Hydrothermal Synthesis and Electrochemical Evaluation of Zn-Tmla MOFs Employing Trimellitic Acid
Authors: Ueda Shotaro , Hirai Yuji , Sun He , Matsushima Yuta , Masuhara Akito , Shiroishi Hidenobu , Yoshida Tsukasa 
Published: February 2020

Metal organic frameworks (MOFs) are widely studied because of their unique structures and wide applications represented by gas separation, gas sensor and electrocatalysis. In our previous research, 4 types of Zn-telephthalic acid (TPA) MOFs were synthesized via microwave hydrothermal reaction by varying pH of precursor solution. One of the Zn-TPA MOFs in a Zn4(OH)6(C8H6O4) composition exhibited a stable reversible redox reaction accompanied with proton exchange owing to its structural rigidity brought by para positional carboxyl groups (–COOH), to show its usefulness as a negative electrode material for redox batteries.
In this study, we have employed trimellitic acid (1,2,4-benzenetricarboxylic acid, abbreviated as TMLA) for synthesis of Zn-based MOFs. TMLA has an extra carboxyl groups to the structure of TPA, so that an increased storage capacity is anticipated.
Aqueous solutions containing 0.1 M Zn (CH3COO) 2 and 0.035 M TMLA with pH adjusted between 7 and 5 by KOH were put into a Teflon-lined vessels for microwave (2.45 GHz) reaction at 150℃ for 30 min. The powder samples were centrifugally collected, washed and dried for analysis by XRD and TG-DTA. Paste containing 35wt% powder was coated F-doped tin oxide (FTO) glass to fabricate mesoporous electrodes with ca. 10 μm thickness for their evaluation by cyclic voltammetry and chronoamperometry in a 0.1 M KCl.
Three types of Zn-TMLA MOFs were synthesized depending on pH and two of them in layered structures with 14.4 and 12.2 Å spacing could be isolated, with their compositions determined as Zn5(OH)7(C9H3O6) and Zn4(OH)5(C9H3O6)•1.5H2O from TG-DTA, named as types A and B, respectively. Both A and B exhibit reversible redox reactions as shown in Fig. 1, with their coulombic reversibility of 85 and 90%, E
1/2 values of -1.12 and -1.10 V vs. Ag/AgCl, respectively (determined from 70 mV s-1 scans), similar to 90% and -1.03 V found for Zn-TPA MOF. Oxidation peaks are positively shifted on the slow end of the potential scanning, unlike the case with Zn-TPA, which might be associated with reconstruction of the MOF structure, upon proton exchange in the redox reactions expressed as follows,
Zn5(OH)7(C9H3O6) + 3H+ + 3e- ⇄ Zn5(OH)7(C9H6O6) (A)
Zn4(OH)5(C9H3O6) + 3H+ + 3e- ⇄ Zn4(OH)5(C9H6O6) (B)

The presence of an extra carboxylate in TMLA could be the reason of the structural flexibility to cause the observed change. An important difference is noticed for kinetic behavior between A and B. While A shows slow reduction and relatively fast oxidation, similar to that found for Zn-TPA MOF, those of B are highly reversible, indicating facile exchange of protons, possibly owing to the presence of hydration water in its structure. Potentiostatic full charging an discharging of the electrodes revealed redox active fractions of 31 and 22% of the total amount of A and B deposited on FTO according to the reactions expressed by Eqs. (A) and (B) above, respectively, being somewhat smaller than the value found for Zn-TPA (35%). However, when the absolute storage capacity per molar amount of Zn is compared, similar values of 1.79 × 104, 1.59 × 104, and 1.69 × 104 C mol-1 are found for A, B and Zn-TPA, respectively. Thus, upon improvements of redox active fractions by optimizing the film preparation procedure as well as the thickness, higher storage capacity is expected for Zn-TMLA MOFs than Zn-TPA, because of the contribution of the third carboxylic acid group.

Figure 1