Microwave Synthesis of Zn-Terephthalate MOF As Anode Material for Redox Batteries
Authors: Hirai Yuji , Nishiumi Nobuyuki , Sun He , Matsushima Yuta , Masuhara Akito , Shiroishi Hidenobu , Yoshida Tsukasa 
Published: February 2020

Over the past few years, electric power generation from renewable resources such as wind and sun light have already become cheaper than those from fossil fuels and nuclear. However, production of such renewable electricity naturally fluctuates depending on weather. The lack of technically and economically viable systems to store the surplus still hampers the renewables to become primary energy sources.
Redox flow batteries (RFBs) are seen as one of the most promising systems of large scale energy storage, because of their separately designable power and capacity, and also its high cycle stability. Vanadium RFBs (VRFBs) are the most established ones, employing redox reactions of vanadium ions in sulfuric acid. Still, high material cost obstructs practical use of VRFBs, especially because proton exchanging Nafion® membrane takes almost half of the total system cost, followed by 1/4 by vanadium. Alternative redox materials out of cheap elements as well as eliminating Nafion® are therefore the challenges of top priority.
We have recently succeeded in microwave-assisted hydrothermal synthesis of crystalline particles of Zn-based MOFs (metalorganic frameworks) [1]. Double-hydroxide layers of Zn(II) are bridged by terephthalic acid (TPA, benzene-1,4-dicarboxylic acid) to form inorganic/organic hybrid crystals in layered structures, whose spacing and composition vary depending of the pH of precursor solution. The inorganic layer could afford a pathway for electron transport, whereas the interlayer space partially occupied by TPA could allow ion (especially proton) exchange. If a proton-selective redox reaction is achieved, this Zn-TPA MOF could work as a storage electrode for redox batteries in a semi-flow configuration and eliminating membranes.
Pastes of Zn-TPA MOFs were prepared by dispersing them into a mixture of 2-butanol and acetylacetone at 35 wt%. The paste was coated on an F-doped SnO2 (FTO) conductive glass by doctor blading and submerged in warm water to promote necking to fabricate mesoporous electrodes. Electrochemical measurements of the MOF electrodes were performed in a 0.1 M KCl or KI aqueous solution under N2, with a Pt wire as counter.
Electrode made from Zn-TPA MOF in a Zn4(OH)6(TPA) composition and with an interlayer distance of 8.18 Å exhibited reversible redox behavior as shown in Fig. 1. Broad cathode peak and relatively sharp anodic peak are observed, both of them being proportional to the square root of the scan rate, but with different slopes. It is most likely that the redox of MOF is coupled with exchange of proton, as expressed by,
Zn4(OH)6(C8H4O4) + 2H+ + 2e- ⇄ Zn4(OH)6(C8H6O4) (1)
so that diffusion of proton within MOF crystals can be limiting current, and be slow and fast on charging and discharging, respectively. Coulombic reversibility was over 90% at all scan rates and the current was fairly stable over multiple scan cycles. Full charging and discharging under potentiostatic conditions revealed exchange of charge that accounts redox active portion of more than 30% of the total amount of MOF deposited onto the FTO substrate, when assuming the reaction of eq. (1) (1 out of 4 Zn ions undergoes redox). It is therefore clear that the redox reaction is not limited to the Zn ions at the surface of MOFs directly in contact with the electrolyte, but involves those within the crystal bulk, owing to its proton exchanging capability.
The same cathodic charging and anodic discharging behavior was observed when the electrolyte was replaced from KCl to KI. On charging, the solution near the counter electrode turned yellow due to formation of I3
- ions.
3I- + 2e- ⇄ I3
- (2)
Since redox potentials for (1) and (2) are around -1.0 and +0.4 V (vs. Ag/AgCl), respectively, the system could be charged at a voltage of around 1.5 V. Although the cell design as well as the operating condition need to be optimized to achieve a good efficiency, it was possible to maintain some voltage on discharge to prove the system to work as a redox battery.
[1] Y. Hirai et al., Microsys. Technol., 24, 1, 699–708, 2018.

Figure 1