Rechargeable zinc-air batteries (ZABs) have drawn extensive attention due to their eco-friendliness and safety. However, the lack of high-performance and low-cost oxygen evolution and reduction reactions (OER/ORR) catalysts has become one of the main stumbling blocks of their development. In this thesis, a series of transition-metal-based bifunctional oxygen electrocatalysts are developed. To improve their activity, different strategies, such as oxygen vacancy engineering on spinel oxides, constructing the Schottky barrier of alloy/metal oxide interfaces, and integrating metallic alloy with hierarchical carbon materials, are adopted. Moreover, to meet the growing demand for flexible electronics, flexible/wearable ZABs are fabricated based on as-prepared solid-state hydrogel electrolytes.

First of all, a facile two-step tuning strategy is investigated to convert NiCoLDH to oxygen vacancy-rich NiCo₂O₄ nanosheet (R-NiCo₂O_4-x). With carefully controlled air annealing and NaBH₄ treatment processes, the optimal R-NiCo₂O_4-x on carbon cloth (CC) exhibits enhanced OER and ORR activities compared with NiCo₂O₄/CC counterpart. Liquid and flexible ZABs based on the binder-free R-NiCo₂O_4-x/CC electrode demonstrate power density and cycling stability that surpass the reference RuO₂+Pt/C/CC catalyst with open-circuit voltage and peak power density of 1.486 and 1.39 V, and 88.6 and 38.6 mW cm^-2, respectively. The flexible batteries with alkali-modified polyacrylic acid hydrogel electrolyte can power LED bulbs of different colors.

Constructing a Schottky barrier of alloy/metal oxide interfaces is an emerging approach to designing electrocatalysts with desired performance. Subsequently, a nitrogenous organic modified bimetal layered hydroxide salts (LHS) precursor is used to prepare CoNi alloy and CoO coupled nitrogen-doped carbon hybrids on carbon paper (CoNi-CoO@NC/CP) via one-step pyrolysis at only 500 °C. Through adjusting the metal molar ratio in the LHS precursor and the pyrolysis temperature, CoO is in-situ produced during pyrolysis. Since both alloy and metal oxide possess potential OER activity, the resulting CoNi-CoO@NC/CP exhibits enriched electrochemical active surface area, small overpotential (309 mV) at current density of 10 mA cm^-2, low Tafel slope (67.7 mV dec^-1), and good durability for 64 h at ~20 mA cm^-2 in 1 M KOH. Furthermore, a ZAB based on this catalyst shows a high specific power of 96.2 W g_cat^-1, which exceeds that of conventional RuO₂+Pt/C catalyst (66.8 W g_cat^-1).

Further, a CoFe nanobubble encapsulated in NC nanocage on wood carbon support (CoFe@NC/WC) is successfully fabricated via pyrolyzing a novel Prussian blue analog/spruce precursor. The hierarchical CoFe@NC/WC catalyst exhibits an excellent potential difference between OER and ORR of 0.74 V in 0.1 M KOH, surpassing recently reported preeminent electrocatalysts. Thus, CoFe@NC/WC shows outstanding electrochemical performance in liquid ZAB, with a peak power density of 138.9 mW cm^-2 and a specific capacity of 763.5 mAh g^-1. More importantly, a bacterial cellulose nanofiber reinforced polyacrylic acid (BC-PAA) hydrogel presents ultrahigh tensile-breaking stress of 1.58 MPa. In conjunction with the as-prepared CoFe@NC/WC catalyst, BC-PAA-based wearable ZAB displays impressive rechargeability and foldability, and can power portable electronics, such as electronic timer and mobile phone, even in bent states.

Overall, this thesis demonstrates cost-effective strategies toward high-activity transition-metal-based electrocatalysts for ZABs. Meanwhile, in conjunction with solid-state hydrogel electrolytes, the flexible batteries are promising for the future development of wearable energy storage devices.