Highly active and stable graphene tubes decorated with FeCoNi alloy nanoparticles via a template-free graphitization for bifunctional oxygen reduction and evolution

Development of highly active and stable bifunctional oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) catalysts from earth-abundant elements remains a grand challenge for highly demanded reversible fuel cells and metal-air batteries. Carbon catalysts have many advantages over others due to their low cost, excellent electrical conductivity, high surface area, and easy functionalization. However, they typically cannot withstand the highly oxidative OER environment. Here, a new class of bifunctional electrocatalyst is reported, consisting of ultralarge sized nitrogen doped graphene tubes (N-GTs) (>500 nm) decorated with FeCoNi alloy particles. These tubes are prepared from an inexpensive precursor, dicyandiamide, via a template-free graphitization process. The ORR/OER activity and the stability of these graphene tube catalysts depend strongly on the transition metal precursors. The best performing FeCoNi-derived N-GT catalyst exhibits excellent ORR and OER activity along with adequate electrochemical durability over a wide potential window (0-1.9 V) in alkaline media. The measured OER current is solely due to desirable O-2 evolution, rather than carbon oxidation. Extensive electrochemical and physical characterization indicated that high graphitization degree, thicker tube walls, proper nitrogen doping, and presence of FeCoNi alloy particles are vital for high bifunctional activity and electrochemical durability of tubular carbon catalysts.