The Design of PEMFC MEAs with Improved Tolerance to Start up / Shut Down Events

Performance degradation during start-up (SU) and shut-down (SD) is an important issue affecting durability and lifetime of proton exchange membrane fuel cells (PEMFCs).  SU/SD processes generate very dynamic (albeit, transient) operating conditions in which the local gas mixtures differ to those of steady state conditions.  Briefly, via diffusional processes during shut down, air can slowly fill the anode flow field of the fuel cell causing an air/H₂ front; conversely, a H₂/air front is generated during SU.  These mixed-gas fronts cause the cathode potential to become abnormally high (potentials of 1.40 – 1.75 V have been reported¹) at which point the cathode carbon is prone to oxidation.  Owing to loss of the electrocatalyst support as CO₂, the resulting changes in layer mass transport properties and reduction of electrochemical area, the negative and irreversible impact on performance can be dramatic.  Though this degradation mode has been known for some years, it has become the subject of increased attention owing to the requirements of the automotive sector.

Recent studies to understand this degradation mechanism includes the measurement and analysis of internal currents using segmented cells,^2,3 reference electrode methods,⁴ and adoption of the dual-cell configuration.⁵ These works suggest that the extent of carbon corrosion is affected by the duration of the mixed-gas front, temperature, humidity and gas flow rates.⁶  The impact of this corrosion on the components of the MEA has also been comprehensively studied.^6,7  Mitigation strategies to minimise the effects of SU/SD have been developed (summarised in a recent review¹) with approaches including the utilisation of a gas purge on the anode and/or cathode before SU / after SD, the application of an auxiliary load to consume residual gases, the use of electrical shorting to eliminate the high potential at the cathode etc. 

Since mitigation at the stack/system level increases cost and complexity, there remains a requirement to increase the tolerance of the MEA to operation under mixed air/fuel fronts; here, comparatively little has been reported.   The anode electrode has a particularly important role since it is the ORR capability of the anode Pt/C (in addition to its primary role of hydrogen oxidation) which drives the corrosion reaction.  In this paper, we present work on anode catalyst layers incorporating novel catalyst materials which have reduced ORR capability but which maintain sufficient HOR functionality – found to have a very significant impact on MEA durability.  Further, selection of a highly stable carbon support material leads to an improvement in MEA durability.  Of course, highly stable carbons can generate other issues – not least that they are challenging to catalyse and at best, offer good, but condition sensitive, polarisation performance.  Here we also report on the approach of adding oxygen evolution functionality to the cathode such that, under high potentials, water is hydrolysed in preference to carbon corrosion.  The benefits of these modified MEA designs are demonstrated under accelerated SU/SD test methods, with the use of CO₂ monitoring, to validate the suppression of cathode corrosion.  After some challenges, solutions were found in which the durability of the MEA was considerably increased, without compromising the basic polarisation performance.

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[1] Yu et al., J. Power Sources, 205 (2012), pp. 10-23.

[2] A. Lamibrac et al., J. Power Sources, 196, (2011), 9451-9458

[3] J. Durst et al., Applied Catal. B: Environmental, 138-139 (2013), 416-426

[4] G. Hinds and E. Brightman, Electrochem. Comm., 17, (2012), pp. 26-29

[5] C. A. Reiser et al., Electrochem and Solid State Lett., 8 (2005), A273-276  and  H. Tang et al., J. Power Sources, 158 (2006) pp. 1306-1312

[6] Y.C. Park et al., Electrochim. Acta, 123 (2014) pp. 84-92.

[7] N. Linse et al., J. Power Sources, 219 (2012), pp. 240-248.