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Electrochimica Acta 56 (2010) 122–127

Contents lists available at ScienceDirect

Electrochimica Acta

journa l homepage: www.e lsev ier .com/ locate /e lec tac ta

haracterizations of proton conducting polymer electrolytes for electrochemicalapacitors

an Gao, Keryn Lian ∗

epartment of Materials Science and Engineering, University of Toronto, 184 College St. Toronto, Toronto, Ont., Canada M5S 3E4

r t i c l e i n f o

rticle history:eceived 6 July 2010eceived in revised form2 September 2010ccepted 13 September 2010vailable online 17 September 2010

a b s t r a c t

Solid polymer electrolytes containing phosphotungstic acid (PWA) and/or silicotungstic acid (SiWA) inpolyvinyl alcohol (PVA) were investigated for their proton conductivities. Enhanced conductivity wasobtained when mixing PWA and SiWA at equal ratio. This polymer electrolyte was found viable forelectrochemical capacitors. Thermal and structural analyses were conducted with DSC, XRD, and FTIR. Thepolymer electrolyte exhibited a different structure and different thermal properties from its respective

eywords:olid polymer electrolyteeteropoly acidlectrochemical capacitorSC

components. The polymer electrolyte retained its original Keggin structure but contained crystallizedprotonated water in the form of H5O2

+. The protonated water may contribute to the proton conductivityof the polymer electrolyte.

© 2010 Elsevier Ltd. All rights reserved.

RDTIR

. Introduction

Solid-polymer electrolytes are important enabling materials fornergy storage devices that require high performance with thinnd flexible form factors, such as batteries and electrochemicalapacitors (EC). Due to their solid-state nature, they can greatlyeduce packaging and sealing of a device, which can further increasenergy and power density. However, the conductivity of polymerlectrolytes is typically a few orders of magnitude lower than thatf their liquid counterparts. Moreover, the ionic conductivity ofany proton conducting polymer electrolytes such as Nafion and

ts derivatives depends strongly on relative humidity (RH) and tem-erature [1–3], which is not desirable for electrochemical energytorage devices that mostly operate in an ambient environment.

A promising low cost proton conductor is Keggin type het-ropoly acid (HPA), for which conductivity as high as 0.18 S/cmas been demonstrated in its solid state [4]. A Keggin structureonsists of a central heteroatom (X = P, Si, Ge, B etc.) bonded withour oxygen to form a tetrahedron that is surrounded by a cage

f 12 octahedral MO6 units (M = W or Mo) linked to one anothery the neighboring oxygen atoms [5–7]; its generic formula isXn+M12O40](8−n)− [7,8]. As a crystalline powder, HPA cannot eas-ly form a film and the earlier conductivity studies therefore relied

∗ Corresponding author. Tel.: +1 416 978 8631.E-mail address: keryn.lian@utoronto.ca (K. Lian).

013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved.oi:10.1016/j.electacta.2010.09.036

on HPA pressed into pellets. Efforts have been made to immobilizeHPA into a thin film matrix such as SiO2 [9] or polymers [1,10–13]to form proton-conducting composites for fuel cell applications.

Solid electrolytes using poly(vinyl alcohol) (PVA) and HPA havebeen demonstrated for solid-state EC and showed promising con-ductivities and stability [14–16]. In previous studies, we havedemonstrated that PVA and silicotungstic acid (SiWA) can forma solid film with a conductivity of 0.01 S/cm, and we have imple-mented these as electrolytes in an EC [14,15]. We further optimizedconductivity by adjusting the ratio of PWA and SiWA in the PVAmatrix and obtained a conductivity of around 0.013 S/cm, exceed-ing that of pure PVA–PWA or PVA–SiWA solid electrolytes [16]. Theobjectives of the current study are: (a) to characterize the struc-tural and thermal behavior of this electrolyte and (b) to develop anunderstanding of the factors contributing to the proton conductiv-ity in this solid polymer electrolyte.

2. Experimental

Two precursors were prepared by mixing a PVA solution (PVA,Aldrich MW = 125,000) with a heteropoly acid solution, eitherH4SiW12O40·xH2O (SiWA, Alfa Aesar) or H3PW12O40·xH2O (PWA,

Alfa Aesar). The precursor solutions were composed of 32.5% SiWAor PWA, 1.5% PVA, and 66% de-ionized water (all in wt.%). ThePVA–PWA and PVA–SiWA precursors were combined in equal vol-umes for a mixed polymer electrolyte (we henceforth refer to thismixture as PVA–Mix).

H. Gao, K. Lian / Electrochimica Acta 56 (2010) 122–127 123

Table 1Materials and conditions for DSC, XRD and FTIR analyses.

Analysis PVA PWA SiWA PVA–Mix

Powder (as received) Cast thin filmPowder (as received) Cast thin film followed by hot pressingPellets with KBr Cast thin film followed by hot pressing

fMacriHafiefr00

maAwiea

f11aiP6ip

3

3

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0.002

0.006

0.010

0.014

0.018

50 10 15 20 25 30 35

Days

Ave

rag

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nd

uct

ivit

y (S

cm

-1)

30%

40%

50%

60%

70%

80%

90%

100%

Relative H

um

idity

SiWA PWA MIX Relative Humidity

were still quite rectangular and showed a capacitance of 50 mF/cmin the cell, which was more than 70% of the capacitance for anequivalent cell in liquid electrolyte [17]. This suggests that the elec-trolyte is viable for high rate capacitive devices. Moreover, the twovoltammograms in Fig. 2 were almost overlapping, suggesting an

-0.03

-0.02

-0.01

0

0.01

0.02

0.03

Cur

rent

(A

)

Cycle No.100 Cycle No.5000

DSC Cast thin film Powder (as received)XRD Cast thin film Powder (as received)FTIR Cast thin film Pellets with KBr

The electrodes were either stainless steel foils or RuO2 on Tioils, both 50 �m thick. The stainless steel foils were obtained from

cMaster-Carr, and degreased and cleaned with acetone, ethanol,nd de-ionized water before being used as electrodes. The pro-ess of manufacturing RuO2 electrodes was described in a previouseport [17]. The loading of RuO2 was about 1.5 mg/cm2, result-ng in 150–170 mF/cm2 capacitance for a single electrode in 1 M

2SO4 electrolyte. The solid-state EC devices were then fabricateds reported in detail in [14,15]: Both sides of the electrodes wererst coated with the precursor solution and dried in air. The coatedlectrodes were then laminated at 12.5 psi and 90 ◦C for 30 min toorm an EC cell. The thickness of the polymer electrolyte was in theange of 0.04–0.06 mm. Since the electrode thickness was between.05 and 0.07 mm, the total thickness of each single cell was about.14–0.20 mm. All solid-state EC cells had an area of 0.8 cm2.

The solid-state EC cells were characterized using cyclic voltam-etry (CV) and electrochemical impedance spectroscopy (EIS). CV

nd EIS were performed on a CHI 760 D bipotentiostat with EIS.ll electrochemical tests were conducted in ambient temperature,hich was around 25 ◦C. The ambient RH was also monitored dur-

ng the EIS measurements. The reported conductivity data for eachlectrolyte composition was based on the average conductivity oft least five single-cell devices measured by EIS.

Differential scanning calorimetry (DSC) analyses were per-ormed on a DSC Q2000 thermal analyzer, with a scan rate of0 ◦C/min in nitrogen purged cell over a temperature range from0 ◦C to 150 ◦C. The X-ray diffraction analyses were performed onSiemens D5000 �/2� diffractometer with a Cu K� source operat-

ng at 50 kV, 35 mA. The infrared (IR) spectra were recorded on aerkin Elmer’s 1000 FT/IR spectrometer in the wavenumber range00–4000 cm−1. The material conditions for these tests are listed

n Table 1. The electrolyte films were “hot-pressed” to mimic therocess conditions during EC cell assembly.

. Results and discussion

.1. Electrochemical characterization

In a previous study, an investigation into the effect of the aque-us PWA/SiWA ratio on proton conductivity was performed. Weound that the proton conductivity of the mixed aqueous electrolyteas higher than that of the individual solutions at the same con-

entration [16]. The maximum was reached at a 1:1 ratio leadings to rely on equal proportions of PWA and SiWA when prepar-

ng polymer electrolytes. The proton conductivities of the polymerlectrolytes for PVA–PWA, PVA–SiWA, and PVA–Mix were mea-ured with EIS and monitored over time as shown in Fig. 1. Inhis figure, we see that PVA–SiWA had a higher conductivity thanVA–PWA. This result agrees with results previously reported bytaiti [12] and Stangar et al. [13], who attributed the higher con-uctivity of PVA–SiWA to the additional proton per molecule iniWA. The conductivity of PVA–Mix was 0.013 S/cm, higher thanhat of either PVA–PWA or PVA–SiWA.

The RH at each measurement was tracked and plotted togetherith the proton conductivity of each solid polymer electrolyte, as

hown in Fig. 1. The conductivity of these polymer electrolyteshowed little dependence on the RH. In spite of the fluctuationsn RH between 30 and 70%, the conductivity values of each mate-

Fig. 1. Proton conductivity of solid polymer electrolytes based on PWA, SiWA, and a1:1 mixture of PWA and SiWA (PVA–Mix), demonstrating stability of the materialsover time and at various levels of relative humidity.

rial remained relatively stable and did not respond to the changein RH. Proton conductivity of the PVA–HPA based polymer elec-trolyte was more stable than for other polymer electrolytes such asNafion®.

Since the intended application of these polymer electrolytes isfor thin film EC and therefore require a long cycle life, the polymerelectrolyte-based EC was subjected to cycle life tests. We assem-bled a symmetric pseudocapacitor with PVA–Mix as the electrolytesandwiched in between two RuO2/TiO2 electrodes, forming a solidcell with a thickness of 0.2 mm. In this case, the polymer elec-trolyte not only acted as proton conductor, but also facilitated theoxidation and reduction reactions of the electrodes. Fig. 2 showsthe voltammograms obtained at the 100th and 5000th cycles fromthis cell. At a voltage scan rate as high as 500 mV/s, the CV profiles

2

0 0.2 0.4 0.6 0.8 1

Cell Voltage (V)

Fig. 2. CVs of a RuO2 based solid-state electrochemical thin film capacitor cell with.PVA–Mix polymer electrolyte based on a 1:1 mixture of PWA and SiWA, at the 100thand 5000th cycle (sweep rate 0.5 V/s). Area = 0.8 cm2.

124 H. Gao, K. Lian / Electrochimica Acta 56 (2010) 122–127

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3

peiTb[Pe

Fig. 3. DSC thermograms of PVA, PWA, SiWA, and PVA–Mix.

xcellent cycle life of the electrolyte and the capacitor system. Goodnd stable proton conductivity as well as the long cycle life makesVA–HPA solid polymer electrolytes attractive for electrochemicalapacitors.

Solid-state proton conductivity was 0.18 S/cm for PWA whensed in the form of pellets [4], where PWA contained 29 waterolecules in its structure. When immobilized in the PVA matrix,

WA and SiWA can be processed into a thin film thus avoiding thebvious drawbacks of pellets for applications as solid electrolytes.owever, the addition of the polymer reduces the conductivity ofWA and SiWA from their pure state. As we achieved a conductivityreater than 0.01 S/cm for PVA–Mix, this solid polymer electrolyteould still be effective for EC when processed via casting into thinlm followed by hot pressing [14–16]. Since such film is much thin-er than pellets, the effective resistance is relatively small, which

s evident in the high rate CV in Fig. 2.

.2. DSC characterization

Further studies were focused on the characterizations of theolymer electrolyte to understand its structural and thermal prop-rties. The DSC thermograms for PVA–Mix as well as for itsndividual components (PVA, PWA, and SiWA) are shown in Fig. 3.he glass transition temperature (Tg) of pure PVA was found to

e around 84 ◦C in agreement with the findings of Wilkes et al.18]. At higher temperatures, there was one endorthermic peak forWA but a split peak for SiWA. In the case of PVA–Mix, two clearndorthermic peaks were observed.

Fig. 4. XRD patterns of PVA, PWA, SiWA, and PVA–Mix.

All four samples showed a continued decrease of water contentin the early phase of the temperature scan. It is known that watermolecules can exist in HPA solids as adsorbed water, loosely bondedwater, and crystallized protonated water [6,9]. The departure ofany water molecules requires the system to absorb energy. There-fore, each endothermic peak in Fig. 3 may be interpreted as a phasetransition or as the escape of certain form of water. Table 2 lists themajor peaks and the possible causes of these peaks for PVA–Mixand each of its components. As Table 2 shows, the complete releaseof crystallized protonated water from PVA–Mix required a highertemperature (122 ◦C) compared to PWA (78 ◦C) and SiWA (106 ◦C).In other words, the crystallized water in the PVA matrix is more sta-ble than in the pure solid forms (PWA or SiWA salts). Table 2 furtherindicates that under the processing conditions, the polymer elec-trolyte hot pressed at 90 ◦C should retain some level of crystallizedwater.

3.3. XRD characterization

The XRD patterns of PVA–Mix and its components are shown inFig. 4. The three strongest peaks for each component are labeled,when applicable. The PVA spectrum showed a broad peak at 19.60◦

suggesting an amorphous phase. The XRD patterns for as-receivedpowder PWA and SiWA matched that of hydrated PWA and SiWAreported by Jalil et al. [19] and Sun et al. [20]. Moreover, the XRD

patterns of PWA and SiWA are very close and indicative of the samecharacteristic Keggin structure. In addition, the XRD patterns of thepowder PWA and SiWA showed very narrow peaks, suggesting along-range order crystal structure.

H. Gao, K. Lian / Electrochimica Acta 56 (2010) 122–127 125

Table 2DSC peaks and potential mechanism for polymer electrolyte (PVA–Mix) and its components.

Material Mechanism

Adsorbed water loss Bonded water loss Crystallized water loss Glass transition

◦ ◦

P(op[wcwiibtmatvoPaocp

PVA 44 C –PWA – –SiWA 72 ◦C 92 ◦CPVA–Mix – 84 ◦C

However, a totally different XRD pattern was obtained fromVA–Mix, exhibiting two notable features: (1) a shift of peaks and2) a broadening of the peaks. Similar XRD patterns were alsobtained from PVA–PWA and PVA–SiWA (not shown here). XRDatterns for PWA and SiWA have been discussed in the literature7,12,21]. We had expected that the XRD of the polymer electrolyteould be related to its individual components, but this was not the

ase. As the DSC results in Table 2 show, several different types ofater existed in PWA, SiWA, and the polymer electrolyte. Thus, it

s likely that the XRD patterns reflect a different degree and bond-ng in hydration. According to Mioc et al. [7] and Sun et al. [20],oth PWA and SiWA show a shift in peaks in XRD patterns duringhe dehydration process. Similar XRD patterns, corresponding to 6

olecules of crystallized water per PWA or SiWA molecule, werelso reported by Staiti et al. [9]. Comparing the reported XRD spec-ra [7,9,12,21] with the spectrum of PVA–Mix, the latter matchesery well with the patterns of PWA·6H2O and SiWA·6H2O. Basedn the results from XRD and DSC, it is likely that the HPA in the

VA matrix have lost some loosely bound water and some but notll crystallized water. The broadening of peaks in the XRD patternsf PVA–Mix indicates that the polymer electrolyte contained HPArystalline structures which were finely dispersed in the amor-hous PVA matrix.

Fig. 5. FTIR spectra of PVA, PW

– 84 C78 ◦C –106 ◦C –84 ◦C, 122 ◦C –

3.4. FTIR characterization

FTIR analyses were performed to obtain structural, com-positional, and bonding information concerning the polymerelectrolyte. Similar to the DSC and XRD studies, PVA–Mix wasinvestigated along with PWA, SiWA, and PVA. Fig. 5 shows theFTIR spectra of individual components and the polymer electrolyte.Table 3 summarizes the wavenumbers of the significant bands withtheir associated bonding interactions for pure PVA, PWA, SiWA, andPVA–Mix.

The PVA film has a similar spectrum as reported in the literature[22,23]. For pure PWA and SiWA, it can be observed that most bandsappeared in the 600–2000 cm−1 region with only a few bands inbetween 2000 and 4000 cm−1. Examining the FTIR spectra of PWAand SiWA in Fig. 5, the primary structures in the 600–1200 cm−1

region represented the characteristic structures of the Keggin unitas reported in the literature [5,6,8,10,11,13,19,24,25]. The struc-ture and interaction of PWA and SiWA in this frequency region

have been well studied and have been described as vibrationalstretching modes of edge sharing oxygen (W–Ob–W), corner shar-ing (W–Oc–W), a center heteroatom with tetrahedral oxygen (P–O,or Si–O), and terminal groups (W O) [5,6,8,10,11,13,19,24,25].

A, SiWA, and PVA–Mix.

126 H. Gao, K. Lian / Electrochimica Acta 56 (2010) 122–127

Table 3FTIR band positions and associated bonding information for polymer electrolyte (PVA–Mix) and its components.

Wavenumbers (cm−1) Band assignments Reference

PVA film PWA powder SiWA powder PVA–Mix film

805 810 744, 808 W–Oc–W stretching [5,8,13,24,25]857 – – 841 C–H rocking [22]– 892 885 901 W–Ob–W stretching [5,8,13,24,25]– 983 942, 980 973, 998 W O stretching [5,8,13,24,25]– – 1020 1027 Si–O–W stretching [13,24]– 1081 – 1075 P–O–W stretching [5,8,13,25]1103 1183 C–O stretching [22,26]

– – 1267 O–H stretching in H5O2+ [28,29]

– – 1375 1375 O–H bending in protonated H5O2+ [29,30]

1455 – – vibrations of–C–H [26,27]– – 1473 1456 H–O–H bending in protonated H5O2

+ [25,28,29,30]1675 1617 1607 – H–O–H bending in loosely bonded water [25,28,29,30]

1740 1734 1734 H–O–H bending in protonated H5O2+ [25,28,29,30]

2929–2948 – – – C–H stretching [23,26,27]– – 2930 2931 a

3306 – – – O–H stretching of PVA [23,26,27]– 3419 3456 – O–H stretching of HPA [5,6,8,13,25]

ing tht 7], weS A–Mixc

ifatsnaPdp

1Fcsaws

eaaPtctbtaibps1tStPrrb

a Similar as in the literature [10,11,30], an unresolved question remains concernhis peak coincides with C–H bond stretching of alkyl group for polymers [23,26,2iWA, as SiWA does not contain carbon. This explanation is also questionable for PVontent of PVA, also observed in XRD.

Comparing the FTIR spectrum of PVA–Mix with the spectra ofts individual components (Fig. 5 and Table 3), almost all bandsor PVA–Mix can be related to its individual components; nodditional bands are observed, indicating that the Keggin struc-ure/composition had been retained in the polymer electrolyte. Thisuggestion is also supported by the results from XRD, where the sig-al from PVA–Mix was similar to that of Keggin-type PWA·6H2Ond SiWA·6H2O [7,12,21]. Furthermore, at higher wavenumbers,VA–Mix was very similar to SiWA. In addition, some band shiftingue to the partial dehydration [8,13,19] has been observed in therocessed polymer electrolyte (Table 3).

For the secondary structures ranging from wavenumber200 cm−1 to 4000 cm−1, only a few peaks were observed (Fig. 5).or PVA film, the main peaks were at 1455, 2948, and 3306 cm−1,orresponding to –CH2– vibration, C–H stretching, and O–Htretching, respectively [23,26,27]. The band around 3500 cm−1 isstrong indication of the presence of adsorbed or loosely bondedater molecules [8,25]. Similar bands were seen in PWA and SiWA,

uggesting that they both contained bonded water as shown in XRD.However, no peak was seen around 3500 cm−1 for the PVA–Mix

lectrolyte, which implies the absence of loosely bonded waterfter the film processing procedures. Instead, there was a peakt 1740 cm−1 together with a satellite peak at 1456 cm−1 forVA–Mix as well as for SiWA. These peaks could be due to pro-onated water in the form of hydrated protons, H5O2

+ (or Zundelation) [6,8,13,25,28–30]. It was proposed by Zecchina et al. [28]hat the IR spectra of H+(H2O)n (n = 1,2,. . .) species can alwayse characterized by two main fingerprint features: (1) an absorp-ion band in the 1650–1750 cm−1 range often accompanied bypartner at 1450 cm−1, and (2) a strong “continuum” extending

nto the 1300 cm−1–3000 cm−1 interval. Indeed, such absorptionand was observed at 1734 cm−1 together with an absorptioneak at 1456 cm−1 in both spectra for PVA–Mix and SiWA. Theseharp bands together with relatively continuous profiles between500 cm−1 and 3000 cm−1 strongly suggest the presence of pro-onated water in the PVA–Mix polymer electrolyte as well as iniWA. The H5O2

+ species are believed to have a high mobility and

hus result in high proton conductivity [25]. That the spectrum ofVA–Mix was very similar to that of SiWA is also consistent with theesults from the thermograms in DSC, where the temperature foremoving protonated water was the highest in PVA–Mix followedy SiWA and then PWA. This reflects the order of the thermal stabil-

e absorption peak observed for PVA, SiWA, and PVA–Mix at 2930 cm−1. Althoughbelieve such is the case for PVA only. C–H stretching cannot explain the peak for, due to its profile being similar to pure SiWA. Moreover, PVA–Mix has only a small

ity of H5O2+ in these three solids, where H5O2

+ is more stable whencrystallized in PVA–Mix than in SiWA or PWA salts. After process-ing the film with a 1:1 ratio of PWA and SiWA in PVA, the resultingfilm appeared to behave more like SiWA than PWA.

Based on activation energy analysis, we earlier had argued thatproton hopping was the mechanism for the PVA–HPA polymerelectrolyte [16]. The presence of protonated water in the HPA struc-ture can form a network with the PWA and SiWA molecules [5,6],which can facilitate the hopping of protons. From both XRD andDSC studies, it is clear that PWA/SiWA in the PVA matrix waspartially hydrated with crystallized water. The crystallized watershowed higher thermal stability than SiWA or PWA according toDSC. Moreover, the XRD results suggest the presence of a long-range crystalline structure in the PVA–Mix polymer electrolyte.Thus, proton hopping will be relatively easier in this polymer elec-trolyte when compared to polymer electrolytes with an amorphousstructure. The FTIR results suggested that the crystallized water islikely protonated with a structure of H5O2

+ and forms a networkwith PWA and SiWA molecules. The crystallized water moleculescould facilitate the proton hopping and thus lead to a high conduc-tivity. Generally, it is desirable to have a high degree of hydration.However, among the three types of water in Table 1, crystallizedH5O2

+ species in solid electrolyte is more desirable to form a stableproton conducting electrolyte for EC applications.

4. Conclusions

In this paper, we have demonstrated a PVA–HPA solid polymerelectrolyte for EC based on a mixture of PWA and SiWA (PVA–Mix),which showed very good proton conductivity (0.013 S/cm) and sta-bility at ambient temperature and RH. The mixture of PWA andSiWA exhibited higher conductivity than its individual compo-nents at equivalent concentration. Thermal analyses showed thatthe PVA–Mix polymer electrolyte retained some crystallized water.This was further supported by the result from XRD, which showedthat the polymer electrolyte had a partially hydrated crystalline

structure composed of a mixture of PWA·6H2O and SiWA·6H2O.FTIR analyses suggested the crystallized water in PVA–Mix is in theform of protonated H5O2

+ species. The crystallized H5O2+ species

could facilitate proton mobility and thus resulted in a higher con-ductivity of the polymer electrolyte.

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H. Gao, K. Lian / Electroch

cknowledgment

We would like to acknowledge NSERC Canada for the financialupport as well as a summer fellowship (H. Gao).

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