Design of Efficient Cobalt-based Bi-functional Catalysts for Zinc-Air Batteries

Due to its high theoretical specific energy and low-cost, rechargeable zinc-air batteries have attracted tremendous attention as a promising next-generation energy conversion system. However, there are some challenges that need to overcome before its practical application. One of the key issues is the slow reaction kinetics in the air cathode of the batteries towards the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). This would cause insufficient charge/discharge efficiency and poor cycle stability of the batteries. Therefore, the development of efficient ORR-OER bi-functional electrocatalysts with high catalytic activity and durability is essential for the development of rechargeable zinc-air batteries. \n In this work, a series of catalyst design strategies have been explored to improve the activity and durability of cobalt-based bi-functional catalysts especially under the oxidative condition of OER reaction. The latter would cause catalyst oxidation and aggregation, and therefore deteriorate the cycling performance of the bi-functional catalysts in zinc-air batteries. In the first study, a surface engineering approach was adopted to prepare efficient bi-functional catalyst consists of mildly oxidized, N-doped Co9S8 catalyst supported on N-doped reduce graphite oxide (O-N-Co9S8@N-RGO). The surface decorated electrocatalyst shows excellent activity for both ORR and OER, and maintains good stability over 900 charge-discharge cycles at 10 mA cm-2 in zinc-air battery. Interestedly, it was found that O-N-Co9S8 nanoparticles responsible for the OER reaction were completely converted into Co3O4 after OER reaction, indicating Co3O4 is the actual active phase for OER. On the basis of this observation, we propose and demonstrate that oxides in-situ generated cobalt oxides during OER reaction are more active than the directly calcined oxides. This work advances fundamental insight and the design of metal chalcogenides-based bi-functional “catalysts”. \n On the recognition of the high catalytic activity of surface-engineered Co9S8 material, a three-dimensionally ordered mesoporous (3DOM) structured surface-engineered Co9S8 catalyst was developed to explore the benefits of the 3DOM structural design for its catalytic performance. Different from the N-RGO supported O-N-Co9S8, the 3DOM-Co9S8 catalyst is self-supported, which contains only an inner carbon layer within its mesoporous structure. Due to the 3D interconnected architecture and large surface area, the air electrode delivers excellent cell performance and cycling durability. However, the partial structure crush of N-Co9S8 after long-time OER testing was observed, demonstrating that the highly oxidative operating condition of rechargeable zinc-air batteries could cause significant structural integrity issues of porous chalcogenide electrocatalysts. \n Thus, in the last study, a new strategy focusing on the oxidation-resistive catalyst support design using oxygen vacancy (OV)-rich, low-bandgap semiconductor was proposed. The OVs promote the electrical conductivity of the semiconductor support, and at the same time offer a strong metal-support interaction (SMSI). The SMSI enables the catalysts with small metal size, high catalytic activity, and high stability. This strategy is demonstrated by successfully synthesizing ultrafine Co metal decorated 3DOM titanium oxynitride (3DOM-Co@TiOxNy). The catalyst not only exhibits good ORR-OER activities, but also shows excellent cycling stability in alkaline conditions, e.g. less than 1% energy efficiency loss over 900 charge-discharge cycles at 20 mA cm-2. Theoretical calculation confirmed that the high stability of this catalyst is attributed to the strong SMSI between Co and 3DOM-TiOxNy. This study will provide an alternative strategy for the design of efficient and durable non-precious electrocatalysts using OV-rich semiconductors as support materials. \nIn summary, a series of catalyst design strategies for efficient and durable bi-functional ORR-OER catalyst were developed in this work. It was found that NH3 treatment is an effective surface-engineering approach to develop highly active ORR-OER catalysts. The in-situ transformation or oxidation of Co9S8 into Co3O4 observed in post-OER analysis advanced our understanding of the chemical, structural transformation and real catalytic phase for OER “catalyst”. Moreover, the results show that the 3DOM design of self-supported Co9S8 catalyst could also benefits the catalytic performance by facilitating the mass and electronic transportation within the 3DOM framework. Finally, based on our up-to-date understanding of the OV in semiconductor physics and heterogeneous catalysis, a novel bi-functional catalyst support design strategy was proposed and demonstrated using OV-rich TiOxNy semiconductor. Excellent cycling stability and activity performance of such semiconductor supported cobalt catalyst in rechargeable zinc-air batteries is achieved.