A unitized electrode assembly (9) for use in the fuel cell comprises a first GDL (23), a PEM (28), and a second GDL (12), with electrode catalysts (27, 30) disposed between said PEM and each of said GDLs, said layers (23, 27, 30, 12) being impregnated with a thermoplastic polymer a sufficient distance from each edge of the UEA so as to form a fluid seal (13). The UEA is formed by a process which comprises making a sandwich of some or all of said layers (23, 27, 28, 30 and 33), with thermoplastic polymer film (22, 25, 32, 35) extending inwardly from the edges of said sandwich a sufficient distance to form the seal, said thermoplastic polymer film being disposed between each electrode and the adjacent GDL and/or between each GDL and release film (21, 36) on the top and bottom of the sandwich. The sandwich, compressed by force between a holding fixture (19) and an anvil (40), is heated by ultrasonic vibration so as to cause said thermoplastic polymer to melt and impregnate the layers (23, 27, 30, 12) of the UEA with the film (22 and/or 25; 32 and/or 35).