Synthesis of Zinc-Terephthalate Metal Organic Frameworks and Their Application to Redox Batteries

Metal-organic frameworks (MOFs) made of inorganic ions and organic linkers are attracting attention for their unique properties because of their widely open porous crystal structure. We have recently succeeded in microwave-assisted hydrothermal synthesis of Zn-Terephthalate (TPA) MOF particles [1]. Several different types such as Zn3(OH)4(TPA)･6H2O (Type I) and Zn4(OH)6(TPA) (Type II) in layered structures were obtained depending on pH of the precursor solution. They exhibited proton-selective reversible redox reactions to indicate possibilities for membrane-free redox batteries [2]. However, still little is known about their electrochemical behavior (such as redox potential, coulombic capacity, cycle stability, and their difference between I and II). In this study, we have undertaken quantitative electrochemical analysis at the Zn-TPA MOF electrodes to evaluate their applicability to batteries. The precursor solution containing 0.1 M zinc acetate and 0.05 M TPA was basified to pH 7.0 and 5.9 to yield Types I and II, respectively. White precipitates after hydrothermal reaction at 150℃ and 30 min. under microwave irradiation were centrifugally collected. 35wt% of MOF pastes were prepared in 2-butanol containing acetylacetone, coated onto an FTO glass substrate to fabricate porous electrodes with a projected area of 2 cm2. While the film thickness for both Types I and II was about 10 µm, the density was 0.25 and 0.45 g cm-3, corresponding to porosity of 90 and 82%, respectively, assuming the density of bulk MOFs of Types I and II to be the same as that of Zn3(OH)4(TPA) reported earlier (2.538 g cm-3) [3]. The redox potentials of Types I and II were -1.16 and -1.08 V vs. Ag/AgCl in a neutral KCl aq. as judged from redox peaks in cyclic voltammograms (CVs, Fig. 1). Although Type II initially exhibited the higher redox activity than Type I, it gradually decreased over the repetition of CVs, whereas coulombic reversibility always was around 100% (Fig. 1). Thus, the redox active fraction calculated by assuming redox reactions as, Zn3(OH)4(TPA)･6H2O + 2H+ + 2e- ⇌ Zn3(OH)4(TPAH2)･6H2O (1) Zn4(OH)6(TPA) + 2H+ + 2e- ⇌ Zn4(OH)6(TPAH2) (2) was about 0.22% for Type I and stayed unchanged, whereas it dropped from 0.89% (initial) to 0.50% (100th cycle) for Type II. In fact, the XRD pattern of Type II electrode after 100 CV cycles revealed its conversion into Type I, described as, Zn4(OH)6(TPA) + 2e- + 2H+ + 6H2O → Zn3(OH)4(TPAH2)･6H2O + Zn(OH)2 (3) Although Type I appears to be the better electrode material for its more negative potential and higher stability, the absolute redox capacity is so small because of its small redox active fraction. That may be caused by poor necking of the particles as suggested from its extremely high porosity. Therefore, control of particle size, conditions of paste preparation and film fabrication need to be reviewed to improve its redox activity. [1] Y. Hirai et al., Microsys. Technol., 24, 699–708 (2017) [2] Y. Hirai et al., ECS Trans., 88(1):259-268 (2018) [3] A. Carton et al., Solid State Sciences., 8 (2006) 958–963 Figure 1