(518a) Microelectrode Study of Oxygen Mass Transport and Its Influence on the Performance of High Temperature Proton Exchange Membrane Fuel Cells

Oxygen mass transport in the cathode catalyst layer (CCL) of high temperature proton exchange membrane fuel cells (HT-PEMFCs) plays an important role in promoting the fuel cell performance. Binders, such as polybenzimidazole (PBI), polytetrafluoroethylene (PTFE) and poly(vinyl pyrrolidone) (PVP) are used to adjust O₂ mass transport in the catalyst layer (CL) of HT-PEMFCs. Up to now, many researches have reported the influence of binders loading on the performance of HT-PEMFCs. However, all of these researches are qualitative, and there are no direct and quantitative evidences to illustrate the effect of binder compositions on O₂ mass transport in CCL. In addition, phosphoric acid (PA) leaching is another crucial issue for HT-PEMFCs. It is generally agreed that PA leaching leads to the proton conductivity decline of PEM, which leads to fuel cell performance degradation. But the quantitative effect of PA leaching on O₂ mass transport in CCL has rarely been investigated.

In this work, we employ a microelectrode technique to quantify the effect of the binder composition and PA leaching on O₂ mass transport in CCL of HT-PEMFCs. A microelectrode electrochemical cell is designed and fabricated for quantitatively characterizing the O₂ mass transport coefficients in CCL of HT-PEMFCs. A series of electrochemical measurements, including high-resolution cyclic voltammetry, potential-step chronoamperometry and sampled chronoamperometry, at the nA scale is conducted on a poly(ethersulfone)-poly(vinyl pyrrolidone) (PES-PVP) binder under various conditions.

A systematic data analysis is used to successfully obtain the diffusion coefficients (D_O2), solubility (C_O2) and permeability (D_O2*C_O2) of O₂ in PES-PVP binders with different PVP contents. We find that the permeability (D_O2*C_O2) decreases as the PVP content increases, indicating that the binder composition has a considerable effect on O₂ mass transport in the binder.

We also find that the permeability first increases as the PA doping level in the binder increases from 2.1 to 5.1, but then decreases upon further increase of the PA doping level. This result indicates an optimal PA doping level in the binder.

The research methods and conclusions presented in this study pave the way for optimizing the cathode catalyst layer (CCL) structure of HT-PEMFCs, thereby realizing highly efficient O₂ mass transport in the CCL and improving the cell performance.