Nature-inspired design and synthesis of metal oxides for energy applications

Abstract

calcium carbonate ($CaCO_3$), which is the most abundant biomineral. Chapter 1 describes and demonstrates the green approach to synthesize porous metal oxides using carboxymethyl cellulose (CMC) fibers. The synthesis of metal oxides has been intensively pursued over many years for diverse industrial applications. A simple but general and green approach for the synthesis of hierarchical metal oxides is introduced using cellulose, the most abundant organic material on Earth. 1D hierarchical metal oxides were prepared using CMC as a sacrificial template. The electrostatic interaction between metal ions and carboxyl groups in CMC contributed to the formation of porous structures of binary (e.g., $CeO_2$) and tertiary (e.g., $CaMn_2O_4$) metal oxide fibers. Moreover, the morphologies of metal oxides could be controlled by varying the experimental conditions. The porous $CeO_2$ fibers were employed for the photocatalytic reaction, which exhibited much enhanced activity over $CeO_2$ nanoparticles due to facile electron transfer. $CaMn_2O_4$ fibers showed unique bi-functional electrocatalytic activities in both reactions of oxygen evolution and reduction. In chapter 2, cellulose-templated metal oxides have been employed as solar energy harvesting materials and energy storage materials. The cellulose templating approach for synthesis of solar energy harvesting materials is fascinating route by which to fabricate hierarchical metal oxides. Solar energy harvesting composite was prepared by incorporation of carbon nitride ($g-C_3N_4$) as a photosensitizer to cellulose-templated ZnO fibers. The $g-C_3N_4$ hybridized ZnO fibers ($g-C_3N_4$/ZnO fibers) were suitable for artificial photosynthesis under visible light irradiation by showing facile directional photo-assisted electron transfer from electron donors to enzymes through electron mediator and cofactor (i.e., NADH). Cellulose-templated $CaFe_2O_4$ that have hierarchical structure consisted of lath-like particles connected into fibers could absorb visible light. Photocatalytic NADH regeneration with the CaFe2O4 was successfully demonstrated with different electron donors. These integrated photocatalytic systems were efficient for solar energy harvesting coupled with biocatalytic reaction producing fine chemicals. In addition, this cellulose templating method was extended to the synthesis of energy storage materials, exhibiting a preparation of carbon and $NaCrO_2$ ($C-NaCrO_2$) composites and its application for Na-ion batteries (NIBs) as a cathode material. Chapter 3 introduces the polydopamine (PDA) as a biomimetic mediator for amorphous $CaCO_3$ mineralization. In nature, living organisms produce hard materials for essential functions such as skeletal support, protection, navigation, and others. For the synthesis of natural inorganic materials, biomineralization occurs through selective concentration of metal ions and spontaneous nucleation by macromolecular assemblies such as proteins and polysaccharides. Biomineralization is a biogenic process that produces elaborate inorganic and organic hybrid materials in nature. Inspired by the natural process, this study explores novel mineralization approach to create nanostructured $CaCO_3$ films composed of amorphous $CaCO_3$ hemispheres using catechol-rich polydopamine (PDA) as a biomimetic mediator. The mechanism of PDA-induced $CaCO_3$ mineralization was elucidated by multiple electrochemical analyses and ex situ microscopic imaging, which suggested PDA-induced CaCO3 mineralization is an analogue pathway of forming mollusk shells. Lastly, chapter 4 present the capability of PDA-induced $CaCO_3$ as a sacrificial template for the synthesis of hierarchical metal oxide minerals (MOMs). The thus-synthesized biomimetic $CaCO_3$ was transformed to nanostructured films of metal oxide minerals, such as FeOOH, $CoCO_3$, $NiCO_3$, and $Mn(OH)_3$, via a simple and environmentally friend procedure. $CaCO_3$-templated metal oxide minerals functioned as an efficient electrocatalyst

$CaCO_3$-templated CoPi (nanoCoPi) film exhibited high stability as a water oxidation electrocatalyst with a current density of $1.5 mA cm^{-2}$. The nanostructure of nanoCoPi consisting of individual nanoparticles (~50 nm) and numerous internal pores (BET surface area: $3.17 m^2g^{-1}$) facilitated an additional charge transfer pathway from the electrode to individual active sites of catalysts. This work demonstrates a plausible strategy for facile synthesis of nanostructured electrocatalysts through biomimetic $CaCO_3$ mineralization.

With the increasing energy issues, energy conversion and storage have become important topics of widespread concern. Researches for the design and development of new materials with better performance for diverse energy applications such as catalysis, energy harvesting and transfer have been extensively studied to meet the growing demands for practical uses. Hierarchical nanomaterials, having increased surface area, directional charge transfer, and distinct surface properties are suitable for a variety of energy applications, exhibiting overwhelming performance compared to bulk materials. Therefore, the rational design and synthesis of hierarchical nanomaterials is fundamental approach in dealing with energy problems. Considering environmental and economic factors, sustainability in the development and synthesis of hierarchical nanomaterials also become an essential prerequisite for a practical and adequate use. This thesis focuses on two different categories of synthesis of hierarchical nanomaterials via the green and environmentally friend way with bio-inspired templates: cellulose, which is the most abundant organic material