검색 상세

Three-Dimensional Construction of Electrode Materials Using Carbon-Based Array Substrates for Highly Efficient Energy Storage and Water Electrolysis Combined with Electrochemical Oxidation

  • Sung-Woo Park (박성우, 대학원 건축사회환경공학과 기후 및 에너지공학전공)

초록/요약

With an increase in the demand for renewable energy resources for reducing the over-dependence on fossil fuels, efficient energy storage and conversion systems have garnered considerable research attention to stabilize the fluctuation in electric power supply and demand. To meet the industrial requirements for cost, energy density and efficiency, rational design of electrodes is indispensable to the electrochemical cells. This thesis focuses on the rational design of vertically aligned (VA) 3-dimensional (3D) substrates to construct powerful 3D electrodes, and their application to electrochemical energy storage and conversion. Especially, the challenges for the cost, energy density and efficiency of conventional state-of-the-art electrodes in lithium-ion batteries (LIBs), electrochemical oxidation (EO) process and electrochemical hydrogen production via water oxidation are handled in this thesis. Carbon-based materials have been considered as an efficient supporting material to enhance electrochemical performances of active materials. Because, they possess remarkable advantages for electrochemical applications such as earth-abundant, low-cost, easy to developing nano-structure, high electrical conductivity, and chemical resistivity. Hence, VA-3D substrates have been constructed by utilizing carbon-based materials. Firstly, scalable graphene oxides (GO) based free-standing substrates are rationally designed and developed via freeze-casting method. Freeze casting can generate unidirectional open channels inside the electrode, which enables fast ion transport and provides large contact area between electrode and electrolyte. Furthermore, S, and N dopants are immobilized in the reduced GO (rGO) lattice structure during thermal reduction of the GO frameworks resulting in highly conductive S, and N co-doped rGO (SN-rGO) frameworks. When pyrrhotite Fe7S8 nanoparticles are decorated on the free-standing substrates, the aligned SN-rGO frameworks provide effective electron transport routes, and sandwiched Fe7S8 within sturdy SN-rGO walls are protected from volume expansion caused by lithiation and sodiation. Consequently, when used in LIBs, Fe7S8@SN-rGO exhibited outstanding rate capability with reversible capacities of 621.1 and 492.1 mA h g-1 at low and high current densities of 0.2 and 4 A g-1, respectively, and excellent cycling stability without drastic capacity decay over 1000 cycles. As an anode for SIBs, the electrode also exhibited a high reversible capacity of 341.3 mA h g-1 at a high current density of 4 A g-1 with a high initial coulombic efficiency of 90.5 % and excellent long-term cycling stability for 2000 cycles. To enhance areal capacity of LIBs anodes, vertically aligned silicon@reduced graphene oxide frameworks (VA-Si@rGO) are furtherly developed via similar freeze-casting method. Briefly, SiO2/rGO frameworks with unidirectional pores can be constructed via the gelation of SiO2/graphene oxide sol and subsequent freeze-casting. Afterwards, the sturdy constructed frameworks are maintained during a series of processes, namely, magnesiothermic reduction, acid etching, and thermal carbon coating, which result in carbon-coated VA-Si@rGO. The electrode exhibits a high specific capacity, reversibility, and cycle stability, which is attributed to its unique inner porous structures, high Si yield, and uniform carbon layers. A high areal capacity of approximately 8 mA h cm-2 could be achieved by increasing the initial sol concentration up to 23.5 wt.%. Furthermore, even at a high current density of 3 mAcm-2, the electrode delivered a high areal capacity of approximately 6 mA h cm-2 and exhibited excellent stability with high capacity retention of 68 % after 150th cycle. Secondly, carbon-coated TiC nanoarrays on a cheap Ti-alloy substrate (C@TiC NAs) are developed via thermochemical reaction, and VA-3D construction of boron-doped diamond (BDD) anodes and Pt cathodes by utilizing the C@TiC NAs are introduced for highly efficient EO process coupled with hydrogen production. Note that fundamental study of cost-effective BDD electrodes is proceeded before the construction of 3D electrodes. Because the high-cost issue for electrochemical degradation of refractory organic pollutants caused by adopting a BDD electrode has been raised. A low-cost Ti substrate is adopted instead of conventional high-cost Nb or Ta substrates. BDD nanofilm on a Ti substrate (BDD@Ti) was prepared via hot-filament chemical vapor deposition under optimum conditions. Critical drawback related to delamination of the BDD on the Ti, caused by difference in coefficients of thermal expansion between BDD and Ti, can be overcome by minimizing thickness of the BDD film down to ~400 nm. When compared with a commercial electrode (C-BDD, micron BDD film on a Nb substrate), inevitable metal-carbide layer formed at interface between BDD film and metal substrate is much thinner than that of C-BDD. The unique structural features of BDD@Ti facilitate a much faster charge transfer than C-BDD, which is attributed to shortening of electron pathway from surface to conductive substrate. Based on the obvious merits of BDD@Ti, both 4-CP and PFOA degradation occurs much faster on the electrode at low potentials. Further investigations into the competitive merits of BDD@Ti are explored in comparison with a commercial dimensionally stable anode. To achieve highly effective and low-cost both anodic treatment of pollutants and simultaneous cathodic hydrogen evolution in a single electrolysis cell containing sulfate-laden wastewaters, first of all, BDD layers are deposited on C@TiC NAs acted as efficient three-dimensional (3D) electroactive supports (BDD@TiC NAs). The BDD@TiC NAs exhibited 1.9-fold higher efficiency for benzoic acid degradation and 4.4-fold higher peroxydisulfate (PDS) generation than the conventional plate type BDD electrode. Moreover, with multi-activity assessment using 8 organic substances, the BDD@TiC NAs enabled successful anodic treatment of all tested refractory organics and the kinetic rate did not depend on the substrate type. For the cathode, Pt nano-dots (NDs) coated on C@TiC NAs (Pt@C@TiC NAs) can be simply prepared via salt coating and subsequent thermal reduction. Uniformly decorated Pt NDs with a low mass loading of 0.029 mg cm-2 on C@TiC NAs can serve the same hydrogen evolution reaction performance as that of a commercial Pt plate. When a single electrolysis cell was configured using BDD@TiC NAs and Pt@C@TiC NAs, the cell performance surpassed a conventional ideal electrolysis cell (flat BDD//Pt plate) in all efficiency aspects: degradation of benzoic acid, sulfate radical generation, and hydrogen production. Overall, this thesis systemically analyzes positive effects of VA-3D electrodes on electrochemical performances of each electrochemical cell, in terms of their morphological, structural, and electrochemical properties of the electrodes. Consequently, I suggest that these electrode design can pave the way for a breakthrough in terms of cost, energy density, and efficiency issues for electrodes in electrochemical cells for effective energy storage and conversion systems.

more

목차

Abstract
Table of Contents
List of Tables
List of Figures

1. Introduction 1
1.1. Overview 1
1.2. Research trend 13
1.3. Aim and strategies 22
1.4. Bibliography 26

2. Graphene-Based Binder-Free 3D Anodes for Electrochemical Energy Storage - 31
2.1. S, N, co-doped reduced graphene oxides/pyrrhotite Fe7S8 as a high-performance free-standing anode - 31
2.1.1. Background 31
2.1.2. Experimental methodology 37
2.1.3. Results and discussion 42
2.1.4. Summary 86
2.2. Vertically aligned silicon frameworks as a binder-free high areal capacity anode - 88
2.2.1. Background 88
2.2.2. Experimental methodology 91
2.2.3. Results and discussion 95
2.2.4. Summary 110
2.3. Bibliography - 111

3. Vertically Aligned 3D Electrodes for Water Electrolysis Combined with Electrochemical Oxidation - 123
3.1. Fundamental study of boron-doped diamond electrodes for electrochemical oxidation process - 123
3.1.1. Background 123
3.1.2. Experimental methodology 130
3.1.3. Results and discussion 135
3.1.4. Summary 173
3.2. 3D construction of electrodes using TiC nanoarrays substrates for highly efficient electrogeneration of sulfate radicals and molecular hydrogen in a single electrolysis cell - 175
3.2.1. Background 175
3.2.2. Experimental methodology 180
3.2.3. Results and discussion 187
3.2.4. Summary 221
3.3. Bibliography 223

4. Conclusion 236

List of papers - 241
Curriculum vitae - 242

more
반출 Meta View 목록