Corrosion Protection By MOF-5 Coatings for Sustained Zinc-Air Battery Anodes
Authors: Singh Ankit , Vedarajan Raman , Matsumi Noriyoshi 
Published: February 2020

Zinc–air is a century-old battery technology but has attracted revived interest recently. With larger storage capacity at a fraction of the cost compared to lithium-ion, zinc–air batteries clearly represent one of the most viable future options to power electric vehicles. Despite of their great potential, the development of zinc–air batteries has been impeded by problems associated with the metal electrode like self-corrosion, dendrite formation on zinc electrode, and poor discharge characteristics. It is therefore urgent to improve their overall device performance. In the present research, an attempt is made to investigate the application of ionic liquid (IL) modified Metal Organic Frameworks (MOFs) in zinc-air batteries. The use of MOF-5 (IL) in zinc-air batteries have never been studied so far, so in this research we will focus on the use of electrochemically modified zinc electrode decorated with MOF-5 (IL) for the fabrication of zinc-air batteries. It can solve issues like self-corrosion and improve discharge performance which would result into high performance zinc-air batteries with prolonged cyclability.

The research also deals with the use of polythiophene in zinc air batteries. Electro-polymerization of a conducting polymer like polythiophene [1] proves to be an efficient approach for modification of the Zn anodes as it provides high thermal and chemical stability. Hence, electropolymerized polythiophene coating can aid in the binding of MOF-5 film over the Zn electrode which would provide stability to the latter.

Synthesis of MOF-5 (IL) decorated zinc anodes - Modified MOF-5 (IL) was successfully synthesized over zinc electrode by a mild in-situ electrochemical method [2] using 1-butyl-3-methylimidazolium chloride (IL) ionic liquid as a templating agent employing a modified procedure of reported method [2]. Synthesis of the MOF-5 (IL) was achieved by a constant dissolution of Zn2+ ion from Zn anode. The dissolution was carried out by applying a constant direct current of 0.20 A. A titanium electrode was employed as cathode. The electrolyte used in this procedure was a solution containing 0.07 M terephthalic acid and 0.04 M zinc nitrate hexahydrate in DMF. The electrochemical galvanostatic procedure was carried out for 1hr to get uniform coating of MOF (IL) over the zinc electrode. After that, the electrode was kept for drying at room temperature for 6 hours.

Synthesis of MOF-5 (IL) / polythiophene decorated zinc anodes
– After the synthesis of MOF-5 (IL), thiophene was added in the solution described above and cyclic voltammetry was performed using Ag/Ag+ as reference electrode for 10 cycles. The electrode was then, kept for drying.

Similarly, controlled electropolymerization of thiophene was also done over the zinc electrodes for polythiophene coated Zn anodes.

The X-ray diffractogram of synthesized MOF-5 (IL) (Fig. 1) was indexed and was found to be in good agreement with the indices of conventional MOF-5. To investigate the corrosion behavior of Zn anode with different surface modifications, potentiodynamic experiment was performed using a three electrode cell including Zn-based electrode as a working electrode, Ag/AgCl as reference electrode and platinum served as counter electrode. The experiment was performed at room temperature in 0.1 M NaOH aq. at a scan rate of 3.6 mV/s in the range of -0.25 V to 0.50 V.

To study discharge characteristics, zinc-air battery was fabricated using carbon cloth as air electrode, zinc based electrodes as anode and nickel mesh as current collector. 5.0 M KOH aq. was used as electrolyte. Chronopotentiometry experiment was performed to see the discharge behavior of different Zn based anodes. The potential went down abruptly in case of pure zinc anode while the potential with zinc anodes decorated with MOF-5 (IL) sustained for a longer cycle. The zinc-air battery with pure zinc as anode showed current density of ~ 7 mA cm-2. On the contrary, zinc-air batteries with zinc anodes decorated with MOF-5 (IL) showed 4 times enhanced current density of ~ 30 mA cm-2.

Considering corrosion current and corrosion potential, MOF-5 (IL) decorated Zn anodes and MOF-5 (IL) / polythiophene coated Zn anodes showed the most favorable characteristics to be used in zinc-air batteries. The enhanced discharge performance may be due to the formation of conducting passive layer of MOF-5 (IL) and MOF-5 (IL) / polythiophene on the surface of pure zinc electrode which prevented the active Zn metal from direct exposure to the KOH electrolyte and thus, minimizing the spontaneous side reactions that occur in conventional zinc-air batteries like hydrogen evolution. Efforts are underway to study the discharge characteristics of MOF-5 (IL) / polythiophene coated Zn and polythiophene coated Zn materials.

Figure 1