Polymer Electrolyte Fuel Cells PEFCs Research Articles

Tomographic Analysis of Polymer Electrolyte Fuel Cell Catalyst Layers: Methods, Validity and Challenges

Tomographic analysis of catalyst layers is one of the key methods to evaluate morphological properties in polymer electrolyte fuel cell PEFC catalyst layers. The most important applications for tomographic reconstruction are process control during manufacturing and investigations of ageing phenomena. We discuss all methods for tomographic imaging that are applicable for PEFC catalyst layers with an emphasis on focused ion beam / scanning electron microscopy tomography (FIB-SEMt). For now, no single tomographic method suffices to image all morphological properties of interest. Multi-tomographic approaches are therefore necessary. Phase fractions and phase-distributions are the most important properties to calculate within a tomographic reconstruction. We review further parameters that may be extracted from tomographic reconstructions and discuss open issues in first modeling approaches. Finally we discuss the validity of analyses within tomographic representations.

Durable Pt Catalyst Using Novel Composite Support of Ordered Mesoporous Carbon and Silicon Carbide for Polymer Electrolyte Fuel Cell

In terms of low durability of membrane electrode assembly (MEA) for polymer electrolyte fuel cell PEFC, the corrosion of carbon support is considered as one of the main factors. Carbon is thermodynamically unstable and may be prone to oxidation under the chemical and electrochemical conditions at both electrodes in the MEA. This corrosion of the carbon support results in the detachment of catalytic metal particles from supported catalyst, which may lead to a loss of catalytic activity and decreasing cell performance.Thus, stable alternatives such as metal carbide, nitride and oxide have been explored to replace the carbon support in the MEA in the past decades [1]. Among the carbides, silicon carbide (SiC) has been proposed to be an interesting possible catalyst support due to high temperature stability up to 1200 oC in an oxidative environment, hardness, as well as chemical inertness [2]. However, the SiC as a support still has the barriers due to a low specific surface area of 55-75 m2/g and a low conductivity around 10-6S/cm.In this study, ordered mesoporous carbon (OMC) [3,4] having large surface area, highly interconnected mesopores and graphitic framework microstructures, is hybridized with SiC via a nano-replication method to stabilize the OMC by SiC under electrochemical environments, which are designated as OMC-SiC. The degradation loss of Pt/OMC-SiC composite for catalytic activity for oxygen reduction reaction (ORR) is lower than that of Pt/OMC and commercial Pt/C catalyst in voltage-cycling test between 0 and 1.4V, which is attributed to the dispersed SiC inside the OMC particles. The OMC-SiC composite were synthesized using ordered mesoporous silica (OMS) as a template. The 1,10-phenanthroline as a carbon source precursor and para-toluene sulfonic acid as a acid catalyst for polymerization was infiltrated into the OMS template and transferred to an alumina crucible. The mixture was then ramped up to 900 oC for general OMC and 1350 oC forOMC-SiC composite, respectively, and kept for 2 h under nitrogen flow. The resulting sample was then washed twice with hydrofluoric acid at room temperature for 1h to remove the OMS template.As shown in Fig. 1, the shape of OMC-SiC showed the rod-like particles, which is the same as the OMC as well as OMS template (not shown here). The elemental mapping images show the Si and C was highly dispersed in the OMC particle, indicating that SiC was formed on the surface of carbon framework.The changes of the ORR activity, described as a current density at the 0.9V of the polarization curve are displayed in the Fig.2 for Pt/OMC, Pt/OMC-SiC, respectively, which was prepared using above OMC and OMC-SiC supports via the polyol method [3]. Comparing the initial ORR activity, the Pt/OMC-SiC showed a comparable activity to the commercial Pt/C catalyst as shown in Fig. 2, but much superior ORR activity than that of Pt/OMC, which indicates the incorporation of SiC in the OMC support can enhance the catalytic activity. The current density of ORR is decreased after 1000 cycles of potential. The decay rate of Pt/OMC and commercial Pt/C is 32.6% and 33.4%, respectively, while that of Pt/OMC-SiC is only 0.16%, which suggests that the electrochemical stability of the catalysts for ORR enhanced significantly by the existence of SiC on the surface of OMC as suggested from the above results. Thus, after the ADT, the ORR activity of Pt/OMC-SiC displayed much higher activity than that of commercial Pt/C and Pt/OMC as shown in Fig. 2. Therefore, the OMC-SiC composite support is a promising support material for highly stable cathode catalysts of PEMFC.In summary, OMC-SiC composite as a novel support material with high surface area and ordered mesopore for cathode catalysts is investigated to improve their electrochemical stability under high potential region by nano replication synthesis using a template such as OMS. It is anticipated that the OMC-SiC composite support can be helpful to develop for cathode catalysts in fuel cell and this hybridization method can expand the diversity of the carbon supports and carbide materials.

In Situ Imaging of Liquid Water and Ice Formation in an Operating PEFC during Cold Start

Cold-start capability and survivability of polymer electrolyte fuel cells PEFCs in a subzero environment remain a major challenge for automotive applications. Its fundamental mechanisms are not fully determined, but it is recognized that product water becomes ice or frost upon startup when the PEFC internal temperature is below the freezing point of water. If the local pore volume of the cathode catalyst layer CL is insufficient to contain all of the accumulated water before the cell operating temperature rises above freezing, solid water may plug the catalyst layer and stop the electrochemical reaction by starving the reagent gases. In addition, ice formation may result in serious damage to the structure of a membrane electrode assembly MEA. In spite of the importance of PEFC cold-start capability and associated MEA durability, very few studies in the literature have focused on PEFC startup dynamics, freeze/thaw cycling,