PROGRAM | Chemical Engineering

By: Jie Zheng Chair: Yushan Yan Co-Chair: Bingjun Xu

ABSTRACT

Commercialization and mass adoption of low temperature fuel cells have been hampered by the large cell voltage loss, which can be largely blamed on the sluggish electrode reaction kinetics even with the state-of-the-art Pt catalysts. Significant progress has been made in the development of cathode catalysts for the oxygen reduction reaction (ORR), whereas the search for efficient anode catalysts has not been as fruitful. Therefore, the rational design and development of efficient anode catalysts are of vital importance, which hinge on two key factors: 1) fundamental understanding of the reaction mechanism and 2) synthesis of catalysts with well-defined structures.

Hydrogen oxidation reaction (HOR, ) is roughly two orders of magnitude slower in base than in acid electrolytes on Pt-group metal (PGM) catalysts, which demands either a substantial anodic overpotential or a high PGM loading for hydroxide exchange membrane fuel cells (HEMFCs). Fundamental understanding HOR kinetics is a prerequisite in the design of highly active HOR catalysts. To achieve this goal, my research established protocols to reliably remove the contribution of diffusion in the HOR/HER activity measurement with the rotating disk electrode (RDE) method, based on which intrinsic kinetic information can be extracted.

The effect of particle size on HOR/HER activities were explored on carbon supported Ir and Pd nanoparticles: the specific HOR/HER activities increase as particle size increase. The most active sites for HOR/HER on Ir/C were identified to be the sites with lowest hydrogen binding energy (HBE) (most likely the low-index facets), based on the observation that the activities normalized to the surface area of weakly binding sites are independent of particle size. Consistent with the results on Ir, the increased HOR/HER activity on larger Pd nanoparticles correlates with an increased ratio of the sites with lower HBE. These findings suggest that future catalyst design should focus on increasing the density of sites with low HBE, e.g., low-index facets.

To establish the generality of the pH effect on the HOR/HER activity, a reliable and easily accessible method to experimentally determine the pH-dependent HBE was developed. In addition, HOR/HER activities on monometallic PGM (Pt, Pd, Ir and Rh) nanoparticles were mapped out over a broad pH range (1-13), which are then correlated with HBE. A universal correlation between the HOR activity and HBE is obtained on all PGMs evaluated, which offers strong evidence that HBE is the dominating descriptor for the performance of HOR catalysts. It follows that tuning of HBE could a key strategy in the future design of HOR catalysts.

Aside from hydrogen, methanol is a promising liguid fuel for fuel cells. A key challenge in the development of active catalysts for methanol oxidation reaction (MOR, ), which is the anode reaction of direct methanol fuel cells (DMFCs), is the structural sensitive nature of the catalytic performance. Hence, synthesis of catalysts with tailored structures is critical. Extended surface nanostructures, e.g., PtRu nanotubes (PtRuNTs) and PtRu coated Cu nanowires (PtRu/CuNWs), were synthesized by galvanically displacing the CuNWs template, which showed higher specific MOR activity than that of the benchmark PtRu/C. We attribute the enhanced activity to the weakened Pt-CO bonding through the modification of d-band center of Pt.

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