Rational design of multifunctional air electrodes for rechargeable Zn–Air batteries: Recent progress and future perspectives

Rechargeable zinc–air batteries (ZABs) are considered as one of the most promising candidates as power sources for the portable electronic devices and electric vehicles (EVs), due to their high energy density, environmentally benignity and low production cost. One of the biggest challenges and the bottleneck in developing high-performance rechargeable ZABs are the design of suitable bifunctional air electrodes, with controlled chemical compositions and well-designed architectures that can efficiently catalyze the key oxygen reactions. Air electrodes are the most significant and complicated part of the rechargeable ZABs that inevitably determine the electrochemical performance and cycling stability. This paper particularly reviews the most recent developments, in designing and fabricating air electrodes with oxygen electrocatalysts. It covers the discussion on the relationships between surface structure and reaction mechanism. It highlights the developments of air electrodes in three main categories: 1) conventional powdery catalysts physically deposited on gas diffusion layer (GDL), 2) catalysts in situ grown on substrates, and 3) freestanding catalyst films or wires. The latter two are also known as self-supported air electrodes with desired properties of advanced nanostructures and binder-free assembly. In this review, various advanced air electrodes composed of non-noble catalysts are presented and discussed. Further, a general perspective for designing and fabricating high-performance air-electrodes is proposed to prolong the lifespan of electrically rechargeable.