Bipolar membranes (BPMs) were prepared by using commercial ion exchange membranes and hydrophilic polymer as a binder to investigate the effects of the interface hydrophilicity on water-splitting capacity. In this study, polyHEMA/MPD cross-linked with TMC was used as a binding material to enhance the BPM interface hydrophilicity. The enhanced hydrophilicity of the BPM interface accelerated the water-splitting reaction because the hydrophilic polymer layer increases the water activity by attracting water from the ion exchange layers to the space charge region. In addition, a mechanism of the metal catalytic reaction was proposed. Metal species were immobilized in the BPM in a hydroxide form and possibly react with water molecules and the quaternary ammonium groups reversibly. It was also observed that metal species immobilized in the membrane improved the water-splitting efficiency by increment of the membrane wetness and enhancement of the membrane conductivity, with an apparent optimum metal concentration for the water-splitting reaction.