新穎能源材料之第一原理計算模擬與研究
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2020
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為了降低石化燃料的使用，科學家們一直致力於尋找乾淨的替代能源，希望在未來使用液態或固態形式的能源。與此同時，也需要發展安全又具經濟效益的新能源儲存系統，最終的目標是尋找具有高能源密度、容易儲存及運輸、並且更為永續的能源。在本論文當中使用了計算化學的方法，在奈米至原子尺度下，藉由電子結構、催化性質和化學反應機構的探討，來改善並發展新的能源材料。總和來說，我們基於第一原理方法的理論模擬，針對不同能源與能源儲存系統的材料表面進行研究，包含了直接甲醇燃料電池(Direct methanol fuel cell, DMFC)、鋰硫(Li-S)電池、質子交換膜燃料電池(Proton exchange membrane fuel cell, PEMFC)和費托合成反應(Fischer-Tropsch synthesis, FTS)等領域。各部分詳細的介紹如下：
第一部份：直接甲醇燃料電池內一氧化碳移除反應在鉑修飾多氧陽極表面(Pt2/o-MO2(110), M = Ru及Ir)的研究
在第三章中將針對液態的直接甲醇燃料電池(DMFC)進行討論。DMFC反應過程中產生的CO或其他碳氫化合物(CmHn)很容易就毒化Pt金屬陽極表面。我們研究CO及H2O於乾淨Pt2/MO2(110)以及多氧Pt2/o-MO2(110)表面(M = Ru及Ir)上的吸附現象。結果顯示使用多氧的表面能夠有效的降低CO及H2O的吸附能，並且讓CO與表面的OH基團以更低的活化能進行類水氣轉換(WGS-like)反應，減緩CO毒化的現象。
第二部分：鋰硫電池中含鋰多硫化物在石墨稀基底材料上的吸附結構研究分析
第四章我們則針對鋰硫(Li-S)二次電池進行研究。近期的文獻顯示，若在陰陽極中間放置以碳為基底的材料做為中間層(interlayer)，能夠有效改善含鋰多硫化物(LiPSs)的飛梭現象並增加電池壽命。我們建構了不同結構形式的異原子(N或S)取代的石墨稀表面，發現當使用含鋰的N及S共同取代石墨稀表面做為鋰硫電池中間層時，能夠讓LiPSs以完整吸附機制吸附，有效的減緩飛梭現象。
第三部分：Pt/v-Tin+1CnT2二維材料表面邊界性質對氧氣還原反應催化的影響
第五章中探討了質子交換膜燃料電池(PEMFC)的陰極氧氣還原反應(ORR)，當使用二維Tin+1CnT2與Pt/v-Tin+1CnT2 (n = 1 ~ 3, T = O and/or F)的材料時，不同取代基對於ORR反應過電壓η的影響。我們的結果顯示F的取代基在表面上鍵結較弱且較不穩定，與實驗上觀察到脫附或被取代的現象符合。但由於F取代基在表面上時，內層的Ti與C具有較高的共價性，有利於吸附物吸附並反應，導致使用含有F取代基的表面進行ORR時可以得到較低的過電壓η。
第四部份：利用雙金屬中心的CNT基底材料促進費托合成中C-C成鍵反應
在費托合成(FTS)中，C-C成鍵的效率是最重要的因素。在第六章中我們模擬了雙金屬中心的M1M2/N6h-CNT (M = Fe, Co, and Mn)表面，分析其電子結構及催化活性，並考慮了三種能夠增長碳鏈長度的C-C成鍵反應：[CO + CH3]、[CO + CH2]和[CH2 + CH2]。結果顯示，CH2單體在2Co/N6h和CoMn/N6h表面上能經由一個近乎為零的活化能，順利進行C-C成鍵反應。整體來說，我們分析了雙金屬中心的系統對於在FTS中增加CO轉換率並降低C1產物比例的可行性。
To reduce the usage of fossil fuels, scientists have constantly been searching for clearer alternative energy sources by using the liquid or gas phase power source in the future. In the meanwhile, the development of new energy storage systems is also necessary to construct a safe and economical energy network. The ultimate goal is to achieve higher energy density, easier storage, more facile transportation, and an overall more sustainable energy supply system. In this thesis, we apply computational chemistry to understand the catalytic chemical and electrochemical reaction mechanisms with an aim to help modify, optimize, and design new energy materials from the nano to the atomic scale. In particular, we have carried out theoretical simulations based on first-principles methods to investigate the surface chemistry on various energy source and energy storage systems, including the direct methanol fuel cell (DMFC), the lithium-sulfur (Li-S) rechargeable batteries, the proton exchange membrane fuel cell (PEMFC), and the catalyst for the Fischer-Tropsch synthesis (FTS). The specific details are summarized below: Part 1: CO Removing Mechanism on Pt-Decorated Oxygen-Rich Anode Surfaces (Pt2/o-MO2(110), M = Ru and Ir) in DMFC In Chapter 3, we focus on the liquid energy source, the direct methanol fuel cell (DMFC). The Pt metal anodes are easily toxified by CO or other hydrocarbons during operation. We apply density functional theory (DFT) to investigate the adsorption of CO and H2O on pristine Pt2/MO2(110) and the oxygen-rich Pt2/o-MO2(110) surfaces (M = Ru and Ir). The resultsshow that the application of the oxygen-rich surfaces significantly reduces the adsorption energies of CO and H2O molecules as well as the major reaction barrier in the water-gas-shift-like (WGS-like) reactions forming CO2, leading to an efficient CO removal. Part 2: Adsorption Mechanisms of Lithium Polysulfides on Graphene-Based Interlayers in Lithium Sulfur Batteries In Chapter 4, we focus on the lithium-sulfur (Li-S) rechargeable batteries. Recent studies reveal that the carbon-based interlayer materials introduced between the cathode and anode can effectively improve the shuttle effect problem and increase the battery life cycles. Here, different types of the heteroatom-doped (N and/or S) graphene surfaces are investigated by theoretical calculations. We find that the Li-trapped N, S co-doped graphene interlayers (NSG1 and NSG2) could efficiently reduce the shuttle effect through the intact adsorption mechanism. Part 3: Termination Effects of Pt/v-Tin+1CnT2 MXene Surfaces for Oxygen Reduction Reaction Catalysis The theoretical investigation of proton exchange membrane fuel cell (PEMFC) and oxygen reduction reaction (ORR) is demonstrated in Chapter 5. We simulate the 2-D Tin+1CnTx and the Pt-decorated Pt/v-Tin+1CnTx (n = 1−3, T = O and/or F) surfaces. Different terminator effects, extent of electron transfer, and the over-potentials of ORR are discussed in this chapter. On the basis of our results, the F-terminated surfaces are predicted to show a better performance for ORR but with a lower stability than the O-terminated counterparts. Part 4: C-C Coupling Reactions Promoted by CNT-Supported Bimetallic Center in Fischer-Tropsch Synthesis C-C coupling efficiency is the most important aspect in Fischer-Tropsch synthesis (FTS). In Chapter 6, we propose a unique bimetallic center based on N-doped CNTs, the M1M2/N6h-CNT (M = Fe, Co, and Mn). We investigate three critical C-C coupling reactions, the [CO + CH3], the [CO + CH2], and the [CH2 + CH2], for the formation of long chain carbons in the FTS, and identify the dominant electronic effects for the catalytic activity. In particular, the 2Co/N6h and the CoMn/N6h surfaces are predicted to catalyze an almost barrierless C-C coupling between the CH2 fragments. The potential of such bimetallic centers is promising in increasing the CO conversion efficiency and suppressing C¬1 product ratio in FTS.
To reduce the usage of fossil fuels, scientists have constantly been searching for clearer alternative energy sources by using the liquid or gas phase power source in the future. In the meanwhile, the development of new energy storage systems is also necessary to construct a safe and economical energy network. The ultimate goal is to achieve higher energy density, easier storage, more facile transportation, and an overall more sustainable energy supply system. In this thesis, we apply computational chemistry to understand the catalytic chemical and electrochemical reaction mechanisms with an aim to help modify, optimize, and design new energy materials from the nano to the atomic scale. In particular, we have carried out theoretical simulations based on first-principles methods to investigate the surface chemistry on various energy source and energy storage systems, including the direct methanol fuel cell (DMFC), the lithium-sulfur (Li-S) rechargeable batteries, the proton exchange membrane fuel cell (PEMFC), and the catalyst for the Fischer-Tropsch synthesis (FTS). The specific details are summarized below: Part 1: CO Removing Mechanism on Pt-Decorated Oxygen-Rich Anode Surfaces (Pt2/o-MO2(110), M = Ru and Ir) in DMFC In Chapter 3, we focus on the liquid energy source, the direct methanol fuel cell (DMFC). The Pt metal anodes are easily toxified by CO or other hydrocarbons during operation. We apply density functional theory (DFT) to investigate the adsorption of CO and H2O on pristine Pt2/MO2(110) and the oxygen-rich Pt2/o-MO2(110) surfaces (M = Ru and Ir). The resultsshow that the application of the oxygen-rich surfaces significantly reduces the adsorption energies of CO and H2O molecules as well as the major reaction barrier in the water-gas-shift-like (WGS-like) reactions forming CO2, leading to an efficient CO removal. Part 2: Adsorption Mechanisms of Lithium Polysulfides on Graphene-Based Interlayers in Lithium Sulfur Batteries In Chapter 4, we focus on the lithium-sulfur (Li-S) rechargeable batteries. Recent studies reveal that the carbon-based interlayer materials introduced between the cathode and anode can effectively improve the shuttle effect problem and increase the battery life cycles. Here, different types of the heteroatom-doped (N and/or S) graphene surfaces are investigated by theoretical calculations. We find that the Li-trapped N, S co-doped graphene interlayers (NSG1 and NSG2) could efficiently reduce the shuttle effect through the intact adsorption mechanism. Part 3: Termination Effects of Pt/v-Tin+1CnT2 MXene Surfaces for Oxygen Reduction Reaction Catalysis The theoretical investigation of proton exchange membrane fuel cell (PEMFC) and oxygen reduction reaction (ORR) is demonstrated in Chapter 5. We simulate the 2-D Tin+1CnTx and the Pt-decorated Pt/v-Tin+1CnTx (n = 1−3, T = O and/or F) surfaces. Different terminator effects, extent of electron transfer, and the over-potentials of ORR are discussed in this chapter. On the basis of our results, the F-terminated surfaces are predicted to show a better performance for ORR but with a lower stability than the O-terminated counterparts. Part 4: C-C Coupling Reactions Promoted by CNT-Supported Bimetallic Center in Fischer-Tropsch Synthesis C-C coupling efficiency is the most important aspect in Fischer-Tropsch synthesis (FTS). In Chapter 6, we propose a unique bimetallic center based on N-doped CNTs, the M1M2/N6h-CNT (M = Fe, Co, and Mn). We investigate three critical C-C coupling reactions, the [CO + CH3], the [CO + CH2], and the [CH2 + CH2], for the formation of long chain carbons in the FTS, and identify the dominant electronic effects for the catalytic activity. In particular, the 2Co/N6h and the CoMn/N6h surfaces are predicted to catalyze an almost barrierless C-C coupling between the CH2 fragments. The potential of such bimetallic centers is promising in increasing the CO conversion efficiency and suppressing C¬1 product ratio in FTS.
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直接甲醇燃料電池, CO毒化, 鋰硫電池, 飛梭效應, 質子交換膜燃料電池, MXene, 費托合成, 奈米碳管, 理論計算, 催化反應, VASP, Direct methanol fuel cell (DMFC), CO poison, Lithium-sulfur (Li-S) batteries, Shuttle effect, Proton exchange membrane fuel cell (PEMFC), MXene, Fischer-Tropsch synthesis (FTS), Carbon nanotube (CNT), Theoratical calculations, Catalysis, VASP