Bi-continuous nanoporous metals from reduction-induced decomposition

Abstract

Bi-continuous nanoporous metals, with unique structural characteristics and properties like high specific surface area and good electrical conductivity, have been applied in a wide range of applications. The most typical method to fabricate nanoporous metals is dealloying, the selective dissolution of one or more components from an alloy. The percolation dissolution mechanism explains the morphology evolution during dealloying. However, the selection of alloy systems and the preparation of uniform alloy precursors always limit the applications of dealloying in preparing bi-continuous nanoporous metals. In my dissertation, I discuss the percolation dissolution of compound precursors to prepare bi-continuous nanoporous metals. The new method is called reduction-induced decomposition (RID). First, the RID of AgCl is chosen as an example to show bi-continuous nanoporous Ag that resembles dealloyed structures can be prepared via RID. The RID also provides high tunability in its length scales, porosity, and structural hierarchy. Then, based on the AgCl RID system, we study the kinetics of RID by controlling the rate of reaction through the concentration of the reductant, NaBH₄. At low concentrations, RID mostly occurs at a constant and low reaction rate under surface diffusion control, whereas at high concentrations and relatively high rates, bulk diffusion controls the reactions. At the early stages of the bulk diffusion-controlled RID, oriented nanoporous Ag evolves at high reaction rates. We identify a rate threshold and elaborate a model for the evolution of the oriented structure. The better understanding of RID kinetics and the formation mechanism of the oriented structure enables us to apply RID in flow cells to prepare nanoporous metal electrodes with an interesting oriented structure. The nanoporous metal electrodes prepared in flow cells have potential in a variety of flow cell applications. In the last chapter of the dissertation, we take the preparation of nanoporous Zn as an example and show the advantage of RID in preparing reactive nanoporous metals. The as-fabricated monolithic nanoporous Zn served as the Zn anode in a Ni-Zn cell and achieved good performance.

Date of Award	2020
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology