In-Situ X-Ray Observation of Bubbles in Porous Transport Layer of PEM Water Electrolysis

Abstract

As renewable energies become increasingly popular as a countermeasure against global warming, hydrogen is attracting attention as a new energy medium. This is because surplus electricity from renewable energy sources, whose generation is unstable, can be used to electrolyze water (water electrolysis) and convert it into storable hydrogen, which can then be reconverted into electricity by fuel cells and used when needed. By using hydrogen for long-term energy storage and utilizing surplus energy, dependence on fossil fuels can be reduced, resulting in a decrease in greenhouse gas emissions. Water electrolysis technology, particularly Proton Exchange Membrane (PEM) water electrolysis, is currently receiving attention due to its high efficiency, high safety, and a large operational range of current density.In PEM water electrolysis cells, bubbles are produced in the liquid water due to an electrochemical reaction in which water is decomposed into oxygen at the catalyst layer inside the anode. There is a concern that the concentration overvoltage increases as these bubbles remain inside the porous transport layer (PTL), resulting in a decrease in the efficiency of electrolysis. However, the behavior of bubbles during the electrolysis has not been fully clarified, and it is necessary to understand the discharge behavior of bubbles in the PTL. This study aims to clarify the bubble retention behavior in the PTL of PEM water electrolysis cell by in-situ bubble visualization using an X-ray microscopy system. Measurements were taken on 2 types of PTL made from carbon or titanium dioxide with different fiber diameters, pore sizes, and wettability to investigate the effect of PTL structure on bubble accumulation and discharge behaviors. Two-dimensional X-ray images were taken before the start of water electrolysis (when the PTLs were filled with liquid water) and during water electrolysis (when bubbles were generated), and the bubble distribution within the PTL and the channel was quantified from the changes in brightness. By tuning the X-ray transmission thickness of the PTL and X-ray energy, the bubble behavior during the electrolysis were successfully visualized. In the case of the carbon PTL (Fig. 1(a), fiber diameter: 10 μm, pore diameter: 150 μm), bubbles were accumulated under the rib area (Fig. 1(b)), while in the case of the titanium PTL (Fig. 1(c), fiber diameter: 25 μm, pore diameter: 50 μm), bubbles were relatively uniformly distributed (Fig. 1(d)). This result is believed to be due to the titanium's superior wettability, which prevents the accumulation of bubbles by inhibiting their bridging between pores. Furthermore, the titanium PTL is attributed to have a lower gas saturation ratio owing to its smaller pore diameter, which effectively prevents bubble coalescence. These results suggested that the pore size and the wettability of the PTL strongly affect the bubble behavior, and the in-situ observation is quite important to understand it. Figure 1