Investigation on the electrode design of hybrid Zn-Co ₃ O ₄ /air batteries for performance improvements

A hybrid Zn battery is developed by integrating a Zn-Co ₃ O ₄ battery and a Zn-air battery at the cell level, which combines the unique advantages of a high working voltage and a high discharge capacity. However, the design of the working electrode, which is the key to the battery performance, is absent in the literature. In this work, the electrode design of hybrid Zn-Co ₃ O ₄ /air batteries for performance improvements is investigated by considering the active material loading and the surface hydrophobicity of the Co ₃ O ₄ nanosheet-decorated carbon cloth electrode. The results demonstrate that the capacity of the Zn-Co ₃ O ₄ battery first dramatically increases with the loading, but the improvement becomes limited when the loading further increases. The discharge voltage plateau of the Zn-air battery increases first and then keeps decreasing. The reason is that although both the active material and the surface area increase, the increased dimension of nanosheets increases the transport resistance, especially for electrons and oxygen. For a given electrode, the discharge voltage of the Zn-air battery first increases with an increase of the hydrophobicity due to the creation of gaseous oxygen transport pathway, but then decreases due to the coverage of active sites. While both the voltage and the capacity of the Zn-Co ₃ O ₄ battery decrease with the hydrophobicity, which is attributed to the decreased contact surfaces between the reactant and the liquid electrolyte. Based on the optimized active material loading and surface hydrophobicity, a hybrid Co ₃ O ₄ /air battery delivers a high energy density of 936 Wh kg ⁻¹ (on the weights of Co ₃ O ₄ and consumed Zn) and stable discharge and charge voltages ranging from 0.94 V to 2.01 V for 400 cycles. This work illustrates that for high-performance hybrid batteries, the active material loading of the electrode should be well designed considering the transport of electrons and oxygen, and a balance between the hydrophobicity and hydrophilicity should be established.