SAXS from supported nanoparticles: useful approximations when studying energy-materials for PEMFCs and PEMWEs

Hierarchical materials are widely used to form the electrodes of the Membrane Electrode Assemblies (MEAs) in both Proton Exchange Membrane Fuel Cells (PEMWEs) and PEM Water Electrolysers (PEMWEs). Here, the catalyst layer is constituted of catalyst nanoparticles (NPs) (usually 2-to-4 nm in size) decorating the surface of supporting materials1 (which size ranges from 10 to 100 nm). In PEMFCs, supports are composed by microporous carbons hosting Pt NPs at the anode electrode, and Pt or Pt-bimetallic alloys NPs at the cathode one. Carbon-supported Pt NPs are also used in cathode electrodes of PEMWEs, while proper support materials for the anode one2 are still missing, and Ir is used at a commercial level. To study catalyst materials in operando conditions, Small-Angle X-ray Scattering (SAXS) has been demonstrated to be a suitable technique for monitoring the evolution of catalyst NPs in FCs3,4. Nonetheless, due to the complexity of the electrode architecture, modelling of the scattering pattern is quite challenging. In fact, in addition to the presence of the contributions due to the mass transport layers and to the Nafion one used as an electrolyte, the presence of an additional cross-correlation term (originated by the mutual interaction among scattered radiation from both supporting and catalyst NPs) cannot be neglected5. From theoretical analysis6, and due to the multiple contributions constituting the scattering pattern of the full MEA, some approximations can be introduced, and the incoherent sum of the form factors used for represent catalyst and support NPs can be applied, simplifying MEA modelling7. In this work, the presented methodology allowed aim to simplify MEA modelling and promote the use of SAXS for the study of both catalyst and support NP evolution in operando analysis for supporting the development of PEM FCs and WEs technology.