Zhiqing Shi

Discrepancies in the Measurement of Ionic Conductivity of PEMs Using Two- and Four-Probe AC Impedance Spectroscopy

Two- and four-probe cells were designed for comparative studies of ionic conductivity of proton conducting membranes (PEMs) using electrochemical impedance spectroscopy. Nafion 115 membrane was employed to examine the influence of cell configuration, probe geometry, and arrangement of probe. Whereas a single arc and linear responses, were observed in Nyquist plots using the 2-probe cell, multiple arcs were observed using the 4-probe cell. The linear response observed in the 2-probe configuration and the low frequency arc observed in the 4-probe configuration are due to transmission line behavior that results from a distributed interfacial capacitance coupled with the membranes ionic resistance at the Pt/PEM interfacial region. Increasing the distance between the probes and/or reducing the electrode contact area reduces or eliminates these low-frequency artifacts so that accurate data for ionic resistance can be obtained. Equivalent circuits for 2-probe and 4-probe cell geometries are constructed and used to extract conductivity data.

Hydrocarbon proton conducting polymers for fuel cell catalyst layers

Proton exchange membrane fuel cells (PEMFCs) employing proton conducting membranes are promising power sources for automotive applications. Perfluorosulfonic acid (PFSA) ionomer represents the state-of-the-art polymer used in both the membrane and catalyst layer to facilitate the transport of protons. However, PFSA ionomer is recognized as having significant drawbacks for large-scale commercialization, which include the high cost of synthesis and use of fluorine-based chemistry. According to published research much effort has been directed to the synthesis and study of non-PFSA electrolyte membranes, commonly referred to as hydrocarbon membranes, which has led to optimism that the less expensive proton conducting membranes will be available in the not-so-distant future. Equally important, however, is the replacement of PFSA ionomer in the catalyst layer, but in contrast to membranes, studies of catalyst layers that incorporate a hydrocarbon polyelectrolyte are relatively sparse and have not been reviewed in the open literature; despite the knowledge that hydrocarbon polyelectrolytes in the catalyst layer generally lead to a decrease in electrochemical fuel cell kinetics and mass transport. This review highlights the role of the solid polymer electrolyte in catalyst layers on pertinent parameters associated with fuel cell performance, and focuses on the effect of replacing perfluorosulfonic acid ionomer with hydrocarbon polyelectrolytes. Collectively, this review aims to provide a better understanding of factors that have hindered the transition from PFSA to non-PFSA based catalyst layers.

Fuel cell cathode catalyst layers from “green” catalyst inks

Fuel cell cathode catalyst layers deposited from a water-based catalyst ink formulation, using high water content and minimum volatile organic compounds, are investigated. Cathodes fabricated from a dispersion medium containing 96 wt% water are compared with cathodes fabricated from conventional alcohol-based inks containing 1-propanol–water 3 : 1 (w/w). The morphology of the two catalyst layers are similar, as are electrochemically-active surface areas at relative humidities of 100, 70 and 30% RH. Oxygen reduction kinetics obtained under fully humidified H2/O2 conditions exhibit similar Tafel slopes, 67 ± 3 mV per dec. However, cathodes prepared from water-based inks exhibit a lower H2/air fuel cell performance under 100, 70 and 30% RH while its porosity, obtained using mercury porosimetry, is slightly higher. EIS measurements obtained under high current density indicate that the mass transport resistance of the water-based catalyst layer is lower, which is consistent with porosimetric data, and suggests that factors other than mass transport limit the performance of the water-based cathode. The protonic resistance of the catalyst layers was found to be 105 and 145 mΩ cm2 for the propanol- and water-based catalyst layers, respectively. The differences are more pronounced when RH is decreased from 100 to 30%. This trend is consistent with the observed decrease in fuel cell performance under conditions of lower RH, and indicates that the higher proton resistance of the water-based catalyst layer is the cause of its lower fuel cell performance.

Factors Influencing Electrochemical Properties and Performance of Hydrocarbon-Based Electrolyte PEMFC Catalyst Layers

Cathode catalyst layers (CLs) for proton exchange membrane fuel cells (PEMFCs) incorporating sulfonated poly(ether ether ketone) (SPEEK) solid polymer electrolyte were prepared and studied in a H 2 /O 2 fuel cell operated at 50°C and 100% relative humidity. SPEEK-based CLs were found to exhibit higher protonic resistance, lower effective usage of Pt, and lower fuel cell performance compared to Nafion-based cathodes. A method of fabrication was developed to achieve a homogeneous distribution of SPEEK and polytetrafluoroethylene (PTFE) throughout the catalyst layer. Homogeneously prepared SPEEK-based CLs exhibited a higher electrochemically active surface area and lower protonic resistance but lower porosity and inferior water management compared to those prepared using a traditional two-step fabrication method, wherein SPEEK ionomer was impregnated into a preformed catalyst layer incorporating sintered PTFE and Pt/C. The choice of SPEEK electrolyte over Nafion was shown to adversely affect the bulk proton conductivity of the electrolyte inside the catalyst layer. This can be offset by increasing the SPEEK content in the catalyst layer but with the penalty of increased flooding and a larger resistance to gas transport.

Microstructure–Performance Relationships of sPEEK-Based Catalyst Layers

Porous catalyst layers containing sulfonated polyetheretherketone (sPEEK, 1.4 meq g ―1 ) were deposited onto membranes to form catalyst-coated membranes. The catalyst layer properties, as a function of sPEEK content, were studied ex situ and in fuel cells. Particle sizes observed for sPEEK-catalyst inks were smaller than for corresponding Nafion-catalyst inks. Consequently, particle sizes in spray-coated catalyst layers were also smaller. Catalyst layers containing sPEEK were observed to densify with increasing ionomer content in contrast to Nafion-catalyst layers. The catalyst layer thickness decreased from 14 to 4 μm when the sPEEK content was increased from 10 to 40 wt %. Pore volumes also decreased and were lower than Nafion-catalyst layers. The optimum sPEEK content of the cathode layer for membrane electrode assemblies was 20 wt %. Lower sPEEK contents compromised proton conductivity, whereas higher sPEEK contents led to excessive flooding of the cathode. Relative humidity of the gases exerted a strong influence on the cathode and fuel cell performance. Too much hydration resulted in excessive flooding; too little led to a significant loss in proton conductivity. Although this paradox is typical of proton exchange membrane fuel cells, it is exacerbated by sPEEK for which water sorption and proton conductivity are highly sensitive to external conditions.

Water Permeation Through Catalyst-Coated Membranes

Water permeabilities, driven by concentration or pressure gradients, through NRE211 and catalyst-coated membranes are reported, and the effect of the catalyst layer (∼ 18 μm thick, 30 wt % Nafion ionomer, carbon-supported Pt, 0.4 mg Pt cm ―2 ) on membrane water permeation is deconvoluted. For the system studied, water permeation is limited by the bulk membrane; the effect of the catalyst layer was insignificant and, despite catalyst layers being deposited on the membranes surface, it does not influence the rates of sorption or desorption at membrane interfaces.

Image analysis and modeling of spherical and channel microstructures of fuel-cell materials

The development of novel fuel-cell materials demands accurate and flexible microstructure characterization techniques. Conventional electron microscopy-based microstructural morphology analysis is carried out through the conceptual interpretation of transmission electron microscope images. With this method, only qualitative information on material morphologies can usually be obtained. This paper presents a digital image analysis system that deals with the automatic measurement and quantitative characterization of the microstructural morphologies of polymer electrolyte membrane fuel-cell materials. In this approach, two types of essential microstructural morphologies (spheral particles and interconnected graft channels) are modeled based on statistical geometry theory, and the statistical analysis schemes of the microstructural morphologies are designed and applied to the characterization of the phase-separated microstructures in fuel-cell components such as solid electrolyte ionomers, catalyst layers, and gas diffusion layers. Experimental results on real fuel-cell materials specimens demonstrate the effectiveness of the method.