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Germanomolybdate (GeMo12O404−) Modified Carbon Nanotube Composites for Electrochemical Capacitors
Electrochimica Acta 117 (2014) 153–158
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Germanomolybdate (GeMo12 O40 4− ) Modified Carbon Nanotube Composites for Electrochemical Capacitors Matthew Genovese, Yee Wei Foong, Keryn Lian ∗ Department of Materials Science and Engineering, University of Toronto Toronto, Ontario, Canada M5S 3E4
a r t i c l e
i n f o
Article history: Received 2 October 2013 Received in revised form 18 November 2013 Accepted 20 November 2013 Available online 4 December 2013 Keywords: Polyoxometalates Germanomolybdate Layer-by-layer Pseudocapacitance Supercapacitor
a b s t r a c t Keggin type germanomolybdate, GeMo12 O40 4− (GeMo), was deposited onto multi-walled carbon nanotubes (MWCNT) via layer-by-layer (LbL) deposition to form composite electrodes for electrochemical capacitors (ECs). The GeMo composite electrode demonstrated charge storage six times greater than that of the bare MWCNT electrode while maintaining excellent conductivity and cycling stability. GeMo also demonstrated charge storage complementary to that of the commercial Keggin type POMs, PMo12 O40 3− (PMo) and SiMo12 O40 4− (SiMo). Dual-layer coatings superimposing GeMo with either PMo or SiMo showed an additive combination of both active layers, which resulted in cyclic voltammograms (CVs) with overlapping redox features and charge storage twelve times greater than that of the bare MWCNT electrode. Scanning Electron Microscopy (SEM) demonstrated successful single and dual layer coating of POMs on MWCNT with high coverage and uniform surface morphologies. © 2013 Elsevier Ltd. All rights reserved.
1. Introduction Carbon nano materials such as graphene, onion-like carbon (OLC), and carbon nanotubes (CNT) have emerged as promising electrode materials for electrochemical double layer capacitors (EDLC), due to their large surface area and metal-like conductivity [1–3]. However, storing charge through the electrochemical double layer alone limits the specific capacitance and energy density of nano-carbon electrodes. One of the most common approaches to improve the energy density of EDLC electrodes is the addition of pseudocapacitive materials. Pseudocapacitive materials allow for reversible Faradaic oxidation/reduction reactions on the electrode surface, and can yield specific capacitance 10 to 100 times greater than EDLC [4]. RuO2 is one of the most effective pseudocapacitive materials due to its fast reversible redox reactions with overlapping peak potentials: Its high cost, however, proves to be a challenge for commercialization. Polyoxometalates (POMs), a class of large metal oxide clusters, have been investigated as low cost RuO2 alternatives [5–8]. Keggin-type heteropolyanions are the most well known and widely used class of POMs. The Keggin structure has the general formula [XM12 O40 ]n− , in which the central heteroatom (i.e., P, Si, or B) is surrounded by twelve addenda atoms (i.e., Mo or W) and forty
∗ Corresponding author. Department of Materials Science and Engineering, University of Toronto, 184 College St. Toronto, ON M5S 3E4 Canada. Tel.: +416 978 8631. E-mail address: [email protected] (K. Lian). 0013-4686/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2013.11.114
oxygen atoms. These POMs exhibit multiple, highly stable redox states, which gives them the ability to act as electron reservoirs, an ideal characteristic for electrochemical storage applications [9]. Furthermore, different combinations of hetero and addenda atoms can lead to different electrochemical properties [10], allowing for the ability to tune the electrochemical behaviour of POMs. By combining these POMs with nano-carbon materials, the electrochemical activity of the former can be leveraged with the surface area of the latter to achieve high performance and low cost inorganic-organic composite electrodes. A number of methods can be used to create POM-carbon composites, including simple mixing [7], chemisorption following carbon oxidation [11], electrodeposition [10], and layer-by-layer (LbL) self assembly [12–14]. LbL self assembly, achieved through the alternate adsorption of positive and negative layers, is a simple and effective technique to deposit monolayers of oppositely charged species. Martel et al. have reported the LbL deposition of 4 different POM chemistries on smooth glassy carbon, and showed a linear increase in charge storage with the number of layers deposited [12]. Kulesza and his group developed composites of POMs with various conductive substrates [5,8,15]. For Keggin-type POMs, charge storage is often concentrated in certain voltage regions that do not overlap [8,10,12]. In order to combat this limitation, the LbL technique can be used to superimpose layers of different POM chemistries to achieve an electrode which exhibits a collective contribution of each layer. Thus, different POM chemistries with different redox peak potentials can be superimposed to mimic the overlapping redox peaks and ideally capacitive profile of RuO2 . Our previous
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work has demonstrated the feasibility of superimposing a simple commercial Keggin type anion PMo12 O40 3− (PMo) with the mixed addenda anion PMo10 V2 O40 5− (PMoV2 ). This approach resulted in a significant increase in capacitance and a combined voltametric profile of both the PMo and PMoV2 molecules [14]. However, the difference in electrochemistry and peak potentials between PMo and PMoV2 was quite small, thus limiting the synergistic effect of combining these molecules. The present work explores an alternate Keggin-type anion, GeMo12 O40 4− (GeMo) and aims to 1) investigate the electrochemistry of GeMo coated MWCNT electrodes, 2) compare the electrochemical behaviour of GeMo to that of previously studied commercial POMs with alternate heteroatoms, and 3) explore how potential differences in electrochemical behaviour can be leveraged when designing multi-layer EC electrodes with synergistic combinations of GeMo and other POMs.
2. Experimental 2.1. POM Synthesis Three different POMs were used in this work: H3 PMo12 O40 (PMo), H4 SiMo12 O40 (SiMo), and H4 GeMo12 O40 (GeMo). PMo and SiMo were both purchased from Alfa Aesar, while GeMo was synthesized according to a procedure modified from that described by Wu et al. [16]. Briefly, GeO2 (0.8 g) was dissolved in 20 ml of a 5 wt% NaOH solution. An aqueous solution (50 ml) of Na2 MoO4 •2H2 O (22.2 g) was then added to the first mixture, and the pH was adjusted to 1.0 with concentrated H2 SO4 . The mixture was stirred continuously for 2 hours at 80 ◦ C. The cooled solution was extracted with ether and the isolated solid was washed with deionized H2 O and dried for 5 hours at 50 ◦ C yielding a yellow crystal product. All reagents for the GeMo synthesis were purchased from Alfa Aesar.
2.2. Carbon Electrode Fabrication Two different types of carbon electrodes were utilized; a MWCNT film electrode and a mixed graphite/MWCNT ink electrode. For the MWCNT film electrode, the “as received” MWCNTs which had been previously characterized [17] were mixed with 4 wt% PTFE binder and rolled into thin films of approximately 100 m thickness. For electrochemical analysis these films were ground and loaded into a cavity microelectrode (CME) [18] which was used as the working electrode in a 3-electrode cell. The CME had a volume of approximately 4.9 × 10−6 cm3 and at least ten different loadings and tests were performed for each sample to ensure the stability of the process. The MWCNT film electrodes were used for all electrochemical tests except for the cycle life analysis. Carbon ink electrodes were used in cycle life analysis and were fabricated according to a procedure modified from that described by Park et al. [19]. The carbon ink was synthesized by adding both graphite and MWCNT to an aqueous solution of poly-vinyl alcohol (PVA), in a proportion 15 wt% PVA and 85 wt% carbon, with a carbon mass ratio of 3:1 graphite to MWCNT. A small amount of glutaric acid (10 wt% of the PVA content) was added for cross-linking. The ink was cast on titanium foils in approximately 60 m thick layers. The electrodes were annealed at 50 ◦ C for 1 hour followed by 140 ◦ C for 1 hour for cross-linking of the PVA. After annealing, the carbon electrodes were cut into pieces with an active area of 1 cm2 for use as the working electrode in a 3-electrode cell. Graphite, PVA, and glutaric acid used to synthesize the carbon ink were purchased from Alfa Aesar. The titanium foils were purchased from McMaster Carr.
Fig. 1. XRD patterns of commercial SiMo and synthesized GeMo.
2.3. LbL Process For LbL coating, all three POMs were dissolved in deionized water to form 10 mM solutions. 4 wt% poly (diallyldimethylammonium chloride) (PDDA) and HNO3 (conc.) were used as part of the LbL process and were both purchased from Alfa Aesar. The LbL coating of the MWCNT films proceeded according to the following steps: i. Soaking in concentrated HNO3 (2 minutes) followed by H2 O washing (2 minutes) ii. Soaking in 4 wt% PDDA solution (10 minutes) followed by H2 O washing (2 minutes) iii. Soaking in 10 mM POM solution (20 minutes) followed by H2 O washing (2 minutes) The process for coating of the carbon ink electrodes was similar. However, conc. HNO3 treatment was replaced with overnight soaking in a more dilute (1 M) HNO3 solution. 2.4. Characterizations Electrochemical analysis was performed by a 3-electrode system with the working electrode as described above. Saturated Ag/AgCl and a platinum mesh were used as the reference and counter electrodes, respectively. The electrolyte for all tests was 1 M sulfuric acid (H2 SO4 ) and electrochemical measurements were conducted using an EG&G 273 potentiostat. Structural and surface characterizations were performed using X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), and Energy-Dispersive X-ray Spectroscopy (EDX). XRD was conducted using a Philips XRD system with a monochromatized Cu-K␣ anode. SEM micrographs were obtained using FEI Quanta FEG 250 large stage environmental SEM with auxiliary EDX detection. XRD and EDX analysis were performed on the POM powder materials, while SEM was performed on the POM coated MWCNT films. 3. Results and Discussion 3.1. Structure and Composition of Polyoxometalates The crystal structure and phases present in GeMo were studied using powder X-ray diffraction (XRD). The XRD pattern for the synthesized GeMo powder is shown in Fig. 1 along with the pattern for commercial SiMo. GeMo and SiMo should have a similar
M. Genovese et al. / Electrochimica Acta 117 (2014) 153–158 Table 1 Theoretical and experimental atomic % ratios of addenda atoms (Mo) and heteroatoms (Si and P) present in both commercial SiMo and synthesized GeMo.
Theoretical Experimental
SiMo Mo:Si
GeMo Mo:Ge
12: 1 12.3: 1
12: 1 12.5: 1
crystal structure as they are both Keggin POMs with group IV heteroatoms. The XRD patterns for both the commercial SiMo and the synthesized GeMo are quite similar, indicating the synthesized material has the Keggin crystal structure. There are some differences between the two patterns especially with regards to the intensity of peaks in the 2=6-8◦ region, but these can be attributed to changes in the crystal structure resulting from different levels of hydration in the SiMo and GeMo powders [20]. Moreover, the pattern for the GeMo powder is very similar to the XRD data obtained by Wu et al. for vanadium substituted germanomolybdate (GeMo11 VO40 ) [16]. The XRD results seemed to confirm the presence of the Keggin structure, so EDX analysis was conducted to study the stoichiometry and composition of the synthesized material (Table 1). The EDX spectra of GeMo revealed that the atomic percent ratio of addenda atoms (Mo) to heteroatoms (Ge) was 12.5:1, only slightly higher than the theoretical 12:1 Keggin ratio and very similar to the 12.3:1 Mo:Si ratio determined for commercial SiMo. Thus, based upon the structural and compositional analysis, the presence of GeMo in the synthesized powder was confirmed. 3.2. Electrochemical Analysis: Single Layer GeMo Coated MWCNT Cyclic voltammograms of bare and single layer GeMo coated MWCNTs at a scan rate of 20 mV/s are shown in Fig. 2A. It is clear from the CVs that coating of GeMo on the carbon substrate was quite effective, as the GeMo coated electrode showed a significant increase in current density from that of the bare film. The CV of the GeMo coated MWCNTs demonstrated three pairs of oxidation/reduction peaks (labeled I, II, and III in Fig. 2A) which represent the characteristic charge transfer reactions of the Keggin molecule. For reactions I, II, and III, the equilibrium oxidation potentials were found to be 0.37, 0.28, and 0.07 V vs. Ag/AgCl, respectively, matching the results obtained by Wang et al. for a GeMo modified glassy carbon electrode [21]. The redox peaks of the GeMo coated MWCNTs were quite sharp and demonstrated a difference in the corresponding oxidation/reduction peak potentials (Ep ) of 30, 26, and 28 mV for reaction I, II, and III, respectively. Ep was close to 30 mV, the theoretical value for a reversible 2 electron transfer process [22], for all reactions. Thus, even after immobilization on a CNT substrate, GeMo demonstrated the fast reversible electron transfer reactions ideal for pseudocapacitive behaviour. The fast charge transfer reactions and pseudocapacitive behaviour of the GeMo modified electrode are further demonstrated in Fig. 2B which shows the anodic peak current density for reaction III plotted vs. the scan rate. The GeMo electrode demonstrated ideal pseudocapacitive behaviour: a linear relationship between peak current and scan rate, up to a high scan rate of 0.5 V/s. From 0.75 to 2 V/s, the peak currents began to deviate slightly from ideal behaviour and follow a non-linear trend, a consequence of high current densities which cause an increased contribution of ohmic resistance and onset of diffusion control. The volumetric capacitance of the GeMo coated electrode at the slowest tested scan rate of 20 mV/s was 109.8 F/cm3 , which represents a six-fold increase compared to the capacitance of the bare MWCNT electrode (17 F/cm3 ). The plot of capacitance vs. scan rate for the GeMo modified electrode (inset Fig. 2B) shows only a small decline between 20
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Table 2 Area Specific Capacitance of bare carbon ink and GeMo coated carbon ink electrodes. Electrode
Area Specific Capacitance (mF/cm2 ) Scan Rate
Bare Carbon Ink GeMo Coated Carbon Ink
0.05 V/s
1 V/s
11.15 48.89
11.08 41.96
and 500 mV/s. Even at a high rate of 2 V/s the capacitance decline of the GeMo coated MWCNT was only about 22%, which again demonstrates the high conductivity and fast charge transfer reactions of this electrode. The pH dependence of the redox reactions for the GeMo coated MWCNT was studied by taking CV measurements in various concentrations of H2 SO4 electrolytes. Plots of the anodic peak potential vs. pH for each GeMo redox reaction are shown in Fig. 2 C. The plots show good linearity and a gradual decrease in peak potential from a pH of 0 to 1. The slope of the plots for peaks I, II, and III are -59, -58, and -61 mV/pH respectively. These slopes are very close to the theoretical value of -59 mV/pH, which would indicate the 2 proton per 2 electron coupled redox process common for Keggin molecules [8,23]. This suggests that for the GeMo coated electrode the highest faradic currents will be achieved in low pH electrolytes. 3.3. GeMo Cycling Stability Since the GeMo modified electrode is intended for EC applications, its stability upon repeated cycling was evaluated. The CME, while valuable for electrochemical characterization, is not well suited for cycle-life tests, as material loss from the electrode cavity may bias the results. For this reason, cycle life analysis was performed with a GeMo modified carbon ink electrode coated onto a titanium current collector. This larger-scale electrode has the added benefit of illustrating that the results demonstrated with the CME can be effectively “scaled-up” for device fabrication. The area specific capacitance of the bare and GeMo coated carbon ink electrodes at a scan rate of 0.05 and 1 V/s are shown in Table 2. Much like the results demonstrated with the CME, the GeMo coated carbon ink electrode showed a significant increase in capacitance from that of the bare electrode and maintained this capacitance at high rates. The cycling stability of the bare and GeMo modified electrode was evaluated by applying 5000 successive potential cycles between -0.15 and 1.0 V at a rate of 1 V/s; the results are shown in Fig. 3. The bare carbon electrode showed only minor capacitance degradation (1.3%), and the decline for the GeMo electrode was also minimal; after 5000 cycles the GeMo electrode demonstrated a capacitance loss of only 6.3%. Furthermore, the majority of this capacitance loss (4.5%) occurred over the first 2500 cycles, while the final 2500 cycles resulted in a decline of only 1.8%, indicating stabilization of the electrode capacitance and good durability. The cycle life tests were conducted in a 3-electrode cell with a liquid electrolyte, and thus the cycling stability of the GeMo electrode could be further improved in a solid state electrochemical device using a proton conducting polymer electrolyte. 3.4. Comparison of Electrochemical Behaviour: GeMo vs. Commercial POMs Capacitive performance and electrochemical characteristics of the synthesized GeMo material were compared to those of commercial POMs (PMo and SiMo). CVs of the bare, single layer PMo coated, and single layer GeMo coated MWCNT are shown in Fig. 4A. The peak current densities of the PMo and GeMo electrodes are quite comparable, although the CV of GeMo has a slightly broader
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a
b
c
Fig. 2. A) Cyclic voltammograms of bare and single layer GeMo-coated MWCNT at 20 mV/s; B) Anodic peak current (reaction III) vs. scan rate for single layer GeMo coated MWCNT and the capacitance vs. scan rate for single layer GeMo coated MWCNT(inset); C) Relationship between anodic peak potential and pH for the GeMo redox processes.
Fig. 3. Variation in area specific capacitance as a function of cycle number for bare and GeMo coated carbon ink electrodes.
profile. Charge storage for both electrodes was very similar; at 50 mV/s the PMo and GeMo coated MWCNT showed a capacitance of 106.9 F/cm3 and 109.5 F/cm3 , respectively. While the capacitance of the two electrodes was similar, it is obvious that the change in heteroatom from P to Ge significantly altered the electrochemistry of the material. None of three redox peaks of GeMo directly overlay those of PMo. For instance, the peak potential for reaction I was shifted negatively by 35 mV for GeMo as compared to PMo, while the potentials for reactions II and III were shifted positively by 30 and 60 mV respectively. Consequently, the charge storage for these
two materials could complement each other, that is, GeMo could store charge in voltage regions where PMo will not. The comparison for GeMo and SiMo was even more interesting, as these are very similar molecules; both share a group IV heteroatom and Keggin anions with a 4− charge. The CVs for the bare, single layer SiMo coated, and single layer GeMo coated MWCNT are shown in Fig. 4B. Not surprisingly, the CVs for both the SiMo and GeMo coated electrodes share a very similar shape and their charge storage is quite comparable. At a scan rate of 50 mV/s, the SiMo electrode showed a capacitance of 111.2 F/cm3 . Although the shape of both CVs was similar, compared to that of SiMo, the entire CV for the GeMo electrode appeared to be shifted to more positive potentials by approximately 40 mV. This consistent change in all three redox peak potentials indicates a fundamental effect brought about by changing the heteroatom from one group IV atom (Si) to another (Ge). This effect may be due to the increased size and electronegativity of the Ge atom compared to Si. Nevertheless, much like with PMo, a complimentary charge storage for SiMo and GeMo coated electrodes is suggested. These significant changes in electrochemistry resulting from changing the heteroatom illustrate the immense potential for tuning POM chemistry to achieve desired electrode properties. 3.5. Dual-layer POM Coated MWCNT The complimentary charge storage demonstrated for GeMo and both commercial POMs suggests that it may be beneficial to combine GeMo with either PMo or SiMo on a dual-layer coated electrode. The effect of superimposing GeMo on top of PMo with a dual-layer electrode was studied. The resulting CV is shown in Fig. 5A, along with the CVs for bare, single layer PMo coated, and single layer GeMo coated MWCNTs for comparison. The CV of the dual-layer coated electrode not only showed significantly
M. Genovese et al. / Electrochimica Acta 117 (2014) 153–158
a
a
b
b
Fig. 4. A) Cyclic voltammograms of bare, single layer GeMo-coated MWCNT, and single layer PMo coated MWCNT at 50 mV/s; B) Cyclic voltammograms of bare, single layer GeMo-coated MWCNT, and single layer SiMo coated MWCNT at 50 mV/s.
higher charge storage compared to both single layer electrodes, but also appeared to be an additive combination of both the PMo and GeMo profiles. For instance, peak III for both GeMo and PMo combined constructively to form a broad plateau in the CV of the dual layer electrode. Furthermore, dual layer peaks I and II occurred at potentials intermediate to the corresponding GeMo and PMo peaks (Fig. 4A) and were much larger and wider than the narrow peaks of the individual CVs of the single layer electrodes. Combining these effects resulted in a CV that resembled an equal contribution of constituent layers and was thus much broader and more ideally capacitive than the single layer profiles. At the scan rate of 50 mV/s, the dual layer PMo-GeMo electrode demonstrated a capacitance of 218.7 F/cm3 , double the capacitance of the single layer electrodes and over twelve times the double layer capacitance of the bare MWCNT. The CV for the dual layer SiMo-GeMo electrode is shown in Fig. 5B along with the CVs for the bare, single layer SiMo coated, and single layer GeMo coated MWCNT for comparison. Once again, the CV of the dual layer coating showed further enhanced charge storage over the single layer electrodes with a shape that resembled a relatively equal and additive combination of both SiMo and GeMo chemistries. Different from the CV of the PMo-GeMo electrode, where 3 pairs of redox peaks were observed, the CV of the SiMoGeMo electrode had only 2 major peaks. This is due to the 40 mV shift observed between the GeMo and SiMo CVs, which resulted in a series of overlapping peaks and a broad dual-layer profile. The
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Fig. 5. A)Cyclic voltammograms of single layer GeMo-coated MWCNT and single layer PMo coated MWCNT compared to dual-layer PMo-GeMo coated MWCNT at 50 mV/s; B) Cyclic voltammograms of single layer GeMo-coated MWCNT and single layer SiMo coated MWCNT compared to dual-layer SiMo-GeMo coated MWCNT at 50 mV/s.
Fig. 6. Volumetric specific capacitance of bare, single layer coated, and dual-layer coated MWCNT.
capacitive performance for the SiMo-GeMo electrode and all single and dual layer electrodes tested is summarized in Fig. 6. At a scan rate of 50 mV/s, the SiMo-GeMo electrode showed a capacitance of 204.2 F/cm3 , slightly less than the PMo-GeMo electrode, but almost double the capacitance of the single layer modified MWCNT. Considering their large capacitance, the dual layer electrodes also
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Fig. 7. SEM micrographs of a) bare MWCNT, b) single-layer GeMo coated MWCNT, and c) dual layer PMo-GeMo coated MWCNT.
demonstrated good high rate performance. At a rate of 1 V/s, the PMo-GeMo electrode maintained 71% of its capacitance at 50 mV/s, while the SiMo-GeMo electrode was slightly more resistive, maintaining 68% of its low-rate capacitance.
dual layer electrodes was approximately double that of the single layer electrodes and twelve times greater than EDLC. These results indicate that GeMo is a promising material for high performance pseudocapacitive electrodes, especially when superimposed synergistically with alternate POM chemistries.
3.6. Surface Morphology Acknowledgments The morphologies of the bare and coated MWCNT are shown in Fig. 7, which compares bare, single layer coated GeMo, and dual layer coated PMO-GeMo MWCNTs. The bare MWCNT are arranged in a course network and displayed smooth surfaces with an average tube diameter of approximately 14 nm. The single layer GeMo coated MWCNT show an obvious increase in thickness over the bare MWCNT with average tube diameters of 19 nm. Similarly, the tube thickness is increased by an additional 5 nm for the dual-layer coated MWCNT. It is clear from the increase in thickness that the single and dual layer coated MWCNT are covered with significant amounts of POM material. The coating does not appear to aggregate in any particular location. Rather, both the single and dual layer coated films displayed relatively uniform surface coverage with a coating that is evenly distributed around the tubes. This indicates that the LbL technique is effective in yielding uniform, nano-meter scale multi-layer coatings. These thin surface layers result in intimate contact between the POM material and the carbon substrate, allowing for enhanced capacitance without sacrificing electrical conductivity. 4. Conclusions GeMo can be effectively coated onto MWCNT to modify the surface and add pseudocapacitive properties. The GeMo coated electrode had a charge storage capacity six times greater than that of bare MWCNT and its conductivity remained high as the electrode showed good rate performance. GeMo also demonstrated good cycling stability, showing only a 6.3% capacitance loss after 5000 cycles. The charge storage profile of GeMo was complementary to that of the commercial POMs tested, with GeMo storing charge in voltage regions where PMo and SiMo did not. Dual-layer electrodes which superimposed GeMo with PMo or SiMo showed further enhanced capacitance and a collective contribution of both constituent layers. This resulted in CVs with overlapping redox peaks and much broader and more ideally capacitive profiles than those of the single layer electrodes. The volumetric capacitance of the
We acknowledge the financial support of NSERC Canada. MG would like to thank NSERC for a postgraduate scholarship and Hatch for a scholarship for sustainable energy research. YF acknowledges a UTEA summer fellowship from University of Toronto. We also appreciate Professor Yury Gogotsi providing MWCNT samples. References [1] P. Simon, Y. Gogotsi, Nat. Mater. 7 (2008) 845. [2] Y. Zhang, H. Feng, X. Wu, L. Wang, A. Zhang, T. Xia, H. Dong, X. Li, L. Zhang, Int. J. Hydrogen Energy 34 (2009) 4889. [3] S.W. Lee, J. Kim, S. Chen, P.T. Hammond, Y. Shao-Horn, ACS Nano 4 (2010) 3889. [4] B.E. Conway, Electrochemical Supercapacitor, Scientific Fundamentals and Technological Applications, Kluwer Academic, New York, 1999. [5] P. Gomez-Romero, M. Chojak, K. Cuentas-Gallegos, J. Asensio, P. Kulesza, N. Casan-Pastor, M. Lira-Cantu, Electrochem. Commun. 5 (2003) 149. [6] S. Park, K. Lian, Y. Gogotsi, J. Electrochem Soc. 156 (2009) A921. [7] V. Ruiz, J. Suárez-Guevara, P. Gomez-Romero, Electrochem. Comm. 24 (2012) 35. [8] M. Skunik, M. Chojak, I. Rutkowska, P. Kulesza, Electrochim. Acta 53 (2008) 3862. [9] B.B. Xu, L. Xu, G.G. Gao, W.H. Guo, S.P. Liu, J. Colloid Interface Sci. 330 (2009) 408. [10] M. Sadakane, E. Steckhan, Chem. Rev. 98 (1998) 219. [11] A.K. Cuentas-Gallegos, R. Martinez-Rosales, M. Baibarac, P. Gomez Romero, M.E. Rincón, Electrochemistry Commun. 9 (2007) 2008. [12] D. Martel, M. Gross, Journal of Solid State Electrochemistry 11 (2007) 421. [13] T. Akter, K. Hu, K. Lian, Electrochim. Acta 56 (2011) 4966. [14] G. Bajwa, M. Genovese, K. Lian, ECS Journal of Solid State Science and Technology 2 (2013) M3046. [15] M. Skunik, B. Baranowska, D. Fattakhova, K. Miecznikowski, M. Chojak, A. Kuhn, P. Kulesza, J. Solid State Electrochem 10 (2006) 168. [16] Q. Wu, G. Meng, Materials Research Bulletin 35 (2000) 85. [17] K. Behler, S. Osswald, H. Ye, S. Dimovski, Y. Gogotsi, J. Nanopart Res. 8 (2006) 615. [18] C. Portet, J. Chmiola, Y. Gogotsi, S. Park, K. Lian, Electrochim. Acta 53 (2008) 7675. [19] B.H. Park, J.H. Choi, Electrochimica Acta 55 (2010) 2888. [20] P.A. Jalil, M.A. Al-Daous, A.R.A. Al-Arfaj, A.M. Al-Amer, J. Beltramini, S.A.I. Barri, Appl. Catal. A 207 (2001) 159. [21] P. Wang, Y. Yuan, Z. Han, G. Zhu, J. Mater. Chem. 11 (2001) 549. [22] J. Wang, Analytical Electrochemistry, VCH, New York, 1994. [23] C. Rocchiccioli-Deltcheff, M. Fournier, R. Franck, R. Thouvenot, Inorg. Chem. 22 (1983) 207.
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Germanomolybdate (GeMo12 O40 4− ) Modified Carbon Nanotube Composites for Electrochemical Capacitors Matthew Genovese, Yee Wei Foong, Keryn Lian ∗ Department of Materials Science and Engineering, University of Toronto Toronto, Ontario, Canada M5S 3E4
a r t i c l e
i n f o
Article history: Received 2 October 2013 Received in revised form 18 November 2013 Accepted 20 November 2013 Available online 4 December 2013 Keywords: Polyoxometalates Germanomolybdate Layer-by-layer Pseudocapacitance Supercapacitor
a b s t r a c t Keggin type germanomolybdate, GeMo12 O40 4− (GeMo), was deposited onto multi-walled carbon nanotubes (MWCNT) via layer-by-layer (LbL) deposition to form composite electrodes for electrochemical capacitors (ECs). The GeMo composite electrode demonstrated charge storage six times greater than that of the bare MWCNT electrode while maintaining excellent conductivity and cycling stability. GeMo also demonstrated charge storage complementary to that of the commercial Keggin type POMs, PMo12 O40 3− (PMo) and SiMo12 O40 4− (SiMo). Dual-layer coatings superimposing GeMo with either PMo or SiMo showed an additive combination of both active layers, which resulted in cyclic voltammograms (CVs) with overlapping redox features and charge storage twelve times greater than that of the bare MWCNT electrode. Scanning Electron Microscopy (SEM) demonstrated successful single and dual layer coating of POMs on MWCNT with high coverage and uniform surface morphologies. © 2013 Elsevier Ltd. All rights reserved.
1. Introduction Carbon nano materials such as graphene, onion-like carbon (OLC), and carbon nanotubes (CNT) have emerged as promising electrode materials for electrochemical double layer capacitors (EDLC), due to their large surface area and metal-like conductivity [1–3]. However, storing charge through the electrochemical double layer alone limits the specific capacitance and energy density of nano-carbon electrodes. One of the most common approaches to improve the energy density of EDLC electrodes is the addition of pseudocapacitive materials. Pseudocapacitive materials allow for reversible Faradaic oxidation/reduction reactions on the electrode surface, and can yield specific capacitance 10 to 100 times greater than EDLC [4]. RuO2 is one of the most effective pseudocapacitive materials due to its fast reversible redox reactions with overlapping peak potentials: Its high cost, however, proves to be a challenge for commercialization. Polyoxometalates (POMs), a class of large metal oxide clusters, have been investigated as low cost RuO2 alternatives [5–8]. Keggin-type heteropolyanions are the most well known and widely used class of POMs. The Keggin structure has the general formula [XM12 O40 ]n− , in which the central heteroatom (i.e., P, Si, or B) is surrounded by twelve addenda atoms (i.e., Mo or W) and forty
∗ Corresponding author. Department of Materials Science and Engineering, University of Toronto, 184 College St. Toronto, ON M5S 3E4 Canada. Tel.: +416 978 8631. E-mail address: [email protected] (K. Lian). 0013-4686/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2013.11.114
oxygen atoms. These POMs exhibit multiple, highly stable redox states, which gives them the ability to act as electron reservoirs, an ideal characteristic for electrochemical storage applications [9]. Furthermore, different combinations of hetero and addenda atoms can lead to different electrochemical properties [10], allowing for the ability to tune the electrochemical behaviour of POMs. By combining these POMs with nano-carbon materials, the electrochemical activity of the former can be leveraged with the surface area of the latter to achieve high performance and low cost inorganic-organic composite electrodes. A number of methods can be used to create POM-carbon composites, including simple mixing [7], chemisorption following carbon oxidation [11], electrodeposition [10], and layer-by-layer (LbL) self assembly [12–14]. LbL self assembly, achieved through the alternate adsorption of positive and negative layers, is a simple and effective technique to deposit monolayers of oppositely charged species. Martel et al. have reported the LbL deposition of 4 different POM chemistries on smooth glassy carbon, and showed a linear increase in charge storage with the number of layers deposited [12]. Kulesza and his group developed composites of POMs with various conductive substrates [5,8,15]. For Keggin-type POMs, charge storage is often concentrated in certain voltage regions that do not overlap [8,10,12]. In order to combat this limitation, the LbL technique can be used to superimpose layers of different POM chemistries to achieve an electrode which exhibits a collective contribution of each layer. Thus, different POM chemistries with different redox peak potentials can be superimposed to mimic the overlapping redox peaks and ideally capacitive profile of RuO2 . Our previous
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work has demonstrated the feasibility of superimposing a simple commercial Keggin type anion PMo12 O40 3− (PMo) with the mixed addenda anion PMo10 V2 O40 5− (PMoV2 ). This approach resulted in a significant increase in capacitance and a combined voltametric profile of both the PMo and PMoV2 molecules [14]. However, the difference in electrochemistry and peak potentials between PMo and PMoV2 was quite small, thus limiting the synergistic effect of combining these molecules. The present work explores an alternate Keggin-type anion, GeMo12 O40 4− (GeMo) and aims to 1) investigate the electrochemistry of GeMo coated MWCNT electrodes, 2) compare the electrochemical behaviour of GeMo to that of previously studied commercial POMs with alternate heteroatoms, and 3) explore how potential differences in electrochemical behaviour can be leveraged when designing multi-layer EC electrodes with synergistic combinations of GeMo and other POMs.
2. Experimental 2.1. POM Synthesis Three different POMs were used in this work: H3 PMo12 O40 (PMo), H4 SiMo12 O40 (SiMo), and H4 GeMo12 O40 (GeMo). PMo and SiMo were both purchased from Alfa Aesar, while GeMo was synthesized according to a procedure modified from that described by Wu et al. [16]. Briefly, GeO2 (0.8 g) was dissolved in 20 ml of a 5 wt% NaOH solution. An aqueous solution (50 ml) of Na2 MoO4 •2H2 O (22.2 g) was then added to the first mixture, and the pH was adjusted to 1.0 with concentrated H2 SO4 . The mixture was stirred continuously for 2 hours at 80 ◦ C. The cooled solution was extracted with ether and the isolated solid was washed with deionized H2 O and dried for 5 hours at 50 ◦ C yielding a yellow crystal product. All reagents for the GeMo synthesis were purchased from Alfa Aesar.
2.2. Carbon Electrode Fabrication Two different types of carbon electrodes were utilized; a MWCNT film electrode and a mixed graphite/MWCNT ink electrode. For the MWCNT film electrode, the “as received” MWCNTs which had been previously characterized [17] were mixed with 4 wt% PTFE binder and rolled into thin films of approximately 100 m thickness. For electrochemical analysis these films were ground and loaded into a cavity microelectrode (CME) [18] which was used as the working electrode in a 3-electrode cell. The CME had a volume of approximately 4.9 × 10−6 cm3 and at least ten different loadings and tests were performed for each sample to ensure the stability of the process. The MWCNT film electrodes were used for all electrochemical tests except for the cycle life analysis. Carbon ink electrodes were used in cycle life analysis and were fabricated according to a procedure modified from that described by Park et al. [19]. The carbon ink was synthesized by adding both graphite and MWCNT to an aqueous solution of poly-vinyl alcohol (PVA), in a proportion 15 wt% PVA and 85 wt% carbon, with a carbon mass ratio of 3:1 graphite to MWCNT. A small amount of glutaric acid (10 wt% of the PVA content) was added for cross-linking. The ink was cast on titanium foils in approximately 60 m thick layers. The electrodes were annealed at 50 ◦ C for 1 hour followed by 140 ◦ C for 1 hour for cross-linking of the PVA. After annealing, the carbon electrodes were cut into pieces with an active area of 1 cm2 for use as the working electrode in a 3-electrode cell. Graphite, PVA, and glutaric acid used to synthesize the carbon ink were purchased from Alfa Aesar. The titanium foils were purchased from McMaster Carr.
Fig. 1. XRD patterns of commercial SiMo and synthesized GeMo.
2.3. LbL Process For LbL coating, all three POMs were dissolved in deionized water to form 10 mM solutions. 4 wt% poly (diallyldimethylammonium chloride) (PDDA) and HNO3 (conc.) were used as part of the LbL process and were both purchased from Alfa Aesar. The LbL coating of the MWCNT films proceeded according to the following steps: i. Soaking in concentrated HNO3 (2 minutes) followed by H2 O washing (2 minutes) ii. Soaking in 4 wt% PDDA solution (10 minutes) followed by H2 O washing (2 minutes) iii. Soaking in 10 mM POM solution (20 minutes) followed by H2 O washing (2 minutes) The process for coating of the carbon ink electrodes was similar. However, conc. HNO3 treatment was replaced with overnight soaking in a more dilute (1 M) HNO3 solution. 2.4. Characterizations Electrochemical analysis was performed by a 3-electrode system with the working electrode as described above. Saturated Ag/AgCl and a platinum mesh were used as the reference and counter electrodes, respectively. The electrolyte for all tests was 1 M sulfuric acid (H2 SO4 ) and electrochemical measurements were conducted using an EG&G 273 potentiostat. Structural and surface characterizations were performed using X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), and Energy-Dispersive X-ray Spectroscopy (EDX). XRD was conducted using a Philips XRD system with a monochromatized Cu-K␣ anode. SEM micrographs were obtained using FEI Quanta FEG 250 large stage environmental SEM with auxiliary EDX detection. XRD and EDX analysis were performed on the POM powder materials, while SEM was performed on the POM coated MWCNT films. 3. Results and Discussion 3.1. Structure and Composition of Polyoxometalates The crystal structure and phases present in GeMo were studied using powder X-ray diffraction (XRD). The XRD pattern for the synthesized GeMo powder is shown in Fig. 1 along with the pattern for commercial SiMo. GeMo and SiMo should have a similar
M. Genovese et al. / Electrochimica Acta 117 (2014) 153–158 Table 1 Theoretical and experimental atomic % ratios of addenda atoms (Mo) and heteroatoms (Si and P) present in both commercial SiMo and synthesized GeMo.
Theoretical Experimental
SiMo Mo:Si
GeMo Mo:Ge
12: 1 12.3: 1
12: 1 12.5: 1
crystal structure as they are both Keggin POMs with group IV heteroatoms. The XRD patterns for both the commercial SiMo and the synthesized GeMo are quite similar, indicating the synthesized material has the Keggin crystal structure. There are some differences between the two patterns especially with regards to the intensity of peaks in the 2=6-8◦ region, but these can be attributed to changes in the crystal structure resulting from different levels of hydration in the SiMo and GeMo powders [20]. Moreover, the pattern for the GeMo powder is very similar to the XRD data obtained by Wu et al. for vanadium substituted germanomolybdate (GeMo11 VO40 ) [16]. The XRD results seemed to confirm the presence of the Keggin structure, so EDX analysis was conducted to study the stoichiometry and composition of the synthesized material (Table 1). The EDX spectra of GeMo revealed that the atomic percent ratio of addenda atoms (Mo) to heteroatoms (Ge) was 12.5:1, only slightly higher than the theoretical 12:1 Keggin ratio and very similar to the 12.3:1 Mo:Si ratio determined for commercial SiMo. Thus, based upon the structural and compositional analysis, the presence of GeMo in the synthesized powder was confirmed. 3.2. Electrochemical Analysis: Single Layer GeMo Coated MWCNT Cyclic voltammograms of bare and single layer GeMo coated MWCNTs at a scan rate of 20 mV/s are shown in Fig. 2A. It is clear from the CVs that coating of GeMo on the carbon substrate was quite effective, as the GeMo coated electrode showed a significant increase in current density from that of the bare film. The CV of the GeMo coated MWCNTs demonstrated three pairs of oxidation/reduction peaks (labeled I, II, and III in Fig. 2A) which represent the characteristic charge transfer reactions of the Keggin molecule. For reactions I, II, and III, the equilibrium oxidation potentials were found to be 0.37, 0.28, and 0.07 V vs. Ag/AgCl, respectively, matching the results obtained by Wang et al. for a GeMo modified glassy carbon electrode [21]. The redox peaks of the GeMo coated MWCNTs were quite sharp and demonstrated a difference in the corresponding oxidation/reduction peak potentials (Ep ) of 30, 26, and 28 mV for reaction I, II, and III, respectively. Ep was close to 30 mV, the theoretical value for a reversible 2 electron transfer process [22], for all reactions. Thus, even after immobilization on a CNT substrate, GeMo demonstrated the fast reversible electron transfer reactions ideal for pseudocapacitive behaviour. The fast charge transfer reactions and pseudocapacitive behaviour of the GeMo modified electrode are further demonstrated in Fig. 2B which shows the anodic peak current density for reaction III plotted vs. the scan rate. The GeMo electrode demonstrated ideal pseudocapacitive behaviour: a linear relationship between peak current and scan rate, up to a high scan rate of 0.5 V/s. From 0.75 to 2 V/s, the peak currents began to deviate slightly from ideal behaviour and follow a non-linear trend, a consequence of high current densities which cause an increased contribution of ohmic resistance and onset of diffusion control. The volumetric capacitance of the GeMo coated electrode at the slowest tested scan rate of 20 mV/s was 109.8 F/cm3 , which represents a six-fold increase compared to the capacitance of the bare MWCNT electrode (17 F/cm3 ). The plot of capacitance vs. scan rate for the GeMo modified electrode (inset Fig. 2B) shows only a small decline between 20
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Table 2 Area Specific Capacitance of bare carbon ink and GeMo coated carbon ink electrodes. Electrode
Area Specific Capacitance (mF/cm2 ) Scan Rate
Bare Carbon Ink GeMo Coated Carbon Ink
0.05 V/s
1 V/s
11.15 48.89
11.08 41.96
and 500 mV/s. Even at a high rate of 2 V/s the capacitance decline of the GeMo coated MWCNT was only about 22%, which again demonstrates the high conductivity and fast charge transfer reactions of this electrode. The pH dependence of the redox reactions for the GeMo coated MWCNT was studied by taking CV measurements in various concentrations of H2 SO4 electrolytes. Plots of the anodic peak potential vs. pH for each GeMo redox reaction are shown in Fig. 2 C. The plots show good linearity and a gradual decrease in peak potential from a pH of 0 to 1. The slope of the plots for peaks I, II, and III are -59, -58, and -61 mV/pH respectively. These slopes are very close to the theoretical value of -59 mV/pH, which would indicate the 2 proton per 2 electron coupled redox process common for Keggin molecules [8,23]. This suggests that for the GeMo coated electrode the highest faradic currents will be achieved in low pH electrolytes. 3.3. GeMo Cycling Stability Since the GeMo modified electrode is intended for EC applications, its stability upon repeated cycling was evaluated. The CME, while valuable for electrochemical characterization, is not well suited for cycle-life tests, as material loss from the electrode cavity may bias the results. For this reason, cycle life analysis was performed with a GeMo modified carbon ink electrode coated onto a titanium current collector. This larger-scale electrode has the added benefit of illustrating that the results demonstrated with the CME can be effectively “scaled-up” for device fabrication. The area specific capacitance of the bare and GeMo coated carbon ink electrodes at a scan rate of 0.05 and 1 V/s are shown in Table 2. Much like the results demonstrated with the CME, the GeMo coated carbon ink electrode showed a significant increase in capacitance from that of the bare electrode and maintained this capacitance at high rates. The cycling stability of the bare and GeMo modified electrode was evaluated by applying 5000 successive potential cycles between -0.15 and 1.0 V at a rate of 1 V/s; the results are shown in Fig. 3. The bare carbon electrode showed only minor capacitance degradation (1.3%), and the decline for the GeMo electrode was also minimal; after 5000 cycles the GeMo electrode demonstrated a capacitance loss of only 6.3%. Furthermore, the majority of this capacitance loss (4.5%) occurred over the first 2500 cycles, while the final 2500 cycles resulted in a decline of only 1.8%, indicating stabilization of the electrode capacitance and good durability. The cycle life tests were conducted in a 3-electrode cell with a liquid electrolyte, and thus the cycling stability of the GeMo electrode could be further improved in a solid state electrochemical device using a proton conducting polymer electrolyte. 3.4. Comparison of Electrochemical Behaviour: GeMo vs. Commercial POMs Capacitive performance and electrochemical characteristics of the synthesized GeMo material were compared to those of commercial POMs (PMo and SiMo). CVs of the bare, single layer PMo coated, and single layer GeMo coated MWCNT are shown in Fig. 4A. The peak current densities of the PMo and GeMo electrodes are quite comparable, although the CV of GeMo has a slightly broader
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a
b
c
Fig. 2. A) Cyclic voltammograms of bare and single layer GeMo-coated MWCNT at 20 mV/s; B) Anodic peak current (reaction III) vs. scan rate for single layer GeMo coated MWCNT and the capacitance vs. scan rate for single layer GeMo coated MWCNT(inset); C) Relationship between anodic peak potential and pH for the GeMo redox processes.
Fig. 3. Variation in area specific capacitance as a function of cycle number for bare and GeMo coated carbon ink electrodes.
profile. Charge storage for both electrodes was very similar; at 50 mV/s the PMo and GeMo coated MWCNT showed a capacitance of 106.9 F/cm3 and 109.5 F/cm3 , respectively. While the capacitance of the two electrodes was similar, it is obvious that the change in heteroatom from P to Ge significantly altered the electrochemistry of the material. None of three redox peaks of GeMo directly overlay those of PMo. For instance, the peak potential for reaction I was shifted negatively by 35 mV for GeMo as compared to PMo, while the potentials for reactions II and III were shifted positively by 30 and 60 mV respectively. Consequently, the charge storage for these
two materials could complement each other, that is, GeMo could store charge in voltage regions where PMo will not. The comparison for GeMo and SiMo was even more interesting, as these are very similar molecules; both share a group IV heteroatom and Keggin anions with a 4− charge. The CVs for the bare, single layer SiMo coated, and single layer GeMo coated MWCNT are shown in Fig. 4B. Not surprisingly, the CVs for both the SiMo and GeMo coated electrodes share a very similar shape and their charge storage is quite comparable. At a scan rate of 50 mV/s, the SiMo electrode showed a capacitance of 111.2 F/cm3 . Although the shape of both CVs was similar, compared to that of SiMo, the entire CV for the GeMo electrode appeared to be shifted to more positive potentials by approximately 40 mV. This consistent change in all three redox peak potentials indicates a fundamental effect brought about by changing the heteroatom from one group IV atom (Si) to another (Ge). This effect may be due to the increased size and electronegativity of the Ge atom compared to Si. Nevertheless, much like with PMo, a complimentary charge storage for SiMo and GeMo coated electrodes is suggested. These significant changes in electrochemistry resulting from changing the heteroatom illustrate the immense potential for tuning POM chemistry to achieve desired electrode properties. 3.5. Dual-layer POM Coated MWCNT The complimentary charge storage demonstrated for GeMo and both commercial POMs suggests that it may be beneficial to combine GeMo with either PMo or SiMo on a dual-layer coated electrode. The effect of superimposing GeMo on top of PMo with a dual-layer electrode was studied. The resulting CV is shown in Fig. 5A, along with the CVs for bare, single layer PMo coated, and single layer GeMo coated MWCNTs for comparison. The CV of the dual-layer coated electrode not only showed significantly
M. Genovese et al. / Electrochimica Acta 117 (2014) 153–158
a
a
b
b
Fig. 4. A) Cyclic voltammograms of bare, single layer GeMo-coated MWCNT, and single layer PMo coated MWCNT at 50 mV/s; B) Cyclic voltammograms of bare, single layer GeMo-coated MWCNT, and single layer SiMo coated MWCNT at 50 mV/s.
higher charge storage compared to both single layer electrodes, but also appeared to be an additive combination of both the PMo and GeMo profiles. For instance, peak III for both GeMo and PMo combined constructively to form a broad plateau in the CV of the dual layer electrode. Furthermore, dual layer peaks I and II occurred at potentials intermediate to the corresponding GeMo and PMo peaks (Fig. 4A) and were much larger and wider than the narrow peaks of the individual CVs of the single layer electrodes. Combining these effects resulted in a CV that resembled an equal contribution of constituent layers and was thus much broader and more ideally capacitive than the single layer profiles. At the scan rate of 50 mV/s, the dual layer PMo-GeMo electrode demonstrated a capacitance of 218.7 F/cm3 , double the capacitance of the single layer electrodes and over twelve times the double layer capacitance of the bare MWCNT. The CV for the dual layer SiMo-GeMo electrode is shown in Fig. 5B along with the CVs for the bare, single layer SiMo coated, and single layer GeMo coated MWCNT for comparison. Once again, the CV of the dual layer coating showed further enhanced charge storage over the single layer electrodes with a shape that resembled a relatively equal and additive combination of both SiMo and GeMo chemistries. Different from the CV of the PMo-GeMo electrode, where 3 pairs of redox peaks were observed, the CV of the SiMoGeMo electrode had only 2 major peaks. This is due to the 40 mV shift observed between the GeMo and SiMo CVs, which resulted in a series of overlapping peaks and a broad dual-layer profile. The
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Fig. 5. A)Cyclic voltammograms of single layer GeMo-coated MWCNT and single layer PMo coated MWCNT compared to dual-layer PMo-GeMo coated MWCNT at 50 mV/s; B) Cyclic voltammograms of single layer GeMo-coated MWCNT and single layer SiMo coated MWCNT compared to dual-layer SiMo-GeMo coated MWCNT at 50 mV/s.
Fig. 6. Volumetric specific capacitance of bare, single layer coated, and dual-layer coated MWCNT.
capacitive performance for the SiMo-GeMo electrode and all single and dual layer electrodes tested is summarized in Fig. 6. At a scan rate of 50 mV/s, the SiMo-GeMo electrode showed a capacitance of 204.2 F/cm3 , slightly less than the PMo-GeMo electrode, but almost double the capacitance of the single layer modified MWCNT. Considering their large capacitance, the dual layer electrodes also
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Fig. 7. SEM micrographs of a) bare MWCNT, b) single-layer GeMo coated MWCNT, and c) dual layer PMo-GeMo coated MWCNT.
demonstrated good high rate performance. At a rate of 1 V/s, the PMo-GeMo electrode maintained 71% of its capacitance at 50 mV/s, while the SiMo-GeMo electrode was slightly more resistive, maintaining 68% of its low-rate capacitance.
dual layer electrodes was approximately double that of the single layer electrodes and twelve times greater than EDLC. These results indicate that GeMo is a promising material for high performance pseudocapacitive electrodes, especially when superimposed synergistically with alternate POM chemistries.
3.6. Surface Morphology Acknowledgments The morphologies of the bare and coated MWCNT are shown in Fig. 7, which compares bare, single layer coated GeMo, and dual layer coated PMO-GeMo MWCNTs. The bare MWCNT are arranged in a course network and displayed smooth surfaces with an average tube diameter of approximately 14 nm. The single layer GeMo coated MWCNT show an obvious increase in thickness over the bare MWCNT with average tube diameters of 19 nm. Similarly, the tube thickness is increased by an additional 5 nm for the dual-layer coated MWCNT. It is clear from the increase in thickness that the single and dual layer coated MWCNT are covered with significant amounts of POM material. The coating does not appear to aggregate in any particular location. Rather, both the single and dual layer coated films displayed relatively uniform surface coverage with a coating that is evenly distributed around the tubes. This indicates that the LbL technique is effective in yielding uniform, nano-meter scale multi-layer coatings. These thin surface layers result in intimate contact between the POM material and the carbon substrate, allowing for enhanced capacitance without sacrificing electrical conductivity. 4. Conclusions GeMo can be effectively coated onto MWCNT to modify the surface and add pseudocapacitive properties. The GeMo coated electrode had a charge storage capacity six times greater than that of bare MWCNT and its conductivity remained high as the electrode showed good rate performance. GeMo also demonstrated good cycling stability, showing only a 6.3% capacitance loss after 5000 cycles. The charge storage profile of GeMo was complementary to that of the commercial POMs tested, with GeMo storing charge in voltage regions where PMo and SiMo did not. Dual-layer electrodes which superimposed GeMo with PMo or SiMo showed further enhanced capacitance and a collective contribution of both constituent layers. This resulted in CVs with overlapping redox peaks and much broader and more ideally capacitive profiles than those of the single layer electrodes. The volumetric capacitance of the
We acknowledge the financial support of NSERC Canada. MG would like to thank NSERC for a postgraduate scholarship and Hatch for a scholarship for sustainable energy research. YF acknowledges a UTEA summer fellowship from University of Toronto. We also appreciate Professor Yury Gogotsi providing MWCNT samples. References [1] P. Simon, Y. Gogotsi, Nat. Mater. 7 (2008) 845. [2] Y. Zhang, H. Feng, X. Wu, L. Wang, A. Zhang, T. Xia, H. Dong, X. Li, L. Zhang, Int. J. Hydrogen Energy 34 (2009) 4889. [3] S.W. Lee, J. Kim, S. Chen, P.T. Hammond, Y. Shao-Horn, ACS Nano 4 (2010) 3889. [4] B.E. Conway, Electrochemical Supercapacitor, Scientific Fundamentals and Technological Applications, Kluwer Academic, New York, 1999. [5] P. Gomez-Romero, M. Chojak, K. Cuentas-Gallegos, J. Asensio, P. Kulesza, N. Casan-Pastor, M. Lira-Cantu, Electrochem. Commun. 5 (2003) 149. [6] S. Park, K. Lian, Y. Gogotsi, J. Electrochem Soc. 156 (2009) A921. [7] V. Ruiz, J. Suárez-Guevara, P. Gomez-Romero, Electrochem. Comm. 24 (2012) 35. [8] M. Skunik, M. Chojak, I. Rutkowska, P. Kulesza, Electrochim. Acta 53 (2008) 3862. [9] B.B. Xu, L. Xu, G.G. Gao, W.H. Guo, S.P. Liu, J. Colloid Interface Sci. 330 (2009) 408. [10] M. Sadakane, E. Steckhan, Chem. Rev. 98 (1998) 219. [11] A.K. Cuentas-Gallegos, R. Martinez-Rosales, M. Baibarac, P. Gomez Romero, M.E. Rincón, Electrochemistry Commun. 9 (2007) 2008. [12] D. Martel, M. Gross, Journal of Solid State Electrochemistry 11 (2007) 421. [13] T. Akter, K. Hu, K. Lian, Electrochim. Acta 56 (2011) 4966. [14] G. Bajwa, M. Genovese, K. Lian, ECS Journal of Solid State Science and Technology 2 (2013) M3046. [15] M. Skunik, B. Baranowska, D. Fattakhova, K. Miecznikowski, M. Chojak, A. Kuhn, P. Kulesza, J. Solid State Electrochem 10 (2006) 168. [16] Q. Wu, G. Meng, Materials Research Bulletin 35 (2000) 85. [17] K. Behler, S. Osswald, H. Ye, S. Dimovski, Y. Gogotsi, J. Nanopart Res. 8 (2006) 615. [18] C. Portet, J. Chmiola, Y. Gogotsi, S. Park, K. Lian, Electrochim. Acta 53 (2008) 7675. [19] B.H. Park, J.H. Choi, Electrochimica Acta 55 (2010) 2888. [20] P.A. Jalil, M.A. Al-Daous, A.R.A. Al-Arfaj, A.M. Al-Amer, J. Beltramini, S.A.I. Barri, Appl. Catal. A 207 (2001) 159. [21] P. Wang, Y. Yuan, Z. Han, G. Zhu, J. Mater. Chem. 11 (2001) 549. [22] J. Wang, Analytical Electrochemistry, VCH, New York, 1994. [23] C. Rocchiccioli-Deltcheff, M. Fournier, R. Franck, R. Thouvenot, Inorg. Chem. 22 (1983) 207.