Renewable energy devices such as fuel cells and metal air batteries involves oxygen reduction reactions (ORR), often the performance-determining process of the device. While Pt and its alloys have been the materials of choice due to their high catalytic activity, intense efforts have been made to find alternatives due to their prohibit cost, limited availability and instability. In this presentation, we report a study of TiO₂–graphene hybrid as a cost effective option and more environmentally friendly alternative ORR catalyst. While TiO₂ has been rarely considered to be an effective ORR electrocatalyst, a recent study by Pei and colleagues reported that oxygen-deficient titanium dioxide rendered surprisingly competitive ORR activity and excellent durability.[1] Motivated by this work and additional advantages of TiO_2-x such as abundance, safety and cost effectiveness, we probed the feasibility of applying self-doped TiO_2-x based on graphene oxide (GO) to ORR catalyst. This hybrid structure leverages the extremely high surface area and excellent electronic conductivity of reduced GO, which is highly advantageous for maximizing catalytically active sites per mass.[2] Since only the wrinkles and edges of GO are populated with active sites (binding functional groups), GO was treated with acid (HBr acid and/or oxalic acid) to functionalize the non-active sites (epoxide groups) on its basal plane. HBr acid is expected to create hydroxyl groups, and additional oxalic acid treatment is used to create carboxyl groups. These functional groups are then used to bind TiO₂nanoparticles. [3]

TiO₂/GO hybrid materials was synthesized by a hydrothermal reaction using three different kinds of GO: as-synthesized, HBr-treated, and HBr + oxalic acid-treated GOs. For simplicity, we named the resulting hybrid materials as GT (i.e. GO-TiO₂), HGT (HBr-treated GO-TiO₂), and HOGT (HBr and oxalic acid-treated GO-TiO₂), respectively. From a series of characterization, we found that HOGT samples show the best catalytic activity among the samples with a four electron process, whereas the other samples show a two-electron processes. It was also observed that HOGT showed the best performance when reacted at the highest temperature (160°C) unlike the other samples. From these observations, it is concluded that high density of carboxyl group is essential in rendering high performance and durability toward ORR catlysis.

This project is funded by NASA Advanced STEM Training and Research (ASTAR) Fellowship (NNX15AW57H) and NASA MUREP Institutional Research Opportunity (MIRO) Program (Grant No. NNX15AQ01A).

 

 

References

[1] Pei, D.-N. et al. Nat. Commun. 6, 8696 (2015).

[2] Sun, Y. et al. Graphene based new energy materials. Energy Environ. Sci. 4, 1113 (2011).

[3] Chen, M. et al. Sci. Rep. 5, 10389 (2015)