(471e) Nano-Structure Analysis of Catalyst Layer in Polymer Electrolyte Fuel Cell

Fuel cell electric vehicle (FCEV) is an effective solution to reduce the consumption of petroleum fuel and the emission of carbon dioxide (CO2). Most of FCEVs are equipped with polymer electrolyte fuel cell (PEFC) power system, which is composed of PEFC stacks and auxiliary instruments. PEFC stack has hundreds of membrane electrode assemblies (MEAs) that polymer electrolyte membrane (PEM) is sandwiched by two catalyst layers. PEM has two functions: transporting protons and separating anode and cathode physically and electrically. A catalyst layer is porous structure composed of platinum loaded carbon (Pt/C) nano-particles and ionomer. Ionomer in catalyst layer has also two functions, that is, transporting protons and binding Pt/C nano-particles.

Perfluorosulfonic acid (PFSA) polymer electrolyte, which is a sort of hydropolymer, is often used for PEM. During operation of PEFC power system, humidity in PEFC stack increases and decreases in proportion to output power of PEFC stack. This phenomena is well-known as dry-wet cycle and it causes degradation of PEM and increases hydrogen cross-leakage from anode to cathode. PFSA polymer electrolyte is also used for ionomer in catalyst layer. Size of ionomer in catalyst layer is sub-micron, so it is difficult to detect ionomer degradation in catalyst layer directly. However, from recent researches on PEM, it is possible that dry-wet cycle also accelerate ionomer degradation and break the structure of catalyst layer. In order to clarify the degradation mechanism, nano-scale structural analysis of catalyst layer is required.

According to above background, we analyzed the nano-scale structure of catalyst layer by experimental, computational and microscopic approaches. As experimental approach, mechanical properties and electron resistivity of catalyst layer were measured in order to know connectivity of Pt/C nano-particles. Based on these measured results, we estimated nano-structures of catalyst layers by 3D geometric model analysis. We also observed cross-sectional structures of catalyst layers by electron microscopy in order to validate estimated structure of catalyst layer.

In this research, we analyzed nano-structures of catalyst layers that were prepared on different ionomer / carbon (I/C) weight ratios. Young modulus and electrical conductivity of catalyst layers increase with increasing I/C weight ratio. 3D geometric model analysis based on measured properties of materials and components of MEA suggested that the network-like structure can be formed by connecting small size aggregates of Pt/C nano-particles at higher I/C weight ratio. This result indicates that ionomer in catalyst layer affects not only proton transportability, but also dispersivity and connectivity of Pt/C nano-particles.

Acknowledgement;

This work was funded by ImPACT Program of Council for Science, Technology and Innovation (Cabinet Office, Government of Japan). The authors would like to thank AGC Asahi Glass for providing sample PEMs.