Electrochemical Evaluation of Zinc Terephthalate Metal-Organic Frameworks for Application in Rechargeable Batteries

Abstract

Introduction

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), Zn₄(OH)₆(TPA) (Type-II) and Zn₂(OH)₂(TPA)･H₂O (Type-IV) 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, their redox potentials, capacity, rate characteristics and electrochemical stability, which are important for the application as a rechargeable battery, have not been clarified. In this study, we investigated the redox activity rate and the crystal structure of Zn-TPA MOFs with different types before and after redox by Cyclic voltammetry (CV) and XRD to evaluate their electrochemical stability.

Experimental

The precursor solution containing 0.1 M zinc acetate and 0.05 M TPA was basified to pH 7.0, 5.9 and 4.9 to yield Types I, II and IV, 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 Types I, II and IV was about 10, 10 and 25 µm the density was 0.25, 0.45 and 0.68 g cm^-3, corresponding to porosity of 90, 82 and 73%, 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]. Cyclic Voltammograms (CVs) were measured at Zn-TPA MOFs electrodes in 0.1 M KCl aqueous solution.

Result and Discussion

The redox reactions of Type-I, II and IV which predicted to undergo proton coupled reversible two electron reduction were assumed 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)

Zn₂(OH)₂(TPA)･H₂O + 2H⁺ + 2e^- ⇌ Zn₂(OH)₂(TPAH₂)･H₂O                                 (3)

The redox potentials of Type-I, II and IV were -1.16, -1.08 and -1.07 V vs. Ag/AgCl as judged from redox peaks in cyclic voltammograms (CVs, Fig. 1). The redox active fraction, which is an index of the percentage of MOF particles in the film that contributed to the reaction, described as,

Redox active fraction [%]                                                                           (4)

where C is the anodic capacity (C), FW is the formula weight (g/mol), m is the total mass (g) of active materials, F and n are the Faraday constant (C/mol) and electron number, respectively. The redox active fraction of Type-I, II and IV calculated from the equation (1)~(3) reaches 0.22, 0.95 and 0.30%, respectively. While Type-I showed almost unchanging in redox active fraction even at the 100th cycle, Type-II and IV that showed initially higher values than Type-I, gradually decrease, eventually reaching 0.53 and 0.21%, respectively. In fact, the XRD patterns of Type II and IV electrode after 100 CV cycles revealed their conversion into Type I (Fig. 2), described as,

Zn₄(OH)₆(TPA) + 2e^- + 2H⁺ + 6H₂O → Zn₃(OH)₄(TPAH₂)･6H₂O + Zn(OH)₂                           (5)

3Zn₂(OH)₂(TPA)･H₂O + 2e^- + 2H⁺ + 11H₂O → 2Zn₃(OH)₄(TPAH₂)･6H₂O + TPAH₂              (6)

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.