Nanoporous Copper Ribbons Prepared By Chemical Dealloying of a Melt-Spun Zn-Cu Alloy

Abstract

Dealloying process is a powerful and versatile method to fabricate three-dimensional nanoporous (np) materials for applications in electrochemical synthesis, sensors, and catalysis. In the last years, several morphologies of np-copper and their dealloying mechanisms have been reported in alkaline and acidic media.[1-4] For instance, the nature of anions such as chloride and phosphate strongly influence the ligament size of np-Cu. The effects of several experimental parameters to control the structural and chemical properties of np-Cu materials via dealloying are not fully understood to date.In our work, Zn80Cu20 alloy ribbons prepared by melt-spinning were dealloyed by free corrosion in 0.1 M HCl and 1.3 M NaOH solutions. Both the surface and cross-section of the obtained np-Cu ribbons were comprehensively characterized by SEM, EDX, STEM, XRD, and XPS. Our results show a clear relation between dealloying conditions (pH, kind of electrolyte and reaction time) and structural parameters (ligament size and chemical composition) for np-Cu ribbons. The ligament size is strongly coupled with the residual Zn content prepared by dealloying in acidic environment. In contrast, the ligaments produced in alkaline media are much smaller, whereas the residual Zn content can be tuned in a broad range without changes in ligament size. For the first time, we point out the distribution of residual Zn within single ligaments of np-Cu. We suggest that the Cu surface atoms diffuse and capture the Zn atoms which are located in the near-subsurface during dealloying. Therefore, the Zn-rich regions seem to be relicts of the master alloy. We have also evaluated the time-resolved evolution of porosity within the ribbons via cross-section analysis. Despite the relatively thick melt-spun ribbons (35 ± 3 μm), kinetics of a bicontinuous ligament-pore structure are controlled by the interfacial process instead of the diffusion of corrosive electrolyte solution in and out of the porous layers, referred to as long-range mass transport. Surface diffusivities of Cu at 25 °C were determined to be 1.4 × 10−18 and 9.4 × 10−21 m2 s−1 in 0.1 M HCl and 1.3 M NaOH, respectively. Therefore, the high Cu surface diffusion rate in the presence of chloride ions enables an enhanced dealloying front propagation into the ribbons, and also coarsening of the ligaments to form larger ligament. On the contrary, the slow surface diffusion rate of Cu (hydr)oxide in 1.3 M NaOH solution strongly limits the dealloying process and forms smaller ligaments. Moreover, dealloying temperature strongly influences the surface diffusivity of Cu, leading to a strong relationship between dealloying temperature and the np-Cu structure. The electrochemically active surface area of np-Cu ribbons was determined by double layer capacity method. The catalytic properties of the np-Cu ribbons were then evaluated for organic electro-synthesis of cyclic carbonates from CO2 and epoxide. The yield of cyclic carbonate could be improved by using np-Cu ribbons for the activation of CO2.Based on our results, we provide deeper insights to the formation processes of np-Cu ribbons with tunable ligament size and Zn content in various electrolyte environments and its application as electrode material for electro-synthesis of cyclic carbonates formation from CO2 and epoxide.