In this dissertation mass, energy, and environmental influences of the low temperature proton exchange membrane fuel cells (LT PEMFC) in its operation phase was studied. We performed targeted accelerated degradation tests in order to establish the degradation mechanisms of the components in the membrane electrode assembly. Based on the obtained results we designed semi-empirical model for the NT PEMFC operation phase and developed a mathematical model for the degradation of LT PEMFC in an operational lifetime, taking into account the degradation of LT PEMFC energy efficiency and maintenance, with more frequent replacement of the fuel cell stack. The latter was implemented into new, innovative LCA model of the NT PEMFC system, which is generally useful for life cycle assessment (LCA) analysis of hydrogen technologies with LT PEMFC. We compared the new, updated Life Cycle Inventory (LCI) with existing LCI from other studies and demonstrated that degradation processes have a significant indirect impact on integrated environmental balances and should be taken into account in evaluations of hydrogen technologies in future energy supply.