The impact of temperature on air-breathing, polymer electrolyte membrane (PEM) fuel cells is investigated using polarization and impedance spectroscopy. Three active area sizes of 5 cm², 10 cm², and 25 cm² in both forced convection and air-breathing cathode configurations are presented. The cell design incorporates a large thermal body which can conduct heat away from the active membrane area and minimize the influence of self-heating; allowing for active and precise control of temperature regardless of the current density. Polarization and electrochemical impedance spectroscopy (EIS) results show that at higher current densities, elevated temperature increases the buoyancy of the air around the cell, which improves the air-breathing fuel cell performance. However, the opposite is true for lower current densities as membrane dehydration becomes more prevalent at higher temperatures. Temperature plays a larger role in air-breathing fuel cell performance than the actual size of the cell, whereas both cell temperature and size influence the cell performance for forced convection fuel cells. The discussions presented here provide guidelines for thermal engineering of practical air-breathing fuel cells as a promising portable energy source for the future.