Modelling Proton Transport Phenomena in Nanopores of Fuel Cell Catalyst Layers

Proton transport through nanoporous media is an important process in electrochemical energy conversion. Proton density and mobility in porous electrodes determine the proton conductivity as well as the rates of interfacial electrochemical processes. In the case of polymer electrolyte fuel cells (PEFCs), the foremost practical objective is to design porous electrodes or catalyst layers with high performance at markedly reduced platinum loading. Achieving this objective demands an understanding of the impact of composition and porous structure as well as surface structure and charging properties of pore walls on the proton density distribution. In the presented work, we consider simple pore geometries to study the concerted effects of ionomer structure and metal charging properties on proton density and transport in nanopores. The approach employs Poisson-Nernst-Planck theory.^1,2 The basic model system is a cylindrical pore with an ionomer shell. In one variant, we consider a core consisting of a solid metal rod. In another variant, we assume a random porous core that represents Pt/C aggregates. In both variants, the remaining pore space is filled with water. The set of ordinary differential equations for transport and reaction in these model structures is formulated and solved. Solutions are analysed by comparing potential and proton density distributions for varying pore geometries and charging properties at interfaces. The effectiveness factor of catalyst utilization is calculated to evaluate the electrocatalytic performance at the pore level.

References

[1] R.Coalson, and M.Kurnikova, Biological membrane Ion channels, Chap 13, Springer, NewYork (2001)

[2] K.Chan and M. Eikerling, J. Electrochem. Soc. 158(1), (2011), B18-B28.