Won Suk Jung

New Method to Synthesize Highly Active and Durable Chemically Ordered fct-PtCo Cathode Catalyst for PEMFCs

In the bottom-up synthesis strategy performed in this study, the Co-catalyzed pyrolysis of chelate-complex and activated carbon black at high temperatures triggers the graphitization reaction which introduces Co particles in the N-doped graphitic carbon matrix and immobilizes N-modified active sites for the oxygen reduction reaction (ORR) on the carbon surface. In this study, the Co particles encapsulated within the N-doped graphitic carbon shell diffuse up to the Pt surface under the polymer protective layer and forms a chemically ordered face-centered tetragonal (fct) Pt-Co catalyst PtCo/CCCS catalyst as evidenced by structural and compositional studies. The fct-structured PtCo/CCCS at low-Pt loading (0.1 mgPt cm-2) shows 6% higher power density than that of the state-of-the-art commercial Pt/C catalyst. After the MEA durability test of 30 000 potential cycles, the performance loss of the catalyst is negligible. The electrochemical surface area loss is less than 40%, while that of commercial Pt/C is nearly 80%. After the accelerated stress test, the uniform catalyst distribution is retained and the mean particle size increases approximate 1 nm. The results obtained in this study indicated that highly stable compositional and structural properties of chemically ordered PtCo/CCCS catalyst contribute to its exceptional catalyst durability.

Titania Supported Platinum Catalyst with High Electrocatalytic Activity and Stability for Polymer Electrolyte Membrane Fuel Cell

Titania supported Pt electrocatalysts (Pt/TiO2) were synthesized and investigated as alternative cathode catalysts for polymer electrolyte membrane fuel cells (PEMFCs). Transmission electron microscope (TEM) images revealed uniform distribution of Pt nanoparticles (dPt = 3-5 nm) on the TiO2 support. In-house developed accelerated durability test (ADT, continuous potential cycling between 0.6 and 1.4 V) in half-cell condition indicated nearly ten-fold higher ORR activity (1.20 mA cm-2) when compared to the Pt/C catalyst (0.13 mA cm-2). The Pt/C catalyst showed no activity in fuel cell testing after 2000 potential cycles due to severe carbon corrosion, Pt dissolution, and catalyst particle sintering. Conversely, the Pt/TiO2 electrocatalyst showed only a small voltage loss (0.09 V at 0.8 A cm-2) even after 4000 cycles. The ADT results showed excellent stability for the Pt/TiO2 electrocatalysts at high potentials in terms of minimum loss in the Pt electrochemical surface area (ECSA).

Development of Supported Bifunctional Oxygen Electrocatalysts with High Performance for Unitized Regenerative Fuel Cell Applications

Titania supported platinum and iridium electrocatalysts (Pt/TiO2 and Ir/TiO2) were synthesized and investigated as bifunctional oxygen electrode (BOE) catalysts for unitized regenerative fuel cells (URFCs). TEM images revealed uniform distribution of Pt and Ir nanoparticles on the TiO2 support. Histogram analysis showed particle sizes of Pt and Ir to be 4.5 and 2.0 nm, respectively, which was also confirmed by the XRD characterization. Among the various Pt-Ir compositions prepared, Pt85Ir15 (with a Pt/Ir weight ratio of 85/15) showed the highest catalyst efficiency towards oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). The URFC testing results showed that the round-trip energy conversion efficiency (eRT) of supported Pt-Ir/TiO2 (42%) was significantly higher than that of unsupported PtIr black (30%). The TiO2 support provided high surface area for uniform dispersion of the catalyst particles. The URFC performance increase was ascribed to the uniform dispersion and better utilization of noble metal catalysts.

High-performance bimetallic alloy catalyst using Ni and N co-doped composite carbon for the oxygen electro-reduction

In this study, a novel synthesis method for the bimetallic alloy catalyst is reported, which is subsequently used as an oxygen reduction catalyst in polymer electrolyte membrane fuel cells (PEMFCs). The support prepared from the Ni-chelate complex shows a mesoporous structure with a specific surface area of ca. 400 m2 g-1 indicating the suitable support for PEMFC applications. Ethylenediamine is converted to the nitrogen and carbon layers to protect the Ni particles which will diffuse into the Pt lattice at 800 °C. The PtNi/NCC catalyst with PtNi cores and Pt-rich shells is successfully formed when acid-treated as evidenced by line scan profiles. The catalyst particles thus synthesized are well-dispersed on the N-doped carbon support, while the average particle size is ca. 3 nm. In the PEMFC test, the maximum power density of the PtNi/NCC catalyst shows approximately 25% higher than that of the commercial Pt/C catalyst. The mass activity of the PtNi/NCC catalyst showed approximately 3-fold higher than that of the commercial Pt/C catalyst. The mass activity strongly depends on the ratio of Pt to Ni since the strain effect can be strong for catalysts due to the mismatch of lattice parameters of the Ni and Pt.

Development of Highly Active Pt2Ni/CCC Catalyst for PEM Fuel Cell

Polymer electrolyte membrane fuel cells (PEMFCs) are attractive for automotive applications because of their low operating temperature (80°C), high power density at 0.6 V, portability, and relatively matured technology when compared to other lowand hightemperature fuel cells [1,2]. The membrane electrode assembly is the important component in a PEMFC which contains a Pt or Pt-alloy based electrocatalysts supported on high surface area carbon and a proton conducting polymer membrane. Studies have shown that the anode Pt loading can be reduced up to 0.05 mgPt/cm due to the faster kinetics at the anode when pure H2 fuel is used. Nearly 4-8 fold higher Pt loadings (0.2 to 0.4 mgPt/cm) are required on the cathode due to the sluggish oxygen reduction reaction (ORR) kinetics in order to achieve the necessary high power density at high cell voltages needed for automotive applications [3-6]. Considering the limited world supply of Pt and its high cost, 2015 DOE target requires Pt group metal content of ≤ 0.125 g/kW, while maintaining the MEA power density. To meet this goal, cathode Pt loadings needs to be reduced to 0.1 mgPt/cm without affecting the performance which requires at least fourfold higher mass activity of Pt-based catalysts. The specific objectives of this work are: (1) to increase the catalyst mass activity by choosing appropriate alloying metal and (2) determine the effect of different catalyst loadings on mass activity and the fuel cell performance. The catalyst support was suitably modified prior to the Pt deposition and alloying process with nickel by a methodology developed at University of South Carolina. A protective coating method was used to avoid Pt-alloy particle size growth during high temperature alloying process. Figure 1 shows the X-ray diffraction patterns of Pt/C, fresh and leached Pt2Ni/C catalysts. In order to avoid the Pt-alloy catalyst particle agglomeration at high temperature treatment, the catalyst was protected using a USC-developed coating process prior to the heat treatment. The particle sizes of Pt/C and leached Pt2Ni/C are 2.2 and 3.5 nm, respectively which confirms the efficient role of the protective coating used to control the particle growth during high temperature pyrolysis. Furthermore, the shift in the 2θ values to higher values indicates the Pt-Ni alloy formation. The mass activities of Pt/C and leached Pt2Ni/C catalysts are compared in Fig. 2. The testing was carried out under the following DOE suggested conditions [80 C/ H2/O2 (2/9.5 stoic.), 100% RH and 150 kPaabs.]. As can be seen from the figure, the alloying process drastically increased the mass activity of the Pt2Ni/C catalyst by ~3 times (0.45 A/mgPt) when compared to the fresh Pt/C catalyst (0.18 A/mgPt). Detailed experimental results and theoretical studies explaining the effect of catalyst loading on the mass activity and high current density performance under H2-air will be presented at the conference.