Cation and/or molecule transfer within nanoporous materials is utilized in lithium-ion secondary battery, ion exchange, hydrogen storage, molecular sensors, molecular filters, and so on. Here, we performed ab initio total energy calculation to derive the alkali cation potential in the Prussian blue analogues, ( , Na, K, Rb, and Cs; , Ni, Mn, and Cd), with jungle-gym-type nanoporous framework. The potential curves of larger cations, that is, K+, Rb+ and Cs+, exhibit a barrier at the window of the host framework, while those of the smaller cations, that is, Li+ and Na+, exhibit no barrier. We will discuss the useful functionalities observed in the Prussian blue analogues, that is, (a) battery properties mediated by Li+ intercalation/de-intercalation, (b) electrochromism mediated by Na+ transfer in all solid device, and (c) the elimination of Cs+ from aqueous solution by precipitation, in terms of the alkali cation potentials. 1. Introduction Nanoporous materials are attracting the increasing interest of material scientists because the materials are utilized in lithium-ion battery, ion exchange, hydrogen storage, molecular sensors, and molecular filters, and so on. To further improve the functionality, we need to comprehend the cation and molecular routes and their potential within the host framework. Among the nanoporous materials, the Prussian blue analogues, ( and are alkali metal and transition metal, resp.), are attracting current interest of material scientists because they exhibit useful functionalities, such as battery properties mediated by Li+ intercalation [1–8], electrochromism mediated by Na+ intercalation [9–16], and the elimination of Cs+ from aqueous solution by precipitation [17]. The Prussian blue analogue is the oldest complex compound that the human being synthesized. The compound is easily synthesized from cheap and ubiquitous source materials and, hence, has great advantage for commercial use of the above-mentioned functionalities. The Prussian blue analogue, , consists of a three-dimensional (3D) cyano-bridged transition metal framework ( ) and guests ( and H2O), as schematically shown in Figure 1. A part of the waters (ligand water: ) locates at the [Fe(CN)6] vacancies and coordinates to the transition metal. The residual waters (zeolite water: ) and alkali cations ( ) locate in the nanocubes of the host framework. Most of the Prussian blue analogues exhibit the face-centered cubic structure ( ; ) [18]. Figure 1: Schematic structure of the Prussian blue analogue, ( and are alkali metal and transition metal, resp.). Transition metal
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