| Summary: | 碩士 === 國立臺灣科技大學 === 化學工程系 === 106 === In recent years, rechargeable zinc-air batteries have attracted much attention owing to its high energy density, promising safety, and economic viability. In air electrode, bi-functional electrocatalysts are desirable since the dual functionality of the oxygen evolution reaction (OER) and oxygen oxygen reduction reaction (ORR) are required on the same electrode under charging and discharging processes, respectively. Unfortunately, both ORR catalyst Pt/C and OER catalyst IrO2 don’t have bifunctional property. The high cost of precious Pt/C and IrO2 catalysts also limit their wide spread application.
In the light of this, this work provides a promising bi-functional electrocatalyst with earth-abundant elements to enable the oxygen conversion reaction efficiently. Carbon supported NiFe layered double hydroxide (NiFe LDH/C) can be synthesized by a facile hydrothermal method which can precisely control the catalyst’s composition. Then, the optimal NiFe LDH/C was used as precursor and further reduced to bi-functional catalyst by hydrogen reduction and thermal ammonolysis. The results show that NiFe/NiFeN/NC nanocomposites, characterized by duel electroactive sites for OER and ORR, can be simultaneously derived by thermal ammonolysis process. According to the electrochemical measurements by linear sweep voltammetry (LSV), NiFe/NiFeN/NC nanocomposite calcined in ammonia at 500 oC demonstrates excellent activities for oxygen conversion reaction, when compared to NiFe LDH and NiFe/C. Its overpotential △E between the ORR current density of 3 mA cm−2 and OER current density of 10 mA cm−2 is 0.91 (V).
In the stability test, a chronoamperometry method was used in 0.1 M KOH. After 6 hours, NiFe/NiFeN/NC catalyst calcined at 500 oC showed high stability with a decline of current of 8.9% and 14.1% in OER and ORR, comparable to 29.1% for IrO2 and 7.7% for Pt/C, respectively. In addition, the ORR stability test in 1 M KOH showed that the activity decayed 18.4% for NiFe/NiFeN/NC, whereas 23.1% for Pt/C. This indicates that the composite catalyst is more suitable for operations under harsh environments.
This study further attempts to establish a rechargeable zinc-air battery test platform and analyze material performance. NiFe/NiFeN/NC shows good stability and its performance is comparable to that of Pt/C+IrO2, confirming its bi-functional property. Considering the cost and mass production, NiFe/NiFeN/NC offers more advantages than the combination of noble materials with Pt/C and IrO2.
Keywords:Alkaline, Bifunctional electrocatalyst, Layered double hydroxide, N-doped carbon, Rechargeable zinc-air battery.
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