The microstructural dependence of ionic transport in bi-continuous nanoporous metal

Abstract

Ionic transport in electrolyte-imbibing nanopores is considered a common bottleneck in the functional applications of bi-continuous nanoporous (NP) metals, which in turn offer a unique opportunity to understand structure-transport relationships at nanoscales. By dealloying an Ag-Au alloy and isothermal coarsening, we can control the pore size of NP Au in the range of 13 nm to 2.4 µm and the porosity between 38 % and 69 %. By reduction-induced decomposition of AgCl, we can further control the structural hierarchy and the pore orientation of NP Ag. In these NP metals, we measure the effective conductivities of 1 M NaClO₄ to range from 7 % to 44 % of that of a free solution. The tortuosity of NP Au displays weak dependences on both the pore size and the porosity, consistent with the observed self-similarity in coarsening, except for those of pores narrower than 25 nm, which we consider deviating from the well-coarsened pore geometry. For NP Ag, rapid transport is observed for the hierarchical and the oriented structures; the former can be explained with the Maxwell-Garnett equation and the latter underlines random orientations as the common cause of slow transport. We then demonstrate the practical significance of the structure-transport relationship in the application of NP Ag in CO₂ reduction.