a Highly Efficient and Durable Hybrid Bifunctional Catalyst of Polymer-Wrapped Pristine Multi-Wall Carbon Nanotubes and NiCo2O4 for Rechargeable Zinc-Air Batteries
Authors: Yang Jun , Fujigaya Tsuyohiko , Nakashima Naotoshi 
Published: February 2020

In recent years, the explosively increasing needs for portable electronic devices and electric vehicles have significantly stimulated the development of energy storage systems with high efficiency, low cost, and long life-circle. Compared to current lithium-ion rechargeable batteries, zinc-air batteries have very high theoretical energy density, low cost, less safety risk and thus have attracted tremendous attention in the past decade. One of the key challenges for zinc-air batteries is the bifunctional air electrode, on which the oxygen reduction and evolution reactions (ORR and OER) occur during discharge and charge processes with very sluggish kinetics, respectively.
Cobalt-based spinel oxides, such as Co3O4, NiCo2O4, and MnCo2O4, etc., have been intensively studied as the electrode materials for rechargeable zinc-air batteries and supercapacitors due to their high electrocatalytic activities. Since the electrical conductivity of these oxides is poor, they are usually supported on carbon or porous metals to act as electrocatalysts. In this work, we employed pristine multi-wall carbon nanotubes (MWNTs) as the support material because of the following advantages. Pristine MWNTs have excellent chemical stability and resistance to electro-oxidation even at high anodic potentials. Moreover, they are easy to be dispersed in some organic solvents, which allows MWNTs to easily form a free-standing film with sufficient gas diffusion path. Such a porous electrode structure will significantly benefits the practical oxygen reduction/evolution on electrode. Unfortunately, it is difficult to decorate pristine MWNTs with metal oxides because MWNTs have rare binding sites on the highly crystallized graphitic surface. The conventional way to create binding sites for MWNTs is to oxidize MWNTs in strong acid. As a result, the surface of MWNTs will be damaged, leading decrease in electrical conductivity and stability.
In this study, we propose a novel strategy for decorating polymer-wrapped pristine multi-wall carbon nanotubes (MWNTs) with metal oxides, including NiO, Co3O4, and NiCo2O4. Uniform NiCo2O4 nanoparticles were homogeneously dispersed on the surface of pyridine-based polybenzimidazole (PyPBI)-wrapped pristine MWNTs via a solvothermal synthesis method. The resulted catalysts exhibited promising activity and durability as the bifunctional catalysts for zinc-air batteries.
The PyPBI-wrapped MWNTs (MWNT/PyPBI) were obtained by a sonication method. Then, nickel, manganese, and cobalt acetylacetonates with corresponding molar ratios were dissolved together with MWNT/PyPBI in a mixed solution of ethanol, water, and ammonia water. The suspension was refluxed at 80 °C for 20 h, and subsequently transferred into a Teflon autoclave and solvothermally treated at 150 °C for 3 h. The products (MWNT/PyPBI/NiCo2O4) were collected by filtration and vacuum drying.
The catalysts were characterized by thermogravimetric analysis (TG), X-ray diffraction analysis (XRD), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and scanning transmission electron microscopy (STEM). Only the crystal phases of MWNTs and spinel oxides were detected by XRD. In addition, the pyridinic and pyrrolic N bondings which originated from PyPBI were detected by XPS. Then, the STEM images reveal that uniform nanoparticles of NiCo2O4 and MnCo2O4were found to be coated on MWNTs homogeneously.
The electrocatalytic properties of the samples were investigated in 0.1 M KOH solution by using a ring rotating disk electrode. The PyPBI-wrapped MWNT-supported spinel oxides showed promising activity for both ORR and OER. Moreover, a durability test was carried on MWNT/PyPBI/NiCo2O4 which exhibited the best activity. A cathodic current of -1.0 mA cm-2 was loaded on MWNT/PyPBI/NiCo2O4­ for 30 min and then an anodic current of 1.0 mA cm-2 was loaded for another 30 min. The galvanic square wave cycle was repeated for 60 cycles and the potential of MWNT/PyPBI/NiCo2O4 was monitored against time. It was confirmed that even after 60 cycles the OER potential did not increase at all while the ORR potential decreased for ca. 80 mV, demonstrating the excellent durability of MWNT/PyPBI/NiCo2O4­. Finally, MWNT/PyPBI/NiCo2O4­ was prepared into a free-standing electrode film by a screen-printing method. The MWNT jungles formed a very porous electrode structure, which significantly benefit the gas diffusion. The MWNT/PyPBI/NiCo2O4­ film electrode was used for a homemade zinc-air battery and a high cell voltage of 1.06 V was obtained at 20 mA cm-2 while the power density at 0.8 V achieved 51 mW cm-2.