Dealloyed Air- and Water-Sensitive Nanoporous Metals and Metalloids for Emerging Energy Applications

Abstract Nanoporous copper (np-Cu) films that feature fine ligament length scales and high surface area are prepared using an all-electrochemical approach. The two-step routine involves initially the electrodeposition of Cu_ x Zn_(100− x ) precursor alloy films with controlled thickness and composition from a pyrophosphate bath. This is followed by a selective oxidative removal or dealloying of Zn that leads to a surface area increase (by up to 22 times per one µm thickness of the precursor alloy). In this article, we present a systematic investigation of the impact of three factors on the as-synthesized np-Cu structure: (1) dealloying bath composition, (2) atomic composition (at.%) of precursor alloy, and (3) addition of Ni. Our results show that varying the dealloying bath composition allows for tunable ligament and pore sizes that are due to the anion-induced (ClO_4^−, SO_4^2−, or Cl^−) modified surface diffusivity of Cu. The increase in surface area is also proportionally scalable by modifying the Cu content (12–42 at.%) in the precursor alloy. Thus, np-Cu films with porosity length scales in the range of 20–33 nm were obtained. A further np-Cu length scale refinement down to 10–12 nm was achieved after dealloying of a ternary Cu–Zn–Ni(3%) alloy. Overall, the strict parameter control yields a wide range of nano-scaled np-Cu films with specific surface characteristics, which may be suitable for various applications including microelectronic packaging, sensing, and catalysis. Impact statement We are presenting the first (to the best of our knowledge) comprehensive study on a proposed all-electrochemical approach for the synthesis of nanoporous (np) Cu films with controllable thickness, porosity length scale, and pore-ligament ratio (pore-volume). The novelty of this approach is the use of electrochemical means to synthesize the precursor alloy (Cu–Zn in this work, also Cu–Mn, Cu–Al, etc.) followed by its subsequent dealloying, as opposed to most other approaches that employ dealloying of commercially available bulk precursor alloys and/or thin alloy films deposited in ultrahigh vacuum. The cost-effective use of electrodeposition routines allows for strict composition, thickness, and shape control of the deposit. Also, the deposition of precursor alloys with a desired composition (in turn) enables the development of np-Cu films with controlled pore-ligament length scale as well as pore-ligament ratio. Finally, the proposed approach allows for fine-tuning of the structure, morphology, and ligament/surface elemental content of the as-synthesized np-Cu structure by introducing additional doping metals (Ni) into the precursor alloy. Overall, the systematic study of the proposed approach largely enables versatile and scalable synthetic routes for the preparation of np-Cu-based architectures for a variety of applications ranging from electrocatalysis ( CO _ 2 fixation, H _ 2 energy, fuel cells ), sensing and actuation ( NO _ 3 ^ − reduction, NO _ 2 ^ − analysis ), environmental remediation ( tap water purification from toxic compounds ), and 2.5/3D electronic packaging ( realization of Cu-on-Cu bonding or defect-free and limited in Sn, Cu-Sn micro-bonding in miniaturized devices ). Graphical abstract Preparation of fine-structured nanoporous copper films using an all-electrochemical method.

All-electrochemical synthesis of tunable fine-structured nanoporous copper films


 Electrolytic dealloying has proven to be a practical technique for fabricating nanoporous metals such as nanoporous Au (NP-Au). This widely studied material has novel properties that are useful in applications ranging from chemical sensing to energy storage. Building off the success of this field, issues concerning the sustainability of fabricating NP-Au through dealloying need to be further explored, such as the large amount of valuable sacrificial Ag material dissolved in solution after dealloying is complete. In this work, an electrolytic cell is created consisting of a Au-Ag alloy foil working electrode and a Ag foil counter and reference electrode immersed in a non-aqueous Ag+ ion electrolyte. When a small overpotential is applied, the working electrode is dealloyed to form NP-Au. Unique to this configuration, Ag removed from the working electrode is simultaneously plated on the counter electrode, resulting in high yield recovery of Ag sacrificial material, which can be used to create new Au-Ag parent alloys. This integrated process of simultaneous dealloying and recovery of the sacrificial material significantly improves the sustainability and economic viability of dealloying. From these results, a generalized electrolytic dealloying fabrication procedure is derived that can be extended to a wide range of nanoporous metals.

Sacrificial Silver Recovery during Nanoporous Gold Formation by Electrolytic Dealloying of Gold-Silver Alloy

While most research efforts on CO2-to-CO reduction electrocatalysts focus on boosting their selectivity, the reduction rate, directly proportional to the reduction current density, is another critical parameter to be considered in practical applications. This is because mass transport associated with the diffusion of reactant/product species becomes a major concern at a high reduction rate. Nanostructured Au is a promising CO2-to-CO reduction electrocatalyst for its very high selectivity. However, the CO2-to-CO reduction current density commonly achieved in conventional nanostructured Au electrocatalysts is relatively low (in the range of 1-10 mA/cm2) for practical applications. In this work, we combine direct ink writing-based additive manufacturing and dealloying to design a robust hierarchical porous Au electrocatalyst to improve the mass transport and achieve high CO2-to-CO reduction current densities on the order of 64.9 mA/cm2 with CO partial current density of 33.8 mA/cm2 at 0.55 V overpotential using an H-cell configuration. Although the current density achieved in our robust hierarchical porous Au electrocatalyst is one order of magnitude higher than the one achieved in conventional nanostructured electrocatalysts, we found that the selectivity of our system is relatively low, namely 52%, which suggests that mass transport remains a critical issue despite the hierarchical porous architecture. We further show that the bulk dimension of our electrocatalyst is a critical parameter governing the interplay between selectivity and reduction rate. The insights gained in this work shed new light on the design of electrocatalysts toward scale-up CO2 reduction and beyond.

Sub-100 mA/cm2 CO2-to-CO Reduction Current Densities in Hierarchical Porous Gold Electrocatalysts Made by Direct Ink Writing and Dealloying.

Nanoporous gold (NP-Au) is used in various energy applications. However, little attention has been paid to the nature of surface films which can grow via side reactions during the formation of NP-Au by selective leaching. These films could significantly impact both the kinetics of NP-Au formation by selective leaching, and the performance of this NP-Au in specific energy applications. While it has been reported that complete removal of Ag from a Au–Ag parent alloy by selective leaching is kinetically more sluggish in HClO4 than in HNO3, and that the resulting structure size of NP-Au is smaller when formed in HClO4 compared to HNO3, these findings were not attributed to the growth of surface film via side reactions during selective leaching. In this work, small-angle and wide-angle X-ray scattering (SAXS and WAXS) are used in real time during NP-Au formation by selective electrolytic leaching of Ag from a Au–Ag parent alloy in 1 M HClO4 and 1 M HNO3 to demonstrate that the sluggish dealloying behavior in HClO4 results from the growth of a surface film during dealloying. In HClO4, Ag+ ions released from the Au–Ag working electrode spontaneously react with Cl− ions released from ClO4− reduction at the counter electrode to form an insoluble AgCl passive film on NP-Au. The AgCl hinders Ag dissolution and surface diffusion of Au atoms, slowing down the reaction kinetics and the rate of ligament growth. Our findings shed light on dealloying reaction mechanisms and will contribute to better control of the morphology and surface chemical state of nanoporous metals for energy applications.

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Asaph Lee

Papers

Effects of side reactions on the kinetics of nanoporous gold formation revealed by real-time X-ray scattering during electrolytic dealloying