A review of X-ray water imaging studies applied to the gas diffusion layer of proton exchange membrane fuel cells

X-rays are a means of monitoring the internal information of a sample in a non-destructive way, which is used to visualize the multi-scale and multi-dimensional structure, chemistry, and dynamic behavior of an object. X-ray imaging has been used to observe the formation of liquid water during electrochemical reactions effectively and to extract information about the liquid water morphology, diffusion mechanisms, etc. This paper reviews the progress of multi-dimensional X-ray visualization of liquid water in the gas diffusion layer region, from X-ray tubes to synchrotron radiation light sources. The water cluster evolution and droplet dynamics from ex-situ experiments to in-situ experiments are described in detail, and the quantification of water distribution and the optimization of the gas diffusion layer structure in the operating environment are outlined for ex-situ and in-situ experiments, respectively. Data from synchronized studies of cell performance and water distribution in in situ experiments are reported. Image processing, imaging parameters, beam intensity, imaging modes, liquid water accuracy, and optimization are discussed. The characteristics of two light sources are systematically compared, and multimodal imaging methods and experimental strategies are explored.