Facile route to achieve bifunctional electrocatalysts towards oxygen reduction and evolution reactions derived from CeO2 encapsulated with zeolitic imidazolate framewok-67

Highly efficient bifunctional electrocatalyst for both oxygen reduction reaction and oxygen evolution reaction is essential for the development of rechargeable metal-air battery. Herein, we demonstrate a novel bifunctional electrocatalyst with CeO2 and Co nanoparticles being embedded in N-doped carbon, which is achieved via pyrolyzing CeO2@zeolitic imidazolate framework-67 particles. In 0.1 M KOH aqueous solution, the CeO2/Co@N-doped carbon shows an excellent electrocatalytic performance with an oxygen reduction reaction onset potential of 0.998 V vs. reversible hydrogen electrode and an oxygen evolution reaction overpotential of 474 mV at 10 mA cm-2. The liquid rechargeable Zn-air battery that is assembled by using CeO2/Co@N-doped carbon as air cathode presents excellent charge/discharge performance, with an open-circuit potential of 1.42 V and maximal power density of 102.69 mW cm-2. Furthermore, the rechargeable Zn-air battery exhibits stable cycling performance at a constant current density of 5 mA cm-2 after 70 hours, with a charge/discharge gap of only 0.8 V and round-trip efficiency of ca. 59 %. The remarkable bifunctional properties are considered to arise from the synergistic effect between the N-doped carbon and CeO2 nanoparticles, where amounts of Ce3+ and oxygen vacancies in CeO2 content are generated and more defect sites are available in N-doped carbon.