Electrochemical Investigations into the Influence of the Outer Helmholtz Plane towards the Oxygen Reduction Reaction on Non-Precious Metal Catalysts in Alkaline Mediums

Abstract

The continuous decrease in the amount of fossil fuels combined with the increasing energy demands of our society has fueled the rapid growth in research for alternative energy sources. Although these energy devices possess ample potential to supply these energy demands, most of these devices require rare and/or expensive materials to operate at highly efficient rates. No electrochemical energy device exemplifies this more than fuel cells. Fuel cells, which harness the chemical energy released from redox reactions and converts it to electrical energy, offer a clean, renewable, and non-toxic energy source and can potentially play a major role in supplying the energy demands of the future. However, the oxygen reduction reaction, which occurs at the cathode, suffers from a high overpotential and is often viewed as the biggest obstacle to fuel cell commercialization. Therefore, research into an affordable and durable catalyst to lower the overpotential and increase the efficiency of fuel cells has been and continues to be a major area of research. Recent reports have shown metal ions coordinated to nitrogen compounds having high catalytic activity for the oxygen reduction in alkaline mediums. Remarkable, even though heat treatment likely destroys these coordinated structures, improvements in catalytic activity are seen after heat treatment. Although these catalysts provide great promise as an alternative to platinum, the active sites or how the oxygen reduction reaction proceeds remains a mystery. In addition, controversies exist in regards to where the active sites are in these non-precious metal catalysts. Therefore, optimization of these catalysts remains impossible until this information is known. This work focused on investigating the active site as well as the possible pathways for the oxygen reduction reaction using copper, cobalt, and iron metal phthalocyanines. Due to differing oxygen adsorption energies between these metal ions, different ORR activities were observed. In addition, poisoning experiments showed a significant decrease in activity, further proving that before heat treatment, the M+-N-C structure is the active site and whether the ORR proceeds via the 2 electron or 4 electron pathway is dependent on the oxygen adsorption strength. However, these structures are bound to decompose during heat treatment and, as a result, the ORR pathway likely does not proceed through the same pathway. Electrochemical measurements using LiOH, NaOH, KOH, and CsOH electrolytes were done on the phthalocyanine catalysts before and after heat treatment to confirm this hypothesis. Changes in the ORR activity trends for cobalt and iron phthalocyanine before and after heat treatment show that different pathways are favored. To determine the changes after heat treatment, physical characterization measurements, such as x-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS), and ultraviolet-visible absorbance (UV-Vis) were conducted. The results of this study show that after heat treatment, oxygen is first converted to a superoxide via the outer Helmholtz plane reaction and is completely reduced at nitrogen doped sites created during heat treatment. These conclusions offer a viable explanation for the high level of activity observed after heat treatment as well as a possibility why this high activity is only observed in alkaline mediums.

Keywords: Non-precious metal catalyst, oxygen reduction reaction, alkaline fuel cell, inner and outer Helmholtz plane reactions. Student Number: 2013-22533