Novel electrodes with enhanced performance for proton exchange membrane fuel cell applications
Date of Issue2013
School of Mechanical and Aerospace Engineering
As a promising substitution to conventional power sources rely on combustion of fossil fuels, proton exchange membrane fuel cells (PEMFC) have attracted plenty of interest for various applications such as power sources for portable electronics, stationary power generation, and onto mobiles. At the present time, PEMFCs are still facing the challenges of cost as well as performance issues before they can be largely commercialized. One of the high cost factors comes from the significant amount of platinum (Pt) used in the PEMFC, especially at the cathode side due to the sluggish kinetic towards oxygen reduction reaction (ORR). The objective of this study is focus on understanding factors limiting the efficient use of the electrocatalyst in PEMFCs, and more importantly developing and optimizing new methods to prepare electrodes that are able to overcome cost and cathode kinetic limitations. More technically and specifically, this study is focused on reducing cathode Pt loading without sacrificing the fuel cell performance due to the sluggish kinetic and high Pt usage at the cathode. In state of the art PEMFCs, Pt supported on carbon black is used as an electrocatalyst. Catalyst support strongly affects the electrical properties of the catalyst layer. In order to reduce the Pt loading as well as improve the catalyst performance, carbon nanofibers (CNFs) directly grown on carbon paper is produced to act as novel catalyst support. This structure ensures that all the catalyst particles are in firm electrical contact with the carbon paper backings. Furthermore, CNFs possess higher electric conductivity, larger surface area and better corrosion resistance compared with conventional carbon black catalyst supports. Due to these advantages the use of CNFs as catalyst support can improve the Pt catalyst utilization efficiency; the mass transfer properties of the catalyst layer and the durability. In the single cell test, Pt/CNFs electrode is able to produce 50% (0.14 mgPt/cm2) and 70% (0.07 mgPt/cm2) higher cathode Pt mass power output than that using Pt/Carbon black catalyst (0.4 mgPt/cm2). To further reduce the cathode Pt loading and improve the performance, a pulse current electrodeposition (PCE) based three-step method was developed and utilized to fabricate membrane-electrode assemblies (MEAs) cathodes with ultra low platinum loading. The purpose of this study is to improve the catalytic activity of platinum by alloying it with transition metals on oxygen reduction reaction occured at cathode side and to investigate the electrochemical and electrocatalytic characteristics of ternary alloys in PEMFCs. This approach involves an electrodeposition process to establish an improved catalyst layer structure with better catalyst utilization efficiency, followed with a galvanic displacement and electrochemical dissolution process to produce core shell structured Pt-based alloy to improve the catalyst mass activity. The control of catalyst distribution uniformity on a 5 × 5 cm2 electrode prepared by this novel three step process was studied. It was found that density and porosity of the carbon black substrate affect the catalyst layer thickness and thus influence the performance, PtFeNi catalyst prepared on a dense carbon black substrate produces a catalyst layer thickness of 3-5 μm, while catalyst prepared on a loose carbon black substrate exhibits a thickness of about 10 μm. The MEA prepared with a loose carbon substrate cathode performs twice higher than that with a dense carbon substrate in a single cell test. Different Pt loadings of 0.02, 0.035, 0.05, and 0.1 mgPt/cm2 were prepared and tested in MEA cathodes, the results demonstrate that the best performance was obtained at a cathode catalyst loading of 0.05 mgPt/cm2. The comparison of a 0.05 mgPt/cm2 PtFeNi cathode with a 0.4 mgPt/cm2 commercial Pt cathode was also studied, indicating a comparable performance for the two electrodes. In terms of mass power output the PtFeNi cathode exhibits a 8 times higher performance than that of Pt cathode, while preliminary durability test was also carried out on the PtFeNi MEA, a good stability of the catalyst was proven.
DRNTU::Engineering::Mechanical engineering::Power resources