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Ce redox flow battery
Materials Chemistry and Physics 220 (2018) 208–215
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Chemically reduced graphene oxide paper as positive electrode for advanced Zn/Ce redox flow battery
T
Zhipeng Xiea,b,∗, Baolu Liua, Chenfan Xiec, Bin Yanga, Yunfen Jiaoa, Dingjian Caia, Liang Yanga, Qi Shua, Anhong Shia a
Engineering Research Institute, Jiangxi University of Science and Technology, Ganzhou, 341000, China State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, 130022, China c School of Petroleum and Chemical Engineering, Dalian University of Technology, Panjin, 124221, China b
H I GH L IG H T S
RGOC paper possesses good flexibility with a porous structure. • The C/O ratio of RGOC is different from that of RGOT. • The • The activity of RGOC towards Ce /Ce is better than that of RGOT. 3+
4+
A R T I C LE I N FO
A B S T R A C T
Keywords: Graphene oxide Redox flow battery Cerium New energy Energy storage
Zn/Ce redox flow battery (ZRFB) is emerging as a promising technology to store large amount of energy economically and efficiently, wherein a highly efficient positive electrode with a continuous and fast electronic and ionic transportation path is urgently desired. The unique nanostructure of reduced graphene oxide (RGO) paper electrode is prepared by a simply chemical reduction method, which facilitates transference of the electron and Ce3+/Ce4+ at the electrolyte/electrode interface. Thus ZRFB exhibits superior extent of charge (81.0%) and energy efficiency (71.3%). The results show RGO paper is a good candidate for positive electrode of ZRFB.
1. Introduction Energy is of importance to all of us. It is necessary to develop new energy resources [1–3] for reducing environmental pollutants [4] and improving human life level. However, the more widespread use of them is dependent upon the development of an affordable and reliable energy storage system. Redox flow batteries (RFBs) have received increasing attention for their storing enormous amount of electrical energy friendly and efficiently [5,6]. For a typical RFB, its electroactive species are dissolved in two electrolytes stored in separate tanks instead of in electrodes, which is different from traditional batteries. The cell reactions occur when electrolyte flows through electrode with the help of pump. The electrode only offers a place where electrode reaction occurs without undergoing any deformation, which is helpful to prolong the service life of the battery. The capacity of RFB is determined by the amount of electroactive species in electrolyte, while the power output by the size of the electrode. For example, the greater the amount of Ce3+/Ce4+ in positive electrolyte, the greater the capacity of ZRFB.
∗
The architecture characteristic of separation of power output from capacity gives RFB considerable design flexibility. It is of fundamental importance to select suitable electrode material for optimization and upgrading of battery performance. The factors to be considered in selection of electrode materials for RFB application include conductivity, mechanical strength, chemical stability and electrochemical activity. Generally, selection of electrode material for RFB application is the process of finding a balance point in the above mentioned factors based on the specific use. The kinetic characteristics of Ce3+/Ce4+ electrode reaction on glassy carbon, platinum, platinized titanium, carbon felt, graphite, porous carbon, and graphene oxide/ graphite composite electrodes were investigated by different researchers [7–18]. Although some achievements have been made in the research of electrode, it is still necessary to search for alternative materials with better performance toward effective positive electrode for advanced ZRFB application. Graphene-based materials have attracted significant attention for their excellent mechanical and electrical properties [19–24], which can
Corresponding author. No.86, Hongqi Ave., Ganzhou, Jiangxi, China. E-mail address: [email protected] (Z. Xie).
https://doi.org/10.1016/j.matchemphys.2018.08.082 Received 29 September 2017; Received in revised form 8 April 2018; Accepted 24 August 2018 Available online 27 August 2018 0254-0584/ © 2018 Elsevier B.V. All rights reserved.
Materials Chemistry and Physics 220 (2018) 208–215
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Fig. 1. a)Working principle of Zn/Ce redox flow battery. b)Assembled ZRFB. 1. negative electrode, 2. positive electrode, 3. Nafion 115 membrane. 209
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electrode is zinc sheet (3 × 3 × 0.2 cm). The measurement of Zn/Ce RFB was carried out under constant current of 180 mA at the voltage range between 2.6 and 0.5 V.
be prepared in large scale by modifying graphene oxide (GO) with reduction reagent and/or thermal treatment method. There are wide range of oxygen functional groups on both basal planes and edges of GO, which provides more possibilities for applications in materials science and nanocomposites [25]. Graphene oxide paper is a novel freestanding paper-like material with good mechanical properties [25–28]. Herein, we report the first use of reduced graphene oxide paper as positive electrode in advanced ZRFB. These results show that the reduced graphene oxide paper is an excellent candidate in the application of ZRFB.
3. Results and discussion 3.1. XRD and Raman characterization As shown in Fig. 2a, there is a big difference among the XRD patterns of GO powder, RGO powder (RGOT, thermally reduced graphene oxide; RGOC, Chemically reduced graphene oxide) and graphite powder. The graphene oxide shows a characteristic diffraction peak (001) at 10.02° with an interplanar spacing of 0.88 nm, which is caused by the atomic scale roughness and the generation of oxygen-containing functional groups attached on GO nanosheets [18]. When the graphene oxide is reduced by thermal treatment under a nitrogen atmosphere at 200 °C for 2 h (corresponding to RGOT) or by NaBH4 (corresponding to RGOC), the disappearance of the characteristic diffraction peak at 10.02° is accompanied by the emergence of a weak wide bulge centered at 22.51° for RGOT and at 24.04° for RGOC. As for graphite powder, it has a strong peak (002) at 26.54° with an interlayer spacing of 0.34 nm and a weak peak (004) at 54.66° with an interlayer spacing of 0.17 nm, which means that the graphite powder was partly oxidized [18]. Raman spectroscopy, a nondestructive and fast method for characterization of the crystal structure, is then used to characterize the disorder and defects in the as-prepared samples. The graphite is also employed for comparison, which consists of fused aromatic ring structures along direction in the plane. As seen in Fig. 2b, the graphite crystal shows two bands: a strong one positioned at a Raman frequency of 1569.9 cm−1 and a much weaker one at 1346.4 cm−1. The former was assigned to the E2gC-C stretching mode, called G band and the latter D band representing a disorderly network of sp2-coordinated clusters. Generally, graphene of high quality also shows a weak D band. In theory, the greater the quantity of the ring near a graphene edge or the more the structure defect of a graphene, the greater the intensity of D band. The material without any defect has a very weak, even no D band. Hence, the disorder in a graphene can be reflected from the D band intensity, and its measurement is often based on the ratio of D to G band intensity. In this work, we prepared two kinds of reduced graphene oxide, which are graphene with many defects. One is chemically reduced graphene oxide (RGOC) prepared by the reduction of GO with NaH4B solution, the other is thermally reduced graphene oxide (RGOT) prepared by thermal treatment of GO under a nitrogen atmosphere at 200 °C for 2 h. GO is prepared by the oxidation of graphite. However, the reduction degree of GO and the oxidation degree of graphite can not be reflected from the ratio of D to G band intensity which can be the result of interaction of other factors such as edges, defects and ripples. The intensity ratios of D to G for GO, RGOT and RGOC are 0.84, 0.89 and 1.03, respectively. The increment of intensity ratio of D to G band is also observed in other study on reduction of graphene oxide [29]. Therefore, the following conclusions can be obtained. The reduction of GO, whether thermal ruduction or chemical reduction, can introduce a large number of edges, defects and ripples to graphene. Furthermore, the amount of edges, defects and ripples introduced by chemical reduction is more than that of by thermal reduction.
2. Experimental 2.1. Electrode preparation The preparation of GO was carried out on the basis of the improved Hummers’ method [28]. Briefly, graphite powders (99.95%, 1200 mesh, aladdin®) were oxidized by the concentrated H2SO4/H3PO4 and KMnO4 to produce precursor, reoxidized by 30%-H2O2, then washed using H2O, HCl and C2H5OH, respectively. The centrifugalization of the mixture was performed at 12000 rpm for 10 min and obtains graphite oxide. Graphite oxide was further diluted in deionized water, ultrasonication for 30 min to obtain graphene oxide aqueous solution [18]. Then, sodium borohydride solution was added into the GO aqueous solution, which was accompanied by a violent reaction and the generation of reduced graphene oxide (i.e., chemically reduced graphene oxide, RGOC). RGOC dispersion requires washing using H2O no less than 3 times, then followed by freeze-drying under vacuum for 24 h. The RGOC paper is obtained by pressing the RGOC powder at a pressure of 10 MPa. All the chemicals are AR grade, purchased from Shanghai Aladdin Bio-Chem Technology Co., LTD (aladdin®, China). 2.2. Sample characterization The characterization of RGOC paper was carried out by field emission scanning electron microscopy (HITACHI S-4800, Shanghai Yong Ming Automation Equipment Co. LTD), X-ray diffraction patterns (Bruker D8 Focus powder XRD, Xiamen Mingda Technology Co. LTD), Raman scattering (Renishaw inVia spectrometer system, Suzhou Ruice Precision Instrument Co. LTD) and X-ray photoelectron spectroscopy (QUANTUM 2000 surface analysis system, Physical Electronics USA). 2.3. Electrochemical measurements Several electrochemical analytical methods (cyclic voltammetry, linear sweep voltammetry and impedance spectroscopy) were selected for characterizing the electrochemical behaviour of Ce3+/Ce4+ redox couple in a solution of 2 mol/L CH3SO3H at RGOC paper electrode at room temperature with a computer-controlled CHI660 (USA). A platinum mesh of 5.3 cm2 was used as counter electrode, and a saturated calomel electrode (SCE) as the reference electrode with a salt bridge eliminating the liquid junction potential [29]. In the experiment, GO paper, thermally reduced graphene oxide (RGOT) paper, and platinum electrodes were compared with RGOC paper electrode. 2.4. Battery test
3.2. SEM The working principle of ZRFB is illustrated in Fig. 1a and an assembled cell is shown in Fig. 1b. The positive electrode and negative electrode of zinc-cerium redox flow test cell are separated by an ion exchange membrane (Nafion 115, Dupont) in positive half-cell (18 mL) and negative half-cell (18 mL), where the electrochemical reactions occurred when the electrolytes (each ca. 200 mL in two external tanks) flowed through with the help of pumps (12.0 mL/min) [18]. The positive electrode is reduced graphene oxide paper (3 × 3 × 0.2 cm3) with flexible graphite sheet served as current collector. The negative
As shown in Fig. 3a, there is a big difference in colour between GO paper and RGOC paper. GO paper is golden, while RGOC paper is black (Fig. 3b). After reduction, the RGOC paper can still possess good flexibility. It should be noted that dense and disordered aggregation and stack problems are generally observed in most of the free standing RGOC papers. Consequently, the favourable properties of individual graphene sheets such as high surface area can not be achieved in RGOC paper. In the present work, when GO dispersion is reduced using 210
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Fig. 2. a)XRD patterns of GO, RGOT, RGOC and graphite powders. b)Raman stpectra of GO, RGOT, RGOC and graphite powders.
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Fig. 3. a) Digital images of GO paper, b) digital images of RGOC paper, c) and d) SEM images of RGOC paper.
Fig. 4. XPS spectra for the GO, RGOT and RGOC. 212
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spectroscopic technique requiring high vacuum or ultra-high vacuum conditions. The C/O ratio would become bigger with the reduction of GO. That is to say, the greater the C/O ratio, the greater the reduction degree of GO. The XPS C1s spectra before and after GO reduction are shown in Fig. 4. There are various oxygen-containing functional groups on the surface and edge of GO including carboxylate, carboxyl, carbonyl, hydroxyl and epoxide. The carbon content in GO, RGOT and RGOC is 67.33%, 68.5% and 84.75%, respectively. So, the reduction degree of RGOC is greater than that of RGOT. It is observed that the peak at about 286.6 eV became weaker after thermal treatment of GO, even disappeared after chemical reduction of GO. The weakening or disappearance of the peak at about 286.6 eV is cosistent with the
NaBH4, the obtained RGOC paper can still maintain a porous structure (Fig. 3c and d), which could contribute to the formation of conductive network in the paper electrode prepared by pressing RGOC powder. The large surface area and easy pathway within RGOC paper electrode for fast electrolyte ions diffusion partly result in fast kinetics of Ce3+/Ce4+ electrode reaction. 3.3. XPS The characterization of reduction degree of GO can be carried out by analysing the C/O ratio on the surface of GO using X-ray photoelectron spectroscopy which is a surface-sensitive quantitative
Fig. 5. Electrochemical analysis of Ce3+/Ce4+ redox couple. a) CV curves of the Ce3+/Ce4+ redox pair, test conditions: aqueous solvent, 20 mM Ce(CH3SO3)3, 2 M CH3SO3H as the supporting electrolyte; RGOC paper as working electrode (1.0 cm2), and Pt mesh and SCE as the counter electrode and the reference electrode, respectively. b) Anodic peak current against the square root of scan rate for the Ce3+/Ce4+ redox pair. c) CV curves of the Ce3+/Ce4+ redox pair at GO, RGOT, RGOC and Pt electrodes(each of 1.0 cm2), scan rate of 50 mV s−1. d)Overpotential-current curves of 10 mM Ce3+ + 10 mM Ce4+ + 2 M CH3SO3 at 1 mV s−1. e)EIS curves and the equivalent circuit used to fit the EIS results. 213
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other conditions, for example, temperature, scanning speed, electrode composition and active substance concentration, it can be concluded that the order of electrochemical activity toward Ce3+/Ce4+ couple from high to low is RGOC, RGOT, Pt and GO. In addition, the same conclusion can be obtained from the data showed in Fig. 5d and e. The change of oxygen-containing functional groups is observed on the surface and edge of GO during thermal reduction or chemical reduction. The descending order of C-O functional groups content is GO, RGOT and RGOC. The obvious increase of the peak potential separation at RGOT and RGOC papers will be observed if Ce3+/Ce4+ redox reactions could be catalysed by the C-O functional groups. But the data obtained in this work are just reversed, indicating these functional groups are of no moment to Ce3+/Ce4+ electrode processes. The fact that the activity of RGOC toward Ce3+/Ce4+ is better than that of RGOT also confirm the above conclusions. Furthermore, the removal of C-O functional groups leads to the decrease of steric hindrance and the introduction of defects in RGOC and RGOT papers. The decrease of steric hindrance is beneficial to the acceleration of adsorption kinetics. The introduction of defects would bring the increase of reactive points for the Ce3+/Ce4+ electrode process. Therefore, RGOC is better than RGOT, and RGOT is better than GO, in terms of electrochemical activity towards Ce3+/Ce4+ electrode reaction.
decrease or depletion of hydroxyl and epoxide on the surface and edge of GO. 3.4. Electrochemical analysis The evaluation of kinetics of Ce3+/Ce4+ redox reaction at RGOC paper electrode is firstly carried out by cyclic voltammetry which is a reversal technique and is the potential-scan equivalent of double potential step chronoamperometry. Fig. 5a shows CV curves of 20 mM Ce (CH3SO3)3 in aqueous solvent with 2 M CH3SO3H as supporting electrolyte under air atmosphere. Regardless of the scan rate, every CV curve of Ce(CH3SO3)3 shows an anodic peak corresponding to the oxidation of Ce3+ to Ce4+ and a cathodic one corresponding to the reduction of Ce4+ to Ce3+. Both the anodic peak current and cathodic one increased with the scan rate, and the diffusion coefficient of Ce3+ was evaluated by employing the Randles-Sevcik equation [30] for reversible systems. Fig. 5b shows the fitting of the anodic peak current against the square root of the scan rate, giving a diffusion coeffient of 3.6 × 10−5 cm2 s−1 for Ce3+ ion. The separation of peak potentials is one of the two important parameters of cyclic voltammogram, the smaller separation of peak potential indicating the faster kinetics of electrode process. As seen in Fig. 5c, the cyclic voltammogram of Ce3+/Ce4+ couple showed a 356 mV of separation of peak potentials at GO paper, while those at RGOT, RGOC and Pt electrodes are 145 mV, 101 mV and 159 mV, respectively. Another important parameter of cyclic voltammogram is the ratio of peak currents. For a nernstian wave with stable product, the ratio of peak current is unity regardless of scan rate. In this work, deviation of the ratio of peak currents from unity was observed, indicating the existence of other complications in the electrode process. The significant increase of peak current was also observed after the reduction of GO. Since the experiment is carried out under the same conditions as
3.5. Battery performance The improvement of utilization rate of active material is beneficial to the improvement of battery energy density, which is an important parameter of battery performance. As shown in Fig. 6a, the Zn/Ce RFB with RGOC paper positive electrode exhibited a charge capacity of 2167.2 mAh, corresponding to a 81.0% utilization rate of active material, while it was 168.6 mAh with GO paper positive electrode and the utilization rate of active material was only 6.3%. An obvious decrease
Fig. 6. Performance of zinc-cerium redox flow cell. a) Cell-voltage profile with respect to capacity during a typical charge/discharge process. b) Coulombic efficiency variety as a function of the cycle number. c) Voltage efficiency variety as a function of the cycle number. d) Energy efficiency variety as a function of the cycle number. 214
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of overpotential with RGOC paper as positive electrode can be responsible for the improvement of utilization rate of active material in the charge-discharge process of Zn/Ce RFB. Under the condition that the charge-discharge voltage range is certain, the smaller the overpotential is, the greater the charge capacity and the utilization rate of active material will be. The Zn/Ce RFB with RGOC paper as positive electrode exhibited an average coulombic efficiency (CE) of 89.6% (Fig. 6b), an average voltage efficiency (VE) of 79.6% (Fig. 6c), and an average energy efficiency (EE) of 71.3% (Fig. 6d), while they were 23.8% (Fig. 6b), 55.7% (Fig. 6c) and 13.3% (Fig. 6d) for the cell with GO paper electrode. In addition, the coulombic efficiency, voltage efficiency and energy efficiency of Zn/Ce RFB with RGOC paper electrode has no obvious decay over 50 cycles, indicating the stability of RGOC paper electrode is very good over repetitive cycling. While those of Zn/Ce RFB with GO paper electrode exists obvious decay over 5 cycles. High coulombic efficiency of RGOC paper should be attributed to low oxygen functional group in materials (Fig. 4) because oxygen functional group can lead to serious oxygen evolution reaction [3]. According to the above result, it could be deduced that RGOC paper could be a candidate of positive electrode material for advanced Zn/Ce RFB.
[7] B. Fang, S. Iwasa, Y. Wei, T. Arai, M. Kumagai, A study of Ce(III)/Ce(IV) redox couple for redox flow battery application, Electrochim. Acta 47 (2002) 3971–3976. [8] Fengjiao Xiong, Debi Zhou, Zhipeng Xie, Yunyang Chen, A study of the Ce3+/Ce4+ redox couple in sulfamic acid for redox battery application, Appl. Energy 99 (2012) 291–296. [9] P.K. Leung, C. Ponce de Lón, C.T.J. Low, F.C. Walsh, Ce(III)/Ce(IV) in methanesulfonic acid as the positive half cell of a redox flow battery, Electrochim. Acta 56 (2011) 2145–2153. [10] Zhipeng Xie, Debi Zhou, Fengjiao Xiong, Shimin Zhang, Kelong Huang, Cerium-zinc redox flow battery: positive half-cell electrolyte studies, J. Rare Earths 29 (6) (2011) 567–573. [11] P. Modiba, A.M. Crouch, Electrochemical study of cerium (IV) in the presence of ethylenediaminetetraacetic acid (EDTA) and diethylenetriaminepentaacetate (DTPA) ligands, J. Appl. Electrochem. 38 (2008) 1293–1299. [12] Zhipeng Xie, Fengjiao Xiong, Debi Zhou, Study of the Ce3+/Ce4+ redox couple in mixed-acid media (CH3SO3H and H2SO4) for redox flow battery application, Energy Fuel. 25 (2011) 2399–2404. [13] Zhipeng Xie, Qingchao Liu, Zhiwen Chang, Xinbo Zhang, The developments and challenges of cerium half-cell in zinc-cerium redox flow battery for energy storage, Electrochim. Acta 90 (2013) 695–704. [14] A. Paulenova, S.E. Creager, J.D. Navratil, Y. Wei, Redox potentials and kinetics of Ce3+/Ce4+ redox reaction and solubility of cerium sulfates in sulfuric acid solutions, J. Power Sources 109 (2002) 431–438. [15] P.K. Leung, C. Ponce de León, C.T.J. Low, A.A. Shah, F.C. Walsh, Characterization of a zinc-cerium flow battery, J. Power Sources 196 (2011) 5174–5185. [16] Georgios Nikiforidis, Walid A. Daoud, Indium modified graphite electrodes on highly zinc containing methanesulfonate electrolyte for zinc-cerium redox flow battery, Electrochim. Acta 168 (2015) 394–402. [17] Zhipeng Xie, Bin Yang, Dingjian Cai, Liang Yang, Hierarchical porous carbon toward effective cathode in advanced zinc-cerium redox flow battery, J. Rare Earths 32 (10) (2014) 973–978. [18] Zhipeng Xie, Bin Yang, Liang Yang, Xiaona Xu, Dingjian Cai, Jianchai Chen, Yujuan Chen, Yanhua He, Ying Li, Xiaochun Zhou, Addition of graphene oxide into graphite toward effective positive electrode for advanced zinc-cerium redox flow battery, J. Solid State Electrochem. 19 (2015) 3339–3345. [19] Sasha Stankovich, Dmitriy A. Dikin, Geoffrey H.B. Dommett, Kevin M. Kohlhaas, Eric J. Zimney, Eric A. Stach, Richard D. Piner, SonBinh T. Nguyen, Rodney S. Ruoff, Graphene-based composite materials, Nature 442 (2006) 282–286. [20] Xiao Huang, Xiaoying Qi, Freddy Boey, Hua Zhang, Graphene-based composites, Chem. Soc. Rev. 41 (2012) 666–686. [21] Yuanlong Shao, Maher F. El-Kady, Lisa J. Wang, Qinghong Zhang, Yaogang Li, Hongzhi Wang, Mir F. Mousavi, Richard B. Kaner, Graphene-based materials for flexible supercapacitors, Chem. Soc. Rev. 44 (2015) 3639–3665. [22] Francois Perreault, Andreia Fonseca de Faria, Menachem Elimelech, Environmental applications of graphene-based nanomaterials, Chem. Soc. Rev. 44 (2015) 5861–5896. [23] Dongdong Li, Lei Zhang, Hongbin Chen, Jun Wang, Liangxin Ding, Suqing Wang, Peter J. Ashman, Haihui Wang, Graphene-based nitrogen-doped carbon sandwich nanosheets: a new capacitive process controlled anode material for high-performance sodium-ion batteries, J. Mater. Chem. 4 (2016) 8630–8635. [24] Xiaoyan Zhang, Artur Ciesielski, Fanny Richard, Pengkun Chen, Eko Adi Prasetyanto, Luisa De Cola, Paolo Samorì, Graphene: modular graphene-based 3D covalent networks: functional architectures for energy applications, Small 12 (8) (2016) 1108. [25] Adrian Hunt, Dmitriy A. Dikin, Ernst Z. Kurmaev, Teak D. Boyko, Paul Bazylewski, Gap Soo Chang, Alexander Moewes, Epoxide speciation and functional group distribution in graphene oxide paper-like materials, Adv. Funct. Mater. 22 (18) (2012) 3950–3957. [26] Dongfei Sun, Xingbin Yan, Junwei Lang, Qunji Xue, High performance supercapacitor electrode based on graphene paper via flame-induced reduction of graphene oxide paper, J. Power Sources 222 (2013) 52–58. [27] Tugrul Cetinkaya, Seyma Ozcan, Mehmet Uysal, Mehmet O. Guler, Hatem Akbulut, Free-standing flexible graphene oxide paper electrode for rechargeable Li-O2 batteries, J. Power Sources 267 (2014) 140–147. [28] D.C. Marcano, D.V. Kosynkin, J.M. Berlin, A. Sinitskii, Z.Z. Sun, A. Slesarev, L.B. Alemany, W. Lu, J.M. Tour, Improved synthesis of graphene oxide, ACS Nano 4 (8) (2010) 4806–4814. [29] Wenyue Li, Jianguo Liu, Chuanwei Yan, Reduced graphene oxide with tunable C/O ratio and its activity towards vanadium redox pairs for an all vanadium redox flow battery, Carbon 55 (2013) 313–320. [30] Ke Gong, Qianrong Fang, Shuang Gu, Sam Fong Yau Li, Yushan Yan, Nonaqueous redox-flow batteries:organic solvents, supporting electrolytes, and redox pairs, Energy Environ. Sci. 8 (2015) 3515–3530.
4. Conclusions RGOC paper with good voids and strength was prepared by a simple method that reduction of GO was carried out using NaBH4. The improved kinetics of Ce3+/Ce4+ electrode process at RGOC paper is attributed to the introduction of more defects into graphitic structure of RGOC paper and the removal of C-O functional groups minimizing the space steric hindrance. RGOC paper is a candidate toward an excellent electrode material for Zn/Ce RFB. Acknowledgements The authors acknowledge the financial support from the National Natural Science Foundation of China (No.21361010), Province Natural Science Foundation of Jiangxi (No.20161BAB206144 and No.20171BAB206001), China Scholarship Council (No.201708360025) as well as JXPCOD-JXSTA (No. 100RYH2017). References [1] Rajiv Giridharagopal, Phillip A. Cox, David S. Ginger, Functional scanning probe imaging of nanostructured solar energy materials, Acc. Chem. Res. 49 (9) (2016) 1769–1776. [2] Xiaolei Huang, Mei Leng, Wen Xiao, Meng Li, Jun Ding, Teck Leong Tan, Wee Siang Vincent Lee, Junmin Xue, Activating basal planes and S-terminated edges of MoS2 toward more efficient hydrogen evolution, Adv. Funct. Mater. 27 (6) (2017) 1604943. [3] Xunyu Lu, Wai-Leung Yim, Bryan H.R. Suryanto, Chuan Zhao, Electrocatalytic oxygen evolution at surface-oxidized multiwall carbon nanotubes, J. Am. Chem. Soc. 137 (2015) 2901–2907. [4] Baixiong Liu, Jinshu Wang, Hongyi Li, Junshu Wu, Meiling Zhou, Tieyong Zuo, Facile synthesis of hierarchical hollow mesoporous Ag/WO3 spheres with high photocatalytic performance, J. Nanosci. Nanotechnol. 13 (6) (2013) 4117–4122. [5] Shuang Gu, Ke Gong, Emily Z. Yan, Yushan Yan, A multiple ion-exchange membrane design for redox flow batteries, Energy Environ. Sci. 7 (9) (2014) 2986–2998. [6] P.K. Leung, C.P. de Leon, F.C. Walsh, An undivided zinc-cerium redox flow battery operating at room temperature (295 K), Electrochem. Commun. 13 (2011) 770–773.
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Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Chemically reduced graphene oxide paper as positive electrode for advanced Zn/Ce redox flow battery
T
Zhipeng Xiea,b,∗, Baolu Liua, Chenfan Xiec, Bin Yanga, Yunfen Jiaoa, Dingjian Caia, Liang Yanga, Qi Shua, Anhong Shia a
Engineering Research Institute, Jiangxi University of Science and Technology, Ganzhou, 341000, China State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, 130022, China c School of Petroleum and Chemical Engineering, Dalian University of Technology, Panjin, 124221, China b
H I GH L IG H T S
RGOC paper possesses good flexibility with a porous structure. • The C/O ratio of RGOC is different from that of RGOT. • The • The activity of RGOC towards Ce /Ce is better than that of RGOT. 3+
4+
A R T I C LE I N FO
A B S T R A C T
Keywords: Graphene oxide Redox flow battery Cerium New energy Energy storage
Zn/Ce redox flow battery (ZRFB) is emerging as a promising technology to store large amount of energy economically and efficiently, wherein a highly efficient positive electrode with a continuous and fast electronic and ionic transportation path is urgently desired. The unique nanostructure of reduced graphene oxide (RGO) paper electrode is prepared by a simply chemical reduction method, which facilitates transference of the electron and Ce3+/Ce4+ at the electrolyte/electrode interface. Thus ZRFB exhibits superior extent of charge (81.0%) and energy efficiency (71.3%). The results show RGO paper is a good candidate for positive electrode of ZRFB.
1. Introduction Energy is of importance to all of us. It is necessary to develop new energy resources [1–3] for reducing environmental pollutants [4] and improving human life level. However, the more widespread use of them is dependent upon the development of an affordable and reliable energy storage system. Redox flow batteries (RFBs) have received increasing attention for their storing enormous amount of electrical energy friendly and efficiently [5,6]. For a typical RFB, its electroactive species are dissolved in two electrolytes stored in separate tanks instead of in electrodes, which is different from traditional batteries. The cell reactions occur when electrolyte flows through electrode with the help of pump. The electrode only offers a place where electrode reaction occurs without undergoing any deformation, which is helpful to prolong the service life of the battery. The capacity of RFB is determined by the amount of electroactive species in electrolyte, while the power output by the size of the electrode. For example, the greater the amount of Ce3+/Ce4+ in positive electrolyte, the greater the capacity of ZRFB.
∗
The architecture characteristic of separation of power output from capacity gives RFB considerable design flexibility. It is of fundamental importance to select suitable electrode material for optimization and upgrading of battery performance. The factors to be considered in selection of electrode materials for RFB application include conductivity, mechanical strength, chemical stability and electrochemical activity. Generally, selection of electrode material for RFB application is the process of finding a balance point in the above mentioned factors based on the specific use. The kinetic characteristics of Ce3+/Ce4+ electrode reaction on glassy carbon, platinum, platinized titanium, carbon felt, graphite, porous carbon, and graphene oxide/ graphite composite electrodes were investigated by different researchers [7–18]. Although some achievements have been made in the research of electrode, it is still necessary to search for alternative materials with better performance toward effective positive electrode for advanced ZRFB application. Graphene-based materials have attracted significant attention for their excellent mechanical and electrical properties [19–24], which can
Corresponding author. No.86, Hongqi Ave., Ganzhou, Jiangxi, China. E-mail address: [email protected] (Z. Xie).
https://doi.org/10.1016/j.matchemphys.2018.08.082 Received 29 September 2017; Received in revised form 8 April 2018; Accepted 24 August 2018 Available online 27 August 2018 0254-0584/ © 2018 Elsevier B.V. All rights reserved.
Materials Chemistry and Physics 220 (2018) 208–215
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Fig. 1. a)Working principle of Zn/Ce redox flow battery. b)Assembled ZRFB. 1. negative electrode, 2. positive electrode, 3. Nafion 115 membrane. 209
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electrode is zinc sheet (3 × 3 × 0.2 cm). The measurement of Zn/Ce RFB was carried out under constant current of 180 mA at the voltage range between 2.6 and 0.5 V.
be prepared in large scale by modifying graphene oxide (GO) with reduction reagent and/or thermal treatment method. There are wide range of oxygen functional groups on both basal planes and edges of GO, which provides more possibilities for applications in materials science and nanocomposites [25]. Graphene oxide paper is a novel freestanding paper-like material with good mechanical properties [25–28]. Herein, we report the first use of reduced graphene oxide paper as positive electrode in advanced ZRFB. These results show that the reduced graphene oxide paper is an excellent candidate in the application of ZRFB.
3. Results and discussion 3.1. XRD and Raman characterization As shown in Fig. 2a, there is a big difference among the XRD patterns of GO powder, RGO powder (RGOT, thermally reduced graphene oxide; RGOC, Chemically reduced graphene oxide) and graphite powder. The graphene oxide shows a characteristic diffraction peak (001) at 10.02° with an interplanar spacing of 0.88 nm, which is caused by the atomic scale roughness and the generation of oxygen-containing functional groups attached on GO nanosheets [18]. When the graphene oxide is reduced by thermal treatment under a nitrogen atmosphere at 200 °C for 2 h (corresponding to RGOT) or by NaBH4 (corresponding to RGOC), the disappearance of the characteristic diffraction peak at 10.02° is accompanied by the emergence of a weak wide bulge centered at 22.51° for RGOT and at 24.04° for RGOC. As for graphite powder, it has a strong peak (002) at 26.54° with an interlayer spacing of 0.34 nm and a weak peak (004) at 54.66° with an interlayer spacing of 0.17 nm, which means that the graphite powder was partly oxidized [18]. Raman spectroscopy, a nondestructive and fast method for characterization of the crystal structure, is then used to characterize the disorder and defects in the as-prepared samples. The graphite is also employed for comparison, which consists of fused aromatic ring structures along direction in the plane. As seen in Fig. 2b, the graphite crystal shows two bands: a strong one positioned at a Raman frequency of 1569.9 cm−1 and a much weaker one at 1346.4 cm−1. The former was assigned to the E2gC-C stretching mode, called G band and the latter D band representing a disorderly network of sp2-coordinated clusters. Generally, graphene of high quality also shows a weak D band. In theory, the greater the quantity of the ring near a graphene edge or the more the structure defect of a graphene, the greater the intensity of D band. The material without any defect has a very weak, even no D band. Hence, the disorder in a graphene can be reflected from the D band intensity, and its measurement is often based on the ratio of D to G band intensity. In this work, we prepared two kinds of reduced graphene oxide, which are graphene with many defects. One is chemically reduced graphene oxide (RGOC) prepared by the reduction of GO with NaH4B solution, the other is thermally reduced graphene oxide (RGOT) prepared by thermal treatment of GO under a nitrogen atmosphere at 200 °C for 2 h. GO is prepared by the oxidation of graphite. However, the reduction degree of GO and the oxidation degree of graphite can not be reflected from the ratio of D to G band intensity which can be the result of interaction of other factors such as edges, defects and ripples. The intensity ratios of D to G for GO, RGOT and RGOC are 0.84, 0.89 and 1.03, respectively. The increment of intensity ratio of D to G band is also observed in other study on reduction of graphene oxide [29]. Therefore, the following conclusions can be obtained. The reduction of GO, whether thermal ruduction or chemical reduction, can introduce a large number of edges, defects and ripples to graphene. Furthermore, the amount of edges, defects and ripples introduced by chemical reduction is more than that of by thermal reduction.
2. Experimental 2.1. Electrode preparation The preparation of GO was carried out on the basis of the improved Hummers’ method [28]. Briefly, graphite powders (99.95%, 1200 mesh, aladdin®) were oxidized by the concentrated H2SO4/H3PO4 and KMnO4 to produce precursor, reoxidized by 30%-H2O2, then washed using H2O, HCl and C2H5OH, respectively. The centrifugalization of the mixture was performed at 12000 rpm for 10 min and obtains graphite oxide. Graphite oxide was further diluted in deionized water, ultrasonication for 30 min to obtain graphene oxide aqueous solution [18]. Then, sodium borohydride solution was added into the GO aqueous solution, which was accompanied by a violent reaction and the generation of reduced graphene oxide (i.e., chemically reduced graphene oxide, RGOC). RGOC dispersion requires washing using H2O no less than 3 times, then followed by freeze-drying under vacuum for 24 h. The RGOC paper is obtained by pressing the RGOC powder at a pressure of 10 MPa. All the chemicals are AR grade, purchased from Shanghai Aladdin Bio-Chem Technology Co., LTD (aladdin®, China). 2.2. Sample characterization The characterization of RGOC paper was carried out by field emission scanning electron microscopy (HITACHI S-4800, Shanghai Yong Ming Automation Equipment Co. LTD), X-ray diffraction patterns (Bruker D8 Focus powder XRD, Xiamen Mingda Technology Co. LTD), Raman scattering (Renishaw inVia spectrometer system, Suzhou Ruice Precision Instrument Co. LTD) and X-ray photoelectron spectroscopy (QUANTUM 2000 surface analysis system, Physical Electronics USA). 2.3. Electrochemical measurements Several electrochemical analytical methods (cyclic voltammetry, linear sweep voltammetry and impedance spectroscopy) were selected for characterizing the electrochemical behaviour of Ce3+/Ce4+ redox couple in a solution of 2 mol/L CH3SO3H at RGOC paper electrode at room temperature with a computer-controlled CHI660 (USA). A platinum mesh of 5.3 cm2 was used as counter electrode, and a saturated calomel electrode (SCE) as the reference electrode with a salt bridge eliminating the liquid junction potential [29]. In the experiment, GO paper, thermally reduced graphene oxide (RGOT) paper, and platinum electrodes were compared with RGOC paper electrode. 2.4. Battery test
3.2. SEM The working principle of ZRFB is illustrated in Fig. 1a and an assembled cell is shown in Fig. 1b. The positive electrode and negative electrode of zinc-cerium redox flow test cell are separated by an ion exchange membrane (Nafion 115, Dupont) in positive half-cell (18 mL) and negative half-cell (18 mL), where the electrochemical reactions occurred when the electrolytes (each ca. 200 mL in two external tanks) flowed through with the help of pumps (12.0 mL/min) [18]. The positive electrode is reduced graphene oxide paper (3 × 3 × 0.2 cm3) with flexible graphite sheet served as current collector. The negative
As shown in Fig. 3a, there is a big difference in colour between GO paper and RGOC paper. GO paper is golden, while RGOC paper is black (Fig. 3b). After reduction, the RGOC paper can still possess good flexibility. It should be noted that dense and disordered aggregation and stack problems are generally observed in most of the free standing RGOC papers. Consequently, the favourable properties of individual graphene sheets such as high surface area can not be achieved in RGOC paper. In the present work, when GO dispersion is reduced using 210
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Fig. 2. a)XRD patterns of GO, RGOT, RGOC and graphite powders. b)Raman stpectra of GO, RGOT, RGOC and graphite powders.
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Fig. 3. a) Digital images of GO paper, b) digital images of RGOC paper, c) and d) SEM images of RGOC paper.
Fig. 4. XPS spectra for the GO, RGOT and RGOC. 212
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spectroscopic technique requiring high vacuum or ultra-high vacuum conditions. The C/O ratio would become bigger with the reduction of GO. That is to say, the greater the C/O ratio, the greater the reduction degree of GO. The XPS C1s spectra before and after GO reduction are shown in Fig. 4. There are various oxygen-containing functional groups on the surface and edge of GO including carboxylate, carboxyl, carbonyl, hydroxyl and epoxide. The carbon content in GO, RGOT and RGOC is 67.33%, 68.5% and 84.75%, respectively. So, the reduction degree of RGOC is greater than that of RGOT. It is observed that the peak at about 286.6 eV became weaker after thermal treatment of GO, even disappeared after chemical reduction of GO. The weakening or disappearance of the peak at about 286.6 eV is cosistent with the
NaBH4, the obtained RGOC paper can still maintain a porous structure (Fig. 3c and d), which could contribute to the formation of conductive network in the paper electrode prepared by pressing RGOC powder. The large surface area and easy pathway within RGOC paper electrode for fast electrolyte ions diffusion partly result in fast kinetics of Ce3+/Ce4+ electrode reaction. 3.3. XPS The characterization of reduction degree of GO can be carried out by analysing the C/O ratio on the surface of GO using X-ray photoelectron spectroscopy which is a surface-sensitive quantitative
Fig. 5. Electrochemical analysis of Ce3+/Ce4+ redox couple. a) CV curves of the Ce3+/Ce4+ redox pair, test conditions: aqueous solvent, 20 mM Ce(CH3SO3)3, 2 M CH3SO3H as the supporting electrolyte; RGOC paper as working electrode (1.0 cm2), and Pt mesh and SCE as the counter electrode and the reference electrode, respectively. b) Anodic peak current against the square root of scan rate for the Ce3+/Ce4+ redox pair. c) CV curves of the Ce3+/Ce4+ redox pair at GO, RGOT, RGOC and Pt electrodes(each of 1.0 cm2), scan rate of 50 mV s−1. d)Overpotential-current curves of 10 mM Ce3+ + 10 mM Ce4+ + 2 M CH3SO3 at 1 mV s−1. e)EIS curves and the equivalent circuit used to fit the EIS results. 213
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other conditions, for example, temperature, scanning speed, electrode composition and active substance concentration, it can be concluded that the order of electrochemical activity toward Ce3+/Ce4+ couple from high to low is RGOC, RGOT, Pt and GO. In addition, the same conclusion can be obtained from the data showed in Fig. 5d and e. The change of oxygen-containing functional groups is observed on the surface and edge of GO during thermal reduction or chemical reduction. The descending order of C-O functional groups content is GO, RGOT and RGOC. The obvious increase of the peak potential separation at RGOT and RGOC papers will be observed if Ce3+/Ce4+ redox reactions could be catalysed by the C-O functional groups. But the data obtained in this work are just reversed, indicating these functional groups are of no moment to Ce3+/Ce4+ electrode processes. The fact that the activity of RGOC toward Ce3+/Ce4+ is better than that of RGOT also confirm the above conclusions. Furthermore, the removal of C-O functional groups leads to the decrease of steric hindrance and the introduction of defects in RGOC and RGOT papers. The decrease of steric hindrance is beneficial to the acceleration of adsorption kinetics. The introduction of defects would bring the increase of reactive points for the Ce3+/Ce4+ electrode process. Therefore, RGOC is better than RGOT, and RGOT is better than GO, in terms of electrochemical activity towards Ce3+/Ce4+ electrode reaction.
decrease or depletion of hydroxyl and epoxide on the surface and edge of GO. 3.4. Electrochemical analysis The evaluation of kinetics of Ce3+/Ce4+ redox reaction at RGOC paper electrode is firstly carried out by cyclic voltammetry which is a reversal technique and is the potential-scan equivalent of double potential step chronoamperometry. Fig. 5a shows CV curves of 20 mM Ce (CH3SO3)3 in aqueous solvent with 2 M CH3SO3H as supporting electrolyte under air atmosphere. Regardless of the scan rate, every CV curve of Ce(CH3SO3)3 shows an anodic peak corresponding to the oxidation of Ce3+ to Ce4+ and a cathodic one corresponding to the reduction of Ce4+ to Ce3+. Both the anodic peak current and cathodic one increased with the scan rate, and the diffusion coefficient of Ce3+ was evaluated by employing the Randles-Sevcik equation [30] for reversible systems. Fig. 5b shows the fitting of the anodic peak current against the square root of the scan rate, giving a diffusion coeffient of 3.6 × 10−5 cm2 s−1 for Ce3+ ion. The separation of peak potentials is one of the two important parameters of cyclic voltammogram, the smaller separation of peak potential indicating the faster kinetics of electrode process. As seen in Fig. 5c, the cyclic voltammogram of Ce3+/Ce4+ couple showed a 356 mV of separation of peak potentials at GO paper, while those at RGOT, RGOC and Pt electrodes are 145 mV, 101 mV and 159 mV, respectively. Another important parameter of cyclic voltammogram is the ratio of peak currents. For a nernstian wave with stable product, the ratio of peak current is unity regardless of scan rate. In this work, deviation of the ratio of peak currents from unity was observed, indicating the existence of other complications in the electrode process. The significant increase of peak current was also observed after the reduction of GO. Since the experiment is carried out under the same conditions as
3.5. Battery performance The improvement of utilization rate of active material is beneficial to the improvement of battery energy density, which is an important parameter of battery performance. As shown in Fig. 6a, the Zn/Ce RFB with RGOC paper positive electrode exhibited a charge capacity of 2167.2 mAh, corresponding to a 81.0% utilization rate of active material, while it was 168.6 mAh with GO paper positive electrode and the utilization rate of active material was only 6.3%. An obvious decrease
Fig. 6. Performance of zinc-cerium redox flow cell. a) Cell-voltage profile with respect to capacity during a typical charge/discharge process. b) Coulombic efficiency variety as a function of the cycle number. c) Voltage efficiency variety as a function of the cycle number. d) Energy efficiency variety as a function of the cycle number. 214
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of overpotential with RGOC paper as positive electrode can be responsible for the improvement of utilization rate of active material in the charge-discharge process of Zn/Ce RFB. Under the condition that the charge-discharge voltage range is certain, the smaller the overpotential is, the greater the charge capacity and the utilization rate of active material will be. The Zn/Ce RFB with RGOC paper as positive electrode exhibited an average coulombic efficiency (CE) of 89.6% (Fig. 6b), an average voltage efficiency (VE) of 79.6% (Fig. 6c), and an average energy efficiency (EE) of 71.3% (Fig. 6d), while they were 23.8% (Fig. 6b), 55.7% (Fig. 6c) and 13.3% (Fig. 6d) for the cell with GO paper electrode. In addition, the coulombic efficiency, voltage efficiency and energy efficiency of Zn/Ce RFB with RGOC paper electrode has no obvious decay over 50 cycles, indicating the stability of RGOC paper electrode is very good over repetitive cycling. While those of Zn/Ce RFB with GO paper electrode exists obvious decay over 5 cycles. High coulombic efficiency of RGOC paper should be attributed to low oxygen functional group in materials (Fig. 4) because oxygen functional group can lead to serious oxygen evolution reaction [3]. According to the above result, it could be deduced that RGOC paper could be a candidate of positive electrode material for advanced Zn/Ce RFB.
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4. Conclusions RGOC paper with good voids and strength was prepared by a simple method that reduction of GO was carried out using NaBH4. The improved kinetics of Ce3+/Ce4+ electrode process at RGOC paper is attributed to the introduction of more defects into graphitic structure of RGOC paper and the removal of C-O functional groups minimizing the space steric hindrance. RGOC paper is a candidate toward an excellent electrode material for Zn/Ce RFB. Acknowledgements The authors acknowledge the financial support from the National Natural Science Foundation of China (No.21361010), Province Natural Science Foundation of Jiangxi (No.20161BAB206144 and No.20171BAB206001), China Scholarship Council (No.201708360025) as well as JXPCOD-JXSTA (No. 100RYH2017). References [1] Rajiv Giridharagopal, Phillip A. Cox, David S. Ginger, Functional scanning probe imaging of nanostructured solar energy materials, Acc. Chem. Res. 49 (9) (2016) 1769–1776. [2] Xiaolei Huang, Mei Leng, Wen Xiao, Meng Li, Jun Ding, Teck Leong Tan, Wee Siang Vincent Lee, Junmin Xue, Activating basal planes and S-terminated edges of MoS2 toward more efficient hydrogen evolution, Adv. Funct. Mater. 27 (6) (2017) 1604943. [3] Xunyu Lu, Wai-Leung Yim, Bryan H.R. Suryanto, Chuan Zhao, Electrocatalytic oxygen evolution at surface-oxidized multiwall carbon nanotubes, J. Am. Chem. Soc. 137 (2015) 2901–2907. [4] Baixiong Liu, Jinshu Wang, Hongyi Li, Junshu Wu, Meiling Zhou, Tieyong Zuo, Facile synthesis of hierarchical hollow mesoporous Ag/WO3 spheres with high photocatalytic performance, J. Nanosci. Nanotechnol. 13 (6) (2013) 4117–4122. [5] Shuang Gu, Ke Gong, Emily Z. Yan, Yushan Yan, A multiple ion-exchange membrane design for redox flow batteries, Energy Environ. Sci. 7 (9) (2014) 2986–2998. [6] P.K. Leung, C.P. de Leon, F.C. Walsh, An undivided zinc-cerium redox flow battery operating at room temperature (295 K), Electrochem. Commun. 13 (2011) 770–773.
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