The capacity of glassy carbon in aprotic salt solutions in propylene carbonate (PC) is investigated as an approach to the optimalisation of electrochemical supercapacitors. The strong influence of water residuum on capacities at negative potentials is indicated. The capacities exhibit a shallow minimum at potential from 0 V to +0.5 V (vss. Cd/Cd2+ reference electrode) and they increase with increasing electrolyte concentration. The admittance of the electrode in pure PC (as received) is by 2 – 3 orders of magnitude smaller. Undoubtedly, the properties of ions in solution influence the capacity by a factor not exceeding 2 to 5

Macalík, Michal

Sedlaříková, Marie

Velická, Jana

Vondrák, Jiří

DSpace at University of West Bohemia

Roník 2010 íslo IVCapacity of a glassy carbon electrode in propylene carbonate basedelectrolytesJ. Vondrák1, M. Sedlaíková1, M. Macalík1, J. Velická21 Department of Electrical and Electronic Technology, The Faculty of Electrical Engineering andCommunication, Brno University of Technology, Technická 10, Brno2 Institute of Inorganic Chemistry of the ASCR, v. v. i., 250 68 e u PrahyE-mail : vondrakj@iic.cas.cz, macalik@feec.vutbr.czAnotation:The capacity of glassy carbon in aprotic salt solutions in propylene carbonate (PC) is investigated as an approachto the optimalisation of electrochemical supercapacitors. The strong influence of water residuum on capacities atnegative potentials is indicated. The capacities exhibit a shallow minimum at potential from 0 V to +0.5 V (vss.Cd/Cd2+ reference electrode) and they increase with increasing electrolyte concentration. The admittance of theelectrode in pure PC (as received) is by 2  3 orders of magnitude smaller. Undoubtedly, the properties of ions insolution influence the capacity by a factor not exceeding 2 to 5.Kapacita skelného uhlíku v bezvodých solných roztocích v propylenkarbonátu (PC) je zkoumána z hlediskaoptimalizace elektrochemických superkondenzátor. Zjitn byl významný vliv zbytkové vody na kapacitu pinegativních potenciálech. Kapacity vykazují mlké minimum pi potenciálech 0 V a +0,5 V (proti Cd/Cd2+referentní elektrod) a zvyují se s rostoucí koncentrací elektrolytu. Admitance elektrody v istém PC je o 2 a 3ády mení ne v elektrolytech se solnými ionty. Vlastnosti iont v roztoku ovlivují kapacitu nejvýe 2 krát a 5krát.INTRODUCTIONElectrochemical supercapacitors [1] are moderncomponents for the use as auxiliary or main powersources in many applications. Beside the capacitorsbased on transition metal oxides, the capacitors usingdouble layer effects on carbon electrodes in aproticsolvents seem to be much favorable.Main components of those capacitors are variouscarbonaceous electrode materials and suitableelectrolytes as it is described in [1]. We try to findoptimal electrolyte and we decided to make anattempt to investigate potential usage of gel polymerelectrolytes, which offer several benefits known fromlithium ion batteries. Therefore, we started tomeasure various propylene carbonate (PC) basedelectrolytes in contact with a glassy carbon electrode.We excluded the variability of carbon used as theelectrode in this way.Our main technique used was the impedancespectroscopy.THEORYAny boundary between ionic and electronicconductors is the place on which potential differencebetween the bulk of both phases exists. If noelectrochemical reactions, i. e., if no transport of massacross the boundary can proceed, then a steadypotential barrier is formed from mobile ionic species.Such a boundary is called ideally polarized boundary.Then we suppose that the conductivity of electronicconductor is very high (in other words, the mobilityof charge carriers is sufficiently high), then thepotential barrier is located in the ionic conductor(electrolyte). The distribution of potential in the layerdirectly beneath the boundary is governed by theequation0022eerex=¶Y¶ (1)where x is the coordinate perpendicular to theboundary. Its general solution is well known (forexample, see [2]). In electrochemistry, the equationsdescribe the properties of diffusion double layer asthe influence of potential drop y in the double layeron the differential capacity of the space chargeexisting in the double layer. In general, this capacityis described by the quantity [Sn.C (y-y0)]-1/2 wherethe potential of zero charge is denoted as y0 and theparameter Sn.C equals to the sum of concentrationsof all ions expressed as their charges.In reality, the differential capacity of a polarizedelectrode is a series connection of the diffuse doublelayer capacity and the capacity of the layer formed byadsorption of ions and polar molecules on theinterface. Further, a layer of organic or inorganicsubstances is usually present on the interface betweenan inert electrode and organic aprotic electrolyte. Thislayer is known as SEI or PEI interfaces and itsdifferential impedance is more or less like to that ofcapacity. Finally, it is generally known that theimpedance of a polarized solid electrode is not a purecapacity, but a constant phase element (C.P.E.)almost in all cases.The investigation of the interface of glassy carbon inaprotic electrolytes seems to be important for theproper choice of the electrolyte and it must be astarting point for the study of gel polymer electrolytein contact with carbon. This paper deals with thebehavior of selected liquid electrolytespredominately, while their usage in gel polymermatrix will be described in a following paper.METHOD AND APPARATUSThe potentiostat AUTOLAB enabled with the moduleFRA-2 and software GPES vor cyclic voltammetrywas used. A three-electrode vessel contained Ptcounter electrode and Cd/Cd2+ pseudoreferenceelectrode [3] based on the gel polymer electrolytewere used.Analysis of results was done using the Boukampsoftware delivered with the potentiostat. Anequivalence circuit containing the series resistance ofthe electrolyte and then a parallel combination of aquasi capacitor (exactly, a C.P.E. with phase anglefrom 80°C to 85°C) was suggested. The parallelresistance was considered to be a residual backgroundcurrent. The main part of investigations coveredfrequency range from 500 Hz to 0,1 Hz. Theelectrode was a glassy carbon rod of Ø5 mm inPTFE holder.MATHERIALSLithium and sodium perchlorates (0.5M to 2M),tetramethylammonium and/or lithium fluoroborateand a saturated solution of potassium perchlorate inpropylene carbonate (PC) were used.PMMA based gel polymer electrolyte was preparedfrom MMA monomer, the salt solution in PC andpolymerization catalyst according to our previouspapers [4], [5]. The salts were dried in vacuum attemperature from 100°C to 110°C before use.Propylene carbonate was used as received fromSigma-Aldrich.RESULTSAn example of a cyclic voltammogram in PC –LiClO4 electrolyte is depicted in Fig. 1. We can seethe behavior similar to a capacitor at potentials morepositive than -0.4V or more. A small peak appearsbetween -0.6 V and -0.4 V, which may correspond tothe formation of the SEI layer by reduction of theelectrolyte.Fig. 1: Voltammogram of a 1M solution of LiClO4 in PC at0.01 V.s-1The impedance spectrum of a PMMA – LiClO4electrolyte is depicted for several potentials in Fig. 2as an example. We can easily see that the spectrumconsists essentially of a lowered semicircle caused bya pseudo capacity. An equivalent circuit as suggestedin the experimental part can describe the lowfrequency part. Further, we can see rather smallcomponent at frequencies exceeding 1 kHz. This issupposed to correspond to the adsorption of thesolvent and will be worth of further examination.Fig. 2: Impedance spektrum at various potentials in 1M LiClO4in PCThe pseudo capacitance (the element C.P.E.) atvarious potentials in several inert salts is plotted inFig. 3. Sodium, lithium, tetraalkylammonium andpotassium – propylene carbonate electrolytes arecompared.0E+0 5E+4 1E+5 2E+5 2E+5 3E+5Z '0E+01E+52E+53E+5Z''1.7 V0.9 V0.8 V- 0.1 VZ‘’/Z’ / Fig. 3: Influence of electrolyte composition (given in thefigure) on electrode kapacitanceApparently, the pseudo capacity passes through aminimum and at potential more negative. As the peakor wave corresponding to the SEI formation appearsbelow -0.4 V, the impedance does no morecorrespond to the pseudocapacity model below thatvalue (see Fig 4). Therefore, we restricted theinvestigation to the potentials from positive end to therange in which this phenomenon does not play anyimportant role. As the exception, TEAClO4 solutiondoes not exhibit this effect.Fig. 4: The impedance of an electrolyte containingtetraalkylammonium perchlorateThe best results in the range of negative potentialswere obtained with the solution if 1M TEAClO4. Wesee there the minimum of pseudo capacity atpotentials in the vicinity of zero.It should be noted that the KClO4 solubility in PC arenot very high and an almost saturated solvent(milimolar approx.) was used. Therefore, thecapacitance is much lower in this case.Further we can see the capacity found in pure PC. Itsimpedance is perhaps due to residual impurities orwater content in PC.The spectrum of LiClO4 in gel polymer electrolyte isshown in following Fig 5. Two depressed semicirclesare quite clear on the curves. The high frequency partis explained as the SEI layer on the interface, similarto those described on the lithium metal electrodes byAurbach and coworkers [6].Fig. 5: Impedance in Li salt containing PMMA gel. Uppercurve: potential 0.8 V, lower curve: potential 0 V (Vs.Cd/Cd2+ reference electrode)CONCLUSIONIn general, the capacity increases towards positivepotentials and its value does differ much from thoseknown in aqueous systems. This is not surprising asthe relative permitivity of propylene carbonate (PC)is fairly high (e = 64.4). It is interesting that thehighest capacity was found in the case of smalllithium cations. Also the wide range of potentialsaccessible in tetraalkylammonium (TEA) saltsindicates their importance in commercialsupercapacitors.The capacity in PC alone is by 2 - 3 orders ofmagnitude lower. The capacities should be inverselyproportional to square root of concentration and sucha result is in accordance with that presumption.Further results will be oriented more towards the gelPMMA-PC based electrolytes and the use of variouscarbonaceous materials with large specific surfacearea.0E+0 1E+5 2E+5 3E+5Z '0E+04E+48E+41E+52E+52E+5Z''Z‘’/Z’ / -2.0 -1.0 0.0 1.0 2.0Potential vs. Cd electrode, V04080120160200Ca pa ci t y[ µF]E / V (vs. Cd/Cd2+)C/ µF-2 -1 0 1 2E (V vs. Cd/Cd2+)1E-91E-81E-71E-61E-5l og( C)LiClO4NaClO4PC onlyKClO4 sat.LiBF4TMA BF4E / V (vs. Cd/Cd2+)l og( C)ACKNOWLEDGEMENTThis work was supported by The Grant Agency of theAcademy of Sciences of the CR, grant no. KJB208130902 and by The Ministry of Education, Youthand Sports, project no. MSM0021630516.REFERENCES[1] B. E. Conway, W. G. Pell, "Dynamic Aspects ofElectrochemical Supercapacitor Behavior", in:Electrochemical Supercapacitor and HybridPower Sources, R. J. Brod (editor), TheElectrochemical Society, NJ, USA, 2002.[2] Z. Stojek, "The Electrical Double Layer and ItsStructure, in: Electroalalytical Methods", R.Scholz (editor), Springer, Berlin, p. 3-8 andreferences therein, 2002.[3] J. Reiter, J. Vondrák, Z. Mika, "Solid-stateCd/Cd2+ reference electrode based on PMMAgel electrolytes", Solid State Ionics, vol.177, p3501-3506, 2007.[4] J. Vondrák, M. Sedlaíková, J. Reiter, T. Hodal,"Polymer gel electrolytes for electrochromicdevices", Electrochimica Acta 44, p. 3067, 1999.[5] J. Vondrák, M. Sedlaíková, J. Velická, B.Klápt,V. Novák, J. Reiter, "Gel polymerelectrolytes based on PMMA", ElectrochimicaActa 46, p. 2047, 2001.[6] D. Aurbach, Y. Einely, A. Zaban, "The SurfaceChemistry of Lithium Electrodes in AlkylCarbonate Solutions", J. Electrochem. Soc. 141,1994.

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Capacity of a glassy carbon electrode in propylene carbonate based electrolytes

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Capacity of a glassy carbon electrode in propylene carbonate based electrolytes

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