產氫反應機制與活性在鈷鐵磷化物上的計算與實驗研究

Abstract

氫氣燃料作為取代化石燃料成為下一代符合永續發展的能源，以電解水方式進行析氫反應可以說是一種有效且可行的產氫方式，本篇研究以非貴金屬材料CoP、FeP和CoFeP作為反應觸媒，分析其反應機制並利用雙金屬的摻雜增進析氫活性。本篇研究第一部分以常見的方法以氫吸附能分析析氫活性，利用氫原子在真空下吸附於觸媒表面計算吸附能，我們以Ag、Au、Co、Cu、Ni 、Pd、Pt、Rh確認吸附能與析氫反應電流的火山圖趨勢，接者以相同方式探討CoP、FeP和CoFeP，發現CoFeP由於Co-Fe雙金屬摻雜影響電子結構，確實有著更好的吸附能(ΔGH*≈0)。除此之外’，我們對於火山圖趨勢中Cu的偏移，我們利用含水層的模型計算其反應的活化能以及電位改變對電子轉移過程的影響，成功解釋了Cu不在火山圖趨勢的原因。利用這個方法，我們發現CoFeP在速率決定步驟Volmer step的活化能低於CoP、FeP，因此我們認為在計算上CoFeP會有更好的析氫活性。最後，我們利用實驗確認CoP、FeP和CoFeP的電化學活性，以共沉澱法和化學氣相沉積合成，再以SEM、EDX、XRD、XPS確認觸媒之晶粒大小皆為6~8 nm，且都為均勻的純相磷化物，透過電化學方法，我們測得CoFeP有更好的電化學活性，在10毫安電流的過電位為50 mV以及117 mV/dec的塔菲爾斜率，在排除活性面積的增加改變電流，我們確認了雙金屬在觸媒間的互相影響不只增強了化學穩定度，也確實增加了反應活性。Hydrogen fuel as a fascinating alternative to traditional fossil energy resources; hydrogen evolution reaction (HER) through water electrolysis is one of the most effective and feasible way for hydrogen production. In this study, we investigated the low-cost phosphides of CoP, FeP and CoFeP to better understand the HER mechanism and improve their activity.Initially, we examined the HER mechanism via the common evaluation of free energy for hydrogen adsorption (ΔGH*) on the catalysts in vacuum. We studied the various metals of Co, Ni, Cu, Rh, Pd, Ag, Pt and Au, as references, as well as our phosphide samples. Our results found that CoFeP has the appropriate ΔGH* for the better HER activity than other phosphides, attributable to the altered electronic structure due to Co-Fe interaction. The results for metallic samples well reproduce the volcano plot of their HER activity, with the deviation of Cu, from previous works. Furthermore, we utilized more realistic water-layer model to examine the reaction barrier and effect from electric potential in the proton transfer process in HER. Our result found that the rate-determining Volmer step has the lowest energy on CoFeP among phosphides, implying its best HER activity; also, the Cu deviation has been well resolved. Finally, we experimentally confirmed the superior activity of CoFeP. CoP, FeP and CoFeP have been synthesized by co-precipitation and chemical vapor deposition methods and characterized by SEM, EDX, XRD, XPS and TEM, in which similar sized (6 ~ 8 nm) and uniformly distributed phosphides without oxide contaminants have been well prepared.Electrochemical experiment indeed shows that CoFeP has the best HER reactivity with the overpotential of 50 mV and Tafel slope of 117 mV/dec.