鎳基奈米結構電極在鹼性電解液之電化學應用

Abstract

本研究採用陽極氧化鋁（Anodic Aluminum Oxide, AAO）為模板，以無電鍍鎳的方式將其金屬化製作出具通孔特性的鎳多孔結構薄膜。AAO因具有有序排列且垂直的孔洞，因此常被應用在製備多種奈米線、奈米管、奈米點等之模板。本研究採用孔洞直徑約350 nm，厚度為40 μm的AAO作為模板，其深寬比為57。由於其高深寬比的結構，因此要在其表面均勻鍍上連續的鎳層並維持通孔結構具有相當的困難度。透過控制無電鍍鎳的添加劑與製程參數，在本研究中成功製備出具有通孔及導電特性的鎳通孔薄膜。此薄膜將可作為電化學的多孔電極材料並應用在電解水，感測器及燃料電池上等。將此電極應用在電解水的電極，由於其具有多孔的結構提供較高的電化學表面積，因此在造氫（Hydrogen Evolution Reaction; HER）及造氧（Oxygen Evolution Reaction; OER）方面會有顯著的性能提升。另外在鹼性酒精的電化學氧化方面，也明顯得到較平版鎳電極更高的性能。為進一步改善其HER效能，本研究利用自發沉積方式，將更具活性的Ru與Pt沉積在奈米結構鎳材料表面得到一複合的電化學電極。憑藉著奈米結構鎳提供的大電化學面積及自發沉積的Ru與Pt提供更優良的電解水性能，將可以更進一步的提高此電極在HER的性能，並在長時間的定電流測試中具有良好的穩定性，應用此方式製作複合材料電極可以用較低成本得到高的電解水性能。另外延伸自發沉積的概念可以在鎳的基材上承載Pt, Pd與PdxPt1-x二元合金，並應用於鹼性乙醇燃料電池的陽極觸媒。在鹼性環境中Pd具有高於Pt的乙醇氧化性能，且Pd的價格僅有Pt的四分之一。在多元的自發沉積過程中，Pt會參雜進Pd中，由於Pt的原子半徑與晶格常數均比Pd大，因此會提升其晶格常數及改變電子結構，也將會改變其吸脫附物種的能力。另一方面Pt與Pd各自會具有不同物種的吸附能力，因此兩者結合會具有雙功能效應，可以增進對中間產物移除的效率並進一步提升對乙醇氧化的性能。

In this study, the fabrication of nanostructured Ni is carried out by electroless deposition on a through-channel anodic aluminum oxide (AAO) template. The AAO is prepared by a two-step anodization process resulting in pore diameter and thickness of 350 nm and 40 μm, respectively. Subsequently, the AAO undergoes repetitive of sensitization and activation, followed by electroless Ni deposition. The Ni is deposited in an acidic bath with NaH2PO2 as a reducing agent. In addition, polyethylene glycol (PEG) is used as an inhibitor to prevent premature closing of pore opening. Under deliberate control in relevant parameters, a conformal Ni overcoat along AAO pore channel is prepared with shrinkage of average pore diameter to 78 nm. Afterward, the AAO skeleton is removed in a KOH solution, forming free-standing Ni tubules in a brush configuration. The nanostructured Ni enhances electro-oxidation of alcohol and water electrolysis in electrochemical measurements because of notable increment in apparent surface area. Moreover, displacement reactions in an acidic solution enable the spontaneous depositions of Ru and Pt in conjunction with corrosive dissolution of Ni. As the result, the nanostructured Ni is modified by Ru and Pt particles by spontaneous deposition. This modified Ni structure increases the efficiency of hydrogen evolution reaction (HER) in both current-potential polarizations and galvanostatic measurements. Furthermore, Pd, Pt, and PdPt bimetallic alloys are deposited on the Ni by displacement reaction. The compositions for the PdPt alloy are adjusted by various concentrations of Pd and Pt cations in the plating solution. From SEM images and XRD patterns, the PdPt alloy deposited on the Ni substrate reveals a uniform coating electro-oxidation of ethanol is conducted in an alkaline electrolyte for Pd, Pt, and PdPt alloys on Ni substrate. In cyclic voltammetric profiles, the Pd77Pt23 revealed the highest electrocatalytic ability in both apparent current and mass activity. We rationalize that the observed catalytic enhancement is possibly caused by the bifunctional effect. The presence of absorbed OH− ions on the Pt promotes the oxidation of adsorbed CH3CO on the Pd. The overall efficiency of ethanol electro-oxidation is enhanced by the accelerated removal of the CH3CO intermediates.