Enhancement of the electrode reactions in vanadium redox flow batteries is highly demanded for improvement of their efficiency. In a recent study, the catalytic activity for the positive electrode discharge reaction was found at a carbonaceous material with an active site consisting of iron coordinated by 4 nitrogens in the square-planar configuration and embedded on the surface. The catalyst was formed as a thin film on the surface of a cup-stack carbon nanotube. In this study, the improvement of the structure of the catalyst layer consisting of the catalyst and ion-exchange polymer as a binder was attempted in order to enhance the positive electrode reactions. The catalyst layer structure was changed in two ways; variation in the binder content and inclusion of magnesium oxide in the catalyst layer as a pore-forming agent. The lower binder content produced a thinner coating on the catalyst, which was favorable for enhancement of the mass transfer in the layer, although the lack of the binder caused an insufficient dispersion of the catalyst in the layer, thus lowering the activity. The pores left after the removal of the magnesium oxide in the layer was also favorable for the mass transfer and effective for the electrode reaction enhancement unless an excessive amount of magnesium oxide was added to generate disorder in the layer. (C) 2016 Elsevier Ltd. All rights reserved.