Nanoporous metal by dealloying for electrochemical energy conversion and storage
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Introduction “Nano,” “porous,” and metal are all necessary ingredients in a recipe for effective conversion of electricity to chemical energy (and vice versa). This may have driven the initial exploitation of nanoporous (NP) metals for energy applications. Ample evidence has shown that these materials are more than just an assembly of buzzwords; they are an embodiment of optimal morphologies and functional defects. Since the MRS Bulletin issue on dealloying,1 we have witnessed burgeoning research on NP metals for electrochemical energy-conversion/storage devices, including fuel cells,2 supercapacitors,3 and batteries.4 The accumulated literature warrants this article, whose intent is to keep researchers updated on the latest progress and, more importantly, to inform readers how NP metals and energy connect. The challenges that remain on the path toward commercial energy devices based on these materials are also discussed.
Nanoporous electrodes by dealloying To understand the merits of NP metals, we need to look at the materials they are intended to replace. Fuel cells and batteries commonly employ electrodes composed of a continuous solid and pore phase. The continuity of these phases ensures the flow of ions and electrons to the interface. The two phases interpenetrate for maximum interface area per unit volume, which enables a large amount of reaction and thereby a high
current. The solid phase, however, may not be considered continuous, at least compositionally, as such a bicontinuous porous electrode is usually constructed with the use of a polymer binder and a conductive additive (e.g., carbon), in addition to the active material. In this structure, the size of the active material determines the specific area, the catalytic activity, and the distance over which a solid-state diffusion process must operate, whereas the size of pores and the porosity affect mass transport and ion conduction in the electrolyte. Typical values of these crucial parameters, namely, the porosity, the diameter of the active material, and the diameter of pores, are schematically illustrated in Figure 1 for rationalizing the use of NP metals in fuel cells,5 supercapacitors,6,7 and batteries.8 The dealloying process requires no additive for creating a uniform bicontinuous structure. The metal atoms stay connected to their neighbors during the course of dissolution, and the pores stay open, otherwise dissolution cannot proceed. (The Introductory article in this issue provides a comprehensive description of the structural evolution mechanism.) The volume fraction of the pore phase (i.e., the porosity) usually evolves to the range of 40–80%, with the lower limit corresponding to the so-called alloy parting limit9 and the upper limit to the percolation threshold. Ligaments and pores in NP metals share one length scale determined by the rates of dealloying and surface diffusion. The length scale (pore or
Qing Chen, Department of Mechanical and Aerospace Engineering, and Department of Chemistry, The Hong Kong University of Science and Techno
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