Electrocatalysts for the reduction of dioxygen and carbon dioxide

The oxygen reduction reaction (ORR) plays a critical role at the cathode of most fuel cells. Due to the slow kinetics of the ORR, research has focused primarily on generating electrocatalysts with improved activity and economic viability. Building on the prior ORR success of a carbon-supported binuclear copper complex of 3,5-diamino-1,2,4-triazole, the first section of this thesis focuses on four copper complexes built from structurally similar ligands which were synthesized and examined for their effectiveness as ORR electrocatalysts: 1,2,4 triazole (CuTri), 3,4,5-triamino-1,2,4-triazole (CuTAT), 3,5-diamino-1,2,4-triazole (CuDAT), and 4-amino-3,5-di-2-pyridyl-4H-1,2,4-triazole (CuABPT). The crystal structures of the copper complexes are readily altered by introducing different ligand substituents. These structural changes directly impact the activity of the electrocatalyst. The ORR onset potential is consistent for each of the complexes; however, both CuDAT and CuTAT reduce O2 in what is mainly a 4 e- process, whereas CuTri and CuABPT are predominantly 2 e- O2 reduction catalysts, producing greater quantities of H2O2 side-product. It is expected that with further structural modifications, one could optimize Cu-Cu spacing, leading to a better ORR catalyst. The second and third sections of this thesis concentrate on the electrochemical reduction of CO2. It is well known that atmospheric CO2 levels are rising at significant rates, thereby demanding the development of multifaceted approaches to avoid further climate change. The electrochemical reduction of CO2 to produce useful chemicals, such as carbon monoxide (CO), provides a potentially carbon-neutral approach to using excess renewable energy from intermittent sources, while reducing our dependence on fossil fuels. However, in order to apply this promising technology in a commercial setting, catalysts with sufficient activity and product selectivity must be developed. The synthesis and application of carbon-supported nitrogen-based organometallic silver catalysts for the reduction of CO2 is studied using an electrochemical flow reactor, as well as three-electrode cell measurements. Their performance towards the selective formation of CO is similar to the performance achieved using silver as the catalyst, but comparatively at much lower silver loading. Faradaic efficiencies of the organometallic catalyst are comparable to those of Ag, demonstrating efficiencies higher than 90%. Furthermore, with the addition of an amine ligand to Ag/C, the partial current density for CO increases significantly, suggesting a possible co-catalyst mechanism. Additional improvements in activity and selectivity may be achieved as greater insight is obtained on the mechanism of CO2 reduction and on how these complexes assemble on the carbon support. Secondly, a new catalyst platform, Au nanoparticles supported on polymer-wrapped multi-walled carbon nanotubes (MWNTs), is applied for the electrochemical reduction of CO2 to CO. This catalyst exhibits a high selectivity for CO over H2 (80-92% CO at most cathode potentials), as well as high activity: a maximum partial current density for CO of 160 mA/cm2 and an up to 8.8x higher current density for CO at intermediate cathode potentials (V= -1.39 V vs. Ag/AgCl) compared to the state-of-the-art silver nanoparticle-based catalysts normally used under identical experimental conditions. Remarkably, this performance is achieved with a very low catalyst loading of 0.17 mg Au/cm2, suggesting that Au nanoparticles are highly dispersed on MWNTs. Indeed, it is shown that this catalyst possesses a high electrochemically-active surface area of 23 m2/g Au. Reducing loadings of precious metal catalysts without sacrificing activity and selectivity offers promise for electrochemical CO2 reduction to become an economically practical process. As well, polymer-wrapped MWNTs may serve as a platform for other metal deposition and can be applied for a variety of different catalytic reactions