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In this thesis, design and engineering of nanostructured transition metal oxides are investigated towards efficient electrochemical oxygen evolution reaction (OER). The thesis is divided into three parts. This first part focuses on studying the effect of coupling silver with ordered mesoporous Co3O4 from silica nanocasting on its electron transfer ability. In the second part, alternative templating methods are developed to construct nanostructured transition metal oxides (TMO, i.e. Co3O4, Ni-Fe oxides) for efficient OER catalysis, as well as for an investigation on the effect of foreign metal incorporation on the structural and electrocatalytic properties of TMO. The third part is devoted to post-structural tuning Co3O4 with the aim of increasing the specific surface area and modifying the intrinsic catalytic activity for OER.
Spinel Co3O4 is one of the most promising OER electrocatalysts, but its application in electrocatalysis is severely hindered by the poor charge transfer ability as a semiconductor. The first part of the thesis offers a solution to this issue by coupling the structure with silver species. Ordered mesoporous Co3O4 was prepared via a typical nanocasting route with a confined structure, which is suitable to be employed as a model system of studying electrocatalytic performance due to its advantages of: (i) high specific surface area, (ii) rigid structure composed of defined nanoparticles and (iii) no contamination of metal and carbon residues except ~1 at. % silica remained in final oxides. Conductive silver was introduced into this system by simply adding a silver precursor during nanocasting. Detailed spectroscopic characterizations revealed that silver exists as not dopants but two different species, which are metallic silver particles intermixed with Co3O4 matrix and well-dispersed Ag2O nanoclusters.
These species have twofold advantages for OER. The metallic Ag with a high electrical conductivity improved the electron transfer ability of Co3O4 with achieving a higher current density. Ag2O nanoclusters provided the sites that are eligible for the uptake of iron from KOH electrolyte and caused a significant activation and boosted performance of the electrocatalyst.
A facile and cost-efficient synthetic method is in great demand for large-scale preparation of nanostructured TMOs. In the second part of the thesis, a sustainable method was developed based on hard templating using biomass wastes (e.g. spent tea leaves and coffee waste grounds). By a simple impregnation-calcination treatment, crystalline TMOs were prepared with nanostructures and high surface areas. Different transition metals, such as cobalt, iron, nickel, and mixed oxides, were then prepared and employed as active OER catalysts. In addition, the effect of foreign metal incorporation on the structural and electrocatalytic properties of TMOs was studied. In the case of Co3O4, the substitution of octahedral and tetrahedral sites of the spinel structure with divalent and trivalent metals (Cu, Ni, Fe, and Cr) increased the OER activity, while the incorporation of W caused a significant enhancement on the electrocatalytic performance. A hard templating process using spent tea leaves was employed as toolbox to study effect of the composition of iron nickel oxide nanocrystals for OER. The optimized composition was found to be Ni/Fe 32, which is associated with lower reaction resistance and higher intrinsic activity that contribute to a high reaction rate of OER. Although this hard templating using biomass wastes is proven to be a cheap and scalable way to prepare nanostructured metal oxides, an unsatisfactory surface area is obtained compared to the oxides prepared from silica nanocasting. The third part of the thesis provides two posttreatment strategies to engineering the nanostructure of Co3O4 to enhance its surface area. Those are selective acid leaching and laser fragmentation treatments. Upon both post treatments, significantly higher specific surface areas (up to 150 m2/g) were achieved for coffee-waste templated Co3O4, which possessed a surface area of around 50 m2/g in the initial form. Moreover, structure defects were induced into Co3O4 with the irradiation of pulsed laser, which improved also the intrinsic activity towards electrochemical OER.
