The threshold method in the analysis of catalyst layer porosity towards oxygen transport resistance in PEMFCs†
The threshold method is introduced to characterize the porosity of a catalyst layer in this work on the basis of the impact analysis of the ionomer content on oxygen transport resistance. The limiting current method is used to measure the transport resistance of fuel cells with various ionomer content levels. The results show that increasing the ionomer content, especially I/C > 0.7, can cause the enlargement of Fickian resistance and local transport resistance. The increase in local oxygen transport resistance is attributed to the thickening of the ultra-thin ionomer film and the improvement of the ionomer coverage degree at the surface of the Pt particles. The decrease in porosity in the catalyst layer caused by the blocking effect of the ionomer is considered to be the reason for the enlargement of the Fickian resistance. However, according to published work, the measurement of the catalyst layer porosity is limited, especially in situ characterization. Here, the threshold method is applied to characterize the porosity in situ through the calculation of the saturated water content in the fuel cell, since water accumulates via capillarity in the pore structure of the catalyst layer. The characterization results are highly consistent with the trend of the Fickian diffusion coefficient, which verifies the availability of the threshold method for the in situ measurement of porosity in catalyst layers.