Local Structure and Proton Transport in HT-PEFCs Measured with Neutron Scattering

The conception, design and optimization of fuel cells in general requires a good knowledge of the underlying physical processes on all length scales. In this contribution we focus on High Temperature Polymer Electrolyte Fuel Cells (HT-PEFCs) based on a proton conducting membrane such as poly2,2’-m-(phenylene)-5,5’-bibenzimidazole (PBI) doped with phosphoric acid (PA) and show how microscopic techniques can be related to macroscopic properties of the fuel cell. A major part of our studies uses different neutron scattering techniques, a tool which is well suited for studying thick objects due to the large penetration depth. Contrast variation by using different isotopes and the sensitivity to light elements are the major advantages of neutron scattering. The self-correlation function of protons is deduced from incoherent scattering, which gives access to the relevant transport processes in fuel cells.

Small angle neutron scattering (SANS) provides insight into the fractal structure of the PA doped membrane on length scales of ~10-500 nm. We present results on the variation of the microscopic structure between different lots of the same material. Small angle X-ray scattering (SAXS) provided insight into the Pt distribution and structure of the electrode layers, a multicomponent system consisting of carbon support, PTFE and the Pt catalyst itself.

With quasielastic neutron scattering (QENS), proton diffusion can be measured on local length scales of about 0.1-10 nm (1). The length scale dependent energy transfer measured with QENS gives insight into local proton transport processes in the PBI membrane and in the adjacent electrode layers and allows to relate microscopic proton mobilities with macroscopic measurements, e.g. of the proton conductivity of the membrane (2).    

Complementary techniques such as X-ray scattering, TEM or PFG-NMR round up the picture on a broad range of length scales. With this approach, the proton conducting membrane and the electrode layers of a HT-PEFC is studied and the results then linked to macroscopic properties of the membranes.

1. O. Holderer,  O. Ivanova, B. Hopfenmüller, M. Zamponi, W. Maier, A. Majerus, W. Lehnert, M. Monkenbusch, R. Zorn, Int. J. Hydrogen Energy 39 (2014) 21657 – 21662
2. A. Majerus, F. Conti, C. Korte, W. Lehnert, D. Stolten, ECS Transactions 50 (2) (2012) 1155-1165