Renewable energy devices such as fuel cells and metal air batteries harness their energy via lethargic oxygen reduction reactions (ORR) whose catalysis is largely relying on expensive and scarce noble metals. To address this issue, in particular, non-precious metal incorporated in carbon nanostructures have been widely explored. In this study, we focus on synthesizing inexpensive and efficient electrode material for ORR catalysis and energy storage using reduced graphene oxide (rGO) incorporated with non-stoichoimetric titanium dioxide by atomic layer deposition (ALD). Although stoichiometric TiO₂ has been rarely considered as an effective ORR electrocatalyst, oxygen-deficient TiO_2-x recently showed surprisingly competitive ORR activity and excellent durability[1]. The work was performed on a single crystalline TiO₂. Motivated by the recent report and the advantages of TiO₂ due to its abundance, safety and cost effectiveness, we probed the feasibility of applying this oxygen-deficient TiO_2-x to the widely employed oxide nanoparticle-incorporated carbon nanostructures. This is to leverage extremely high surface area and excellent electronic conductivity of carbon-based 3D structures, which is highly advantageous for maximized catalytically active sites per mass.

We used electrophoretically deposited rGO as the matrix structure. Graphene oxides prepared by the modified Hummer’s method are first deposited onto a stainless steel by electrophoresis. The resulting structure was deposited by highly dispersed nanoparticles or ultrathin films of TiO_2-x via ALD. The degree of non-stoichiometry is controlled by either changes in ALD temperature or a separate post-thermal process. ALD is a newer variant of chemical vapor deposition in which materials are deposited on a surface by a multiple sequence of ‘self-limiting’ chemical reactions. This unique sequential process enables a highly uniform deposition of dots/films and accurate control over not only the size/thickness at the atomic scale but also chemical composition and location of deposition.

In this presentation, we will present 1) the feasibility of using TiO_2-x incorporated rGO as ORR electrode in terms of catalytic activity and cyclability and 2) the correlation among the ALD process parameters, resulting chemical composition/structure and ORR catalytic activity.

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