Atomic metal on graphene for electrochemistry
Tour, James M
Doctor of Philosophy
As the limit in downsizing metal morphology is sought, the concept of single-atom catalysts (SACs) has emerged since it maximizes transition metal atom utilization and includes high exposed atom efficiency for a range of reactions. However, the preparation of SACs remains challenging because the high free energy of individual metal atoms leads to metal aggregation, affording nanoclusters or nanoparticles. A strong interaction between metal atoms and the supporting substrate is a desirable approach to anchor and stabilize these atomically dispersed metal sites. Because of its high specific area, large electron mobility and tunable surface chemistry, graphene, a well-defined 2D structure, is a promising substrate to support these single-atomic active sites for electrocatalytic applications. This thesis begins with the investigation of a general synthetic approach towards the atomic dispersion of metal atoms on nitrogen-doped graphene derived from graphene oxide. Herein, a series of atomic transition metal dispersed on nitrogen-doped graphene are synthesized and investigated as efficient electrocatalyst for the oxygen reduction reaction (ORR), CO2 reduction reaction (CO2RR) and nitrogen reduction reaction (NRR). In Chapter 1, single atomic dispersed rutheniums on nitrogen doped graphene are disclosed as efficient catalytic sites for ORR in acidic medium. This reaction is important in proton-exchange membrane fuel cells (PEMFC) that convert chemical energy liberated from the electrochemical reaction between hydrogen and oxygen into electrical energy, which is considered as a possible main source of power for next-generation automobiles. Chapter 2 describes the electrocatalytic performance of atomic iron on nitrogen-doped graphene for direct reduction of CO2 to CO in aqueous solution. Nitrogen-doping were also found to play vital roles in the enhancement of CO conversion. Finally, a preliminary result in Chapter 3 demonstrates that atomic Mo catalytic sites anchoring on a holey nitrogen-doped graphene framework possess an intriguing activity toward electrochemical N2 reduction to NH3 with excellent selectivity under ambient conditions. The holey nitrogen-doped graphene is an efficiency substrate for dispersing and bonding atomic Mo species, which are considered to be the active site for NRR. In general, aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and X-ray absorption fine structure analysis (XAFS) reveals the existence of nitrogen-coordinated atomic metal moieties embedded on the nitrogen-doped graphene substrate. The electrochemical reaction mechanism on those isolated metal atoms surrounded by nitrogen atoms embedded in nitrogen-doped graphene is further investigated through density functional theory calculations. The nitrogen doped graphene substrate not only provides a stabilizing matrix for the metal atoms, but also impacts the electronic density of the metal atoms due to strong nitrogen-metal interactions, which may lead to their enhanced electrocatalytic activity in ORR, CO2RR and NRR. This work has built a bridge between homogenous and heterogenous catalysts, expanding the possibility of designing and engineering atomic structures of graphene with transition metals toward electrochemical applications.
electrochemistry; graphene; single-atom catalysis