-1 can be tuned by controlling the carbonization conditions, e.g., temperature and time [42,43]. Herein, we developed a simple and efficient strategy for preparing GF-based hierarchically porous carbon electrodes for use in high performance VRFBs, with fairly uniform and wellinterconnected multimodal pore architectures. We synergistically combined the use of PIM-1 polymer as a carbon and nitrogen precursor and the vapor induced phase separation (VIPS) method. A PIM-1 solution was coated on the surface of the GF fibers by a simple dip-coating method, and then the coated solution was phase-inversed using VIPS, where the complex diffusion and convection behavior of solvent/vapor molecules introduced hierarchical porosity into the carbon precursors [44,45]. After film stabilization and carbonization, a carbon skin with multimodal porosity featuring both micro- and mesoporous characteristics was successfully produced on the surface of GF fibers. Importantly, during the carbonization, N atoms can be simultaneously incorporated into the resulting carbon skin layer due to the presence of two nitrile groups in the repeating unit of PIM-1, this results in a heteroatom doping effect that increases the final electrochemical activity of the material. Therefore, nanostructuring and subsequent carbonization provide a simple, binder-free and template-free methodology for producing GF-based multimodal porous electrodes, while avoiding complicated and tedious templating and activation processes. Increased specific surface area, wettability,
and electrochemical reactivity, and excellent electronic conductivity of the prepared hybrid electrode resulted in outstanding performance of a VRFB single cell using the produced electrode material. To the best of our knowledge, this is the first time that a carbon electrode with multimodal porous architecture has been applied for VRFB applications. The polymerization of PIM-1 (hereafter abbreviated to PIM) was carried out following procedures in the literature and confirmed using nuclear magnetic resonance (NMR) spectroscopy (Fig. S1) [42]. The molecular weight and polydispersity index (PDI) of the synthesized PIM was analyzed using gel-permeation chromatography, yielding a number-average molar mass of 42 300 g mol 1 and a PDI of 1.94. Fig. 1a illustrates the basic fabrication process of the novel GF material with the carbonized PIM (cPIM) skin layer. Pristine GFs were first treated with ozone to remove excess surface-adsorbed O2 molecules and then immersed into PIMs solutions with various concentrations dissolved in chloroform (nonpolar solvent). Higher concentrations of the polymer solution increased the thickness of the cPIM skin layer deposited on the GF. During the VIPS process, the PIM solution on the surface of the GFs was immediately exposed to water vapor (polar solvent), where instantaneous precipitation and separation occurred due to the phase separation phenomena, producing mesoporous, but mechanically robust, cPIM skin layers. The obtained precursors were then dried at 25 C, followed by carbonization at 900 C in an argon atmosphere, and further treatment at 300 C for 2 h in an O2 atmosphere to generate the oxygen functional groups; the carbonization temperature was determined to optimize both the electrical conductivity as well as nitrogen content of the resulting cPIM skin layer (Fig. S2 and Table S1). The resulting GF electrodes with cPIM skin layers were labeled PIMVC@GF-Xs, where X (0.5, 1.0, or 2.0) is the concentration of the PIM coating solution. It should be noted that no binder was necessary to produce this porous carbon skin layer on the GF electrode. We hypothesize that this binder-free process can significantly reduce the Ohmic resistance of the resulting electrode, as discussed later. To evaluate the effect of the VIPS-induced mesoporous structure, a carbonized PIM-coated GF material was prepared without VIPS processing in a similar way to [email protected]; this control sample is referred to as [email protected]. Details of the polymer synthesis and sample preparation are described later in the Experimental section. 2. Results and discussion The surface morphology of the pristine GF and PIMVC@GF-Xs were studied using field-emission scanning electron microscopy (FESEM) (Fig. 1bef). The pristine GF showed a smooth and featureless surface (Fig. 1b); such a defect-free surface has been reported to be unsuitable for VRFB applications due to its poor electrochemical activity [46]. Meanwhile, the PIMVC@GF-Xs samples clearly showed the presence of thin cPIM skin layers on the surface of the GF fibers (Fig. 1cee). It is important to note that the concentration of PIM in the coating solution greatly affects the coating uniformity, skin layer thickness, and surface morphology. When a low concentration (0.5 wt%) of PIM was used, the cPIM skin layer did not uniformly cover the surface of the GF fibers; residual uncoated areas were observed in Fig. 1c. However, a high concentration of the coating solution (2.0 wt%) resulted in the formation of a thicker cPIM skin layer; however, this concentration was thought to be excessive as the film uniformity was poor, with agglomerated particles and thorn-like flakes (Fig. 1e). However, the [email protected] sample was considered to have the optimal PIM concentration, as the formed cPIM skin layer had a moderate thickness and relatively uniform surface morphology, as shown in Fig. 1d. Although a textured surface with obvious spots (thought to be
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Fig. 1. (a) Schematic illustration of the PIMC @ GF-X electrode fabrication process. SEM images of the (b) pristine GF (c) PIMC@ GF-0.5, (d) PIMC@ GF-1.0, (e) PIMC@ GF-2.0, (f) high magnification image of PIMC@ GF-1.0 highly mesoporous structures can be seen.
formed during the coating process) was observed by SEM on the [email protected] surface, we observed full surface coverage of the GF by the cPIM skin layer using high-magnification SEM (Fig. S3). Moreover, from the high-magnification SEM image of the [email protected] surface (Fig. 1f), it was confirmed that the cPIM skin layer had a mesoporous structure with pores of tens of nanometers, as expected. It is important to note that this mesoporous structure of the carbonized PIM skin layer was observed only for PIMVC@GFXs prepared using the VIPS process; the control sample prepared without the VIPS process ([email protected]) showed no such mesopores (Fig. S4). This multimodal pore structure of the PIMVC@GF-X samples is expected to decrease the transport resistance of the vanadium ions and expedite their adsorption under an applied potential. The pore characteristics of the prepared electrodes were further studied using N2 adsorption. Fig. S5 shows the N2 adsorptione desorption isotherms for all prepared electrodes. The specific surface area of the electrodes was calculated using the BrunauereEmmetteTeller (BET) equation. For the pristine GF, a negligible specific surface area (<1 m2g-1) was observed, confirming the absence of surface micropores. However, the specific surface area values of the PIMVC@GF-X samples significantly increased with increasing PIM precursor content (157, 240, and 301 m2g-1, for [email protected], [email protected], and [email protected], respectively). This demonstrates that the addition of the cPIM skin layer effectively introduces microporosity on the GF surface. Considering the very small relative weight of the cPIM skin layer compared to the total weight of [email protected] (~1.0 wt%), such large increases in the specific surface area for [email protected] are unprecedented. We believe that the high surface areas of PIMVC@GF-X samples mainly originated from the intrinsically microporous nature of the PIM precursor; the specific surface area of the pure carbonized PIM-1 sample (without GF) prepared under the same carbonization conditions was as high as 1297 m2g-1. Unlike pristine GF, which did not
show any isotherms or a hysteresis loop, all PIMVC@GF-Xs showed typical type-IV isotherms with H3 hysteresis loops, demonstrating the presence of micro- and mesopores, in good agreement with SEM observations. The textural parameters of all PIMVC@GF-X, pristine GF, and [email protected] samples are summarized in Table S2. Notably, [email protected] synthesized under optimal conditions showed a considerable volume of mesopores. The mesoporous structure of the PIMVC@GF-Xs is expected to be beneficial for enhancing the redox reaction kinetics by providing efficient pathways for the fast absorption/desorption of the vanadium ions through mesopores. Raman spectroscopy was conducted to investigate the crystallinity of the carbon in the PIMVC@GF-X samples. As seen in Fig. S6, all PIMVC@GF-Xs showed the D and G bands typical of carbon materials at 1350 cm1 and 1590 cm1, respectively. It was seen that the peak intensity of the D band increased with increasing X in the PIMVC@GF-X samples, indicating the amorphous nature of the PIM-derived carbon. In contrast to pristine GF, which showed a highly crystalline (graphitic) nature, carbonized PIM-1 has been shown to have an amorphous structure [42]. The chemical nature of the cPIM-coated GF materials was studied using X-ray photoelectron spectroscopy (XPS). Fig. 2a shows the XPS survey spectra of the pristine GF and [email protected] samples. Typical C, O, and N peaks were obtained for the [email protected] electrode, which clearly showed that the N atoms were simultaneously incorporated into the cPIM skin layer during carbonization. Therefore, the N content increased with increasing cPIM skin layer volume, as shown in Table S3; [email protected], [email protected], and [email protected] had N contents of 0.54 at.%, 1.18 at.%, and 1.54 at.%, respectively. The binding arrangements of the N dopant were identified using high-resolution XPS. The deconvolution of the N spectra of [email protected] yielded four subpeaks with binding energies of 398.1, 399.6, 401.2, and 403.2 eV, which corresponded to pyridinic-N, pyrrolic-N, quaternary N, and
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Fig. 2. (a) XPS survey scan of pristine GF and the PIMC@ GF-1.0 and (b) high resolution N 1S and (c) O 1s spectra. Contact angle measurement for (d) pristine GF and e) PIMC@ GF-1.0 taken by pouring deionized (DI) water onto each CFE surface. (f) Electrical conductivity measured against pressure for all the prepared samples.
terminal N-O, respectively (Fig. 2b). Since pyridinic and pyrrolic N have a lone pair of electrons, they can donate their electrons to a carbon p conjugated system and consequently reduce the band gap of the resulting doped carbon [47]. Therefore, the nitrogen species can have a profound effect on enhancing the electrical conductivity of the doped carbon. Furthermore, the nitrogen functionality is believed to promote the reduction reaction due to the presence of the extra electrons from the N atoms, and also faciliate oxidation by positively charging neighboring carbon atoms [48]. Hence, we propose that the presence of such nitrogen species in the prepared PIMVC@GF-X electrodes can significantly enhance their activity for the redox reactions involving vanadium ions. Moreover, [email protected] showed significant oxygen functionalities, as seen from the O1s spectra in Fig. 2c. The amount of oxygen moieties in [email protected] and pristine GF was found to be 9.64 and 6.64 at.%, respectively. Importantly, from the deconvolution of the O spectra, it was concluded the higher oxygen functionality of [email protected]:[email protected] compared to pristine GF mainly arised from higher number of C]O bonds. As C]O functional groups are directly associated with the redox reactions of vanadium ions, the resulting electrode is expected to have improved electrocatalytic performance [9]. We examined the influence of these chemical and structural changes due to the introduction of cPIM skin layer on the hydrophilicity/wettability and electrical conductivity of the electrodes. Fig. 2d and e compare the contact angles of the pristine GF and [email protected]. Pristine GF, composed of graphitic carbon with high hydrophobicity, showed a contact angle of 106.2 , indicating its poor wettability in an aqueous electrolyte system. Surprisingly, in the case of [email protected], the water droplet was immediately absorbed, demonstrating its excellent wettability. Such clear improvement in the wettability of the electrode was attributed to the enhanced interaction between water molecules and the nitrogen and oxygen functional groups present in the cPIM skin layer [9,22].
Electronic conductivity measurements (conducted using a pressure-variable cell to ensure the accuracy) showed that the PIMVC@GF-X samples maintained the excellent conductivity of the GF-based material despite the various modification processes, including coating and heating steps. Thermal treatments are often employed to modify the surface properties of GF electrodes, but they generally degrade the conductivity of GF [19]. The pristine GF electrode used this study also showed a drastic decrease in conductivity after thermal treatment (Fig. 2f). However, all PIMVC@GFX samples showed higher conductivities than the heat-treated GF; in particular, [email protected] and [email protected] showed only slightly lower values than that of the pristine GF. The binder-free fabrication process allowed the high electronic conductivity to be maintained in our system. Thus, multimodal porous carbon electrodes with both high surface functionality and electronic conductivity were successfully realized. The catalytic activity of the prepared electrodes was measured using cyclic voltammetry (CV) experiments, for both negative and positive reactions, i.e., the V2þ/V3þ and VO2þ/VOþ 2 conversion, respectively, as shown in Fig. 3. Using CV analysis, the catalytic activity for the vanadium redox reactions can be analyzed through parameters such as the potential difference (DV ¼ Vpa-Vpc), oxidation and reduction peak current density (Ipa and Ipc), and redox onset potential. The CV curves for the negative electrode reaction of the VRFB, corresponding to V2þ/V3þ redox couples, are shown in Fig. 3a. No reduction peak and a very small oxidation peak were obtained for the pristine GF electrode, indicating that the pristine GF electrode had very poor catalytic activity and irreversibly oxidized the vanadium ions. On the contrary, a pronounced anodic peak, between 0.3 to 0.6 V, and a reversible cathodic peak, between 0.9 to 1.0 V, was observed for the PIMVC@GF-X electrodes, demonstrating that the presence of the cPIM skin layer enhanced the electrocatalytic activities of the electrodes. The improvement in the reversibility of the ion adsorption on PIMVC@GF-Xs was attributed to suppression of undesirable side
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Fig. 3. CV curves for the Pristine GF, and PIMVC@ GF at a scan rate of 5 mV s1 within a potential window of a) 1.1 to 0.5 V in the anolyte and b) 0.3e1.3 V in the catholyte (0.1 M V3þ and VO2þ dissolved in 1 M H2SO4) versus an Ag/AgCl reference electrode.
reactions, such as the hydrogen evolution reaction. Fig. 3b shows the CV curves for the positive electrode reaction, i.e., for VO2þ/VOþ 2 redox couples, for the prepared electrodes. The Ipa and Ipc values for the PIMC@GF-X electrodes were much higher than those of the pristine GF electrode. The DV and redox onset potential values of the PIMVC@GF-X and pristine GF electrodes showed the following order: [email protected] > [email protected] > [email protected] > [email protected] > pristine GF. To further elucidate the effect of the mesoporous structure, the CV performance of [email protected] was observed, which showed that this electrode had the most facile reversibility for the VO2þ/VOþ 2 redox reaction. Furthermore, the improved onset potentials for both the cathodic and anodic reactions on the [email protected] electrode, indicated favorable electron transfer kinetics and improved energy storage capability. Superior electrocatalytic performance of the [email protected] electrode, as compared to its similarly prepared counterparts, was attributed to the high quality mesoporous structure, stable PIM-1 coating, and optimized N content. As discussed earlier, for the [email protected] electrode, the volume of the cPIM skin layer was too low and could not provide a sufficient number of active sites for the electrocatalytic reactions. However, for the [email protected] electrode, the cPIM skin layer was excessively thick, resulting in a decreased mesopore volume and poor film quality, resulting in lower activity of this electrode compared to the others. The corresponding electrochemical parameters of all electrodes are summarized in Table S4. To understand the electronic charge transfer mechanism in the prepared electrodes, electrochemical impedance spectroscope (EIS) measurements were performed at 0.49 V (Fig. S7). The EIS parameters were estimated by fitting the Nyquist plots using an equivalent circuit diagram, as shown in the inset of Fig. S6. The Nyquist plots showed a semicircle in the high-frequency region and a linear trend in the low-frequency region, indicating the charge transfer and diffusion processes at the interface between the electrolyte and the electrode. The semicircle of the PIMC@GF-X sample was much smaller than that of the pristine GF, indicating that the active barrier for the vanadium redox reaction was greatly reduced. Furthermore, the charge transfer resistance was the lowest for the [email protected] electrode, followed by [email protected], [email protected], and [email protected]. Given the higher N content and surface area of [email protected], we propose that the charge transfer resistance in the prepared material was mainly governed by the N content and surface area. The VRFB single cell testing was performed using PIMVC@GF-X samples and pristine GF as both negative and positive electrodes and Nafion 115 as a membrane. The rate performances of the
electrodes were measured by varying the current density from 50 to 150 mA cm2. Fig. 4 shows the coulombic efficiency (CE), voltage efficiency (VE), and energy efficiency (EE) of the VRFBs during 30 chargeedischarge cycles. As shown in Fig. 4a, the average CE values for all prepared electrodes were nearly equal, implying that the differences between the charge and discharge capacities were similar for all prepared electrodes. Fig. 4b shows VE curves for the prepared electrodes obtained at different current densities, where [email protected] showed the highest VE of all samples. As VE indicates the ratio of charge and discharge voltages, the change in the VE mainly occurs as a result of potential losses due to overpotentials and internal resistances. The lower VE value for the [email protected] electrode indicated that this electrode had a lower potential loss than the other samples. Fig. 4c shows the EE vs. current density plots, which confirmed that the [email protected] electrode had the highest efficiency for all current densities. It is interesting to note that when the current density was reversed and scanned from 150 to 50 mA cm2, the EE and VE of the [email protected] electrode almost recovered to its initial values, indicating good electrochemical and chemical robustness of the prepared electrodes in the vanadium electrolyte. Fig. 5 shows the results of longterm stability tests for [email protected] at a current density of 100 mA cm2. No apparent changes in the EE, VE, and CE values were observed, even after 100 cycles, clearly demonstrating the robustness of the [email protected] electrode under highly acidic conditions and a high current density. Such promising activity was attributed to the multimodal porous structure, optimal N doping, and stable and robust films. 3. Conclusions In summary, here we have demonstrated a unique methodology for preparing robust and stable PIM-coated GF for VRFB applications. The present binder-free process is advantageous as binder residues can increase the Ohmic resistance. The PIM-1 polymer was simply dissolved into a non-polar solvent and then dip-coated onto the GF electrodes. We observed that PIM-1 generated mesopores only when the films were treated with polar solvents. The obtained PIM-coated GF was then carbonized to convert the polymer film into multimodal-porous-N-doped carbon. The obtained PIMC@GF was then used as both anode and cathode in VRFB devices. Surprisingly, the [email protected] electrode demonstrated long-term stability and excellent rate capability up to 150 mA cm2 with significantly reduced electron transfer and polarization resistance compared to the other samples. The excellent performance was attributed to the formation of N-doped multimodal porous carbon
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Fig. 5. Long-term stability test of PIMC@ GF-1.0 at 100 mA/cm2
Data availability statement The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study. Acknowledgements This work was supported by the Program through the National Research Foundation of Korea (NRF) Grant funded by the Korean government (MSIP: Ministry of Science, ICT and Future Planning) (NRF-2015M1A2A2056722) and the Korea Research Institute of Chemical Technology (KRICT) core project (KK1802C00, SK01806M04, SI1803). Appendix B. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.mtener.2018.11.003. References
Fig. 4. VRFB single cell performance of PIMC@ GFs and pristine-GF at various current densities (50e150 mA cm 2): (a) coulombic efficiency, (b) voltage efficiency, and (c) energy efficiency.
on the GF surface without any binder. We propose that PIMC@GF materials are highly promising for high-performance electrodes for VRFB applications. Due to the ease of synthesis and high activity of the prepared catalyst, the present approach is expected to open new avenues in the study and commercialization of VRFBs.
[1] Z. Yang, J. Zhang, M.C.W. Kintner-Meyer, X. Lu, D. Choi, J.P. Lemmon, J. Liu, Electrochemical energy storage for green grid, Chem. Rev. 111 (2011) 3577e3613. [2] J.D. Milshtein, A.P. Kaur, M.D. Casselman, J.A. Kowalski, S. Modekrutti, P.L. Zhang, N. Harsha Attanayake, C.F. Elliott, S.R. Parkin, C. Risko, F.R. Brushett, S.A. Odom, High current density, long duration cycling of soluble organic active species for non-aqueous redox flow batteries, Energy Environ. Sci. 9 (2016) 3531e3543. [3] C. Chen, Y. Zhang, Y. Li, J. Dai, J. Song, Y. Yao, Y. Gong, I. Kierzewski, J. Xie, L. Hu, All-wood, low tortuosity, aqueous, biodegradable supercapacitors with ultrahigh capacitance, Energy Environ. Sci. 10 (2017) 538e545. [4] W. Wang, Q. Luo, B. Li, X. Wei, L. Li, Z. Yang, Recent progress in redox flow battery Research and development, Adv. Funct. Mater. 23 (2013) 970e986. [5] L. Ji, P. Meduri, V. Agubra, X. Xiao, M. Alcoutlabi, Graphene-based nanocomposites for energy storage, Advanced Energy Materials 6 (2016) 1502159. [6] J.-K. Jang, T.-H. Kim, S.J. Yoon, J.Y. Lee, J.-C. Lee, Y.T. Hong, Highly proton conductive, dense polybenzimidazole membranes with low permeability to vanadium and enhanced H2SO4 absorption capability for use in vanadium redox flow batteries, J. Mater. Chem. 4 (2016) 14342e14355. [7] Q. Liu, A. Turhan, T.A. Zawodzinski, M.M. Mench, In situ potential distribution measurement in an all-vanadium flow battery, Chem. Commun. 49 (2013) 6292e6294. [8] M. Park, J. Ryu, J. Cho, Nanostructured electrocatalysts for all-vanadium redox flow batteries, Chem. Asian J. 10 (2015) 2096e2110. [9] K.J. Kim, H.S. Lee, J. Kim, M.-S. Park, J.H. Kim, Y.-J. Kim, M. Skyllas-Kazacos, Superior electrocatalytic activity of a robust carbon-felt electrode with oxygen-rich phosphate groups for all-vanadium redox flow batteries, ChemSusChem 9 (2016) 1329e1338.
D.-S. Yang et al. / Materials Today Energy 11 (2019) 159e165 [10] X. Li, H. Zhang, Z. Mai, H. Zhang, I. Vankelecom, Ion exchange membranes for vanadium redox flow battery (VRB) applications, Energy Environ. Sci. 4 (2011) 1147e1160. [11] J.-Z. Chen, W.-Y. Liao, W.-Y. Hsieh, C.-C. Hsu, Y.-S. Chen, All-vanadium redox flow batteries with graphite felt electrodes treated by atmospheric pressure plasma jets, J. Power Sources 274 (2015) 894e898. [12] L. Shi, S. Liu, Z. He, H. Yuan, J. Shen, Synthesis of boron and nitrogen co-doped carbon nanofiber as efficient metal-free electrocatalyst for the VO2þ/VO2þ Redox Reaction, Electrochim. Acta 178 (2015) 748e757. [13] Y. Shen, H. Xu, P. Xu, X. Wu, Y. Dong, L. Lu, Electrochemical catalytic activity of tungsten trioxide- modified graphite felt toward VO2þ/VO2þ redox reaction, Electrochim. Acta 132 (2014) 37e41. [14] G. Hu, M. Jing, D.-W. Wang, Z. Sun, C. Xu, W. Ren, H.-M. Cheng, C. Yan, X. Fan, F. Li, A gradient bi-functional graphene-based modified electrode for vanadium redox flow batteries, Energy Storage Materials 13 (2018) 66e71. [15] D.-S. Yang, J.Y. Lee, S.-W. Jo, S.J. Yoon, T.-H. Kim, Y.T. Hong, Electrocatalytic activity of nitrogen-doped CNT graphite felt hybrid for all-vanadium redox flow batteries, Int. J. Hydrogen Energy 43 (2018) 1516e1522. [16] W. Li, J. Liu, C. Yan, The electrochemical catalytic activity of single-walled carbon nanotubes towards VO2þ/VO2þ and V3þ/V2þ redox pairs for an all vanadium redox flow battery, Electrochim. Acta 79 (2012) 102e108. [17] S. Wang, X. Zhao, T. Cochell, A. Manthiram, Nitrogen-doped carbon nanotube/ graphite felts as advanced electrode materials for vanadium redox flow batteries, J. Phys. Chem. Lett. 3 (2012) 2164e2167. [18] J. Ryu, M. Park, J. Cho, Catalytic effects of B/N-co-Doped porous carbon incorporated with Ketjenblack nanoparticles for all-vanadium redox flow batteries, J. Electrochem. Soc. 163 (2016) A5144eA5149. [19] S. Zhong, C. Padeste, M. Kazacos, M. Skyllas-Kazacos, Comparison of the physical, chemical and electrochemical properties of rayon- and polyacrylonitrile-based graphite felt electrodes, J. Power Sources 45 (1993) 29e41. [20] Y.I. Cho, S.J. Park, H.J. Hwang, J.G. Lee, Y.K. Jeon, Y.H. Chu, Y.-g. Shul, Effects of microwave treatment on carbon electrode for vanadium redox flow battery, ChemElectroChem 2 (2015) 872e876. [21] M.H. Chakrabarti, N.P. Brandon, S.A. Hajimolana, F. Tariq, V. Yufit, M.A. Hashim, M.A. Hussain, C.T.J. Low, P.V. Aravind, Application of carbon materials in redox flow batteries, J. Power Sources 253 (2014) 150e166. [22] K.J. Kim, S.-W. Lee, T. Yim, J.-G. Kim, J.W. Choi, J.H. Kim, M.-S. Park, Y.-J. Kim, A new strategy for integrating abundant oxygen functional groups into carbon felt electrode for vanadium redox flow batteries, Sci. Rep. 4 (2014) 6906. [23] K.J. Kim, Y.-J. Kim, J.-H. Kim, M.-S. Park, The effects of surface modification on carbon felt electrodes for use in vanadium redox flow batteries, Mater. Chem. Phys. 131 (2011) 547e553. [24] M. Park, I.-Y. Jeon, J. Ryu, H. Jang, J.-B. Back, J. Cho, Edge-halogenated graphene nanoplatelets with F, Cl, or Br as electrocatalysts for all-vanadium redox flow batteries, Nanomater. Energy 26 (2016) 233e240. [25] X. Wu, H. Xu, P. Xu, Y. Shen, L. Lu, J. Shi, J. Fu, H. Zhao, Microwave-treated graphite felt as the positive electrode for all-vanadium redox flow battery, J. Power Sources 263 (2014) 104e109. lez, S. Vizireanu, G. Dinescu, C. Blanco, R. Santamaría, Carbon nano[26] Z. Gonza walls thin films as nanostructured electrode materials in vanadium redox flow batteries, Nanomater. Energy 1 (2012) 833e839. [27] Z. Niu, P. Luan, Q. Shao, H. Dong, J. Li, J. Chen, D. Zhao, L. Cai, W. Zhou, X. Chen, S. Xie, A "skeleton/skin" strategy for preparing ultrathin free-standing singlewalled carbon nanotube/polyaniline films for high performance supercapacitor electrodes, Energy Environ. Sci. 5 (2012) 8726e8733. [28] Y. Shao, X. Wang, M. Engelhard, C. Wang, S. Dai, J. Liu, Z. Yang, Y. Lin, Nitrogendoped mesoporous carbon for energy storage in vanadium redox flow batteries, J. Power Sources 195 (2010) 4375e4379. [29] H.T. Thu Pham, C. Jo, J. Lee, Y. Kwon, MoO2 nanocrystals interconnected on mesocellular carbon foam as a powerful catalyst for vanadium redox flow battery, RSC Adv. 6 (2016) 17574e17582.
165
[30] Z. Zhang, J. Xi, H. Zhou, X. Qiu, KOH etched graphite felt with improved wettability and activity for vanadium flow batteries, Electrochim. Acta 218 (2016) 15e23. [31] J. Jin, X. Fu, Q. Liu, Y. Liu, Z. Wei, K. Niu, J. Zhang, Identifying the active site in nitrogen-doped graphene for the VO2þ/VO2þ redox reaction, ACS Nano 7 (2013) 4764e4773. [32] A. Mahmood, W. Guo, H. Tabassum, R. Zou, Metal-organic framework-based nanomaterials for electrocatalysis, Adv. Energy Mater. 6 (2016). [33] B. Sun, X. Huang, S. Chen, P. Munroe, G. Wang, Porous graphene nanoarchitectures: an efficient catalyst for low charge-overpotential, long life, and high capacity lithiumeoxygen batteries, Nano. Lett. 14 (2014) 3145e3152. [34] M. Kim, E. Yoo, W.-S. Ahn, S.E. Shim, Controlling porosity of porous carbon cathode for lithium oxygen batteries: influence of micro and meso porosity, J. Power Sources 389 (2018) 20e27. [35] D.-S. Yang, D. Bhattacharjya, S. Inamdar, J. Park, J.-S. Yu, Phosphorus-doped ordered mesoporous carbons with different lengths as efficient metal-free electrocatalysts for oxygen reduction reaction in alkaline media, J. Am. Chem. Soc. 134 (2012) 16127e16130. [36] N. Du, H.B. Park, G.P. Robertson, M.M. Dal-Cin, T. Visser, L. Scoles, M.D. Guiver, Polymer nanosilver membranes for CO2-capture applications, Nat. Mater. 10 (2011) 372e375. [37] E. Lasseuguette, M.-C. Ferrari, Development of microporous electrospun PIM-1 fibres, Mater. Lett. 177 (2016) 116e119. [38] K. Halder, G. Bengtson, V. Filiz, V. Abetz, Catalytically active (Pd) nanoparticles supported by electrospun PIM-1: influence of the sorption capacity of the polymer tested in the reduction of some aromatic nitro compounds, Appl. Catal. Gen. 555 (2018) 178e188. [39] Y. Kinoshita, K. Wakimoto, A.H. Gibbons, A.P. Isfahani, H. Kusuda, E. Sivaniah, B. Ghalei, Enhanced PIM-1 membrane gas separation selectivity through efficient dispersion of functionalized POSS fillers, J. Membr. Sci. 539 (2017) 178e186. [40] S.-Y. Son, Y.-J. Noh, C. Bok, S. Lee, B.G. Kim, S.-I. Na, H.-I. Joh, One-step synthesis of carbon nanosheets converted from a polycyclic compound and their direct use as transparent electrodes of ITO-free organic solar cells, Nanoscale 6 (2014) 678e682. [41] P. Kaur, J.T. Hupp, S.T. Nguyen, Porous organic polymers in catalysis: opportunities and challenges, ACS Catal. 1 (2011) 819e835. [42] H.J. Kim, D.-G. Kim, K. Lee, Y. Baek, Y. Yoo, Y.S. Kim, B.G. Kim, J.-C. Lee, A carbonaceous membrane based on a polymer of intrinsic microporosity (PIM-1) for water treatment, Sci. Rep. 6 (2016) 36078. [43] J.W. Jeon, J.H. Han, S.-K. Kim, D.-G. Kim, Y.S. Kim, D.H. Suh, Y.T. Hong, T.H. Kim, B.G. Kim, Intrinsically microporous polymer-based hierarchical nanostructuring of electrodes via nonsolvent-induced phase separation for high-performance supercapacitors, J. Mater. Chem. A 6 (2018) 8909e8915. [44] H. Fan, F. Ran, X. Zhang, H. Song, W. Jing, K. Shen, L. Kong, L. Kang, A hierarchical porous carbon membrane from polyacrylonitrile/polyvinylpyrrolidone blending membranes: preparation, characterization and electrochemical capacitive performance, J. Energy Chem. 23 (2014) 684e693. [45] P. van de Witte, P.J. Dijkstra, J.W.A. van den Berg, J. Feijen, Phase separation processes in polymer solutions in relation to membrane formation, J. Membr. Sci. 117 (1996) 1e31. [46] L. Yue, W. Li, F. Sun, L. Zhao, L. Xing, Highly hydroxylated carbon fibres as electrode materials of all-vanadium redox flow battery, Carbon 48 (2010) 3079e3090. [47] P. Wu, Y. Qian, P. Du, H. Zhang, C. Cai, Facile synthesis of nitrogen-doped graphene for measuring the releasing process of hydrogen peroxide from living cells, J. Mater. Chem. 22 (2012) 6402e6412. [48] Y. Zhao, L. Yang, S. Chen, X. Wang, Y. Ma, Q. Wu, Y. Jiang, W. Qian, Z. Hu, Can boron and nitrogen Co-doping improve oxygen reduction reaction activity of carbon nanotubes? J. Am. Chem. Soc. 135 (2013) 1201e1204.
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Materials Today Energy journal homepage: www.journals.elsevier.com/materials-today-energy/
Multimodal porous and nitrogen-functionalized electrode based on graphite felt modified with carbonized porous polymer skin layer for all-vanadium redox flow battery Dae-Soo Yang a, Jae Hee Han a, b, Jun Woo Jeon a, Jang Yong Lee a, Dong-Gyun Kim a, Dong Hack Seo b, Byoung Gak Kim a, Tae-Ho Kim a, *, Young Taik Hong a, ** a b
Advanced Materials Division, Korea Research Institute of Chemical Technology (KRICT) 141 Gajeong-ro, Yuseong-gu, Daejeon 305-600, Republic of Korea Department of Chemical Engineering, Hanyang University, 17 Haengdang-dong, Seongdong-gu, Seoul 133-791, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 13 August 2018 Received in revised form 13 November 2018 Accepted 15 November 2018
Novel nitrogen-functionalized multimodal porous graphite felt (GF) electrodes for all-vanadium redox flow batteries (VRFB) have been developed using a simple binder-free fabrication method. We synergistically combined a polymer of intrinsic microporosity (PIM), as a carbon and nitrogen precursor, with vapor-induced phase separation. The GF with a carbonized PIM skin layer showed a high surface area and multimodal pore architecture featuring interconnected micro-, meso-, and macropores. The enhanced electrochemical reactivity and wettability, and excellent electronic conductivity of the prepared electrodes successfully improved the overall kinetics of the redox reactions of the vanadium ion species by providing highly active catalytic sites and efficient ion and electron transport pathways. This resulted in the outstanding performance of VRFB single cells using this material as electrodes. © 2018 Elsevier Ltd. All rights reserved.
Keywords: Vanadium redox flow battery Porous carbon High performance electrode Redox chemistry
1. Introductions Due to the growing integration of renewable energy sources into the existing electricity grid, optimization of the storage and subsequent release of electrical energy plays a significant role in improving the whole power system [1e4]. Vanadium redox flow batteries (VRFBs) have attracted considerable attention due to their high capacity, fast response time, long cycle life, flexible cell configuration, and low maintenance, making them an ideal energy storage solution for load shifting in power grids, as well as in wind and solar energy conversion systems [5e10]. VRFBs exploit the fact that vanadium can exist in solution with four different oxidation states and store electrical energy via the redox reactions of the vanadium ion species contained in two separate electrolyte solutions. As a key component, the electrodes in each half-cell provide active sites for the redox reactions involving the vanadium ions, which directly affect the cell
* Corresponding author. ** Corresponding author. E-mail address: [email protected] (T.-H. Kim). https://doi.org/10.1016/j.mtener.2018.11.003 2468-6069/© 2018 Elsevier Ltd. All rights reserved.
performance of the VRFB system [11e14]. According to previous studies of the redox reaction mechanism for VRFBs, the electrochemical processes during charging/discharging comprise multiple electron transfer and ion exchange steps between vanadium ions in the electrolyte and the protons from functional groups on the electrode surface. This highlights the significance of developing electrode materials with both high electronic conductivity and a large specific surface area and surface functionality [15e18]. Among the many carbon-based materials evaluated as VRFB electrode materials, carbon felt (CF)- and graphite felt (GF)-based electrodes are preferable due to their high electronic conductivity, stability under acidic conditions, and corrosion resistance. In addition, their three-dimensional macroporous structure lowers the electrolyte flow resistance and provides mechanical stability at a relatively low cost [8,19,20]. Nevertheless, pristine CF and GF have porous structures with low specific surface areas and poor surface functionality, limiting their electrochemical activity in vanadium electrolytes [21]. Various techniques have been used to increase the activity of CF and GF electrodes, including thermal and chemical treatments, electrochemical activation, plasma or microwave treatments, and nitrogen doping; these methods focus on the development of oxygenated and/or nitrogenated functional groups on the surface of
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CF and GF, which act as reactive sites towards vanadium redox reactions [9,18,22e27]. However, these approaches face the persistent challenge of providing a well-balanced pore architecture combining micro-, meso-, and macropores [28,29]. For example, the use of strong bases as activating agents creates microporous structures with high oxygen functionality, but is not effective in the formation of mesopores [30]. Considering the mass transport through micropores, the reaction kinetics also depend on the mass transfer rate of the diffusion of vanadium ions at the electrolyte and electrode interface [31]. During charging (discharging), fast diffusion of V3þ(V2þ) and VO2þ(VOþ 2 ) ions from the bulk anolyte and catholyte, respectively, into the active sites on each electrode surface is beneficial to improve the kinetic reversibility of the redox reactions. In this regard, optimal pore networks should allow efficient vanadium ion transport while hosting sufficient active sites, which can be realized in multimodal porous structures featuring interconnected micro-, meso-, and macropores. Although microporous and mesoporous materials such as Ketjenblack carbon, porous graphenes, mesoporous carbon, and metal-organic frameworks (MOFs) have been investigated for various electrochemical applications [32e35], the development of carbon electrodes with multimodal porous architectures, particularly for VRFB applications, remains relatively immature. Polymers of intrinsic microporosity (PIMs) feature a continuous microporous network as a consequence of their fused-ring and ladder-like structures integrated with contortion sites. This structure provides uniform and inter-connected micropores (<2 nm) and a high surface area (300e1000 m2 g1) [36]. Typically the chemical structure of PIM-1 is shown in Fig. S1. In contrast to other intrinsically microporous materials, such as mesoporous silica, zeolite, and MOFs, PIMs have advantageous form-factor flexibility, as well as good solubility/processability in organic solvents, facilitating the use of methods such as spin-coating, solution casting, and electrospinning [37e40]. Recently, it was found that these materials can maintain their microporosity and structural integrity even after being carbonized (due to their intrinsic bonding characteristics), which broadens their use as precursors for microporous carbon materials [41]. Carbonized PIM-1 shows uniformly sized micropores with an average diameter <1 nm, while the specific surface area of carbonized PIM-1 can be tuned by controlling the carbonization conditions, e.g., temperature and time [42,43]. Herein, we developed a simple and efficient strategy for preparing GF-based hierarchically porous carbon electrodes for use in high performance VRFBs, with fairly uniform and wellinterconnected multimodal pore architectures. We synergistically combined the use of PIM-1 polymer as a carbon and nitrogen precursor and the vapor induced phase separation (VIPS) method. A PIM-1 solution was coated on the surface of the GF fibers by a simple dip-coating method, and then the coated solution was phase-inversed using VIPS, where the complex diffusion and convection behavior of solvent/vapor molecules introduced hierarchical porosity into the carbon precursors [44,45]. After film stabilization and carbonization, a carbon skin with multimodal porosity featuring both micro- and mesoporous characteristics was successfully produced on the surface of GF fibers. Importantly, during the carbonization, N atoms can be simultaneously incorporated into the resulting carbon skin layer due to the presence of two nitrile groups in the repeating unit of PIM-1, this results in a heteroatom doping effect that increases the final electrochemical activity of the material. Therefore, nanostructuring and subsequent carbonization provide a simple, binder-free and template-free methodology for producing GF-based multimodal porous electrodes, while avoiding complicated and tedious templating and activation processes. Increased specific surface area, wettability,
and electrochemical reactivity, and excellent electronic conductivity of the prepared hybrid electrode resulted in outstanding performance of a VRFB single cell using the produced electrode material. To the best of our knowledge, this is the first time that a carbon electrode with multimodal porous architecture has been applied for VRFB applications. The polymerization of PIM-1 (hereafter abbreviated to PIM) was carried out following procedures in the literature and confirmed using nuclear magnetic resonance (NMR) spectroscopy (Fig. S1) [42]. The molecular weight and polydispersity index (PDI) of the synthesized PIM was analyzed using gel-permeation chromatography, yielding a number-average molar mass of 42 300 g mol 1 and a PDI of 1.94. Fig. 1a illustrates the basic fabrication process of the novel GF material with the carbonized PIM (cPIM) skin layer. Pristine GFs were first treated with ozone to remove excess surface-adsorbed O2 molecules and then immersed into PIMs solutions with various concentrations dissolved in chloroform (nonpolar solvent). Higher concentrations of the polymer solution increased the thickness of the cPIM skin layer deposited on the GF. During the VIPS process, the PIM solution on the surface of the GFs was immediately exposed to water vapor (polar solvent), where instantaneous precipitation and separation occurred due to the phase separation phenomena, producing mesoporous, but mechanically robust, cPIM skin layers. The obtained precursors were then dried at 25 C, followed by carbonization at 900 C in an argon atmosphere, and further treatment at 300 C for 2 h in an O2 atmosphere to generate the oxygen functional groups; the carbonization temperature was determined to optimize both the electrical conductivity as well as nitrogen content of the resulting cPIM skin layer (Fig. S2 and Table S1). The resulting GF electrodes with cPIM skin layers were labeled PIMVC@GF-Xs, where X (0.5, 1.0, or 2.0) is the concentration of the PIM coating solution. It should be noted that no binder was necessary to produce this porous carbon skin layer on the GF electrode. We hypothesize that this binder-free process can significantly reduce the Ohmic resistance of the resulting electrode, as discussed later. To evaluate the effect of the VIPS-induced mesoporous structure, a carbonized PIM-coated GF material was prepared without VIPS processing in a similar way to [email protected]; this control sample is referred to as [email protected]. Details of the polymer synthesis and sample preparation are described later in the Experimental section. 2. Results and discussion The surface morphology of the pristine GF and PIMVC@GF-Xs were studied using field-emission scanning electron microscopy (FESEM) (Fig. 1bef). The pristine GF showed a smooth and featureless surface (Fig. 1b); such a defect-free surface has been reported to be unsuitable for VRFB applications due to its poor electrochemical activity [46]. Meanwhile, the PIMVC@GF-Xs samples clearly showed the presence of thin cPIM skin layers on the surface of the GF fibers (Fig. 1cee). It is important to note that the concentration of PIM in the coating solution greatly affects the coating uniformity, skin layer thickness, and surface morphology. When a low concentration (0.5 wt%) of PIM was used, the cPIM skin layer did not uniformly cover the surface of the GF fibers; residual uncoated areas were observed in Fig. 1c. However, a high concentration of the coating solution (2.0 wt%) resulted in the formation of a thicker cPIM skin layer; however, this concentration was thought to be excessive as the film uniformity was poor, with agglomerated particles and thorn-like flakes (Fig. 1e). However, the [email protected] sample was considered to have the optimal PIM concentration, as the formed cPIM skin layer had a moderate thickness and relatively uniform surface morphology, as shown in Fig. 1d. Although a textured surface with obvious spots (thought to be
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Fig. 1. (a) Schematic illustration of the PIMC @ GF-X electrode fabrication process. SEM images of the (b) pristine GF (c) PIMC@ GF-0.5, (d) PIMC@ GF-1.0, (e) PIMC@ GF-2.0, (f) high magnification image of PIMC@ GF-1.0 highly mesoporous structures can be seen.
formed during the coating process) was observed by SEM on the [email protected] surface, we observed full surface coverage of the GF by the cPIM skin layer using high-magnification SEM (Fig. S3). Moreover, from the high-magnification SEM image of the [email protected] surface (Fig. 1f), it was confirmed that the cPIM skin layer had a mesoporous structure with pores of tens of nanometers, as expected. It is important to note that this mesoporous structure of the carbonized PIM skin layer was observed only for PIMVC@GFXs prepared using the VIPS process; the control sample prepared without the VIPS process ([email protected]) showed no such mesopores (Fig. S4). This multimodal pore structure of the PIMVC@GF-X samples is expected to decrease the transport resistance of the vanadium ions and expedite their adsorption under an applied potential. The pore characteristics of the prepared electrodes were further studied using N2 adsorption. Fig. S5 shows the N2 adsorptione desorption isotherms for all prepared electrodes. The specific surface area of the electrodes was calculated using the BrunauereEmmetteTeller (BET) equation. For the pristine GF, a negligible specific surface area (<1 m2g-1) was observed, confirming the absence of surface micropores. However, the specific surface area values of the PIMVC@GF-X samples significantly increased with increasing PIM precursor content (157, 240, and 301 m2g-1, for [email protected], [email protected], and [email protected], respectively). This demonstrates that the addition of the cPIM skin layer effectively introduces microporosity on the GF surface. Considering the very small relative weight of the cPIM skin layer compared to the total weight of [email protected] (~1.0 wt%), such large increases in the specific surface area for [email protected] are unprecedented. We believe that the high surface areas of PIMVC@GF-X samples mainly originated from the intrinsically microporous nature of the PIM precursor; the specific surface area of the pure carbonized PIM-1 sample (without GF) prepared under the same carbonization conditions was as high as 1297 m2g-1. Unlike pristine GF, which did not
show any isotherms or a hysteresis loop, all PIMVC@GF-Xs showed typical type-IV isotherms with H3 hysteresis loops, demonstrating the presence of micro- and mesopores, in good agreement with SEM observations. The textural parameters of all PIMVC@GF-X, pristine GF, and [email protected] samples are summarized in Table S2. Notably, [email protected] synthesized under optimal conditions showed a considerable volume of mesopores. The mesoporous structure of the PIMVC@GF-Xs is expected to be beneficial for enhancing the redox reaction kinetics by providing efficient pathways for the fast absorption/desorption of the vanadium ions through mesopores. Raman spectroscopy was conducted to investigate the crystallinity of the carbon in the PIMVC@GF-X samples. As seen in Fig. S6, all PIMVC@GF-Xs showed the D and G bands typical of carbon materials at 1350 cm1 and 1590 cm1, respectively. It was seen that the peak intensity of the D band increased with increasing X in the PIMVC@GF-X samples, indicating the amorphous nature of the PIM-derived carbon. In contrast to pristine GF, which showed a highly crystalline (graphitic) nature, carbonized PIM-1 has been shown to have an amorphous structure [42]. The chemical nature of the cPIM-coated GF materials was studied using X-ray photoelectron spectroscopy (XPS). Fig. 2a shows the XPS survey spectra of the pristine GF and [email protected] samples. Typical C, O, and N peaks were obtained for the [email protected] electrode, which clearly showed that the N atoms were simultaneously incorporated into the cPIM skin layer during carbonization. Therefore, the N content increased with increasing cPIM skin layer volume, as shown in Table S3; [email protected], [email protected], and [email protected] had N contents of 0.54 at.%, 1.18 at.%, and 1.54 at.%, respectively. The binding arrangements of the N dopant were identified using high-resolution XPS. The deconvolution of the N spectra of [email protected] yielded four subpeaks with binding energies of 398.1, 399.6, 401.2, and 403.2 eV, which corresponded to pyridinic-N, pyrrolic-N, quaternary N, and
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Fig. 2. (a) XPS survey scan of pristine GF and the PIMC@ GF-1.0 and (b) high resolution N 1S and (c) O 1s spectra. Contact angle measurement for (d) pristine GF and e) PIMC@ GF-1.0 taken by pouring deionized (DI) water onto each CFE surface. (f) Electrical conductivity measured against pressure for all the prepared samples.
terminal N-O, respectively (Fig. 2b). Since pyridinic and pyrrolic N have a lone pair of electrons, they can donate their electrons to a carbon p conjugated system and consequently reduce the band gap of the resulting doped carbon [47]. Therefore, the nitrogen species can have a profound effect on enhancing the electrical conductivity of the doped carbon. Furthermore, the nitrogen functionality is believed to promote the reduction reaction due to the presence of the extra electrons from the N atoms, and also faciliate oxidation by positively charging neighboring carbon atoms [48]. Hence, we propose that the presence of such nitrogen species in the prepared PIMVC@GF-X electrodes can significantly enhance their activity for the redox reactions involving vanadium ions. Moreover, [email protected] showed significant oxygen functionalities, as seen from the O1s spectra in Fig. 2c. The amount of oxygen moieties in [email protected] and pristine GF was found to be 9.64 and 6.64 at.%, respectively. Importantly, from the deconvolution of the O spectra, it was concluded the higher oxygen functionality of [email protected]:[email protected] compared to pristine GF mainly arised from higher number of C]O bonds. As C]O functional groups are directly associated with the redox reactions of vanadium ions, the resulting electrode is expected to have improved electrocatalytic performance [9]. We examined the influence of these chemical and structural changes due to the introduction of cPIM skin layer on the hydrophilicity/wettability and electrical conductivity of the electrodes. Fig. 2d and e compare the contact angles of the pristine GF and [email protected]. Pristine GF, composed of graphitic carbon with high hydrophobicity, showed a contact angle of 106.2 , indicating its poor wettability in an aqueous electrolyte system. Surprisingly, in the case of [email protected], the water droplet was immediately absorbed, demonstrating its excellent wettability. Such clear improvement in the wettability of the electrode was attributed to the enhanced interaction between water molecules and the nitrogen and oxygen functional groups present in the cPIM skin layer [9,22].
Electronic conductivity measurements (conducted using a pressure-variable cell to ensure the accuracy) showed that the PIMVC@GF-X samples maintained the excellent conductivity of the GF-based material despite the various modification processes, including coating and heating steps. Thermal treatments are often employed to modify the surface properties of GF electrodes, but they generally degrade the conductivity of GF [19]. The pristine GF electrode used this study also showed a drastic decrease in conductivity after thermal treatment (Fig. 2f). However, all PIMVC@GFX samples showed higher conductivities than the heat-treated GF; in particular, [email protected] and [email protected] showed only slightly lower values than that of the pristine GF. The binder-free fabrication process allowed the high electronic conductivity to be maintained in our system. Thus, multimodal porous carbon electrodes with both high surface functionality and electronic conductivity were successfully realized. The catalytic activity of the prepared electrodes was measured using cyclic voltammetry (CV) experiments, for both negative and positive reactions, i.e., the V2þ/V3þ and VO2þ/VOþ 2 conversion, respectively, as shown in Fig. 3. Using CV analysis, the catalytic activity for the vanadium redox reactions can be analyzed through parameters such as the potential difference (DV ¼ Vpa-Vpc), oxidation and reduction peak current density (Ipa and Ipc), and redox onset potential. The CV curves for the negative electrode reaction of the VRFB, corresponding to V2þ/V3þ redox couples, are shown in Fig. 3a. No reduction peak and a very small oxidation peak were obtained for the pristine GF electrode, indicating that the pristine GF electrode had very poor catalytic activity and irreversibly oxidized the vanadium ions. On the contrary, a pronounced anodic peak, between 0.3 to 0.6 V, and a reversible cathodic peak, between 0.9 to 1.0 V, was observed for the PIMVC@GF-X electrodes, demonstrating that the presence of the cPIM skin layer enhanced the electrocatalytic activities of the electrodes. The improvement in the reversibility of the ion adsorption on PIMVC@GF-Xs was attributed to suppression of undesirable side
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Fig. 3. CV curves for the Pristine GF, and PIMVC@ GF at a scan rate of 5 mV s1 within a potential window of a) 1.1 to 0.5 V in the anolyte and b) 0.3e1.3 V in the catholyte (0.1 M V3þ and VO2þ dissolved in 1 M H2SO4) versus an Ag/AgCl reference electrode.
reactions, such as the hydrogen evolution reaction. Fig. 3b shows the CV curves for the positive electrode reaction, i.e., for VO2þ/VOþ 2 redox couples, for the prepared electrodes. The Ipa and Ipc values for the PIMC@GF-X electrodes were much higher than those of the pristine GF electrode. The DV and redox onset potential values of the PIMVC@GF-X and pristine GF electrodes showed the following order: [email protected] > [email protected] > [email protected] > [email protected] > pristine GF. To further elucidate the effect of the mesoporous structure, the CV performance of [email protected] was observed, which showed that this electrode had the most facile reversibility for the VO2þ/VOþ 2 redox reaction. Furthermore, the improved onset potentials for both the cathodic and anodic reactions on the [email protected] electrode, indicated favorable electron transfer kinetics and improved energy storage capability. Superior electrocatalytic performance of the [email protected] electrode, as compared to its similarly prepared counterparts, was attributed to the high quality mesoporous structure, stable PIM-1 coating, and optimized N content. As discussed earlier, for the [email protected] electrode, the volume of the cPIM skin layer was too low and could not provide a sufficient number of active sites for the electrocatalytic reactions. However, for the [email protected] electrode, the cPIM skin layer was excessively thick, resulting in a decreased mesopore volume and poor film quality, resulting in lower activity of this electrode compared to the others. The corresponding electrochemical parameters of all electrodes are summarized in Table S4. To understand the electronic charge transfer mechanism in the prepared electrodes, electrochemical impedance spectroscope (EIS) measurements were performed at 0.49 V (Fig. S7). The EIS parameters were estimated by fitting the Nyquist plots using an equivalent circuit diagram, as shown in the inset of Fig. S6. The Nyquist plots showed a semicircle in the high-frequency region and a linear trend in the low-frequency region, indicating the charge transfer and diffusion processes at the interface between the electrolyte and the electrode. The semicircle of the PIMC@GF-X sample was much smaller than that of the pristine GF, indicating that the active barrier for the vanadium redox reaction was greatly reduced. Furthermore, the charge transfer resistance was the lowest for the [email protected] electrode, followed by [email protected], [email protected], and [email protected]. Given the higher N content and surface area of [email protected], we propose that the charge transfer resistance in the prepared material was mainly governed by the N content and surface area. The VRFB single cell testing was performed using PIMVC@GF-X samples and pristine GF as both negative and positive electrodes and Nafion 115 as a membrane. The rate performances of the
electrodes were measured by varying the current density from 50 to 150 mA cm2. Fig. 4 shows the coulombic efficiency (CE), voltage efficiency (VE), and energy efficiency (EE) of the VRFBs during 30 chargeedischarge cycles. As shown in Fig. 4a, the average CE values for all prepared electrodes were nearly equal, implying that the differences between the charge and discharge capacities were similar for all prepared electrodes. Fig. 4b shows VE curves for the prepared electrodes obtained at different current densities, where [email protected] showed the highest VE of all samples. As VE indicates the ratio of charge and discharge voltages, the change in the VE mainly occurs as a result of potential losses due to overpotentials and internal resistances. The lower VE value for the [email protected] electrode indicated that this electrode had a lower potential loss than the other samples. Fig. 4c shows the EE vs. current density plots, which confirmed that the [email protected] electrode had the highest efficiency for all current densities. It is interesting to note that when the current density was reversed and scanned from 150 to 50 mA cm2, the EE and VE of the [email protected] electrode almost recovered to its initial values, indicating good electrochemical and chemical robustness of the prepared electrodes in the vanadium electrolyte. Fig. 5 shows the results of longterm stability tests for [email protected] at a current density of 100 mA cm2. No apparent changes in the EE, VE, and CE values were observed, even after 100 cycles, clearly demonstrating the robustness of the [email protected] electrode under highly acidic conditions and a high current density. Such promising activity was attributed to the multimodal porous structure, optimal N doping, and stable and robust films. 3. Conclusions In summary, here we have demonstrated a unique methodology for preparing robust and stable PIM-coated GF for VRFB applications. The present binder-free process is advantageous as binder residues can increase the Ohmic resistance. The PIM-1 polymer was simply dissolved into a non-polar solvent and then dip-coated onto the GF electrodes. We observed that PIM-1 generated mesopores only when the films were treated with polar solvents. The obtained PIM-coated GF was then carbonized to convert the polymer film into multimodal-porous-N-doped carbon. The obtained PIMC@GF was then used as both anode and cathode in VRFB devices. Surprisingly, the [email protected] electrode demonstrated long-term stability and excellent rate capability up to 150 mA cm2 with significantly reduced electron transfer and polarization resistance compared to the other samples. The excellent performance was attributed to the formation of N-doped multimodal porous carbon
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Fig. 5. Long-term stability test of PIMC@ GF-1.0 at 100 mA/cm2
Data availability statement The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study. Acknowledgements This work was supported by the Program through the National Research Foundation of Korea (NRF) Grant funded by the Korean government (MSIP: Ministry of Science, ICT and Future Planning) (NRF-2015M1A2A2056722) and the Korea Research Institute of Chemical Technology (KRICT) core project (KK1802C00, SK01806M04, SI1803). Appendix B. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.mtener.2018.11.003. References
Fig. 4. VRFB single cell performance of PIMC@ GFs and pristine-GF at various current densities (50e150 mA cm 2): (a) coulombic efficiency, (b) voltage efficiency, and (c) energy efficiency.
on the GF surface without any binder. We propose that PIMC@GF materials are highly promising for high-performance electrodes for VRFB applications. Due to the ease of synthesis and high activity of the prepared catalyst, the present approach is expected to open new avenues in the study and commercialization of VRFBs.
[1] Z. Yang, J. Zhang, M.C.W. Kintner-Meyer, X. Lu, D. Choi, J.P. Lemmon, J. Liu, Electrochemical energy storage for green grid, Chem. Rev. 111 (2011) 3577e3613. [2] J.D. Milshtein, A.P. Kaur, M.D. Casselman, J.A. Kowalski, S. Modekrutti, P.L. Zhang, N. Harsha Attanayake, C.F. Elliott, S.R. Parkin, C. Risko, F.R. Brushett, S.A. Odom, High current density, long duration cycling of soluble organic active species for non-aqueous redox flow batteries, Energy Environ. Sci. 9 (2016) 3531e3543. [3] C. Chen, Y. Zhang, Y. Li, J. Dai, J. Song, Y. Yao, Y. Gong, I. Kierzewski, J. Xie, L. Hu, All-wood, low tortuosity, aqueous, biodegradable supercapacitors with ultrahigh capacitance, Energy Environ. Sci. 10 (2017) 538e545. [4] W. Wang, Q. Luo, B. Li, X. Wei, L. Li, Z. Yang, Recent progress in redox flow battery Research and development, Adv. Funct. Mater. 23 (2013) 970e986. [5] L. Ji, P. Meduri, V. Agubra, X. Xiao, M. Alcoutlabi, Graphene-based nanocomposites for energy storage, Advanced Energy Materials 6 (2016) 1502159. [6] J.-K. Jang, T.-H. Kim, S.J. Yoon, J.Y. Lee, J.-C. Lee, Y.T. Hong, Highly proton conductive, dense polybenzimidazole membranes with low permeability to vanadium and enhanced H2SO4 absorption capability for use in vanadium redox flow batteries, J. Mater. Chem. 4 (2016) 14342e14355. [7] Q. Liu, A. Turhan, T.A. Zawodzinski, M.M. Mench, In situ potential distribution measurement in an all-vanadium flow battery, Chem. Commun. 49 (2013) 6292e6294. [8] M. Park, J. Ryu, J. Cho, Nanostructured electrocatalysts for all-vanadium redox flow batteries, Chem. Asian J. 10 (2015) 2096e2110. [9] K.J. Kim, H.S. Lee, J. Kim, M.-S. Park, J.H. Kim, Y.-J. Kim, M. Skyllas-Kazacos, Superior electrocatalytic activity of a robust carbon-felt electrode with oxygen-rich phosphate groups for all-vanadium redox flow batteries, ChemSusChem 9 (2016) 1329e1338.
D.-S. Yang et al. / Materials Today Energy 11 (2019) 159e165 [10] X. Li, H. Zhang, Z. Mai, H. Zhang, I. Vankelecom, Ion exchange membranes for vanadium redox flow battery (VRB) applications, Energy Environ. Sci. 4 (2011) 1147e1160. [11] J.-Z. Chen, W.-Y. Liao, W.-Y. Hsieh, C.-C. Hsu, Y.-S. Chen, All-vanadium redox flow batteries with graphite felt electrodes treated by atmospheric pressure plasma jets, J. Power Sources 274 (2015) 894e898. [12] L. Shi, S. Liu, Z. He, H. Yuan, J. Shen, Synthesis of boron and nitrogen co-doped carbon nanofiber as efficient metal-free electrocatalyst for the VO2þ/VO2þ Redox Reaction, Electrochim. Acta 178 (2015) 748e757. [13] Y. Shen, H. Xu, P. Xu, X. Wu, Y. Dong, L. Lu, Electrochemical catalytic activity of tungsten trioxide- modified graphite felt toward VO2þ/VO2þ redox reaction, Electrochim. Acta 132 (2014) 37e41. [14] G. Hu, M. Jing, D.-W. Wang, Z. Sun, C. Xu, W. Ren, H.-M. Cheng, C. Yan, X. Fan, F. Li, A gradient bi-functional graphene-based modified electrode for vanadium redox flow batteries, Energy Storage Materials 13 (2018) 66e71. [15] D.-S. Yang, J.Y. Lee, S.-W. Jo, S.J. Yoon, T.-H. Kim, Y.T. Hong, Electrocatalytic activity of nitrogen-doped CNT graphite felt hybrid for all-vanadium redox flow batteries, Int. J. Hydrogen Energy 43 (2018) 1516e1522. [16] W. Li, J. Liu, C. Yan, The electrochemical catalytic activity of single-walled carbon nanotubes towards VO2þ/VO2þ and V3þ/V2þ redox pairs for an all vanadium redox flow battery, Electrochim. Acta 79 (2012) 102e108. [17] S. Wang, X. Zhao, T. Cochell, A. Manthiram, Nitrogen-doped carbon nanotube/ graphite felts as advanced electrode materials for vanadium redox flow batteries, J. Phys. Chem. Lett. 3 (2012) 2164e2167. [18] J. Ryu, M. Park, J. Cho, Catalytic effects of B/N-co-Doped porous carbon incorporated with Ketjenblack nanoparticles for all-vanadium redox flow batteries, J. Electrochem. Soc. 163 (2016) A5144eA5149. [19] S. Zhong, C. Padeste, M. Kazacos, M. Skyllas-Kazacos, Comparison of the physical, chemical and electrochemical properties of rayon- and polyacrylonitrile-based graphite felt electrodes, J. Power Sources 45 (1993) 29e41. [20] Y.I. Cho, S.J. Park, H.J. Hwang, J.G. Lee, Y.K. Jeon, Y.H. Chu, Y.-g. Shul, Effects of microwave treatment on carbon electrode for vanadium redox flow battery, ChemElectroChem 2 (2015) 872e876. [21] M.H. Chakrabarti, N.P. Brandon, S.A. Hajimolana, F. Tariq, V. Yufit, M.A. Hashim, M.A. Hussain, C.T.J. Low, P.V. Aravind, Application of carbon materials in redox flow batteries, J. Power Sources 253 (2014) 150e166. [22] K.J. Kim, S.-W. Lee, T. Yim, J.-G. Kim, J.W. Choi, J.H. Kim, M.-S. Park, Y.-J. Kim, A new strategy for integrating abundant oxygen functional groups into carbon felt electrode for vanadium redox flow batteries, Sci. Rep. 4 (2014) 6906. [23] K.J. Kim, Y.-J. Kim, J.-H. Kim, M.-S. Park, The effects of surface modification on carbon felt electrodes for use in vanadium redox flow batteries, Mater. Chem. Phys. 131 (2011) 547e553. [24] M. Park, I.-Y. Jeon, J. Ryu, H. Jang, J.-B. Back, J. Cho, Edge-halogenated graphene nanoplatelets with F, Cl, or Br as electrocatalysts for all-vanadium redox flow batteries, Nanomater. Energy 26 (2016) 233e240. [25] X. Wu, H. Xu, P. Xu, Y. Shen, L. Lu, J. Shi, J. Fu, H. Zhao, Microwave-treated graphite felt as the positive electrode for all-vanadium redox flow battery, J. Power Sources 263 (2014) 104e109. lez, S. Vizireanu, G. Dinescu, C. Blanco, R. Santamaría, Carbon nano[26] Z. Gonza walls thin films as nanostructured electrode materials in vanadium redox flow batteries, Nanomater. Energy 1 (2012) 833e839. [27] Z. Niu, P. Luan, Q. Shao, H. Dong, J. Li, J. Chen, D. Zhao, L. Cai, W. Zhou, X. Chen, S. Xie, A "skeleton/skin" strategy for preparing ultrathin free-standing singlewalled carbon nanotube/polyaniline films for high performance supercapacitor electrodes, Energy Environ. Sci. 5 (2012) 8726e8733. [28] Y. Shao, X. Wang, M. Engelhard, C. Wang, S. Dai, J. Liu, Z. Yang, Y. Lin, Nitrogendoped mesoporous carbon for energy storage in vanadium redox flow batteries, J. Power Sources 195 (2010) 4375e4379. [29] H.T. Thu Pham, C. Jo, J. Lee, Y. Kwon, MoO2 nanocrystals interconnected on mesocellular carbon foam as a powerful catalyst for vanadium redox flow battery, RSC Adv. 6 (2016) 17574e17582.
165
[30] Z. Zhang, J. Xi, H. Zhou, X. Qiu, KOH etched graphite felt with improved wettability and activity for vanadium flow batteries, Electrochim. Acta 218 (2016) 15e23. [31] J. Jin, X. Fu, Q. Liu, Y. Liu, Z. Wei, K. Niu, J. Zhang, Identifying the active site in nitrogen-doped graphene for the VO2þ/VO2þ redox reaction, ACS Nano 7 (2013) 4764e4773. [32] A. Mahmood, W. Guo, H. Tabassum, R. Zou, Metal-organic framework-based nanomaterials for electrocatalysis, Adv. Energy Mater. 6 (2016). [33] B. Sun, X. Huang, S. Chen, P. Munroe, G. Wang, Porous graphene nanoarchitectures: an efficient catalyst for low charge-overpotential, long life, and high capacity lithiumeoxygen batteries, Nano. Lett. 14 (2014) 3145e3152. [34] M. Kim, E. Yoo, W.-S. Ahn, S.E. Shim, Controlling porosity of porous carbon cathode for lithium oxygen batteries: influence of micro and meso porosity, J. Power Sources 389 (2018) 20e27. [35] D.-S. Yang, D. Bhattacharjya, S. Inamdar, J. Park, J.-S. Yu, Phosphorus-doped ordered mesoporous carbons with different lengths as efficient metal-free electrocatalysts for oxygen reduction reaction in alkaline media, J. Am. Chem. Soc. 134 (2012) 16127e16130. [36] N. Du, H.B. Park, G.P. Robertson, M.M. Dal-Cin, T. Visser, L. Scoles, M.D. Guiver, Polymer nanosilver membranes for CO2-capture applications, Nat. Mater. 10 (2011) 372e375. [37] E. Lasseuguette, M.-C. Ferrari, Development of microporous electrospun PIM-1 fibres, Mater. Lett. 177 (2016) 116e119. [38] K. Halder, G. Bengtson, V. Filiz, V. Abetz, Catalytically active (Pd) nanoparticles supported by electrospun PIM-1: influence of the sorption capacity of the polymer tested in the reduction of some aromatic nitro compounds, Appl. Catal. Gen. 555 (2018) 178e188. [39] Y. Kinoshita, K. Wakimoto, A.H. Gibbons, A.P. Isfahani, H. Kusuda, E. Sivaniah, B. Ghalei, Enhanced PIM-1 membrane gas separation selectivity through efficient dispersion of functionalized POSS fillers, J. Membr. Sci. 539 (2017) 178e186. [40] S.-Y. Son, Y.-J. Noh, C. Bok, S. Lee, B.G. Kim, S.-I. Na, H.-I. Joh, One-step synthesis of carbon nanosheets converted from a polycyclic compound and their direct use as transparent electrodes of ITO-free organic solar cells, Nanoscale 6 (2014) 678e682. [41] P. Kaur, J.T. Hupp, S.T. Nguyen, Porous organic polymers in catalysis: opportunities and challenges, ACS Catal. 1 (2011) 819e835. [42] H.J. Kim, D.-G. Kim, K. Lee, Y. Baek, Y. Yoo, Y.S. Kim, B.G. Kim, J.-C. Lee, A carbonaceous membrane based on a polymer of intrinsic microporosity (PIM-1) for water treatment, Sci. Rep. 6 (2016) 36078. [43] J.W. Jeon, J.H. Han, S.-K. Kim, D.-G. Kim, Y.S. Kim, D.H. Suh, Y.T. Hong, T.H. Kim, B.G. Kim, Intrinsically microporous polymer-based hierarchical nanostructuring of electrodes via nonsolvent-induced phase separation for high-performance supercapacitors, J. Mater. Chem. A 6 (2018) 8909e8915. [44] H. Fan, F. Ran, X. Zhang, H. Song, W. Jing, K. Shen, L. Kong, L. Kang, A hierarchical porous carbon membrane from polyacrylonitrile/polyvinylpyrrolidone blending membranes: preparation, characterization and electrochemical capacitive performance, J. Energy Chem. 23 (2014) 684e693. [45] P. van de Witte, P.J. Dijkstra, J.W.A. van den Berg, J. Feijen, Phase separation processes in polymer solutions in relation to membrane formation, J. Membr. Sci. 117 (1996) 1e31. [46] L. Yue, W. Li, F. Sun, L. Zhao, L. Xing, Highly hydroxylated carbon fibres as electrode materials of all-vanadium redox flow battery, Carbon 48 (2010) 3079e3090. [47] P. Wu, Y. Qian, P. Du, H. Zhang, C. Cai, Facile synthesis of nitrogen-doped graphene for measuring the releasing process of hydrogen peroxide from living cells, J. Mater. Chem. 22 (2012) 6402e6412. [48] Y. Zhao, L. Yang, S. Chen, X. Wang, Y. Ma, Q. Wu, Y. Jiang, W. Qian, Z. Hu, Can boron and nitrogen Co-doping improve oxygen reduction reaction activity of carbon nanotubes? J. Am. Chem. Soc. 135 (2013) 1201e1204.