Influence of the PTFE content in the diffusion layer of low-Pt loading electrodes for polymer electrolyte fuel cells

The influence of the structure and composition of the diffusion layer on polymer electrolyte fuel cell (PEFC) cathode performance was investigated. Electrodes were prepared with different poly-tetrafluoroethylene (PTFE) content in the diffusion layer and maintaining a constant composition for the catalytic layer with a low-Pt loading (0.11 mg cm⁻²). Electrodes were characterized by Hg-intrusion porosimetry, scanning electron microscopy and electrochemical techniques (cyclic voltammetry, galvanostatic polarization and ac-impedance spectroscopy).     

Using electrochemical impedance spectroscopy to compensate for errors when measuring polarisation curves during three-electrode measurements of solid oxide fuel cell electrodes

The use of three-electrode techniques involving an independent reference electrode is invaluable in determining the overpotential losses at solid oxide fuel cell (SOFC) electrodes. However, there are numerous barriers to achieve the accurate measurement of such overpotentials in an SOFC. Furthermore electrochemical impedance spectroscopy (EIS) is commonly used to analyse the processes occurring on SOFC electrodes, and there has been considerable work in establishing viable three-electrode techniques for EIS experiments under open circuit conditions. However, the three-electrode techniques currently developed for EIS experiments are not well suited for conditions of high load, or changing gas compositions; either intentionally or under diffusion limiting conditions. This paper reports a solution for commonly used pellet cells, which mitigates these problems. The paper presents a method using EIS to correct for errors when measuring the working electrode overpotential during polarisation arising from a shift in the electrolyte current distribution from the primary to the secondary current distribution under load. This technique enables meaningful overpotentials to be calculated using experimentally simple cell geometries under conditions where they cannot normally be accurately measured.     

Transient analysis of polymer electrolyte fuel cells

A three-dimensional, transient model has been developed to study the transient dynamics of polymer electrolyte fuel cell (PEFC) operation. First, various time constants are estimated for important transient phenomena of electrochemical double-layer discharging, gas transport through the gas diffusion layer (GDL) and membrane hydration. It is found that membrane hydration occurs over a period of 10 s, the gas transport of 0.01-0.1 s, with the double-layer discharging being negligibly fast. Subsequently, extensive numerical simulations, with the transient processes of membrane hydration and gas transport taken into consideration, are carried out to characterize the dynamic response of a singe-channel PEFC with N112 membrane. The results show that the time for fuel cells to reach steady state is in the order of 10 s due to the effect of water accumulation in the membrane, consistent with theoretical estimation. In addition, overshoot or undershoot of the current densities is found during the step changes in some operating conditions, and detailed results are provided to reveal the dynamic physics of these phenomena.     

Investigation of fuel and media flexible laminar flow-based fuel cells

We investigate the performance of air-breathing laminar flow-based fuel cells (LFFCs) operated with five different fuels (formic acid, methanol, ethanol, hydrazine, and sodium borohydride) in either acidic or alkaline media. The membraneless LFFC architecture enables interchangeable operation with different fuel and media combinations that are only limited by the actual anode catalyst used. Furthermore, operating under alkaline conditions significantly improves methanol and ethanol oxidation kinetics and stabilizes sodium borohydride. LFFCs operated with hydrazine and sodium borohydride as fuels exhibit power densities of 80 and 101 mW/cm², respectively. To optimize anode performance, particularly for ethanol electro-oxidation, we introduced a hydrogen cathode to the membraneless LFFC design which renders the cell an ideal platform for anode investigation. Here, we highlight two simple diagnostic methods, in situ single electrode studies and electrochemical impedance spectroscopy (EIS), for characterizing and optimizing the performance of a direct ethanol LFFC anode.     

Modeling of CO2 gasification of carbon for integration with solid oxide fuel cells

This modeling study focuses on gasification of carbon by CO₂ in a minimally fluidized bed containing a solid oxide fuel cell (SOFC). Kinetic parameters for a five‐step reaction mechanism characterizing the Boudouard reaction (C + CO₂ → 2CO) were determined thermogravimetrically at 1 atm from 973 to 1273 K. Experimentally determined kinetic parameters are employed in a transport model that predicts velocities and gas concentration profiles established in the carbon bed as a consequence of convection, diffusion, and heterogeneous reaction. The model is used to simulate the effect of an imbedded SOFC, in contact with the carbon bed. Although the model does not assume particular I‐V characteristics for the fuel cell, it indicates that current densities in the practical range of 100–1000 mA/cm² can be supported. Results show that temperature strongly affects the current density, whereas CO₂ flow rate has only a weak effect. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

Simulation of membrane microreactor for fuel cell with methane feed

Microreactors are composed of channels with a dimension of 1–1000 μm in width and depth. Three types of membrane microreactors for production of electric power by fuel cells with methane feed, i.e. (1) Pd membrane microreactor (Pd-MMR) followed by polymer electrolyte fuel cell (PEFC), (2) oxide-ion conducting solid oxide fuel cell (SOFC) and (3) proton conducting solid oxide fuel cell (H⁺-SOFC) were simulated and compared.

The Pd-MMR followed by PEFC was the most effective system for electric power generation compared with the other two types of SOFC although the system was rather complex. However, the performances were dependent on the permeation properties of membranes (Pd, YSZ, perovskite) as well as kinetics of catalysts.     

Investigation of solid oxide fuel cell short stacks for mobile applications by electrochemical impedance spectroscopy

Solid oxide fuel cells (SOFC) for mobile applications are developed and investigated at the German Aerospace Center (DLR) in Stuttgart. Therefore a light-weight stack design was developed in cooperation with the automotive industry (BMW/Munich, Elring-Klinger/Dettingen, ThyssenKrupp/Essen) and the Research Center Jülich (FZJ). This concept is based on the application of stamped metal sheet bipolar plates, into which the SOFC cells are integrated by brazing technology. For the development and the investigation of the SOFC cells and short stacks, the electrochemical impedance spectroscopy (EIS) is an important and useful characterization method. The paper concentrates on the investigation and on the electrochemical testing of the SOFC short stacks with sintered anode-supported cells (ASC). The short stacks were electrochemically characterized mainly by electrochemical impedance spectroscopy, by current-voltage measurements and by long-term measurements. The cells and stacks were operated at different temperatures, varying fuel gas compositions, different fuel gas flow rates and at different electrical current loads. The influence of these operating conditions on the electrochemical performance of the short stacks is outlined. The nature of losses, e.g. ohmic and the polarization resistances of the electrodes were examined and determined by fitting of the impedance spectra to an equivalent circuit.     

Internally humidified polymer electrolyte fuel cells using water absorbing sponge

Polymer electrolyte fuel cell (PEFC) mounted with two strips of polyvinyl alcohol (PVA) sponge is presented and the effect of operating conditions on the cell performance is investigated. Mounting the sponge wicks is advantageous for the humidification of dry inlet air and for the removal of liquid water in the cell. It was found that dry inlet hydrogen could be internally humidified by water diffusion from the cathode to anode when operating in a counterflow mode. The results show that the relative humidity of the inlet gases could have little effect on the performance of the cell mounted with two sponge wicks under certain operating conditions. At a cell potential of 0.5 V, the current densities of the sponge-mounted PEFC operated with dry air are 5% and 31% higher than those of the conventional one without wicks operated with saturated and dry air, respectively. The molar percentage of water vapor to total water exiting the cathode (R_gas) is an important parameter to gauge the cell performance with dry gases. A very large R_gas may cause the membrane dehydration and subsequently a low cell performance.     

Effect of yttrium-stabilized bismuth bilayer electrolyte thickness on the electrochemical performance of anode-supported solid oxide fuel cells

Lowering operating temperature and optimizing electrolyte thickness, while maintaining the same high efficiencies are the main considerations in fabricating solid oxide fuel cells (SOFCs). In this study, the effect of yttrium-stabilized bismuth bilayer electrolyte thickness on the electrical performance was investigated. The yttrium-stabilized bismuth bilayer electrolyte was coated on the nickel–samarium-doped composite anode/samarium-doped ceria electrolyte substrate with varying bilayer electrolyte thicknesses (1.5, 3.5, 5.5, and 7.5 μm) via dip-coating technique. Electrochemical performance analysis revealed that the bilayer electrolyte with 5.5 μm thickness exhibited high open circuit voltage, current and power densities of 1.068 V, 259.5 mA/cm² and 86 mW/cm², respectively at 600 °C. Moreover, electrochemical impedance spectroscopy analysis also exhibited low total polarization resistance (4.64 Ωcm²) at 600 °C for the single SOFC with 5.5 μm thick yttrium-stabilized bismuth bilayer electrolyte. These findings confirm that the yttrium-stabilized bismuth bilayer electrolyte contributes to oxygen reduction reaction and successfully blocks electronic conduction in Sm_0.2Ce_0.8O_1.9 electrolyte materials. This study has successfully produced an Y_0.25Bi_0.75O_1.5/Sm_0.2Ce_0.8O_1.9 bilayer system with an extremely low total polarization resistance for low-temperature SOFCs.     

Studies of the performance of PEM fuel cell cathodes with the catalyst layer directly applied on Nafion membranes

The performance of a proton exchange membrane fuel cell (PEMFC) with gas diffusion cathodes having the catalyst layer applied directly onto Nafion membranes is investigated with the aim at characterizing the effects of the Nafion content, the catalyst loading in the electrode and also of the membrane thickness and gases pressures. At high current densities the best fuel cell performance was found for the electrode with 0.35 mg Nafion cm⁻² (15 wt.%), while at low current densities the cell performance is better for higher Nafion contents. It is also observed that a decrease of the usual Pt loading in the catalyst layer from 0.4 to ca. 0.1 mg Pt cm⁻² is possible, without introducing serious problems to the fuel cell performance. A decrease of the membrane thickness favors the fuel cell performance at all ranges of current densities. When pure oxygen is supplied to the cathode and for the thinner membranes there is a positive effect of the increase of the O₂ pressure, which raises the fuel cell current densities to very high values (>4.0A cm⁻², for Nafion 112—50 μm). This trend is not apparent for thicker membranes, for which there is a negligible effect of pressure at high current densities. For H₂/air PEMFCs, the positive effect of pressure is seen even for thick membranes.     

Characterization of membrane electrode assemblies in polymer electrolyte fuel cells using a.c. impedance spectroscopy

The most common methods used to characterize the electrochemical performance of fuel cells are to record current–voltage U(i) curves. However, separation of electrochemical and ohmic contributions to the U(i) characteristics requires additional experimental techniques. The application of electrochemical impedance spectra (EIS) is an approach to determine parameters which have proved to be indispensable for the development of fuel cell electrodes and membrane electrode assemblies (MEAs). This paper proves that it is possible to split the cell impedance into electrode impedances and electrolyte resistance by varying the operating conditions of the fuel cell (current load) and by simulation of the measured EIS with an equivalent circuit. Furthermore, integration in the current density domain of the individual impedance elements enables the calculation of the individual overpotentials in the fuel cell and the determination of the voltage loss fractions.     

A new technique for two-dimensional current distribution measurements in electrochemical cells

A new technique has been developed using a magnetic loop array to measure current distribution in electrochemical cells. The main advantage of this approach is the combination of high spatial and time resolution and stack integration with an easily handled measurement carried out independently of the cell operation. A polymer electrolyte fuel cell (PEFC) of technically relevant dimensions (about 600 cm² electrode area) with 5 × 8 current sensors has been constructed and operated, thus confirming the feasibility of the measurement technique.     

Ameliorating effect of silica addition in the anode-catalyst layer of the membrane electrode assemblies for polymer electrolyte fuel cells

Incorporation of silica particles through a sol-gel process into the anode-catalyst layer with a sol-gel modified Nafion-silica composite membrane renders easy retention of back-diffused water from the cathode to anode through the composite membrane electrolyte, increases the catalyst-layer wettability and improves the performance of the Polymer Electrolyte Fuel Cell (PEFC) while operating under relative humidity (RH) values ranging between 18% and 100% with gaseous hydrogen and oxygen reactants at atmospheric pressure. A peak power density of 300 mW cm⁻² is achieved at a load current-density value of 1200 mA cm⁻² for the PEFC employing a sol-gel modified Nafion-silica composite membrane and operating at 18% RH. Under similar operating conditions, the PEFC with a Membrane Electrode Assembly (MEA) comprising Nafion-silica composite membrane with silica in the anode-catalyst layer delivers a peak power density of 375 mW cm⁻². By comparison, the PEFC employing commercial Nafion membrane fails to deliver satisfactory performance at 18% RH due to the limited availability of water at its anode, acerbated electro-osmotic drag of water from anode to cathode and insufficient water back diffusion from cathode to anode causing the MEA to dehydrate.     

Deconvolution of electrochemical impedance spectra for the identification of electrode reaction mechanisms in solid oxide fuel cells

The polarization processes occurring at the electrode–electrolyte interfaces of solid oxide fuel cells (SOFC) were investigated by electrochemical impedance spectra measured at single cells under realistic operating conditions. The approach presented is based on distributions of relaxation times which are the basic quantity of interest in electrochemical impedance data analysis. A deconvolution method was developed and implemented that yields these characteristic distribution patterns directly from the impedance spectra. In contrast to nonlinear least squares curve fit of equivalent circuit models, no a priori circuit choice has to be made. Even more importantly, the excellent resolving capacity allows the untangling of the impedance contributions of up to three physically distinct processes within one frequency decade. With the method, processes with the highest polarization losses can be identified and targeted to improve cell performance. Based on the distributions, a general strategy for the identification of the reaction mechanisms is given. The evaluation of the distributions in terms of peak parameters is illustrated by a physical model for oxygen reduction at the SOFC cathode–electrolyte interface. The method is expected to find many applications in electrochemistry beyond the field of solid oxide fuel cell development.     

H2/Cl2 fuel cell for co-generation of electricity and HCl

A H₂/Cl₂ fuel cell system with an aqueous electrolyte and gas diffusion electrodes has been investigated and the effects of electrolyte concentration and temperature on the open circuit voltage (OCV) and cell performance have been evaluated. Furthermore, the kinetics and long-term stability of Pt as electrocatalyst have been studied under various conditions. In addition, the long-term stability of Rh electrocatalyst has been evaluated. The OCV obtained showed that the Cl₂ reduction is more reversible than O₂ reduction. The ohmic drop was determining the cell performance at high current densities. An output power of about 0.5 W cm^–2 was achieved with this system.     

Mo oxide modified catalysts for direct methanol, formaldehyde and formic acid fuel cells

Pt black and PtRu black fuel cell anodes have been modified with Mo oxide and evaluated in direct methanol, formaldehyde and formic acid fuel cells. Mo oxide deposition by reductive electrodeposition from sodium molybdate or by spraying of the fuel cell anode with aqueous sodium molybdate resulted in similar performance gains in formaldehyde cells. At current densities below ca. 20 mA cm⁻², cell voltages were 350–450 mV higher when the Pt catalyst was modified with Mo oxide, but these performance gains decreased sharply at higher current densities. For PtRu, voltage gains of up to 125 mV were observed. Modification of Pt and PtRu back catalysts with Mo oxide also significantly improved their activities in direct formic acid cells, but performances in direct methanol fuel cells were decreased.     

Performance degradation study of a direct methanol fuel cell by electrochemical impedance spectroscopy

A stability test of a direct methanol fuel cell (DMFC) was carried out by keeping at a constant current density of 150 mA cm⁻² for 435 h. After the stability test, maximum power density decreased from 68 mW cm⁻² of the fresh membrane-electrode-assembly (MEA) to 34 mW cm⁻² (50%). Quantitative analysis on the performance decay was carried out by electrochemical impedance spectroscopy (EIS). EIS measurement of the anode electrode showed that the increase in the anode reaction resistance was 0.003 Ω cm². From the EIS measurement results of the single cell, it was found that the increase in the total reaction resistance and IR resistance were 0.02 and 0.05 Ω cm², respectively. Summarizing the EIS measurement results, contribution of each component on the performance degradation was determined as follows: IR resistance (71%) > cathode reaction resistance (24%) > anode reaction resistance (5%). Transmission electron microscopy (TEM) results showed that the average particle size of the Pt catalysts increased by 30% after the stability test, while that of the PtRu catalysts increased by 10%.     

Experimental results on the direct electrochemical oxidation of methanol in PEM fuel cells

The cell performance of direct methanol fuel cells (DMFC) is 0.5 V at 0.5 A cm^–2 under high pressure oxygen operation (3 bar abs.) at 110 °C. However, high oxygen pressure operation at high temperatures is only useful in special market niches. Therefore, our work has now focused on air operation of a DMFC under low pressure (up to 1.5 bar abs.). At present, a power density of more than 100 mW cm^–2 can be achieved at 0.5 V on air operation at 110 °C. These measurements were carried out in single cells with an electrode area of 3 cm² and the air stoichiometry only amounted to 10. The effects of methanol concentration and temperature on the anode performance were studied by pseudo half cell measurements and the results are presented together with their impact on the cell voltage. A cell design with an electrode area of 550 cm², which is appropriate for assembling a DMFC stack, was tested. A three-celled stack based on this design revealed nearly the same power densities as in the small experimental cells at low air excess pressure and the voltage–current curves for the three cells were almost identical. At 110 °C a power output of 165 W at a stack voltage of 1.5 V can be obtained in the air mode.     

Development and operation of a 150 W air-feed direct methanol fuel cell stack

A five-cell 150 W air-feed direct methanol fuel cell (DMFC) stack was demonstrated. The DMFC cells employed Nafion 117^® as a solid polymer electrolyte membrane and high surface area carbon supported Pt-Ru and Pt catalysts for methanol electrooxidation and oxygen reduction, respectively. Stainless steel-based stack housing and bipolar plates were utilized. Electrodes with a 225 cm² geometrical area were manufactured by a doctor-blade technique. An average power density of about 140 mW cm^–2 was obtained at 110 °C in the presence of 1 M methanol and 3 atm air feed. A small area graphite single cell (5 cm²) based on the same membrane electrode assembly (MEA) gave a power density of 180 mW cm^–2 under similar operating conditions. This difference is ascribed to the larger internal resistance of the stack and to non-homogeneous reactant distribution. A small loss of performance was observed at high current densities after one month of discontinuous stack operation.     

Polymer electrolyte fuel cells based on main-chain-type sulfonated polyimides

Main-chain-type sulfonated polyimides (SPIs) from 1,4,5,8-naphthalenetetracarboxylic dianhydride (NTDA), 4,4′-bis(4-aminophenoxy)biphenyl-3,3′-disulfonic acid (BAPBDS) and 4,4′-bis(4-aminophenoxy)biphenyl (BAPB) were used as electrolyte membranes for polymer electrolyte fuel cells (PEFCs). The SPI membranes swelled anisotropically in water by four to five times more largely in thickness direction than in plane direction of membrane. The cells with the SPI membranes showed excellent polarization performances, which were a little better than that for Nafion 112, i.e., using highly humidified H₂ and O₂ gases with a pressure of 0.3 MPa, the cell voltages with current densities of 0.5 and 1 A/cm² were 0.76 and 0.69 V, respectively, at 90 °C. The polarization performance was rather weakly dependent on the cell temperature and on the humidifier temperature. The cells showed good short-term stability up to 50 h. The proton conductivities in thickness direction of membrane were calculated from the membrane resistances evaluated from the complex impedance spectra of the operating cells. They were 0.24 and 0.16 S/cm for the homo- and co-SPI membranes (2/1 in molar ratio of BABPDS/BAPB), respectively. The SPI membranes were rather isotropic in relation to proton conductivity in spite of their anisotropic morphology.