(Invited) Boosting Non-Precious Metal Catalysts Toward the Oxygen Reduction Reaction through Ionic Liquid Modification

Abstract

Finding cost effective catalysts for the oxygen reduction reaction (ORR) is one of the most overriding challenges in the field of electrochemistry [1]. Carbon-based materials emerge as one of the promising candidates due to their remarkable advantages of low cost, abundant structural variety, tailorable surface chemistry, and good conductivity. Tremendous efforts have been made to improve the performance of carbon-based materials for the ORR, mainly through modifying the inherent structures of carbon by doping heteroatoms (e.g., B, N, P) or combining with metal/metal oxides. We have demonstrated recently that modifying Pt-based catalysts by a minor amount of ionic liquid (IL, e.g., [BMIM][NTf2], [MTBD][NTf2]) could significantly boost their ORR activity [2-5]. This innovative strategy has also been successfully transferred to some non-precious metal Fe-N-C catalysts, on which an ionic liquid ([BMMIM][NTf2]) modification is found to boost both their activity and stability [6]. In the current contribution, we will present our latest progress toward boosting the emerging zeolitic imidazolate framework (ZIF)-derived carbons (ZDC) for the ORR using IL modification strategy. First we verified again the boosting effect of IL toward the ORR on the as-synthesized ZDC materials. The half wave potential of ORR on these ZDCs is positively shifted by up to 18 mV and their intrinsic kinetic current (@ 0.85 V) is increased by a factor of 2 after being modified with [BMMIM][NTf2]. At the same time, it is observed that the boosting effect from IL modification is highly sensitive to the exact loading amount of the IL. Exceeding a specific loading amount, however, leads to a mass transport related activity drop. Moreover, it is also unraveled that the IL takes effect by increasing electrochemically accessible surface area, as reflected by the significantly enhanced double layer capacitance of ZDCs after IL modification. These results again demonstrate the great promise of the IL modification strategy as a generic method to improve ORR catalysts, and at the same time are anticipated to form the basis for an unprecedented perspective in developing high-performing non-precious metal catalysts for low temperature fuel cell applications.