Zinc-air batteries (ZABs) are promising energy systems due to their safe and environmentally friendly nature, high theoretical energy density and low cost. However, the slow oxygen reduction (ORR) and oxygen evolution reactions (OER) that drive these batteries, impose a challenge for their use and upscaling. Hence, an electrocatalyst is necessary to help promoting these reactions. Additionally, the benchmark noble metal-based catalysts, not only are costly and rare but also suffer from stability issues. Therefore, this thesis was focused on the development of alternative electrocatalysts for efficient Zn-air batteries, including their synthesis, characterization, and larger scale cell application. Some key findings include123:
• The development of a metal-free and carbon-based ORR electrocatalyst by a facile strategy that allowed the introduction of N functionalities onto a carbon framework. This catalyst (CNDA900) was obtained due to the interaction of polydopamine with C3N4 while avoiding the use of templates, catalysts, or harsh chemicals. The battery containing the prepared catalyst presented a comparable performance to that of the ORR benchmark Pt-based catalyst.
• The synthesis of an efficient bifunctional electrocatalyst (ZIFCNDA) prepared by the introduction of Co ions during the polymerisation of dopamine, which lead to the direct doping of a carbon framework and the localized growth of Co-based MOFs polyhedrons, generating Co-based NPs upon pyrolysis. The C3N4-controlled polydopamine growth formed thin graphitized carbon nanosheets which can facilitate the transfer of electrons to the catalytically active sites. The ZAB containing the developed catalyst presented a long-term discharge of 600 h (at 5 mA cm-2) and a high specific capacity of 686 mAh g-1. The cell was even able to cycle at a high current density of 15 mA cm-2 for 65h.
• The development of polydopamine-coated electrospun fibers as a bifunctional ORR and OER catalyst. The polydopamine coating was crucial to provide a graphitic carbon coating and control the structural integrity of the catalyst composite during battery cycling. This strategy formed a composite with a large BET surface area, porosity and abundant catalytically active sites. Additionally, the ZAB containing prepared air cathode catalyst exhibited excellent rechargeable performance for 100h, a steady and long-term discharge performance for about 230h and a high specific capacity of 530 mAh g-1 at a high current density of 10 mA cm-2.