The feasibility and restrictions of feeding oxygen to a PEM fuel cell through free-convection mass transfer were studied through theoretical analysis and experimental testing. It was understood through the theoretical analysis that the free-convection mass-transfer coefficient strongly depends on the difference in mass fraction or concentration of oxygen at the cathode surface and in the quiescent air. Thus, the mass-transfer rate has a strong dependence on the oxygen concentration at the cathode surface, which can be viewed in terms of the relationship of the fuel cell current density and the fuel cell voltage. Through this analysis, the maximum applicable current density was derived, beyond which there will be an abrupt drop in the output voltage, which results in excessively low power in the fuel cell. Experimental tests were conducted for one PEM fuel cell stack and two single PEM fuel cell units. An excessive drop in output voltage was observed when the free-convection mass-transfer mode was utilized. It was also found that the orientation of the cathode surface affects the performance of the fuel cell, which is mainly due to the fact that the free-convection mass-transfer coefficient depends on the orientation of the involved mass-transfer surface, which is analogous to free-convection heat transfer.