Synthesis and Characterization of Bi-functional (ORR/OER) Nano-catalysts in Alkaline Media

by Nalin Iris Andersen

Institution: University of New Mexico
Year: 2014
Keywords: bi-functional; catalyst; CNT; spinel
Record ID: 2025445
Full text PDF: http://hdl.handle.net/1928/23598


The catalysts described in this thesis can be utilized in the systems that work by: storing energy, adding to the energy economy and directly converting chemical energy into electrical energy. This thesis presents the synthesis methods and characterization of three families of catalysts that are bi-functionally active, with high performance in the oxygen reduction reactions (ORR) and oxygen evolution reactions (OER) at the cathode and anode, respectively. All catalysts have been derived from transition metal oxides; an analysis of the synthesis and characterization has been conducted. The objectives are: (i) explore synthetic approaches to achieving highly porous, phase pure nano-sized metal oxides in the form of spinels, (ii) engineer functionalized carbon nanotubes (CNTs) that support metal oxide species and understand the intrinsic catalytic activity of different types of metal oxides deposited on them, and (iii) combine data about the metal oxide species and synthesis methods from objectives (i) and (ii) to synthesize another set of catalysts and infer bi-functional catalyst properties. To meet the first research objective, four synthetic approaches were explored to produce CuCo2O4 spinel catalysts. Based on their electrochemical activity, CuCo2O4 synthesized by the sacrificial support method (SSM) was found superior compared to the other materials. The sacrificial support method involves the incorporation of oxide precursors onto the surface of fumed silica (or other sacrificial support), post-heat treatment, followed by removing silica in a manner that does not affect the chemical composition of the final catalyst. The resulting unsupported material has been found to be highly active in ORR with a half-wave potential of 0.80V and a limiting current density of -3.66 mA cm-2. It was measured to have an onset potential of 1.52V versus the reversible hydrogen electrode (RHE) during the OER, which makes it a state-of-the-art bi-functional electrocatalyst. The second research objective was achieved by the development of a modified SSM application to synthesize functionalized CNTs. Furthermore, various metal oxides were deposited onto the CNT surfaces (MOx/CNT) with a loading of 50wt%, resulting in catalysts where the CNTs act as durable, highly electronic conductive carbon supports. All catalysts in this series have high activity in the OER and ORR cycles. It was found that the 25wt% Ni + 25wt% Mn/CNT catalyst was the best performer for the OER with an onset potential of 1.41V versus RHE; the best ORR catalyst is the 50wt% MnO2/CNT catalyst with a half wave potential of 0.84V at a current density of -2.1 mA cm-2. Both materials show high stability and durability in ORR and OER conditions. The third research objective was to synthesize catalysts by combining the methods that yielded the best catalysts from the first two research objectives. Two new sets of catalysts were derived and the catalysts were electrochemically analyzed. For the first set of catalysts, the…