Plasma-Enhanced Copper Sulfide/Oxide Electrode for Zinc-Air/Copper Dual Mode Battery

Abstract

As search for novel electrochemical energy storage systems continues, metal-air batteries are one of the possible solution. Consisting of a metal anode and an air-breathing cathode, by design, they achieve much higher gravimetric energy density in theory. However, s-orbital metals suffer passivation issues and poses safety threats due to their reactivity with air and humidity. As a solution to this issue, safer d-block metals are being studied as an alternative. Zinc air batteries (ZABs) utilizes earth-abundant and cost-effective zinc as its anode, and has high theoretical capacity. At its cathode, oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) occur during charging and discharge processes. Sluggish kinetics of ORR degrades the discharge performance, and is the bottleneck in commercial application of the zinc air battery. Additionally, irreversibility of the cathode reaction results in dissatisfactory retention of battery capacity. Current state-of-the-art noble metal catalysts achieves nearly 80% of the theoretical capacity. However, its cycling stability is severely limited to less than 200 cycles despite low discharge rate applied.Herein, we present a nitrogen plasma-enhanced copper sulfide/oxide catalyst (referred to as N-CuS) that exhibits good catalytic activity for its application as the air-breathing cathode for ZABs. The material is grown directly on metal foam surface using anodization, which is then treated using nitrogen plasma. Anodization creates defect-rich flakes of copper sulfide on the oxide surface, highly increasing surface area and creating the active sites. The material has good electrical conductivity and highly open porous structure that alleviates mass transport issues. Additionally, nitrogen plasma treatment further increases the surface area about 80%, and is expected to introduce nitrogen dopants to the surface, enhancing conductivity of the material. ECSA estimated by measuring Cdl shows that each process increases the ECSA by over 40 times and about 1.8 times, respectively.LSV of CuS and N-CuS half-cells have been performed to measure and compare catalytic performance of the material. N-CuS shows far higher current density and better catalytic activity (35mV less overpotential) than CuS, and higher output voltage is observed with N-CuS cells with the same current density. By assessing the GCPL (potential limited galvanostatic cycling) results, it was concluded that N-CuS requires lower charging voltage and exhibits higher discharge voltages than CuS.The overall performance was evaluated in a stack cell configuration, utilizing thin zinc foil as its anode to minimize mass error. The measured specific capacity of the cell was 789.3 mA·h·g-1, which approximately is 85% of the theoretical specific capacity of zinc-air cells (820 mA·h·g-1). This cell yields gravimetric energy density of about 666 Wh·kg-1. It is notable that these ZAB characteristics are comparable to Pt/C, the state-of-the-art catalyst as of now. N-CuS may be a viable replacement for the Pt/C electrode, resulting in the cost-saving effect. The cycling stability of both N-CuS and Pt/C-based ZAB has been assessed to show that N-CuS-based ZAB has much prolonged (more than twice the cycles) cyclability compared to Pt/C-based ZAB (Fig. S11).Additionally, ZAB constructed using N-CuS can operate in oxygen-deficient environments. As the counter reaction it utilizes redox reaction of copper oxides (Zn-Cu mode) in the substrate when operating in anaerobic conditions. The reduction of divalent and monovalent copper oxides appears as plateaus at 1.1 V and 0.8 V versus the zinc anode during GCPL. Due to this alternate reaction, the cell can operate even at low concentrations of oxygen or even in its absence.DFT calculations have been performed to elucidate the reaction mechanism and the intrinsic catalytic activity of the material, and shows nitrogen dopants improve reactivity of the active sites, thus reducing the overpotential.In summary, we have synthesized N-CuS as the OER/ORR catalyst for ZAB. Through anodization the catalyst was directly grown on the substrate and nitrogen plasma treatment further enhances the catalyst properties to render it higher surface area, conductivity, and better stability. This allows better reversibility in the OER/ORR and results in enhanced cyclability of the overall cell. The constructed RZAB exhibited high specific capacity of 789.3 mA·h·g-1, which, has higher capacity than the analogue using Pt/C catalyst. In addition, N-CuS based RZAB is capable of operating under oxygen-deficient conditions utilizing the metal oxide substrate. This allows the dual mode operation of zinc-air and zinc-copper modes, and results in a more consistent output from the RZAB, as short-term air supply issues could be buffered by this effect. Figure 1