This work analyses the operating performance of polymer-electrolyte-membrane fuel cells (PEMFC) for the use as portable energy converter. Especially, mass transport in the liquid and gaseous phase inside the PEMFC are experimentally examined. Water produced during the reaction is measured and liquid water transport in the air flow field of a semi-transparent PEMFC is visualized. Besides this, investigations of different flow field designs are used to develop two portable PEMFC systems. To understand the physical and electrochemical behaviour inside a PEMFC numerical simulations are performed. The used mathematical model is provided by the company Fluent. It verifies spatially resolved current distribution measurements of a PEMFC with straight, parallel gas channels. Thereby, the calculations can be used for the optimisation of the operating performance of PEMFC systems.