Proton-conducting membranes based on CsH2PO4 and copolymer of tetrafluoroethylene with vinylidene fluoride published by Ural Federal University eISSN 2411-1414; chimicatechnoacta.ru LETTER 2022, vol. 9(3), No. 20229303 DOI: 10.15826/chimtech.2022.9.3.03 1 of 5 Proton-conducting membranes based on CsH2PO4 and copolymer of tetrafluoroethylene with vinylidene fluoride Irina N. Bagryantseva a , Yuri E. Kungurtsev ab, Valentina G. Ponomareva a* a: Institute of Solid State Chemistry and Mechanochemistry SB RAS, Novosibirsk 630090, Russia b: Novosibirsk State University, Novosibirsk 630090, Russia * Corresponding author: ponomareva@solid.nsc.ru This paper belongs to the CTFM'22 Special Issue: https://www.kaznu.kz/en/25415/page. Guest Editors: Prof. N. Uvarov and Prof. E. Aubakirov. © 2022, the Authors. This article is published open access under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/). Abstract In this work, proton conductivity, morphology and mechanical prop- erties of (1–x)CsH2PO4–xF-42 (x=0.05–0.3, weight ratio) membranes were investigated for the first time. Thin flexible membranes for x≥0.15 with the uniform distribution of the components were ob- tained by a tape casting method. Mechanical properties of the mem- branes were measured by Vickers microhardness tests for a low pol- ymer content (x˂0.15), also the tensile strength for membranes with high polymer content x=0.2–0.3 were evaluated. Proton conductivity of the (1–x)CsH2PO4–xpF-42 composite polymer electrolytes decreas- es monotonically with increasing x due to the effect of a «conductor- insulator» percolation. The combination of conductivity, mechanical strength and hydrophobic properties of (1–x)CsH2PO4–xF-42 makes certain compositions of proton-conducting membranes (x~0.2–0.25) promising for their use in intermediate-temperature fuel cells, de- spite decreased conductivity. Keywords proton conductivity cesium dihydrogen phosphate fluoropolymer p(VDF/TFE) tape casting Received: 27.06.22 Revised: 15.07.22 Accepted: 15.07.22 Available online: 19.07.22 1. Introduction Solid Acid Fuel Cell (SAFC) is a promising new type of fuel cells with a CsH2PO4 acid salt as a membrane [1, 2]. Inter- est in solid acid compounds, such as alkali metal dihy- drogenphosphates and dihydrogensulfates, is constantly growing, a number of salts of this family is increasing; as a result, new compounds with Cs3(H1.5PO4)2 [3–5] and Cs7(H4PO4)(H2PO4)8 [6] compositions have been recently discovered. However, CsH2PO4 remains the salt with the highest value of proton conductivity. The CsH2PO4 phase at room temperature is characterized by a low conductivity, while at 230 °C there exists its sharp increase by several orders of magnitude due to the phase transition to a Pm- 3m superionic phase, which is characterized by a high de- gree of structural disorder and high proton conductivity, 6·10–2 S/cm [7]. To achieve a more efficient and stable operation of H2/O2 FCs based on CsH2PO4 in a wider tem- perature range, it is necessary to maintain humidity level of pH2O ~0.3 atm. to prevent salt dehydration [8, 9]. The disadvantages of the CsH2PO4 membranes (such as a nar- row temperature range of the existence of a highly con- ductive state, time-dependent plasticity of acid salt in su- perionic phase, solubility in water, low mechanical strength) can be significantly improved by methods of homogeneous and heterogeneous doping. The search for polymer additives for the synthesis of thin membranes, which combine the flexibility and hydrophobicity of a pol- ymer additive with a high proton conductivity of the acid salt, is being intensively developed. A number of compo- site membranes based on CsH2PO4 and different polymers such as PVDF, SPEEK, epoxy resin, UPTFE, Butvar B98, p(VDF/HFP) have been investigated [10–16]. At a high concentration of polymer in such systems, the mechanical properties of the membranes are improved, while the high content of the non-conductive component results in a de- crease in conductivity due to the effect of the «conductor- insulator» percolation. The «CsH2PO4 – polymer» compo- sites can combine the required mechanical, conductive, and hydrophobic properties. In addition, the solubility of polymer additives in various solvents makes it possible to switch from the solid-phase methods of membrane’s syn- thesis to the production of thin and flexible films by tape casting technique. Fluoropolymers are considered as a chemically inert, thermally stable, effective additive to the acid salt CsH2PO4. The previous studies have included PVDF [10], ultrafine PTFE [14], and a VDF/HFP copolymer [15]. Fluoroplast 42 (F-42, similar to Kynar 7200) copolymer of vinylidene fluoride (VDF) with tetrafluoroethylene (TFE) http://chimicatechnoacta.ru/ https://doi.org/10.15826/chimtech.2022.9.3.03 mailto:ponomareva@solid.nsc.ru http://creativecommons.org/licenses/by/4.0/ http://orcid.org/0000-0003-1800-3421 http://orcid.org/0000-0002-8661-5799 https://crossmark.crossref.org/dialog/?doi=https://doi.org/10.15826/chimtech.2022.9.3.03&domain=pdf&date_stamp=2022-7-19 Chimica Techno Acta 2022, vol. 9(3), No. 20229303 ARTICLE 2 of 5 is a promising polymer matrix for CsH2PO4. TFE and VDF can be polymerized in any ratio giving rise to a broad va- riety of copolymers. A composition of F-42 corresponds to the approximate TFE:VDF component ratio of 29:71. The polymer has a high hydrophobicity due to the high content of fluorine atoms 65.8 (wt.%). F-42 is a robust polymer with a high tensile strength (~14.6 MPa), high specific surface resistance (~1010 Ohm/m). It is chemically re- sistant in acidic and basic media, and soluble in esters, ketones and N,N-dimethylformamide (DMF) [17–19]. F-42 polymer has high thermal stability with decomposition temperatures above 360 °C. Melting of F-42 at tempera- tures of 150–160 °C corresponds to the crystallites, but the polymer does not pass into a viscous state. In present work, morphology, mechanical characteristics, transport and structural properties of (1–x)CsH2PO4–xF42 polymer composite electrolytes have been studied. 2. Materials and Methods The CsH2PO4 salt was obtained by a slow solvent evapora- tion of an aqueous solution of Cs2CO3 and H3PO4 in a ratio of 1:2. Composite electrolytes with a polymer weight ratio of x=0.05–0.15 were synthesized by a thorough homogeni- zation of a suspension of CsH2PO4 particles in a solution of p(VDF/TFE) in DMF with a mortar and a pestle with a fur- ther drying and uniaxial pressing of formed powder at 300 MPa. For the x=0.2–0.3, a viscous suspension of CsH2PO4 particles in polymer solution was spread on a fluoroplastic substrate using a TOB-VFC-150 tape casting machine and dried to form the film. Proton conductivity measurements were carried out on the thin films ~150 μm for x=0.2 or pellet (x=0.05–0.15) with silver or platinum paste or pressed electrodes. Mem- branes were subjected to repetitive heating-cooling cycles in the temperature range from 50 to 245 °C. Humid condi- tions (pH2O ~0.3 atm) were used at temperatures higher 180 °C to prevent the CsH2PO4 dehydration. The conduc- tivity was measured by electrochemical impedance spec- troscopy using an P-5X impedance meter (frequency range of 1 mHz to 0.5 MHz) and Instek (12 Hz-200 kHz) in a cooling regime. Scanning electron microscopy (SEM) images of compo- sites were obtained on the gold sputter-coated membranes using a Hitachi TM 1000 microscope. X9ray diffraction (XRD) analysis was performed on a Bruker D8 Advance diffractometer (λ Cu Kα1 = 1.5406 Å) with a one- dimensional Lynx-Eye detector and Kβ filter. Vickers hardness for (1–x)CsH2PO4–xF42 (x≤0.15) was determined using a DuraScan 50 microhardness tester EMCO-TEST with an application time of load (0.5 kgf (4.9 N)) for 10 seconds. The measurements were repeated at least ten times for each sample. Dense tablets (5 mm in diameter and 1 mm in thick) were obtained by uniaxial pressing at 300 MPa. The tensile strength of thin-film polymer composite electrolytes with x >0.15 was measured using an Instron 5944 mechanical testing machine. For the preparation of samples, a punching die of certain sizes was used to obtain the samples with a form of a double blade with a 5 mm wide and 20 mm high working area. The thin-film mem- brane was stretched at a constant rate of 5 mm/min under atmospheric conditions and the applied load and elonga- tion was recorded. For each x at least six measurements were made, and average value was calculated. 3. Results and Discussion For the synthesis of composites, the x<0.38 composition range was chosen, since the proton conductivity drops sharply with an increase in the ratio of the polymer addi- tive due to the predominance of the nonconductive com- ponent in the membrane volume. Composites with a low content of F-42 x=0.05–0.15 were obtained in the form of pellets. For higher polymer content (x>0.15) it was possi- ble to produce thin flexible films with a thickness ~150 µm by tape casting method. The search for optimal conditions for tape casting process such as solvent used, application speed, the height of the gap, number of layers and the temperature regime of drying has been carried out. DMF was used as a solvent with a high boiling point (T=153 °C) that provides high quality of films obtained. According to X-ray diffraction data, a monoclinic CsH2PO4 (P21/m) phase is retained in composite electro- lytes over the entire range of compositions. Figure 1 XRD data for the (1–x)CsH2PO4–xF42 membranes of vari- ous composition. Chimica Techno Acta 2022, vol. 9(3), No. 20229303 ARTICLE 3 of 5 Figure 2 SEM images of (1–х)CsH2PO4–xF-42 for х=0.05 (a, c), x=0.1 (b), x=0.15 (d), x=0.25(e), cross-section of x=0.25 (f). With an increase in the weight ratio of a polymer addi- tive, the intensity of the CsH2PO4 reflexes decreases in accordance with the change in the mass fraction of the salt (Figure 1). The F-42 fluoropolymer has a sufficiently high degree of crystallinity; the XRD pattern has reflections in a region of 2θ~19 and 40º. The structure of the β-phase of PVDF is realized in the F-42 polymer [20]. For composite electrolytes with x≥0.1, the most intense F-42 reflex ap- pears in vicinity of 19°. The distribution of components in the volume signifi- cantly affects the proton conductivity and mechanical properties of membranes. CsH2PO4 is practically insoluble in most known organic solvents, and composite mem- branes present a polymer matrix with salt particles dis- persed in its volume. The study of membrane morphology and determination of the size of salt particles in the poly- mer matrix was performed using SEM. The resulting membranes exhibit a uniform distribution of components with the salt particles size less than 5 μm (Figure 2). The nature of the temperature dependences of the composites is close to that of pure salt. The conductivity of the high-temperature phase decreases by less than an or- der of magnitude for compositions with x≤0.15 (Figure 3). Compared to the initial salt of CsH2PO4, the proton con- ductivity of the composites decreases even at a low vol- ume fraction of the polymer (7.87 vol.% for x=0.05) due to its dielectric nature. A further increase in the polymer content (x>0.2) results in a close-to-linear decrease of conductivity. To assess the mechanical properties of the membranes, the Vickers microhardness was determined for the acid salt CsH2PO4 and hybrid polymer compounds with F-42. Figure 3 Temperature dependences of conductivity for the (1–x)CsH2PO4–xF-42 composites. CsH2PO4 x=0.05 (7.87 vol.%) x=0.1 (15.3 vol.%) x=0.15 (22.3 vol.%) Figure 4 Microscopic images of the indenter's imprint on the surface of (1–x)CsH2PO4–xF42 membranes. The Vickers microhardness test evaluates the mechani- cal properties of composite polymer electrolytes in the form of tablets with a low polymer content (x≤0.15) in comparison with initial CsH2PO4 salt. The relative density of the obtained tablets "CsH2PO4-polymer" was close to 95%. Vickers hardness (HV) was determined by division of the load by the area of the sloping surface of the indenta- tion. With an increase in the content of the polymer addi- tive, the diagonals of the indentation from the diamond pyramid increase (Figure 4). Thus, the Vickers numbers, HV, for the initial salt had the values HV~34, which corre- sponds to 333.4 MPa, for hybrid membranes containing Chimica Techno Acta 2022, vol. 9(3), No. 20229303 ARTICLE 4 of 5 F-42 polymer x=0.15, the microhardness decreases by more than two times reaching value ~13 HV. For hybrid compounds, it was shown that with an increase in the mass fraction of the polymer, the ability to resist plastic deformation and, as a result, the mechanical strength in- crease. For thin-film membranes with x>0.15, the tensile strength was measured as the load at which the sample failed, referred to the initial cross-sectional area of the sample. Films with x=0.2–0.3 and thickness of ~150 μm were obtained by applying a two layer of suspension and drying until the solvent evaporation. The magnitude of the breaking stress increased with the increase in the mass fraction of F-42. The maximum value of the breaking stress of a thin-film polymer-composite membrane with x=0.3 was 0.7 MPa. 4. Conclusions The synthesis method of the (1–x)CsH2PO4–xF-42 compo- site electrolytes by tape casting technique was developed. A study of the proton conductivity, structural properties, mechanical characteristics, and morphology of the system was carried out for the first time. According to XRD, F-42 is the chemically inert polymer matrix for CsH2PO4. Com- posite electrolytes in the form of thin flexible films with the thickness 100–150 μm can be obtained for high poly- mer content. The (1–x)CsH2PO4–x-42 composites are char- acterized by a decrease in superionic conductivity in com- parison with the initial salt within 1 order of magnitude for x=0.15. A further increase in the polymer content (x>0.2) results in a conductivity decrease close to linear. Polymer content x˂0.15 results in the low HV values corre- sponding to the high robustness of the membranes to plas- tic deformation. The improvement of mechanical proper- ties and hydrolytic stability makes the investigated com- posite polymer electrolytes promising for use as proton- conducting membranes in the medium-temperature range fuel cells. Supplementary materials No supplementary materials are available. Funding This work was supported by the Russian Science Founda- tion (grant no. 21-73-00298), https://www.rscf.ru/en. Acknowledgments None. Author contributions Conceptualization: I.N.B, V.G.P. Data curation: I.N.B, Y.E.K. Formal Analysis: I.N.B, V.G.P., Y.E.K. Funding acquisition: I.N.B. Investigation: Y.E.K., I.N.B. Methodology: V.G.P., I.N.B. Project administration: I.N.B. Resources: V.G.P., I.N.B. Software: V.G.P., I.N.B. Supervision: I.N.B. Validation: I.N.B, V.G.P. Visualization: I.N.B, Y.E.K. Writing – original draft: I.N.B. Writing – review & editing: V.G.P. Conflict of interest The authors declare no conflict of interest. Additional information Author IDs: Bagryantseva I.N., Scopus ID 41461057600; Ponomareva V.G., Scopus ID 56186783300. Websites: Institute of Solid State chemistry and Mechanochemis- try SB RAS, www.solid.nsc.ru; Novosibirsk State University, www.nsu.ru. References 1. Haile SM, Chisholm CRI, Sasaki K, Boysen DA, Uda T. Solid acid proton conductors: from laboratory curiosities to fuel cell electrolytes. Faraday Discuss. 2007;134:17-39. doi:10.1039/B604311A 2. Uda T, Haile SM. Electrochem. Thin-membrane solid-acid fuel cell. Solid State Lett. 2005;8:A245. doi:10.1149/1.1883874 3. Ponomareva V, Bagryantseva I, Zakharov B, Bulina N, Lavrova G, Boldyreva E. Crystal structure and proton conduc- tivity of a new Cs3(H2PO4)(HPO4)·2H2O phase in the caesium di- and monohydrogen orthophosphate system. Acta Cryst. 2017;C73:773–779. doi:10.1107/S2053229617012335 4. Sanghvi S, Haile SM. Crystal structure, conductivity, and phase stability of Cs3(H1.5PO4)2 under controlled humidity. Solid State Ionics. 2020;349:115291. doi:10.1016/j.ssi.2020.115291 5. Weil M, Stöger B. The caesium phosphates Cs3(H1.5PO4)2(H2O)2, Cs3(H1.5PO4)2, Cs4P2O7(H2O)4, and CsPO3 MonatsheftefürChemie – Chemical Monthly. 2020;151:1317– 1328. doi:10.1007/s00706-020-02675-6 6. Wang LS, Patel SV, Sanghvi SS, Hu YY, Haile SM. Structure and Properties of Cs7(H4PO4)(H2PO4)8: A New Superprotonic Solid Acid Featuring the Unusual Polycation (H4PO4) +. JACS. 2020;142(47):19992–20001. doi:10.1021/jacs.0c08870 7. Baranov AI, Khiznichenko VP, Sandler VA, Shuvalov LA. Fre- quency Dielectric Dispersion in the Ferroelectric and Superi- onic Phases of CsH2PO4. Ferroelectrics. 1988;81:1147–1150. doi:10.1080/00150198808008840 8. Boysen DA, Uda T, Chisholm CRI, Haile SM. High- performance solid acid fuel cells through humidity stabiliza- tion. Sci. 2004;303:68–70. doi:10.1126/science.109092 https://www.rscf.ru/en https://www.scopus.com/authid/detail.uri?authorId=41461057600 https://www.scopus.com/authid/detail.uri?authorId=56186783300 http://www.solid.nsc.ru/ http://www.nsu.ru/ https://doi.org/10.1039/B604311A https://doi.org/10.1149/1.1883874 https://doi.org/10.1107/S2053229617012335 https://doi.org/10.1016/j.ssi.2020.115291 https://doi.org/10.1007/s00706-020-02675-6 https://doi.org/10.1021/jacs.0c08870 https://doi.org/10.1080/00150198808008840 https://doi.org/10.1126/science.109092 Chimica Techno Acta 2022, vol. 9(3), No. 20229303 ARTICLE 5 of 5 9. Taninouchi Y, Uda T, AwakuraY, Ikeda A, Haile SM. Dehydra- tion behavior of the superprotonic conductor CsH2PO4 at moderate temperatures: 230 to 260 °С. J Mater Chem. 2007;17:3182–3189. doi:10.1039/B704558C 10. Qing G, Kikuchi R, Takagaki A, Sugawara T, Oyama ST. CsH2PO4/polyvinylidene fluoride composite electrolytes for intermediate temperature fuel cells. J Electrochem Soc. 2014;161:451–457. doi:10.1149/2.052404jes 11. Xie Q, Li Y, Hu J, Chen X, Li H. A CsH2PO4-based composite electrolyte membrane for intermediate temperature fuel cells. J Membrane Sci. 2015;489:98–105. doi:10.1016/j.memsci.2015.03.083 12. Qing G, Kikuchi R, Takagaki A, Sugawara T, Oyama ST. CsH2PO4/epoxy composite electrolytes for intermediate tem- perature fuel cells. Electrochim Acta. 2015;169:219–226. doi:10.1016/j.electacta.2015.04.089 13. Bagryantseva IN, Gaydamaka AA, Ponomareva VG. Interme- diate temperature proton electrolytes based on cesium dihy- drogen phosphate and Butvar polymer. Ionics. 2020;26:1813– 1818. doi:10.1007/s11581-020-03505-9 14. Bagryantseva IN, Ponomareva VG, Lazareva NP. Proton- conductive membranes based on CsH2PO4 and ultradispersed polytetrafluoroethylene. Solid State Ionics. 2019;329:61–66. doi:10.1016/j.ssi.2018.11.010 15. Bagryantseva IN, Ponomareva VG, Khusnutdinov VR. Inter- mediate temperature proton electrolytes based on cesium di- hydrogen phosphate and poly (vinylidene fluoride-co- hexafluoropropylene). J Materi Sci. 2021;56(25):14196– 14206. doi:10.1007/s10853-021-06137-0 16. Navarrete L, Yoo CY, Serra JM. Comparative study of epoxy‐ CsH2PO4 composite electrolytes and porous metal based elec- trocatalysts for solid acid electrochemical cells. Membranes. 2021;11:196. doi:10.3390/membranes11030196 17. Koch EC. Metal-fluorocarbon based energetic materials. Wiley-VCH Verlag GmbH & Co. KGaA;Germany, 2012. P. 360. 18. Drobny JG. Technology of fluoropolymers. Taylor & Francis Group:LLC:Boca Raton;2009.248. 19. Ameduri B. From vinylidene fluoride (VDF) to the applica- tions of vdf-containing polymers and copolymers: recent de- velopments and future trends. Chem Rev. 2009;109:6632– 6686. doi:10.1021/cr800187m 20. Lovinger AJ, Davis DD, Cais RE, Kometani JM. Compositional variation of the structure and solid-state transformations of vinylidene fluoride/tetrafluoroethylene copolymers. Macro- molec. 1988;21:78–83. doi:10.1021/ma00179a017 https://doi.org/10.1039/B704558C https://doi.org/10.1149/2.052404jes https://doi.org/10.1016/j.memsci.2015.03.083 https://doi.org/10.1016/j.electacta.2015.04.089 https://doi.org/10.1007/s11581-020-03505-9 https://doi.org/10.1016/j.ssi.2018.11.010 https://doi.org/10.1007/s10853-021-06137-0 https://doi.org/10.3390/membranes11030196 https://doi.org/10.1021/cr800187m https://doi.org/10.1021/ma00179a017