Polymer Electrolyte Membrane Fuel Cell Cathode Research Articles

Novel non-Pt PEM Fuel Cell Cathode Catalysts Based on Heteropoly Acid Materials

Heteropoly acids were investigated to determine their activity as non-Pt oxygen reduction reaction (ORR) catalysts in polymer electrolyte membrane fuel cell cathodes (PEMFCs). Polarization curves, cyclic voltammetry, and impedance spectroscopy results determined that, of the HPAs tested, only Mo based HPAs are active for the ORR and that V substitutions can improve the activity. Power densities of 67mW at 0.2V were obtained using H5PMo10V2O40. The catalysts must first be reduced insitu before the HPA can reduce oxygen. The reduction potentials of the HPAs in the fuel cell environment were determined by cyclic voltammetry and provide an explanation for why no activity is seen above 0.55V. Impedance spectroscopy confirmed that V substitutions increase catalytic activity. The potential at which the HPA can be reduced by 4 electrons has been determined to be the limiting factor when using these catalysts for ORR in PEMFCs.

Process modification for coating SnO 2:F on stainless steels for PEM fuel cell bipolar plates

Our previous procedure for depositing SnO 2:F on stainless steels for interfacial contact resistance (ICR) reduction and as a protective coating for metal bipolar plates was modified by pre-etching and coating characterization. Only ferrite stainless steels acquired a good quality SnO 2:F coating. The modified SnO 2:F coating decreased the ICR over our previous coating results, confirming the beneficial effect of the pre-etching; however, the corrosion resistance we obtained was significantly reduced. For the pre-etched and coated AISI444 and AISI446, both dynamic and potentiostatic polarizations revealed that the corrosion resistance of the modified coated steels is not as good as that obtained with the original coating process. The pre-etched and coated AISI444 showed higher currents in the polymer electrolyte membrane fuel cell (PEMFC) anode environment, as compared with earlier coated (original) AISI444 results. It took 10–50 min to reach a stabilized current for the pre-etched and coated AISI446; moreover, the stable currents are higher than those for original coated AISI446. Inductive coupled plasma (ICP) analysis for dissolved metallic ions in the test solutions was in agreement with the polarization results. Additionally, both the polarization and ICP analysis results indicate that the PEMFC cathode environment is more corrosive to the coated steels (especially AISI444) than the PEMFC anode environment. The Auger electron spectroscopy (AES) investigation showed that the pre-etched and coated AISI444 had heavy dissolution in the PEMFC cathode environment, confirming that for this process it has enhanced corrosion over the PEMFC anode environment. Both coated AISI444 and coated AISI446 showed cracks and peel-off of the modified coating after the polarization. This is apparently the source for the higher anodic currents in the polarization curves as compared to our original process for coating the steels. The conclusion is that both the corrosion resistance in PEMFC environments and the adhesion of the modified SnO 2:F coating are challenges for further development.

Novel PEMFC Cathodes Prepared by Pulse Deposition

A pulse electrodeposition method of preparing thin platinum catalyst layers for polymer electrolyte membrane fuel cell (PEMFC) cathodes has been developed through surface activation of the gas diffusion layer (GDL) by a wetting agent. The performance of the catalyst layer was optimized by wetting agent type, immersion time in the wetting agent, and pulse deposition parameters such as total charge density, peak current density, and duty cycle ratio. The time played a more important role than the time in determining the electrode characteristics such as high concentration of Pt, smaller particle size, and loading. Pt cathodes prepared using a peak current density of with a duty cycle of 10.7% and total charge density of resulted in a thin platinum catalyst layer and uniformly distributed platinum nanoparticles on the GDL surface. Novel cathodes with Pt loading of prepared in the present study exhibited at .

SnO 2:F coated austenite stainless steels for PEM fuel cell bipolar plates

Austenite stainless steels (316L, 317L, and 349™) have been coated with 0.6 μm thick SnO 2:F by low-pressure chemical vapor deposition and investigated in simulated polymer electrolyte membrane fuel cell (PEMFC) environments. The results showed that substrate steel has a significant influence on the behavior of the coating. Coated 316L showed a steadily increasing anodic current in PEMFC environments, indicating that it is not suitable for this alloy/coating combination. Coated 349™ showed a cathodic current in the PEMFC anode environment, demonstrating its stability in the PEMFC cathode environment. Coated 317L exhibited a stable anodic current after a current peak (at ca. 14 min) in the PEMFC anode environment, and showed an extremely stable low current in PEMFC cathode environment, suggesting the possibility of using SnO 2:F coated 317L for PEMFC bipolar plate applications. ICP results on the corrosion solutions showed that the PEMFC anode environment is much more corrosive than the cathode one. Fresh 316L showed the highest Fe, Cr, and Ni dissolution rates, and coating with SnO 2:F significantly reduced the dissolution. Coating the 317L also showed a significant beneficial effect on the corrosion resistance in the PEMFC environments. Coating 349™ steel further improved the already excellent corrosion resistance of this alloy. Trace Sn ions were detected for all coated steels in PEMFC anode environment, but not in the cathode one. The influence of SnO 2:F on the interfacial contact resistance (ICR) is mixed. For 316L and 317L steels, a SnO 2:F coating reduced the ICR. For 349™ steel, the SnO 2:F coating increased the ICR.

On the Passivation of 349TM Stainless Steel in Simulated PEMFC Cathode Environment

Austenite stainless steel 349TM has been tested in a simulated polymer electrolyte membrane fuel cell (PEMFC) cathode environment, and a stable passive film was formed within 15 minutes of polarization. X-ray photoelectron spectroscopy (XPS) has been utilized to investigate these surface films on the steel. The surface oxide film formed in air (air-formed film) composed of iron oxides and chromium oxide, with a Fe-rich outer layer and a Cr-rich inner layer. Neither of them was a dominating species in the air-formed film. On the other hand, the passive film formed in the simulated PEMFC cathode environment was dominated by Cr-oxide. The thickness of the films was estimated to be 2.4 nm for air-formed film, and 3.0 nm for the passive film. The difference in morphology between fresh and passived steels was identified by means of atomic force microscopy (AFM).

On effective transport coefficients in PEM fuel cell electrodes: Anisotropy of the porous transport layers

This paper reviews the approach taken in the literature to model the effective transport coefficients – mass diffusivity, electrical conductivity, thermal conductivity and hydraulic permeability – of carbon-fibre based porous electrode of polymer electrolyte membrane fuel cells (PEMFCs). It is concluded that current PEMFC model do not account for the inherent anisotropic microstructure of the fibrous electrodes. Simulations using a 2-D PEMFC cathode model show that neglecting the anisotropic nature and associated transport coefficients of the porous electrodes significantly influences both the nature and the magnitude of the model predictions. This emphasizes the need to appropriately characterize the relevant anisotropic properties of the fibrous electrode.

Effect of Varying Electrolyte Conductivity on the Electrochemical Behavior of Porous Electrodes

Ionic conductivity in the air cathode of a polymer electrolyte membrane (PEM) fuel cell changes with distance depending on the ionomer loading [ J. Electrochem. Soc. , 150 , A1440 (2003) ]. When the ionic conductivity changes with distance inside a porous electrode, existing models in the literature fail to capture the behavior of porous electrodes. In this paper, we show how existing models in the literature can be modified to take care of varying ionic conductivity. The modified model is shown to be efficient in handling mathematical singularity arising from the distribution of ionic conductivity inside a porous electrode. In addition, the model developed was used to optimize the functionality of ionic conductivity (ionomer loading) to minimize the ohmic drop across a porous electrode. It is shown that depending on the system parameters, there might be an optimum way to distribute the ionomer loading inside the PEM fuel cell cathode.

2D and 3D Modeling of a PEMFC Cathode with Interdigitated Gas Distributors

A steady-state, two-dimensional model of the cathode of a polymer electrolyte membrane fuel cell (PEMFC) with interdigitated gas distributor is formulated with the intention of studying the effects of various operating parameters on its performance. The model describes the fluid dynamics of reactant gases and gives an insight to their distribution in the electrode. The model is verified with published data [ J. S. Yi and T. V. Nguyen , J. Electrochem. Soc. , 146 , 38 (1999) ], but an error has been detected in the data. This work corrects the error, and the results are presented. The effect of changes in electrode permeability, thickness, and shoulder width are then studied. It is observed that changes in permeability, ranging from to , has little impact on the cell performance. Both increasing the electrode thickness and the shoulder width result in poorer performance due to greater resistance to flow. In addition, results show that liquid water tends to form near the outlet of the electrode when the current density is greater than . The 2D model is then extended to a 3D one to determine the flow distribution in a fuel cell for an interdigitated gas distributor. A comparison in performance is made with this 3D model between an interdigitated/conventional gas distributor and it is found that an interdigitated gas distributor does improve cell performance over a conventional gas distributor.

Extent of PEMFC Cathode Surface Oxidation by Oxygen and Water Measured by CV

The influence of gas-phase oxygen on platinum surface oxidation in polymer electrolyte membrane fuel cell (PEMFC) cathodes was investigated. Catalyst layers were exposed to 4, 21, and 100% O 2 for a fixed time at +0.85 and +0.95 V R H E . The extent of surface oxidation was characterized using cyclic voltammetry (CV). The extent of Pt surface oxidation increased relative to the corresponding O 2 -free potential hold at +0.85 V R H E . The extent of oxidation was directly proportional to oxygen concentration, and increased with time exposed to the oxygen-containing gas. Pt surface oxidation was not influenced by gas-phase O 2 concentration at +0.95 V R H E .

Ferritic stainless steels as bipolar plate material for polymer electrolyte membrane fuel cells

Abstract Both interfacial contact resistance (ICR) measurements and electrochemical corrosion techniques were applied to ferritic stainless steels in a solution simulating the environment of a bipolar plate in a polymer electrolyte membrane fuel cell (PEMFC). Stainless steel samples of AISI434, AISI436, AISI441, AISI444, and AISI446 were studied, and the results suggest that AISI446 could be considered as a candidate bipolar plate material. In both polymer electrolyte membrane fuel cell anode and cathode environments, AISI446 steel underwent passivation and the passive films were very stable. An increase in the ICR between the steel and the carbon backing material due to the passive film formation was noted. The thickness of the passive film on AISI446 was estimated to be 2.6 nm for the film formed at −0.1 V in the simulated PEMFC anode environment and 3.0 nm for the film formed at 0.6 V in the simulated PEMFC cathode environment. Further improvement in the ICR will require some modification of the passive film, which is dominated by chromium oxide.