Bifunctional Electrocatalyst for Aqueous Type Rechargeable Li-Air Batteries Derived from Copper Chelate Polymer

Abstract

1. Introduction Lithium-air (Li-O2) batteries have attracted focus of many research groups worldwide due to their higher theoretical gravimetric energy density in comparison to the current Li-ion batteries. With the three times higher energy density, such batteries could power electrical vehicles up to 500 km distance on a single charge. Commercialization of these batteries is hindered by the development of bifunctional catalyst that can efficiently catalyze both oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Various metal and oxide nanoparticles have been studied as a catalyst. Recently, metal-organic framework structures have shown good catalytic properties. Here, we propose to use a chelate polymer as the template that provides well-dispersed metal oxides. Chelate polymers are peculiar metal-organic hybrid structures. The most extensively studied chelate polymer is copper dithiooxamide (Cu(dto).1) Herein, a new method is proposed for preparing an efficient bifunctional electrocatalyst well dispersed in a carbon matrix. The method includes in situ electrochemical transformation of Cu(dto) compound into copper oxides in alkaline solutions. 2. Experimental Cu(dto) was synthesized by mixing corresponding aqueous solutions of metal salts and ethanolic solution of dto in 1:1 stoichiometric ratio. The compounds were characterized by powder X-ray diffraction (XRD), SEM-EDX, and cyclic voltammetry (CV). The active material slurry was deposited on the surface of a glassy carbon electrode (GCE) used as a working electrode. The catalytic activities of the electrodes were studied by linear swept voltammetry (LSV) and rotating disk electrode (RDE) methods. All experiments were performed in an aqueous 1M KOH solution, with/without oxygen feeding in a three-electrode glass cell. 3. Results and Discussion Cu(dto) compounds were obtained using several copper precursors. The yields obtained from CuSO4, Cu(NO3)2, Cu(CH3COO)2, and CuCl2 precursors were 100%, 99%, 94%, and 100%, respectively. Upon electrochemical cycling, the compound underwent in-situ decomposition to Cu(I) and Cu(II) resulting in the formation of nanostructured copper oxides dispersed on acetylene black, as shown in Fig.1. In this Cu(dto)-derived (Cu(dto)-DO), copper oxide electrocatalyst, CuO and Cu2O are proposed to be the active catalytic species for OER and ORR, respectively. This catalytic system exhibited superior OER performance, indicated by a low overpotential of 400 mV at 10 mA/cm2 and a small Tafel slope of 81 mV/dec in a nitrogen-saturated 1 M KOH solution. It gives a seven-fold higher current density per gram of the metal with comparison to the best-performing IrO2/C as shown in Figure 2.2) The ORR performance of this catalyst showed an onset potential and a half-wave potential of 0.81 and 0.72 V vs. RHE, respectively, in an oxygen-saturated 1 M KOH. The half-wave potential of this catalyst is comparable with the best-performing Pt/C catalyst.3) The results reveal a new way to produce a highly active noble-metal-free bifunctional catalytic system in a carbon network on a large scale using in-situ electrochemical transformation. This catalytic system has the potential to be applied as the active component of a cathode in aqueous type metal-air batteries. References 1) Abboudi M. et al., Synthesis of d-Transition Metal Sulfides from Amorphous Dithiooxamide Complexes. J. Solid State Chem., 109, 70 (1994).2) R.P. Putra, H. Horino, I.I. Rzeznicka, An Efficient Electrocatalyst for Oxygen Evolution Reaction in Alkaline Solutions Derived from a Copper Chelate Polymer via in situ Electrochemical Transformation, Catalysts, 10, 233 (2020).3) R.P. Putra, Y. Samejima, S. Nakabayashi, H. Horino, I.I. Rzeznicka, Copper-based Electrocatalyst Derived from a Copper Chelate Polymer for Oxygen Reduction Reaction in Alkaline Solutions, Catalysis Today, (2020). Acknowledgements The authors would like to thank Shibaura Institute of Technology and Japan Society for Promotion of Science under the JSPS KAKENHI Grant Number JP 20K05689 for financial supports. Figure 1