Heat-treated metal phthalocyanine complex as an oxygen reduction catalyst for non-aqueous electrolyte Li/air batteries
Highlights
► Studied the catalytic effect of heat-treated metal phthalocyanine complex (MPc) on oxygen reduction in Li/air cells. ► Heat-treated FeCuPc complex with carbon not only promotes two-electron oxygen reduction but also catalyzes non-redox Li2O2 disproportionation. ► In Li/air cells the FeCu/C catalyst reduces cell's polarization and increases open-circuit voltage recovery rate. ► The catalytic effect of FeCu/C catalyst on the Li2O2 disproportionation is not as effective as on the two-electron oxygen reduction.
Introduction
Li/air batteries are more complicated than the traditional metal/air batteries since their discharge products (Li2O2 and Li2O) are insoluble in non-aqueous electrolytes. During discharge, the Li2O2 and Li2O deposit and accumulate on the surfaces of a carbon air cathode, which eventually blocks the access of oxygen to catalytic sites and halts the operation of the battery. Therefore, the amount that the carbon air cathode can accommodate for the discharge products determines the specific capacity of Li/air cells [1], [2], [3]. To achieve high specific capacity, a liquid/solid two-phase reaction zone between the liquid electrolyte and solid catalytic sites is required for the Li/air cells to achieve the maximum reaction area and the largest accommodation capacity [3]. However, such cells are intrinsically low power since diffusion of the dissolved oxygen in liquid electrolyte is slow and the discharge products cannot be removed from the catalytic sites. For these low power Li/air cells, the pristine carbon without any other catalysts has been directly used as the catalyst for the oxygen reduction [2], [3], [4], [5], [6], [7], [8].
Catalytic reduction of oxygen in Li/air battery takes place through such three steps as: (1) dissolution of oxygen from gas phase into liquid electrolyte, (2) diffusion of the dissolved oxygen into catalytic sites on the carbon surface, and (3) catalytic reduction of oxygen into Li2O2 or Li2O. Of them, the last step might become the rate-determining step to limit the discharge performance of Li/air cells when the discharge current rate is increased. Therefore, more effective catalysts are needed to increase the power capability of Li/air cells. Transition metal N4-macrocycle complexes have long been known to be highly active for the catalytic reduction of oxygen. However, these compounds are either soluble or chemically instable in aqueous acidic or alkaline electrolyte solutions. For these reasons, the technique of heat-treatment with carbon has been used to enhance catalytic activity and increase chemical stability against the aqueous electrolyte environments [9], [10], [11]. The heat-treated transition metal N4-macrocycle complexes have been considered as an excellent catalyst for the oxygen reduction in alkaline metal/air cells and fuel cells [9], [10], [11], [12], [13], [14], [15]. Keeping this knowledge in mind, in this work we study the effect of oxygen reduction catalyst on the discharge performance of Li/air cells by selecting a heat-treated iron and copper phthalocyanine (FeCuPc) complex as the catalyst and loading it onto a high surface area Ketjenblack EC-600JD carbon black.
Section snippets
Experimental
Ketjenblack EC-600JD carbon black (AkzoNobel) was used as the control and supporting carbon for catalyst. FeCu/C catalyst was received from Acta S.p.A, Italy, which was prepared by first absorbing a FeCuPc complex solution onto Ketjenblack EC-600JD carbon black and then heating the mixture at temperatures between 800 °C and 900 °C in an argon atmosphere. The final FeCu/C catalyst contained 1.5 wt.% Fe and 1.7 wt.% Cu, respectively, which corresponds to a 1:1 molar ratio of Fe to Cu. In this work we
Air cathode structure and Li/air cell's OCV
Ketjenblack EC-600JD is a highly branched carbon black consisting of 30–100 nm long aggregates [19]. After loading with catalyst, the surface area and pore volume of the carbon are significantly reduced as shown in Table 1. This is because many of pores in Ketjenblack carbon were filled with the thermal decomposition products of FeCuPc, including metal oxides and pyrolytic carbon [20]. Therefore, the FeCu/C air cathode has much lower pore volume than the pristine carbon air cathode.
It is
Conclusions
Based on the results of this work, the following conclusions can be made: (1) the unusually high OCV of the newly assembled Li/air cells with FeCu/C catalyst corresponds to the OCV of catalyst itself, rather than to the oxygen reduction, (2) the FeCu/C catalyst not only accelerates the two-electron oxygen reduction, but also catalyzes the non-redox Li2O2 disproportionation. In Li/air cells, the former reduces cell's polarization and the latter increases OCV recovery rate, (3) since the
Acknowledgements
The authors would like to thank Dr. K. Xu for his supply of TFP solvent and Dr. D. Tran for his assistance in BET measurement.
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