A PEM water electrolyser module comprising: a plurality of structural plates each having a sidewall extending between opposite end faces with a half cell chamber opening, at least one oxygen degassing chamber opening, and at least one hydrogen gas collection manifold opening, extending through said structural plate between said opposite end faces; said structural plates being arranged in face to face juxtaposition between opposite end plates; each said half cell chamber opening at least partially housing electrolytic half cell components comprising at least a MEA, a bipolar plate in electrical communication with said MEA, said structural plates and half cell components defining an array of series connected electrolytic cells surmounted by at least one oxygen degassing chamber, and at least one hydrogen gas collection manifold; said structural plates defining, at least when in said face to face juxtaposition, respective gas-liquid passages extending between a top part of the anode half cell chambers and a bottom part of an upper section of at least one of said at least one oxygen degassing chamber to provide fluid communication between said anode half cell chambers and said at least one of said at least one oxygen degassing chamber; said structural plates further defining, at least when in said face to face juxtaposition, discrete degassed liquid passages extending between a bottom part of a lower section of said at least one oxygen degassing chamber and a bottom part of said anode half cell chambers for degassed liquid return from at least one of said at least one oxygen degassing chamber to said anode half cell chambers; said structural plates further defining, at least when in face to face juxtaposition hydrogen gas passages extending between a top part of the cathode half cell chambers and at least one of said at least one gas collection manifold to provide fluid communication between said cathode half cell chambers and at least one of said at least one hydrogen gas collection manifold; said PEM water electrolyser module further comprising oxygen gas discharge and feed water passages extending therethrough and fluidly communicating with said degassing chamber for oxygen gas discharge from said at least one oxygen degassing chamber and for feed water introduction into said degassing chamber; said PEM water electrolyser module further comprising hydrogen gas passages extending therethrough for hydrogen gas discharge from said at least one hydrogen gas collection manifold; and said PEM water electrolyser module further comprising cooling tubes or cooling coils contained in the lower section of said at least one oxygen degassing chamber for cooling incoming gas-liquid mixtures. a plurality of structural plates each having a sidewall extending between opposite end faces with a half cell chamber opening, at least one oxygen degassing chamber opening, and at least one hydrogen gas collection manifold opening, extending through said structural plate between said opposite end faces; said structural plates being arranged in face to face juxtaposition between opposite end plates; each said half cell chamber opening at least partially housing electrolytic half cell components comprising at least a MEA, a bipolar plate in electrical communication with said MEA, said structural plates and half cell components defining an array of series connected electrolytic cells surmounted by at least one oxygen degassing chamber, and at least one hydrogen gas collection manifold; said structural plates defining, at least when in said face to face juxtaposition, respective gas-liquid passages extending between a top part of the anode half cell chambers and a bottom part of an upper section of at least one of said at least one oxygen degassing chamber to provide fluid communication between said anode half cell chambers and said at least one of said at least one oxygen degassing chamber; said structural plates further defining, at least when in said face to face juxtaposition, discrete degassed liquid passages extending between a bottom part of a lower section of said at least one oxygen degassing chamber and a bottom part of said anode half cell chambers for degassed liquid return from at least one of said at least one oxygen degassing chamber to said anode half cell chambers; said structural plates further defining, at least when in face to face juxtaposition hydrogen gas passages extending between a top part of the cathode half cell chambers and at least one of said at least one gas collection manifold to provide fluid communication between said cathode half cell chambers and at least one of said at least one hydrogen gas collection manifold; said PEM water electrolyser module further comprising oxygen gas discharge and feed water passages extending therethrough and fluidly communicating with said degassing chamber for oxygen gas discharge from said at least one oxygen degassing chamber and for feed water introduction into said degassing chamber; said PEM water electrolyser module further comprising hydrogen gas passages extending therethrough for hydrogen gas discharge from said at least one hydrogen gas collection manifold; and said PEM water electrolyser module further comprising cooling tubes or cooling coils contained in the lower section of said at least one oxygen degassing chamber for cooling incoming gas-liquid mixtures.