Spectroscopic verification of extended temperature stability of superionic phase obtained in mechanosyntehsis process for CsHSO4/Phospho-silicate glass composite Authors: Maja Mroczkowska-Szerszeń1, Maciej Siekierski2 ,Rafał Letmanowski2, Michał Piszcz2, Renata Cicha-Szot1, Lidia Dudek1, Sławomir Falkowicz1, Grażyna Żukowska2, Magda Dudek 3 1 Oil and Gas Institute, ul. Lubicz 25a, 31-503 Cracow, Poland 2Warsaw University of Technology Faculty of Chemistry, Inorganic Chemistry and Solid State Technology Division ul.Noakowskiego 3, 00-640 Warsaw, Poland 3AGH – University of Science and Technology, Faculty of Fuels and Energy, al. Mickiewicza 30, 30-059 Cracow, Poland corresponding :
[email protected] 1. Introduction:
4b. SUBSTRATE 1: Inorganic glass
Solid acids and especially acid sulfates are expected to be one of the most suitable substances for the purpose of preparing middle and low temperature fuel cell membranes for PEM – proton exchange membrane type fuel cells operating below 300oC [1],[2]. Among the whole family of compounds described by the chemical formula MHXO4 and M3 H(XO4)2 where (M=Cs, NH4,Rb ; X=S,Se), cesium hydrogen sulfate (CsHSO4) - CHS is most promising one. Its structure, phase transitions character and protonic conductivity mechanisms has been recently investigated thoroughly in case of pure CHS as well as for some of its composites [3–6]. So called first phase of the cesium hydrogen sulfate CsHSO4, high temperature one, is also a high conducting phase. Tetragonal anions present in the structure are responsible for the formation of the hydrogen bonds with the mobile protons, This superionic phase appears above 154oC and its conductivity reaches 10-2 do 10-6 S/cm range. But recently it was proven also for intermediate temperature monoclinic phase II CHS /SiO2 composites to be a very good protonic conductors specially in its disordered state [7,8]. The CHS most interesting characteristic (for the potential applications) is the superprotonic conductivity in the non hydrated state.
SiO2-P2O5
with polymeric additive PEO poly(ethylene oxide) Superionic conductor in (30-140oC) For PEM – proton exchange embrane in fuel cell devices
Extending the conductivity results of the protonically conducting phases of CHS in to the lower temperature regimes, is a subject of efforts of many research groups. This would allow to use CHS or its composite as wide range operating superionic proton conducting membrane. One of possible ways to improve the CHS characteristics is amorphization processes. The best results so far, which means the highest thermal stability and high conductivity in the range from room temperature to 180oC, were achieved by Mastuda et al [9] in mechanosynthesis process leading to obtaining CsHSO4-CsH2PO4 composite. There were trials of filling up porous alumina (Al2O3) from cesium sulfate solution [10]. In this case the results were worse than for the SiO2 composites. Combining cesium acidic sulfate with zeolites also yielded in interesting results [11] and also some other way of CHS modifications where proposed.
2.Experimental: In our case the CsHSO4/Phospho-silicate polymer doped glass composite, was obtained by machanosynthetic process. Planetary mill (Fritsch Pulverisette 7 premium line) with high energy grinding option, allowed the solid state synthesis and probably amorphization process in the material. The CsHSO4, has been previously synthesized in its third phase (phase III, monoclinic, P21/C) to act as a substrate (its structure was verified by XRD method, FTIR, FT-Raman spectroscopy). The second substrate was a glassy composite 70SiO2-30P2O5, doped with PEO poly(ethylene oxide) polymer also spectroscopically investigated as well by means of XRD Impedance spectroscopy was used for its XRD. conductivity verification. The substrate was prepared by optimized synthetic technique [12]. After ball milling process the final product (CsHSO4/Phospho-silicate polymer doped composite) was verified again by the same techniques and in case of its sulfate part the phase transition between the substrate and a mechanosinthethically modified product was recognized by the methods.
5. SUBSTRATE 2: RT (phase III) CsHSO4
Superionic conductor (above 90oC) in this temperature range appears as phase II or I For PEM – proton exchange membrane in fuel cell devices
time: 24h+ Ph=3-4 minimum 2 weeks in oC 40 stirring: hours 4a. Synthesis
For inorganic substrate
SiO2-P2O5 EtOH, Formamide, HCl H2O
main reagents:
Fig.2 FT-Raman spectrum of cesium hydrogen sulfate (CsHSO4) phase III,substrate
6. Mechanosynthesis 720 rpm, time of minutes
temperature:
~60˚C
conditions (sol-gel method)
synthesis environment:
Fig.1 FT-IR spectrum of cesium hydrogen sulfate (CsHSO4) phase III ,substrate
molar ratio 70 SiO230 P2O5
7. Product: CsHSO4 Intermediate temperature (phase II) of high protonic conductivity stabilized in RT in a glassy matrix of SiO2-P2O5
3b. Results:
TEOS (Tetraetylo ortho silicate), TMP (Trimetylo phospate)
3a. Results/Conclusions: After ball milling process the intermediate temperature monoclinic structure (normally normally appearing above 96oC) - phase II was present in the structure, (previously present phase III was replaced by phase II). All the measurements was provided in room temperature range, so we observed intermediate temperature phase II stabilization in the room temperature in case of phospho phospho-silicate with PEO and the CHS composite. The result was best pronounced in FT-Raman spectroscopy technique. One of the reasons was due to fact that FT-Raman method is not very sensitive for silicate part of the composite so other components like CsHSO4 are more pronounced in the spectrum. Recognized Raman shift maxima positions were corresponding very well with results shown in Otomo CHS/ silica phases by Raman spectroscopy [7]. The same author reports very good electrical results for the phase two which is also present in our composite so again, not only in case of so called phase one (tetragonal high temperature phase appearing above 154K) but also in its monoclinic phase II the superionic conductivity was present. Further conductivity investigation, BET specific surface area measurements and other, in case of our new composite as well as different mechanosynthetically obtained compositions are also researched One of the additives will be sodium silicate glass– prepared by water glass gelation researched. with additives) which is less expensive alternative than previously used in synthesis TEOS (Tetra Ethylo Orthosilicate). First step was the synthesis optimization and gelling process kinetics recognition for new sodium silicate compositions which is especially important for further quick silicate part obtaining development.
Acknowledgments: T research was partly supported by PBS 177070/PBS/2012-2015 grant The
Fig.3 FT-Raman spectra of substrates – room temperature measurement (A- glass 30P2O5-70SiO2 B- crystalline CsHSO4 – phase III – low ionic conductivity C – spectrum of hand grinded sample (CsHSO4 + 30P2O5-70SiO2 ) – the spectrum consists of the substrate bands. CsHSO4 phase III is marked by blue triangles and 30P2O5-70SiO2. phase is marked by green pentagons.
FT-Raman bands positins (cm-1) 413 427 574 591 741 847 997 1020 2861 2961
Bands assigments for CSH phase II (monoclinic) n2SO4 n2SO4 n3SO4 n4SO4 TMP (trimethylphosphate) n(S-OH) n(S-OH) n(S-O) TMP TMP
.
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Fig.4 FT-Raman spectra of substrates – room temperature measurement (A- glass 30P2O5-70SiO2 B- crystalline CsHSO4 – phase III – low ionic conductivity; C – spectrum of sample prepared by ball milling method 720 rpm (mechanosynthetic way) (CsHSO4 + 30P2O5-70SiO2 ) – the spectrum represents phase II – of high proton conductivity.
