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For a long time, people believed that the Pt/C particles aggregation in proton exchange membrane fuel cell (PEMFC) catalyst layer will inevitably lead to lower catalyst utilization. However, our research showed that Pt/C particles aggregation sometimes made catalyst utilization increases. PEMFC catalyst layer microstructure two-phase lattice models, in which we hypothesized that these catalyst layers was composed of C phase (Pt/C particles) and IP phase (the mixture phase of pore and ionic polymer), were used to simulate the effect of Pt/C particles aggregation on catalyst utilization, IP phase tortuosity and cell performance. It was found that when IP phase volume fraction ε < 0.6, catalyst utilization indeed decreased with the increase of aggregation coefficient φ . But, when ε > 0.65, the catalyst utilization of electrodes with Pt/C particles aggregation was larger than the electrode without Pt/C particles aggregation. At the same time, IP phase tortuosity decreased with increasing Pt/C particles aggregation coefficient. In overall consideration of the above factors, the best value of Pt/C particles aggregation coefficient was 5. [ABSTRACT FROM AUTHOR]