Murat Ünlü

Anion Exchange Membrane Fuel Cells: Experimental Comparison of Hydroxide and Carbonate Conductive Ions

Anion exchange membrane fuel cells operating near room temperature using hydroxide or carbonate as the conductive ions have been demonstrated and compared. A membrane electrode assembly was fabricated using a poly(arylene ether sulfone) polymer membrane functionalized with quaternary ammonium cations. The maximum power density using hydroxide as the conductive ion was 2.1 mW/cm 2 . When CO 2 was introduced to the cathode stream, the maximum power density increased to 4.1 mW/cm 2 . CO 2 was shown to be materially involved in the oxygen reduction reaction at room temperature and was transported from the cathode to the anode. CO 2 was shown to be consumed at the cathode and evolved from the anode. The performance was analyzed by electrochemical impedance spectroscopy.

Hybrid Polymer Electrolyte Fuel Cells: Alkaline Electrodes with Proton Conducting Membrane

Fuel cells have the potential to provide clean and efficient energy for stationary, traction, and portable applications. Among the various types of fuel cells, proton-exchange membrane (PEM) fuel cell has several desirable features including well established membranes and cell designs. Although PEM fuel cells have been used in numerous applications, there are several obstacles that impede widescale commercialization. These issues include the high cost of noble-metal catalysts and perfluorinated membranes, carbon monoxide poisoning, and limited lifetime due to membrane and electrode degradation. Recently, there is a growing interest in anion-exchange membrane (AEM) fuel cells operating at high pH. The highpH environment addresses many of the shortfalls experienced with PEM fuel cells. Alkaline cells can function with nickel and silver, and are resistant to CO poisoning. 7] Although AEM fuel cells have several advantages compared to protonbased fuel cells, the lower ionic conductivity of AEMs compared to commercially available Nafion is a concern because it may lower the performance. Moreover, the strong dependence of the AEM conductivity on humidity and the need for water in the cathode reaction are significant challenges that limit the performance of current AEM fuel cells. In an effort to use the high conductivity and established infrastructure of PEMs and still exploit the advantages of high-pH electrode operation—e.g., resistance to CO poisoning at the anode and use of non-platinum catalysts at the anode and cathode—we report here a hybrid fuel cell comprised of AEM electrodes and PEM core. The high-pH electrode (AEM electrode) was made using an anionexchange ionomer (AEI), poly(aryleneether sulfone), functionalized with quaternary ammonium groups synthesized for this study, as described previously. The AEM electrodes were pressed onto a PEM membrane, Nafion 212. The cell configuration is shown in Figure 1. Under alkaline conditions, the anode and cathode reactions are as follows:

Study of Alkaline Electrodes for Hybrid Polymer Electrolyte Fuel Cells

Hybrid polymer electrolyte fuel cells combine a proton exchange membrane (PEM) and anion exchange membrane (AEM) and exhibit unique advantages. The electrochemical performance and current limitations of a hybrid cell with an alkaline electrode fabricated on a Nafion membrane was characterized at 70°C as a function of relative humidity. To separate the individual characteristics of the AEM anode and cathode electrodes, two semihybrid cell configurations, each with an AEM and PEM electrode, were constructed and compared to a conventional PEM fuel cell. Electrochemical impedance spectroscopy was used to diagnose the cell component characteristics. In particular, low catalyst utilization in the AEM electrode was a primary factor in the performance losses in the hybrid cells.

Characterization of Anion Exchange Ionomers in Hybrid Polymer Electrolyte Fuel Cells

Anion exchange ionomers (AEI) synthesized here were characterized by use of a novel fuel cell configuration. The new analysis method involves assembling the AEI electrode of interest as the cathode in a hybrid, acid/alkaline, fuel cell configuration. The hybrid cell includes a conventional proton conducting anode/membrane half-cell along with the anionic conductor of interest at the cathode. Electrochemical impedance spectroscopy and voltammetry were used to evaluate the performance of the hybrid AEI-containing fuel cell with H₂ and O₂. In particular, the AEI electrode response in impedance spectroscopy was easily identified because the contributions from other components are largely minimized in the presented hybrid cell configuration. The properties of ionomers used in the AEI electrode were shown to have a substantial effect on the electrode performance. Low catalyst utilization, due to high water uptake and low conductivity, was identified as the major causes of poor performance in AEI electrodes.