novel middle temperature pem fuel cell … · novel middle temperature pem fuel cell membranes -...

NOVEL MIDDLE TEMPERATURE PEM FUEL CELL MEMBRANES - POLYBENZIMIDAZOLE

CONTAINING IMMOBILIZED PHOSPHONIC OR SULFONIC ACID GROUPS

V. Sinigersky1, H. Penchev1, F. Ublekov1, M. Staneva1,

D. Budurova1, I.Radev2, V. Peinecke2

1 Institute of Polymers – Bulgarian Academy of Sciences, Sofia, Bulgaria 2 Zentrum für BrennstoffzellenTechnik ZBT GmbH, Duisburg, Germany

PIERO LUNGHI EFC CONFERENCE 11-13.12.2013, ROME, ITALY

OUTLOOK

Ø  Introduction – Nafion 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


Nafion® and PA Doped PBI – Limitations

Nafion®: - Low working temperatures – up to 80oC, 100% RH - Water management (min. 7 molecules H2O/-SO3H group) - Relatively high methanol cross-over - Expensive (over 600 USD/m2)

Phosphoric acid doped PBI : - Can be operated only at temperatures above 100oC (usually 160oC) - If operated under 100oC the drained water (reaction product) washes the electrolyte (phosphoric acid) out – the proton conductivity decreases

y

SO3H

CF2CF2 x

CF

CF3

O OCF2 CF2 n

CF2CF

m

N

NN

N* *n P

H

H OH

OHO OH

pKa = - 6 Superacid !

pKa1= 2.1 pKa2 = 7.2 pKa3 = 12.6 Middle strong, buffering acid

Piero Lunghi EFC Conference 11-13.12.2013, Rome, Italy

PBI/PA N

NN

N* *n P

H

H OH

OHO OH 5 – 40 H3PO4

per PBI unit

Polymeric acid alternative: Polyvinylphosphonic acid (PVPA) Problem: PVPA is water soluble

n CH2 CH

P(O)(OH)2

CH2 CH

P(O)(OH)2

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 Nafion in therm of methanol cross-over. Proton conductivity – 100 mS.cm-1 (100oC, wet)

In

PBI containing –PO3H2 groups


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.

Objectives


N

NN

N*n

N

NH

N

NH

n

Starting materials

NN

CH3

CH3CH3

CH3

NH

NH2

NH

NH2.2HCl

N

N

N

O

O

O

PO3H2

SO3Na

m-PBI

p-PBI

VPA

VSNa

Initiator (V50) Cross-linker Trially triazin trion


Membranes, containing cross-linked PVPA (PBI crossPVPA membranes)

Dry PBI film

PBI containing VPA, Initiator and Cross-linker

PBI membrane, containing

cross-linked PVPA

VPA

swelling PBI containing VPA

exchange VPA, Initiator Cross-linker

UV irradiation, Δ

Polymerization/ cross-linking

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 oC) . 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 60-80°C. Irradiation time: 5-15 min

Membranes, containing cross-linked PVSA (PBI crossPVSA membranes)

p-PBI/PA membrane

PBI containing VSNa, Initiator and Cross-linker


cross-linked PVSNa

exchange

VSNa, Initiator Cross-linker

UV irradiation, Δ

Polymerization/ cross-linking


Porous PBI filled with

water

Washing with water

5% HCl

Water


cross-linked PVSA

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 VSNa, initiator and cross-linker: The film prepared in step 1 was transferred into a bath, containing 25%water solution of VSNa, cross-linker (2-5 wt.%), initiator (1-2% wt.) and ethanol. Exchange was completed in 3h at RT

Step 3: Polymerization/cross-linking of VSNa in the PBI matrix: The film prepared in Step 2 was irradiated with UV light (up to 4h) or heated in a furnace (80 oC) for up to 96h.

Step 4: Acidification – transfer of the –SO3Na group in –SO3H: The film from step 3 was washed subsequently with 5%HCl and water.


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. 1H NMR (H2SO4-d2, 80 oC),

ppm (f1) 1.02.03.04.05.06.07.08.09.0

8.686

8.475

8.345

8.259

6.337

6.121

5.904

3.112

3.077

2.528

2.515

2.497

10.00

14.04

Ar H -CH2-CH-


TGA

Membranes prepared

Using the procedures described membranes of very

good quality (smooth, flexible) were obtained. Very high contents of immobilized –PO3H2 and –SO3H

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 - 0.8 to

4.6 VSA groups per PBI repeat unit

All membranes exhibit good mechanical properties and thermal stability


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 80oC and 100oC, RH 20-100%,

Membranes, containing cross-linked PVSA:

Measurements at 60oC, 80oC and 95oC,

Wet cell (the measuring cell immersed in water)



Membrane Weight ratio PBI/Cross-

linked PVPA

VPA units per PBI

σ,

mS.cm-1

PC 1 1/3.1 8.8 65.0

PC 2 1/4 11.4 62.1 PC 3 1/4.7 13.3 80.2

PC 4 1/5.4 15.4 86.7

Nafion 117 105.0

Proton conductivity of PBI membranes, containing cross-linked PVPA and Nafion 117, measured at 80 ºС и 100% RH


Membrane

VPA units per PBI

σ, mS.cm-1

RH 100% RH 80% RH 50% RH 20%

PC 1 8.8 59.3 50.5 15.3 3.2

PC 2 11.4 53.9 44.7 37.1 5.0

PC 3 13.3 80.3 69.1 27.8 8.7

PC 4 15.4 82.6 80.8 60.5 8.3

Proton conductivity of PBI membranes, containing cross-linked PVPA measured at 100 ºС and 20 - 100% RH



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 oC 80 oC 95 oC

T1 Thermal, 80 oC, 72h

3.0 25.0 29.2 31.5

T2 Thermal, 80 oC, 96h

3.6 28.0 38.2 45.2

UV1 UV, 3h 3.7 29.1 37.3 42.2

UV2 UV, 4h 4.6 51.3 55.3 61.8


Summary

Ø  Using an original procedure, PBI membranes, containing cross- linked PVPA or PVSA have been prepared

Ø  High concentrations of immobilised (water insoluble) –PO3H2 and -SO3H 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 –PO3H2 and -SO3H groups in the mebrane

Ø  The highest proton conductivity was measured for the membrane with 15 VPA groups per PBI repeating unit - 86.7 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 -SO3H exhibit high proton conductivity only in the wet state and 80-95 oC

Ø  In the fully hydrated state the PBI membranes, containing cross- linked PVPA exhibit proton conductivities close to these of the Celtec V® (BASF Fuel Cell GmbH) and Nafion 117 membranes



Acknowledgement: The authors would like to thank the Bulgarian Science Fund

(Project PemHydroGen, ДТК 02/68) for funding the research

and EU project POLINNOVA, № 316086 for funding this

presentation

THANK YOU VERY MUCH FOR YOUR KIND ATTENTION

novel middle temperature pem fuel cell … · novel middle temperature pem fuel cell membranes -...

Documents