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Development of Electrocatalysts in Solid Acid Fuel Cells


Paik, Haemin (2019) Development of Electrocatalysts in Solid Acid Fuel Cells. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/C96K-Z389.


Solid acid fuel cells (SAFCs) can operate at intermediate temperature (near 250 ºC) using a non-toxic, solid proton-conducting electrolyte, CsH2PO4, which allows for fuel flexibility, high efficiency, inexpensive auxiliary components, and easy on-off cycling. Despite these features, large activation overpotentials at the electrodes require high Pt loadings in order to achieve acceptable power output. Few alternatives to Pt have emerged for either the hydrogen oxidation reaction or the oxygen reduction reaction in SAFCs. This thesis explores the use of Pd and Pd-containing alloys for electrocatalysis in SAFCs to reduce overall precious metal loading and therefore reduce cost to commercialization.

First, this work explores the use of Pd at the SAFC anode, assessing both catalytic activity for hydrogen electro-oxidation and reactivity with the CsH2PO4 electrolyte. A thin film geometry, in which nanometric layers of metal were deposited onto a polycrystalline disk of CsH2PO4 was used to simplify the device and facilitate interpretation electrochemical behavior. Using a symmetric geometry, the cells were examined under a uniform hydrogen-rich gas. It was found that Pd reacts with CsH2PO4, forming palladium phosphide (Pd-P) at the metal-electrolyte interface. With the aim studying the behavior of Pd in the absence of this reactivity, Pd overlain on Pt was examined in a bilayer geometry of Pd | Pt | CsH2PO4 | Pt | Pd. The bilayer Pt | Pd films show much higher activity for hydrogen electro-oxidation than films of Pt alone, as measured by AC impedance spectroscopy. Ex-situ low energy ion scattering and scanning transmission electron microscopy revealed that Pd diffused into the Pt layer under operating conditions. The extremely high activity of the interdiffused films suggest that Pd catalyzes reactions at both the metal-gas and metal-electrolyte interfaces, and furthermore facilitates rapid hydrogen diffusion rates through the films.

The high activity of Pt | Pd films, in which Pd eventually contacts the underlying electrolyte due to interdiffusion of the metals, motivates an investigation of Pd-based catalysts (Pd and Pd-P) for hydrogen electro-oxidation in a fuel cell relevant configuration. Working electrodes were formed from a mixture of Pd on carbon and the electrolyte material. The hydrogen oxidation kinetics from Pd, Pd6P, and Pd3P0.8 were observed to be comparable. The result is consistent with the observation that Pd catalyst reacts with CsH2PO4 and converts into Pd-P during cell operation. Both Pd and Pd-P appear to be more effective electrocatalysts for hydrogen oxidation than the equivalent mole percent of Pt supported on carbon. Further enhancement of Pd catalytic activity is achieved by reducing its crystallite size.

Lastly, this work examines the catalytic activity of Pd for oxygen reduction at the SAFC cathode. Evaluation of this system is complicated by the instability of Pd on CsH2PO4 under oxidizing conditions, which causes microstructure collapse and performance degradation. A SnO2 thin film was introduced as a barrier layer to inhibit Pd reactivity with CsH2PO4 and as a structural support for the catalyst. Employing atomic layer deposition, a SnO2 thin film was deposited either between the Pd and CsH2PO4 interface, or over the Pd catalyst. Both Pd-SnO2 bilayers show improved fuel cell performance stability compared to a Pd-only control, forming Pd-Sn alloys under cathode conditions. This suggests that the formation of Pd-Sn alloy stabilizes the metallic phase of Pd, improving catalytic activity. This work presents a new approach for designing the cathode materials for SAFCs.

Item Type:Thesis (Dissertation (Ph.D.))
Subject Keywords:Fuel cells; electrocatalyst; solid acid; palladium; palladium phosphide
Degree Grantor:California Institute of Technology
Division:Engineering and Applied Science
Major Option:Materials Science
Thesis Availability:Public (worldwide access)
Research Advisor(s):
  • Haile, Sossina M.
Thesis Committee:
  • Johnson, William Lewis (chair)
  • Goddard, William A., III
  • Rossman, George Robert
  • Haile, Sossina M.
Defense Date:3 May 2019
Non-Caltech Author Email:haemin813 (AT)
Record Number:CaltechTHESIS:06072019-011829568
Persistent URL:
Related URLs:
URLURL TypeDescription adapted for Ch 3 adapted for appendix A
Paik, Haemin0000-0001-8358-6067
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:11701
Deposited By: Haemin Paik
Deposited On:10 Jun 2019 22:53
Last Modified:12 Dec 2019 21:41

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