Electrodeposited Electrocatalysts For Rechargeable Zinc-air Batteries

Abstract

Rechargeable zinc-air (Zn-air) batteries have gained renewed interest among the various technologies available with their high theoretical energy density and low cost. However, large-scale industrial deployment of Zn-air batteries is limited by several issues; the most concerning of these are low round-trip energy efficiency and performance degradation. Both problems are intimately related to the low activity and stability of electrocatalysts at the air electrode for catalyzing the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Many transition metal-based catalysts have been developed to replace precious metal catalysts. However, most of them require complex procedures to fabricate and need to be mixed with additives to work as electrodes. These processes can add extra cost and are difficult to scale-up. Therefore, a simple way to prepare air electrodes with active catalysts is desired. The purpose of this work was to electrodeposit transition metal (Co, Fe, Mn) based ORR/OER active catalysts on a gas diffusion layer (GDL) as the air electrode of Zn-air batteries. The as-deposited samples were characterized by several techniques including scanning electron microscopy (SEM), transmission electron microscopy (TEM), Auger electron spectroscopy (AES), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The electrochemical properties were investigated by a variety of electrochemical tests, such as cyclic voltammetry (CV), linear sweep voltammetry (LSV) and electrochemical impedance spectroscopy (EIS). Samples with the best performance were assembled into a Zn-air battery for further evaluation. The first study involved electrodeposition of cobalt-iron (Co-Fe) OER catalysts on GDL as the air electrode. The morphology and mass loading were directly controlled by adjusting deposition time and the deposits evolved from single crystal nanocubes into continuous films. The Co-Fe catalysts exhibited a low overpotential (0.29 V at 10 mA cm-2) and good durability during testing. A Zn-air battery using Co-Fe showed the same cycling efficiency as one using Pt/Ru catalysts. The second study followed up the first study to various deposit Co-Fe solid solutions. Different electrolyte compositions were explored so that deposits with a full range of compositions, from pure Co to pure Fe, were obtained. Electron microscopy and AES were used to investigate the morphology and composition of the electrodeposits. The Fe content in the deposits increased with increasing Fe concentration in the electrolyte and Fe segregated to the particle surfaces. Electrochemical tests demonstrated that the deposit Co/Fe ratio influences OER activity by altering the electrochemically active surface area (ECSA) and charge transfer resistance. The OER activity increased with increasing Fe content up to ~65 at% Fe, with a minimum overpotential of 0.33 V at 10 mA cm-2. In the third study, manganese oxide (MnOx) and Co-Fe were sequentially electrodeposited onto a GDL as bifunctional electrocatalysts for rechargeable Zn-air batteries. The fabricated material was characterized by SEM, TEM, XRD and XPS. The sequentially deposited MnOx/Co-Fe catalysts, tested using CV, showed activity for both the ORR and OER, with better performance than either MnOx or Co-Fe alone. A Zn-air battery fabricated using MnOx/Co-Fe catalysts exhibited good performance and a cycling efficiency of 59.6% at 5 mA cm-2, which is comparable to Pt/C catalysts. In addition, the electrodeposited MnOx/Co-Fe layer showed strong adhesion to the GDL and was structurally stable throughout 40 h of battery cycling. In the fourth study, a horizontal Zn-air battery has been designed to enable the use of physically decoupled ORR and OER electrodes for discharge and charge, respectively. The horizontal design features a horizontally positioned ORR electrode and a vertically positioned OER electrode, allowing effective management of oxygen transport and pressure. The ORR catalyst (MnOx) and OER catalyst (Co-Fe alloy) are fabricated via one-step electrodeposition on carbon paper and Ni foam, respectively. MnOx was identified as a combination of Mn3O4 nanorods and α-Mn2O3 spheres. Co-Fe was deposited as a solid solution film with an oxidized surface. Electrochemical tests showed that both catalysts have comparable or even better activity than their commercial Pt-Ru catalyst counterpart. Cycling tests at 20 mA cm-2 show that the potential affects catalyst durability, with improved lifetime under separate ORR and OER conditions compared with the full ORR-OER voltage range. The fabricated catalysts were tested in Zn-air battery and show the same average efficiency (58%) as Pt-Ru catalysts at 10 mA cm-2.