Leveraging atomic layer deposition in oxygen reduction electrodes for proton exchange membrane fuel cells
Keywords: Chemical Engineering.

Through powering our vehicles and supplying the raw materials for pharmaceuticals, detergents, and plastics, fossil fuels have erected much of the industrialized world. However, fossil fuels are not a functionally renewable resource, and their continued use threatens climate health through the release of greenhouse gases. Therefore, clean energy technologies are urgently needed to sustainably uphold the global economy. To this end, impressive feats of science and engineering have enabled the extraction and storage of energy from the sun and wind, but deployment remains limited and some sectors are technologically underserviced. For example, lithium-ion batteries have notably electrified automobiles for many commuters but still occupy a small fraction of the global fleet, and their low energy densities and relatively long charging times currently prohibit their use in heavy duty applications like aircraft and large ships. As a transportable fluid with a high intrinsic energy density, hydrogen is an attractive option to help diversify renewable energy and service demanding machinery. In the select applications where hydrogen is already deployed—warehouse forklifts and automobiles with access to fueling stations—proton exchange membrane fuel cells (PEMFCs) are the preferred technology for extracting energy from hydrogen. One limitation of PEMFCs, which employ Pt-based catalysts and a proton-conducting polymer membrane to mediate the requisite electrochemistry, is the heat lost to the environment due to the sluggish kinetics of the cathodic oxygen reduction reaction (ORR). This dissertation joins a large body of academic work focused on raising the PEMFC efficiency through improved catalysis. Broadly, it describes three distinct efforts to study and advance Pt-based PEMFC electrochemistry, linked by the use of atomic layer deposition to fabricate various components of the cathode. Atomic layer deposition (ALD) is a precise material synthesis technique characterized by self-terminating reactants and gas-phase chemistry. ALD finds most of its commercial utility in the fabrication of microelectronics but has recently received attention in the field of catalysis. This work describes the first high-performing PEMFC cathode prepared with ALD-synthesized Pt. Through a novel fabrication architecture wherein the cathode is prepared with the sequential deposition of carbon, Pt, and the solid ionomer electrolyte, a high mass-normalized activity (MA) of over 0.3 A/mgPt at 0.9 V vs. RHE resulted in a 0.65 V power density of 1.0 W/cm2 at 150 kPa and 80 degrees C. This sequential, "bottom-up" fabrication architecture was also amenable to empirical studies of ionomer morphology and carbon support nanostructure, which supported the hypothesis that Pt nanoparticles are effectively shielded from agglomerated ionomers in ~4--7 nm carbon pores. With a methodology in place for preparing high-performing PEMFCs with ALD, strategies for elevating the activity of the Pt active center were subsequently explored. First, Zn—an inexpensive but underexplored transition metal dopant—was alloyed with Pt to beneficially weaken the adsorption of key intermediates in the ORR. When Zn was deposited via ALD, the material system was readily incorporated into fuel cell tests and delivered an MA of ~0.4 A/mgPt, 30% higher than pure Pt. To elevate the activity further, Zn electroplating was employed to increase the Zn content in the Pt-Zn nanoparticles. This modified approach led to an MA of nearly 2.5 A/mgPt in benchtop rotating disk electrode (RDE) tests, but the material could not be reproduced in a membrane electrode assembly (MEA) for full PEMFC testing. To identify reasons beyond fabrication issues that catalysts tend to underperform in MEAs relative to RDEs, an Arrhenius analysis was performed on the Pt-Zn alloys. Based on the extracted activation energies, it was determined that a ~40% convergence in activity between the operating conditions of RDEs and MEAs is expected for highly active materials. Other architectural differences are posited to be responsible for the remaining discrepancy. ALD was subsequently leveraged in a core-shell-like structure to improve Pt atom utilization, which not only increases mass activity but also extends the reactant transport efficiencies in the PEMFC. In a series of samples, Pt was deposited at different ALD cycles on high-surface-energy metallic Ru nanoparticles. At very low cycle numbers, an amorphous dispersion of Pt populates the Ru surface and delivers MAs exceeding 0.4 A/mgPt despite a decrease in ORR turnover frequency. Thus, ALD is firmly established as a platform for achieving highly dispersed catalysts, with a significant opportunity to adjust performance through the modulation of substrate properties and synthesis parameters. This dissertation advances the field of acidic ORR catalysis through the presentation of kinetics and computational results as well as new materials and electrode fabrication techniques. Natural extensions of the work discussed in some detail include optimization and pilot scale demonstrations of the designs and structures disclosed