Electrocatalysis of Oxygen Reduction in Acidic Media with Non-platinum Group Metal Catalysts and a Study of Anion Poisoning

Concerns of climate change and depleting fossil fuel resources have encouraged researchers to develop a replacement for the internal combustion engine in automotive transportation. The Hydrogen/air polymer electrolyte fuel cell (PEMFC) is a promising technology, but it suffers from several technical challenges. One of the most critical of these challenges is the high production cost of the fuel cell stack. A significant portion of the cost of the system can be attributed to the use of platinum as the catalyst material in both the anode and cathode. The slow kinetics of the oxygen reduction reaction (ORR) at the cathode requires more platinum than the more facile hydrogen oxidation at the anode, and therefore finding a non-platinum group metal (non-PGM) catalyst with high ORR activity would have the greatest payoff. Traditional platinum based electrocatalyst also suffer poisoning effects from various anions that further inhibit ORR. Anion adsorption is structure dependent and it has been shown that some Pt-alloys exhibit a heightened tolerance to poisoning. Additionally, fuel cell lifetime has been shown to be limited by factors such as Pt-particle dissolution and particle agglomeration.;Significant performance advancements have been made with non-PGM catalysts in the last several years, but the structure of the active site and the resulting ORR mechanism are still heavily debated. The current state of the art non-PGM catalysts are made by mixing and heat-treating a metal and nitrogen precursors on a carbon support. The heat treatment step has led to improvement of the durability and performance, but the composite nature of the derived catalyst material imposes significant challenges in its characterization.;This dissertation focuses on the synthesis and characterization of two novel types of non-PGM cathode electrocatalysts for use in acidic media. Chapter 1 introduces the electrochemical and spectroscopic methods that are used in Chapter 2-5 to investigate the ORR activity and stability of the electrocatalysts. A Co-based non-PGM is introduced in Chapter 2, and the rotating ring disk electrode (RRDE) technique was used to determine the reactivity and selectivity of the material towards the individual steps of ORR. Synchrotron based X-ray absorption spectroscopy (XAS) analysis of the electronic and structural properties of the cobalt after heat treatment. Chapter 3 introduces a Fe-based non-PGM that is very active for ORR in alkaline and acidic media and lacks the Fe-N x coordination that has been heavily debated as the active site of non-PGMs. The combination of ex situ and in situ characterization techniques reveal the Fe is present as Fe/FexC particles subsurface to a carbon overlayer. The lack of Fe-Fe-Nx coordination and isolation of the Fe-particles allows us to attribute the ORR activity to the nitrogen-doped carbon. Chapter 4-5 further probes the Fe-based non-PGM introduced in Chapter 3. The traditional Pt-based catalysts are poisoned by the phosphate anion and Chapter 4 investigates the effect of anion poisoning on the Fe-based catalyst. Electrochemical measurements combined with a subtractive technique, 'Deltamu', provided insight into the site specific adsorption and the electrocatalytic pathways. Durability of non-PGMs is a significant issue so fuel cell durability of the most active material was also investigated in Chapter 5 using the above mentioned techniques. Correlation of catalyst degradation with electrochemical oxidation of the carbon is obtained. In situ XAS revealed that the Fe is stable during normal fuel cell operating conditions, eliminating concerns of any Fenton type process related to onset of peroxide induced free radical formation. Chapter 6 summarizes the findings and future directions.