高效水分解電極之研究;Efficient Catalytic Electrodes for Water Dissociation

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题名:	高效水分解電極之研究;Efficient Catalytic Electrodes for Water Dissociation
作者:	芭納利;MALLIKARJUN, BHAVANARI
贡献者:	能源工程研究所
关键词:	析氫反應;水分解;電催化劑;陰極;層狀氫氧化物;MoSx;石墨烯;CuFe 層狀氫氧化物;Hydrogen evolution reaction;water splitting;electrocatalyst;cathode;layered double hydroxide;MoSx;graphene;CuFe layered double hydroxide
日期:	2021-05-31
上传时间:	2021-12-07 12:10:17 (UTC+8)
出版者:	國立中央大學
摘要:	為滿足未來大量可再生能源的間歇性發電造成的儲能問題，從能耗最小化的角度來探討能源的轉換，氫能被認為是最重要的解決方案，可滿足當前與未來的能源需求。利用再生能源電解水產氫僅會產生副產物氧氣，是生產潔淨氫的最佳方法。電解槽以工作酸鹼條件作區分，可分為質子交換膜proton exchange membrane, PEM）型和鹼性電解(alkaline electrolysis, AE) 型兩種。電解槽的電極須具備高效率、可自支撐性與低成本。為了降低電解水的能耗與提升效率，需降低催化劑分解水的電化學反應電位以降低電極工作電壓、增加觸媒表面積以增加活性部位 (active site)。同時，觸媒支撐材料需具備合適的三維結構，提供電解質傳輸與氣泡擴散的路徑。本論文包含兩項高效且經濟的電解水觸媒之研究並成功應用於PEM與AE電解槽的陰極。第一項研究以熱退火與浸塗製程製備高電導性與具硫邊緣(S-edge)反應位MoSx奈米顆粒於具有氮參雜石墨烯碳布的表面作為高效催化電解產氫的電極。研究首先以X光光電子能譜、X光近邊光譜與電子能量損失能譜分析MoSx奈米顆粒沉積位置與石墨烯不同氮官能基 (nitrogen-functionality) 的組成奈米結構與電子組態。研究發現相較位於?咯氮位的MoSx奈米顆粒，位於石墨氮位與?啶氮位的MoSx奈米顆粒具有較佳的量子傳導與電子傳輸率。當MoSx奈米顆粒成長於石墨氮位較多的樣品時，具有較低的塔菲爾斜率 (Tafel slope) ，39.6 mV dec-1於0.5 M H2SO4硫酸溶液工作電壓215 mV與電流密度 10 mA cm-2 。Mo-N-C鍵結的高效電荷傳導性直接促進了MoSx奈米顆粒觸媒S-edge反應位的反應活性，並且多孔性的石墨烯與具三維結構的碳布進一步提升電解液與氣泡擴散的傳輸能力，在5000圈的加速老化測試中維持了95% 的性能，此研究開發的電解電極可應用於PEM電解槽。第二項研究為製備CuFe層狀雙氫氧化合物 (Layered double hydroxide, LDH) 於三維鎳金屬發泡材應用於鹼性電解產氫。LDH是一種具有大量電催化活性部位的三維材料結構，金屬材料因容易與酸反應故較適合應用於鹼性電解槽。較低空d-軌域元素具有高的質子吸附能力與較高電子傳導能力，本研究製備之CuFe LDH於1.0 M NaOH工作電壓達159 mV時，電流密度達 10 mA cm-2。交互相連的LDH三維層狀結構使電荷與氣體有效傳輸，進一步提升了觸媒穩定性，於電流密度 30 mA cm-2可維持5和10小時以上。這項研究高性能的CuFe層狀雙氫氧化合物可大幅降低鹼性電解槽的成本。我們開發了一種具有高反應面積、快速電荷轉移和具協同催化的電解觸媒，該電解觸媒具有優異的催化活性和良好的耐久性。這項工作有助降低PEM和AE電解槽的工作電壓，降低能源轉換消耗，能進一步降低觸媒成本，此觸媒可應用於各種儲能與發電裝置。本研究工作有助於促進減少使用化石燃料，並過渡到清潔能源發電系統。 ;In the perspective of clean energy transition with minimal losses, hydrogen is considered as a clean and sustainable energy carrier for reaching the current and future energy demands. Water dissociation is a favorable way to generate clean hydrogen with oxygen as byproduct. But it requires more external energy to start the water splitting. The hydrogen evolution and water oxidation reactions rely on highly effective electrocatalysts. The pH favored dissociation for low energy promoted two types of electrolysis cells namely, proton exchange membrane (PEM) and alkaline electrolysis (AE). Both of the types requires an efficient, low cost and robust self-supported electrodes. Moreover, lowering the potential required for generating hydrogen also reduces the energy required to initiate the reaction. The abundant exposed active sites with facile charge transport to active sites is one of the ways to enhance the efficiency of the catalyst. Also, the 3D structure of the catalyst and support provides larger surface area for catalytic process with electrolyte transport and bubble diffusion. This dissertation is an account of two detailed studies on developing highly efficient and cost effective electrocatalysts as cathode for water dissociation in PEM and AE. MoSx nanoparticles with excellent conductivity and exposed reactive S-edge sites is functionally grown on nitrogen-doped-graphene pre-engrafted on a flexible carbon cloth by a thermal annealing and dip coating process as model system for developing an efficient electrocatalyst for hydrogen evolution reaction. The specific fraction of N is analyzed by X-ray photoelectron spectroscopy, X-ray near edge spectroscopy and energy electron loss spectroscopy to analyze the selective positioning of MoSx on nitrogen-functionality present in graphene structure. It was found that, MoSx is positioned on the graphitic ‘N’ and pyridinic ‘N’ site, where the quantum conductance and electron transport required is higher compared to pyrrolic ‘N’. MoSx nanoparticles grown on nitrogen doped graphene with higher amount of Graphitic ‘N’ require extremely low Tafel slope of 39.6 mV dec-1 with an overpotential of 214 mV at 10 mA cm-2 in 0.5 M H2SO4 solution. The Mo-N-C bond facilitates highly effective charge transfer directly to the active sulfur sites on the edges of MoSx. The porous structure of graphene and three dimensional structure of carbon cloth facilitates easy electrolyte and bubble diffusion required in continuous water dissociation process. The conducting support, graphene, functional bonding for constant electron transport to the S-edge site of MoSx helped in maintaining the excellent stability of the catalyst upto 5000 cycles with 95% retention. The electrocatalyst developed for PEM electrolysis is promising for current day clean energy generation. Layered double hydroxide (LDH) structures are three dimensional materials with larger exposed active sites for electrocatalytic process. The reaction of metals with acid hinders the utilization of LDH for PEM electrolysis. Transition metal LDH possess excellent stability for oxygen evolution reaction in AE. CuFe layered double hydroxide is synthesized on a three dimensional nickel foam to promote HER in AE. The higher proton adsorption kinetics of lower empty d-orbital containing transition elements and their easy electron transfer outperformed CuFe LDH to generate 10 mA cm-2 with 159 mV for HER in 1M NaOH solution. The efficient charge transfer with interconnected LDH layers, favorable three dimensional structure facilitates easy electron transfer to active site and gas diffusion helped in maintaining the stability of catalyst at 30 mA cm-2 for 5 and 10 hours. This work helps in development of low cost and efficient hydroxide catalyst for HER. We developed an electrocatalyst with high effective surface area, fast charge transfer kinetics, and strong synergistic coupled electrocatalyst with excellent catalytic activity and good durability for water dissociation. This work helps in reducing the potentials of water dissociation in both PEM and AE. Also, the strategical approach of designing an efficient and low cost catalyst helps in development of efficient electrocatalysts for various energy storage and generation application. This work helps in transition of the fossil fuel to clean energy generation systems.
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