Electro-Chemo-Mechanical Coupling in Hierarchical Nanoporous Gold-Polypyrrole-Water Hybrids
Authors: Matts Olga , Mameka Nadiia 
Published: February 2024

Nanoporous (np) metals made by dealloying is a novel class of nanostructured materials that offers numerous functionalities, ranging from sensing, energy storage, to (electro-)catalysis and beyond. A substantial amount of research in np materials has been focused on np gold due to its excellent structural and chemical stability, adjustable pore size, high conductivity, biocompatibility, and easy fabrication process via free corrosion or electrochemical dealloying. A considerable number of studies have been conducted to investigate np gold infiltrated with an aqueous electrolyte [1]. In that way, the material is considered as a hybrid so that one can reversibly tune the gold surface state by manipulating the superficial charge densities or adsorbate coverages via the applied electric potential [1]. The above-mentioned approach results in a number of intriguing properties of np Au revealed upon electrode potential variation in aqueous electrolytes such as actuation [2, 3], sensing [4], tunable strength [5] and elastic modulus [6]. The behavior is well-studied in case of weak ion adsorption and surface oxidation.
One of the promising methods towards further improvement of functional and mechanical properties of np metals was implemented by Roschning and Weissmueller [7]. It was shown that the actuation strain of np Au can be enhanced via electrodeposition of a conductive polymer polypyrrole (PPy) on the gold surfaces. The actuation strain of the np Au-PPy hybrids strongly scales with the PPy phase fraction. Moreover, Li et al. [8] observed variations of effective elastic modulus in np Au-PPy-water hybrids. Thus, introducing the conductive polymer into a pore space of the np metal yields a hybrid actuator with a significantly improved actuation amplitude and yield strength.
Here, we employ PPy for surface functionalization of hierarchical (hc) np Au (Figure 1) to explore the impact of the conductive polymer coating on the functional behavior of the hierarchically structured np metals. Np metals with structural hierarchy made by dealloying [9] open up new opportunities for multifunctional hard-soft hybrids. The larger pores at the higher hierarchical level (characterized by diameters around 120 nm) can promote fast mass exchange, while nanopores at the lower hierarchy level (below 30 nm) provide a large surface area and high chemical activity. Actuation behavior of the hybrid materials was analyzed in situ in a dilatometer upon potential cycling in a 1 M HClO4 aqueous electrolyte. The elastic response was examined in situ in a dynamic mechanical analyzer in the same electrolyte. We revealed pronounced changes of the macroscopic length and Young’s modulus in response to the voltage-induced redox reactions of the polymer films at the np electrodes. In the contribution, we discuss the origin of the electro-chemo-mechanical coupling phenomena.
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Figure 1. Scanning electron micrograph of hierarchical nanoporous Au-PPy hybrid microstructure.

Figure 1