Designing a nickel(ii) thiourea-formaldehyde polymer/nanocarbon bifunctional molecular catalyst with superior ORR, OER activities and its application to Zn-air battery

As efficient electrodes in energy conversion and storage devices, we focus on the development of a polymer-type non-precious metal-coordinated eco-friendly catalyst with high performance, which is of importance in flexible wearable energy storage electronic devices with adaptable shapes. In the present study, we describe the design and synthesis of a nickel (Ni)-coordinated poly(thiourea-formaldehyde) polymer, then a nanocarbon (Vulcan, porous nanocarbon CNovel™, 2-different multi-walled carbon nanotubes, or single-walled carbon nanotubes) as the conducting support was combined to prepare seven different molecular catalysts. The chemical structure of the Ni-coordinated polymer was characterized by elemental analysis, ¹³C NMR, FT-IR and XPS. Furthermore, EXAFS studies and theoretical calculations revealed that this polymer consists of two Ni-O (2.17 Å) and two Ni-S (2.17 Å) coordination bonds. This Ni-coordination polymer was stable in an oxidative and reductive environment in an alkaline medium over a long term due to its strong Ni-coordination. The catalysts were found to act as an efficient catalyst for an oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Especially, among the catalysts, the CNovel™-based catalyst showed the highest oxygen electrode performance for the ORR: E_1/2: 0.81 V vs. RHE, and OER: 1.57 V vs. RHE at 10 mA cm⁻². The catalyst acted as an efficient and durable cathode for a rechargeable Zn-air battery (charge-discharge overpotential gap of 0.45 V). Such outstanding air-cathode performance is explained by the cooperative mechanism between water on the axial site of the Ni in the polymer and superoxide ion on the porous carbon. The present study is of importance for the development of advanced energy materials in batteries and molecular catalysts.