Transition Metal Oxide/ Metal-Organic Framework Hybrid Structures for Efficient Bifunctional Oxygen Electrocatalysis in Alkaline Environments

Abstract

Use of regenerative fuel cells (RFC) requires bifunctionality in oxygen electrocatalysis of oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) to decrease weight and cost for efficient energy conversion. Many RFC based technologies have used noble metals such as Pt and its alloys (Pt/Ir or Pt/Ru) due to their efficient catalytic activity, selectivity and stability in harsh environments. However, their susceptibility to fast degradation during operation, as a result of catalyst agglomeration and poisoning, has driven research to find alternatives. Non-precious transition metal oxides (TMO) including Fe3O4, MnOx and Co3O4 have attracted significant attention as a potential candidate [1].We utilized a hybrid structure where terephthalic acid ligand, cobalt and cerium nitrate salts, and solvents are subjected to a hydrothermal process at 160ºC for 24 h, a phosphoric acid treatment, an atomic layer deposition (ALD) of titania or zirconia, and finally a heat treatment in air. The phosphate group formed by the acid treatment is believed to have formed strong anchoring sites for ALD nanodots on the novel metal organic framework (MOF)-derived structure, which also results in a dispersed distribution of catalytically active sites.The hybrid catalyst where TiO2 nanodots are uniformly anchored on phosphorylated MOF by ALD showed good ORR and OER performance in 0.1 M KOH in terms of onset/half-wave potential, electron transfer number, and current density when comparing to the performance of Pt/C (for ORR) and IrO2 (for OER). In addition, an activation of catalytic sites can be realized by a proper engineering of interfaces in each hybrid systems. Materials characterization emphasizes a strong adhesion of metal oxides upon MOF structures, thus providing ample surface interactions for favorable reaction route is important.This project was funded by NASA Advanced STEM Training and Research (ASTAR) Fellowship.