Oxygen transfer at mesoscale catalyst layer in proton exchange membrane fuel cell: Mechanism, model and resistance characterization

Oxygen transfer in catalyst layer (CL) is an important issue affecting the oxidation-reduction reaction (ORR) and the output voltage of fuel cells, which is significant for guiding high-performance catalyst and CL design, and steady fuel cell operation for avoiding water flooding and degradation. Apart from the research on catalyst with low- and zero- loading platinum, high oxygen-transfer CL is another hot spot, the result of which will be beneficial to both proton exchange membrane (PEM) fuel cell and its inverse water electrolysis. In this paper, oxygen transfer is systematically reviewed involving oxygen transfer mechanism, model description, oxygen transfer resistance characterization, model solution method, and finally the oxygen transfer investigation in water-filled CL. For the transfer mechanism, in two scales that the studies are consecutively reviewed, which are Pt/C-ionomer agglomerate and porous CL. Among three main transfer paths, different factors influencing oxygen transfer are also clarified. At the point of fuel cell application, the oxygen transfer resistance and impedance, structure modeling and solution for oxygen transfer model, especially the oxygen transfer in water-involved CL are reviewed as well. Some view points on future research are proposed. Firstly, direct experimental validating oxygen transfer mechanism in agglomerate, CL, and water-involved CL should be given more attention. Secondly, the oxygen transfer and water content under transient conditions in CL still need to be closely monitored in terms of oxygen transfer resistance via mathematical methods that combining the basic transfer mechanism and proper structural establishment, where the traditional low-frequency impedance affecting oxygen transfer at large water amount needs more investigations. This paper is of great significance for the forward design of CL as well as fuel cell operation management.