- Email: [email protected]
Twin-cocoon-derived self-standing nitrogen-oxygen-rich monolithic carbon material as the cost-effective electrode for redox flow batteries
Journal of Power Sources 421 (2019) 139–146
Contents lists available at ScienceDirect
Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Twin-cocoon-derived self-standing nitrogen-oxygen-rich monolithic carbon material as the cost-effective electrode for redox flow batteries
T
Rui Wang, Yinshi Li∗ Key Laboratory of Thermo-Fluid Science and Engineering of MOE, School of Energy and Power Engineering, Xi'an Jiaotong University, Xi'an, Shaanxi, 710049, China
H I GH L IG H T S
G R A P H I C A L A B S T R A C T
self-standing • Twin-cocoon-derived monolithic carbon catalyst material was proposed.
monolithic carbon • Nitrogen-doped contains oxygen-rich functional groups.
carbon catalyst material pro• NO-MC mises a high-performance and lowcost VRFB.
carbon catalyst material yields • NO-MC a 50% decrease in redox potential difference.
A R T I C LE I N FO
A B S T R A C T
Keywords: Flow battery Carbon material Twin cocoon Nitrogen-oxygen-doped Self-standing
Developing cost-effective and environmental-friend electrode material is critical to accelerate the large-scale commercialization of the all-vanadium redox flow batteries. Herein, we propose and develop a twin-cocoonderived self-standing carbon material by a green and safe preparation method. This facile degumming and carbonization process allows the twin-cocoon-derived monolithic carbon material to be decorated with nitrogen defects and oxygen functional groups. It has been demonstrated that this nitrogen-oxygen-rich monolithic carbon electrode material yields a promoted electrochemical activity for both V2+/V3+ and VO2+/VO2+ couples, causing a 50% decrease in redox potential difference and a 192% increase in diffusion slope as compared with the commercial carbon paper, thereby improving the electrochemical reversibility and enhancing the mass transfer kinetics. It is also found that the flow battery using nitrogen-oxygen-rich monolithic carbon electrode material shows 83% higher average discharge capacity and 20% higher energy efficiency than that with carbon paper at the current density of 100 mA cm−2, promising a high-performance and low-cost all-vanadium redox flow battery. This work opens a new way to developing monolithic carbon electrode material that possesses great potential applications in flow battery and other electrochemical energy conversion and storage systems.
1. Introduction The energy shortage and environmental pollution call for utilizing renewable and sustainable energy, e.g. solar and wind [1–4]. However, the feature of the fluctuation and intermittence of renewable energy has brought great challenges to practical applications [5,6]. Redox flow
∗
batteries as one of the most promising large-scale energy storage system meet this requirement, owing to their advantages of balancing the supply and demand of electricity with rapid response, high safety and flexibility [7–9]. Among flow batteries, the all-vanadium redox flow battery (VRFB) has its unique superiority in the light of the minimal cross-contamination by running the same vanadium element in both
Corresponding author. E-mail address: [email protected] (Y. Li).
https://doi.org/10.1016/j.jpowsour.2019.03.023 Received 7 December 2018; Received in revised form 14 February 2019; Accepted 7 March 2019 0378-7753/ © 2019 Elsevier B.V. All rights reserved.
Journal of Power Sources 421 (2019) 139–146
R. Wang and Y. Li
2. Experimental section
positive and negative electrolytes, resulting in a long cycle life [10–12]. Although promising, the low efficiency and rate capability of VRFB at high charge/discharge current density hinders the large-scale commercialization [13,14]. Therefore, it is vital to develop cost-effective electrode materials [15–19]. Carbon materials, typically carbon paper (CP) and graphite felt (GF), have attracted considerable attention in VRFBs because of the high stability, high conductivity and large corrosion resistance [13,17,20,21]. However, the pristine CP and GF that are composed of C-C bonds with weak wettability and chemical inertness possess the sluggish electrochemical activity and high kinetic irreversibility, lowering efficiency and current density of the VRFBs [20,22,23]. One effective solution for these issues is to introduce functional groups to carbon materials [24–27], such as doping nitrogen into carbon framework [28–30]. The appearance of N atoms not only forms vacancy and defects in the carbon matrix but also changes the charge distribution between N and C atoms, thereby increasing the mass transfer rate and electron transfer kinetics toward vanadium redox couples [28,31,32]. Moreover, the electronegative N atoms can be ionized out free electrons, raising active sites for vanadium ion exchange and redox reaction [23,29,33]. Currently, doping nitrogen into carbon is mainly conducted by treating carbon base in ammonia media at high temperature [20,34,35]. As seen, this treatment still suffers from both the toxic environment and high cost, limiting mass production [29,36]. An alternative approach is to utilize nitrogenous biomaterials [36,37]. It has been revealed that the corn-protein-derived graphite felt demonstrates high catalytic activity for vanadium redox reactions [36]. However, it should be mentioned that the commercial CP and GF as the carbon framework are typically fabricated by carbonizing polyacrylonitrile (PAN) at elevated temperature (> 2000 °C) [34,38,39], suggesting high cost as a result of the expensive precursor and severe preparing conditions. Obviously, developing the all-biomaterial-derived monolithic carbon materials that hold abundant functional groups is an ideal solution for the large-scale commercialization of VRFBs. To this end, our attention is paid to twin cocoon (Fig. S1), which is a natural polymer composite with a nitrogen content as high as 15% [40–43]. Inspired by the unique feature of twin cocoon, herein, a biomaterial-derived selfstanding monolithic carbon (MC) electrode material with enriched nitrogen defects and oxygen functional groups (NO-MC) was proposed and fabricated by the green, safe, and facile hydrothermal treatment and carbonization of the twin cocoon (Fig. 1). It is proved that this NOMC electrode material shows 83% higher average discharge capacity and 20% higher energy efficiency than the conventional carbon paper at the current density of 100 mA cm−2 in VRFBs. To the best of our knowledge, this is the first attempt to use twin cocoon as precursor for fabricating nitrogen-doped and oxygen functional group-rich monolithic electrode material.
2.1. Materials Twin cocoon of bombyx mori from a farm in southern China. Sodium carbonate (Na2CO3), vanadyl sulfate (VOSO4) and sulfuric acid (H2SO4) were purchased from Aladdin, ZhongTian Chemical Ltd., Sigma-Aldrich, respectively. Nafion 212 membrane was received from DuPont. Carbon paper from Toray. 2.2. Preparation of NO-MC electrode material The twin-cocoon-derived carbon electrode material was prepared by the green and facile hydrothermal treatment and carbonization of the twin cocoon as shown in Fig. 1. Prior to carbonizing twin cocoon, the sericin needs to be removed from silk, which is called degumming. The twin cocoon was first cut open, and then the inside pupa was removed as shown in Fig. S1. The tailored sample was immersed into the 100-mL Teflon-lined stainless steel autoclave that contains 0.06 M Na2CO3 solution. Then, the autoclave was sealed and heated in an electric oven at 120 °C for 2 h. The degummed twin cocoon was washed by DI water and dried at 65 °C for 24 h. Subsequently, the clean sample was burned at a given temperature (600 °C, 800 °C and 1000 °C) for 4 h with a heating rate of 5 °C min−1 in a quartz tube furnace (OTF-1200X) under Ar flow, thereby achieving the NO-MC electrode material that has the areal density of 11.56 mg cm−2. In addition, the as-received carbon paper (TGP-H-090, 14.33 mg cm−2) and the oxidized carbon paper (OCP, 13.82 mg cm−2) that was treated in air at 500 °C for 6 h were used for comparison. 2.3. Physical characterization The surface morphology and elemental composition of the samples were observed by the scanning electron microscopy (SEM, Gemini SEM500, Zeiss, Germany) at 20 kV. Energy-dispersive X-ray spectroscopy coupled to the SEM was used for elemental mapping. Raman spectra were measured by the Raman spectrometer (Lab RAM, Jobin Yvon, France). X-ray diffraction (XRD) patterns were obtained on the X-ray diffractometer (XRD-6100, Shimadzu, Japan) with scanning rate of 5° min−1 in an angle range from 15° to 75°. The Brunauer-Emmett-Teller (BET) measurement was used to collect surface area and pore size of the samples (Auto ChemTM II 2920, Micromeritics, USA). Surface chemical states of the samples were achieved by X-ray photoelectron spectroscopy (XPS, AXIS ULtrabld, Kratos, UK). The wettability of CP, OCP and NO-MC was observed by the contact angle tester (Theta, Attension, Sweden). 2.4. Electrochemical characterization Cyclic voltammetric (CV) and electrochemical impedance spectroscopy (EIS) measurements were performed by a conventional threeelectrode system connected to an electrochemical workstation (CHI
Fig. 1. Schematic diagram of fabricating NO-MC electrode material. 140
Journal of Power Sources 421 (2019) 139–146
R. Wang and Y. Li
Fig. 2e that the surface of the carbonized silk fiber becomes rough due to generating a large number of even-distributed protuberances, which is capable of increasing active surface area of the NO-MC. To offer an illuminating insight into the elemental distribution in the NO-MC electrode material, the mapping of C, N and O was obtained by the EDS analysis and is shown in Fig. 2f–i. As is seen, the distributions of N and O are rich and even in the skeleton of the NO-MC, suggesting forming the functionalized NO-MC so as to enhance material potentials. XRD analysis reveals the bulk structural information of the CP, OCP and NO-MC as shown in Fig. 3a. In comparison with CP and OCP that have a conspicuous diffraction peak at 26.5°, NO-MC possesses two broad and weak peaks at 26.5° and 43.1° (see the inset in Fig. 3a), corresponding respectively to the (002) and (100) lattice planes of graphitic carbon, which indicates the existence of graphitic structure and structural defects in NO-MC [38,46]. Raman spectra as shown in Fig. 3b confirms the rich structural defects in NO-MC due to the fact that the ratio of the intensity between the defect-induced D band and the graphite-induced G band (ID/IG) for NO-MC is higher than that of CP and OCP. The bulk structure of CP, OCP and NO-MC was also investigated by cryo-nitrogen adsorption: the BET surface area of NO-MC (21.16 m2 g−1) is much larger than that of both CP (0.79 m2 g−1) and OCP (1.30 m2 g−1). A strong adsorption in low pressure region (P/ P0 = 0–0.1) and an obvious hysteresis loop in medium pressure region (P/P0 = 0.4–0.8) for the NO-MC (Fig. 3c) are respectively attributed to the appearance of the mesopore and micropore [47], as evidenced by the distinct peaks at 2–5 nm in the pore size distribution curves shown in Fig. 3d. As seen, the rich structural defects, high BET surface area and multi-porous structure in NO-MC lead to more active sites for the redox reaction of vanadium ions [29]. The chemical structures of oxygen and nitrogen atoms on the surface of CP, OCP and NO-MC were analyzed by XPS spectra (Fig. 4). The C1s peak of carbon at 284.5 eV was used to calibrate all spectra. Although carbonized in a moderate heating temperature, exhibits sharp peak and high composition ratio of graphitic double-bonded carbon at 284.5 eV (Fig. 4b and Fig. S3) [48], which benefits from the rich β-sheet in twin cocoon [40,49]. The large O1s region and atomic content ratio of oxygen to carbon atoms (ca. 0.10) for the NO-MC indicate higher oxygen content on its surface (Fig. 4c and d). The O1s region can be differentiated into three peaks including C-C=O (533.2 eV), C-OH (532.1 eV) and C=O (530.9 eV), respectively (Fig. 4c) [50]. The oxygen functional groups could accelerate the formation of active sites for vanadium redox reaction, especially, the content of C-C=O is directly associated with the quantity of active sites [36]. It shows that the proportion of C-C=O in oxygen functional groups of NO-MC (ca. 31.05%) is higher than CP (ca. 0.00%) and OCP (ca. 24.38%) (Fig. S4). Moreover, as shown in Fig. 4a and e, no significant peaks were found in
760E). The samples with a diameter of 6 mm, platinum mesh and Ag/ AgCl electrode were used as working electrode, counter electrode and reference electrode, respectively. For CV measurement, the positive tests were carried out between 0.3 V and 1.4 V while the negative tests were performed between −0.9 V and 0.0 V in 1.0 M VOSO4 + 3.0 M H2SO4. AC impedance was conducted in a frequency ranging from 10−2 to 10−5 Hz with an excitation signal of 5 mV at 0.85 V. 2.5. Flow battery performance test The CP, OCP and NO-MC with the same active area of 4 cm2 (2 × 2 cm) were employed as positive and negative electrodes in flow battery. 20 mL solution containing 1.0 M VO2+ + 3.0 M H2SO4 and 20 mL solution containing 1.0 M V3+ + 3.0 M H2SO4 were used as the positive and negative electrolytes, respectively. Nafion NR-212 (Dupont, USA) was applied as the membrane. The battery performance was tested in a zero-gap battery, which was assembled with serpentine flow-field structured graphite plates, copper current collectors, aluminum support plate and bakelite end plates. The electrolytes in the batteries were circulated with a peristaltic pump (WT600-2J, Longer) at a fixed flow rate of 46 mL min−1. The performances of the VRFBs were assessed in a battery test system (Arbin BT-G, Arbin, USA) under the operating potential range between 1.7 and 0.9 V with a current density range between 40 and 100 mA cm-2. All measurements were conducted at room temperature. 3. Results and discussion Fig. 2 presents the surface morphologies of pristine, degummed and carbonized twin cocoons at different macroscopic and microscopic levels. As observed in Fig. 2a and b, the pristine twin cocoon that has a multi-layer structure is unbending. This is because the twin cocoon is composed of silk fibers fixed by sericin (see the inset in Fig. 2b) [42–45]. The question is how to make the twin cocoon-derived carbon material possess porous structure and flexible mechanical property. To meet this need, as shown in Fig. 1, i) in the degumming process, twin cocoon was heated in an alkaline aqueous solution under a high pressure that prevents the solution from boiling, resulting in forming a porous (Fig. 2c) but loose (see the inset in Fig. 2c) structure; ii) in carbonizing process, a moderate heating temperature was applied to the degummed twin cocoon, as a result, the cross-linked structure (see the inset in Fig. 2d) was created, thereby enhancing the mechanical property and electronic conductivity as well as ensuring the reasonable flexibility and porosity of the carbonized twin cocoon (Fig. 2d and Fig. S2). As such, an all-biomaterial-derived self-standing monolithic carbon material was successfully prepared. Interestingly, it can be seen from
Fig. 2. (a) The images of the pristine twin cocoon, hydrothermal treated twin cocoon and carbonized twin cocoon. SEM images of (b) the pristine twin cocoon, (c) the hydrothermal treated twin cocoon, and (d) the carbonized twin cocoon. (e) SEM image of the carbonized silk fiber, and (f–i) the EDS elemental mapping of the carbonized silk fiber.
141
Journal of Power Sources 421 (2019) 139–146
R. Wang and Y. Li
Fig. 3. (a) XRD patterns, (b) Raman spectra, (c) adsorption/desorption isotherms, and (d) pore size distribution of CP, OCP and NO-MC.
nitrogen, owning to that it can be ionized out free electrons and is highly stable under acidic conditions, has been proved to be more effective for vanadium redox reaction [29]. To obtain a suitable carbonizing temperature for the material with a high content of graphitic-N, twin cocoons with different treating temperature were analyzed by XPS (Fig. S5). It is demonstrated that the proportion of graphitic-N in nitrogen species for NO-MC with 1000 °C (ca. 21.65%) is higher than that with 600 °C (ca. 17.79%) and 800 °C (ca. 19.59%) (Fig. 4f).
the N1s region for CP and OCP, whereas the nitrogen-containing groups in the NO-MC were observed with an obvious increase in N1s region at 400 eV. Carbon materials containing nitrogen have been known to significantly promote the vanadium redox reaction [28,29], according to XPS analysis, the nitrogen content in NO-MC was ca. 5.81%. The N1s peak of oxygenated-N, graphitic-N, pyrrolic-N, and pyridinic-N were observed at 403.2, 401.4, 400.3 and 398.6 eV in NO-MC spectrum, respectively (Fig. 4e) [51]. Among the nitrogen species, graphitic-
Fig. 4. (a) XPS spectra of the CP, OCP and NO-MC. XPS analysis and its fitting from high resolution (b) C1s peak, (c) O1s peak, (d) chemical composition ratio of oxygen and carbon atoms for CP, OCP and NO-MC, (e) N1s peak, and (f) chemical composition ratio of NO-MC with different carbonized temperature (600, 800 and 1000 °C) from N1s spectra; (g) contact angles of the CP, OCP and NO-MC.
142
Journal of Power Sources 421 (2019) 139–146
R. Wang and Y. Li
Fig. 5. (a) Cyclic voltammograms of the CP and NO-MC at a scan rate of 5 mV s−1 with a potential window of 0.3–1.4 V versus Ag/ AgCl, (b) Nyquist plots of CP, OCP and NOMC in the frequency range from 10−2–105 Hz at 8.5 V, (c–d) CV of the CP and NO-MC at different scan rates (Insets: plot of the anodic peak current (Ipa) versus the square root of each scan rate) in 1.0 M VOSO4 + 3.0 M H2SO4 solution.
determination (R2) for both CP (99.95) and NO-MC (99.87) are close to 100%, indicating that the vanadium redox reactions are primarily controlled by the diffusion process in both CP and NO-MC [52,53]. In this regard, the slope of NO-MC is 195, 192% larger than that of CP with a value of 66.7, significantly lowering the mass transfer resistance for the vanadium redox reaction. This is ascribed to the better hydrophilic surface in NO-MC due to the formation of oxygen functional groups and nitrogen defects. The flow batteries assembled with CP, OCP and NO-MC electrodes were tested at different current densities as displayed in Fig. 6a–d. It was found that the VRFB with NO-MC electrodes has lower charge/ discharge overpotential and higher capacities as compared with CP and OCP electrodes. The battery associated with the NO-MC electrodes achieves the average discharge capacity from 18.62 to 10.50 Ah L−1 with current density from 40 to 100 mA cm−2, great higher than that with the CP from 15.97 to 5.75 Ah L−1 and OCP from 17.67 to 10.04 Ah L−1 (Fig. 7a). Fig. 7b shows the coulombic efficiency (CE) and voltage efficiency (VE) for VRFBs. It can be seen the CEs for all three kind of electrodes are more than 90%, demonstrating good airtightness of the battery setup [25]. As for the VE, agreeing with the trend of charge–discharge curves, as increasing current density from 40 to 100 mA cm−2, the VE of VRFB with NO-MC decreases from 90% to 75%, while higher than that with CP reducing from 80% to 63% and OCP ranging from 80% to 69%, because of the enhanced electrochemical reversibility and improved activity of the nitrogen-doped oxygen-rich NO-MC electrode. To further evaluate the overall efficiency and cycling stability of redox flow batteries with CP, OCP and NO-MC electrodes, the energy efficiency (EE) was achieved during long-term charge-discharge cycling test as presented in Fig. 7c, when setting the current density at 40 mA cm−2, 60 mA cm−2, 80 mA cm−2, and 100 mA cm−2, the battery with NO-MC electrodes yields the respective EE of 84%, 81%, 77% and 73%, which are 14%, 15%, 18% and 20% higher than that with the pristine CP electrodes. Moreover, it can be clearly seen that battery with NO-MC electrode is more stable than that with CP and OCP electrodes. The improved performance and stability of the battery with NO-MC electrodes are mainly attributed to the nitrogen-doping oxygen-rich surface and the monolithic porous structure.
Furthermore, thanks to the rational content of oxygen and nitrogen functional groups on the surface, the NO-MC with a contact angle of 0° exhibits the better wettability than CP (130.55°) and OCP (102.98°) do (Fig. 4g), implying the high hydrophilic surface for the NO-MC to increase the mass transfer rate. Therefore, the nitrogen-doped NO-MC material with oxygen-rich functional groups not only provides abundant active sites but also promises a fast electrolyte diffusion so as to improve the kinetics of vanadium redox reaction, thus boosting the performance of VRFBs [23,36]. Cyclic voltammetry tests were carried out to show the electrochemical properties of CP, OCP and NO-MC (Fig. 5a) in terms of the ratio of oxidation and reduction peak current (Ipa/Ipc), the redox potential difference (ΔEp), and the redox onset potential: i) The ratio of oxidation and reduction peak current of NO-MC (Ipa/Ipc = 1.25) gets close to CP (Ipa/Ipc = 1.23) and OCP (Ipa/Ipc = 1.21), indicating the stable vanadium redox reaction for NO-MC; ii) NO-MC presents a smaller redox potential difference (ΔEp = 166 mV) compared to both the CP (ΔEp = 329 mV) and OCP (ΔEp = 293 mV), resulting in a better electrochemical reversibility; iii) The lower oxidization onset potential and higher reduction onset potential of NO-MC (see the insets of Fig. 5a) implies its high electrochemical catalytic activity for the vanadium redox reaction. Moreover, with regard to the negative redox reaction toward V2+ and V3+ ions, there exist the remarkable oxidation and reduction peaks for NO-MC (Ipa = 107.18 mA, Ipc = 140.48 mA) at −0.28 mV and −0.75 mV while no significant cathodic peaks for CP (Fig. S6), which demonstrate the better electrochemical activity for NOMC in the negative redox reaction. Electrochemical impedance spectroscopy (EIS) analysis for CP, OCP and NO-MC (Fig. 5b) also agreed with the conclusion of the cyclic voltammetry tests. As seen, in contrast, NO-MC that owns the lower charge transfer resistance and the faster mass transfer process exhibits the best electrochemical activity, as proven by the smaller diameter of the semi-circle at high-frequency region and the larger line slope at low-frequency region. To further ascertain the mass transfer properties of CP and NO-MC electrode materials, the linear relationship between peak current density and square root of scan rate was quantified by the Randles-Sevcik equation [52]. As shown in Fig. 5c and d, the coefficients of 143
Journal of Power Sources 421 (2019) 139–146
R. Wang and Y. Li
Fig. 6. (a–c) Charge/discharge curves of batteries with CP, OCP and NO-MC electrodes at various current densities, and (d) with CP, OCP and NO-MC electrodes at 40 mA cm−2 during cycling test.
Fig. 7. (a) Discharge capacities of VRFBs as a function of cycle number at different current densities with CP and NO-MC electrodes; (b) coulombic and voltage efficiencies of VRFBs with CP, OCP and NO-MC electrodes at various current densities; (c) energy efficiencies of CP, OCP and NO-MC electrodes during cycling test. 144
Journal of Power Sources 421 (2019) 139–146
R. Wang and Y. Li
superiority of the NO-MC electrode material is mainly attributed to two aspects: i) the monolithic and cross-linked structure enables the NO-MC to possess not only the enhanced mechanical property and electronic conductivity but also the reasonable flexibility and porosity; ii) the rough surface of the skeleton with oxygen functional groups and nitrogen defects allows the NO-MC to hold the high electrochemical activity and reversibility. Therefore, the all-biomaterial-derived N-doped O-rich monolithic carbon electrode material promises a great potential in redox flow battery, even other electrochemical energy devices. Conflicts of interest There are no conflicts to declare. Acknowledgments This work was fully supported by the National Natural Science Foundation of China (51776156). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.jpowsour.2019.03.023. References [1] J.W. Ager, A.A. Lapkin, Chemical storage of renewable energy, Science 360 (6390) (2018) 707–708. [2] X.D. Sun, Y.S. Li, Understanding mass and charge transports to create anion-ionomer-free high-performance alkaline direct formate fuel cells, Int. J. Hydrog. Energy 44 (14) (2019) 7538–7543. [3] Y.S. Li, Y. Feng, X.D. Sun, Insight into interface behaviors to building a phaseboundary-matched Na-ion direct liquid fuel cell, ACS Sustain. Chem. Eng. 6 (10) (2018) 12827–12834. [4] Y. Qiu, M.J. Li, K. Wang, Z.B. Liu, X.D. Xue, Aiming strategy optimization for uniform flux distribution in the receiver of a linear fresnel solar reflector using a multi-objective genetic algorithm, Appl. Energy 205 (2017) 1394–1407. [5] B. Dunn, H. Kamath, J.M. Tarascon, Electrical energy storage for the grid: a battery of choices, Science 334 (6058) (2011) 928–935. [6] Z. Yang, J. Zhang, M.C. Kintner-Meyer, X. Lu, D. Choi, J.P. Lemmon, J. Liu, Electrochemical energy storage for green grid, Chem. Rev. 111 (5) (2011) 3577–3613. [7] J. Zhang, G. Jiang, P. Xu, A.G. Kashkooli, M. Mousavi, A. Yu, Z. Chen, An allaqueous redox flow battery with unprecedented energy density, Energy Environ. Sci. 11 (2018) 2010–2015. [8] H. Jiang, L. Wei, X. Fan, J. Xu, W. Shyy, T. Zhao, A novel energy storage system incorporating electrically rechargeable liquid fuels as the storage medium, Sci. Bull. 64 (4) (2019) 270–280. [9] P. Leung, X. Li, C.P. de Leon, L. Berlouis, C.T.J. Low, F.C. Walsh, Progress in redox flow batteries, remaining challenges and their applications in energy storage, RSC Adv. 2 (27) (2012) 10125–10156. [10] L. Li, S. Kim, W. Wang, M. Vijayakumar, Z. Nie, B. Chen, J. Zhang, G. Xia, J. Hu, G. Graff, J. Liu, Z. Yang, A stable vanadium redox-flow battery with high energy density for large-scale energy storage, Adv. Energy Mater. 1 (3) (2011) 394–400. [11] R. Wang, Y.S. Li, Y.L. He, Achieving gradient–pore–oriented graphite felt for vanadium redox flow batteries: meeting improved electrochemical activity and enhanced mass transport from nano– to micro–scale, J. Mater. Chem. A (2019), https://doi.org/10.1039/c9ta00807a. [12] Q. Deng, P. Huang, W. Zhou, Q. Ma, N. Zhou, H. Xie, W. Ling, C. Zhou, Y. Yin, X. Wu, X. Lu, Y. Guo, A high-Performance composite electrode for vanadium redox flow batteries, Adv. Energy Mater. 7 (18) (2017) 1700461. [13] B. Li, M. Gu, Z. Nie, Y. Shao, Q. Luo, X. Wei, X. Li, J. Xiao, C. Wang, V. Sprenlde, W. Wang, Bismuth nanoparticle decorating graphite felt as a high-performance electrode for an all-vanadium redox flow battery, Nano Lett. 13 (3) (2013) 1330–1335. [14] Q. Liu, A. Turhan, T.A. Zawodzinski, M.M. Mench, In situ potential distribution measurement in an all-vanadium flow battery, Chem. Commun. 49 (56) (2013) 6292–6294. [15] R. Kumar, T. Bhuvana, A. Sharma, Tire waste derived turbostratic carbon as electrode for vanadium redox flow battery, ACS Sustain. Chem. Eng. 6 (7) (2018) 8238–8246. [16] S. Abbas, H. Lee, J. Hwang, A. Mehmood, H. Shin, S. Mehboob, J. Lee, H.Y. Ha, A novel approach for forming carbon nanorods on the surface of carbon felt electrode by catalytic etching for high-performance vanadium redox flow battery, Carbon 128 (2018) 31–37. [17] C. Yao, H. Zhang, T. Liu, X. Li, Z. Liu, Carbon paper coated with supported tungsten trioxide as novel electrode for all-vanadium flow battery, J. Power Sources 218 (2012) 455–461.
Fig. 8. Schematic diagram of an alternative sustainable energy ecosystem.
As observed, the all-biomaterial-derived monolithic carbon material has been proven to be a promising high-performance and low-cost electrode in VRFBs. The abundant oxygen-containing active sites with nitrogen defects play an important role in the vanadium redox reaction as described in Fig. 8: i) in the charge process, the positively charged vanadium ions (VO2+ or V3+) were easily absorbed by the negatively charged nitrogen atoms to form the N-V bond; and ii) after the oxidation or reduction reaction, the reaction products (VO2+ or V2+) easily diffuse to electrolyte solution from the nitrogen active sites by ion exchange [29]. Let us place the present cocoon-derived carbon electrode material in a large-scale electrical power system as demonstrated in Fig. 8. The cocoon, a protein-rich natural polymer fiber composite, can be easily obtained in silkworm life cycle, including egg, larva, pupa and moth [42,43,54]. All of them are natural and sustainable. More importantly, after more than 4000 years utilization by humanity, currently, cocoon is cheap (ca. $14 per kg) and abundant in the textile raw material market [41,55–57]. The proposed hydrothermal treatment and carbonization process enables the twin cocoon readily to be a cost-effective carbon electrode material, as a result boosting the performance of redox flow battery, thereby meeting the requirement of utilizing renewable energy, e.g. solar and wind. As such, an alternative sustainable energy ecosystem could be established. More appealingly, this all-biomaterial-derived self-standing monolithic carbon material also can be applied for other energy conversion and storage devices, such as lithium-ion battery [58,59], fuel cell [60,61], supercapacitor [62,63]. 4. Conclusion To summarize, twin-cocoon-derived self-standing nitrogen-doped monolithic carbon materials with oxygen-rich functional groups was proposed and developed as the high-performance and low-cost electrode for VRFBs by a facile, well-controlled and environmental-friendly method in this work. Compared with CP and OCP, NO-MC electrode material exhibits higher electrochemical activity and reversibility in vanadium redox reaction, as well as high energy efficiency, large discharge capacity and better stability in practical redox flow battery. The 145
Journal of Power Sources 421 (2019) 139–146
R. Wang and Y. Li
[18] L. Wei, T.S. Zhao, G. Zhao, L. An, L. Zeng, A high-performance carbon nanoparticledecorated graphite felt electrode for vanadium redox flow batteries, Appl. Energy 176 (2016) 74–79. [19] P.C. Ghimire, R. Schweiss, G.G. Scherer, N. Wai, T.M. Lim, A. Bhattarai, Titanium carbide-decorated graphite felt as high performance negative electrode in vanadium redox flow batteries, J. Mater. Chem. 6 (15) (2018) 6625–6632. [20] M.H. Chakrabarti, N.P. Brandon, S.A. Hajimolana, E. 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) 150–166. [21] A. Parasuraman, T.M. Lim, C. Menictas, M. Skyllas-Kazacos, Review of material research and development for vanadium redox flow battery applications, Electrochim. Acta 101 (2013) 27–40. [22] 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 (8) (2013) 970–986. [23] L. Wu, Y. Shen, L. Yu, J. Xi, X. Qiu, Boosting vanadium flow battery performance by Nitrogen-doped carbon nanospheres electrocatalyst, Nanomater. Energy 28 (2016) 19–28. [24] P. Huang, W. Ling, H. Sheng, Y. Zhou, X. Wu, X. Zeng, X. Wu, Y. Guo, Heteroatomdoped electrodes for all-vanadium redox flow batteries with ultralong lifespan, J. Mater. Chem. 6 (1) (2018) 41–44. [25] H. Jiang, W. Shyy, L. Zeng, R. Zhang, T.S. Zhao, Highly efficient and ultra-stable boron-doped graphite felt electrodes for vanadium redox flow batteries, J. Mater. Chem. 6 (2018) 13244–13253. [26] B. Sun, M. Skyllas-Kazacos, Modification of graphite electrode materials for vanadium redox flow battery application-I. Thermal treatment, Electrochim. Acta 37 (7) (1992) 1253–1260. [27] B. Sun, M. Skyllas-Kazacos, Chemical modification of graphite electrode materials for vanadium redox flow battery application-part II. Acid treatments, Electrochim. Acta 37 (13) (1992) 2459–2465. [28] 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 (16) (2012) 2164–2167. [29] J. Jin, X. Fu, Q. Liu, Y. Liu, Z. Wei, K. Niu, J. Zhang, Identifying the active site in nitrogen-doped graphene for the VO2+/VO2+ redox reaction, ACS Nano 7 (6) (2013) 4764–4773. [30] J. Liu, P. Song, Z. Ning, W. Xu, Recent advances in heteroatom-doped metal-free electrocatalysts for highly efficient oxygen reduction reaction, Electrocatalysis-US 6 (2) (2015) 132–147. [31] F. Zheng, Y. Yang, Q. Chen, High lithium anodic performance of highly nitrogendoped porous carbon prepared from a metal-organic framework, Nat. Commun. 5 (2014) 5261. [32] M. Park, I.Y. Jeon, J. Ryu, J.B. Baek, J. Cho, Exploration of the effective location of surface oxygen defects in graphene-based electrocatalysts for all-vanadium redoxflow batteries, Adv. Energy Mater. 5 (5) (2015) 1401550. [33] A. Ejigu, M. Edwards, D.A. Walsh, Synergistic catalyst-support interactions in a graphene–Mn3O4 electrocatalyst for vanadium redox flow batteries, ACS Catal. 5 (12) (2015) 7122–7130. [34] T. Wu, K. Huang, S. Liu, S. Zhuang, D. Fang, S. Li, D. Lu, A. Su, Hydrothermal ammoniated treatment of PAN-graphite felt for vanadium redox flow battery, J. Solid State Electrochem. 16 (2) (2012) 579–585. [35] 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 (13) (2010) 4375–4379. [36] M. Park, J. Ryu, Y. Kim, J. Cho, Corn protein-derived nitrogen-doped carbon materials with oxygen-rich functional groups: a highly efficient electrocatalyst for allvanadium redox flow batteries, Energy Environ. Sci. 7 (11) (2014) 3727–3735. [37] 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 (1) (2016) A5144–A5149. [38] Z.H. Zhang, T.S. Zhao, B.F. Bai, L. Zeng, L. Wei, A highly active biomass-derived electrode for all vanadium redox flow batteries, Electrochim. Acta 248 (2017) 197–205. [39] N. Yusof, A.F. Ismail, Post spinning and pyrolysis processes of polyacrylonitrile (PAN)-based carbon fiber and activated carbon fiber: a review, J. Anal. Appl. Pyrolysis 93 (2012) 1–13. [40] S.Y. Cho, Y.S. Yun, S. Lee, D. Jang, K.Y. Park, J.K. Kim, H.J. Jin, Carbonization of a
[41]
[42] [43]
[44]
[45] [46] [47] [48]
[49]
[50]
[51]
[52]
[53]
[54] [55] [56] [57] [58]
[59]
[60]
[61]
[62]
[63]
146
stable β-sheet-rich silk protein into a pseudographitic pyroprotein, Nat. Commun. 6 (2015) 7145. H. Yamada, H. Nakao, Y. Takasu, K. Tsubouchi, Preparation of undegraded native molecular fibroin solution from silkworm cocoons, Mat. Sci. Eng. C-Mater. 14 (1–2) (2001) 41–46. O. Hakimi, D.P. Knight, F. Vollrath, P. Vadgama, Spider and mulberry silkworm silks as compatible biomaterials, Compos. B Eng. 38 (3) (2007) 324–337. J. Zhang, J. Kaur, R. Rajkhowa, J.L. Li, X.Y. Liu, X.G. Wang, Mechanical properties and structure of silkworm cocoons: a comparative study of Bombyx mori, Antheraea assamensis, Antheraea pernyi and Antheraea mylitta silkworm cocoons, Mat. Sci. Eng. C-Mater. 33 (6) (2013) 3206–3213. J. Hou, C. Cao, F. Idrees, X. Ma, Hierarchical porous nitrogen-doped carbon nanosheets derived from silk for ultrahigh-capacity battery anodes and supercapacitors, ACS Nano 9 (3) (2015) 2556–2564. F. Chen, D. Porter, F. Vollrath, Silk cocoon (Bombyx mori): multi-layer structure and mechanical properties, Acta Biomater. 8 (7) (2012) 2620–2627. Z.Q. Li, C.J. Lu, Z.P. Xia, Y. Zhou, Z. Luo, X-ray diffraction patterns of graphite and turbostratic carbon, Carbon 45 (8) (2007) 1686–1695. B. Xu, H. Duan, M. Chu, G. Cao, Y. Yang, Facile synthesis of nitrogen-doped porous carbon for supercapacitors, J. Mater. Chem. 1 (14) (2013) 4565–4570. C. Gao, N. Wang, S. Peng, S. Liu, Y. Lei, X. Liang, S. Zeng, H. Zi, Influence of Fenton's reagent treatment on electrochemical properties of graphite felt for all vanadium redox flow battery, Electrochim. Acta 88 (2013) 193–202. Y.S. Yun, K.Y. Park, B. Lee, S.Y. Cho, Y.U. Park, S.J. Hong, Y.W. Park, Sodium-ion storage in pyroprotein-based carbon nanoplates, Adv. Mater. 27 (43) (2015) 6914–6921. S. Park, H. Kim, Fabrication of nitrogen-doped graphite felts as positive electrodes using polypyrrole as a coating agent in vanadium redox flow batteries, J. Mater. Chem. 3 (23) (2015) 12276–12283. K. Gong, F. Du, Z. Xia, M. Durstock, L. Dai, Nitrogen-doped carbon nanotube arrays with high electrocatalytic activity for oxygen reduction, Science 323 (5915) (2009) 760–764. R. Huang, C. Sun, T. Tseng, W. Chao, K. Hsueh, F. Shieu, Investigation of active electrodes modified with platinum/multiwalled carbon nanotube for vanadium redox flow battery, J. Electrochem. Soc. 159 (10) (2012) A1579–A1586. J. Kim, K.J. Kim, M. Park, N.J. Lee, U. Hwang, H. Kim, Y. Kim, Development of metal-based electrodes for non-aqueous redox flow batteries, Electrochem. Commun. 13 (9) (2011) 997–1000. J. Zhang, R. Rajkhowa, J.L. Li, X.Y. Liu, X.G. Wang, Silkworm cocoon as natural material and structure for thermal insulation, Mater. Des. 49 (2013) 842–849. Y. Takahashi, M. Gehoh, K. Yuzuriha, Crystal structure of silk (Bombyx mori), J. Polym. Sci., Polym. Phys. Ed. 29 (7) (1991) 889–891. W. Liu, Y. Yu, E. Liu, Analysis on operation of Chinese cocoon silk industry in 2016 and prospect in 2017, J. Silk 54 (6) (2017) 1–7. F. Chen, D. Porter, F. Vollrath, Morphology and structure of silkworm cocoons, Mat. Sci. Eng. C-Mater. 32 (4) (2012) 772–778. L. Qie, W.M. Chen, Z.H. Wang, Q.G. Shao, X. Li, L.X. Yuan, Y.H. Huang, Nitrogendoped porous carbon nanofiber webs as anodes for lithium ion batteries with a superhigh capacity and rate capability, Adv. Mater. 24 (15) (2012) 2047–2050. J. Song, M.L. Gordin, T. Xu, S. Chen, Z. Yu, H. Sohn, D. Wang, Strong lithium polysulfide chemisorption on electroactive sites of nitrogen-doped carbon composites for high-performance lithium-sulfur battery cathodes, Angew. Chem. Int. Ed. 127 (14) (2015) 4399–4403. K. Gong, F. Du, Z. Xia, M. Durstock, L. Dai, Nitrogen-doped carbon nanotube arrays with high electrocatalytic activity for oxygen reduction, Science 323 (5915) (2009) 760–764. D. Guo, R. Shibuya, C. Akiba, S. Saji, T. Kondo, J. Nakamura, Active sites of nitrogen-doped carbon materials for oxygen reduction reaction clarified using model catalysts, Science 351 (6271) (2016) 361–365. B. Li, F. Dai, Q. Xiao, L. Yang, J. Shen, C. Zhang, M. Cai, Nitrogen-doped activated carbon for a high energy hybrid supercapacitor, Energy Environ. Sci. 9 (1) (2016) 102–106. T. Lin, I.W. Chen, F. Liu, C. Yang, H. Bi, F. Xu, F. Huang, Nitrogen-doped mesoporous carbon of extraordinary capacitance for electrochemical energy storage, Science 350 (6267) (2015) 1508–1513.
Contents lists available at ScienceDirect
Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Twin-cocoon-derived self-standing nitrogen-oxygen-rich monolithic carbon material as the cost-effective electrode for redox flow batteries
T
Rui Wang, Yinshi Li∗ Key Laboratory of Thermo-Fluid Science and Engineering of MOE, School of Energy and Power Engineering, Xi'an Jiaotong University, Xi'an, Shaanxi, 710049, China
H I GH L IG H T S
G R A P H I C A L A B S T R A C T
self-standing • Twin-cocoon-derived monolithic carbon catalyst material was proposed.
monolithic carbon • Nitrogen-doped contains oxygen-rich functional groups.
carbon catalyst material pro• NO-MC mises a high-performance and lowcost VRFB.
carbon catalyst material yields • NO-MC a 50% decrease in redox potential difference.
A R T I C LE I N FO
A B S T R A C T
Keywords: Flow battery Carbon material Twin cocoon Nitrogen-oxygen-doped Self-standing
Developing cost-effective and environmental-friend electrode material is critical to accelerate the large-scale commercialization of the all-vanadium redox flow batteries. Herein, we propose and develop a twin-cocoonderived self-standing carbon material by a green and safe preparation method. This facile degumming and carbonization process allows the twin-cocoon-derived monolithic carbon material to be decorated with nitrogen defects and oxygen functional groups. It has been demonstrated that this nitrogen-oxygen-rich monolithic carbon electrode material yields a promoted electrochemical activity for both V2+/V3+ and VO2+/VO2+ couples, causing a 50% decrease in redox potential difference and a 192% increase in diffusion slope as compared with the commercial carbon paper, thereby improving the electrochemical reversibility and enhancing the mass transfer kinetics. It is also found that the flow battery using nitrogen-oxygen-rich monolithic carbon electrode material shows 83% higher average discharge capacity and 20% higher energy efficiency than that with carbon paper at the current density of 100 mA cm−2, promising a high-performance and low-cost all-vanadium redox flow battery. This work opens a new way to developing monolithic carbon electrode material that possesses great potential applications in flow battery and other electrochemical energy conversion and storage systems.
1. Introduction The energy shortage and environmental pollution call for utilizing renewable and sustainable energy, e.g. solar and wind [1–4]. However, the feature of the fluctuation and intermittence of renewable energy has brought great challenges to practical applications [5,6]. Redox flow
∗
batteries as one of the most promising large-scale energy storage system meet this requirement, owing to their advantages of balancing the supply and demand of electricity with rapid response, high safety and flexibility [7–9]. Among flow batteries, the all-vanadium redox flow battery (VRFB) has its unique superiority in the light of the minimal cross-contamination by running the same vanadium element in both
Corresponding author. E-mail address: [email protected] (Y. Li).
https://doi.org/10.1016/j.jpowsour.2019.03.023 Received 7 December 2018; Received in revised form 14 February 2019; Accepted 7 March 2019 0378-7753/ © 2019 Elsevier B.V. All rights reserved.
Journal of Power Sources 421 (2019) 139–146
R. Wang and Y. Li
2. Experimental section
positive and negative electrolytes, resulting in a long cycle life [10–12]. Although promising, the low efficiency and rate capability of VRFB at high charge/discharge current density hinders the large-scale commercialization [13,14]. Therefore, it is vital to develop cost-effective electrode materials [15–19]. Carbon materials, typically carbon paper (CP) and graphite felt (GF), have attracted considerable attention in VRFBs because of the high stability, high conductivity and large corrosion resistance [13,17,20,21]. However, the pristine CP and GF that are composed of C-C bonds with weak wettability and chemical inertness possess the sluggish electrochemical activity and high kinetic irreversibility, lowering efficiency and current density of the VRFBs [20,22,23]. One effective solution for these issues is to introduce functional groups to carbon materials [24–27], such as doping nitrogen into carbon framework [28–30]. The appearance of N atoms not only forms vacancy and defects in the carbon matrix but also changes the charge distribution between N and C atoms, thereby increasing the mass transfer rate and electron transfer kinetics toward vanadium redox couples [28,31,32]. Moreover, the electronegative N atoms can be ionized out free electrons, raising active sites for vanadium ion exchange and redox reaction [23,29,33]. Currently, doping nitrogen into carbon is mainly conducted by treating carbon base in ammonia media at high temperature [20,34,35]. As seen, this treatment still suffers from both the toxic environment and high cost, limiting mass production [29,36]. An alternative approach is to utilize nitrogenous biomaterials [36,37]. It has been revealed that the corn-protein-derived graphite felt demonstrates high catalytic activity for vanadium redox reactions [36]. However, it should be mentioned that the commercial CP and GF as the carbon framework are typically fabricated by carbonizing polyacrylonitrile (PAN) at elevated temperature (> 2000 °C) [34,38,39], suggesting high cost as a result of the expensive precursor and severe preparing conditions. Obviously, developing the all-biomaterial-derived monolithic carbon materials that hold abundant functional groups is an ideal solution for the large-scale commercialization of VRFBs. To this end, our attention is paid to twin cocoon (Fig. S1), which is a natural polymer composite with a nitrogen content as high as 15% [40–43]. Inspired by the unique feature of twin cocoon, herein, a biomaterial-derived selfstanding monolithic carbon (MC) electrode material with enriched nitrogen defects and oxygen functional groups (NO-MC) was proposed and fabricated by the green, safe, and facile hydrothermal treatment and carbonization of the twin cocoon (Fig. 1). It is proved that this NOMC electrode material shows 83% higher average discharge capacity and 20% higher energy efficiency than the conventional carbon paper at the current density of 100 mA cm−2 in VRFBs. To the best of our knowledge, this is the first attempt to use twin cocoon as precursor for fabricating nitrogen-doped and oxygen functional group-rich monolithic electrode material.
2.1. Materials Twin cocoon of bombyx mori from a farm in southern China. Sodium carbonate (Na2CO3), vanadyl sulfate (VOSO4) and sulfuric acid (H2SO4) were purchased from Aladdin, ZhongTian Chemical Ltd., Sigma-Aldrich, respectively. Nafion 212 membrane was received from DuPont. Carbon paper from Toray. 2.2. Preparation of NO-MC electrode material The twin-cocoon-derived carbon electrode material was prepared by the green and facile hydrothermal treatment and carbonization of the twin cocoon as shown in Fig. 1. Prior to carbonizing twin cocoon, the sericin needs to be removed from silk, which is called degumming. The twin cocoon was first cut open, and then the inside pupa was removed as shown in Fig. S1. The tailored sample was immersed into the 100-mL Teflon-lined stainless steel autoclave that contains 0.06 M Na2CO3 solution. Then, the autoclave was sealed and heated in an electric oven at 120 °C for 2 h. The degummed twin cocoon was washed by DI water and dried at 65 °C for 24 h. Subsequently, the clean sample was burned at a given temperature (600 °C, 800 °C and 1000 °C) for 4 h with a heating rate of 5 °C min−1 in a quartz tube furnace (OTF-1200X) under Ar flow, thereby achieving the NO-MC electrode material that has the areal density of 11.56 mg cm−2. In addition, the as-received carbon paper (TGP-H-090, 14.33 mg cm−2) and the oxidized carbon paper (OCP, 13.82 mg cm−2) that was treated in air at 500 °C for 6 h were used for comparison. 2.3. Physical characterization The surface morphology and elemental composition of the samples were observed by the scanning electron microscopy (SEM, Gemini SEM500, Zeiss, Germany) at 20 kV. Energy-dispersive X-ray spectroscopy coupled to the SEM was used for elemental mapping. Raman spectra were measured by the Raman spectrometer (Lab RAM, Jobin Yvon, France). X-ray diffraction (XRD) patterns were obtained on the X-ray diffractometer (XRD-6100, Shimadzu, Japan) with scanning rate of 5° min−1 in an angle range from 15° to 75°. The Brunauer-Emmett-Teller (BET) measurement was used to collect surface area and pore size of the samples (Auto ChemTM II 2920, Micromeritics, USA). Surface chemical states of the samples were achieved by X-ray photoelectron spectroscopy (XPS, AXIS ULtrabld, Kratos, UK). The wettability of CP, OCP and NO-MC was observed by the contact angle tester (Theta, Attension, Sweden). 2.4. Electrochemical characterization Cyclic voltammetric (CV) and electrochemical impedance spectroscopy (EIS) measurements were performed by a conventional threeelectrode system connected to an electrochemical workstation (CHI
Fig. 1. Schematic diagram of fabricating NO-MC electrode material. 140
Journal of Power Sources 421 (2019) 139–146
R. Wang and Y. Li
Fig. 2e that the surface of the carbonized silk fiber becomes rough due to generating a large number of even-distributed protuberances, which is capable of increasing active surface area of the NO-MC. To offer an illuminating insight into the elemental distribution in the NO-MC electrode material, the mapping of C, N and O was obtained by the EDS analysis and is shown in Fig. 2f–i. As is seen, the distributions of N and O are rich and even in the skeleton of the NO-MC, suggesting forming the functionalized NO-MC so as to enhance material potentials. XRD analysis reveals the bulk structural information of the CP, OCP and NO-MC as shown in Fig. 3a. In comparison with CP and OCP that have a conspicuous diffraction peak at 26.5°, NO-MC possesses two broad and weak peaks at 26.5° and 43.1° (see the inset in Fig. 3a), corresponding respectively to the (002) and (100) lattice planes of graphitic carbon, which indicates the existence of graphitic structure and structural defects in NO-MC [38,46]. Raman spectra as shown in Fig. 3b confirms the rich structural defects in NO-MC due to the fact that the ratio of the intensity between the defect-induced D band and the graphite-induced G band (ID/IG) for NO-MC is higher than that of CP and OCP. The bulk structure of CP, OCP and NO-MC was also investigated by cryo-nitrogen adsorption: the BET surface area of NO-MC (21.16 m2 g−1) is much larger than that of both CP (0.79 m2 g−1) and OCP (1.30 m2 g−1). A strong adsorption in low pressure region (P/ P0 = 0–0.1) and an obvious hysteresis loop in medium pressure region (P/P0 = 0.4–0.8) for the NO-MC (Fig. 3c) are respectively attributed to the appearance of the mesopore and micropore [47], as evidenced by the distinct peaks at 2–5 nm in the pore size distribution curves shown in Fig. 3d. As seen, the rich structural defects, high BET surface area and multi-porous structure in NO-MC lead to more active sites for the redox reaction of vanadium ions [29]. The chemical structures of oxygen and nitrogen atoms on the surface of CP, OCP and NO-MC were analyzed by XPS spectra (Fig. 4). The C1s peak of carbon at 284.5 eV was used to calibrate all spectra. Although carbonized in a moderate heating temperature, exhibits sharp peak and high composition ratio of graphitic double-bonded carbon at 284.5 eV (Fig. 4b and Fig. S3) [48], which benefits from the rich β-sheet in twin cocoon [40,49]. The large O1s region and atomic content ratio of oxygen to carbon atoms (ca. 0.10) for the NO-MC indicate higher oxygen content on its surface (Fig. 4c and d). The O1s region can be differentiated into three peaks including C-C=O (533.2 eV), C-OH (532.1 eV) and C=O (530.9 eV), respectively (Fig. 4c) [50]. The oxygen functional groups could accelerate the formation of active sites for vanadium redox reaction, especially, the content of C-C=O is directly associated with the quantity of active sites [36]. It shows that the proportion of C-C=O in oxygen functional groups of NO-MC (ca. 31.05%) is higher than CP (ca. 0.00%) and OCP (ca. 24.38%) (Fig. S4). Moreover, as shown in Fig. 4a and e, no significant peaks were found in
760E). The samples with a diameter of 6 mm, platinum mesh and Ag/ AgCl electrode were used as working electrode, counter electrode and reference electrode, respectively. For CV measurement, the positive tests were carried out between 0.3 V and 1.4 V while the negative tests were performed between −0.9 V and 0.0 V in 1.0 M VOSO4 + 3.0 M H2SO4. AC impedance was conducted in a frequency ranging from 10−2 to 10−5 Hz with an excitation signal of 5 mV at 0.85 V. 2.5. Flow battery performance test The CP, OCP and NO-MC with the same active area of 4 cm2 (2 × 2 cm) were employed as positive and negative electrodes in flow battery. 20 mL solution containing 1.0 M VO2+ + 3.0 M H2SO4 and 20 mL solution containing 1.0 M V3+ + 3.0 M H2SO4 were used as the positive and negative electrolytes, respectively. Nafion NR-212 (Dupont, USA) was applied as the membrane. The battery performance was tested in a zero-gap battery, which was assembled with serpentine flow-field structured graphite plates, copper current collectors, aluminum support plate and bakelite end plates. The electrolytes in the batteries were circulated with a peristaltic pump (WT600-2J, Longer) at a fixed flow rate of 46 mL min−1. The performances of the VRFBs were assessed in a battery test system (Arbin BT-G, Arbin, USA) under the operating potential range between 1.7 and 0.9 V with a current density range between 40 and 100 mA cm-2. All measurements were conducted at room temperature. 3. Results and discussion Fig. 2 presents the surface morphologies of pristine, degummed and carbonized twin cocoons at different macroscopic and microscopic levels. As observed in Fig. 2a and b, the pristine twin cocoon that has a multi-layer structure is unbending. This is because the twin cocoon is composed of silk fibers fixed by sericin (see the inset in Fig. 2b) [42–45]. The question is how to make the twin cocoon-derived carbon material possess porous structure and flexible mechanical property. To meet this need, as shown in Fig. 1, i) in the degumming process, twin cocoon was heated in an alkaline aqueous solution under a high pressure that prevents the solution from boiling, resulting in forming a porous (Fig. 2c) but loose (see the inset in Fig. 2c) structure; ii) in carbonizing process, a moderate heating temperature was applied to the degummed twin cocoon, as a result, the cross-linked structure (see the inset in Fig. 2d) was created, thereby enhancing the mechanical property and electronic conductivity as well as ensuring the reasonable flexibility and porosity of the carbonized twin cocoon (Fig. 2d and Fig. S2). As such, an all-biomaterial-derived self-standing monolithic carbon material was successfully prepared. Interestingly, it can be seen from
Fig. 2. (a) The images of the pristine twin cocoon, hydrothermal treated twin cocoon and carbonized twin cocoon. SEM images of (b) the pristine twin cocoon, (c) the hydrothermal treated twin cocoon, and (d) the carbonized twin cocoon. (e) SEM image of the carbonized silk fiber, and (f–i) the EDS elemental mapping of the carbonized silk fiber.
141
Journal of Power Sources 421 (2019) 139–146
R. Wang and Y. Li
Fig. 3. (a) XRD patterns, (b) Raman spectra, (c) adsorption/desorption isotherms, and (d) pore size distribution of CP, OCP and NO-MC.
nitrogen, owning to that it can be ionized out free electrons and is highly stable under acidic conditions, has been proved to be more effective for vanadium redox reaction [29]. To obtain a suitable carbonizing temperature for the material with a high content of graphitic-N, twin cocoons with different treating temperature were analyzed by XPS (Fig. S5). It is demonstrated that the proportion of graphitic-N in nitrogen species for NO-MC with 1000 °C (ca. 21.65%) is higher than that with 600 °C (ca. 17.79%) and 800 °C (ca. 19.59%) (Fig. 4f).
the N1s region for CP and OCP, whereas the nitrogen-containing groups in the NO-MC were observed with an obvious increase in N1s region at 400 eV. Carbon materials containing nitrogen have been known to significantly promote the vanadium redox reaction [28,29], according to XPS analysis, the nitrogen content in NO-MC was ca. 5.81%. The N1s peak of oxygenated-N, graphitic-N, pyrrolic-N, and pyridinic-N were observed at 403.2, 401.4, 400.3 and 398.6 eV in NO-MC spectrum, respectively (Fig. 4e) [51]. Among the nitrogen species, graphitic-
Fig. 4. (a) XPS spectra of the CP, OCP and NO-MC. XPS analysis and its fitting from high resolution (b) C1s peak, (c) O1s peak, (d) chemical composition ratio of oxygen and carbon atoms for CP, OCP and NO-MC, (e) N1s peak, and (f) chemical composition ratio of NO-MC with different carbonized temperature (600, 800 and 1000 °C) from N1s spectra; (g) contact angles of the CP, OCP and NO-MC.
142
Journal of Power Sources 421 (2019) 139–146
R. Wang and Y. Li
Fig. 5. (a) Cyclic voltammograms of the CP and NO-MC at a scan rate of 5 mV s−1 with a potential window of 0.3–1.4 V versus Ag/ AgCl, (b) Nyquist plots of CP, OCP and NOMC in the frequency range from 10−2–105 Hz at 8.5 V, (c–d) CV of the CP and NO-MC at different scan rates (Insets: plot of the anodic peak current (Ipa) versus the square root of each scan rate) in 1.0 M VOSO4 + 3.0 M H2SO4 solution.
determination (R2) for both CP (99.95) and NO-MC (99.87) are close to 100%, indicating that the vanadium redox reactions are primarily controlled by the diffusion process in both CP and NO-MC [52,53]. In this regard, the slope of NO-MC is 195, 192% larger than that of CP with a value of 66.7, significantly lowering the mass transfer resistance for the vanadium redox reaction. This is ascribed to the better hydrophilic surface in NO-MC due to the formation of oxygen functional groups and nitrogen defects. The flow batteries assembled with CP, OCP and NO-MC electrodes were tested at different current densities as displayed in Fig. 6a–d. It was found that the VRFB with NO-MC electrodes has lower charge/ discharge overpotential and higher capacities as compared with CP and OCP electrodes. The battery associated with the NO-MC electrodes achieves the average discharge capacity from 18.62 to 10.50 Ah L−1 with current density from 40 to 100 mA cm−2, great higher than that with the CP from 15.97 to 5.75 Ah L−1 and OCP from 17.67 to 10.04 Ah L−1 (Fig. 7a). Fig. 7b shows the coulombic efficiency (CE) and voltage efficiency (VE) for VRFBs. It can be seen the CEs for all three kind of electrodes are more than 90%, demonstrating good airtightness of the battery setup [25]. As for the VE, agreeing with the trend of charge–discharge curves, as increasing current density from 40 to 100 mA cm−2, the VE of VRFB with NO-MC decreases from 90% to 75%, while higher than that with CP reducing from 80% to 63% and OCP ranging from 80% to 69%, because of the enhanced electrochemical reversibility and improved activity of the nitrogen-doped oxygen-rich NO-MC electrode. To further evaluate the overall efficiency and cycling stability of redox flow batteries with CP, OCP and NO-MC electrodes, the energy efficiency (EE) was achieved during long-term charge-discharge cycling test as presented in Fig. 7c, when setting the current density at 40 mA cm−2, 60 mA cm−2, 80 mA cm−2, and 100 mA cm−2, the battery with NO-MC electrodes yields the respective EE of 84%, 81%, 77% and 73%, which are 14%, 15%, 18% and 20% higher than that with the pristine CP electrodes. Moreover, it can be clearly seen that battery with NO-MC electrode is more stable than that with CP and OCP electrodes. The improved performance and stability of the battery with NO-MC electrodes are mainly attributed to the nitrogen-doping oxygen-rich surface and the monolithic porous structure.
Furthermore, thanks to the rational content of oxygen and nitrogen functional groups on the surface, the NO-MC with a contact angle of 0° exhibits the better wettability than CP (130.55°) and OCP (102.98°) do (Fig. 4g), implying the high hydrophilic surface for the NO-MC to increase the mass transfer rate. Therefore, the nitrogen-doped NO-MC material with oxygen-rich functional groups not only provides abundant active sites but also promises a fast electrolyte diffusion so as to improve the kinetics of vanadium redox reaction, thus boosting the performance of VRFBs [23,36]. Cyclic voltammetry tests were carried out to show the electrochemical properties of CP, OCP and NO-MC (Fig. 5a) in terms of the ratio of oxidation and reduction peak current (Ipa/Ipc), the redox potential difference (ΔEp), and the redox onset potential: i) The ratio of oxidation and reduction peak current of NO-MC (Ipa/Ipc = 1.25) gets close to CP (Ipa/Ipc = 1.23) and OCP (Ipa/Ipc = 1.21), indicating the stable vanadium redox reaction for NO-MC; ii) NO-MC presents a smaller redox potential difference (ΔEp = 166 mV) compared to both the CP (ΔEp = 329 mV) and OCP (ΔEp = 293 mV), resulting in a better electrochemical reversibility; iii) The lower oxidization onset potential and higher reduction onset potential of NO-MC (see the insets of Fig. 5a) implies its high electrochemical catalytic activity for the vanadium redox reaction. Moreover, with regard to the negative redox reaction toward V2+ and V3+ ions, there exist the remarkable oxidation and reduction peaks for NO-MC (Ipa = 107.18 mA, Ipc = 140.48 mA) at −0.28 mV and −0.75 mV while no significant cathodic peaks for CP (Fig. S6), which demonstrate the better electrochemical activity for NOMC in the negative redox reaction. Electrochemical impedance spectroscopy (EIS) analysis for CP, OCP and NO-MC (Fig. 5b) also agreed with the conclusion of the cyclic voltammetry tests. As seen, in contrast, NO-MC that owns the lower charge transfer resistance and the faster mass transfer process exhibits the best electrochemical activity, as proven by the smaller diameter of the semi-circle at high-frequency region and the larger line slope at low-frequency region. To further ascertain the mass transfer properties of CP and NO-MC electrode materials, the linear relationship between peak current density and square root of scan rate was quantified by the Randles-Sevcik equation [52]. As shown in Fig. 5c and d, the coefficients of 143
Journal of Power Sources 421 (2019) 139–146
R. Wang and Y. Li
Fig. 6. (a–c) Charge/discharge curves of batteries with CP, OCP and NO-MC electrodes at various current densities, and (d) with CP, OCP and NO-MC electrodes at 40 mA cm−2 during cycling test.
Fig. 7. (a) Discharge capacities of VRFBs as a function of cycle number at different current densities with CP and NO-MC electrodes; (b) coulombic and voltage efficiencies of VRFBs with CP, OCP and NO-MC electrodes at various current densities; (c) energy efficiencies of CP, OCP and NO-MC electrodes during cycling test. 144
Journal of Power Sources 421 (2019) 139–146
R. Wang and Y. Li
superiority of the NO-MC electrode material is mainly attributed to two aspects: i) the monolithic and cross-linked structure enables the NO-MC to possess not only the enhanced mechanical property and electronic conductivity but also the reasonable flexibility and porosity; ii) the rough surface of the skeleton with oxygen functional groups and nitrogen defects allows the NO-MC to hold the high electrochemical activity and reversibility. Therefore, the all-biomaterial-derived N-doped O-rich monolithic carbon electrode material promises a great potential in redox flow battery, even other electrochemical energy devices. Conflicts of interest There are no conflicts to declare. Acknowledgments This work was fully supported by the National Natural Science Foundation of China (51776156). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.jpowsour.2019.03.023. References [1] J.W. Ager, A.A. Lapkin, Chemical storage of renewable energy, Science 360 (6390) (2018) 707–708. [2] X.D. Sun, Y.S. Li, Understanding mass and charge transports to create anion-ionomer-free high-performance alkaline direct formate fuel cells, Int. J. Hydrog. Energy 44 (14) (2019) 7538–7543. [3] Y.S. Li, Y. Feng, X.D. Sun, Insight into interface behaviors to building a phaseboundary-matched Na-ion direct liquid fuel cell, ACS Sustain. Chem. Eng. 6 (10) (2018) 12827–12834. [4] Y. Qiu, M.J. Li, K. Wang, Z.B. Liu, X.D. Xue, Aiming strategy optimization for uniform flux distribution in the receiver of a linear fresnel solar reflector using a multi-objective genetic algorithm, Appl. Energy 205 (2017) 1394–1407. [5] B. Dunn, H. Kamath, J.M. Tarascon, Electrical energy storage for the grid: a battery of choices, Science 334 (6058) (2011) 928–935. [6] Z. Yang, J. Zhang, M.C. Kintner-Meyer, X. Lu, D. Choi, J.P. Lemmon, J. Liu, Electrochemical energy storage for green grid, Chem. Rev. 111 (5) (2011) 3577–3613. [7] J. Zhang, G. Jiang, P. Xu, A.G. Kashkooli, M. Mousavi, A. Yu, Z. Chen, An allaqueous redox flow battery with unprecedented energy density, Energy Environ. Sci. 11 (2018) 2010–2015. [8] H. Jiang, L. Wei, X. Fan, J. Xu, W. Shyy, T. Zhao, A novel energy storage system incorporating electrically rechargeable liquid fuels as the storage medium, Sci. Bull. 64 (4) (2019) 270–280. [9] P. Leung, X. Li, C.P. de Leon, L. Berlouis, C.T.J. Low, F.C. Walsh, Progress in redox flow batteries, remaining challenges and their applications in energy storage, RSC Adv. 2 (27) (2012) 10125–10156. [10] L. Li, S. Kim, W. Wang, M. Vijayakumar, Z. Nie, B. Chen, J. Zhang, G. Xia, J. Hu, G. Graff, J. Liu, Z. Yang, A stable vanadium redox-flow battery with high energy density for large-scale energy storage, Adv. Energy Mater. 1 (3) (2011) 394–400. [11] R. Wang, Y.S. Li, Y.L. He, Achieving gradient–pore–oriented graphite felt for vanadium redox flow batteries: meeting improved electrochemical activity and enhanced mass transport from nano– to micro–scale, J. Mater. Chem. A (2019), https://doi.org/10.1039/c9ta00807a. [12] Q. Deng, P. Huang, W. Zhou, Q. Ma, N. Zhou, H. Xie, W. Ling, C. Zhou, Y. Yin, X. Wu, X. Lu, Y. Guo, A high-Performance composite electrode for vanadium redox flow batteries, Adv. Energy Mater. 7 (18) (2017) 1700461. [13] B. Li, M. Gu, Z. Nie, Y. Shao, Q. Luo, X. Wei, X. Li, J. Xiao, C. Wang, V. Sprenlde, W. Wang, Bismuth nanoparticle decorating graphite felt as a high-performance electrode for an all-vanadium redox flow battery, Nano Lett. 13 (3) (2013) 1330–1335. [14] Q. Liu, A. Turhan, T.A. Zawodzinski, M.M. Mench, In situ potential distribution measurement in an all-vanadium flow battery, Chem. Commun. 49 (56) (2013) 6292–6294. [15] R. Kumar, T. Bhuvana, A. Sharma, Tire waste derived turbostratic carbon as electrode for vanadium redox flow battery, ACS Sustain. Chem. Eng. 6 (7) (2018) 8238–8246. [16] S. Abbas, H. Lee, J. Hwang, A. Mehmood, H. Shin, S. Mehboob, J. Lee, H.Y. Ha, A novel approach for forming carbon nanorods on the surface of carbon felt electrode by catalytic etching for high-performance vanadium redox flow battery, Carbon 128 (2018) 31–37. [17] C. Yao, H. Zhang, T. Liu, X. Li, Z. Liu, Carbon paper coated with supported tungsten trioxide as novel electrode for all-vanadium flow battery, J. Power Sources 218 (2012) 455–461.
Fig. 8. Schematic diagram of an alternative sustainable energy ecosystem.
As observed, the all-biomaterial-derived monolithic carbon material has been proven to be a promising high-performance and low-cost electrode in VRFBs. The abundant oxygen-containing active sites with nitrogen defects play an important role in the vanadium redox reaction as described in Fig. 8: i) in the charge process, the positively charged vanadium ions (VO2+ or V3+) were easily absorbed by the negatively charged nitrogen atoms to form the N-V bond; and ii) after the oxidation or reduction reaction, the reaction products (VO2+ or V2+) easily diffuse to electrolyte solution from the nitrogen active sites by ion exchange [29]. Let us place the present cocoon-derived carbon electrode material in a large-scale electrical power system as demonstrated in Fig. 8. The cocoon, a protein-rich natural polymer fiber composite, can be easily obtained in silkworm life cycle, including egg, larva, pupa and moth [42,43,54]. All of them are natural and sustainable. More importantly, after more than 4000 years utilization by humanity, currently, cocoon is cheap (ca. $14 per kg) and abundant in the textile raw material market [41,55–57]. The proposed hydrothermal treatment and carbonization process enables the twin cocoon readily to be a cost-effective carbon electrode material, as a result boosting the performance of redox flow battery, thereby meeting the requirement of utilizing renewable energy, e.g. solar and wind. As such, an alternative sustainable energy ecosystem could be established. More appealingly, this all-biomaterial-derived self-standing monolithic carbon material also can be applied for other energy conversion and storage devices, such as lithium-ion battery [58,59], fuel cell [60,61], supercapacitor [62,63]. 4. Conclusion To summarize, twin-cocoon-derived self-standing nitrogen-doped monolithic carbon materials with oxygen-rich functional groups was proposed and developed as the high-performance and low-cost electrode for VRFBs by a facile, well-controlled and environmental-friendly method in this work. Compared with CP and OCP, NO-MC electrode material exhibits higher electrochemical activity and reversibility in vanadium redox reaction, as well as high energy efficiency, large discharge capacity and better stability in practical redox flow battery. The 145
Journal of Power Sources 421 (2019) 139–146
R. Wang and Y. Li
[18] L. Wei, T.S. Zhao, G. Zhao, L. An, L. Zeng, A high-performance carbon nanoparticledecorated graphite felt electrode for vanadium redox flow batteries, Appl. Energy 176 (2016) 74–79. [19] P.C. Ghimire, R. Schweiss, G.G. Scherer, N. Wai, T.M. Lim, A. Bhattarai, Titanium carbide-decorated graphite felt as high performance negative electrode in vanadium redox flow batteries, J. Mater. Chem. 6 (15) (2018) 6625–6632. [20] M.H. Chakrabarti, N.P. Brandon, S.A. Hajimolana, E. 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) 150–166. [21] A. Parasuraman, T.M. Lim, C. Menictas, M. Skyllas-Kazacos, Review of material research and development for vanadium redox flow battery applications, Electrochim. Acta 101 (2013) 27–40. [22] 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 (8) (2013) 970–986. [23] L. Wu, Y. Shen, L. Yu, J. Xi, X. Qiu, Boosting vanadium flow battery performance by Nitrogen-doped carbon nanospheres electrocatalyst, Nanomater. Energy 28 (2016) 19–28. [24] P. Huang, W. Ling, H. Sheng, Y. Zhou, X. Wu, X. Zeng, X. Wu, Y. Guo, Heteroatomdoped electrodes for all-vanadium redox flow batteries with ultralong lifespan, J. Mater. Chem. 6 (1) (2018) 41–44. [25] H. Jiang, W. Shyy, L. Zeng, R. Zhang, T.S. Zhao, Highly efficient and ultra-stable boron-doped graphite felt electrodes for vanadium redox flow batteries, J. Mater. Chem. 6 (2018) 13244–13253. [26] B. Sun, M. Skyllas-Kazacos, Modification of graphite electrode materials for vanadium redox flow battery application-I. Thermal treatment, Electrochim. Acta 37 (7) (1992) 1253–1260. [27] B. Sun, M. Skyllas-Kazacos, Chemical modification of graphite electrode materials for vanadium redox flow battery application-part II. Acid treatments, Electrochim. Acta 37 (13) (1992) 2459–2465. [28] 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 (16) (2012) 2164–2167. [29] J. Jin, X. Fu, Q. Liu, Y. Liu, Z. Wei, K. Niu, J. Zhang, Identifying the active site in nitrogen-doped graphene for the VO2+/VO2+ redox reaction, ACS Nano 7 (6) (2013) 4764–4773. [30] J. Liu, P. Song, Z. Ning, W. Xu, Recent advances in heteroatom-doped metal-free electrocatalysts for highly efficient oxygen reduction reaction, Electrocatalysis-US 6 (2) (2015) 132–147. [31] F. Zheng, Y. Yang, Q. Chen, High lithium anodic performance of highly nitrogendoped porous carbon prepared from a metal-organic framework, Nat. Commun. 5 (2014) 5261. [32] M. Park, I.Y. Jeon, J. Ryu, J.B. Baek, J. Cho, Exploration of the effective location of surface oxygen defects in graphene-based electrocatalysts for all-vanadium redoxflow batteries, Adv. Energy Mater. 5 (5) (2015) 1401550. [33] A. Ejigu, M. Edwards, D.A. Walsh, Synergistic catalyst-support interactions in a graphene–Mn3O4 electrocatalyst for vanadium redox flow batteries, ACS Catal. 5 (12) (2015) 7122–7130. [34] T. Wu, K. Huang, S. Liu, S. Zhuang, D. Fang, S. Li, D. Lu, A. Su, Hydrothermal ammoniated treatment of PAN-graphite felt for vanadium redox flow battery, J. Solid State Electrochem. 16 (2) (2012) 579–585. [35] 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 (13) (2010) 4375–4379. [36] M. Park, J. Ryu, Y. Kim, J. Cho, Corn protein-derived nitrogen-doped carbon materials with oxygen-rich functional groups: a highly efficient electrocatalyst for allvanadium redox flow batteries, Energy Environ. Sci. 7 (11) (2014) 3727–3735. [37] 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 (1) (2016) A5144–A5149. [38] Z.H. Zhang, T.S. Zhao, B.F. Bai, L. Zeng, L. Wei, A highly active biomass-derived electrode for all vanadium redox flow batteries, Electrochim. Acta 248 (2017) 197–205. [39] N. Yusof, A.F. Ismail, Post spinning and pyrolysis processes of polyacrylonitrile (PAN)-based carbon fiber and activated carbon fiber: a review, J. Anal. Appl. Pyrolysis 93 (2012) 1–13. [40] S.Y. Cho, Y.S. Yun, S. Lee, D. Jang, K.Y. Park, J.K. Kim, H.J. Jin, Carbonization of a
[41]
[42] [43]
[44]
[45] [46] [47] [48]
[49]
[50]
[51]
[52]
[53]
[54] [55] [56] [57] [58]
[59]
[60]
[61]
[62]
[63]
146
stable β-sheet-rich silk protein into a pseudographitic pyroprotein, Nat. Commun. 6 (2015) 7145. H. Yamada, H. Nakao, Y. Takasu, K. Tsubouchi, Preparation of undegraded native molecular fibroin solution from silkworm cocoons, Mat. Sci. Eng. C-Mater. 14 (1–2) (2001) 41–46. O. Hakimi, D.P. Knight, F. Vollrath, P. Vadgama, Spider and mulberry silkworm silks as compatible biomaterials, Compos. B Eng. 38 (3) (2007) 324–337. J. Zhang, J. Kaur, R. Rajkhowa, J.L. Li, X.Y. Liu, X.G. Wang, Mechanical properties and structure of silkworm cocoons: a comparative study of Bombyx mori, Antheraea assamensis, Antheraea pernyi and Antheraea mylitta silkworm cocoons, Mat. Sci. Eng. C-Mater. 33 (6) (2013) 3206–3213. J. Hou, C. Cao, F. Idrees, X. Ma, Hierarchical porous nitrogen-doped carbon nanosheets derived from silk for ultrahigh-capacity battery anodes and supercapacitors, ACS Nano 9 (3) (2015) 2556–2564. F. Chen, D. Porter, F. Vollrath, Silk cocoon (Bombyx mori): multi-layer structure and mechanical properties, Acta Biomater. 8 (7) (2012) 2620–2627. Z.Q. Li, C.J. Lu, Z.P. Xia, Y. Zhou, Z. Luo, X-ray diffraction patterns of graphite and turbostratic carbon, Carbon 45 (8) (2007) 1686–1695. B. Xu, H. Duan, M. Chu, G. Cao, Y. Yang, Facile synthesis of nitrogen-doped porous carbon for supercapacitors, J. Mater. Chem. 1 (14) (2013) 4565–4570. C. Gao, N. Wang, S. Peng, S. Liu, Y. Lei, X. Liang, S. Zeng, H. Zi, Influence of Fenton's reagent treatment on electrochemical properties of graphite felt for all vanadium redox flow battery, Electrochim. Acta 88 (2013) 193–202. Y.S. Yun, K.Y. Park, B. Lee, S.Y. Cho, Y.U. Park, S.J. Hong, Y.W. Park, Sodium-ion storage in pyroprotein-based carbon nanoplates, Adv. Mater. 27 (43) (2015) 6914–6921. S. Park, H. Kim, Fabrication of nitrogen-doped graphite felts as positive electrodes using polypyrrole as a coating agent in vanadium redox flow batteries, J. Mater. Chem. 3 (23) (2015) 12276–12283. K. Gong, F. Du, Z. Xia, M. Durstock, L. Dai, Nitrogen-doped carbon nanotube arrays with high electrocatalytic activity for oxygen reduction, Science 323 (5915) (2009) 760–764. R. Huang, C. Sun, T. Tseng, W. Chao, K. Hsueh, F. Shieu, Investigation of active electrodes modified with platinum/multiwalled carbon nanotube for vanadium redox flow battery, J. Electrochem. Soc. 159 (10) (2012) A1579–A1586. J. Kim, K.J. Kim, M. Park, N.J. Lee, U. Hwang, H. Kim, Y. Kim, Development of metal-based electrodes for non-aqueous redox flow batteries, Electrochem. Commun. 13 (9) (2011) 997–1000. J. Zhang, R. Rajkhowa, J.L. Li, X.Y. Liu, X.G. Wang, Silkworm cocoon as natural material and structure for thermal insulation, Mater. Des. 49 (2013) 842–849. Y. Takahashi, M. Gehoh, K. Yuzuriha, Crystal structure of silk (Bombyx mori), J. Polym. Sci., Polym. Phys. Ed. 29 (7) (1991) 889–891. W. Liu, Y. Yu, E. Liu, Analysis on operation of Chinese cocoon silk industry in 2016 and prospect in 2017, J. Silk 54 (6) (2017) 1–7. F. Chen, D. Porter, F. Vollrath, Morphology and structure of silkworm cocoons, Mat. Sci. Eng. C-Mater. 32 (4) (2012) 772–778. L. Qie, W.M. Chen, Z.H. Wang, Q.G. Shao, X. Li, L.X. Yuan, Y.H. Huang, Nitrogendoped porous carbon nanofiber webs as anodes for lithium ion batteries with a superhigh capacity and rate capability, Adv. Mater. 24 (15) (2012) 2047–2050. J. Song, M.L. Gordin, T. Xu, S. Chen, Z. Yu, H. Sohn, D. Wang, Strong lithium polysulfide chemisorption on electroactive sites of nitrogen-doped carbon composites for high-performance lithium-sulfur battery cathodes, Angew. Chem. Int. Ed. 127 (14) (2015) 4399–4403. K. Gong, F. Du, Z. Xia, M. Durstock, L. Dai, Nitrogen-doped carbon nanotube arrays with high electrocatalytic activity for oxygen reduction, Science 323 (5915) (2009) 760–764. D. Guo, R. Shibuya, C. Akiba, S. Saji, T. Kondo, J. Nakamura, Active sites of nitrogen-doped carbon materials for oxygen reduction reaction clarified using model catalysts, Science 351 (6271) (2016) 361–365. B. Li, F. Dai, Q. Xiao, L. Yang, J. Shen, C. Zhang, M. Cai, Nitrogen-doped activated carbon for a high energy hybrid supercapacitor, Energy Environ. Sci. 9 (1) (2016) 102–106. T. Lin, I.W. Chen, F. Liu, C. Yang, H. Bi, F. Xu, F. Huang, Nitrogen-doped mesoporous carbon of extraordinary capacitance for electrochemical energy storage, Science 350 (6267) (2015) 1508–1513.