(Invited) Ion-Conductive Metal-Organic Frameworks

Abstract

Proton conductivity in solids is of importance not only for useful applications such as fuel cells but also for better understanding of the nature of proton-transport phenomena. Recently, metal–organic frameworks (MOFs) have been emerged as a novel class of proton-materials because the ability to rationally design and chemically tune their architecture allows chemists to establish various methods to control the proton conductivity. We have proposed three types (types I–III) of concepts to give high proton conductivity to MOFs. In type I, proton carriers are introduced as counterions into the pores (e.g., H3O+). In type II, acidic groups are placed on the framework (e.g., –COOH). In type III, charge-neutral proton-conducting species are incorporated within the pores (e.g., H3PO4).1 We have synthesized (NH4)2(adp)[Zn2(ox)3]·3H2O (adp = adipic acid; ox = oxalate), having both type I and type III features; (NH4)2(adp)[Zn2(ox)3]·3H2O shows high proton conductivity because of the existence of two-dimensional (2-D) hydrogen-bonding networks which act as a crystalline proton-conducting pathways.2 Intentional control of proton conductivity and the proton dynamics in this MOF has been also studied.On another front, hydroxide-ion conductors have received a great deal of interest as electrolytes for alkaline fuel cells that can operate without precious metal catalysts such as Pt. We have also focused on creating novel hydroxide ion conductors using the MOFs. We proposed a new strategy to introduce hydroxide ions into MOFs, i.e., which is “hydroxide salt inclusion into alkaline-stable MOFs”. We succeeded in synthesizing a novel MOF including alkylammonium hydroxide salt by employing an alkaline-stable ZIF-85 framework (NBu4-ZIF-8-OH) (Figure 1).6 ,7 Ionic conductivity of NBu4-ZIF-8-OH was found to be 2.3 × 10–8 S cm–1 at maximum, which is four orders of magnitude higher than that of blank ZIF-8. Considering that the framework structure of NBu4-ZIF-8-OH is the same as that of ZIF-8, the enhancement in the ionic conductivity can be attributed to the included salt.(References) (1) M. Sadakiyo et al., J. Am. Chem. Soc. 2009, 131, 9906. (2) M. Sadakiyo et al., J. Am. Chem. Soc. 2014, 136, 7701. (3) M. Sadakiyo et al., J. Am. Chem. Soc. 2014, 136, 13166. (4) S. Miyatsu, M. Kofu, O. Yamamuro, M. Sadakiyo, T. Yamada, H. Kitagawa et al., Phys. Chem. Chem. Phys. 2014, 16, 17295. (5) Park, K. S.; Ni, Z.; Cote, A. P.; Choi, J. Y.; Huang, R.; Uribe-Romo, F. J.; Chae, H. K.; O’Keeffe, M.; Yaghi, O. M. PNAS 2006, 103, 10186. (6) M. Sadakiyo et al., J. Am. Chem. Soc. 2014, 136, 1702. (7) M. Sadakiyo et al., Chem. Lett. 2017, 46, 1004. Figure 1