Transformation of a new polyoxometalate into multi-metal active sites on ZIF-derived carbon nanotubes as bifunctional cathode catalyst and dendrite-free anode coating for Zn-air batteries

Abstract

To improve the kinetics of the ORR/OER at the cathode and inhibit dendrite formation on the anode of a Zn-Air battery (ZAB), a new polyoxometalate (POM) with a basic structrual unit of [Zn{P4Mo6}2] was designed and converted to well-separated Zn/Mo2C catalytic sites on Co, N doped carbon nanotubes (Co-NCNTs) derived from ZIF-67. This conversion was achieved at a relatively low pyrolysis temperature of 600 °C, which circumvented the collapse of catalyst structures, burial of catalytic sites and volatilization of metals. The experimental results demonstrate that the pyrolysis of [Zn{P4Mo6}2] can etch Co-NCNTs-800 to produce a porous structure with an enlarged specific surface area, thus increasing the number of catalytic sites and promoting the transfer of electrons/ions. The high-valence Mo atoms from the POM precursor are inclined to hybridize with O active intermediates produced during ORR/OER, which can improve electron exchange and the ORR/OER performance. Furthermore, the multi-catalytic sites (Zn and Mo2C, etc.) produced from the [Zn{P4Mo6}2] precursor can not only facilitate ORR/OER at the battery cathode, but also inhibit the formation of zinc dendrites by forming a large number of nucleation sites on the battery anode. Accordingly, the ZABs assembled with the POM-derived catalyst exhibit a high open circuit voltage of 1.506 V and a peak power density of 223.54 mW cm−2. Moreover, the assembled all-solid coin cell ZABs also display high capacity and long-term charge–discharge stability. Furthermore, density functional theory calculations demonstrate that the synergy between Zn/Mo2C and Co-NCNTs active sites mainly contributes to the superior ORR/OER catalytic performance.