Investigation of PEMFC start-up/shut-down

degradation using reference electrode array

Gareth Hinds

National Physical Laboratory

United Kingdom

Tel: + 44 20 8943 7147 Email: gareth.hinds@npl.co.uk

London

UK’s national standards laboratory (~ 700 scientists)

Based in Teddington, South West London

Top 3 among 54 National Measurement Institutes

The importance of measurement

“In physical science the first essential step in the direction of learning any subject is to find principles of numerical reckoning and practicable methods for measuring some quality connected with it. I often say that when you can measure what you are speaking about, and express it in numbers, you know something about it; but when you cannot measure it, when you cannot express it in numbers, your knowledge is of a meagre and unsatisfactory kind; it may be the beginning of knowledge, but you have scarcely in your thoughts advanced to the state of science, whatever the matter may be.”

Lord Kelvin, 3 May 1883

“If you can’t measure it, you can’t improve it.”

Other things Lord Kelvin said….

“Heavier-than-air flying machines are impossible.”

“The Earth is between 20 million and 100 million years old.” “There is nothing new to

be discovered in physics now. All that remains is more and more precise

measurement.” “Only 400 years of

oxygen supply remain on the planet.”

“Large increases in cost with questionable increases in performance…

…can be tolerated only in racehorses and women.”

Automotive

Stationary

Portable

Barriers

Durability

Refuelling

Cost

Degradation

mechanisms

poorly understood

Lack of standardised

test methodology

Lack of in situ

measurement capability

PEMFCs Applications

Expertise in fuel cell measurement, modelling

and test method development

PEMFC research at NPL

Fuel cell modelling

Catalyst characterisation Durability testing

In situ diagnostics

Start-up/shut-down degradation

C.A. Reiser, L. Bregoli, T.W. Patterson, J.S. Yi, J.D. Yang, M.L. Perry, T.D. Jarvi, Electrochem. Solid State Lett., 8, A273 (2005).

Reverse current decay mechanism: corrosion of the carbon support on the cathode can occur during PEMFC start-up/shut-down due to the presence of an air/fuel boundary at the anode, which leads to a gradual decrease in available catalyst surface area on the cathode

Start-up degradation

Cathode

Shut-down degradation

Cathode

Mitigation strategies (engineering)

Cathode

Inert gas purge

Mitigation strategies (engineering)

R

Cathode

e

Application of

external load

Reduced lateral current

Mitigation strategies (materials)

Cathode

More corrosion resistant carbon support materials

OER catalysts

Mitigation strategies (materials)

Cathode

Decrease lateral

electronic conductivity

×

What do we measure?

Evolved CO2

Electrode potential

A reference electrode is an electrode against which the potential of an electrode of interest may be measured. Requirements for reliable measurement are:

Constant, stable potential

No source of contamination

No perturbation of system being studied

Liquid electrolytes

Luggin capillary (salt bridge) placed in ionic current path close to working electrode

If current and electrolyte resistance are known, correction can be made for the potential drop in solution

Thin solid electrolytes

Positioning of reference electrode is hampered by geometric constraints

Very difficult to obtain reliable measurements

Reference electrodes

Fuel cell reference electrodes

Conventional fuel cell reference electrodes may be divided into two categories:

External (edge) type – electrode attached to edge of membrane

most are of this type

Internal (sandwich) type – electrode sandwiched between two membranes

Fuel cell reference electrodes

Disadvantages:

External – far from main ionic current path, dominated by edge effects

Internal – perturb charge and water transport in membrane

Both – take no account of potential drop in membrane

cf. Piela et al, J. Phys. Chem. C, 111, 6512 (2007)

Salt bridge consists of

Nafion tubing (ID 0.64 mm, OD 0.84 mm) supplied by Perma Pure

(New Jersey)

Nafion tubing encased in

PTFE tubing (ID 1.01 mm, OD 1.27 mm)

which is filled with deionised water to maximise proton

conductivity in the Nafion

Ion-conducting

path to catalyst layer is made by impregnating GDL

with Nafion over a small ‘landing

area’ for salt bridge

NPL reference electrode

G. Hinds and E. Brightman, Electrochem. Commun. 17 (2012) p.26–29

Reference electrode array

Nafion tubing

sheathed in PTFE Miniature O-ring

RE1 RE2 RE3

RE6 RE5 RE4

RE7 RE8 RE9

Anode gas

INLET

OUTLET

7 c

m

Gaskatel Hydroflex™ ET070

Platinised gas diffusion electrode with replaceable hydrogen cartridge

Cell temperature: 80 °C

Anode flow rate: 0.2 sL/min

Cathode flow rate: 1.0 sL/min

5 cycles (OCP) 10 cycles (0.013 load) 5 cycles (OCP)

RH values: 100%, 66%, 30%

CO2 Probe: Vaisala GMP343 IR-probe H2 or air in H2 or air out

Zero Air in

(< 1 ppm CO2)

Exhaust

Air out

3-way

valve

Reference Electrodes

Condenser

Measurement of evolved CO2

Anode

Cathode

Measurement of evolved CO2

0 50 100 150 200

0

20

40

60

80

CO

2 c

on

ce

ntr

atio

n (

pp

m)

Time (s)

Start-up

Shut-down

100% RH

Measurement of evolved CO2

Contradictory literature results

Authors Institution Publication More severe

corrosion?

S. Kreitmeier, A.

Wokaun, F.N. Buchi

Paul Scherrer

Institute

JES 159 (2012)

F787-F793 Start-up

N. Linse, G.C.

Scherer, A. Wokaun,

L. Gubler

Paul Scherrer

Institute

JPS 219 (2012)

240-248 Shut-down

W. Gu, R.N. Carter,

P.T. Yu, H.A.

Gasteiger

General Motors ECS Transactions

11 (2007) 963-973 Start-up

This work NPL - Start-up

Potential transients on cathode during start-up

0.0

0.5

1.0

1.5

Cath

ode p

ote

ntial

vs R

HE

(V

)

Anode inlet

(H2 in)

Anode outlet

Potential transients on cathode during shut-down

0.0

0.5

1.0

1.5

Cath

ode p

ote

ntial

vs R

HE

(V

)

Anode inlet

(air in)

Anode outlet

Potential transients

on cathode

RE1 RE2 RE3

RE6 RE5 RE4

RE7 RE8 RE9

Anode gas

INLET

OUTLET

Start-up

Shut-down

Potential transients on anode during start-up

0.0

0.5

1.0

Cath

ode p

ote

ntial

vs R

HE

(V

)

Anode inlet

(H2 in)

Anode outlet

Potential transients on anode during shut-down

Cath

ode p

ote

ntial

vs R

HE

(V

)

0.0

0.5

1.0

Anode inlet

(air in)

Anode outlet

Potential transients on cathode (open circuit)

Start-up

Shut-down

Increasing RH

Potential transients on cathode (external load)

Start-up

Shut-down

Increasing RH

Duration of potential transients on cathode

Open circuit

External load

Dotted lines show calculated residence time in flow-field

Maximum potential of transients on cathode

Open circuit

External load

Minimum potential of transients on cathode

Open circuit

External load

Comparison of anode and cathode (open circuit)

Cathode

Anode

Potential transients on cathode (external load)

Start-up

Shut-down

Decreasing anode flow rate 66% RH

Combination of in situ potential mapping using NPL reference electrode array and CO2 measurement at cathode outlet has been applied to study of SU/SD in an operating PEMFC

Powerful new technique for the evaluation of SU/SD tolerant catalysts, optimisation of hardware design and assessment of mitigation strategies

Technique is now being applied to commercial hardware (both fuel cells and electrolysers) in collaboration with our industrial partners and is available under H2FC project via transnational access activities

Summary

UK Department of Business, Innovation & Skills

Industrial Advisory Group

Acal Energy

AFC Energy

C Tech Innovation

Intelligent Energy

ITM Power

Johnson Matthey

Logan Energy

Fuel cell components supplied by Johnson Matthey

Acknowledgements