electrode

Ekctroch#mlca Acta, Vol 35. No 6, pp 1031-1034 I!490 Pnnted m Great Bratan

0013-4686/905300+000 Q 1990 PeQ!amon Pnrr plc

IMPEDANCE AND SIMULATION STUDIES OF THE CELL ELECTRODE/HYDROUS ANTIMONIC ACID/ELECTRODE G E BAJARS,* P LINHARDT and M W BREITER Instltutfur Techrusche Elektrochemle, TU Wlen, 9 Getreldemarkt,

1060 Wlen, Austria

(Recemed1 June 1989,in rewed form 29 August 1989) Abstract-Measurements of the Impedance of the cells Pt/HAA/Pt and C/HAA/C were carned out at temperatures between - 80 and 40°C m a high frequency range by a Hewlett Packard Instrument and m a low frequency range by a Zahner-elektnk Instrument The sohd electrolyte had the cornposItIon HSbO, 1 1H,O The bulk resistance of the sobd electrolyte IS mamly determmed by the gram boundary resistance The frequency slmulatlon by a simple analogue clrcult works fairly well The bulk reststance can be determmed m a good approxlmatlon by graph& extrapolation or by the slmulatlon procedure The values of the other elements of the analogue clrcult depend upon the electrode matenal and the frequency range used m the slmulatlon An mterpretatlon of these elements IS dBicult

INTRODUCTION Hydrous antlmomc acid (HAA) belongs to the sohd electrolytes which display a relatively good proton conductlon[l, 21 The conductlon depends largely upon the water content of the aad[l-61 In contrast, the nature of the samples (polycrystallme or amorphous) beems to have little influence Impedance measurements of the cell El/HAA/El with El = platinum or.graphlte were carned out here at constant temperatures between -80 and 40°C m a large frequency range because the system appears suitable for an evaluation of the mterpretablhty of the parameters for the different elements m analogue ctrcmts for the simulation of the frequency dependence of the expenmental data Various models and eqmvalent clrcmts were proposedC7, 83 It was found[9, lo] that a relatrvely simple clrcmt, used mltlally for another system, allows a satisfactory slmulatlon It 1s investigated now if this analogue clrcult only yields a phenomenologlcal descnptlon or If the elements m it can be gven a phyclal interpretation A solid electrolyte of the composltlon HSbO, 1 1 H,O was chosen m this study because most of the water 1s present as crystal water[ 11)

EXPERIMENTAL Powders of the said composltlon were produced by the hydrolysis of SbC15 m distilled water[l l] The prectpltate was washed and dned at room temperature Disks with a diameter of 1 cm and a thickness of 02fO03cm were hydrostatically pressed at 20 kNcm-* The composltron of the samples was determined from the wetght loss after heatmg them to 1ooo”c Platmum electrodes were sputtered onto both flat surfaces on some of the samples Graphite from a rod

*On leave from the Institute of Solid State Physics, Latvian State Umverslty, Rlga, U S S R

was put on some other samples as electrodes by hght rubbmg Sdver pamt was used as contact material between the electrodes and the leads of the Impedance meter The cells were kept mslde a cryostat made of stainless steel and cooled by means of hqurd mtrogen m a Dewar contamer A disk of Al,O, and the cell were placed on top of a thick copper rod located m the axls of the vessel An insulated heating cod around the copper rod allowed adjustment of test temperature, using a temperature controller The copper rod passed through the bottom of the stainless steel vessel It protruded mto the &war vessel This portion of the copper rod had to be wrapped by Teflon tape to reduce the rate of heat transfer to an acceptable level, imposed by the maxlmum DC current whrch could pass through the heating cod The stamless steel vessel was flushed with nitrogen at a low rate The impedance measurements were made from low to high temperature The IM SE automated Impedance meter of Zahner-electnc (F R G ) was employed m the frequency range 1O-2-1O4 Hz The measurements were also made by the Hewlett Packard 4192A LF Impedance Analyzer m conJunctIon with an XT computer m the frequency range between 5 Hz and 13 MHz The agreement of the experimental data m the overlapping frequency range was found to be satisfactory m most cases When the cell impedance becomes large at low temperatures, the upper frequency hmlt of the IM 5E meter decreases more rapidly than that of the Hewlett Packard instrument Both instruments were checked by the measurement of ohmic resistances at which high frequencies phase shifts begm to appear as artefacts The results of this check limited the impedance measurements to the frequency ranges, stated above, and to a lower temperature of - 80°C The data could be prmted out m a hst or plotted The data were also stored on discs The optlmlzatlon program FIRDAC[lZ] which determmes the parameters of the elements for a chosen analogue circuit from the best fit between the theoretrcal and expenmental frequency dependence was run on a

1031

G E BAJARSet

1032

separate obtamed

AT computer

Suitable

plots could

The clrcmt, used here for the slmulatlon, was practically the same as that m[9, lo] For slmphclty the geometnc capacity C, which IS m parallel to the other components of the clrcutt (Fig 3) was dropped The admittance of the constant phase element CPE (m) IS gven by

be

RESULTS The results of the graphite electrodes composltlon at 5°C example The data Packard instrument

al

measurements with platmum or on different disks of the same are shown m Figs 1 and 2 as an were obtamed by the Hewlett and are presented m a Bode plot

Ycpqm) = Y,(W# -a(m)

(1)

The constant phase elements are shown m Fig 3 by the symbol of a capacitor with lines between the plates

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Fig 1 Impedance data for the cell Pt/HAA/Pt m a Bode plot at 5°C

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Fig 2 Impedance data for the cell C/HAA/C m a Bode plot at 5°C

1 64

I 66

Impedance of the El/HAA/EI cell

1033

Y2.a(2) I ,

Y,.a(l)

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Fig 3 Analogue clrcmt for the slmulatlon of the frequency dependence of the Impedance (a)

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ResIstonce (E+4) Fig 4 Plots of the negatwe values of the capacltlve component us those of the ohmic component for the cell Pt/HAA/Pt at -25°C m the high frequency range (a) and the low frequency range(b) The theoretlcal curves of the slmulat!on are gwen by the sohd he

1034

G

E BAJARS et al

A comparison between the expenmental data and those from the slmulatlon 1s presented as an example for the measurements at -25°C m Figs 4a and b for platmum electrodes and m Figs Sa and b for graphite electrodes The measurements on a cell with gven electrodes were made at the same temperature by both Impedance meters m the different frequency ranges, mentloned before The letter a designates the mstrument 4192A and the letter b the instrument IM 5E The negative values of the capacitive components are plotted us the values of the ohmic component The parameters for the elements m the analogue clrcult are compiled for different temperatures m Table 1 Plots of the negative values of the capacltlve component us those of the ohmic component were also produced by the AT computer with suitable scales for ordmate and abscissa for a graphlcal extrapolation procedure The depressed semlclrcle was extended to the abscissa at frequencies before the onset of the nearly linear portion (compare Figs 4 and 5) due to the interface An extrapolation of the linear portion was also carried out and led to practically the same value on the abscissa as the extrapolation of the semlclrcle The extrapolated values R,,,agreed well Hrlth the values of the parameter R, from the slmulatlon The R, values are gven m Fig 6 us 1000/T m a semllogarrthmlc plot The values of R, are of the same order of magnitude as those at HBSO, 1 2H10 m Fig 8 of[9]

DISCUSSION The results m Fig la suggest that the ohmtc bulk resistance of the solid electrolyte m the cell Pt/HAA/Pt IS measured between about 10’ and lo4 Hz Here the term bulk resistance designates the sum of gram boundary resistance and crystal reslstance[ 131 The influence of the interfaces becomes visible at lower frequencies than 10’ Hz At frequencles larger than lo4 Hz the capacltlve component of the gram boundary lmpedance[13] leads to a decrease of 2 and an increase of the phase angle The results for the cell C/HAA/C m Fig 2 can be mterpreted quahtatlvely m a slmllar fashion However, the frequency range m which the bulk resistance 1s obtained 1s narrower This 1s understandable for frequencies below about 10’ Hz because the interface C/I-IAA 1s not the same as that of the interface Pt/HAA In contrast, the vastly different behavior m the increase of the phase angle at large frequencies m Figs 1 and 2 1s difficult to interpret d the increase 1s attnbuted solely to the capacitive component of the gram boundary Impedance It should not depend upon the nature of the electrode Two possible mterpretatlons are advanced (a) an influence of the gram boundary 1s also present at high frequencies because the surface of the compacted solid electrolyte 1s rough and porous, (b) although the disks had the same compoation, their properties might differ because of the lack of good reproduclblhty m the pressmg of the pellets Interpretation (a) IS considered more hkely Unfortunately, a possible influence of(b) cannot be ruled out with the result, gven for In agreement HSbO, 128H,O at -12°C m Fig 10 of[91, the

Impedance of the EI/HAA/EI 54-

cell

1035

(a)

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I 16

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I 48

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I 20

1 25

I 30

I 35

I 40

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I 50

1 55

1 60

I 65

I 70

I 75

(E+4)

5 Plots of the negative values of the capacltlve component versus those of the ohmic component cell C/HAA/C at -25°C In the high frequency range (a) and the low frequency range (b)

depressed semlclrcle extrapolates to very small values on the abscissa for high frequencies m all of our measurements This lmphes that the crystal resistance IS very small R,,,represents mamly the gram boundary resistance A second semlclrcle at high frequencies which was reported for larger water contents m[lO] was not observed here The comparison between Figs 4a and b demonstrates that the frequency dependence can be slmulated m a satisfactory fashion for the cell Pt/HAA/Pt in both frequency ranges A slrmlar statement holds

for the

for the other temperatures m agreement with the results, stated m[9] and given there for one temperature m Fig 10 However, there are deviations m the parameters for the elements of the analogue clrcult (Fig 3) This ~111be discussed m connectlon with the data m Table 1 For graphite electrodes a satisfactory slmulatlon IS only actuevable for the data obtamed m the higher frequency range by the mstrument 4192A The statements, made previously m a quahtatlve fashion m the dlscusslon of the results of Fig 1 and Fig 2, can be extended on the basis of the data m

G E BAJARSet al

1036

2r -I -I -I

-2 -2 -2 -2 -2 -3 -3 -3 -3 -3 -4 -4 -4

Fig 6

Senuiogarlthmlc plots of R, us lOOO/~, El, + Pt, 0 C

Table 1 The parameters Y, and a (1) for the constant phase element m Fig 3 which 1smainly determined by the interface differ considerably for the interfaces Pt/HAA and C/HAA The interface C/HAA displays a more capacitive behavior than that of the interface Pt/HAA The Y, values decrease with temperature Since the mterfaclal behavior of the cells becomes more pronounced at lower frequencies, the results m the low frequency range should be considered the more reliable ones On the other hand, the values of YZ and a (2) should be more rehable m the high frequency range d they represent mainly the gram boundary impedance A comparison of the said parameters for Pt and C electrodes m the high frequency range demonstrates a considerable deviation from each other It appears that Y, and a (2) are also affected by the interface for the reasons gven m the first paragraph of Dlscusslon Finally it IS pointed out that the R, values agree fairly well with each other at a gven temperature Although the optlmlzatlon programm allows the determmatlon of R, for the system, studied here, it does not gve values for Y2 and a (2) which are clearly interpretable The slmulatlon approach remains a phenomenolopcal one The numerical values of R,, obtained for the two measurements with Pt electrodes and the one measurement with graphite, agree approximately with each other However, an exammatlon of the plots m Fig 6 reveals a systematic deviation which becomes the largest at temperatures above - 10°C The type of interface affects the R, values to a small, but noticeable extent At a given temperature the Rb values are somewhat larger here than those for HSbO, 1 12 HZ0

m Fig 8 of[9] The curves m Fig 6 consist of a lrnear portion at lower temperatures and a curved one between about - 10 and 40°C This 1s m agreement with earher results[3, 93 Unambiguous mterpretations of the said curvature have not been advanced [3,9] and cannot be made on the basis of the present results REFERENCES Chowdry, J R Barkley, A D English and A W Shght, Mater Res Bull 17, 917 (1982)

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4 5 6 I 8 9 10

11 12 13

D J Dnmltrowlcz, J B Goodenough and P J Wiseman, Mater Res Bull 17, 971 (1982) G Ya Petrowskls, G E BaJais and Yu’L Lagzdons, Vestmk LGU. Ser Phvs Chem VINITI No 130-V (1987) ’ M Caseola and D Blanchi, S&d St lomcs 17, 287 (1985) C Forano, J P Besse, J P Battut, J Dupuls and A, HaUlmohamad, Sold St Iomcs 34,7 (1989) R C T Slade. G P Hall and E Skou. Sohd St Iomcs 35. 29 (1989) J R Macdonald, J electroanal Chem 53, 1 (1974) J R Macdonald. J them Phvs 61. 391111974) C Forano and J. P Besse, Eui J S&d St ‘Inorb Chem 25, 141 (1988) N Mmra, Y Ozawa and N Yamazone, J them Sot Jpn 12, 1954 (1988) M Abe and T Ito, Bull them Sot Jpn 41,33 (1968) J R Dygas, Ph D Thesis, Northwestern Umverslty, Evanston (1986) W I Archer and R D Armstrong, Electrochemistry, Vol 7, Speczahst Penodal Reports, p 157, The Chemical Society, London, (1980)