Competitive costs, instant recharge, and high energy density make fuel cells ideal for supplanting batteries in portable electronic devices. In this study, we derive a semi-analytical model to elucidate the transport of ions, heat and mass in air-breathing fuel cells. The model includes an empirical correlation for membrane conductivity that improves accuracy when modelling membrane dry-out. A detailed comparison with experimental data demonstrates that the model accurately predicts fuel cell performance through detailed accounting of catalyst layer specifications, including variable width, and electrochemical parameters. A comprehensive parametric study resolves the trends associated with a variety of design specifications and operating conditions. Membrane dry-out is identified as the primary limitation on current density, and is shown to be strongly dependent on heat transfer. The study also identifies some unique effects of coupling between ambient air temperature and humidity on the performance of air-breathing fuel cells.