Fe-N-C with Intensified Exposure of Active Sites for Highly Efficient and Stable Direct Methanol Fuel Cells.

Fe-N-C catalysts are promising candidates to replace expensive and scarce Pt-based catalysts for oxygen reduction reaction (ORR) in fuel cell devices. Herein, simultaneous improvement of activity and stability of Fe-N-C is achieved through exposing active sites via a surface modification strategy. Concretely, EDTAFe groups are anchored on the external surface of zeolitic imidazolate framework-8 (ZIF-8) through size limitation, followed by pyrolysis to obtain ZIF@EDTAFe-1%-950, whose surface active site density increases more than 1.7 times as detected by X-ray photoelectron spectroscopy (XPS) and 57Fe Mössbauer spectra. Consequently, 1.7 times improvement of active site utilization efficiency in electrochemical measurements and more than 2 times performance enhancement in direct methanol fuel cells (DMFCs) are achieved due to facilitated mass transport as revealed by oxygen gain voltage and electrochemical impedance spectroscopy (EIS). Furthermore, through engineering robust drainage channels around exposed active sites to alleviate flooding, the assembled DMFC exhibits better stability than that of Pt/C in the first 3 h and remains 83.9% voltage after 24 h at 100 mA cm-2.