Abstract: A composite membrane that is suitable for use in a molten alkaline water electrolyzer. In one embodiment, the composite membrane includes a porous support, the porous support being in the form of a matrix of metal oxide particles randomly arranged to form a plurality of tortuous pores. The composite membrane also includes molten electrolyte filling the pores of the porous support, the molten electrolyte having hydroxide ion conductivity. The molten electrolyte may be a single species of an alkali hydroxide or of an alkaline earth hydroxide. Alternatively, the molten electrolyte may be a eutectic or non-eutectic mixture of two or more species of alkali hydroxides or alkaline earth hydroxides. The composite membrane may further include one or more additives, such as a coarsening inhibitor, a crack attenuator, and a reinforcing material. The composite material may be used to make a molten alkaline membrane water electrolyzer with high electrical efficiencies.