Prabhu Ganesan

Development of a Titanium Dioxide-Supported Platinum Catalyst with Ultrahigh Stability for Polymer Electrolyte Membrane Fuel Cell Applications

A significant decrease in performance was observed for commercial Pt/C due to electrochemical oxidation of the carbon support and subsequent detachment and agglomeration of Pt particles. The Pt/TiO(2) cathode catalyst exhibited excellent fuel cell performance and ultrahigh stability under accelerated stress test conditions and can be considered as a promising alternative for improving the reliability and durability of PEMFCs.

Study of cobalt-doped lithium-nickel oxides as cathodes for MCFC

Cobalt substituted lithium–nickel oxides were synthesized by a solid-state reaction procedure using lithium nitrate, nickel hydroxide and cobalt oxalate precursor and were characterized as cathodes for molten carbonate fuel cell (MCFC). LiNi0.8Co0.2O2 cathodes were prepared using non-aqueous tape casting technique followed by sintering in air. The X-ray diffraction (XRD) analysis of sintered LiNi1� xCoxO2 indicated that lithium evaporation occurs during heating. The lithium loss decreases with an increase of the cobalt content in the mixed oxides. The stability studies showed that dissolution of nickel into the molten carbonate melt is smaller in the case of LiNi1� xCoxO2 cathodes compared to the dissolution values reported in the literature for state-of-the-art NiO. Pore volume analysis of the sintered electrode indicated a mean pore size of 3 mm and a porosity of 40%. A current density of 160 mA/cm 2 was observed when LiNi0.8Co0.2O2 cathodes were polarized at 140 mV. The electrochemical impedance spectroscopy (EIS) studies done on LiNi0.8Co0.2O2 cathodes under different gas conditions indicated that the rate of the cathodic discharge reaction depends on the O2 and CO2 partial pressures. # 2002 Published by Elsevier Science B.V.

Preparation and Characterization of Pt/NbTiO2 Cathode Catalysts for Unitized Regenerative Fuel Cells (URFCs).

Cathode Catalysts for Unitized Regenerative Fuel Cells (URFCs) Prabhu Ganesan, Shenyang Huang and Branko N. Popov Department of Chemical Engineering University of South Carolina, Columbia, SC 29208 Unitized regenerative fuel cells (URFCs) are electrochemical cells which operate both as a fuel cell and water electrolyzer using hydrogen as the energy medium. The URFC is an excellent, clean, quite energy source and a promising energy storage system for uninterrupted power supplies, solar powered aircrafts and satellites. Despite their apparent advantage, URFCs are still in their early stages of development because of several limiting factors. In conventional PEM fuel cells, catalysts are supported on porous carbon black with high surface area to reduce the noble metal loadings. However, high potentials at the BOE in URFCs during electrolysis mode lead to severe carbon corrosion via the following reaction:

Analysis of Molten Carbonate Fuel Cell Performance Using a Three-Phase Homogeneous Model

In this study a three-phase homogeneous model was developed to simulate the performance of the molten carbonate fuel cell ~MCFC! cathode. The homogeneous model is based on volume averaging of different variables in the three phases over a small volume element. This approach can be used to model porous electrodes as it represents the real system much better than the conventional agglomerate model. Using the homogeneous model the polarization characteristics of the MCFC cathode was studied under different operating conditions.

Development of High Durability Bi-functional Oxygen Electrode for Unitized Regenerative Fuel Cell (URFC)

Performance, durability and cost are the most important considerations for the successful application of unitized regenerative fuel cell (URFC). Among them, the durability of the URFC system can be improved by developing structurally stable bi-functional oxygen electrode (BOE), gas diffusion layer (GDL), and bipolar plate which are capable of withstanding the high potentials when the URFC is operated in the water electrolyzer mode. In one of our previous studies [1] we optimized the electrochemical performance of Pt black with varying amounts of Ir black as BOE catalysts. Rotating ring disc electrode (RRDE) and URFC studies indicated that 85 wt.% Pt black with 15 wt.% Ir black is an optimum catalyst composition and can be used as a BOE catalyst. The unsupported Pt85Ir15 black catalyst showed the highest round-trip energy conversion efficiency (e RT = 49%), as shown in Table 1. The conventional carbon based bipolar plates have poor mechanical strength, poor manufacturability, high cost and high corrosion rate in the high potential region [2, 3] and are not suitable for URFC application. Specifically, carbon corrosion decreases the thickness of the bipolar plate leading to poor electrical contact with the gas diffusion media and subsequent increase in the ohmic resistance. In addition, oxidation of the carbon surface decreases the hydrophobic property and affects water management, leading to increased mass transport losses. This effect causes the stability of URFC performance to degrade quickly. In order to solve these problems, noble metal coated titanium metal bipolar plate was developed and used in this study. Fig. 1 shows the surface morphology of titanium based bipolar plate after noble metal coating. Fig. 2 shows the performance of URFC performance obtained using titanium bipolar plates before and after noble metal coating. The performance of the unit cell is significantly increased with the noble metal coating on titanium bipolar plate due to the decreased ohmic resistance. Thus it is found that the application of nanolayers of noble metal on the titanium bipolar plates can be effective for the improvement of cell performance by inhibiting the formation of passive oxide layer on the surface of titanium based bipolar plates. The overall objective of this research is to develop high durability Pt85Ir15 black based BOE catalyst layer which will have no or minimum performance degradation during cycling test (continuous operation in both fuel cell and water electrolyzer modes). For these studies, new electrode with high performance and durability, high sustainable GDL, and high corrosion resistant bipolar plate are essential. In order to make high durability BOE, the structural stability and loss of hydrophobicity in the BOE will be improved by introducing the hydrophobic PTFE into the catalyst layer [4]. This study will involve: (i) optimization of the hydrophobic/hydrophilic character by selecting appropriate amounts of Teflon® and Nafion® (ii) decrease of the catalyst loading and the catalyst layer thickness. For the preparation of high sustainable GDL, graphitized carbon, which is less corrosive in the high potential region, will be used in the microporous layer. The performance of the developed catalyst layer will be evaluated in an URFC. Cycling test will be carried out according to the conditions exist at the low earth orbit (LEO). The performance and the cycling test results of the developed catalyst layer will be presented at the meeting.

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.

OPTIMIZATION OF THE CATHODE LONG-TERM STABILITY IN MOLTEN CARBONATE FUEL CELLS: EXPERIMENTAL STUDY AND MATHEMATICAL MODELING

This project focused on addressing the two main problems associated with state of art Molten Carbonate Fuel Cells, namely loss of cathode active material and stainless steel current collector deterioration due to corrosion. We followed a dual approach where in the first case we developed novel materials to replace the cathode and current collector currently used in molten carbonate fuel cells. In the second case we improved the performance of conventional cathode and current collectors through surface modification. States of art NiO cathode in MCFC undergo dissolution in the cathode melt thereby limiting the lifetime of the cell. To prevent this we deposited cobalt using an electroless deposition process. We also coated perovskite (La{sub 0.8}Sr{sub 0.2}CoO{sub 3}) in NiO thorough a sol-gel process. The electrochemical oxidation behavior of Co and perovskites coated electrodes is similar to that of the bare NiO cathode. Co and perovskite coatings on the surface decrease the dissolution of Ni into the melt and thereby stabilize the cathode. Both, cobalt and provskites coated nickel oxide, show a higher polarization compared to that of nickel oxide, which could be due to the reduced surface area. Cobalt substituted lithium nickel oxide (LiNi{sub 0.8}Co{sub 0.2}O{sub 2}) and lithium cobalt oxide were also studied. LiNi{sub x}Co{sub 1-x}O{sub 2} was synthesized by solid-state reaction procedure using lithium nitrate, nickel hydroxide and cobalt oxalate precursor. LiNi{sub x}Co{sub 1-x}O{sub 2} showed smaller dissolution of nickel than state of art nickel oxide cathode. The performance was comparable to that of nickel oxide. The corrosion of the current collector in the cathode side was also studied. The corrosion characteristics of both SS304 and SS304 coated with Co-Ni alloy were studied. This study confirms that surface modification of SS304 leads to the formation of complex scales with better barrier properties and better electronic conductivity at 650 C. A three phase homogeneous model was developed to simulate the performance of the molten carbonate fuel cell cathode and the complete fuel cell. The homogeneous model is based on volume averaging of different variables in the three phases over a small volume element. This approach can be used to model porous electrodes as it represents the real system much better than the conventional agglomerate model. Using the homogeneous model the polarization characteristics of the MCFC cathode and fuel cell were studied under different operating conditions. Both the cathode and the full cell model give good fits to the experimental data.

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.