Abstract:In this study, a semi-empirical model is presented that correlates to previously obtained experimental overpotential data for a high temperature polymer electrolyte membrane fuel cell (HT-PEMFC). The goal is to reinforce the understanding of the performance of the cell from a modeling perspective. The HT-PEMFC membrane electrode assemblies (MEAs) were constructed utilizing an 85 wt. % phosphoric acid doped Advent TPS ® membranes for the electrolyte and gas diffusion electrodes (GDEs) manufactured byReactive Spray Deposition Technology (RSDT). MEAs with varying ratios of PTFE binder to carbon support material (I/C ratio) were manufactured and their performance at various operating temperatures was recorded. The semi-empirical model derivation was based on the coated film catalyst layer approach and was calibrated to the experimental data by a least squares method. The behavior of important physical parameters as a function of I/C ratio and operating temperature were explored. OPEN ACCESSCatalysts 2015, 5 1674
Novel tri-N-heterocyclic carbene (triNHC)-coordinated iridium catalysts (Ir(triNHC)) were employed for generating clean hydrogen and useful C n acids from biomass-derived alcohols. The high efficiency and fast rate of hydrogen production (maximum turnover frequency = 13,080 h–1 and 485 L of H2/g-cat·h) from sustainable ethylene glycol were elucidated by the increased electron-richness of Ir(triNHC) complexes where three NHC ligands were coordinated to an iridium(I) ion. Elaborated mechanistic studies support the proposed reaction mechanisms forming H2 and C n acids. This Ir(triNHC)-catalyzed process converting sustainable alcohols to useful fuels and chemicals is a promising carbon-neutral process.
In this study, catalyst layers with varying ratios of binder to carbon support material (I/C) for use in a high temperature phosphoric acid fuel cell were examined. The catalyst layers were deposited onto gas diffusion electrodes (GDEs) using Reactive Spray deposition technology (RSDT). RSDT is a flame based process which produces nanoscale particles of platinum through the combustion of a precursor solution containing metal organic compounds. The resulting GDEs were used to construct a 5x5 cm single cell utilizing an 85wt% phosphoric acid doped Advent membrane. Polarization scans were measured in the presence of oxygen for varying ratios of binder to carbon support material. Numerical tools have been developed based on the thin film modeling approach for catalyst layer performance and links between important physical parameters of the model and the observed performance behavior are discussed.
Recent progress has been made on PEMFC operating from 100oC to 200oC under ambient pressure. This strategy is recognized as a promising solution to the current limitation of low temperature (below 100oC) technique. The advantages of operation at such elevated temperature are: 1) improved kinetics of cathode and anode reaction; 2) improved tolerance of anode fuel contaminants; 3) no humidification of fuel needed and better life time in sub-zero temperature; 4) improved gas diffusion, solubility and permeability; 5) simplified cooling system and increase of power density [1-3]. However, many challenges for materials come with the advantages. For example, carbon support corrosion is accelerated, especially for phosphoric acid-doped membrane fuel cells. Catalyst sintering and dissolution also happen along with the carbon corrosion [3]. For >150oC temperature operation, Nafion membrane suffer from dehydration and loss of proton conductivity. PBI-based membranes, together with other sulfonated polyaromatic electrolytes, are typically used because of the high mechanical strength at high temperature and retain of phosphoric acid in the polymer chain network for proton transport [3]. Long-time durability has also been proved form PBI-based membranes [3]. Although we may have the right membrane to work with, the role of ionomer played in the catalyst layer (CL) and its compatibility with the membrane cannot be neglected [3]. In Nafion-based membrane fuel cells, the ionomer serves as a proton conductor forming a triple-phase boundary [5], whereas in phosphoric acid-doped membrane fuel cells, the ionomer is a binder that helps maintaining the phosphoric acid in the CL [6]. In addition, the interfacial resistance between the CL and proton exchange membrane (PEM) is found to be dependent on the choice of ionomer [3]. For phosphoric acid-doped PBI-based PEMFC, different types of ionomer have been reported, such as Nafion, polyvinylidene difluoride (PVDF), PBI and a combination of PBI and PVDF [6,7]. Studies on the PBI content in the CL have shown that the ionomer is able to penetrate to the primary pores of carbon and the mean pore size increase slightly with the increase of ionomer content. However, an optimal content of around 3 wt% [6] and 10-20 wt% [7] is independently concluded from the two studies. In this study, we attempt to investigate the effect of ionomer type and content on the pore structure, oxygen and air diffusivity and overall cell performance. We employed a one-step dry CL deposition method, reactive spray deposition technology (RSDT), to fabricate catalyst coated membrane (CCM), in which a thin-film CL was directly deposited onto the PEM through a flame-initiated mechanism. RSDT not only has more precise control over the electrode microstructure and component-level dispersion, but also eliminates several traditional manufacturing steps than ink-based deposition process [8]. An Air quench was incorporated into the RSDT process that allows for the introduction of carbon and ionomer at lower temperatures than the primary flame zone, which prevents thermal damage during deposition. The membranes under study included a Celtec-P PBI-based membrane and a pyridine and polyaromatic ether membrane. Nafion, PVDF and PBI were selected as the binder. Catalyst layers with platinum loading of 0.3 mg/cm2 and electrode thickness of ~15 µm were produced; see Figure 1. Transmission electron microscopy and energy dispersive X-ray spectroscopy were used to investigate the catalyst particles and the ionomer distribution. Pore size distribution was obtained through nitrogen adsorption. Single-cell test was performed on a 25 cm2CCM at 160oC at ambient pressure. Gas transport property during the fuel cell test was investigated by electrochemical impendence spectroscopy. Figure 1. a)CCM cross section structure prepared by RSDT with PVDF as binder; b) Pore size distributions for CCM with varying Nafion/Carbon weight ratio. References: [1] Chandan, A. et al. J. Power Sources, 2013, 231, 264-278. [2] Zhang, J. et al. J. Power Sources, 2006, 160, 872-891. [3] Shao, Y. et al. J. Power Sources, 2007, 160, 872-891. [4] Li, Q. et al. Prog. Polym. Sci., 2009, 34, 449-477. [5] Song, Y. et al. J. Power Sources, 2006, 154, 138-144 [6] Lobato, J. et al. Int. J. Hydrogen Energy, 2010, 35, 1347-1355. [7] Kim, J. et al. J. Power Sources, 2007, 170, 275-280. [8] Roller, J. et al J. Electrochem.Soc., 2013,160, F716-F730 (2013).
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