Anode catalyst development for low-temperature fuel cells: fundamentals and synthesis
Date
2016
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
University of Delaware
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, 𝐻2 ↔ 2𝐻+ + 2𝑒) 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,
𝐶𝐻3𝑂𝐻 + 𝐻2𝑂 → 𝐶𝑂2 + 6𝐻+ + 6𝑒), 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.