<p>The proton exchange membrane electrolysis cells (PEMECs) are electrochemical devices that efficiently produce high-purity hydrogen via electrical energy conversion, making them widely applicable in renewable energy storage and hydrogen infrastructure development. However, the external sodium ion (Na⁺) contamination can severely damage the catalyst layer and membrane in PEMEC, causing significant performance degradation. Therefore, a segmented diagnostic platform for PEMEC is developed to analyze the poisoning effects of Na⁺ contamination on a large scale PEMEC under various operating conditions. The results demonstrate that during the cycle test, the Na⁺ poisoning process is defined as three distinct stages of initial, sustained and stable contamination stages. An increased Na⁺ concentration enhances the occupations of active sites on the catalyst layer, resulting in significant voltage spike, dynamic voltage fluctuations, non-uniformity distributions of current density and temperature. Both the low water flow rate and high operating temperature improve the chemical reaction and PEMEC performance at high current density. The deionized water flushing will dissolve Na⁺ on the catalyst layer surface and realize 2.17% decrease in voltage at 2.0 A cm⁻² after three cycles. This study is beneficial to consolidate the understanding of poisoning effects of sodium ion contamination in PEMEC under various operating conditions, thereby overcoming the obstacles for commercial application of green hydrogen production technology. </p>