Polymer based materials for solid electrolyte sensors

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<ul><li><p>yt</p><p>siv</p><p>rma4, D</p><p>gstra</p><p>lytemeoxie peporeses i</p><p>demonstrated, that CPC, consisting of PANI and different metal oxides or metal salts can be used at high temper-peratures of 450 C because of improved thermal properties.</p><p>nsors at of dier temectivityance</p><p>Solid State Ionics 225 (2012) 337341</p><p>Contents lists available at SciVerse ScienceDirect</p><p>Solid Stat</p><p>j ourna l homepage: www.etial sensors [35], which are based on the kinetically controlledelectrochemical conversion of oxidizable gases with oxide ions fromthe solid electrolyte [3,4]. Despite this intensivework there is still a con-siderable variety of potential applications for these sensors, which can-not be addressed due to insufcient sensor performance. One of themost important issues concerns the long-term stability [6,7], which isstill critical for measuring electrodes based on gold and other noblemetals if measuring times of several months without calibration arestrived for. Therefore, new conductive llers for composite mixed po-tential electrodes are needed, which provide sufcient electrochemicalactivity, thermal stability and low catalytic activity. Thiswork is focused</p><p>this purpose [915].The synthesised newmaterials have been characterisedwith respect</p><p>to their thermal stability, their structure andmorphology, their catalyticactivity and their sensing behaviour in different non equilibrated gasmixtures in correlation to their impedances.</p><p>2. Experimental</p><p>Pure PANI has been prepared by two different methods. Onemethod is described elsewhere [16], the other one serves as point ofdeparture for synthesising the polymer composites. It includes the in-on the preparation of new composite mater</p><p> Corresponding author at: Universitt Leipzig, InsJohannisallee 29, D-04103 Leipzig, Germany.</p><p>E-mail address: cvonau@ksi-meinsberg.de (C. Vonau</p><p>0167-2738/$ see front matter 2012 Elsevier B.V. Alldoi:10.1016/j.ssi.2012.04.015of the electrode materialorts were directed espe-aterials for mixed poten-</p><p>(CPC) could be improved signicantly. Because of its relatively highthermal stability, low costs, ease of synthesis and good environmentalbehaviour, among the conductive polymers PANI is most suited for[2]. During the last two decades extensive effcially on the development of new electrode m1. Introduction</p><p>Electrochemical solid electrolyte sehighly sensitive and rapid measuremenronments and also applications at lowsensor parameters like sensitivity, selare correlated mainly with the performre suited for the reliable,fferent gases in hot envi-peratures [1]. Importantand long-term stability</p><p>sensors made of conductive polyaniline (PANI) lled with differentmetal oxides or salts such as Nb2O5 or FeCl3 and Co(NO3)2 during thepolymerisation process. Intrinsically conducting polymers such asPANI have been the subject of extensive theoretical and experimentalstudies in recent years [8]. By lling the polymer with i.e. metal oxides,the thermal stability of the resulting conductive polymer compositeMixed potential 2012 Elsevier B.V. All rights reserved.Solid electrolytePotentiometric gas sensor ature measurement up to temPolymer based materials for solid electrol</p><p>Corinna Vonau a,b,, Jens Zosel b, Muthusamy ParamaVladimir Vashook d, Ulrich Guth b,d</p><p>a Universitt Leipzig, Institut fr Analytische Chemie, Johannisallee 29, D-04103 Leipzig, Geb Kurt-Schwabe-Institut fr Mess-und Sensortechnik e.V. Meinsberg, Kurt-Schwabe-Straec Central Electrochemical Research Institute, Karaikudi 630 006, Tamil Nadu, Indiad Technische Universitt Dresden, Institut fr Physikalische Chemie und Elektrochemie, Ber</p><p>a b s t r a c ta r t i c l e i n f o</p><p>Article history:Received 9 September 2011Received in revised form 29 February 2012Accepted 17 April 2012Available online 12 May 2012</p><p>Keywords:Conductive polymer composite</p><p>Electrochemical solid electroare suited for in-situmeasuredrocarbons and also nitrogenare correlatedmainly with thnewly developed conductivehave been characterised withbehaviour on solid electrolytials for mixed potential</p><p>titut fr Analytische Chemie,</p><p>).</p><p>rights reserved.e sensors</p><p>am c, Kristina Ahlborn b, Frank Gerlach b,</p><p>ny-04720 Ziegra-Knobelsdorf, Germany</p><p>e 66b, D-01062 Dresden, Germany</p><p>sensors based on Yttria Stabilised Zirconia (YSZ) with mixed potential electrodesnts of low concentrations of combustibles like hydrogen, carbonmonoxide or hy-des. The parameters sensitivity, selectivity and long-term stability of these sensorsrformance of the electrodematerial. Thiswork is directed on the investigation oflymer composite materials (CPC) based on polyaniline (PANI). These materialspect to their morphology, thermal stability, catalytic activity and electrochemicaln gases containing hydrogen, ethene and propene. For the rst time it could be</p><p>e Ionics</p><p>l sev ie r .com/ locate /ss itroduction of 0.1 mol aniline into 1 M HCl and the following polymer-isation by adding 0.05 mol ammonium peroxidisulfate solution. Thepolymerisation of aniline in an acidic medium results in the formationof a protonated, partially oxidised form of PANI, the conductive emer-aldine salt 4 (Fig. 1).</p><p>Two composites have been prepared by adding Nb2O5-powder tothe solution described above before and after starting the polymerisa-tion respectively. The molar ratio aniline/Nb2O5 has been adjusted at</p></li><li><p>1n4 5</p><p>n+2H</p><p>N</p><p>N</p><p>N</p><p>N</p><p>N</p><p>N</p><p>N</p><p>N</p><p>H</p><p>H H</p><p>H</p><p>H</p><p>H</p><p>H</p><p>H</p><p>H H</p><p>n</p><p>32N</p><p>N</p><p>N</p><p>N</p><p>N</p><p>N</p><p>N</p><p>N</p><p>n</p><p>-2H</p><p>-4H</p><p>-2H</p><p>-2H</p><p>+4H</p><p>+2H</p><p>+2H</p><p>ine (</p><p>338 C. Vonau et al. / Solid State Ionics 225 (2012) 3373412:1. After stirring for 14 h, the precipitates have been washed till thepH-value of the wash water reached pH=4. To characterise the effectof washing procedure a further composite has been prepared addition-ally, where the Nb2O5 was added after starting the polymerisation, butwashing the precipitate less intensively than the two other CPC till acolour change from green/white to blue/white. A last composite hasbeen prepared by adding 0.075 mol FeCl3 and 0.025 mol Co(NO3)2 tothe solution after starting the polymerisation of aniline. This compositewas washed till the wash water became colourless.</p><p>All materials have been characterised with respect to their specicsurface (BET, COULTER SA 3100), grain size (Laser particle seizer,COULTER LS230), structure (SEM, Quanta 200, FEI and TEM, PhilipsEM 208), thermal stability (Setsys TG-DTA 12, Setaram), conductivityand catalytic activity. The catalytic activity was investigated bymeasur-ing propene oxidation at powder samples with the absolute surface of2 m2 in a gas mixture, containing 0.5 vol.% C3H6 and 2.5 vol.% O2 byquantifying CO2 formation at increasing temperature levels with anFTIR-spectrometer (NICOLET 8700, Thermo Scientic). The experimen-tal procedure of the catalytic investigations is described elsewhere [17].</p><p>Pastes have been made by mixing the powders with a commercialbinder (ESL Europe, 242SB) based on an epoxy resin and printed onYSZ tubes. Alternatively, a binder based on cellulose acetate was tested.These types of binders are commonly utilised in thick lm technique.The binders strongly inuence printing results and therefore layer mor-phology. They have to be removed during drying or ring procedures atelevated temperatures. The electrodes were dried 1 h after printing at150 C and investigated with potentiometric measurements against a</p><p>N NH H</p><p>Fig. 1. Different forms of polyaniline.1 leucoemeraldine (PANI, white/clear), 2 pernigranilgreen), 5 emeraldine base (PANI-EB, blue).platinum air reference at the inner side of the YSZ tube according tothe experimental setup, shown in Fig. 2. The potentiometric measure-ments were carried out at 450 and 500 C in different gas mixtures, con-taining 1.5 vol.% O2 and 0 890 vol.ppmH2 or 0 890 vol.ppmC3H6 inN2, applied at the ow rate 50 ml/min at standard conditions. Moreover,a gas mixture, containing 3.9 vol.% O2, 7.2 vol.% CO2 and 0 19 vol.ppmC2H4 in N2 was used for the potentiometric investigations.</p><p>gasinlet</p><p>Pt100-sensor</p><p>quartz pipe</p><p>heater</p><p>reference elect(Pt-air)</p><p>Fig. 2. Setup of YSZ cell with conducting polymer3. Results</p><p>3.1. Structure and morphology</p><p>The preparations of pure PANI as well as those of the composites re-sult in agglomerates. The metal salt containing composite looks similarto pure PANI, leading to the assumption that the metal ions build an in-tercalation compound with the polymer. The metal oxide containingcomposite conrms the assumption that PANI is adsorbed at theNb2O5 particles. The TEM images in Fig. 3 illustrate a difference betweenthe two samples prepared by adding the Nb2O5 before or after startingof the polymerisation respectively. Adding Nb2O5 to the solution beforethe start of polymerisation results in the adsorption of monomers toNb2O5 and start of polymerisation at every single Nb2O5 particle,which is encased than by pure, crystalline PANI. Contrary to that the ad-mixture of Nb2O5 after starting the polymerisation results in the rando-mised adsorption of polymeric macromolecules to the oxide particles,which are not completely encased after this procedure.</p><p>The grain sizes of the different composites differ with a particlesize distribution around 0.5 m for the metal oxide contanining com-posites to 1.1 m for the metal salt containing composite. For compar-ison, the size distribution of pure PANI is about 1.2 m.</p><p>3.2. Thermal behaviour</p><p>As it is well known from the literature adequate thermal stability ofthe pure PANI is limited to temperatures below 250 C which is outside</p><p>n</p><p>N NH H</p><p>salt) (PANI-PS, blue),3 pernigraniline base (PANI-PB, violet), 4 emeraldine salt (PANI-ES,the range of working temperatures of YSZ based solid electrolyte sensorsbetween 450 800 C. The thermogravimetric investigation given inFig. 4a shows, that pure PANI looses more than 90% of its weight at thetemperature 500 C. The cracking products of the PANI pyrolysis couldnot be characterised. According to [18] the thermal decomposition canresult in several carbazoles, aminocarbazoles, phenylamines and otherintermediates. According to Fig. 4b the thermal behaviour of the polymer</p><p>gasoutlet</p><p>YSZ-pipe</p><p>measuringelectrode</p><p>rode</p><p>electrodes for potentiometric measurements.</p></li><li><p>can be inuenced by the formation of a composite. The added metaloxide Nb2O5 increases the thermal stabilty signicantly, resulting in thereduced weight loss 12% at 500 C.</p><p>Related to the amount of pure PANI in the PANI/Nb2O5 composite(2:1 molar mixing ratio), the weight loss at 500 C can be calculatedby the following equation:</p><p>PANI mass loss in composite 2MPANI MNb2O5 </p><p> 12%2MPANI</p><p> 29% :</p><p>As it can also be seen from Fig. 4, the composite with metal oxideadded before starting the polymerisation shows a slightly higherthermal stability than the composite with Nb2O5 added after start ofpolymerisation. One reason of that lower thermal stability might berelated to the incomplete covering of the grains with PANI.</p><p>Also the more acidic composite shows that pronounced effect ofthermal stabilisation, revealing that the pH value of the compositedoes not inuence the thermal behaviour of the Nb2O5 containingCPC. In contrast to that the composite with the incorporated metal</p><p>Fig. 3. TEM images of Nb2O5 containing PANI composites.a) Nb2O5 is added beforestarting the polymerisation.b) Nb2O5 is added after starting the polymerisation.</p><p>Fig. 4. Thermal characterization curves of pure PANI (4a) and its Nb2O5 containing compos-ites (4b), sample 1: pure PANIwith 0.1mol aniline in 1 l 1 NHCl, sample 2: preparation acc.to [10], sample 3: Nb2O5 added before start of polymerization, sample 4: Nb2O5 added afterstart of polymerization.</p><p>339C. Vonau et al. / Solid State Ionics 225 (2012) 337341salt ions exhibits nearly the same thermal behaviour like pure PANIresulting in more than 90% pyrolysed composite at 500 C.</p><p>The reasons for thermal stabilisation could not be claried, so far.One approach to be followed in future research consists in examiningthe reactive chain ends of the polymer, occuring by heating themixture.These reactive ends might bind to the metal oxide and stabilise thecomposite, which might result in the observed colour change. Due tothe fact that pure PANI as well as the FeCl3/Co(NO3)2 containing com-posite did not change their colours after the thermal treatment it canbe concluded that a large amount of pure polymer remained in thethermally treated samples. The metal oxide containing compositeshave been changed their colours from blue/white to grey/white, but itcould not be claried, why the polymer colour disappeared, althoughit was veried that PANI remaines in the composite.</p><p>3.3. Catalytic behaviour and conductivity</p><p>Catalytic activities of electrodematerials for the oxidation of the gas-eous components to be measured with mixed pontential sensors inu-ence their sensitivity and gas consumption signicantly [19]. Thiscomplementary investigation is therefore carried out to screen promis-ing electrodematerials. Fig. 5 shows signicant differences between theinvestigated materials. The composites with acidic character showhigher catalytic activities than the ones prepared by intensive washingwith end pH value=4. Conductivity measurements at pressed powder</p><p>Fig. 5. Catalytic activity of investigatedmaterials to propene conversion, inset: conductivityof pressed powders, sample 14 see Fig. 4, sample 5 Nb2O5 CPCwashed till colour change of</p><p>precipitate, sample 6: PANI with FeCl3/Co(NO3)2.</p></li><li><p>samples (see inset in Fig. 5) show correlations to the results of catalyticactivity. As expected the acidic composites exhibit higher conductivitiesthan the intensively washed composites due to their much higher con-centration of remaining highly conductive protons. Therefore, acidity isnecessary to provide sufcient electrode conductivity and to enableappropriate charge transfer, which is required for the adjustment ofthe mixed potential.</p><p>3.4. Potentiometric investigation</p><p>Electrodes made from the samples with pure PANI, the compositeswith Nb2O5, washed till pH=4 and pure Nb2O5-electrodes show nomeasurable sensitivities in the investigated gas mixtures. In contrastto that the potentiometric investigations indicate comparably highsensitivities for the more acidic PANI composites as given in Fig. 6.</p><p>While the metal oxides and salts in the composites are responsiblefor oxygen exchange with the gas phase and the solid electrolyte, cata-lytic behaviour and thermal stabilisation, the PANI in the compositesplays an important role for electrode conductivity. The interactionsbetween niobium oxide and polyaniline may reect their chemicalidentities. If the acidity is not too strong, anionic niobate may exist atthe interface with positively charged PANI species. The ratio of thecomposite mixture results from an optimization procedure describedelsewhere [20].</p><p>The results of the FeCl3/Co(NO3)2/PANI-electrode (FCP-electrode)show that the logarithmic response expected from the mixed potentialtheory [3] is established at concentrations above 100 vol.ppm in thecase of H2, 50 ppm for C3H6 and 10 ppm for C2H4. The limit of detection</p><p>oten</p><p>tial [m</p><p>V]</p><p>140</p><p>170</p><p>200</p><p>63</p><p>a</p><p>340 C. Vonau et al. / Solid State Ionics 225 (2012) 33734...</p></li></ul>

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