2020, Vol.6 No.4 The cover picture shows component optimization for catalyst layers in proton exchange membrane fuel cells. As the key part of proton exchange membrane fuel cells, the optimization of catalyst layer has attracted abundant researches from various views. The daedal interaction between catalyst, supported materials, ionomer, solvent and reaction gas has been reviewed briefly in this article. More details are discussed in the article by Peng et al. on page 200016.