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polymer binders as superior electrocatalytic materials for vanadium bromide redox flow batteries
Electrochimica Acta 85 (2012) 175–181
Contents lists available at SciVerse ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Graphene oxide nanosheets/polymer binders as superior electrocatalytic materials for vanadium bromide redox flow batteries Xianhong Rui a,b,1 , Moe Ohnmar Oo a,1 , Dao Hao Sim a , Subash chandrabose Raghu b , Qingyu Yan a,c,∗ , Tuti Mariana Lim b,d,∗∗ , Maria Skyllas-Kazacos e,∗ ∗ ∗ a
School of Materials Science and Engineering, Nanyang Technological University, 639798, Singapore School of Civil and Environmental Engineering, Nanyang Technological University, 639798, Singapore c Energy Research Institute, Nanyang Technological University, 637459, Singapore d School of Life Sciences and Chemical Technology, Ngee Ann Polytechnic, 599489, Singapore e School of Chemical Engineering, The University of New South Wales, UNSW Sydney, NSW 2052, Australia b
a r t i c l e
i n f o
Article history: Received 6 July 2012 Received in revised form 24 August 2012 Accepted 28 August 2012 Available online 6 September 2012 Keywords: Vanadium bromide redox flow batteries Graphene oxide Polymer binder High electrocatalytic activity
a b s t r a c t Few layered graphene oxide (GO) nanosheets with large specific surface area (42.1 m2 g−1 ) are successfully prepared by a modified Hummers method for use as electrodes in the vanadium bromide redox battery. The structure and physicochemical properties of GO are investigated by X-ray diffraction, Raman spectroscopy, Fourier transform infrared spectroscopy, scanning electron microscopy, transmission electron microscopy, and atomic force microscopy. Cyclic voltammetry results indicate that GO nanosheets with polymer binder (i.e., polyvinylidiene fluoride (PVDF) or sulfonated poly(ether ether ketone) (SPEEK)) hybrids demonstrate more favorable electrocatalytic activity towards the Br− /Br3− and V3+ /V2+ redox couples than the pure graphite. This is attributed to the large numbers of oxygen-containing functional groups on the GO nanosheet surface which can generate more active sites to catalyze the redox reactions. For the binder-based electrodes, the SPEEK based electrode gives the best electrochemical performance (e.g., lower overvoltage for both Br− /Br3− and V3+ /V2+ redox couple reactions and higher peak currents for the V3+ /V2+ redox couple). © 2012 Elsevier Ltd. All rights reserved.
1. Introduction The original all-vanadium redox flow battery (VRB), employing VO2+ /VO2 + and V2+ /V3+ redox couples in sulphuric acid as the positive and negative half-cell electrolytes, respectively, has shown great potential for large-scale energy storage applications with long cycle life and high energy efficiency [1–4]. However, owing to the relatively low solubility of vanadium ions in acidic sulfate solutions (1.6–2 M) within the temperature ranges of 0–45 ◦ C [5], the VRB suffers from low energy density, which limits its use in certain stationary applications as well as in electric vehicles. Fortunately, the vanadium bromide redox battery (G2V/Br, as schematically illustrated in Fig. 1) can overcome this limitation while still using
∗ Corresponding author at: School of Materials Science and Engineering, Nanyang Technological University, 639798, Singapore. Tel.: +65 6790 4583. ∗∗ Corresponding author at: School of Civil and Environmental Engineering, Nanyang Technological University, 639798, Singapore. Tel.: +65 6790 4583. ∗ ∗ ∗Corresponding author at: School of Chemical Engineering, The University of New South Wales, UNSW Sydney, NSW 2052, Australia. Tel.: +61 2 9385 4335. E-mail addresses: [email protected] (Q. Yan), [email protected] (T.M. Lim), [email protected] (M. Skyllas-Kazacos). 1 These authors contributed equally to this work. 0013-4686/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2012.08.119
the same elements in both half-cell electrolytes [6,7]. It employs V3+ /V2+ couple in the negative half-cell and Br− /Br3 − couple in the positive half-cell, using a mixture of vanadium bromide, hydrobromic acid and hydrochloric acid as the electrolyte in both half-cells. The charge–discharge reactions are shown as below: At the negative electrode : V3+ + e− ↔ V2+ −
−
At the positive electrode : 3Br − 2e ↔
Br− 3
(1) (2)
The higher solubility of vanadium bromide (3–4 M) allows the energy density to be almost doubled (∼50 Wh kg−1 ) and the V/Br cell can operate at lower-temperatures as compared with the VRB system. The electrode is a key component of redox flow batteries. Commonly, the typical electrodes for VRB are carbon or graphite-based materials (e.g., graphite felt [8] and graphite powder composites [9]). This is due to the relative stability of carbon and graphite-based materials in the highly oxidizing V(V) electrolytes that oxidizes most metallic electrodes. The poor electrochemical reversibility of the vanadium reactions on carbon electrodes however, necessitates the use of high surface carbon or graphite felts in order to reduce current density and thereby decrease the overvoltage losses. The use of graphite felts introduces high pumping energy losses
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mixture changed to brilliant yellow along with bubbling. The resulting mixture was washed with diluted HCl aqueous (1/10, v/v) solution and DI water. The graphite oxide was finally obtained after drying in a vacuum oven at 30 ◦ C. To obtain the graphene oxide nanosheet powder, exfoliation was carried out by sonicating the as-prepared graphite oxide in an ethanol solution and then drying in air at 80 ◦ C. 2.2. Materials characterization
Fig. 1. Schematic diagram of the V/Br system.
in the flow battery stacks that is a parasitic loss for the system. Considerable effort has thus been devoted to improving the electrochemical properties of graphite-based electrodes through activation and surface functionalization [10–12] and to develop new carbon materials [13–15]. Among these, electrodes fabricated by coating a thin graphene oxide (GO) layer on the surface of graphite plate has shown enhanced electrocatalytic activity towards VO2+ /VO2 + and V2+ /V3+ redox couples in the vanadium sulphate electrolytes of the VRB [14]. GO nanosheets, possessing a quasi-two-dimensional layered structure, possess a relatively large surface area and a large amount of hydroxyl and carboxyl acid active groups on basal planes and sheet edges [14–18], resulting in more reactive ions adsorbed onto the electrode surface. However, the GO film is easily stripped from the graphite plate surface after several cycles. It is thus necessary to develop a stable electrode with high electrocatalytic activity. In the present study, a similar approach to that applied to the electrodes in lithium ion batteries [19–21], was used to improve the stability of GO coatings on graphite plates for use in the vanadium halide electrolyte of the G2V/Br cell. Two different polymer binders, i.e., polyvinylidiene fluoride (PVDF) and sulfonated poly(ether ether ketone) (SPEEK) were evaluated as binding agents for the GO coatings. Cyclic voltammetry was used to investigate the electrochemical performance of the GO-based coatings in the V/Br electrolyte and the results are presented here. 2. Experimental
X-ray powder diffraction (XRD) patterns were recorded on a Shimadzu XRD-6000 X-ray diffractometer at the 2 range of 5–60◦ using Cu K␣ radiation. The morphology was investigated by using a field-emission scanning electron microscopy (FESEM) system (JEOL, Model JSM-7600F), and the nanostructure was characterized by using a transmission electron microscopy (TEM) system (JEOL, Model JEM-2100F) operating at 200 kV. Raman spectra were obtained by using a WITec CRM200 confocal Raman microscopy system with a laser wavelength of 488 nm and a spot size of 0.5 mm. FT-IR spectrum was recorded on a Fourier transform infrared spectrometer (PerkinElmer) with a DGTS detector. The specific surface area of the samples was determined with an automated adsorption apparatus (ASAP 2020) using liquid nitrogen adsorption at 77 K and calculated on the basis of Brunauer–Emmett–Teller (BET) equations. Atomic force microscopy (AFM) (Digital Instruments) was used to determine the thickness of the graphene oxide nanosheet. 2.3. Electrochemical measurements Cyclic voltammograms were recorded in the potential range between −1.0 V and 1.0 V at scan rates of 50, 20, 10 and 5 mV s−1 using a three-electrode electrochemical cell and a Solartron Electrochemical Instrument potentiostat (Sol-1470E, Solartron (UK)). The graphite plate electrode and graphite plate modified electrodes with different coatings such as graphene oxide (GO) and GO with two kind polymer binders (i.e., polyvinylidiene fluoride (PVDF) and sulfonated poly(ether ether ketone) (SPEEK) at a weight ratio of 90:10) were used as working electrodes. All potentials were measured using platinum and Ag/AgCl as the counter and reference electrodes, respectively. The electrolyte was a 2 M vanadium solution (V (IV):V (III) = 50:50, referred as V3.5+ ) in 8 M HBr plus 2 M HCl as supporting electrolyte. A mixture of 0.375 M N-ethylN-methyl-pyrrolidinium bromide (MEP) plus 0.375 M N-ethylN-methyl-morpholinium bromide (MEM) added as complexing agent to bind any bromine formed in the positive half-cell during charging.
2.1. Preparation of graphene oxide nanosheets 3. Results and discussion Graphite oxide was prepared from natural graphite (SP-1) by a modified Hummers method [22–24]. Briefly, 1.5 g graphite powder was added into a solution of 10 mL H2 SO4 (98 wt%), 1.25 g K2 S2 O8 and 1.25 g P2 O5 and the mixture was kept at 80 ◦ C for 4.5 h under vigorous stirring. Cooling to room temperature, the pre-oxidized graphite was obtained by washing with de-ionized (DI) water and drying in a vacuum oven at 50 ◦ C. Successively, pretreated graphite was put into 60 mL concentrated H2 SO4 with an ice bath, and then 7.5 g KMnO4 was added gradually under stirring and the reaction temperature was kept below 20 ◦ C. After adding all of KMnO4 , the mixture was stirred at 35 ◦ C for 2 h and then slowly diluted with 125 mL DI water. During the diluting process, a large amount of heat was released, so the addition of water was carried out in an ice bath to maintain the temperature below 50 ◦ C. The mixture was stirred for 2 h and further diluted with additional 200 mL DI water. After that, 30 wt% H2 O2 (10 mL) was slowly added into the mixture to completely react with the excess KMnO4 , and the color of
The XRD analyses of as-prepared GO and natural graphite are shown in Fig. 2a. Natural graphite shows a very sharp diffraction peak at 2 = 26.7◦ corresponding to the (0 0 2) plane, indicating a highly organized crystal structure with a layer-to-layer distance (d-spacing) of 0.336 nm. As for GO, the (0 0 2) peak is observed at 2 = 10.9◦ , corresponding to the d-spacing of 0.811 nm. Such value is much larger than that of natural graphite due to the generation of oxygen-containing functional groups between layers [25,26]. Raman spectroscopy is a powerful non-destructive tool to characterize ordered and disordered crystal structures of carbon. G band is usually assigned to the E2g phonon of C sp2 atoms, while D band is a breathing mode of -point phonons of A1g symmetry [27]. Fig. 2b shows the Raman spectra of GO and natural graphite. Raman spectrum of the natural graphite displays a strong G band at 1566 cm−1 and a weak D band at 1346 cm−1 . After oxidation, in the Raman spectrum of GO, the G band is broadened and shifted upward to
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Fig. 3. FT-IR transmittance spectrum of the as-prepared graphene oxide.
Fig. 2. XRD patterns (a) and Raman spectra (b) of as-prepared graphene oxide and natural graphite.
1580 cm−1 , which was mainly caused by stress [28]. At the same time, the intensity of D band at 1351 cm−1 increases substantially, which indicates that the size of in-plane sp2 domains is decreased, possibly arising from the extensive oxidation and ultrasonic exfoliation. Fig. 3 shows the FT-IR transmittance spectrum (KBr) of the as-prepared GO. The spectrum illustrates a strong and broad absorption in the range of 3700–2000 cm−1 corresponding to the stretching vibrations of structural OH groups and physisorbed water molecules [29]. The peak at 1735 cm−1 is the C O stretching vibration of COOH groups. The absorptions arising from the O H bending vibration, epoxide groups and skeletal ring vibrations are observed at 1622 cm−1 [30]. The absorption at 1375 cm−1 is possibly originated from the bending of tertiary C OH groups [29]. The appearance of absorption at 1228 cm−1 is related to phenolic groups [29]. The 1062 cm−1 absorption is ascribed to C O C stretching vibration. Field emission scanning electron microscopy (FESEM) images of the natural graphite and GO are shown in Fig. 4. It can be observed (Fig. 4a and b) that the natural graphite consists of flakes with flat and smooth surface. The lateral size is 60–200 m, and the thickness is 5–10 m. With a careful observation, the lamellar structure at the edges of flakes can be seen. After oxidation, the GO (Fig. 4c and d) shows an agglomeration of the
exfoliated flakes with some wrinkles and folding on the surfaces and edges. In Fig. 5, the obtained GO was further analyzed by transmission electron microscopy (TEM), high resolution (HR) TEM, and selected area electron diffraction (SAED) observations. The lowand high-magnification TEM images (Fig. 5a) reveal that the GO consists of a thin wrinkled paper-like structure, suggesting that the GO nanosheet has been fully exfoliated. The intrinsic ripples of GO nanosheets may develop into wavy structures in the macroscopic scale (Fig. 4c and d). In addition, GO nanosheets are transparent and exhibit a very stable nature under the high-energy electron beam. From the HRTEM image (Fig. 5b), the scrolled edges giving a cross-section view of the stacked GO nanosheets, only 2–3 layers of GO sheets can be observed. A SAED pattern of the featureless region was recorded (inset in Fig. 5b). The well-defined diffraction spots can be indexed to the hexagonal graphite crystal structure, confirming the graphitic crystalline nature of the GO nanosheets. The obtained GO nanosheets were also analyzed by atomic force microscopy (AFM). Fig. 6 shows an AFM image of a 5 m × 5 m Si surface deposited with GO suspension. The flat sheet exhibits an average thickness of about 1.1 nm, corresponding to 2–3 layers of GO nanosheet. The specific surface area of GO nanosheets and pristine graphite was measured to be 42.1 and 1.2 m2 g−1 , respectively, using the Brunauer–Emmett–Teller (BET) method. The relatively low surface area for few-layered GO nanosheets is due to the intersheet van der Waals attractions, aggregation or restacking inevitably occurring in graphene oxide assemblies. Fig. 7 presents the cyclic voltammetric (CV) behaviors of the graphite, GO, GO-PVDF and GO-SPEEK electrodes in a V/Br electrolyte comprising 2 M V3.5+ , 8 M HBr, 2 M HCl and 0.75 M QBr (MEM + MEP) at a scan rate of 50 mV s−1 . In each of these CVs, the forward scan commences at the initial potential of −1 V. It can be seen from Fig. 7 during the anodic sweep, bromide ions are oxidized to bromine at potentials above 0.59 V. On reversal of the potential scan, the bromine formed is reduced back to bromide ions, giving rise to a cathodic peak in the potential range 0.16–0.65 V. As the potential is scanned further in the negative direction, the V(III) and V(IV) ions in solution are reduced to V(II) below −0.3 V giving rise to an cathodic peak in the potential range −0.6 to −0.95 V. On reversal of the potential scan, anodic peaks corresponding to the re-oxidation of the V(II) ions are observed between −0.3 and 0 V vs Ag/AgCl. The low cathodic peak current is associated with the consumption of the V(II) ions by reaction with V(IV) ions in the electrolyte, effectively reducing the concentration of V(II) at the electrode surface for the reverse anodic scan.
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Fig. 4. FESEM images of the natural graphite (a and b) and as-prepared graphene oxide (c and d).
The peak currents, peak potentials and onset potentials of the redox reactions were used as the key criteria in evaluating the performance of the electrodes [13]. From Fig. 7, the onset potentials for the bromide ion oxidation process are 0.75, 0.75, 0.73 and 0.59 V vs Ag/AgCl for the graphite, GO, GO-PVDF and GO-SPEEK electrodes, respectively with corresponding anodic peak currents of 47, 48, 90 and 100 mA. By comparison, the standard reduction potential of the Br− /Br3 − redox couple is 0.87 V vs Ag/AgCl. The onset potential for bromide ion oxidation on the GO and GO-PVDF electrodes is about the same as that on the graphite electrode. On
the other hand, the GO-SPEEK electrode shows the lowest onset oxidation potential with the higher anodic peak current (100 mA). This has been attributed to its high ionic conductivity in dilute acid solutions that increases the mobility of ionic charge carriers leading to an accelerated bromide oxidation reaction rate [31]. In contrast, the oxidation–reduction potentials of the Br− /Br3 − couple at the graphite and GO electrodes are almost the same because the GO coating film does not adhere to the surface of graphite plate and can thus be easily peeled off from the surface during the CV testing (Fig. 8). From Fig. 7, the bromine reduction
Fig. 5. (a) TEM images of the as-prepared graphene oxide. Inset: high-magnification. (b) Corresponding HRTEM and SAED pattern (inset).
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Fig. 8. Digital Photos of pure GO electrode before and after CV testing.
Fig. 6. AFM image of GO sheets and corresponding AFM height profile.
peak appears at potentials of 0.57, 0.63, 0.55 and 0.29 V with corresponding peak currents of −40, −28, −60 and −80 mA for graphite, GO, GO-PVDF and GO-SPEEK, respectively. The higher reduction peak currents on the GO-PVDF and GO-SPEEK modified electrodes is due to the higher oxidation currents at the upper voltage limit that leads to the formation of a larger quantity of bromine during the anodic sweep. This implies that the energy storage efficiency can be improved with GO modified electrodes used as positive electrode for V/Br flow battery. Compared with the graphite, GO modified electrodes have higher electrochemical activity since the GO has more oxygen functional groups, e.g., C OH and COOH, which can catalyze the redox reaction by generating more active sites [32,33]. In addition,
Fig. 7. Cyclic voltammograms of graphite (a), GO (b), GO-PVDF (c) and GO-SPEEK (d) electrodes in the electrolyte of 2 M V3.5+ , 8 M HBr, 2 M HCl and 0.75 M QBr (MEM + MEP) at a scan rate of 50 mV s−1 .
owing to the hydrophilic properties of these functional groups, the wettability of the GO modified electrodes also improves and hence facilitates charge transfer. The GO modified electrodes can be easily wetted by the aqueous electrolyte, thereby increasing the interfacial contact area, leading to higher oxidation and reduction peak currents for Br− /Br3 − redox couple compared with the graphite electrode. Furthermore, as shown in Fig. 7, the oxidation and reduction reactions for the V3+ /V2+ redox couple are also obvious, indicating that the conversion reactions between the V(II) and V(III) ions can take place at each of the graphite and GO modified electrodes. This implies that these GO modified electrodes can also be used as negative electrodes in the V/Br flow battery. It is observed that the ratio of the anodic to cathodic peak currents for GO modified electrodes are higher than the graphite electrode. In addition, the peak potential separations are higher than graphite electrode. On the other hand, the GO-SPEEK exhibits the lowest oxidation potential (−0.14 V) for re-oxidation of V(II) to V(III), implying the lowest charge voltage and highest energy storage efficiency. Fig. 9 shows cyclic voltammograms (CVs) for all electrodes at different scan rates of 50, 20, 10 and 5 mV s−1 . The decomposition potential for the Br− oxidation reaction in the anodic region of the CVs in Fig. 9 is very similar for all of the electrodes except for the GO-SPEEK which displays a lower onset potential for Br− oxidation, along with a higher cathodic peak current for the Br2 reduction reaction on scan reversal. In contrast, the peak potential separation for the V3+ /V2+ redox couple changes with scan rate and transformation of V(II) to V(III) appears at 0 V, indicating that the transformation between V(II) and V(III) is quasi-reversible. On completion of the CV tests, both GO-PVDF and GO-SPEEK coatings were found to be still intact on the graphite surface and there were no significant changes in appearance confirming the durability of the GO coatings, although further work is need to evaluate their long term stability in the V/Br. From the above results, it can be seen that the introduction of GO with conductive binder can form an electrocatalytic hybrid with an effective mixed conducting layer. These modified electrodes facilitate the electron transfer processes for the Br− /Br3− and V3+ /V2+ redox couples especially for the oxidation of both bromide ions to bromine and V2+ to V3+ ions, leading to lower overvoltage losses and potentially higher energy efficiencies for the V/Br flow battery. This represents a significant step forward in the development of highly effective V/Br electrode materials.
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Fig. 9. Cyclic voltammograms of graphite (a), GO (b), GO-PVDF (c) and GO-SPEEK (d) electrodes at different scan rate of 50, 20, 10 and 5 mV s−1 .
4. Conclusions
References
In summary, few layered GO nanosheets have been successfully prepared by a modified Hummers method. Mixing GO with two different polymer binders (PVDF and SPEEK) can serve as excellent electrochemical active materials for V/Br flow battery, demonstrating a more favorable electrocatalytic activity towards the Br− /Br3− and V3+ /V2+ redox couples than the pristine graphite. The improved performance for GO modified electrodes is attributed to the formation of large numbers of oxygen-containing functional groups (e.g., C OH and COOH) on the GO surfaces, which can generate more active sites to catalyze the redox reactions. Among the binder-based electrodes, SPEEK binder exhibits the best properties, e.g., showing lower oxidation potential for both Br− /Br3− and V3+ /V2+ redox couples, doubled oxidation (100 mA) and reduction (−80 mA) peak currents for Br− /Br3 − redox couple due to its high ionic conductivity in dilute acid solutions and improved ionic charge carriers mobility leading to increase bromide oxidation reaction.
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Acknowledgements The authors gratefully acknowledge AcRF Tier 1 RG 31/08 of MOE (Singapore), NRF2009EWT-CERP001-026 (Singapore), Singapore Ministry of Education (MOE2010-T2-1-017), A*STAR SERC grant 1021700144 and Singapore MPA 23/04.15.03 RDP 009/10/102 and MPA 23/04.15.03 RDP 020/10/113 grant.
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Contents lists available at SciVerse ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Graphene oxide nanosheets/polymer binders as superior electrocatalytic materials for vanadium bromide redox flow batteries Xianhong Rui a,b,1 , Moe Ohnmar Oo a,1 , Dao Hao Sim a , Subash chandrabose Raghu b , Qingyu Yan a,c,∗ , Tuti Mariana Lim b,d,∗∗ , Maria Skyllas-Kazacos e,∗ ∗ ∗ a
School of Materials Science and Engineering, Nanyang Technological University, 639798, Singapore School of Civil and Environmental Engineering, Nanyang Technological University, 639798, Singapore c Energy Research Institute, Nanyang Technological University, 637459, Singapore d School of Life Sciences and Chemical Technology, Ngee Ann Polytechnic, 599489, Singapore e School of Chemical Engineering, The University of New South Wales, UNSW Sydney, NSW 2052, Australia b
a r t i c l e
i n f o
Article history: Received 6 July 2012 Received in revised form 24 August 2012 Accepted 28 August 2012 Available online 6 September 2012 Keywords: Vanadium bromide redox flow batteries Graphene oxide Polymer binder High electrocatalytic activity
a b s t r a c t Few layered graphene oxide (GO) nanosheets with large specific surface area (42.1 m2 g−1 ) are successfully prepared by a modified Hummers method for use as electrodes in the vanadium bromide redox battery. The structure and physicochemical properties of GO are investigated by X-ray diffraction, Raman spectroscopy, Fourier transform infrared spectroscopy, scanning electron microscopy, transmission electron microscopy, and atomic force microscopy. Cyclic voltammetry results indicate that GO nanosheets with polymer binder (i.e., polyvinylidiene fluoride (PVDF) or sulfonated poly(ether ether ketone) (SPEEK)) hybrids demonstrate more favorable electrocatalytic activity towards the Br− /Br3− and V3+ /V2+ redox couples than the pure graphite. This is attributed to the large numbers of oxygen-containing functional groups on the GO nanosheet surface which can generate more active sites to catalyze the redox reactions. For the binder-based electrodes, the SPEEK based electrode gives the best electrochemical performance (e.g., lower overvoltage for both Br− /Br3− and V3+ /V2+ redox couple reactions and higher peak currents for the V3+ /V2+ redox couple). © 2012 Elsevier Ltd. All rights reserved.
1. Introduction The original all-vanadium redox flow battery (VRB), employing VO2+ /VO2 + and V2+ /V3+ redox couples in sulphuric acid as the positive and negative half-cell electrolytes, respectively, has shown great potential for large-scale energy storage applications with long cycle life and high energy efficiency [1–4]. However, owing to the relatively low solubility of vanadium ions in acidic sulfate solutions (1.6–2 M) within the temperature ranges of 0–45 ◦ C [5], the VRB suffers from low energy density, which limits its use in certain stationary applications as well as in electric vehicles. Fortunately, the vanadium bromide redox battery (G2V/Br, as schematically illustrated in Fig. 1) can overcome this limitation while still using
∗ Corresponding author at: School of Materials Science and Engineering, Nanyang Technological University, 639798, Singapore. Tel.: +65 6790 4583. ∗∗ Corresponding author at: School of Civil and Environmental Engineering, Nanyang Technological University, 639798, Singapore. Tel.: +65 6790 4583. ∗ ∗ ∗Corresponding author at: School of Chemical Engineering, The University of New South Wales, UNSW Sydney, NSW 2052, Australia. Tel.: +61 2 9385 4335. E-mail addresses: [email protected] (Q. Yan), [email protected] (T.M. Lim), [email protected] (M. Skyllas-Kazacos). 1 These authors contributed equally to this work. 0013-4686/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2012.08.119
the same elements in both half-cell electrolytes [6,7]. It employs V3+ /V2+ couple in the negative half-cell and Br− /Br3 − couple in the positive half-cell, using a mixture of vanadium bromide, hydrobromic acid and hydrochloric acid as the electrolyte in both half-cells. The charge–discharge reactions are shown as below: At the negative electrode : V3+ + e− ↔ V2+ −
−
At the positive electrode : 3Br − 2e ↔
Br− 3
(1) (2)
The higher solubility of vanadium bromide (3–4 M) allows the energy density to be almost doubled (∼50 Wh kg−1 ) and the V/Br cell can operate at lower-temperatures as compared with the VRB system. The electrode is a key component of redox flow batteries. Commonly, the typical electrodes for VRB are carbon or graphite-based materials (e.g., graphite felt [8] and graphite powder composites [9]). This is due to the relative stability of carbon and graphite-based materials in the highly oxidizing V(V) electrolytes that oxidizes most metallic electrodes. The poor electrochemical reversibility of the vanadium reactions on carbon electrodes however, necessitates the use of high surface carbon or graphite felts in order to reduce current density and thereby decrease the overvoltage losses. The use of graphite felts introduces high pumping energy losses
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mixture changed to brilliant yellow along with bubbling. The resulting mixture was washed with diluted HCl aqueous (1/10, v/v) solution and DI water. The graphite oxide was finally obtained after drying in a vacuum oven at 30 ◦ C. To obtain the graphene oxide nanosheet powder, exfoliation was carried out by sonicating the as-prepared graphite oxide in an ethanol solution and then drying in air at 80 ◦ C. 2.2. Materials characterization
Fig. 1. Schematic diagram of the V/Br system.
in the flow battery stacks that is a parasitic loss for the system. Considerable effort has thus been devoted to improving the electrochemical properties of graphite-based electrodes through activation and surface functionalization [10–12] and to develop new carbon materials [13–15]. Among these, electrodes fabricated by coating a thin graphene oxide (GO) layer on the surface of graphite plate has shown enhanced electrocatalytic activity towards VO2+ /VO2 + and V2+ /V3+ redox couples in the vanadium sulphate electrolytes of the VRB [14]. GO nanosheets, possessing a quasi-two-dimensional layered structure, possess a relatively large surface area and a large amount of hydroxyl and carboxyl acid active groups on basal planes and sheet edges [14–18], resulting in more reactive ions adsorbed onto the electrode surface. However, the GO film is easily stripped from the graphite plate surface after several cycles. It is thus necessary to develop a stable electrode with high electrocatalytic activity. In the present study, a similar approach to that applied to the electrodes in lithium ion batteries [19–21], was used to improve the stability of GO coatings on graphite plates for use in the vanadium halide electrolyte of the G2V/Br cell. Two different polymer binders, i.e., polyvinylidiene fluoride (PVDF) and sulfonated poly(ether ether ketone) (SPEEK) were evaluated as binding agents for the GO coatings. Cyclic voltammetry was used to investigate the electrochemical performance of the GO-based coatings in the V/Br electrolyte and the results are presented here. 2. Experimental
X-ray powder diffraction (XRD) patterns were recorded on a Shimadzu XRD-6000 X-ray diffractometer at the 2 range of 5–60◦ using Cu K␣ radiation. The morphology was investigated by using a field-emission scanning electron microscopy (FESEM) system (JEOL, Model JSM-7600F), and the nanostructure was characterized by using a transmission electron microscopy (TEM) system (JEOL, Model JEM-2100F) operating at 200 kV. Raman spectra were obtained by using a WITec CRM200 confocal Raman microscopy system with a laser wavelength of 488 nm and a spot size of 0.5 mm. FT-IR spectrum was recorded on a Fourier transform infrared spectrometer (PerkinElmer) with a DGTS detector. The specific surface area of the samples was determined with an automated adsorption apparatus (ASAP 2020) using liquid nitrogen adsorption at 77 K and calculated on the basis of Brunauer–Emmett–Teller (BET) equations. Atomic force microscopy (AFM) (Digital Instruments) was used to determine the thickness of the graphene oxide nanosheet. 2.3. Electrochemical measurements Cyclic voltammograms were recorded in the potential range between −1.0 V and 1.0 V at scan rates of 50, 20, 10 and 5 mV s−1 using a three-electrode electrochemical cell and a Solartron Electrochemical Instrument potentiostat (Sol-1470E, Solartron (UK)). The graphite plate electrode and graphite plate modified electrodes with different coatings such as graphene oxide (GO) and GO with two kind polymer binders (i.e., polyvinylidiene fluoride (PVDF) and sulfonated poly(ether ether ketone) (SPEEK) at a weight ratio of 90:10) were used as working electrodes. All potentials were measured using platinum and Ag/AgCl as the counter and reference electrodes, respectively. The electrolyte was a 2 M vanadium solution (V (IV):V (III) = 50:50, referred as V3.5+ ) in 8 M HBr plus 2 M HCl as supporting electrolyte. A mixture of 0.375 M N-ethylN-methyl-pyrrolidinium bromide (MEP) plus 0.375 M N-ethylN-methyl-morpholinium bromide (MEM) added as complexing agent to bind any bromine formed in the positive half-cell during charging.
2.1. Preparation of graphene oxide nanosheets 3. Results and discussion Graphite oxide was prepared from natural graphite (SP-1) by a modified Hummers method [22–24]. Briefly, 1.5 g graphite powder was added into a solution of 10 mL H2 SO4 (98 wt%), 1.25 g K2 S2 O8 and 1.25 g P2 O5 and the mixture was kept at 80 ◦ C for 4.5 h under vigorous stirring. Cooling to room temperature, the pre-oxidized graphite was obtained by washing with de-ionized (DI) water and drying in a vacuum oven at 50 ◦ C. Successively, pretreated graphite was put into 60 mL concentrated H2 SO4 with an ice bath, and then 7.5 g KMnO4 was added gradually under stirring and the reaction temperature was kept below 20 ◦ C. After adding all of KMnO4 , the mixture was stirred at 35 ◦ C for 2 h and then slowly diluted with 125 mL DI water. During the diluting process, a large amount of heat was released, so the addition of water was carried out in an ice bath to maintain the temperature below 50 ◦ C. The mixture was stirred for 2 h and further diluted with additional 200 mL DI water. After that, 30 wt% H2 O2 (10 mL) was slowly added into the mixture to completely react with the excess KMnO4 , and the color of
The XRD analyses of as-prepared GO and natural graphite are shown in Fig. 2a. Natural graphite shows a very sharp diffraction peak at 2 = 26.7◦ corresponding to the (0 0 2) plane, indicating a highly organized crystal structure with a layer-to-layer distance (d-spacing) of 0.336 nm. As for GO, the (0 0 2) peak is observed at 2 = 10.9◦ , corresponding to the d-spacing of 0.811 nm. Such value is much larger than that of natural graphite due to the generation of oxygen-containing functional groups between layers [25,26]. Raman spectroscopy is a powerful non-destructive tool to characterize ordered and disordered crystal structures of carbon. G band is usually assigned to the E2g phonon of C sp2 atoms, while D band is a breathing mode of -point phonons of A1g symmetry [27]. Fig. 2b shows the Raman spectra of GO and natural graphite. Raman spectrum of the natural graphite displays a strong G band at 1566 cm−1 and a weak D band at 1346 cm−1 . After oxidation, in the Raman spectrum of GO, the G band is broadened and shifted upward to
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Fig. 3. FT-IR transmittance spectrum of the as-prepared graphene oxide.
Fig. 2. XRD patterns (a) and Raman spectra (b) of as-prepared graphene oxide and natural graphite.
1580 cm−1 , which was mainly caused by stress [28]. At the same time, the intensity of D band at 1351 cm−1 increases substantially, which indicates that the size of in-plane sp2 domains is decreased, possibly arising from the extensive oxidation and ultrasonic exfoliation. Fig. 3 shows the FT-IR transmittance spectrum (KBr) of the as-prepared GO. The spectrum illustrates a strong and broad absorption in the range of 3700–2000 cm−1 corresponding to the stretching vibrations of structural OH groups and physisorbed water molecules [29]. The peak at 1735 cm−1 is the C O stretching vibration of COOH groups. The absorptions arising from the O H bending vibration, epoxide groups and skeletal ring vibrations are observed at 1622 cm−1 [30]. The absorption at 1375 cm−1 is possibly originated from the bending of tertiary C OH groups [29]. The appearance of absorption at 1228 cm−1 is related to phenolic groups [29]. The 1062 cm−1 absorption is ascribed to C O C stretching vibration. Field emission scanning electron microscopy (FESEM) images of the natural graphite and GO are shown in Fig. 4. It can be observed (Fig. 4a and b) that the natural graphite consists of flakes with flat and smooth surface. The lateral size is 60–200 m, and the thickness is 5–10 m. With a careful observation, the lamellar structure at the edges of flakes can be seen. After oxidation, the GO (Fig. 4c and d) shows an agglomeration of the
exfoliated flakes with some wrinkles and folding on the surfaces and edges. In Fig. 5, the obtained GO was further analyzed by transmission electron microscopy (TEM), high resolution (HR) TEM, and selected area electron diffraction (SAED) observations. The lowand high-magnification TEM images (Fig. 5a) reveal that the GO consists of a thin wrinkled paper-like structure, suggesting that the GO nanosheet has been fully exfoliated. The intrinsic ripples of GO nanosheets may develop into wavy structures in the macroscopic scale (Fig. 4c and d). In addition, GO nanosheets are transparent and exhibit a very stable nature under the high-energy electron beam. From the HRTEM image (Fig. 5b), the scrolled edges giving a cross-section view of the stacked GO nanosheets, only 2–3 layers of GO sheets can be observed. A SAED pattern of the featureless region was recorded (inset in Fig. 5b). The well-defined diffraction spots can be indexed to the hexagonal graphite crystal structure, confirming the graphitic crystalline nature of the GO nanosheets. The obtained GO nanosheets were also analyzed by atomic force microscopy (AFM). Fig. 6 shows an AFM image of a 5 m × 5 m Si surface deposited with GO suspension. The flat sheet exhibits an average thickness of about 1.1 nm, corresponding to 2–3 layers of GO nanosheet. The specific surface area of GO nanosheets and pristine graphite was measured to be 42.1 and 1.2 m2 g−1 , respectively, using the Brunauer–Emmett–Teller (BET) method. The relatively low surface area for few-layered GO nanosheets is due to the intersheet van der Waals attractions, aggregation or restacking inevitably occurring in graphene oxide assemblies. Fig. 7 presents the cyclic voltammetric (CV) behaviors of the graphite, GO, GO-PVDF and GO-SPEEK electrodes in a V/Br electrolyte comprising 2 M V3.5+ , 8 M HBr, 2 M HCl and 0.75 M QBr (MEM + MEP) at a scan rate of 50 mV s−1 . In each of these CVs, the forward scan commences at the initial potential of −1 V. It can be seen from Fig. 7 during the anodic sweep, bromide ions are oxidized to bromine at potentials above 0.59 V. On reversal of the potential scan, the bromine formed is reduced back to bromide ions, giving rise to a cathodic peak in the potential range 0.16–0.65 V. As the potential is scanned further in the negative direction, the V(III) and V(IV) ions in solution are reduced to V(II) below −0.3 V giving rise to an cathodic peak in the potential range −0.6 to −0.95 V. On reversal of the potential scan, anodic peaks corresponding to the re-oxidation of the V(II) ions are observed between −0.3 and 0 V vs Ag/AgCl. The low cathodic peak current is associated with the consumption of the V(II) ions by reaction with V(IV) ions in the electrolyte, effectively reducing the concentration of V(II) at the electrode surface for the reverse anodic scan.
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Fig. 4. FESEM images of the natural graphite (a and b) and as-prepared graphene oxide (c and d).
The peak currents, peak potentials and onset potentials of the redox reactions were used as the key criteria in evaluating the performance of the electrodes [13]. From Fig. 7, the onset potentials for the bromide ion oxidation process are 0.75, 0.75, 0.73 and 0.59 V vs Ag/AgCl for the graphite, GO, GO-PVDF and GO-SPEEK electrodes, respectively with corresponding anodic peak currents of 47, 48, 90 and 100 mA. By comparison, the standard reduction potential of the Br− /Br3 − redox couple is 0.87 V vs Ag/AgCl. The onset potential for bromide ion oxidation on the GO and GO-PVDF electrodes is about the same as that on the graphite electrode. On
the other hand, the GO-SPEEK electrode shows the lowest onset oxidation potential with the higher anodic peak current (100 mA). This has been attributed to its high ionic conductivity in dilute acid solutions that increases the mobility of ionic charge carriers leading to an accelerated bromide oxidation reaction rate [31]. In contrast, the oxidation–reduction potentials of the Br− /Br3 − couple at the graphite and GO electrodes are almost the same because the GO coating film does not adhere to the surface of graphite plate and can thus be easily peeled off from the surface during the CV testing (Fig. 8). From Fig. 7, the bromine reduction
Fig. 5. (a) TEM images of the as-prepared graphene oxide. Inset: high-magnification. (b) Corresponding HRTEM and SAED pattern (inset).
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Fig. 8. Digital Photos of pure GO electrode before and after CV testing.
Fig. 6. AFM image of GO sheets and corresponding AFM height profile.
peak appears at potentials of 0.57, 0.63, 0.55 and 0.29 V with corresponding peak currents of −40, −28, −60 and −80 mA for graphite, GO, GO-PVDF and GO-SPEEK, respectively. The higher reduction peak currents on the GO-PVDF and GO-SPEEK modified electrodes is due to the higher oxidation currents at the upper voltage limit that leads to the formation of a larger quantity of bromine during the anodic sweep. This implies that the energy storage efficiency can be improved with GO modified electrodes used as positive electrode for V/Br flow battery. Compared with the graphite, GO modified electrodes have higher electrochemical activity since the GO has more oxygen functional groups, e.g., C OH and COOH, which can catalyze the redox reaction by generating more active sites [32,33]. In addition,
Fig. 7. Cyclic voltammograms of graphite (a), GO (b), GO-PVDF (c) and GO-SPEEK (d) electrodes in the electrolyte of 2 M V3.5+ , 8 M HBr, 2 M HCl and 0.75 M QBr (MEM + MEP) at a scan rate of 50 mV s−1 .
owing to the hydrophilic properties of these functional groups, the wettability of the GO modified electrodes also improves and hence facilitates charge transfer. The GO modified electrodes can be easily wetted by the aqueous electrolyte, thereby increasing the interfacial contact area, leading to higher oxidation and reduction peak currents for Br− /Br3 − redox couple compared with the graphite electrode. Furthermore, as shown in Fig. 7, the oxidation and reduction reactions for the V3+ /V2+ redox couple are also obvious, indicating that the conversion reactions between the V(II) and V(III) ions can take place at each of the graphite and GO modified electrodes. This implies that these GO modified electrodes can also be used as negative electrodes in the V/Br flow battery. It is observed that the ratio of the anodic to cathodic peak currents for GO modified electrodes are higher than the graphite electrode. In addition, the peak potential separations are higher than graphite electrode. On the other hand, the GO-SPEEK exhibits the lowest oxidation potential (−0.14 V) for re-oxidation of V(II) to V(III), implying the lowest charge voltage and highest energy storage efficiency. Fig. 9 shows cyclic voltammograms (CVs) for all electrodes at different scan rates of 50, 20, 10 and 5 mV s−1 . The decomposition potential for the Br− oxidation reaction in the anodic region of the CVs in Fig. 9 is very similar for all of the electrodes except for the GO-SPEEK which displays a lower onset potential for Br− oxidation, along with a higher cathodic peak current for the Br2 reduction reaction on scan reversal. In contrast, the peak potential separation for the V3+ /V2+ redox couple changes with scan rate and transformation of V(II) to V(III) appears at 0 V, indicating that the transformation between V(II) and V(III) is quasi-reversible. On completion of the CV tests, both GO-PVDF and GO-SPEEK coatings were found to be still intact on the graphite surface and there were no significant changes in appearance confirming the durability of the GO coatings, although further work is need to evaluate their long term stability in the V/Br. From the above results, it can be seen that the introduction of GO with conductive binder can form an electrocatalytic hybrid with an effective mixed conducting layer. These modified electrodes facilitate the electron transfer processes for the Br− /Br3− and V3+ /V2+ redox couples especially for the oxidation of both bromide ions to bromine and V2+ to V3+ ions, leading to lower overvoltage losses and potentially higher energy efficiencies for the V/Br flow battery. This represents a significant step forward in the development of highly effective V/Br electrode materials.
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Fig. 9. Cyclic voltammograms of graphite (a), GO (b), GO-PVDF (c) and GO-SPEEK (d) electrodes at different scan rate of 50, 20, 10 and 5 mV s−1 .
4. Conclusions
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In summary, few layered GO nanosheets have been successfully prepared by a modified Hummers method. Mixing GO with two different polymer binders (PVDF and SPEEK) can serve as excellent electrochemical active materials for V/Br flow battery, demonstrating a more favorable electrocatalytic activity towards the Br− /Br3− and V3+ /V2+ redox couples than the pristine graphite. The improved performance for GO modified electrodes is attributed to the formation of large numbers of oxygen-containing functional groups (e.g., C OH and COOH) on the GO surfaces, which can generate more active sites to catalyze the redox reactions. Among the binder-based electrodes, SPEEK binder exhibits the best properties, e.g., showing lower oxidation potential for both Br− /Br3− and V3+ /V2+ redox couples, doubled oxidation (100 mA) and reduction (−80 mA) peak currents for Br− /Br3 − redox couple due to its high ionic conductivity in dilute acid solutions and improved ionic charge carriers mobility leading to increase bromide oxidation reaction.
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Acknowledgements The authors gratefully acknowledge AcRF Tier 1 RG 31/08 of MOE (Singapore), NRF2009EWT-CERP001-026 (Singapore), Singapore Ministry of Education (MOE2010-T2-1-017), A*STAR SERC grant 1021700144 and Singapore MPA 23/04.15.03 RDP 009/10/102 and MPA 23/04.15.03 RDP 020/10/113 grant.
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