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: [email protected]
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