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. Only 400 years of oxygen supply remain on the planet. There is nothing new to be discovered in physics now. All that remains is more and more precise measurement. Large increases in cost with questionable increases in performance can be tolerated only in racehorses and women.
Barriers PEMFCs Cost Applications Portable Stationary Degradation mechanisms poorly understood Durability Automotive Lack of in situ measurement capability Refuelling Expertise in fuel cell measurement, modelling and test method development Lack of standardised test methodology
PEMFC research at NPL Fuel cell modelling In situ diagnostics Durability testing Catalyst characterisation
Start-up/shut-down degradation 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 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).
Start-up degradation Cathode
Shut-down degradation Cathode
Mitigation strategies (engineering) Inert gas purge Cathode
Application of external load Mitigation strategies (engineering) e R Cathode Reduced lateral current
Mitigation strategies (materials) OER catalysts More corrosion resistant carbon support materials Cathode
Decrease lateral electronic conductivity Mitigation strategies (materials) Cathode
Electrode potential What do we measure? Evolved CO 2
Reference electrodes 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
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)
NPL reference electrode Ion-conducting path to catalyst layer is made by impregnating GDL with Nafion over a small landing area for salt bridge 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 G. Hinds and E. Brightman, Electrochem. Commun. 17 (2012) p.26 29
7 cm Reference electrode array Gaskatel Hydroflex ET070 Platinised gas diffusion electrode with replaceable hydrogen cartridge Nafion tubing sheathed in PTFE Miniature O-ring RE7 RE8 RE9 OUTLET RE6 RE5 RE4 RE1 RE2 RE3 Anode gas INLET
Measurement of evolved CO 2 CO 2 Probe: Vaisala GMP343 IR-probe H 2 or air in H 2 or air out Condenser Air out 3-way valve Anode Cathode Zero Air in (< 1 ppm CO 2 ) 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% Exhaust Reference Electrodes
CO 2 concentration (ppm) Measurement of evolved CO 2 80 Start-up 60 40 Shut-down 20 0 100% RH 0 50 100 150 200 Time (s)
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
Cathode potential vs RHE (V) Potential transients on cathode during start-up 1.5 Anode outlet 1.0 0.5 0.0 Anode inlet (H 2 in)
Cathode potential vs RHE (V) Potential transients on cathode during shut-down 1.5 1.0 0.5 0.0 Anode outlet Anode inlet (air in)
Potential transients on cathode Start-up RE7 RE8 RE9 OUTLET RE6 RE5 RE4 RE1 RE2 RE3 Anode gas INLET Shut-down
Cathode potential vs RHE (V) Potential transients on anode during start-up Anode outlet 1.0 0.5 0.0 Anode inlet (H 2 in)
Cathode potential vs RHE (V) Potential transients on anode during shut-down Anode outlet 1.0 0.5 0.0 Anode inlet (air in)
Start-up Potential transients on cathode (open circuit) Shut-down Increasing RH
Start-up Potential transients on cathode (external load) Shut-down Increasing RH
Duration of potential transients on cathode Open circuit Dotted lines show calculated residence time in flow-field External load
Maximum potential of transients on cathode Open circuit External load
Minimum potential of transients on cathode Open circuit External load
Cathode Comparison of anode and cathode (open circuit) Anode
Start-up Potential transients on cathode (external load) Shut-down 66% RH Decreasing anode flow rate
Summary Combination of in situ potential mapping using NPL reference electrode array and CO 2 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
UK Department of Business, Innovation & Skills Industrial Advisory Group Acal Energy AFC Energy C Tech Innovation Intelligent Energy ITM Power Johnson Matthey Logan Energy Acknowledgements Fuel cell components supplied by Johnson Matthey