In this paper, an extensive characterisation of a range of carbon blacks (CB) with similar surface area but different surface chemistry is carried out by flow calorimetry, Raman spectroscopy, dynamic water vapour sorption, instrumental gas analysis, nitrogen adsorption/desorption and high potential chronoamperometry. Using these carbon materials as supports, Pt/CB electrocatalysts are prepared by microwave-assisted polyol-mediated synthesis in gram scale. Structural, morphological and electrochemical properties of the prepared electrocatalysts are evaluated by X-ray diffraction, transmission electron microscopy, rotating disc electrode and in situ fuel cell characterisation of the corresponding membrane-electrode assemblies. The obtained results allow to establish a relationship between surface chemistry and electrochemical properties useful for the design of Pt/C catalyst layers with high performance and stability.
K E Y W O R D Scarbon black, electrocatalyst support, fuel cell cathodes, surface properties
| INTRODUCTIONCathodes of proton-exchange membrane fuel cells (PEMFC), where the oxygen reduction reaction (ORR) takes place, face many challenges to meet activity, durability and cost requirements: the reduction of noble metal loading while keeping high electroactivity, 1,2 the mitigation of catalyst and support degradation to enhance the lifetime of the devices 3,4 and the improvement in water management and mass transport to enhance their performance at high current density. 5,6
Here,
we report a study on the structural characteristics of membrane
electrode assembly (MEA) samples obtained from a low-temperature (LT)
polymer electrolyte membrane (PEM) fuel cell (FC) stack subjected
to long-term durability testing for ∼18,500 h of nominal operation
along with ∼900 on/off cycles accumulated over the operation
time, with the total power production being 3.39 kW h/cm2 of MEA and the overall degradation being 87% based on performance
loss. The chemical and physical states of the degraded MEAs were investigated
through structural characterizations aiming to probe their different
components, namely the cathode and anode electrocatalysts, the Nafion
ionomer in the catalyst layers (CLs), the gas diffusion layers (GDLs),
and the PEM. Surprisingly, X-ray diffraction and electron microscopy
studies suggested no significant degradation of the electrocatalysts.
Similarly, the cathode and anode GDLs exhibited no significant change
in porosity and structure as indicated by BET analysis and helium
ion microscopy. Nevertheless, X-ray fluorescence spectroscopy, elemental
analysis through a CHNS analyzer, and comprehensive investigations
by X-ray photoelectron spectroscopy suggested significant degradation
of the Nafion, especially in terms of sulfur content, that is, the
abundance of the −SO3
– groups
responsible for H+ conduction. Hence, the degradation of
the Nafion, in both of the CLs and in the PEM, was found to be the
principal mechanism for performance degradation, while the Pt/C catalyst
degradation in terms of particle size enlargement or mass loss was
minimal. The study suggests that under real-life operating conditions,
ionomer degradation plays a more significant role than electrocatalyst
degradation in LT-PEMFCs, in contrast to many scientific studies under
artificial stress conditions. Mitigation of the ionomer degradation
must be emphasized as a strategy to improve the PEMFC’s durability.
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