Despite of the high complexity of the IL, active metal and support interaction the methodology of the project allowed to corroborate fundamental structure-performance relationships, which are summarized subsequently:
1) An IL with higher hydrophobicity can more efficiently reduce the coverage/blockage of Pt by non-reactive oxygenated species and can result in a more pronounced boosting effect on activity.
2) ILs with too long hydrocarbon side chains (e.g. above 6 C-atoms) can hinder surface accessibility of active sites, probably arising from the spontaneous self-assembly of IL molecules with long non-polar side chains and the consequent formation of lipid-like structure on Pt surfaces. This effect leads to reduced activity of the Pt catalyst despite the increase in hydrophobicity of long chain ILs.
3) The interplay of support pore size distribution and where the catalyst nanoparticles are accommodated is essential for determining the amount of IL needed for the boosting effect.
4) The interplay of IL (e.g. chain length size), pore structure and surface functionalization of the carbon support is essential for a stable immobilization.
6) Bi- and tri-metallic Pt-based alloys can also show an activity increase after IL modification.
7) For tri-metallic Pt-based alloys the third metal for stabilization (e.g. Co or Mo) is dissolved faster in presence of the IL and thus deactivation progresses faster.
8) Tri-metallic Pt-based catalysts show a complex activation procedure, and classical pre-activation procedures don’t reach a stable activity plateau. The IL has a strong influence on this pre-activation phase.
9) The IL is distributed homogeneously in the carbon support and not selectively on the active metal. Compared to ionomers it shows a more homogeneous distribution
Second aspect of the project and more related to engineering science was to study these new IL modified catalysts at high current densities and also within membrane electrode assemblies (MEA). The major findings on these aspects are:
10) Gas-diffusion-electrode half cell experiments suit very good for high current density studies. For trustful data on these new technique new best practice protocols was established.
11) IL modified ORR catalyst are more hydrophobic and for preparation of membrane electrode assemblies by spray coating the ink composition needs to be adapted strongly. The alcohol employed a strong influence.
12) Membrane electrode assemblies with IL modified ORR catalysts show a reduced activity at low relative humidity. At high relative humidity an activity increase in the kinetic region can be observed while mass transport limitation starts at lower current densities.
13) The mass loading of the IL on the ORR catalyst has a strong influence on the structure of the whole catalyst layer of the membrane electrode assembly and thus to the mass transfer. Thus, an optimum loading is not only determined by the kinetics, but also by the mass transfer.
