Alternative Me-N-C Based Carbon Foams As Fe-Free and Non-PGM Electrocatalysts

Abstract

There is a demand for reducing the environmental load of automobiles to solve the current environmental problems. Fuel cell electric vehicles (FCEVs) which use hydrogen as fuel emit no carbon dioxide, and have already been commercialized. platinum-based materials are widely used as oxygen reduction reaction (ORR) catalysts. However, their high cost and limited durability impedes large-scale implementation. Until now, non-platinum group metal (non-PGM) Fe and N doped carbon (Fe-N-C) catalysts have achieved promising ORR activity [1]. However, these Fe-N-C catalysts still suffer from poor durability, due to the formation of reactive peroxide radicals as a side reaction during the ORR. Meanwhile, the use of Co species avoids the production of such radicals, potentially preserving the electrocatalyst [2]. Therefore, in this study we investigate the dependence of ORR activity on a variety of different metal ions by synthesizing alternative Me-N-C carbon foam electrocatalysts (Me = Co, Ni, Sn).Pt-free electrocatalysts were synthesized from nitrogen-doped carbon foams (N-C) and various metal salt and small molecule precursors. Porous nitrogen-doped carbon foams with large surface area were made in-house. These were adsorbed with metal acetate and then heated in inert atmosphere, followed by acid washing to remove metallic phases. Finally, heat treatment under ammonia was performed. The effect of heat treatment temperature and metal loading on the microstructure and porosity was evaluated. The resulting catalysts were evaluated for catalytic activity using rotating-ring disk electrode voltammetry (RRDE) in acid.For Co-N-C electrocatalysts it was found that heat treatment of the nitrogen-doped carbon foam support has a significant effect on catalyst performance, largely due to changes in surface area and porosity. The metal loading also has a strong effect on the performance, with an optimum Co loading of 0.1 wt% resulting in an onset potential of 0.8 VRHE and a half-wave potential of around 0.65 VRHE. Different metal ions were also systematically investigated, and it was found that Sn is highly promising as an alternative to Fe in non-PGM electrocatalysts.