Abstract: To reveal the performance evolution behaviors and failure mechanisms of nickel-based cathodes for alkaline water electrolysis under extreme polarization conditions, NiMo alloy catalytic electrodes were prepared via atmospheric plasma spraying in this study. Relying on a single-cell electrolyzer platform, multiple extreme accelerated stress tests-including extreme current densities （2500−10000 A/m²）, power fluctuations （high-frequency start-stop cycles and reverse current）, and impurity （Cr/Cl） synergistic poisoning—were systematically conducted to quantitatively analyze their microstructural collapse and phase lattice reconstruction characteristics. The results indicate that under steady-state high-load operation, the extremely high gas evolution rate triggers turbulence, and high-frequency bubble dynamics generate strong interfacial hydrodynamic shear stress, which directly causes microcrack propagation and large-scale brittle physical spalling of the catalytic layer. When the current density increases to 10000 A/m², the mass fraction of the core active element Mo drops sharply from 6.97% to 1.07%, leading to a severe loss of catalytic sites. Under frequent start-stop and reverse polarization impacts, the high-voltage oxidizing electric field drives the selective electrochemical dissolution of Mo and induces deep lattice reconstruction of the substrate, irreversibly transforming it into dense, high-barrier, and inert β-Ni（OH）₂ and high-valence γ-NiOOH phases. This deep passivation severely retards the reaction kinetics, causing the Tafel slope to increase sharply from 48.56 mV to 120.36 mV. Furthermore, impurity tests confirm that Cl⁻ penetrates the surface passivation film and accelerates Mo dissolution. The resulting pitting defects then act as preferential nucleation sites for the deposition of Cr species, and the insulating layer formed by their combination triggers strong synergistic poisoning and pore-blocking effects. The underlying mechanisms indicate that the ultimate failure of the cathode is caused by the deep coupling of three major mechanisms: mechanical physical spalling, chemical passivation corrosion, and synergistic impurity poisoning. The insufficient mechanical binding stability of the coating restricts continuous operation at high currents, whereas lattice phase transition deactivation and high-intensity impurity deposition induced by potential fluctuations fully dominate the accelerated damage process under non-steady-state conditions.