A multi-dimensional, non-isothermal, two-phase numerical model is used to understand the role of the microporous layer (MPL) in improving polymer electrolyte fuel cell (PEFC) performance. The model is validated under varying operating conditions against experimental data from conventional PEFCs in literature and low loading electrodes measured in-house with and without an MPL. Under dry conditions, the MPL is found to have a minimal effect on cell performance, except for improving ohmic transport and performance stability. Under wet conditions, results show that the MPL increases the temperature in the catalyst coated membrane, thereby enhancing evaporation in the cathode and creating a larger sorbed water gradient across the membrane which results in improved water vapor transport out of the cathode and increased diffusion from cathode to anode, respectively. A mild improvement in performance is also observed due to improved in-plane diffusion once an MPL is introduced as a result of the smaller pore size and hydrophobic nature of the MPL. A parametric study suggests that gas diffusion layer and MPL thermal conductivity are the most critical parameters to improve fuel cell performance followed by thickness and hydrophilic percentage. Other microstructural parameters appear to have minimal effect. An optimal thermal conductivity and hydrophilic percentage exist that achieve optimal fuel cell performance under fully humidified conditions. (c) 2018 Elsevier Ltd. All rights reserved.