The article discusses a significant breakthrough in the field of green hydrogen production through seawater electrolysis powered by renewable electricity. While this method is attractive, challenges such as corrosion and competing reactions at the anode have hindered its efficiency. The focus has primarily been on anode development, as the cathode operates at reducing potentials and is not subject to certain reactions during electrolysis. The research introduces a novel approach by unveiling the dynamic evolution and degradation of the seawater splitting cathode during intermittent electrolysis. The proposal involves constructing a passivation layer on the cathode to maintain hydrogen evolution performance. Specifically, an in situ-formed phosphate passivation layer on the NiCoP–Cr2O3 cathode protects metal active sites against oxidation and repels halide ion adsorption, enabling electrodes to endure fluctuating operations in alkaline seawater without significant voltage increase. The study showcases electrodes optimized with this strategy operating at 0.5 A cm−2 for 10,000 hours with minimal voltage rise. This discovery provides new insights to enhance seawater splitting technologies, making them more practical and sustainable with the use of renewable electricity.