3d Interface Engineered Transition Metal Oxide Carbon Hybrid Structures For Efficient Bifunctional Oxygen Electrocatalysis In Alkaline And Acidic Environments

Use of regenerative fuel cells requires efficient bifunctionality in oxygen electrocatalysis: oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Commonly used noble metals like Pt and its alloys (Pt/Ir or Pt/Ru) are often used for their catalytic activity, selectivity and stability in harsh environments. However, Pt can degrade during operation from catalyst agglomeration and poisoning. Therefore, researchers have used non-precious transition metal oxides (TMO) including Fe3O4, MnOx and Co3O4 and/or nanocarbon structures (NC) as potential catalyst. Composite structures where TMO nanoparticles are deposited onto a NC, derived from either graphene oxide (GO) or metal-organic frameworks (MOFs), have often been used. NCs have high surface area and excellent electronic conductivity, and while many studies assert these types of composite materials exhibiting synergistic effects in oxygen electrocatalysis, efforts to elucidate the origin of the synergy is lacking. This doctoral research explores how functional groups present on the surface of NCs affect synergy (reaction route and kinetics) of these electrocatalysis. To incur catalytically active sites between the metal oxides and carbon, the NCs basal plane were functionalized using acid treatments, after which various types of TMO/NC hybrids were synthesized using either wet process or vacuum deposition techniques. The hydroxylated CeO2/graphene hybrids showed the best ORR and OER performance in both alkaline and acidic media, in terms of onset/half-wave potential, electron transfer number, and current density when compared to the performance of benchmark catalysts: Pt/C (for ORR) and IrO2 (for OER). From a series of material and electrochemical analyses, it was determined that a strong tethering of TMOs on graphene's basal plane prohibited restacking and particle-carbon interfaces dictates the performance and reaction route, as indicated in density functional theory calculations. In addition, a hybrid catalyst of TiO2 nanodots, uniformly anchored on phosphorylated carbon by atomic layer deposition (ALD), showed even better ORR and OER performance in alkaline media when compared the aforementioned CeO2/graphene hybrid. Materials characterization emphasized a strong adhesion of TMOs on MOF structures; thus providing ample surface interactions for a favorable reaction route. Therefore, an activation of catalytic sites can be realized by proper engineering of interfaces in each hybrid system.