Ionics has emerged as an important scientific area for realizing the key materials for the development of advanced electrochemical devices that would support a sustainable society. Prof. Masayoshi Watanabe has focused his interest on new ion-conducting materials such as ionic liquids (ILs) and polymer electrolytes, since conventional aqueous and organic electrolyte solutions have several disadvantages that prove to be a bottleneck for making a breakthrough in electrochemical devices. He has performed a detailed investigation of the ion dynamics in these materials and at the interfaces with electrodes and has subsequently established an area of "organic ionics" and has realized new materialization with distinguished ideas. The outline of his main research achievements is as follows.

1. Fast ion transport by side-chain relaxation in polyethers
In conventional polymer electrolytes based on polyethers, the simultaneous increase in the number of carrier ions and mobility (diffusivity) is inconsistent due to the reduced polymer dynamics upon ion doping and the coupling transport of the ions with main-chain relaxation, which limits their conductivity and utility at ambient temperatures. Prof. Watanabe conceived the idea of coupling the ion transport with side-chain relaxation, which is faster than the main-chain relaxation. He demonstrated fast ion transport in branched polyethers by comparing the same in linear polyethers having similar glass transition temperatures. The introduction of the side chains also increased the frequency factor of the interfacial charge transfer reactions.

2. Fundamental characterization of ionic liquids and ion gels
His fundamental interest was to investigate why ILs behave as ionized liquids even in the absence of solvents. In order to separate the carrier generation and transport processes in ILs, pulse-field-gradient NMR was utilized for the first time for ILs. Molar conductivity was determined by conductometry and was also calculated from the measured diffusivity of the cation and anion using the Nernst-Einstein eq., and the values were compared. The deviation in the molar conductivities determined using the two methods was defined as ionicity, and this was explored in detail as a function of the structures of the ILs. The ionicity is a metric of the correlated motion of the ions in ILs and was found to decrease with increasing Lewis acidity of the cations, increasing Lewis basicity of the anions, directionality of interactions between the cations and anions, and alkyl chain length attached to the ions. These findings clearly demonstrate that the properties of the ILs are dominated by the subtle balance between the Coulombic and molecular interactions, thereby making a significant impact on the IL research community. To break down the limit of the coupling ion transport in polyethers, he proposed polymer electrolytes consisting of polymers and ILs, and named them as "ion gels." The distinct differences from the polyether electrolytes are the plasticizing effects of the ILs toward the polymers and the decoupling ion transport from the segmental motion, by which fast ion transport comparable to that in electrolyte solutions is achieved in polymers.

3. Proposal of functional ionic liquids and their transport mechanisms
Prof. Watanabe realized early in his studies that protic ILs can be H⁺-conducting electrolytes for non-humidified H₂/O₂ fuel cells and found a protic IL with not only excellent bulk properties as a H⁺-conductor but also fascinating electrochemical properties (facile O₂ reduction and H₂ oxidation). He constructed a polymer electrolyte fuel cell using the protic IL and a sulfonated polyimide and demonstrated the fuel cell operation under non-humidified conditions and at temperatures higher than 100 ℃.
He also revealed the screening of Coulombic repulsion in ILs with high ionic strength by studying the charge transport of an I^-/I₃^- redox couple. The diffusivity of I^-/I₃^- in ILs linearly increased with increasing concentration, which was attributed to the exchange reaction between I^- and I₃^- (structural diffusion), and this was not observed in molecular solvents. When ILs containing the I^-/I₃^- redox couple were used in dye-sensitized solar cells, fast charge transport could be enabled despite the high viscosity. He also found that equimolar mixtures of a certain glyme and a lithium salt are liquids at room temperature and exhibit properties similar to those of conventional ILs. The liquids were found to consist of the solvate (complex) cation and anion and could be categorized into Li⁺-conducting solvate ILs. Since the formation of the solvate ILs is governed by the competitive coordination of Lewis basic anions and glymes with Lewis acidic Li⁺ cations, the solvate ILs were obtained when the Lewis basicity of the anions was low and the number of oxygen atoms in the glymes was 4-5, which could satisfy the coordination number of Li⁺. He is now involved in an ongoing project on the development of next-generation Li-S batteries with the solvate IL electrolytes by utilizing the negligible activity of the free solvents.

4. Phase behavior of polymers and colloids in ionic liquids and their materialization
Prof. Watanabe discovered for the first time the unique phase behavior of certain polymers in ILs, i.e., their compatible, upper critical solution temperature (UCST), and lower critical solution temperature (LCST) behaviors. The UCST and LCST were significantly affected by a slight change in the polymer and IL structures, which could be ascribed to small changes in the mixing enthalpy and entropy compared with those in conventional molecular solvents and were determined by the subtle balance between them. These findings led to the development of stimuli-responsive materials such as polymer actuators and stimuli-responsive molecular assembly and also led to the discovery of volume-phase transition of ion gels and temperature-induced sol/gel transitions in ILs. The stimuli-responsiveness has been expanded to photo-healing materials using photochromic compounds and self-healing materials based on intermolecular multiple interactions.

In summary, Prof. Watanabe's work has been supported by his distinct ideas, and the proposals on the design and electrochemical applications of ILs are based on a deep understanding of their nature. He established the area of organic ionics and made a significant contribution to the knowledge and understanding of the ionic materials, thereby he certainly merits the Chemical Society of Japan Award.