Ion Transport in Bi-continuous Nanoporous Metals

Abstract

Bi-continuous nanoporous metals, characterized by interconnected open pores and metal ligaments, offer high surface area, good conductivity, and tunable characteristic length scales. They are widely employed as electrodes in electrochemical energy storage and conversion, where ion and mass transport play critical roles. However, systematic studies establishing clear structure-transport relationships in bi-continuous nanoporous metals remain scarce.

In this dissertation, we reveal how electrolyte permeation and ion diffusion depend on the structures of nanoporous metals. The work starts with the fabrication of nanoporous Cu, hierarchical nanoporous Cu, Ni-filled nanoporous Cu, and ultrafine nanoporous Au leaves, with the guidance of percolation theory. We can control the length scale varying from ~200nm to ~5nm by tailoring the alloy composition and dealloying conditions. By electrodeposition, we can use Ni to reduce the pore size of nanoporous Cu from ~89nm to ~35nm and the porosity from 57.5% to 15.8%, consistent with a percolation threshold. With the hierarchical nanoporous Cu, we show that the hydraulic permeability scales proportionally with the square of pore width, consistent with the classic Hagen–Poiseuille law. In the Ni-filled nanoporous Cu, we observe an enhancement of salt rejection when the pore width is reduced to the order of the Debye length of the electrolyte. With the ultrafine nanoporous Au, we carry out an extensive study of ion transport and reveal a strong selectivity dependent on the charge carried by the anions in the electrolyte. For anions like [Fe(CN)₆]^4-, the diffusivity decreases by six orders of magnitude compared to that in a free solution.

Based on structure-transport relationships, we further use nanoporous metals to address technological challenges in electrochemical devices. The hierarchical nanoporous Cu is applied as a flow-by electrode for electro-organic synthesis, and it achieves a high faradic efficiency and a productivity owing to the high surface area and the high permeability. The ultrafine nanoporous Au is applied as an ion-selective membrane in Zn-air batteries to prevent zincate-ion crossover while maintaining a low ionic resistance, leading to high cycling stability. The thesis provides valuable insights into the transport properties in nanoporous metals, suggesting strong effects of surface charge that are both fundamentally intriguing and significant for practical applications.

Date of Award	2025
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology
Supervisor	Qing CHEN (Supervisor)