Highly active ZIF-8@CNT composite catalysts as cathode materials for anion exchange membrane fuel cells†

Abstract

Developing non-precious metal-based inexpensive and highly active electrocatalysts for the oxygen reduction reaction (ORR) in alkaline media is important for fuel cell applications. Herein, we report a simple and effective synthesis of transition-metal-doped zeolitic imidazolate framework-8 (ZIF-8) and carbon nanotube (CNT) composite catalysts (ZIF-8@CNT) prepared via high-temperature pyrolysis at 900 °C. The catalysts were characterized using different physicochemical techniques and employed as cathode materials in anion exchange membrane fuel cells (AEMFC). The prepared metal-free (ZNT-900), single-metal-doped (Fe-ZNT-900, Co-ZNT-900) and binary-metal-doped (Fe₁Co₁-ZNT-900, Fe₁Co₂-ZNT-900) catalysts had a sufficient amount of N-doping with the presence of FeCo moieties in the carbon skeleton of the latter two materials. N₂ adsorption–desorption isotherms showed that all the prepared catalysts possess a sufficient Brunauer–Emmett–Teller surface area with more micropores present in ZNT-900, while a combined micro–mesoporous structure was obtained for transition-metal-doped catalysts. Binary-metal-doped catalysts showed the highest number of ORR-active sites (pyridinic-N, pyrrolic-N, graphitic-N, M–N_x) and exhibited a half-wave potential (E_1/2) of 0.846 and 0.847 V vs. RHE for Fe₁Co₁-ZNT-900 and Fe₁Co₂-ZNT-900, respectively, which surpassed that of the commercial Pt/C catalyst (E_1/2 = 0.834 V). In H₂–O₂ AEMFCs, the Fe₁Co₂-ZNT-900 catalyst delivered a maximum power density (P_max) of 0.171 W cm⁻² and current density at 0.5 V (j_0.5) of 0.326 A cm⁻², which is very close to that of the Pt/C catalyst (P_max = 0.215 W cm⁻² and j_0.5 = 0.359 A cm⁻²). The prepared ZIF-8@CNT catalysts showed remarkable electrocatalytic ORR activity in 0.1 M KOH solution and fuel cell performance comparable to that of the benchmark Pt/C catalyst.

Keywords: Rotating disk electrode; Anion exchange membrane fuel cell; Zeolitic imidazolate framework; Non-precious metal catalyst; Oxygen reduction reaction.