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Monovalent silicotungstate salts as electrolytes for electrochemical supercapacitors
Electrochimica Acta 138 (2014) 240–246
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Monovalent silicotungstate salts as electrolytes for electrochemical supercapacitors Han Gao, Alvin Virya, 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 14 May 2014 Received in revised form 22 June 2014 Accepted 23 June 2014 Available online 30 June 2014 Keywords: electrical double layer capacitor neutral aqueous electrolytes alkali cations heteropolyacid salts.
a b s t r a c t Lithium, sodium, and potassium salts of silicotungstic acid were synthesized and characterized as aqueous neutral electrolytes for electrochemical supercapacitors. The acidity of the aqueous solution and the structure of the solid-state anion were examined to confirm the presence of SiW salts. Ionic conductivity and the electrochemical stability potential window were characterized and compared to a silicotungstic acid solution using metallic blocking electrodes. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were used to investigate the performance of carbon EDLC cells enabled by the neutral electrolytes and revealed a 1.5 V cell voltage and good cycle life. The similarities and differences among the three salts are explained based on the properties of cations in these neutral electrolytes. © 2014 Elsevier Ltd. All rights reserved.
1. Introduction Electric double layer capacitors (EDLC) are electrochemical capacitors (EC) that have attracted significant attention as electrical energy storage devices [1,2]. In an EDLC, ions in the electrolyte are transferred between electrodes by migration and/or diffusion and energy is stored in the form of charge separation between the electrodes. Ionic conductivity and the stability of the electrolytes govern the power and energy density of the EDLC devices. The majority of commercial EDLCs are based on porous carbon electrodes in organic electrolytes [3,4]. While possessing a high voltage window, organic electrolytes generally are volatile, flammable, and moisture sensitive, requiring an oxygen-free environment for cell assembly. Alternatively, the most common aqueous electrolytes are H2 SO4 and KOH as they have high ionic conductivity. However, they tend to be corrosive and have a limited voltage window. Aqueous neutral electrolytes have been investigated for EC applications as they are non-corrosive, have good conductivity, and have high oxygen and hydrogen evolution overpotentials on carbon electrodes [5–8]. The high overpotential for oxygen and hydrogen evolution leads to an increase in cell voltage, which in turn can improve the energy density of the EDLC devices. A stability window of ca. 2.0 V could be achieved with a carbon electrode in Na2 SO4 , enabling a cell voltage of 1.6 V for a symmetric carbon/carbon EDLC with good cycle life [9]. Other aqueous electrolytes such as Li2 SO4
∗ Corresponding author. E-mail address: [email protected] (K. Lian). http://dx.doi.org/10.1016/j.electacta.2014.06.127 0013-4686/© 2014 Elsevier Ltd. All rights reserved.
and K2 SO4 have been reported for EDLCs [5,6,10,11]. In addition, neutral aqueous electrolytes have been explored in asymmetric ECs to expand the voltage window, using activated carbon and pseudocapacitive electrodes [12–16] or two pseudocapacitive electrodes [17,18]. Although currently studied neutral aqueous electrolytes show great promise to enhance the energy density of ECs, the device power performance is still limited at high charge/discharge rates due to their relatively low ionic conductivity at room temperatures. In our pervious study, Keggin-type heteropoly acids, such as silicotungstic acid (H4 SiW12 O40 , SiWA) and phosphotungstic acid (H3 PW12 O40 , PWA), were characterized as aqueous acidic electrolytes for ECs [19]. The generic formula of the anion is [Xn+ M12 O40 ](8-n)− where the central heteroatom X (e.g. P, Ge, Si, B, etc.) is bonded with four oxygen atoms that is surrounded by a cage of 12 MO6 units (where M is W, Mo, or V) linked to one another by the neighboring oxygen atoms [20]. This configuration is referred as primary Keggin structure. Both SiWA and PWA showed similar ionic conductivity and self-discharge to H2 SO4 [19]. Their high ionic conductivity resulted from (a) the high ionic strength of the electrolyte solutions due to the complete dissociation of ions and (b) the high mobility of the Keggin anions due to their un-solvated nature in aqueous systems [21,22]. In aqueous systems, Keggin anions can be transferred much faster than other anions, such as Cl− or SO4 2− . Therefore, it is of interest to investigate the salts of Keggintype heteropoly anions as aqueous neutral electrolytes for EDLCs to achieve a wider electrochemical stability window and higher ionic conductivity. The salts of Keggin-type heteropoly anions are commonly classified into two groups based on the size of cation: small cations like
H. Gao et al. / Electrochimica Acta 138 (2014) 240–246
Li+ and large ones like Cs+ [23]. The first group shows high solubility in water while the salts in the second group are insoluble [20,24]. In this study, monovalent lithium, sodium, and potassium salts of silicotungstate are synthesized and characterized as aqueous neutral electrolytes for EDLCs. The ionic conductivity as well as the potential window of the neutral electrolytes are determined. Using multiwall carbon nanotube-graphite (referred as CNT-graphite) composite electrodes, the performance of single electrodes and carbon EDLC cells with these aqueous neutral salt electrolytes is examined and correlated to the electrolyte properties. In addition, the cycle life of EDLCs enabled by these neutral electrolytes was investigated to demonstrate their feasibility for high energy EDLCs.
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Table 1 Cation contents and degree of cation exchange in lithium, sodium, and potassium salts of silicotungstate. Sample
Cation content (gmL−1 )
Percentage of cation exchange
Li-SiW Na-SiW K-SiW
Theoretical 894.76 2952.27 4824.00
101% 105% 115%
ICP-AES 901.32 3088.45 5528.19
2. Experimental 2.1. Preparation of salts SiWA was purchased from Alfa Aesar and used as-received. The lithium, sodium, and potassium salts of SiWA (referred to as Li-SiW, Na-SiW, and K-SiW, respectively) were prepared following the methods by Tsigdinos [25]. For example, a predetermined amount of a 0.01 M lithium carbonate solution was slowly added to a 0.05 M SiWA solution (dropwise at room temperature with agitation), leading to the following reaction: Li2 CO3 + H4 SiW12 O40 → Li4 SiW12 O40 + 2H2 O + 2CO2 [reaction a]
Fig. 1. Comparison of pH of aqueous Li-SiW, Na-SiW, K-SiW, and SiWA electrolytes in 0.01, 0.03, and 0.05 M concentrations at room temperature.
Na-SiW and K-SiW were prepared in a similar way. All salts were obtained by evaporating water from the resulting solutions at 50 to 60 ◦ C. 2.2. Material characterizations Inductively coupled plasma atomic emission spectroscopy (ICPAES) (Perkin Elmer Optima 7300) was used to analyze the Li+ , Na+ , and K+ contents of the salts in aqueous solutions and the degrees of cation exchange. Infrared (IR) spectra were recorded on a Thermo Scientific Nicolet iS5 FT-IR spectrometer with iD1 transmission module at room temperature. A few drops of salt solutions were added on the central portion of an IR transparent silicon window. Water was allowed to evaporate under ambient conditions to form solid salts with uniform film thickness. This method minimizes the influence of the external matrix, such as KBr or nujol mull. An HI 9811 pH/EC/TDS meter (HANNA instruments) was used to measure the pH of the aqueous electrolytes. A pH buffer solution (HI 7010, at pH 7.01) was used for calibration. 2.3. Electrochemical characterizations The electrolytes were characterized using two types of working electrodes, Ti foil as metallic electrodes and CNT-graphite as EDLC electrodes, in either 3- or 2-electrode cell configurations. To prepare CNT-graphite electrodes, chemically cross-linked polyvinyl alcohol was used as binder and mixed with multi-wall carbon nanotubes and graphite powders in DI water [26]. The resultant CNT-graphite electrodes were composed of 22% CNT, 63% graphite, and 15% cross-linked polyvinyl alcohol (all in wt. %). The geometric area of the Ti and CNT-graphite electrodes was 1 cm2 . A 3-electrode cell was used to determine the potential stability window of the electrolytes. Pt mesh and Ag/AgCl were used as counter electrode and reference electrode, respectively. The ionic conductivity and the performance of EDLC devices were characterized by employing 2-electrode cell test vehicles, where either two Ti or two CNT-graphite electrodes were used.
Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were performed using a CHI 760 C bipotentiostat. The EIS spectra were recorded from 100 kHz to 0.1 Hz with 5 mV amplitude. Areal capacitance was calculated using charge (Q) divided by voltage window (U) and the geometric surface area of the electrode. The conductivity of the electrolytes was calculated using the cell constant and the equivalent series resistance (ESR), which was extracted from the EIS measurements. The reported conductivity was based on an average of at least five cells. All electrochemical tests were conducted at room temperature.
3. Results and discussion 3.1. Cation content and acidity of the silicotungstate salts The lithium, sodium, and potassium salts of SiWA were first analyzed for the content of alkali metal cations. The cation contents and degree of cation exchange of the salts are summarized in Table 1. The theoretical values were calculated based on fully exchanged reactions as shown in [reaction a]. All three salts showed a complete cation exchange as compared to the theoretical contents. Aqueous solutions from these salts were prepared in the above mentioned concentrations. The pH value of each salt solution was measured and compared to the original SiWA acid. Fig. 1 shows the pH values of the salts as well as the acid at the three concentrations. The pH of SiWA was in a range from 1.5 to 2.2, whereas the pH of the silicotungstate salts ranged between 5.7 and 7.3. The high pH of the salts is evidence of the complete replacement of the protons in H4 SiW12 O40 , resulting in neutral properties. The pH of the salts increased in the order of Li-SiW < Na-SiW < K-SiW. This is because Li+ has the highest polarizing power and demonstrates a certain extent of Lewis acidity in aqueous solutions. Conversely, K+ has the lowest polarizing power among the three salts and therefore K-SiW showed the highest pH.
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Fig. 2. (a) Schematic example of the reversible decomposition process of a Keggin anion at neutral conditions; and (b) IR spectra of SiWA and its alkali metal salts: Li-SiW, Na-SiW, and K-SiW.
Table 2 FTIR band positions and associated bonding information for SiWA, Li-SiW, Na-SiW, and K-SiW. Wavenumber
summarizes the wavenumbers of the bands with their associated bonding interactions for the four spectra. In Fig. 2b, all four IR spectra have similar structures. The spectra of crystallized solid salts on the silicon IR window resembled typical Keggin structures after the evaporation of water. In the 700 cm−1 to 1100 cm−1 region, the structure and interaction of heteropoly acids are well established as vibrational stretching modes of four types of oxygen: a center heteroatom with tetrahedral oxygen (Si-Oa ), edge sharing oxygen (W-Ob -W), corner sharing oxygen (W-Oc -W), and terminal oxygen (W = Od ). The tetrahedral oxygen connects the central heteroatom Si to a W. The two types of sharing oxygen atoms (Ob and Oc ) bridge two W atoms in neighboring octahedrals, whereas the terminal oxygen atom (Od ) is bonded to only one W atom. To determine the interactions between the alkali metal cations and the Keggin anion, the vibrations involving these oxygen atoms were investigated. The terminal oxygen groups (W = Od ) and the tetrahedral oxygen (Si-Oa ) in Fig. 2 did not show any significant shifts, suggesting only a limited influence of cations on these bonds. However the bands involving sharing oxygen atoms (Ob and Oc ) displayed a certain extent of displacement. All three salts showed a shift towards lower wavenumbers for both W-Oc -W stretching and W-Ob -W stretching when compared to the original SiWA acid. This may be due to the presence of dimeric anions in the silicotungstate salts which result in that sharing oxygen atoms are less restricted and more flexible to vibrate. Accordingly, less energy is required to stretch W-Oc -W and W-Ob -W bonds in the silicotungstate salts than in SiWA, where all neighboring octahedrals are connected by Ob and Oc . The similar IR spectra of all three salts compared to SiWA confirmed the presence of the primary Keggin structure in the synthesized salts.
Band assignment
SiWA
Li-SiW
Na-SiW
K-SiW
1634 1016 979 928 889 804
1634 1016 976 928 887 789
1617 1017 978 928 885 797
1614 1016 974 921 879 774
O-H bending of H2 O N/A W = Od stretching Si-Oa stretching W-Ob -W stretching W-Oc -W stretching
3.2. Primary Keggin structure of the silicotungstate salts FTIR analyses were performed to obtain structural, compositional, and bonding information of the silicotungstate salts. Fig. 2 shows a schematic example of the reversible decomposition process of a Keggin anion in neutral solutions and the FTIR spectra of the three salts together with SiWA as a baseline. Table 2
3.3. Ionic conductivity A test vehicle using two Ti electrodes was employed to measure the ionic conductivities of the neutral salt solutions at room temperature. Fig. 3a shows the ionic conductivities of Li-SiW, Na-SiW, K-SiW and SiWA as a function of three concentrations. As expected, SiWA acid showed the highest ionic conductivity across the tested concentration range. This is due to the fast hydrogen bond dynamics involving H+ in acidic conditions resulting from a small limiting ˚ [27]. Among the three silicohydrodynamic radius of H+ (0.28 A) tungstate salt solutions, the conductivity decreased in the order of K-SiW > Na-SiW > Li-SiW. Assuming similar transference numbers and anion mobility for the salts, the difference in ionic conductivity can be attributed to the different limiting hydrodynamic radii of K+ ˚ Na+ (1.84 A), ˚ and Li+ (2.38 A) ˚ [27]. (1.25 A),
Fig. 3. Comparison of (a) ionic conductivity of Li-SiW, Na-SiW, K-SiW, and SiWA in 0.01, 0.03, and 0.05 M concentrations; and (b) molar conductivity (at 0.05 M) of SiWA salts and other common salts with the same counter cations.
H. Gao et al. / Electrochimica Acta 138 (2014) 240–246
Fig. 3b depicts the molar ionic conductivity of the salts and compares it to that of other common salts such as LiCl and NaCl with the same cations. All silicotungstate salts showed higher molar conductivity, due to the larger amount of cations dissociated and the much higher anion mobility of the Keggin anion. It has been reported that the mono-valent anion of SiWA ¼[SiW12 O40 ]− demonstrates a high limiting molar conductivity of 0.0237 Sm2 mol−1 , much higher than that of Cl− (0.0076 Sm2 mol−1 ), ½[SO4 ]− (0.008 Sm2 mol−1 ), or NO3 − (0.0071 Sm2 mol−1 ) [28]. The high limiting molar conductivity of the Keggin anion is due to its un-solvate nature, in which its hydrodynamic radius is similar to its crystallographic radius [21,22]. The high ionic conductivity of the silicotungstate salt solutions can be explained analogously. Even with the decomposition of the primary Keggin structure and the removal of W = O units at neutral pH environments, the size of the dimeric anion, such as [SiW11 O39 ]8- , is relatively large and remains un-solvated, resulting in a much higher ion mobility.
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Fig. 4. Electrochemical stability window of 0.05 M aqueous Li-SiW, Na-SiW, and K-SiW electrolytes compared with SiWA (scan rate of 10 mVs−1 ).
3.4. Electrochemical stability potential window One of the main advantages of using aqueous neutral electrolyte for EDLCs over SiWA is their wider electrochemical stability window. The electrochemical properties of all three silicotungstate salts as well as SiWA were measured in a 3-electrode cell using Ti as the working electrode. The potential stability window of each electrolyte was determined by CV as shown in Fig. 4. All three salt solutions showed a wider stability window than SiWA in terms of hydrogen and oxygen revolution reactions. This significant extension in the stability window can be attributed to the neutral salt solutions, which require additional energy to evolve H2 and O2 . However, due to the presence of W6+ in the Keggin anion, the electrolytes became electrochemically active at negative potentials [29] and the negative limit of the potential windows is only slightly increased in salt solutions. Among the three silicotungstate salts,
the electrochemical stability window increased in the order of KSiW < Na-SiW < Li-SiW, in a good agreement with the strength of solvation for the counter cations in water [30]. The stronger the solvation of cations in water is, the more energy is required to break their bonds. Thus Li-SiW demonstrated the widest stability window among the three neutral electrolytes due to the strongest solvation of Li+ in water. 3.5. Electrochemical characterization of carbon EDLC Fig. 5 shows the CVs of the CNT-graphite electrode in a 3electrode cell for Li-SiW (Fig. 5a), Na-SiW (Fig. 5b), and K-SiW (Fig. 5c), respectively, as well as an overlaid CV (Fig. 5d) comparing the three electrolytes. All three electrolytes showed a stability window of ca. 1.5 V, wider than that of SiWA as expected. The
Fig. 5. Cyclic voltammograms of a CNT-graphite electrode in 0.05 M aqueous (a) Li-SiW, (b) Na-SiW, (c) K-SiW electrolytes, and (d) comparison of these electrolytes (scan rate = 100 mVs−1 ).
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Fig. 6. Cyclic voltammograms of EC cells with 2 CNT-graphite electrodes in 0.05 M aqueous (a) Li-SiW, (b) Na-SiW, (c) K-SiW electrolytes, and (d) comparison of these electrolytes at 1.5 V as well as SiWA at 1.3 V (scan rate = 100 mVs−1 ).
broad anodic peak observed at negative potentials is related to the redox reaction of the Keggin anion and its corresponding cathodic peak overlapping with the hydrogen evolution peak. The electrochemical activity of the Keggin anion and the potential of their redox peaks are highly dependent on their primary Keggin structure. Since the reduction potential of Keggin anions decreases with a decrease in the valence of the central heteroatom or with an increase in the negative charge of the Keggin anion, the following trend of reduction potential has been characterized: PW12 O40 3− > SiW12 O40 4− > BW12 O40 5− > CoW12 O40 6− > CuW12 O40 7− [23,31]. To further increase the stability window, investigations are underway to synthesize other Keggin anions with different central heteroatoms. Similar to the results in Fig. 4, among the three salts Li-SiW showed the widest stability window while K-SiW showed the narrowest. All three salt electrolytes reached their negative potential limits at between -0.2 to -0.3 V (vs. Ag/AgCl) and their positive potential limits at between 1.2 to 1.3 V (vs. Ag/AgCl), resulting in a total stability window of 1.4 to 1.5 V. In order to understand the effects of the different cations in the neutral electrolytes on cell capacitance, EDLC devices were constructed using two identical CNT-graphite electrodes. The opencircuit potential (OCP) of the carbon electrodes in these neutral electrolytes was about 0.45 V (vs. Ag/AgCl). The CVs of the cells in LiSiW, Na-SiW, and K-SiW with incremental cell voltage increase are shown in Fig. 6a-c. The cells had a capacitance of about 5 mFcm−2 , which is approximately 50% of that of a single electrode (Fig. 5). An increase in current density was observed with an increase in cell voltage. This distortion limited the maximum cell voltage to 1.5 V for all three EDLCs. This value agrees well with the results in the 3-electrode cell (Fig. 5) and confirmed the voltage window of the EDLC cells with neutral salts electrolytes, which is greater than that of SiWA electrolyte-enabled cell. Although the EDLC with the SiWA electrolyte could deliver more capacitance (ca. 6 mFcm−2 ),
the higher voltage window of the cells with Li-SiW, Na-SiW, and K-SiW could lead to a higher energy density. The capacitance of the EC cells at different scan rates is shown in Fig. 7. The capacitance decreased with the increase of scan rates for all electrolytes. At low scan rates, the cell with Li-SiW showed the highest capacitance while the cell with K-SiW appeared to be the lowest. This can be explained by the difference in ionic radius ˚ Na+ (0.95 A), ˚ and K+ (1.33 A) ˚ [27] as (without solvation) of Li+ (0.6 A), more ions can access the inside of the pores of carbon, where desolvation occurs [32]. A similar trend has been reported by Qu et al. in the Li2 SO4 , Na2 SO4 , and K2 SO4 electrolytes [10]. The trend in alkali metal ionic radius can be correlated with the trend in capacitance obtained at low scan rates. However, the difference in capacitance become less visible with an increase in scan rate, and eventually,
Fig. 7. Capacitance variation of EC cells with two CNT-graphite electrodes at different scan rates in 0.05 M aqueous Li-SiW, Na-SiW, and K-SiW electrolytes.
H. Gao et al. / Electrochimica Acta 138 (2014) 240–246
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Table 3 Extracted resistance and constant phase element values from EIS for EC cells with 2 CNT-graphite electrodes in 0.05 M aqueous Li-SiW, Na-SiW, and K-SiW electrolytes. Sample
Rel (ohmcm2 )
R1 (ohmcm2 )
Constant phase
Capacitance (mFcm−2 )
Chi-squared
Li-SiW Na-SiW K-SiW
13.58 10.81 8.39
9.5 11.0 11.6
0.91 0.91 0.91
3.154 3.020 2.845
0.0024 0.0022 0.0021
Fig. 8. Nyquist plots of EC cells with two CNT-graphite electrodes at the open-circuit voltage in 0.05 M aqueous Li-SiW, Na-SiW, and K-SiW electrolytes. The inset shows the fitted EIS data at the high frequency region as well as the equivalent circuit model.
Fig. 9. Cyclability of EC cells with two CNT-graphite electrodes in 0.05 M aqueous LiSiW, Na-SiW, and K-SiW electrolytes at 1.5 V and 1.6 V cell voltages (scan rate = 100 mVs−1 ).
the three devices showed very similar capacitance. This can be due to the charge separation process being more dominated by ionic diffusion in the electrolyte at high scan rates. EIS analysis was performed to complement the dc characterization and to confirm the trend in Fig. 3a and Fig. 7. The Nyquist plots of the capacitors at open-circuit voltage are shown in Fig. 8. The response of all three electrolytes is similar with a depressed semi-circle at high frequencies and a nearly vertical line at low frequencies. The extracted capacitance and resistance using the equivalent circuit model (inset) are summarized in Table 3. In the equivalent circuit model, Rel is the total high frequency series resistance; the first constant phase element (CPE1) describes the high frequency behavior of the cell; the parallel resistance R1 describes the diffusion resistance of electrolytic ions towards carbon electroldes; the second constant phase element (CPE2) represents the low frequency behavior of the EC cell; and the parallel resistance R2 represents a leakage resistance of the charge separation process. Since the electrodes and the cell setup are all identical, the difference in Rel was mainly due to the ionic conductivity of the electrolytes. The Rel increased in the order of K-SiW < Na-SiW < LiSiW, which agrees with the trend in their ionic conductivity shown in Fig. 3. In contrast, the diffusion resistance (R1) showed the opposite trend, decreasing in the order of K+ > Na+ > Li+ , because Li+ can penetrate into the carbon pores more easily than K+ due to its smaller ionic radius. Also shown in Table 3 are the capacitance and the constant phase values from CPE2 at low frequencies. Similar to the trend in diffusion resistance, the cell with Li-SiW demonstrated the highest capacitance whereas the cell with K-SiW showed the lowest, in good agreement with the low scan rate capacitance (Fig. 7). CPE1 and R2 are not discussed here because the high frequency capacitance is not useful for EDLC applications and R2 is an infinite large number, due to the highly capacitive response and low leakage of the EC cells. The capacitance estimated from EIS at open-circuit potential were smaller compared to the values obtained from CV. The higher apparent capacitance from CV is due to the gas evolution reaction or other pseudo-capacitive reactions on the electrode at higher voltage region.
A criterion for a good electrolyte for ECs is that it enables long term stability and cycle life. The cycling behavior of the capacitors using Li-SiW, Na-SiW, or K-SiW electrolyte is presented in Fig. 9. When a device (e,g, using Li-SiW) was cycled to 1.6 V, the capacitance dropped by ca. 50% over 5000 cycles. In contrast, when cycled to 1.5 V, all devices showed very stable performance in the neutral electrolytes, showing no significant reduction in capacitance. This observation confirms the voltage operating window of the electrolytes. High ionic conductivity, neutral pH, wide electrochemical stability window, as well as long cycle life render the silicotungstate salts an attractive electrolyte system for EDLCs with high energy density. 4. Conclusions In this work, lithium, sodium, and potassium salts of silicotungstic acid were synthesized and characterized as aqueous neutral electrolytes for EDLCs. All neutral salts retained their primary Keggin structure. These electrolytes exhibit high ionic conductivity due to their high ionic strength as well as the high anion mobility in the solution. The neutral electrolytes demonstrated an increase in their electrochemical stability window in terms of both hydrogen and oxygen evolutions. Carbon based EDLCs utilizing these neutral electrolytes were able to achieve a cell voltage of 1.5 V and showed excellent cycle life without significant reduction in capacitance. These silicotungstate salts are promising electrolytes for high energy density and power density EDLCs. Acknowledgments We appreciate the financial support from NSERC Canada. H. Gao would like to acknowledge an NSERC Alexander Graham Bell Canada Graduate Scholarship and a Hatch Graduate Scholarships for Sustainable Energy Research. A. Virya would like to acknowledge a University of Toronto Nanotechnology Network Summer Fellowship.
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Monovalent silicotungstate salts as electrolytes for electrochemical supercapacitors Han Gao, Alvin Virya, 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 14 May 2014 Received in revised form 22 June 2014 Accepted 23 June 2014 Available online 30 June 2014 Keywords: electrical double layer capacitor neutral aqueous electrolytes alkali cations heteropolyacid salts.
a b s t r a c t Lithium, sodium, and potassium salts of silicotungstic acid were synthesized and characterized as aqueous neutral electrolytes for electrochemical supercapacitors. The acidity of the aqueous solution and the structure of the solid-state anion were examined to confirm the presence of SiW salts. Ionic conductivity and the electrochemical stability potential window were characterized and compared to a silicotungstic acid solution using metallic blocking electrodes. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were used to investigate the performance of carbon EDLC cells enabled by the neutral electrolytes and revealed a 1.5 V cell voltage and good cycle life. The similarities and differences among the three salts are explained based on the properties of cations in these neutral electrolytes. © 2014 Elsevier Ltd. All rights reserved.
1. Introduction Electric double layer capacitors (EDLC) are electrochemical capacitors (EC) that have attracted significant attention as electrical energy storage devices [1,2]. In an EDLC, ions in the electrolyte are transferred between electrodes by migration and/or diffusion and energy is stored in the form of charge separation between the electrodes. Ionic conductivity and the stability of the electrolytes govern the power and energy density of the EDLC devices. The majority of commercial EDLCs are based on porous carbon electrodes in organic electrolytes [3,4]. While possessing a high voltage window, organic electrolytes generally are volatile, flammable, and moisture sensitive, requiring an oxygen-free environment for cell assembly. Alternatively, the most common aqueous electrolytes are H2 SO4 and KOH as they have high ionic conductivity. However, they tend to be corrosive and have a limited voltage window. Aqueous neutral electrolytes have been investigated for EC applications as they are non-corrosive, have good conductivity, and have high oxygen and hydrogen evolution overpotentials on carbon electrodes [5–8]. The high overpotential for oxygen and hydrogen evolution leads to an increase in cell voltage, which in turn can improve the energy density of the EDLC devices. A stability window of ca. 2.0 V could be achieved with a carbon electrode in Na2 SO4 , enabling a cell voltage of 1.6 V for a symmetric carbon/carbon EDLC with good cycle life [9]. Other aqueous electrolytes such as Li2 SO4
∗ Corresponding author. E-mail address: [email protected] (K. Lian). http://dx.doi.org/10.1016/j.electacta.2014.06.127 0013-4686/© 2014 Elsevier Ltd. All rights reserved.
and K2 SO4 have been reported for EDLCs [5,6,10,11]. In addition, neutral aqueous electrolytes have been explored in asymmetric ECs to expand the voltage window, using activated carbon and pseudocapacitive electrodes [12–16] or two pseudocapacitive electrodes [17,18]. Although currently studied neutral aqueous electrolytes show great promise to enhance the energy density of ECs, the device power performance is still limited at high charge/discharge rates due to their relatively low ionic conductivity at room temperatures. In our pervious study, Keggin-type heteropoly acids, such as silicotungstic acid (H4 SiW12 O40 , SiWA) and phosphotungstic acid (H3 PW12 O40 , PWA), were characterized as aqueous acidic electrolytes for ECs [19]. The generic formula of the anion is [Xn+ M12 O40 ](8-n)− where the central heteroatom X (e.g. P, Ge, Si, B, etc.) is bonded with four oxygen atoms that is surrounded by a cage of 12 MO6 units (where M is W, Mo, or V) linked to one another by the neighboring oxygen atoms [20]. This configuration is referred as primary Keggin structure. Both SiWA and PWA showed similar ionic conductivity and self-discharge to H2 SO4 [19]. Their high ionic conductivity resulted from (a) the high ionic strength of the electrolyte solutions due to the complete dissociation of ions and (b) the high mobility of the Keggin anions due to their un-solvated nature in aqueous systems [21,22]. In aqueous systems, Keggin anions can be transferred much faster than other anions, such as Cl− or SO4 2− . Therefore, it is of interest to investigate the salts of Keggintype heteropoly anions as aqueous neutral electrolytes for EDLCs to achieve a wider electrochemical stability window and higher ionic conductivity. The salts of Keggin-type heteropoly anions are commonly classified into two groups based on the size of cation: small cations like
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Li+ and large ones like Cs+ [23]. The first group shows high solubility in water while the salts in the second group are insoluble [20,24]. In this study, monovalent lithium, sodium, and potassium salts of silicotungstate are synthesized and characterized as aqueous neutral electrolytes for EDLCs. The ionic conductivity as well as the potential window of the neutral electrolytes are determined. Using multiwall carbon nanotube-graphite (referred as CNT-graphite) composite electrodes, the performance of single electrodes and carbon EDLC cells with these aqueous neutral salt electrolytes is examined and correlated to the electrolyte properties. In addition, the cycle life of EDLCs enabled by these neutral electrolytes was investigated to demonstrate their feasibility for high energy EDLCs.
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Table 1 Cation contents and degree of cation exchange in lithium, sodium, and potassium salts of silicotungstate. Sample
Cation content (gmL−1 )
Percentage of cation exchange
Li-SiW Na-SiW K-SiW
Theoretical 894.76 2952.27 4824.00
101% 105% 115%
ICP-AES 901.32 3088.45 5528.19
2. Experimental 2.1. Preparation of salts SiWA was purchased from Alfa Aesar and used as-received. The lithium, sodium, and potassium salts of SiWA (referred to as Li-SiW, Na-SiW, and K-SiW, respectively) were prepared following the methods by Tsigdinos [25]. For example, a predetermined amount of a 0.01 M lithium carbonate solution was slowly added to a 0.05 M SiWA solution (dropwise at room temperature with agitation), leading to the following reaction: Li2 CO3 + H4 SiW12 O40 → Li4 SiW12 O40 + 2H2 O + 2CO2 [reaction a]
Fig. 1. Comparison of pH of aqueous Li-SiW, Na-SiW, K-SiW, and SiWA electrolytes in 0.01, 0.03, and 0.05 M concentrations at room temperature.
Na-SiW and K-SiW were prepared in a similar way. All salts were obtained by evaporating water from the resulting solutions at 50 to 60 ◦ C. 2.2. Material characterizations Inductively coupled plasma atomic emission spectroscopy (ICPAES) (Perkin Elmer Optima 7300) was used to analyze the Li+ , Na+ , and K+ contents of the salts in aqueous solutions and the degrees of cation exchange. Infrared (IR) spectra were recorded on a Thermo Scientific Nicolet iS5 FT-IR spectrometer with iD1 transmission module at room temperature. A few drops of salt solutions were added on the central portion of an IR transparent silicon window. Water was allowed to evaporate under ambient conditions to form solid salts with uniform film thickness. This method minimizes the influence of the external matrix, such as KBr or nujol mull. An HI 9811 pH/EC/TDS meter (HANNA instruments) was used to measure the pH of the aqueous electrolytes. A pH buffer solution (HI 7010, at pH 7.01) was used for calibration. 2.3. Electrochemical characterizations The electrolytes were characterized using two types of working electrodes, Ti foil as metallic electrodes and CNT-graphite as EDLC electrodes, in either 3- or 2-electrode cell configurations. To prepare CNT-graphite electrodes, chemically cross-linked polyvinyl alcohol was used as binder and mixed with multi-wall carbon nanotubes and graphite powders in DI water [26]. The resultant CNT-graphite electrodes were composed of 22% CNT, 63% graphite, and 15% cross-linked polyvinyl alcohol (all in wt. %). The geometric area of the Ti and CNT-graphite electrodes was 1 cm2 . A 3-electrode cell was used to determine the potential stability window of the electrolytes. Pt mesh and Ag/AgCl were used as counter electrode and reference electrode, respectively. The ionic conductivity and the performance of EDLC devices were characterized by employing 2-electrode cell test vehicles, where either two Ti or two CNT-graphite electrodes were used.
Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were performed using a CHI 760 C bipotentiostat. The EIS spectra were recorded from 100 kHz to 0.1 Hz with 5 mV amplitude. Areal capacitance was calculated using charge (Q) divided by voltage window (U) and the geometric surface area of the electrode. The conductivity of the electrolytes was calculated using the cell constant and the equivalent series resistance (ESR), which was extracted from the EIS measurements. The reported conductivity was based on an average of at least five cells. All electrochemical tests were conducted at room temperature.
3. Results and discussion 3.1. Cation content and acidity of the silicotungstate salts The lithium, sodium, and potassium salts of SiWA were first analyzed for the content of alkali metal cations. The cation contents and degree of cation exchange of the salts are summarized in Table 1. The theoretical values were calculated based on fully exchanged reactions as shown in [reaction a]. All three salts showed a complete cation exchange as compared to the theoretical contents. Aqueous solutions from these salts were prepared in the above mentioned concentrations. The pH value of each salt solution was measured and compared to the original SiWA acid. Fig. 1 shows the pH values of the salts as well as the acid at the three concentrations. The pH of SiWA was in a range from 1.5 to 2.2, whereas the pH of the silicotungstate salts ranged between 5.7 and 7.3. The high pH of the salts is evidence of the complete replacement of the protons in H4 SiW12 O40 , resulting in neutral properties. The pH of the salts increased in the order of Li-SiW < Na-SiW < K-SiW. This is because Li+ has the highest polarizing power and demonstrates a certain extent of Lewis acidity in aqueous solutions. Conversely, K+ has the lowest polarizing power among the three salts and therefore K-SiW showed the highest pH.
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Fig. 2. (a) Schematic example of the reversible decomposition process of a Keggin anion at neutral conditions; and (b) IR spectra of SiWA and its alkali metal salts: Li-SiW, Na-SiW, and K-SiW.
Table 2 FTIR band positions and associated bonding information for SiWA, Li-SiW, Na-SiW, and K-SiW. Wavenumber
summarizes the wavenumbers of the bands with their associated bonding interactions for the four spectra. In Fig. 2b, all four IR spectra have similar structures. The spectra of crystallized solid salts on the silicon IR window resembled typical Keggin structures after the evaporation of water. In the 700 cm−1 to 1100 cm−1 region, the structure and interaction of heteropoly acids are well established as vibrational stretching modes of four types of oxygen: a center heteroatom with tetrahedral oxygen (Si-Oa ), edge sharing oxygen (W-Ob -W), corner sharing oxygen (W-Oc -W), and terminal oxygen (W = Od ). The tetrahedral oxygen connects the central heteroatom Si to a W. The two types of sharing oxygen atoms (Ob and Oc ) bridge two W atoms in neighboring octahedrals, whereas the terminal oxygen atom (Od ) is bonded to only one W atom. To determine the interactions between the alkali metal cations and the Keggin anion, the vibrations involving these oxygen atoms were investigated. The terminal oxygen groups (W = Od ) and the tetrahedral oxygen (Si-Oa ) in Fig. 2 did not show any significant shifts, suggesting only a limited influence of cations on these bonds. However the bands involving sharing oxygen atoms (Ob and Oc ) displayed a certain extent of displacement. All three salts showed a shift towards lower wavenumbers for both W-Oc -W stretching and W-Ob -W stretching when compared to the original SiWA acid. This may be due to the presence of dimeric anions in the silicotungstate salts which result in that sharing oxygen atoms are less restricted and more flexible to vibrate. Accordingly, less energy is required to stretch W-Oc -W and W-Ob -W bonds in the silicotungstate salts than in SiWA, where all neighboring octahedrals are connected by Ob and Oc . The similar IR spectra of all three salts compared to SiWA confirmed the presence of the primary Keggin structure in the synthesized salts.
Band assignment
SiWA
Li-SiW
Na-SiW
K-SiW
1634 1016 979 928 889 804
1634 1016 976 928 887 789
1617 1017 978 928 885 797
1614 1016 974 921 879 774
O-H bending of H2 O N/A W = Od stretching Si-Oa stretching W-Ob -W stretching W-Oc -W stretching
3.2. Primary Keggin structure of the silicotungstate salts FTIR analyses were performed to obtain structural, compositional, and bonding information of the silicotungstate salts. Fig. 2 shows a schematic example of the reversible decomposition process of a Keggin anion in neutral solutions and the FTIR spectra of the three salts together with SiWA as a baseline. Table 2
3.3. Ionic conductivity A test vehicle using two Ti electrodes was employed to measure the ionic conductivities of the neutral salt solutions at room temperature. Fig. 3a shows the ionic conductivities of Li-SiW, Na-SiW, K-SiW and SiWA as a function of three concentrations. As expected, SiWA acid showed the highest ionic conductivity across the tested concentration range. This is due to the fast hydrogen bond dynamics involving H+ in acidic conditions resulting from a small limiting ˚ [27]. Among the three silicohydrodynamic radius of H+ (0.28 A) tungstate salt solutions, the conductivity decreased in the order of K-SiW > Na-SiW > Li-SiW. Assuming similar transference numbers and anion mobility for the salts, the difference in ionic conductivity can be attributed to the different limiting hydrodynamic radii of K+ ˚ Na+ (1.84 A), ˚ and Li+ (2.38 A) ˚ [27]. (1.25 A),
Fig. 3. Comparison of (a) ionic conductivity of Li-SiW, Na-SiW, K-SiW, and SiWA in 0.01, 0.03, and 0.05 M concentrations; and (b) molar conductivity (at 0.05 M) of SiWA salts and other common salts with the same counter cations.
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Fig. 3b depicts the molar ionic conductivity of the salts and compares it to that of other common salts such as LiCl and NaCl with the same cations. All silicotungstate salts showed higher molar conductivity, due to the larger amount of cations dissociated and the much higher anion mobility of the Keggin anion. It has been reported that the mono-valent anion of SiWA ¼[SiW12 O40 ]− demonstrates a high limiting molar conductivity of 0.0237 Sm2 mol−1 , much higher than that of Cl− (0.0076 Sm2 mol−1 ), ½[SO4 ]− (0.008 Sm2 mol−1 ), or NO3 − (0.0071 Sm2 mol−1 ) [28]. The high limiting molar conductivity of the Keggin anion is due to its un-solvate nature, in which its hydrodynamic radius is similar to its crystallographic radius [21,22]. The high ionic conductivity of the silicotungstate salt solutions can be explained analogously. Even with the decomposition of the primary Keggin structure and the removal of W = O units at neutral pH environments, the size of the dimeric anion, such as [SiW11 O39 ]8- , is relatively large and remains un-solvated, resulting in a much higher ion mobility.
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Fig. 4. Electrochemical stability window of 0.05 M aqueous Li-SiW, Na-SiW, and K-SiW electrolytes compared with SiWA (scan rate of 10 mVs−1 ).
3.4. Electrochemical stability potential window One of the main advantages of using aqueous neutral electrolyte for EDLCs over SiWA is their wider electrochemical stability window. The electrochemical properties of all three silicotungstate salts as well as SiWA were measured in a 3-electrode cell using Ti as the working electrode. The potential stability window of each electrolyte was determined by CV as shown in Fig. 4. All three salt solutions showed a wider stability window than SiWA in terms of hydrogen and oxygen revolution reactions. This significant extension in the stability window can be attributed to the neutral salt solutions, which require additional energy to evolve H2 and O2 . However, due to the presence of W6+ in the Keggin anion, the electrolytes became electrochemically active at negative potentials [29] and the negative limit of the potential windows is only slightly increased in salt solutions. Among the three silicotungstate salts,
the electrochemical stability window increased in the order of KSiW < Na-SiW < Li-SiW, in a good agreement with the strength of solvation for the counter cations in water [30]. The stronger the solvation of cations in water is, the more energy is required to break their bonds. Thus Li-SiW demonstrated the widest stability window among the three neutral electrolytes due to the strongest solvation of Li+ in water. 3.5. Electrochemical characterization of carbon EDLC Fig. 5 shows the CVs of the CNT-graphite electrode in a 3electrode cell for Li-SiW (Fig. 5a), Na-SiW (Fig. 5b), and K-SiW (Fig. 5c), respectively, as well as an overlaid CV (Fig. 5d) comparing the three electrolytes. All three electrolytes showed a stability window of ca. 1.5 V, wider than that of SiWA as expected. The
Fig. 5. Cyclic voltammograms of a CNT-graphite electrode in 0.05 M aqueous (a) Li-SiW, (b) Na-SiW, (c) K-SiW electrolytes, and (d) comparison of these electrolytes (scan rate = 100 mVs−1 ).
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Fig. 6. Cyclic voltammograms of EC cells with 2 CNT-graphite electrodes in 0.05 M aqueous (a) Li-SiW, (b) Na-SiW, (c) K-SiW electrolytes, and (d) comparison of these electrolytes at 1.5 V as well as SiWA at 1.3 V (scan rate = 100 mVs−1 ).
broad anodic peak observed at negative potentials is related to the redox reaction of the Keggin anion and its corresponding cathodic peak overlapping with the hydrogen evolution peak. The electrochemical activity of the Keggin anion and the potential of their redox peaks are highly dependent on their primary Keggin structure. Since the reduction potential of Keggin anions decreases with a decrease in the valence of the central heteroatom or with an increase in the negative charge of the Keggin anion, the following trend of reduction potential has been characterized: PW12 O40 3− > SiW12 O40 4− > BW12 O40 5− > CoW12 O40 6− > CuW12 O40 7− [23,31]. To further increase the stability window, investigations are underway to synthesize other Keggin anions with different central heteroatoms. Similar to the results in Fig. 4, among the three salts Li-SiW showed the widest stability window while K-SiW showed the narrowest. All three salt electrolytes reached their negative potential limits at between -0.2 to -0.3 V (vs. Ag/AgCl) and their positive potential limits at between 1.2 to 1.3 V (vs. Ag/AgCl), resulting in a total stability window of 1.4 to 1.5 V. In order to understand the effects of the different cations in the neutral electrolytes on cell capacitance, EDLC devices were constructed using two identical CNT-graphite electrodes. The opencircuit potential (OCP) of the carbon electrodes in these neutral electrolytes was about 0.45 V (vs. Ag/AgCl). The CVs of the cells in LiSiW, Na-SiW, and K-SiW with incremental cell voltage increase are shown in Fig. 6a-c. The cells had a capacitance of about 5 mFcm−2 , which is approximately 50% of that of a single electrode (Fig. 5). An increase in current density was observed with an increase in cell voltage. This distortion limited the maximum cell voltage to 1.5 V for all three EDLCs. This value agrees well with the results in the 3-electrode cell (Fig. 5) and confirmed the voltage window of the EDLC cells with neutral salts electrolytes, which is greater than that of SiWA electrolyte-enabled cell. Although the EDLC with the SiWA electrolyte could deliver more capacitance (ca. 6 mFcm−2 ),
the higher voltage window of the cells with Li-SiW, Na-SiW, and K-SiW could lead to a higher energy density. The capacitance of the EC cells at different scan rates is shown in Fig. 7. The capacitance decreased with the increase of scan rates for all electrolytes. At low scan rates, the cell with Li-SiW showed the highest capacitance while the cell with K-SiW appeared to be the lowest. This can be explained by the difference in ionic radius ˚ Na+ (0.95 A), ˚ and K+ (1.33 A) ˚ [27] as (without solvation) of Li+ (0.6 A), more ions can access the inside of the pores of carbon, where desolvation occurs [32]. A similar trend has been reported by Qu et al. in the Li2 SO4 , Na2 SO4 , and K2 SO4 electrolytes [10]. The trend in alkali metal ionic radius can be correlated with the trend in capacitance obtained at low scan rates. However, the difference in capacitance become less visible with an increase in scan rate, and eventually,
Fig. 7. Capacitance variation of EC cells with two CNT-graphite electrodes at different scan rates in 0.05 M aqueous Li-SiW, Na-SiW, and K-SiW electrolytes.
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Table 3 Extracted resistance and constant phase element values from EIS for EC cells with 2 CNT-graphite electrodes in 0.05 M aqueous Li-SiW, Na-SiW, and K-SiW electrolytes. Sample
Rel (ohmcm2 )
R1 (ohmcm2 )
Constant phase
Capacitance (mFcm−2 )
Chi-squared
Li-SiW Na-SiW K-SiW
13.58 10.81 8.39
9.5 11.0 11.6
0.91 0.91 0.91
3.154 3.020 2.845
0.0024 0.0022 0.0021
Fig. 8. Nyquist plots of EC cells with two CNT-graphite electrodes at the open-circuit voltage in 0.05 M aqueous Li-SiW, Na-SiW, and K-SiW electrolytes. The inset shows the fitted EIS data at the high frequency region as well as the equivalent circuit model.
Fig. 9. Cyclability of EC cells with two CNT-graphite electrodes in 0.05 M aqueous LiSiW, Na-SiW, and K-SiW electrolytes at 1.5 V and 1.6 V cell voltages (scan rate = 100 mVs−1 ).
the three devices showed very similar capacitance. This can be due to the charge separation process being more dominated by ionic diffusion in the electrolyte at high scan rates. EIS analysis was performed to complement the dc characterization and to confirm the trend in Fig. 3a and Fig. 7. The Nyquist plots of the capacitors at open-circuit voltage are shown in Fig. 8. The response of all three electrolytes is similar with a depressed semi-circle at high frequencies and a nearly vertical line at low frequencies. The extracted capacitance and resistance using the equivalent circuit model (inset) are summarized in Table 3. In the equivalent circuit model, Rel is the total high frequency series resistance; the first constant phase element (CPE1) describes the high frequency behavior of the cell; the parallel resistance R1 describes the diffusion resistance of electrolytic ions towards carbon electroldes; the second constant phase element (CPE2) represents the low frequency behavior of the EC cell; and the parallel resistance R2 represents a leakage resistance of the charge separation process. Since the electrodes and the cell setup are all identical, the difference in Rel was mainly due to the ionic conductivity of the electrolytes. The Rel increased in the order of K-SiW < Na-SiW < LiSiW, which agrees with the trend in their ionic conductivity shown in Fig. 3. In contrast, the diffusion resistance (R1) showed the opposite trend, decreasing in the order of K+ > Na+ > Li+ , because Li+ can penetrate into the carbon pores more easily than K+ due to its smaller ionic radius. Also shown in Table 3 are the capacitance and the constant phase values from CPE2 at low frequencies. Similar to the trend in diffusion resistance, the cell with Li-SiW demonstrated the highest capacitance whereas the cell with K-SiW showed the lowest, in good agreement with the low scan rate capacitance (Fig. 7). CPE1 and R2 are not discussed here because the high frequency capacitance is not useful for EDLC applications and R2 is an infinite large number, due to the highly capacitive response and low leakage of the EC cells. The capacitance estimated from EIS at open-circuit potential were smaller compared to the values obtained from CV. The higher apparent capacitance from CV is due to the gas evolution reaction or other pseudo-capacitive reactions on the electrode at higher voltage region.
A criterion for a good electrolyte for ECs is that it enables long term stability and cycle life. The cycling behavior of the capacitors using Li-SiW, Na-SiW, or K-SiW electrolyte is presented in Fig. 9. When a device (e,g, using Li-SiW) was cycled to 1.6 V, the capacitance dropped by ca. 50% over 5000 cycles. In contrast, when cycled to 1.5 V, all devices showed very stable performance in the neutral electrolytes, showing no significant reduction in capacitance. This observation confirms the voltage operating window of the electrolytes. High ionic conductivity, neutral pH, wide electrochemical stability window, as well as long cycle life render the silicotungstate salts an attractive electrolyte system for EDLCs with high energy density. 4. Conclusions In this work, lithium, sodium, and potassium salts of silicotungstic acid were synthesized and characterized as aqueous neutral electrolytes for EDLCs. All neutral salts retained their primary Keggin structure. These electrolytes exhibit high ionic conductivity due to their high ionic strength as well as the high anion mobility in the solution. The neutral electrolytes demonstrated an increase in their electrochemical stability window in terms of both hydrogen and oxygen evolutions. Carbon based EDLCs utilizing these neutral electrolytes were able to achieve a cell voltage of 1.5 V and showed excellent cycle life without significant reduction in capacitance. These silicotungstate salts are promising electrolytes for high energy density and power density EDLCs. Acknowledgments We appreciate the financial support from NSERC Canada. H. Gao would like to acknowledge an NSERC Alexander Graham Bell Canada Graduate Scholarship and a Hatch Graduate Scholarships for Sustainable Energy Research. A. Virya would like to acknowledge a University of Toronto Nanotechnology Network Summer Fellowship.
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