Carbon Nanomaterials As Metal-Free Catalysts for Energy Conversion

Oxygen reduction reaction (ORR) is an important process in many fields, including energy conversion (fuel cells, metal-air batteries), corrosion, and biosensing. Although Pt nanoparticles have been regarded as the best electrocatalyst for ORR in fuel cells, they are still suffered from multiple problems. For instance, Pt-based cathode electrocatalysts are susceptible to the cross-over effect, long-term instability, and CO poisoning. Furthermore, the high cost of Pt, together with its limited reserve in nature, has been shown to be the “bottleneck” for large-scale commercialization of the fuel cell technology.

Along with the recent intensive research efforts in reducing or replacing Pt-based electrode in fuel cells, we have previously demonstrated that vertically aligned nitrogen-doped carbon nanotubes (VA-NCNTs) produced by pyrolysis of iron (II) phthalocyanine could actively catalyze ORR via a four-electron process free from the crossover and CO poisoning effects with a 3-time higher electrocatalytic activity and better long-term durability than that of commercially available Pt/C electrocatalysts. Similar ORR electrocatalytic activity was also observed for nitrogen-doped graphene (N-graphene). On the basis of these experimental observations and quantum mechanics calculations, we have attributed the observed ORR catalytic activities of the VA-NCNTs and N-graphene to the electron-accepting ability of the chemically-bonded nitrogen atoms, which create a net positive charge (via intramolecular charge-transfer) on adjacent carbon atoms in the nanocarbon structures to readily attract electrons from the anode for facilitating the O₂ adsorption and ORR on the cathode. These findings prompted us to develop carbon-based metal-free ORR catalysts by positively charging carbon atoms in the nitrogen-free carbon nanotubes and graphene plane through intermolecular charge-transfer with functionalized/adsorbed moieties. In particular, we have recently used poly(diallyldimethylammonium chloride), PDDA, as an electron acceptor for functionalizing N-free carbon nanotubes and graphene to show remarkable electrocatalytic activity toward ORR. We have also demonstrated that carbon nanotubes and graphene co-doped with N and B showed a synergetic effect toward ORR, and that nitrogen-doped 3D graphene foam (N-GF) could be used as a metal-free electrocatalyst for the reduction of triiodide to replace the Pt cathode in dye-sensitized solar cells (DSSCs). In this talk, we will summarize some of our work on the metal-free ORR catalysts based on carbon nanomaterials, along with an overview on the recent developments in this exciting field.