For exemplary catalytic activity for oxygen reduction reaction (ORR) platinum and its alloys have been widely used. This, however, remains challenging to develop as platinum suffers from both costs and degradative activity. Alternatives have recently attracted attention, and being more abundant at lower costs, non-precious transmission metal oxides (TMO) including MnOx, Co3O4, and Fe3O4 provide a larger surface area and maximize catalytically active sites per volume and mass1,2. This is achieved by dispersing TMOs on a highly conductive carbon structure, thereby creating a synergetic effect in the generation of electrons.
In this study we use a conductive 3D carbon structure like graphene oxide and TiO2 or ZrO2 to create a hybrid structure to increase electronic conductivity and surface area. GO, however, contains large quantities of O-containing functional groups which do not bind to nanoparticles (non-reactive) unlike the wrinkles and edges of GO. To combat the non-reactive O-containing functional groups species, functionalization of the basal plane of GO using acid treatments could be used. Specifically, the acid treatments performed using hydrobromic and/or oxalic acid to create hydroxyl and/or carboxyl groups would better react with nanoparticles (P25) or precursors (ZrOCl2) to form hybrid structures.
Recent results of ZrO2/GO hybrids indicated that hydroxylated GO had the best ORR performance in 0.1 M KOH as demonstrated with an increase of electron transfer number, current density and onset/half-wave potential. This was comparable to the performance of Pt/C while TiO2 on carboxylated GO exhibited the best ORR performance compared to other TiO2/GO hybrids. Analysis using X-ray diffraction indicated a reoccurring (002) diffraction GO peaks among most hybrids with the exception of hydroxylated ZrO2/GO and carboxylated TiO2/GO. This affinity of metal oxides to the basal planes of graphene flakes prohibits graphene restacking for both hybrid structures. Other experimental analysis indicates the strong ORR performance and reaction route is the result of strong tethering of metal oxide particles on the basal plane of graphene and the particle-graphene interfaces (compared to graphene alone). These phenomena were also observed in density theory calculations.
This work was supported by NASA ASTAR Fellowship (NNX15AW57H). In addition, S.G. and M.H.L acknowledge the NASA MIRO Program (NNX15AQ01A).
1 G. Wu and P. Zelenay, Acc. Chem. Res. 46, 1878 (2013).
2 Y. Sun, Q. Wu, and G. Shi, Energy Environ. Sci. 4, 1113 (2011).