Layered Metal Hydroxide Nanomaterials for Energy Conversion, Energy Storage, and Environmental Remediation

Abstract

The increasing global energy crisis and environmental pollution caused by population growth, industrialization, and dependence on fossil fuels have accelerated the development of clean energy technologies. Among diverse technologies, catalysis and rechargeable batteries have attracted significant attention as sustainable strategies for energy and environmental applications. Progress in these fields requires tailored materials for each system by considering performance-enhancing parameters, which underscores the significance of rational material design for their development. Layered metal hydroxides, consisting of edge-sharing octahedral units in which metal cations are coordinated by hydroxyl ions, have emerged as promising materials for numerous energy conversion and storage applications owing to their high structural and chemical flexibility. The layers can contain a wide range of metal cations with homogeneous metal distribution and precise compositional control. Layered double hydroxides (LDHs) are representative two-dimensional (2D) materials comprising positively charged metal hydroxide layers and interlayer anions with water molecules. The charged layers are formed by partial substitution of divalent metal cations with trivalent cations in layered metal hydroxides. LDHs can be readily exfoliated into individual layers, which electrostatically interact with other materials to form nanocomposites. Furthermore, thermal or chemical treatment enables selective transformation into mixed metal oxides (MMOs) or multi-metal alloys with well-distributed metal elements, and the chemical bonding of silicate species onto metal hydroxide structures results in phyllosilicate-type structures. The structural flexibility and broad compositional range of metal hydroxides enable the development of numerous synthetic strategies. This thesis explores the metal hydroxide structures as a key precursor to construct tailored nanomaterials exhibiting excellent performance in energy conversion, energy storage, and environmental remediation. In Chapter 1, the unique properties and wide applicability of layered metal hydroxides are introduced. Specifically, the hydroxide-derived layered structures exhibit significant advantages for exfoliation processes, expanding the potential range of synthetic strategies. Chapter 2 reports a novel approach for controlling the metal cation distribution within LDHs to improve water oxidation catalysis by utilizing the structural properties of LDHs—anion exchange and exfoliation. Oxygen evolution reaction (OER) activities of unary (oxy)hydroxides containing Ni, Fe, Co, or Al, and binary LDHs (NiFe-LDHs and CoAl-LDHs) were first investigated. The binary LDHs exhibited distinct properties and enhanced OER activities compared with unary (oxy)hydroxides owing to synergistic effects between the different metal cations. However, excessively high degrees of metal- cation mixing in quaternary LDHs resulted in inferior OER performance compared with binary LDHs. To overcome this limitation and effectively utilize the characteristics of both binary LDHs, the two metal hydroxides were exfoliated and restacked to form a new type of LDH through anion exchange and exfoliation processes. The restacked LDHs exhibited superior catalytic activity to both the quaternary and binary LDHs, owing to the optimized cation distribution and the synergistic effects with complementary properties of the binary LDHs. As a result, the restacked LDHs achieved unprecedented electrocatalytic performance among powder-type (hydr)oxide and alloy electrocatalysts with more than three different metal cations. Although exfoliation can maximize the exposure of surfaces in bulk 2D materials and facilitate various synthetic approaches, exfoliation procedures often suffer from low yields, complicated processing, and a tendency to reaggregate during drying or heat-treatment. Such characteristics are unsuitable for large-scale production and thermocatalytic environments. Chapter 3 reports a scalable synthesis strategy to produce highly efficient thermocatalysts with strong resistance to particle aggregation by utilizing the intrinsic properties of 2D materials. A uniformly mixed 2D nanocomposite, composed of exfoliated MnCoAl-LDH nanosheets and hexagonal boron nitride (h-BN) nanosheets was synthesized by combining continuous microfluidization with a single-layer synthesis process. Subsequent heat treatment produced a nanocomposite in which MnCoAl MMO nanoparticles were evenly anchored on h-BN, and the nanocomposite was employed in the selective catalytic reduction of NOx with NH3 (NH3–SCR). As a result, the catalyst achieved the highest TOF of 0.772 h−1 among reported Mn-based NH3-SCR catalysts, high NOx conversion and N2 selectivity. Chapter 4 presents a facile and scalable strategy for fabricating a high-performance bifunctional electrocatalyst for zinc–air batteries (ZABs) through the integration of two types of 2D materials—each active for one of the reactions—with well-designed conductive substrates. Through a high-shear exfoliation and single-layer synthesis, exfoliated CoNiFe-LDH and iron-phthalocyanine (FePc) nanosheets were integrated onto a porous conductive substrate with numerous anchoring sites, resulting in a nanocomposite catalyst. The catalyst exhibited outstanding bifunctionality for both the OER and the oxygen reduction reaction (ORR) with the lowest overpotential difference of 0.63 V among reported oxygen bifunctional catalysts, indicating strong synergistic effects among the active components and the conductive substrate. Moreover, in scaled-up ZAB applications, the catalyst exhibited an extremely high peak power density and cycling stability over 300 h, confirming the reliability of the well-designed bifunctional nanocomposite for practical applications. In previous chapters, nanocomposites derived from metal hydroxides were primarily formed through electrostatic interactions using the positively charged LDH nanosheets. However, this property limited their compatibility with other positively charged species. Chapter 5 explores the use of saponite, a representative 2:1 layered phyllosilicates composed of a metal hydroxide nanosheet between two tetrahedral silicate layers. Saponite structure can have negatively charged nanosheets by the isomorphous substitution of trivalent metal cations for silicon in the tetrahedra, enabling electrostatic interactions with cationic species. In addition, saponite can be easily exfoliated into individual 2D nanosheets. These exfoliated nanosheets can attract and incorporate mobile metal cations owing to their high cation exchange capacity. By leveraging these structural and charge characteristics, exfoliated saponite nanosheets could be electrostatically interacted with cetyltrimethylammonium cations (CTA+), resulting in various types of organo-modified saponite with diverse interlayer configurations. In addition, the precise control of cation arrangement within the saponite interlayers revealed that the optimized interlayer configuration is a pseudo-trilayer arrangement, which was applied as a high- performance quasi-solid-state gel electrolyte additive in Li metal batteries. Thus, the negatively charged saponite nanosheets serve as fast Li+ transport channels, while the optimized arrangement of cations effectively traps counter-anions, resulting in high Li+ transference number and excellent cycling performance. This thesis provides synthetic approaches for designing tailored nanomaterials with remarkable performance across diverse technological fields via a comprehensive understanding of layered metal hydroxides.