PEFC Cathode Research Articles

Impacts of Carbon Corrosion on Cell Performance Decay

In a previous work, authors demonstrated a new carbon corrosion mode caused by rapid voltage rises in PEFC cathode. In this study detailed investigations for the carbon corrosion are conducted with full-scale cells. Test results suggest that higher potential sweep rate and larger potential gain which relates to rapid voltage raises during start-up could accelerate carbon corrosion significantly, without local or global fuel starvation. Impacts of carbon corrosion on material degradation and cell performance decay are also evaluated. The results indicate that voltage cycles between start-up and shutdown, which advances carbon corrosion on the cathode, accelerates cell performance decay with electrochemical surface area reduction and mass transport loss on the cathode. The voltage cycle testing focusing on carbon corrosion is expected to be one of the most promising test protocols for accelerated durability testing.

Dissolution of Platinum and Platinum Alloy PEFC Cathode Electrocatalysts

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Non-Platinum Cathode Based on Tantalum for PEFC

Ta-based compounds for PEFC cathode were prepared by R.F. sputter using TaC target under Ar (Ta-C) or Ar+N2 gas mixture (Ta-C-N) by heating a substrate in the range from room temperature to 800oC. The Ta-C-N compounds were found to be stable in 0.1moldm-3 H2SO4. The catalytic activity of the Ta- C-N compounds for the oxygen reduction reaction (ORR) increased with the increase in the substrate temperature. The ORR current at 0.4V of the Ta-C-N compound deposited at 800oC was about 75mAcm-2 and 125 times larger than that at room temperature. The ORR of the Ta-C-N compound deposited at 800oC was observed at about 0.73V vs. RHE. The physical properties, such as a crystallinity, crystal structure and chemical composition, of the Ta-C-N compounds were investigated by XRD and RBS analysis. The relationship between the physical properties and the catalytic activity for the ORR was discussed.

Hydrogen Peroxide Formation as a Degradation Factor of Polymer Electrolyte Fuel Cells

20wt.% Pt/C catalyst was loaded at different densities (0.28- 56.7 mgcarbon cm-2) on glassy carbon (GC) disk electrode, and the effect of its agglomeration on hydrogen peroxide formation in oxygen reduction reaction (ORR) was investigated by the rotating ring-disk electrode technique. The formation of H2O2 was enhanced with a reduction in agglomeration of Pt/C. Even in the operating potential range of PEFC cathodes (0.6-0.8 V), 20% of hydrogen peroxide was formed on Pt/C loaded at densities lower than 1.41 mgcarbon cm-2 on GC. These results revealed that series 2-electron reduction pathway, which is negligible on clean bulk Pt surface, exists on Pt nanoparticles supported on carbon. A large amount of H2O2 formation was observed on Pt nanoparticles deposited not only on GC, but also on Au, which indicated that H2O2 formation is not specific to Pt nanoparticles on carbon support.

A New Non-Precious Cobalt-Composite Electrocatalyst for PEFC Cathodes

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Surface conductivity and stability of metallic bipolar plate materials for polymer electrolyte fuel cells

Different types of commercial stainless steels, Ni-based alloys and nitride-coated steels were evaluated as metallic bipolar plate in terms of interfacial contact resistance (ICR) and corrosion resistance in conditions typical of PEFC anode and cathode environments. Results show that stainless steel have a high ICR and undergo corrosion in both anode and cathode. Moreover, although Ni-based alloys showed an ICR comparable with graphite, their behaviour was not satisfactory in corrosive acidic medium. Only nitride-coated stainless steel demonstrated to have low ICR and very good corrosion resistance.

Influence of the composition on the structure and electrochemical characteristics of the PEFC cathode

The influence the composition of the cathode has on its structure and electrochemical performance was investigated for a Nafion content spanning from 10 to 70 wt.%. The cathodes were formed on a Nafion membrane by the spray method and using 20 wt.% Pt on Vulcan (E-TEK). Materials characterisation (SEM, STEM, gas and mercury porosimetry, electron conductivity) and electrochemical characterisation (steady-state polarisation curve, impedance spectroscopy in O 2 and current-pulse measurements in N 2) were performed. The impedance spectra were analysed using our dynamic agglomerate model. The results indicate that the agglomerate model is valid until a Nafion content of about 45 wt.%. Pt/C and Nafion are homogeneously mixed for any composition and no Nafion film was observed. The cathodes containing 36–43 wt.% Nafion display a single or double Tafel slope behaviour ascribed to diffusion limitations in the agglomerates. At larger Nafion content, the agglomerate model can describe the curves only by assuming a diffusion coefficient 3–4 decades smaller than that of gases. At such compositions, the porosity was only 10%. These results were interpreted as a blocking of the pores and a non-percolating pore system for too large Nafion contents.

PEFC Cathode Alloy Catalysts Prepared by a Novel Process Using a Chelating Agent

A new Fe-Pt alloy catalyst for the oxygen reduction reaction of PEMFCs was prepared using a chelating agent, EDTA-Fe(III), as its non-precious metal source. Characterization showed that while the conventional catalyst showed particle aggregation over 5 nm, the new catalyst had a uniform particle size and a larger surface area. This new catalyst showed a higher catalytic mass activity than a conventional alloy catalyst although the specific activities are in the same range.

Simulations of PEFC cathodes : an effectiveness factor approach

The key parameters describing the operation mechanism of an active layer of PEFC cathodes were discussed.The macrohomogeneous and the agglomerate model were applied to different kinds of oxygen cathodes. From the simulations, it appears that gas pores must exist, even within real active layers thinner than 10 μm. The more appropriate model is the agglomerate model. An effectiveness factor approach allowed determination of the main factors limiting the oxygen cathode performance. Actual solutions for further improvements of the catalyst utilization were derived from simulations.