Modified Carbon Nanomaterials as Active Electrocatalysts

Abstract

Water splitting (WS) has attracted increasing attention for producing highly pure hydrogen and oxygen. WS is also a promising technique to store intermittent electrical energy from renewable resources such as solar and wind energy in the form of H2 fuel. WS consists of two half-reactions: hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). For HER, efficient non-precious catalysts are required to replace the rare and expensive Pt-based catalysts. For OER, more efficient low-cost catalysts should be designed to improve the sluggish reaction rate of OER and reduce the reaction overpotential. In this thesis, a few synthesis methods have been developed to introduce novel hybrid carbon nanomaterials with efficient catalytic activity toward HER and OER. We have developed a one-step floating catalyst chemical vapor deposition synthesis process to grow carbon-encapsulated iron nanoparticles (CEINs) supported on carbon nanotubes (CNTs), as highly active electrocatalysts. In CEINs, the active metallic core is protected from atmospheric-oxygen-induced degradation and agglomeration with neighboring nanoparticles. Single-shell CEINs decorated on single-walled CNTs (SWNTs) exhibit a high catalytic activity for HER in acidic media comparable to that of platinum. Furthermore, the structure of the CEINs has been transformed to crystalline maghemite (γ-Fe2O3) nanoparticles using a simple electrochemical technique. The electrochemically modified CEINs decorated on CNTs have been introduced as active and durable electrocatalysts for alkaline OER. It has been also shown that CNTs are selectively opened during OER, suggesting a technique for selective opening of CNTs which is beneficial for endohedral functionalization of CNTs. We have further introduced SWNTs as promising supports to stabilize individual atoms or subnano clusters of Pt. An atomic-scale Pt catalyst system allows minimizing the Pt loading. However, ultra-small structures of Pt are not stable unless they are immobilized on a suitable support. The SWNTs have been activated with pseudo-atomic scale Pt, including mostly the active surface atoms, using a simple electroplating method. These activated SWNTs exhibit a similar activity toward HER in acidic media to that of commercial Pt/C with a significantly higher Pt loading. We have also investigated that immobilization of organometallic Ni complexes on the sidewalls of multi-walled CNTs (MWNTs) is a promising strategy to synthesize a new class of efficient non-precious electrocatalysts. A synthesis method has been developed to covalently functionalize MWNTs with Ni bipyridine complexes. The Ni-bipyridine-MWNT shows a high activity for alkaline OER, ranking the material among the most active OER electrocatalysts reported so far.