Green Hydrogen energy is promising alternative of fossil fuels under the global warming crisis. However, the efficiency and lifespans of state-of-the-art low temperature water electrolysis cells (LTWEs) are highly influenced by their two-phase mass transport behaviors, e.g., gas bubble evolutions ranging from micro/nanoscopic range to the macroscopic. Therefore, developing diagnostic/monitoring systems for studying these phenomena are crucial to promote the development of green Hydrogen energy systems. In this work, we focus on the macroscale, and we propose a Scholte wave based non-destructive monitoring scheme of gas bubbles in flow channels of LTWEs. In short, Scholte waves are able to propagate along the interface of water and flow channels incorporated in a bipolar plate, and they are in parallel excited and received only along each straight section of flow channels to avoid complicated reflections of excitation waves at flow fields' boundaries. These excited waves are partially scattered by appearances of gas bubbles in flow channels, and the evolution of gas bubbles can be deduced from acquired echo signals. The proposed method is experimentally demonstrated using an ex-situ setup with a single straight flow channel of 115 mm length on a 400 μm thick Aluminum sheet. The Scholte waves were generated by mode conversions of Lamb waves on a fluid coupled plate-like structure, using angle beam transducers. We show that gas bubbles can be accurately localized by the time-of-flight of acquired echo signals within an error of 200 μm. The proposed method opens new horizons on diagnosis of flow regimes and monitoring of gas bubble evolutions towards more efficient and reliable industrial hydrogen production.