非貴金屬中熵材料於電催化乙醇氧化反應之研究

Abstract

氫氣能源具有零碳排放與高能量密度的優勢，是極具潛力的綠色能源，未來可望取代傳統化石燃料以減緩環境污染。其中，水電解為最潔淨的產氫技術，降低反應過程中的能量損耗，減輕陽極端反應動力學障礙，是提升效率的關鍵策略。透過乙醇氧化反應取代傳統電解水陽極的析氧反應（Oxygen Evolution Reaction OER），不僅可顯著降低電解所需電位，亦能產生具經濟價值的陽極產物。高熵材料近年於電催化領域備受關注，因其獨特的多元組成與混亂結構賦予優異的催化活性與穩定性。然而，多數文獻往往僅以巨觀組成符合高熵材料定義為依據，即直接歸因其催化性能於高熵效應，對其實際活性來源缺乏深入探討。本研究採用符合相同定義之非貴金屬中熵材料薄膜NiCrZnMo作為乙醇氧化反應之電催化劑，進一步探討其催化活性來源是否真正來自高熵效應，或源於材料內部更細緻的結構與協同機制。實驗結果顯示，中熵材料薄膜在泡沫鎳基材上展現優異催化活性，起始電位只需要1.28 V vs. RHE，超越目前文獻所發表的非貴金屬催化劑表現；於碳紙基材上則展現優於現階段之OER催化劑的催化表現。於催化劑合成步驟，採用濺鍍技術將多種金屬均勻沉積於基材表面形成分層金屬薄膜，並經高溫熱處理促進原子垂直方向上擴散，成功製備出具有巨觀上三維均勻度之薄膜。經由能量色散光譜（EDS）與X光光電子能譜（XPS）縱深分析，確認元素均勻分散與混合情形。進一步探討催化機制，透過電化學表現確認Ni為主要反應活性位點。藉由臨場拉曼與XPS分析，發現材料表面於反應中產生結構轉變。結合X光吸收光譜（XAS）解析各元素的配位環境，配合電化學結果指出：Mo於催化過程中扮演質子轉移位點；Cr則增強乙醇吸附能力，有利於反應動力學；Zn則誘導Cr共同形成尖晶石結構，穩定催化劑結構並顯著降低腐蝕，提升長時間操作穩定性。 本研究結果顯示，催化劑的優異性能來自元素間的協同效應及結構穩定效應，而非傳統高熵效應本身。此發現對設計穩定且高活性之電催化材料的設計方向，應更注重於協同效應的影響，對開發高效率乙醇氧化反應之催化劑具有重要意義。

Hydrogen energy is a green energy source with zero carbon emissions and high energy density. Among hydrogen production methods, water electrolysis remains the cleanest approach. Replacing the sluggish oxygen evolution reaction (OER) with the ethanol oxidation reaction (EOR) significantly reduces the electrolysis voltage while generating value-added products at the anode. In this study, we employed a noble metal free medium-entropy materials (MEMs) composed of Ni, Cr, Zn, and Mo as an electrocatalyst for EOR. The catalyst demonstrated an impressively low onset potential of 1.28 V vs. RHE on a nickel foam substrate, outperforming reported state-of-art EOR catalysts. The MEMs catalyst was synthesized via a sputtering technique, followed by thermal annealing to promote atomic diffusion and achieve a medium-entropy structure. Using energy-dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS) confirmed the uniform elemental distribution. Subsequent mechanistic investigations revealed that Ni serves as the primary active site. In-situ Raman spectroscopy and XPS analyses verified surface reconstruction during the catalytic process. X-ray absorption spectroscopy (XAS) provided insights: Mo functions as a proton transfer mediator that activates Ni even in its divalent state; Cr enhances ethanol adsorption on the surface; and the formation of a Cr-Zn spinel structure improves the catalyst’s long-term electrochemical stability. Unlike the widely held view that entropy effects inherently yield superior catalytic performance, our results highlight that the excellent activity and stability from the synergistic effects among the constituent elements. This work not only presents a high-performance MEMs catalyst for EOR but also offers a new perspective on the role of entropy in catalytic design.