以化學與氣相去合金法製備多層次奈微米多孔銅 應用於電催化二氧化碳還原與機械性質分析

Abstract

多層次奈微米多孔銅(Hierarchically micro-nanoporous copper, HM-NPC)因具備高反應表面積與良好的質傳擴散性，使其具有極大的潛力應用於電化學二氧化碳還原(electrochemical CO2 reduction reaction, eCO2RR)。其中，以去合金法(dealloying method)製備HM-NPC不僅製程快速，同時也使材料具備豐富缺陷(defect)以利於eCO2RR。 本研究結合三種不同合金製程方式搭配對應三種不同去合金法製備HM-NPC材料，並探討不同製程對於eCO2RR與電化學表現之影響。第一部分研究方式為透過3D列印之模板法設計出支架尺寸120 μm之週期性八角桁架晶格結構(periodical octet-truss lattice)，再利用磁控共濺鍍 Cu-Al 合金薄膜(~1 μm) 與隨後之去合金法合成支架尺寸為16至28 nm之奈米多孔銅薄膜。此結構與製程不僅能增強其機械性質表現，同時也增加其電化學反應表現。此製程製備之八角桁架晶格披覆奈米多孔銅(octet-truss lattice with nanoporous copper, L-NPC)之降伏強度3倍高於Gibson-Ashby公式預測低密度銅。同時，L-NPC之電雙層電容值為純銅批覆之八隅體結構的10倍大。除了奈米支架效應外，L-NPC的微米列印支架還提供了高捲曲的表面，使NPC薄膜附著在桁架上，這可能是造成不同力學和電化學行為的原因。 在了解HM-NPC之結構能增強電化學表面積與機械性質後，第二部分研究進一步利用銅管高捲曲表面特性，以熱浸鍍鋅法(hot-dip galvanization)擴散鋅使銅管產生多層成分之銅鋅合金層，再利用氣相去合金法(vapor phase dealloying, VPD)合成多層次多孔銅(hierarchical porous copper, HPC)。在調整VPD時間0.5 ~ 30分鐘後、溫度區間為723 ~ 973 K，可以合成支架尺寸為0.61 ~ 1.97µm且殘餘鋅含量為29 ~ 2 at%的HPC。其粗化指數(coarsening exponent)為4.099，代表支架的形成和粗化是由表面擴散引起的。同時，在此氣相合金體系中，0.29 eV的活化能(activation energy)進一步證實了銅支架的粗化是由銅原子的表面擴散所主導。在電化學反應中，HPC的雙層電容量是電拋光銅管的34倍。此外，HPC的eCO2RR的電流密度是電拋光銅管的2倍。通過銅–鋅的相互作用，eCO2RR的產物由甲酸(HCOOH)轉化為一氧化碳(CO)和乙醇(C2H5OH)。 為了進一步發揮多層次結構之效益，第三部分研究將反應系統更換為flow cell此外，材料選用共晶相Cu18Al82與單相Cu33Al67前驅物系統，經過-0.7 VAg/AgCl定電位化學脫合金(electrochemical dealloying)，分別開發了多層次奈米多孔銅(hierarchically nanoporous copper, Hi-NPC)和均質奈米多孔銅(homogeneously nanoporous copper, Ho-NPC)。在相似的過電位下，Hi-NPC在eCO2RR中的 C2+電流密度為 510 mA/cm2，顯著高於 Ho-NPC的72 mA/cm2 C2+電流密度。在標準化電化學活性表面積的產物部分電流後，Hi-NPC和Ho-NPC的CO 電流密度呈現出相似的電化學行為趨勢。然而，兩者在C2H4和C2H5OH 的電流密度趨勢卻有顯著差異。這可能是由於多層次結構的擴散性促進了C-C偶合進而合成出C2產物。在Tafel斜率顯示三個電極具有相同的eCO2RR動力學，但eCO2RR 實驗的線性掃描伏安法顯示Hi-NPC中電流變化的斜率具有最大的梯度。這一結果說明 Hi-NPC的擴散率提供了最好的質傳效應，這使得Hi-NPC的電流上升最快。此外，利用不同O2/N2流速的氧還原反應進一步證實了在電化學系統中，多層次奈米多孔結構可以增強擴散率。

Hierarchically micro-nanoporous copper (HM-NPC) has excellent potential applied to the electrochemical CO2 reduction reaction (eCO2RR) because of its high reaction surface area and good mass transfer and diffusivity. Among all synthesized procedures, the preparation of HM-NPC by dealloying method is not only facile but also makes the material have abundant defects for the benefit of eCO2RR. In this study combined three different alloying processes with three different dealloying methods to prepare HM-NPC electrodes and investigated the effects of different processes on eCO2RR and electrochemical properties. In the first part, a periodical octet-truss lattice with a 120 μm truss size was designed using 3D printing. Subsequently, the nanoporous copper films with ligament sizes ranging from 16 nm to 28 nm were synthesized by chemically dealloyed Cu-Al (~1 μm) films coated by the magnetron co-sputtering method. This structure and process can enhance the mechanical properties and increase the electrochemical performance. The yielding strength of octet-truss lattice with nanoporous copper (L-NPC) is three times higher than that of low relative density copper predicted by the Gibson-Ashby equation. At the same time, the electrochemical double layer capacitance (Cdl) of L-NPC is ten times larger than that of a lattice coated with pure copper film. In addition to the nano-scale ligament effect, L-NPC's micro-printed truss provides a high curvature that allows NPC films to adhere to the truss, which may enhance the mechanical and electrochemical properties. After understanding HM-NPC can enhance the Cdl and mechanical properties. In the second part of the study, we further utilized the hot-dip galvanization method to diffuse Zn on the high curvature Cu tube to synthesize a multi-layer Cu-Zn alloy tube and subsequently synthesized the hierarchically porous copper (HPC) by vapor phase dealloying (VPD) method. After turning the VPD time for 0.5 to 30 min and the temperature ranging from 723-973 K, HPC with a ligament size of 0.61 to 1.97 µm and residual Zn content of 29 to 2 at% could be synthesized. The coarsening exponent is 4.099, indicating that the formation and coarsening of the ligament are caused by surface diffusion. In addition, the activation energy of 0.29 eV in this vapor system further confirmed that the coarsening of the copper ligament is dominated by the Cu atoms diffused on the Cu surface. In the electrochemical behavior, the Cdl of HPC is 34 times that of an electropolished Cu tube. In addition, the current density of HPC's eCO2RR is twice that of electropolished Cu tubes. Through Cu-Zn interaction, the products of eCO2RR are converted from formic acid (HCOOH) to carbon monoxide (CO) and ethanol (C2H5OH). In order to further perform the benefit of hierarchically micro-nanoporous structure, the third part of the study alters the system to a flow cell configuration. The eutectic-phase Cu18Al82 and single-phase Cu33Al67 were selected as precursors. After -0.7 VAg/AgCl electrochemically dealloying, the hierarchically nanoporous copper (Hi-NPC) and homogeneously nanoporous copper (Ho-NPC) were synthesized, respectively. With a similar overpotential, the C2+ partial current density of Hi-NPC in eCO2RR was 510 mA/cm2, significantly higher than that of Ho-NPC, which was 72 mA/cm2. After normalizing the partial current of productions by electrochemical active surface area, the CO current density of Hi-NPC and Ho-NPC showed similar electrochemical behavior trends. However, there are significant differences in current density trends between C2H4 and C2H5OH. It might be explained that the diffusivity of the hierarchical structure promotes the C-C coupling to synthesize C2 products. The Tafel slope showed that three electrodes had the same CO2RR kinetics, but the linear sweep voltammetry of the eCO2RR experiment showed that the slope of the current variation of Hi-NPC has the most significant slope. This result showed that the diffusivity of Hi-NPC provides the best mass transfer effect, which makes the current density of Hi-NPC rise the fastest. In addition, oxygen reduction reactions using different O2/N2 flow rates further demonstrated that hierarchical nanoporous structures can enhance diffusivity in electrochemical systems.