Slide 1

Membranes for recombination and electro-oxidation of permeated hydrogen in PEM electrolysis

Dmitri Bessarabov, Andries KrugerHySA Infrastructure CoC, NWU, South Africa

International Conference on Electrolysis (ICE 2017), Copenhagen, June 12-15, 2017

http://www.hysainfrastructure.org/dmitri.bessarabov@nwu.ac.za

Slide 2

Presentation Outline

HySA: backgroundSystem vs. Membranes trade-offs: ElectrolysisGas permeation in PEM FC and Electrolyzres:

challengesMembranes with recombination catalystResults

HySA Infrastructure: Location and Facilities

HySAInfrastructure

headquarters at the North-West

University (Potchefstroom)

HySAInfrastructure at the CSIR

"Platinum" province of South Africa

System vs. Membranes trade-offs: Electrolysis – safety first !

The system design is largely determined by the gas permeation properties vs. discharge pressure of hydrogen, operating current density and durability in PEM electrolyzer. Membrane advancements enable system capability. Trade-off between system and membrane needs must be achieved.

Safety operation(hydrogen in oxygen)

Gas permeabilityHydrogen discharge pressure

Gas permeability Operating at high currentdensity (low CAPEX)

Thin membrane leadingto high gas permeation

Membrane thicknessOperating at high currentdensity (low CAPEX)Membrane durability

Safety operation(hydrogen in oxygen)

Gas permeability

Recent trends: MW-scale systems

Hydrogen discharge pressure

Private communication with militaries: traces of H2 in air results in nausea and sickness

Definition of Permeability

What are the challenges

How it is measured

Challenges with Permeability measurements

Schematic of hydrated PFSA matrix

Solute in solvent  Order of magnitude of diffusion coefficients in cm2/sGas in gases  ~ 1101  Gas in liquids  ~ 5101  (e.g., H2 in water: 

5105.4 ) Gas in polymers and glasses ~ 8/7101 (e.g., H2 in PTFE 

71047.1 ) Gas in solids  ~ 10101   

pDSpPA

J

Definition of Permeability Coefficient, P

SDP

S is the solubility coefficient, cm3(gas) cm–3(polymer) cm–1 Hg,D is the apparent diffusion coefficient (cm2 sec–1), L is the membrane thickness, A is the membrane area, and ∆p is the differential partial pressure.The permeability coefficient for gases in polymeric membranes can be expressed in a variety of system units. One of the most common - “Barrer” permeability unit: cm3(STP) cm sec-1 cm-2 cmHg-1

P could also be expressed in terms of steady-state crossover current density.

pnFPi

(n is assumed to be 2 for hydrogen), F is the Faraday constant, P is the permeability coefficient

Boundary conditions: H2 (and He) transport properties in water, PTFE, dry PFSA (Nafion and Hyflon Ion SSC)

*Higher crystallinity at given equivalent weight of the SSC ionomer compared to the LSC

Most consistent permeation measurements are done without electrolysis mode (e.g, in dry and semi-wet PFSA membranes)

Diffusion media  Temperature, C D, cm2/s S, cm3(STP)/cm3cmHg P, Barrer Ref Water 25 5105.4   4105.2   103 [1] PTFE 25   9.8 [2] PTFE 25 71047.1   31059.6   9.7 [3] PTFE 25 71047.1   31023.7   10.7 [4] Dry Nafion  35 71058.9 4107.9   9.3 [5] Dry Nafion  35   8.14 [6] Dry Nafion  35   11 [7] Dry Nafion  30 6100.2   4100.7   14 [8] Dry SSC Hyflon*  35 He gas He gas 28 [9] Dry Nafion  30 He gas He gas 41 [5]  

[1] Gevantman, L.H. , Solubility of selected gases in water, CRC Handbook of Chemistry and Physics, 95th Edition, edited by William M. Haynes, 2014-2015, pages 5-134[2]. R. A. Pasternak, M. V. Christensen, and J. Heller, Diffusion and Permeation of Oxygen, Nitrogen, Carbon Dioxide, and Nitrogen Dioxide through Polytetrafluoroethylene, Macromolecules, Vol. 3, No. 3, May – June, 366 - 371 (1970)[3]. H. Miyake, M. Matsuyama, K. Ashida, K. Watanabe, Permeation, diffusion, and solution of hydrogen isotopes, methane, and inert gases in/through tetrafluoroethylene and polyethylene, J. Vac. Sci. Technol. A-I, 3, 1447 (1983). http://doi.org/10.1116/1.572038[4]. C. San Marchi, Sandia National Laboratories, Livermore CA Technical Reference on Hydrogen Compatibility of Materials, 2012 http://www.ca.sandia.gov/matlsTechRef/chapters/8100TechRef_polymers.pdf[5]. Jeffrey S. Chiou and Donald R. Paul. Gas Permeation in a Dry Nafion Membrane, Ind. Eng. Chem. Res. 1988, 27, 2161-2164 [6]. Charles W. James Jr., Abhishek Roy, James E. McGrath, Eva Marand, Determination of the effect of temperature and humidity on the O2 sorption in sulfonated poly(arylene ether sulfone) membranes, Journal of Membrane Science 309 (2008) 141–145, [7]. Hamdy F.M. Mohamed, K. Ito, Y. Kobayashi , N. Takimoto , Y. Takeoka , A. Ohira, Free volume and permeabilities of O2 and H2 in Nafion membranes for polymer electrolyte fuel cells, Polymer 49 (2008) 3091–3097[8]. Tetsuo Sakai, Hiroyasu Takenaka, and Eiichi Torikai, Gas Diffusion in the Dried and Hydrated Nations, J. Electrochem. Soc.: ELECTROCHEMICAL SCIENCE AND TECHNOLOGY, Vol. 133, No. 1, January 1986, 88-92[9]. Jacopo Catalano, Marco Giacinti Baschetti, Maria Grazia De Angelis, Giulio Cesare Sarti, Aldo Sanguineti, Paolo Fossati, Gas and water vapor permeation in a short-side-chain PFSI membrane, Desalination 240 (2009) 341-346

Some related references

Permeability coefficient of H2 in water: upper boundary

Electrolysis mode

0.0

100.0

200.0

300.0

400.0

500.0

600.0

0 10 20 30 40 50 60 70 80 90

Perm

eability, Barrer

Temperature, C

Courtesy of Prof. Hiroshi Ito

Measurement accuracy of P decreases from A to D regimes

Most reliable data are for A and B regions (e.g., relatively dry membranes or well controlled RH% with no polarization)

C region – high water content, experimental difficulties in measurements

"D region" – electrolysis regime under polarization: dynamic system with possible gas "oversaturation"

Permeability coefficient (P) as a function of water uptake

Challenges with P= D*S measurements

1. Dynamic behaviour of water sorption in PFSA membranes2. Long equilibration time needed to establish semi-steady state3. Uncertainty re membrane thickness (swelling, CL – if applicable)4. Presence of ionomer in catalyst layer5. Electro-osmotic water transport under electrolysis mode6. Uncertainty re feed-gas partial pressure value, effect of (RH%)7. Possible "over-saturation" of cathode with hydrogen, if measured

in-situ under polarization8. Possible recombination of hydrogen during in-situ under

polarization

Cathode H2 over-saturation: not received adequate attention by academics. Anode over-saturation with O2 : not received adequate attention by academics

Cathode H2 oversaturationObserved hydrogen flux increase with current density

"The origin of the positive deviation of the H2 fluxes at increasing current densities is the build up of hydrogen oversaturation at the cathode catalyst. This oversaturation can reach values in the order of 20 bars at 1 A cm-2 "

The earliest ref found: "Gas permeation through an SPE membrane during electrolysis", R. Oberlin, S. Stucki, H.J. Christen, ISE Meeting, pp.434-436, 1982

Oversaturation variable depends on (1) current density, (2) cathode CL structure, (3) CL thickness, (4) current collector structure (porosity, etc)

In our recent work with 3M we found some electrode structures are of a very high H2flux. Could be due to unique NSTF structure. More research is needed.

Schematic of hydrogen concentration profile with cathode oversaturation under electrolysis mode

322222111

pSDpSDpSDAi

AAAlocal

121

where DA1 is apparent diffusion coefficient in polymer phase, DA2 is diffusion coefficient in water-rich phases, S1 and S2 are gas solubility in respected phases.

.C. Mittelsteadt, M. Umbrell, (Abstract #770), 207th ECS Meeting, Quebec City, Canada, (2005).

D=D0exp(−ED/RT) D=S0exp(−∆HS/RT )

D(H2 PTFE) ~ 10-7 D(H2 H2O) ~ 10-5

444333222111

pSDpSDpSDpSDAilocal

Simplified case

1 Volume fraction of a phase 1.

2

3Volume fraction of a phase 2.

Volume fraction of a phase 2 (water) = 0, in no electrolysis, function of a current density.

Recombination catalyst (Pt) inside the membrane

Anode Cathode

HySA MEMBRANE DESIGN

H2O2

Ptn

n

AAlocal

rec

kBnkpSDA

i

12

)1(21 2

2

DB ***

2

2

kCxCD

tС

AipSDpSDpSDpSDA

i reclocallocal

444333222111

"Good" Pt Recombination Membranes

SEM/EDX line scans

Carbon Oxygen

Fluorine Sulfur

Platinum

"Not-so-Good" Pt Recombination Membranes

Experimental section

SRI GC

BRUKER GC

Element Weight% Atomic%

C K 4.71 30.94Ti K 23.55 38.82Ni K 0.81 1.09Ir M 70.94 29.15

Totals 100.00

PFSA, 190 micron, 80° C, EW 1200, 25 cm2Ir on Ti anode

Pt on Ti cathode

GDL/membrane arrangements schematic,2 experiments: A,B

Membrane

GDL/membrane arrangements schematic: A,B

PFSA membrane, 190 micron, 80° C, EW 1200, 25 cm2

Pt profile

O2

H2

O2

H2

A B

+ - + -

0.00

0.50

1.00

1.50

2.00

2.50

3.00

3.50

4.00

0 0.5 1 1.5 2

%

Current Density (a/cm2)

Anode H2 in O2

Plain Membrane A

With Pt B

0.000.100.200.300.400.500.600.700.800.90

0 0.1 0.2 0.3 0.4 0.5 0.6

%

Current Density (A/cm2)

Cathode O2 in H2

Plain Membrane A

With Pt B

Effect of Pt in the membrane on the H2 cross-over

1

1.2

1.4

1.6

1.8

2

2.2

0 0.5 1 1.5 2

Volta

ge (V

)

Current Density (A/cm2)

Note: experiment was done with "catalyst free" membrane

Challenges associated with gas permeation measurements for PEM electrolysis process have been discussed. Data presented in C% do not describe actual membrane permeability, but practically useful for comparison purposes.

Measurement protocols still need to be refined. Some preliminary data reported suggested that permeation of

hydrogen during electrolysis are unusually large – could be explained by "oversaturation"

Mitigation strategies may include various approaches, including active solutions, passive solutions, and combinations of the above

Membranes with Pt recombination catalyst were produced and tested

Conclusions

Masahiro Watanabe, Hiroyuki Uchida and Masaomi Emori, Analyses of Self-Humidification and Suppression of Gas Crossover in Pt-Dispersed Polymer Electrolyte Membranes for Fuel Cells, J. Electrochem. Soc., Vol. 145, No. 4, April, 1998