NOVEL MIDDLE TEMPERATURE PEM FUEL CELL MEMBRANES - POLYBENZIMIDAZOLE CONTAINING IMMOBILIZED PHOSPHONIC OR SULFONIC ACID GROUPS

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1 OVEL MIDDLE TEMPERATURE PEM FUEL CELL MEMBRAES - POLYBEZIMIDAZOLE COTAIIG IMMOBILIZED PHOSPHOIC OR SULFOIC ACID GROUPS V. Sinigersky 1, H. Penchev 1, F. Ublekov 1, M. Staneva 1, D. Budurova 1, I.Radev 2, V. Peinecke 2 1 Institute of Polymers Bulgarian Academy of Sciences, Sofia, Bulgaria 2 Zentrum für BrennstoffzellenTechnik ZBT GmbH, Duisburg, Germany

2 OUTLOOK Ø Introduction afion and PA doped PBI Ø Objectives Ø Starting materials Ø Preparation of membranes: m-pbi containing cross-linked PVPA p-pbi containing cross-linked PVSA Ø Characterization of the membranes Ø Proton conductivity measurements Ø Summary

3 Piero Lunghi EFC Conference , Rome, Italy afion and PA Doped PBI Limitations afion : CF 2 CF 2 x CF 2 CF O - Low working temperatures up to 80 o C, 100% RH - Water management (min. 7 molecules H 2 O/-SO 3 H group) - Relatively high methanol cross-over - Expensive (over 600 USD/m 2 ) y CF 2 CF CF 3 O m CF SO 3 H 2 n pka = - 6 Superacid! Phosphoric acid doped PBI : H OH O OH * *n P H OH pka 1 = 2.1 pka 2 = 7.2 pka 3 = 12.6 Middle strong, buffering acid - Can be operated only at temperatures above 100 o C (usually 160 o C) - If operated under 100 o C the drained water (reaction product) washes the electrolyte (phosphoric acid) out the proton conductivity decreases

4 PBI containing PO 3 H 2 groups PBI/PA H OH O OH 5 40 H3 PO * 4 *n P per PBI unit H OH Polymeric acid alternative: CH 2 CH Polyvinylphosphonic acid (PVPA) P(O)(OH) 2 In CH 2 CH n P(O)(OH) 2 Problem: PVPA is water soluble Polymerisation of Vinylphosphonic acid Solution: In order to become water insoluble PVPA has to be - Grafted to the PBI backbone - Cross-linked in the PBI matrix Known: Membrane Celtec V (BASF Fuel Cell GmbH) PBI, containing grafted/ crosslinked PVPA. Prepared by Electron beam irradiation of a PBI film, containing VPA. Outperforms afion in therm of methanol cross-over. Proton conductivity 100 ms.cm- 1 (100 o C, wet)

Objectives Preparation of membranes, comprising semi interpenetrating networks PBI, containing cross-linked polyvinyl phosphonic or polyvinylsulfonic acid (PVPA or PVSA) Synthetic

5 Objectives Preparation of membranes, comprising semi interpenetrating networks PBI, containing cross-linked polyvinyl phosphonic or polyvinylsulfonic acid (PVPA or PVSA) Synthetic concept: Introduction of vinylphosphonic acid (VPA) or polyvinylsulfonic acid (VSA), cross-linker and initiator in porous PBI film. Polymerization/cross-linking of the acid in the PBI matrix.

6 Starting materials m-pbi n * VPA PO 3 H 2 H p-pbi H n O VSa SO 3 a CH 3 H 2.2HCl H H 2 CH 3 CH 3 CH3 H O O Initiator (V50) Cross-linker Trially triazin trion

7 Preparation of membranes Membranes, containing cross-linked PVPA (PBI crosspvpa membranes) Dry PBI film VPA swelling PBI containing VPA exchange VPA, Initiator Cross-linker PBI membrane, containing cross-linked PVPA UV irradiation, Δ Polymerization/ cross-linking PBI containing VPA, Initiator and Cross-linker

Preparation of membranes Step 1: Introduction of VPA in the dry PBI film In a closed vessel a piece of dry PBI film, was swollen in a bath containing VPA and 3-5%

Dimensions increase: up to 100% Weight uptake: up to 1000% Step 2: Introduction of VPA, initiator and cross-linker The film prepared in Step 1 was transferred in a a

8 Preparation of membranes Step 1: Introduction of VPA in the dry PBI film In a closed vessel a piece of dry PBI film, was swollen in a bath containing VPA and 3-5% water for 2 hours (70 o C). Dimensions increase: up to 100% Weight uptake: up to 1000% Step 2: Introduction of VPA, initiator and cross-linker The film prepared in Step 1 was transferred in a a bath, containing VPA, Cross-linker (5%), initiator (1.5%) and 10% water. Exchange was completed for 3h at RT. Step 3: Polymerization/cross-linking of VPA in the PBI matrix The film prepared in Step 2 was irradiated with UV light (intensity 0.08 W/cm2) The temperature rises to C. Irradiation time: 5-15 min

9 Preparation of membranes Membranes, containing cross-linked PVSA (PBI crosspvsa membranes) p-pbi/pa membrane Washing with water Porous PBI filled with water exchange VSa, Initiator Cross-linker PBI containing VSa, Initiator and Cross-linker PBI membrane, containing cross-linked PVSA 5% HCl Water PBI membrane, containing cross-linked PVSa UV irradiation, Δ Polymerization/ cross-linking

10 Preparation of membranes Step 1: Preparation of porous PBI film, filled with water: A PBI membrane, doped with PA (prepared by the sol-gel method from the reaction mixture (5 wt.% PBI) was washed with ammonia and water. Step 2: Introduction of VSa, initiator and cross-linker: The film prepared in step 1 was transferred into a bath, containing 25%water solution of VSa, cross-linker (2-5 wt.%), initiator (1-2% wt.) and ethanol. Exchange was completed in 3h at RT Step 3: Polymerization/cross-linking of VSa in the PBI matrix: The film prepared in Step 2 was irradiated with UV light (up to 4h) or heated in a furnace (80 o C) for up to 96h. Step 4: Acidification transfer of the SO 3 a group in SO 3 H: The film from step 3 was washed subsequently with 5%HCl and water.

11 Characterization Before characterization the membranes were washed subsequently with aqueous ammonia (removing unreacted reagents and homopolymer PVPA) and water, then dried. Contents of cross-linked polyacid (water insoluble): determined gravimetrically from the weight uptake according to the Weight of the starting PBI film. From the weight ratio cross-linked polyacid /PBI the number of PVPA/ PVSA units per PBI repeating unit was calculated. 1 H MR (H 2 SO 4 -d2, 80 o C), Ar H -CH 2 -CH ppm (f1) TGA

12 Membranes prepared Using the procedures described membranes of very good quality (smooth, flexible) were obtained. Very high contents of immobilized PO 3 H 2 and SO 3 H groups in the PBI matrix has been achieved: - Membranes, containing cross-linked PVPA 8.8 to 15.4 VPA groups per PBI repeat unit - Membranes, containing cross-linked PVSA to 4.6 VSA groups per PBI repeat unit All membranes exhibit good mechanical properties and thermal stability

13 Proton conductivity measurements All proton conductivity measurements were performed at Zentrum für BrennstoffzellenTechnik ZBT GmbH, Duisburg, Germany. The EasyCellTest method, developed at Institute of Electrochemistry and Electrical Sources, Bulgarin Academy of Sciences, has been used. Membranes, containing cross-linked PVPA: Measurements at 80 o C and 100 o C, RH %, Membranes, containing cross-linked PVSA: Measurements at 60 o C, 80 o C and 95 o C, Wet cell (the measuring cell immersed in water)

14 Proton conductivity measurements Proton conductivity of PBI membranes, containing cross-linked PVPA and afion 117, measured at 80 ºС и 100% RH Membrane Weight ratio PBI/Crosslinked PVPA VPA units per PBI σ, ms.cm -1 PC 1 1/ PC 2 1/ PC 3 1/ PC 4 1/ afion

15 Proton conductivity measurements Proton conductivity of PBI membranes, containing cross-linked PVPA measured at 100 ºС and % RH Membrane VPA units per PBI σ, ms.cm-1 RH 100% RH 80% RH 50% RH 20% PC PC PC PC

16 Proton conductivity measurements Proton conductivity of PBI membranes, containing cross- linked PVSA measured at different temperatures (wet cell) Mem- brane Method of polymeriza4on/ Cross- linking VPA units per PBI σ, ms.cm-1 60 o C 80 o C 95 o C T1 Thermal, oc, 72h T2 Thermal, oc, 96h UV1 UV, 3h UV2 UV, 4h

17 Summary Ø Using an original procedure, PBI membranes, containing crosslinked PVPA or PVSA have been prepared Ø High concentrations of immobilised (water insoluble) PO 3 H 2 and -SO 3 H groups have been achieved up to 15 VPA and up to 4.6 VSA groups per PBI repeating unit Ø The membranes prepared are of good qulity and thesmaly stable Ø Proton conductivity increases with increasing the contents of PO 3 H 2 and -SO 3 H groups in the mebrane Ø The highest proton conductivity was measured for the membrane with 15 VPA groups per PBI repeating unit ms.cm -1 at 100%RH, 60.5 ms.cm -1 at 50% RH and 8.3 ms.cm -1 at 20% RH Ø The membranes, containing -SO 3 H exhibit high proton conductivity only in the wet state and o C Ø In the fully hydrated state the PBI membranes, containing crosslinked PVPA exhibit proton conductivities close to these of the Celtec V (BASF Fuel Cell GmbH) and afion 117 membranes

18 Acknowledgement: The authors would like to thank the Bulgarian Science Fund (Project PemHydroGen, ДТК 02/68) for funding the research and EU project POLIOVA, for funding this presentation THAK YOU VERY MUCH FOR YOUR KID ATTETIO

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