NOVELTY - A first coolant plate of an electrochemical cell is sealed with an adjacent plate by welding a mating region of the first coolant plate to a complementary region on the adjacent plate to create a seal formed by the polymer at the mating region and the complementary region.    USE - For sealing a first coolant plate of an electrochemical cell with an adjacent plate.    ADVANTAGE - The invention seals a plate, e.g. coolant plate to an adjacent coolant plate or an adjacent bi-polar plate without using any external gasket or sealant. There is no problem of material compatibility during sealing and long-term degradation issues. There is no contamination expected from the seal. The method is cheaper and faster compared to conventional sealing processes. The seal can be made immediately after the plate molding process without handling any adhesive or glue-like materials to form the seal on the plates.    DETAILED DESCRIPTION - Sealing of a first coolant plate of an electrochemical cell with an adjacent plate involves welding a mating region of the first coolant plate to a complementary region on the adjacent plate to create a seal formed by the polymer at the mating region and the complementary region. The first coolant plate comprises at least one mating region for mating with the complementary region on the adjacent plate. The adjacent plate is a second coolant plate or a bipolar plate of the electrochemical cell. The first coolant plate and the adjacent plate comprise a polymer and conductive filler.    INDEPENDENT CLAIMS are also included for:    (a) an electrochemical cell component comprising a first coolant plate sealed to an adjacent plate using the inventive process;    (b) an electrochemical cell comprising the fuel cell component; and    (c) an electrochemical cell stack comprising the electrochemical cells.    DESCRIPTION OF DRAWING(S) - The figure shows a top view of a coolant plate showing flow field channels.    Flow field channel (23)    Manifold hole (42)