(Invited) Highly Stable Nanocarbon Catalysts for Reversible Energy Storage and Conversion Via Oxygen Electrocatalysis

Abstract

Energy conversion and storage via direct electrochemical oxidation and reduction processes, such as the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER), can provide a clean, efficient, and economically viable means of meeting this sustainability challenge, by allowing stored renewable energy to be reliably delivered where and when it is needed. During the development of the highly demanded nonprecious metal catalysts to replace precious metals for these applications, carbon catalysts have many advantages over metal oxides, including excellent electrical conductivity, high surface area, and easy functionalization. However, most carbon-based catalysts suffer from poor activity and stability for the more challenging OER under a highly oxidative conditions. This represents one of grand challenges in energy conversion research societies.For the first time, this work demonstrated that a new class of large-diameter tubular nanocarbon (>500 nm in diameter) catalyst is capable of catalyzing both the ORR and the OER with activity comparable to Pt and exceeding Ir, respectively, for these two reactions. Most importantly, the tube catalyst exhibited good stability upon cycling across a wide potential window (0 to 1.9 V vs RHE) in 0.1 M NaOH electrolyte. The high OER activity was verified using well-designed electrochemical tests, demonstrating that the current generated during the OER arose solely from oxygen evolution, rather than carbon oxidation. This is the first example of such excellent OER activity and durability of a nanostructured carbon-based electrocatalyst across the complete ORR-OER potential window. The newly developed carbon tubes were synthesized via a one-step template-free graphitization process using inexpensive dicyandiamide (DCDA) precursor as carbon/nitrogen source and a ternary FeCoNi alloy as a catalyst. Generation of a mixed metal catalyst, i.e., FeCoNi during the graphitization process is the key factor to yield the largest tube size for maximum electrochemically accessible surface areas, dominant graphitic nitrogen doping, and thicker tube walls, which are associated with the significantly improved activity and durability. The cost effective, easily scalable, single step and environmentally benign synthesis procedure further favors this nanocarbon’s application as a practical bifunctional electrocatalyst for the ORR and OER, with potential to advance the development and commercialization of regenerative fuel cell systems and rechargeable metal-air batteries. The use of nanostructured carbon as bifunctional ORR/OER catalysts will open up new research direction in this field and further advance the role of carbon nanomaterials in oxygen electrocatalysis for energy conversion and storage.