UC Irvine

Decarbonization of energy technologies is crucial for reducing greenhouse gas emissions and their effect on climate change. An increased spotlight has been placed on eliminating emissions stemming from the transportation sector towards zero-carbon energy landscapes. Recent efforts have focused on the deployment of proton exchange membrane fuel cells (PEMFCs) in heavy duty vehicles, which contribute an extremely disproportionally high percentage of CO2 emissions into the atmosphere. This application of PEMFCs, however, requires highly durable oxygen reduction reaction (ORR) catalysts. One significant roadblock is the susceptibility of carbon supports of ORR catalysts towards electrochemical oxidation and corrosion. Hence, major improvements in corrosion-resistant carbon supports are required in order to reach the goal of implementing PEMFCs in heavy duty vehicles.

The interactions between carbon supports and the platinum nanoparticles deposited on them are key towards mitigating ORR catalytic activity losses. Amorphous carbon contains a high density of platinum anchoring sites but is extremely susceptible to oxidation. Graphitic carbon is oxidation-resistant but lacks platinum anchoring sites, which leads to agglomeration of the active material and reduced electrochemical performance. Bulk graphitic content is commonly cited as a measure of a carbon support’s resistance to electrochemical oxidation, yet this value provides little to no insight into the actual morphology of the carbon. Thus, rational design of carbon supports entails thorough understanding of the physical, chemical, and electronic structure of the supports. This dissertation addresses this knowledge gap by probing various novel carbon materials and focusing on the structure-to-property relationships between support morphology and composition and ORR catalytic performance. The specific studies are detailed as follows:

i) Adopting Strategies to Integrate Corrosion-resistant Carbon-based Materials as Supports for Polymer Electrolyte Membrane Fuel Cell Catalysts The mechanism behind electrochemical oxidation of carbon during fuel cell operation and its effect on the cathode catalyst layer morphology and mass transport are summarized. Various strategies to improve oxidation resistance have been presented to inform rational design of carbon supports.

ii) Easily Synthesized Nitrogen-Doped Carbon Nanospheres as Model Supports for Oxygen Reduction Reaction CatalystsA series of monodisperse nitrogen-doped carbon nanosphere supports (NCS1-3) was synthesized through a one-pot process. These were characterized to establish that the differences between the supports were near-surface concentrations of certain nitrogen moieties and minor differences in pore size distribution. A large batch of platinum nanoparticles was then synthesized and decorated onto the NCS supports. Pt/NCS3 exhibited the best Pt dispersion, exhibiting an electrochemically active surface area more than double that of the other catalysts. This enhanced dispersion was attributed to NCS3 having the highest surface concentration of pyridinic-N, lowest surface concentration of amine-N, and moderate microporous surface area.

iii) Novel FCX Carbon Surface Modification with 1-Pyrene Carboxylic AcidCommercial FCX carbons with high degrees of crystallinity were extensively characterized, establishing key differences in specific surface area and pore size distribution. Platinum loss was observed when decorating FCX300 and FCX500 with Pt nanoparticles. 1-pyrene carboxylic acid was then introduced as a support surface modifier to increase the density of Pt anchoring sites while preserving the surface crystallinity of the carbon supports. Electrochemical rotating disk electrode measurements showed that post modification, the 20 wt% Pt/FCX800P catalyst achieved an extremely high ECSA of 169 ± 40 m2/gPt and outperformed Pt/FCX800 with mass activity and specific activity of 166 ± 6 A/gPt and 376 ± 21 μA/cm2Pt at 0.9 V. Thermogravimetric analysis confirmed that the surface modification with 1-pyrene carboxylic acid increased the catalyst Pt loading due to enhanced interactions with the support.

iv) Impregnation of FCX Carbons with Pt and Pt3Co NanoparticlesAn incipient wetness synthesis procedure was implemented to anchor monodisperse Pt nanoparticles uniformly within the interior pore structure of FCX800 By increasing the Pt loading to 40 wt%, a mass activity of 162 ± 23 A/gPt and a specific activity of 255 ± 34 μA/cm2Pt have been achieved at 0.9 V. By modifying the support with 1-pyrene carboxylic acid beforehand, the catalytic performance improved to 201 ± 57 A/gPt and 255 ± 55 μA/cm2Pt at 0.9 V. The synthesis method was extended to Pt3Co nanoparticles. The catalytic activity of this alloyed catalyst was improved ca. 3 times after undergoing a second pyrolysis step to alter the atomic configuration of the nanoparticles. This 20 wt% Pt3Co/FCX800 catalyst exhibited a mass activity of 121 ± 2 A/gPt and a specific activity of 554 ± 23 μA/cm2Pt at 0.9 V.