Fabrication of catalyst-coated membrane-electrode assemblies by doctor blade method and their performance in fuel cells

Membrane-electrode assemblies (MEAs) have been fabricated with a direct coating of the catalyst slurry by a doctor blade method on the pre-swollen Nafion membrane for proton exchange membrane (PEMFC) and direct methanol fuel cells (DMFC). The effects of various swelling agents with different boiling points such as ethylene glycol (EG), diethylene glycol (DEG), triethylene glycol (TEG), tetraethylene glycol (TEEG), and glycerol in the swelling step of the membrane and the drying step of the coated catalyst have been investigated. Also, the use of dimethyl sulfoxide (DMSO) as a dispersing agent in the catalyst slurry has been investigated. Among the various swelling agents investigated, EG gives the best results with the dispersing agent DMSO offering further improvement. The MEAs fabricated with the EG-swollen membranes and DMSO as a dispersing agent in the catalyst layer show good performance in single fuel cells with hydrogen and methanol fuels.     

Effect of ionomer content and relative humidity on polymer electrolyte membrane fuel cell (PEMFC) performance of membrane-electrode assemblies (MEAs) prepared by decal transfer method

Membrane-electrode assemblies (MEAs) were fabricated by the decal transfer method with various Nafion ionomer contents (10–40 wt%) and their single cell performance and electrochemical characteristics were examined in atmospheric air at relative humidities of 25–95%. At high humidity (95%), the MEA performance was the highest with a cathode ionomer content of 30 and 20 wt% at 0.6 and 0.4 V, respectively. The optimum ionomer content of the decal MEAs increased with decreasing humidity, because of the change in the oxygen transport rate (water flooding) and number of active sites (ionic resistance). The concentration overpotential gradually increased with relative humidity up to about 0.4 V at 0.8 A/cm², which was not considered in previous studies using pressurized air and oxygen. The combined effect of the electrochemical active surface area and ionic resistance of the cathodes on the activation overpotential was also investigated, focusing on intermediate and low humidity levels, using a newly developed impedance analysis method.     

Effect of freeze-thaw cycles on the properties and performance of membrane-electrode assemblies

The effect of freezing of a membrane-electrode assembly on its physical properties and performance was investigated. It was found that freeze-thaw cycles caused the electrode (i.e., catalyst layer) of a fully hydrated membrane-electrode assembly (MEA), either as a freestanding piece or as assembled in a cell, to crack. Accompanying the cracking was a reduction in the electrochemical active surface areas of the electrodes as measured by cyclic voltammetry, but the short-term performance of the fuel cell did not show much effect. When dry reactants were used to remove some water from a cell that had been previously tested at fully hydrated condition, freeze-thaw cycling did not cause apparent damage to the appearance of the electrodes. Also, for freestanding MEAs that were taken directly from the manufacturing line and only exposed to ambient temperature (e.g., 23 °C) and relative humidity (e.g., <50% RH), freezing did not cause apparent damage to the appearance of the electrodes.     

Dependence of the performance of a high-temperature polymer electrolyte fuel cell on phosphoric acid-doped polybenzimidazole ionomer content in cathode catalyst layer

Phosphoric acid-doped polybenzimidazole is used as a fuel cell membrane and an ionomer in the catalyst layer of a high-temperature polymer electrolyte fuel cell. Single-cell tests are performed to find the optimum ionomer content in the cathode catalyst layer. To determine the effects of the ionomer in the catalyst layer, the potential loss in the cell is separated into activation, ohmic and concentration losses. Each of these losses is examined by means of impedance and morphological analyses. With the weight ratio of ionomer to Pt/C of 1:4 (20 wt.% ionomer in catalyst layer), the fuel cell shows the lowest ohmic resistance. The activation loss in the fuel cell is lowest when the ratio is 1:9 (10 wt.% ionomer in the catalyst layer). The cell performance is dependent on this ratio, and the best cell performance is obtained with a ratio of 1:4.     

A study on fabrication of sulfonated poly(ether ether ketone)-based membrane-electrode assemblies for polymer electrolyte membrane fuel cells

The porosity effect of catalyst electrodes in membrane-electrode assemblies (MEAs) using a hydrocarbon-based polymer as electrolyte and ionomer was investigated on physical and electrochemical properties by varying the content of ionomer binder (dry condition) in the catalyst electrodes. The MEAs were compared with the Nafion^®-based MEA using Nafion^® 112 and 5 wt.% ionomer solution (EW = 1100) in terms of porosity values, scanning electron microscopic images, Nyquist plots, dielectric spectra and I–V polarization curves. In this study, sulfonated poly(ether ether ketone) (SPEEK) membranes with 25 ± 5 μm of thickness and 5 wt.% ionomer solutions have been prepared. The prepared membranes were characterized in terms of FT-IR, DSC and proton conductivity. Proton conductivity of the SPEEK membranes was compared with one of the Nafion^® membranes with relative humidity. The porosity of the catalyst electrodes was calculated using the properties of catalyst, ionomer solution and solvent. As a result, the performance of the new type polymer (i.e., SPEEK in this study)-based MEA with the similar membrane conductivity and porosity of the catalyst electrode in the Nafion^® MEA was similar to that of the Nafion^® MEA.     

Improvement of cell performance in catalyst layers with silica-coated Pt/carbon catalysts for polymer electrolyte fuel cells

Carbon-supported Pt catalysts (Pt/Cs) for use of cathode catalyst layers (CLs) for PEFCs were covered with silica layers in order to improve performance. CLs with low ratio of ionomer to carbon (I/C) for Pt/C and silica-coated Pt/C were fabricated using an inkjet printing (denoted as Pt/C(IJ) and SiO₂-Pt/C(IJ)) to reduce oxygen diffusion resistance. Compared to Pt/C(IJ), SiO₂-Pt/C(IJ) ink maintained good dispersion and high stability under the lower I/C. The performance of SiO₂-Pt/C(IJ) was significantly higher than Pt/C(IJ) at 0.6 V under all humidity conditions. In particular, the performance of SiO₂-Pt/C(IJ) under low humidity conditions showed noticeable improvement regardless of current density area. From FIB-SEM, it was confirmed that the morphologies and porosities of both catalysts were the same. Thus, these results indicate that oxygen diffusion resistance, related to structure of CLs, hardly affects the performance, whereas improved performance is attributed to increased proton conductivity by silica layers containing hydrophilic groups.     

Optimal catalyst layer structure of polymer electrolyte membrane fuel cell

In a membrane electrode assembly (MEA) of polymer electrolyte membrane fuel cells, the structure and morphology of catalyst layers are important to reduce electrochemical resistance and thus obtain high single cell performance. In this study, the catalyst layers fabricated by two catalyst coating methods, spraying method and screen printing method, were characterized by the microscopic images of catalyst layer surface, pore distributions, and electrochemical performances to study the effective MEA fabrication process. For this purpose, a micro-porous layer (MPL) was applied to two different coating methods intending to increase single cell performances by enhancing mass transport. Here, the morphology and structure of catalyst layers were controlled by different catalyst coating methods without varying the ionomer ratio. In particular, MEA fabricated by a screen printing method in a catalyst coated substrate showed uniformly dispersed pores for maximum mass transport. This catalyst layer on micro porous layer resulted in lower ohmic resistance of 0.087 Ω cm² and low mass transport resistance because of enhanced adhesion between catalyst layers and a membrane and improved mass transport of fuel and vapors. Consequently, higher electrochemical performance of current density of 1000 mA cm^-2 at 0.6 V and 1600 mAcm⁻² under 0.5 V came from these low electrochemical resistances comparing the catalyst layer fabricated by a spraying method on membranes because adhesion between catalyst layers and a membrane was much enhanced by screen printing method.     

Polymer electrolyte membrane fuel cell contamination: Testing and diagnosis of toluene-induced cathode degradation

The effects of toluene contamination on the performance of polymer electrolyte membrane (PEM) fuel cells were investigated, using various levels of toluene concentration in the air streams, under different operational conditions and with different catalyst loadings. Constant-current polarization and electrochemical impedance spectroscopy (EIS) were conducted to analyze the poisoning behaviour of toluene. The severity of the contamination effect increased with an increase in both the current density and the toluene concentration, but decreased with an increase in both the relative humidity (RH) and the cathode-side Pt loading. The toluene-poisoned fuel cell could not be fully recovered by replacing toluene-contaminated air with pure air. EIS measurements revealed that both kinetic resistance and mass transfer resistance increased as a result of toluene contamination, while membrane resistance remained unchanged. However, the increase in kinetic resistance was a major contributor to cell performance degradation.     

An experimental overview of the effects of hydrogen impurities on polymer electrolyte membrane fuel cell performance

Hydrogen quality is critical for increasing the reliability, stability, and durability of polymer electrolyte (PEM) fuel cells. In this work, several hydrogen impurities have been studied to understand their effects on PEM fuel cell performance at various operating concentrations. Our studies have shown that the following impurities suggested by industry stakeholders do not result in substantial fuel cell degradation when they are the sole impurity in hydrogen: 5 ppm formaldehyde, 2 ppm formic acid, 19 ppm chloromethane, 30 ppm acetaldehyde, 5% ethylene, 20 ppm toluene, and 10 ppm benzene. In addition, a specific mixture of impurities called the “specification concentration level cocktail” consisting of 0.2 ppm carbon monoxide, 4 ppb hydrogen sulphide, 0.2 ppm formic acid, 2 ppm benzene, and 0.1 ppm ammonia in hydrogen, also does not show significant effects on cell performance. In comparison, when a cocktail having five times the specification concentration is introduced into the cell, significant performance loss is evident.     

An improved one-dimensional membrane-electrode assembly model to predict the performance of solid oxide fuel cell including the limiting current density

The limiting current density is an important characteristic quantity in solid oxide fuel cells (SOFCs). High concentration overpotential is often used to explain the limiting current density assuming a high tortuosity or limited surface diffusion in the vicinity of the three-phase boundary. Most membrane-electrode assembly models of SOFC fail to predict the limiting current density, even for hydrogen, when using physically reasonable values of tortuosity and considering the short residence time of the adsorbed species near the three-phase boundary. In this paper, a one-dimensional model for the transport–chemistry interactions in SOFCs is described. The model is based on a comprehensive approach that includes the dusty-gas model for gas transport in the porous electrodes, detailed heterogeneous elementary reaction kinetics for the thermo-chemistry in the anode, and detailed electrode kinetics for the electrochemistry. Correct values for the Knudsen diffusion coefficients are used. We apply the unsteady form of the conservation equations, allowing for the analysis of the response of the cell to external dynamics. Results of our model are compared with experimental data, showing good agreement over a wide range of the current density, but fail to predict the limiting current density accurately when the hydroxyl oxidation charge-transfer reaction is assumed to be the rate limiting reaction. To obtain accurate predictions of the limiting current density, we analyze the possibility that different steps can be rate-limiting reactions in the electrochemistry model of hydrogen oxidation. We use recent measurements on the three-phase boundary area and take into account the surface diffusion and competitive adsorption to determine possible rate limiting reactions at high current density. We show that a rate-limiting switchover model, in which the reaction limiting the overall kinetic rate becomes the hydrogen adsorption at the anode, may be required to explain the experimentally measured limiting current density over a range of operating conditions.     

Effect of dynamic operation on chemical degradation of a polymer electrolyte membrane fuel cell

Dynamic operation is known as one of the factors for accelerating chemical degradation of the polymer electrolyte membrane in a polymer electrolyte membrane fuel cell (PEMFC). However, little effort has been made dealing with the quantification of the degradation process. In this investigation, cyclic current operation is carried out on a fuel cell system, and the frequency effect of cyclic operation on chemical degradation is investigated. The dynamic behavior of a fuel cell system is analyzed first with the modified Randles model, where the charge double layer is modeled by three components; a charge transfer resistance (R_ct), and two RC cells for the Warburg impedance. After calculating each parameter value through exponential curve fitting, the dynamic behaviors of the three components are simulated using MATLAB Simulink^®. Fluoride release as a function of the frequency of cyclic operation is evaluated by measuring the concentration of fluoride ion in effluent from a fuel cell exhaust. The frequency effect on chemical degradation is explained by comparing the simulated results and the fluoride release results. Two possible reasons for the accelerated degradation at cyclic operation are also suggested.     

Effect of alcohol content on the ionomer adsorption of polymer electrolyte membrane fuel cell catalysts

Catalyst layers (CL) composed of catalyst composites and an ionomer are key components in polymer electrolyte membrane fuel cells (PEMFCs). In particular, the preparation conditions of the CL, starting from the dispersion of the catalyst composite dispersion with an ionomer, largely affect the PEMFC performance. In this study, the effects of alcohol content in the dispersion solvent were investigated using two binary mixtures composed of water and ethanol. In addition, Pt-loaded carbon black (CB) and Pt-loaded polymer-wrapped CB were used as the catalyst composites to study the effects of the alcohol contents on the interaction between ionomer and surface of the carbon supports. The CL prepared using the water-rich (80 wt% water) solvent achieved a higher PEMFC performance compared to that using the alcohol-rich (13 wt% water) solvent, which is ascribed to the stronger interaction between the ionomer and CB surface under water-rich conditions. Using the polymer-wrapped CB, the difference of the PEMFC performance between the CLs from the water-rich and alcohol-rich dispersions was minimal because of the comparable interaction between the ionomer and wrapping polymer surface in both solvents. Therefore, the control of the interaction between the ionomer and catalyst composites is crucial to controlling the PEMFC performance.     

Temperature-dependent performance of the polymer electrolyte membrane fuel cell using short-side-chain perfluorosulfonic acid ionomer

We report on polymer electrolyte membrane fuel cells (PEMFCs) that function at high temperature and low humidity conditions based on short-side-chain perfluorosulfonic acid ionomer (SSC-PFSA). The PEMFCs fabricated with both SSC-PFSA membrane and ionomer exhibit higher performances than those with long-side-chain (LSC) PFSA at temperatures higher than 100 °C. The SSC-PFSA cell delivers 2.43 times higher current density (0.524 A cm⁻¹) at a potential of 0.6 V than LSC-PFSA cell at 140 °C and 20% relative humidity (RH). Such a higher performance at the elevated temperature is confirmed from the better membrane properties that are effective for an operation of high temperature fuel cell. From the characterization technique of TGA, XRD, FT-IR, water uptake and tensile test, we found that the SSC-PFSA membrane shows thermal stability by higher crystallinity, and chemical/mechanical stability than the LSC-PFSA membrane at high temperature. These fine properties are found to be the factor for applying Aquivion™ E87-05S membrane rather than Nafion^® 212 membrane for a high temperature fuel cell.     

Chloride contamination effects on proton exchange membrane fuel cell performance and durability

Chlorine is a major fuel contaminant when by-product hydrogen from the chlor-alkali industry is used as the fuel for proton exchange membrane (PEM) fuel cells. Understanding the effects of chlorine contamination on fuel cell performance and durability is essential to address fuel cell applications for the automotive and stationary markets. This paper reports our findings of chloride contamination effects on PEM fuel cell performance and durability, as our first step in understanding the effects of chlorine contamination.Fuel cell contamination tests were conducted by injecting ppm levels of contaminant into the fuel cell from either the fuel stream or the air stream. In situ and ex situ diagnosis were performed to investigate the contamination mechanisms. The results show that cell voltage during chloride contamination is characterized by an initial sudden drop followed by a plateau, regardless of which side the contaminant is introduced into the fuel cell. The drop in cell performance is predominantly due to increased cathode charge transfer resistance as a result of electrochemical catalyst surface area (ECSA) loss attributable to the blocking of active sites by Cl⁻ and enhanced Pt dissolution.     

Coupling effects of water content,temperature, oxygen density,and polytetrafluoroethylene loading on oxygen transport through ionomer thin film on platinum surface in catalyst layer of proton exchange membrane fuel cell

In this work, coupling effects of water content, temperature, oxygen density, and polytetrafluoroethylene (PTFE) loading on oxygen transport through an ionomer thin film on a platinum surface in a catalyst layer of a proton exchange membrane (PEM) fuel cell are investigated using molecular dynamics approach. Taguchi orthogonal algorithm is employed to comprehensively analyze the coupling effects in a limited number of cases. It is found that the effect of operation temperature is the weakest among the four factors, which has the smallest effect index 14.4. Coupling effects including the PTFE loadings on the oxygen transfer through the ionomer thin film is uncovered. Less PTFE loadings should be beneficial for the oxygen transfer. The chemical potential gradient is considered as the major driven force for the oxygen transport through the ionomer thin film, and oxygen density is the dominating factor, significantly affecting the chemical potential in the thin film.     

Effects of ionomer content and oxygen permeation of the catalyst layer on proton exchange membrane fuel cell cold start-up

We have verified the effectiveness of ionomer as a carrier of oxygen to improve cold start-up of proton exchange membrane fuel cells. Galvanostatic cold start was performed on proton exchange membrane fuel cells to evaluate the effect of ionomer content in the catalyst layer on the durability of power generation at −30 °C. Cell voltage and internal resistance were measured, and polarization analysis was conducted to evaluate the cell voltage reduction. Cold start-up durability improved significantly with higher ionomer content in the catalyst layer because of higher oxygen permeation of ice formed in the catalyst layer. These results enable robust design of membrane electrode assemblies for cold start-up.     

Evaluation of electrochemical performance of solid-oxide fuel cell anode with pillar-based electrolyte structures

Among the gas, ion, and electron diffusion processes in solid-oxide fuel cell (SOFC) electrodes, it is generally known that ionic conduction has the most impact on their electrochemical performance. Therefore, enhancement of the effective ionic conductivity of electrodes is a useful approach to reduce the overpotential. Yttria-stabilized zirconia (YSZ) pillars can be effective solutions to enhance the effective ionic conductivity of SOFC anodes. In this study, the influence of YSZ pillar structures on the electrochemical performance of SOFC anodes was evaluated by numerical simulation and experiments. First, to reveal the electrochemical reaction kinetics of anodes with pillar structures, a three-dimensional electrochemical simulation was conducted by the lattice Boltzmann method. The microstructure without pillars obtained by a focused ion beam scanning electron microscopy (FIB-SEM) measurement was used as the reference structure. Then, the original structure was replaced with YSZ phase to obtain virtual microstructures with YSZ pillars. With YSZ pillars, predicted area specific resistance became smaller than that of the reference structure, in spite of decrease in percolated TPB density. The electrochemical potential distribution of oxide ion and charge-transfer currents clearly show increase in the effective ionic conductivity. Relationships between overpotential and pillar geometries were parametrically discussed. Then, electrochemical performance of Ni-YSZ anode with the YSZ pillar structure formed by modifying the YSZ electrolyte surface was evaluated. By sputtering Ni-YSZ on pillar structures, stable electrochemical performance was obtained.     

Effect of platinum amount in carbon supported platinum catalyst on performance of polymer electrolyte membrane fuel cell

The performance of polymer electrolyte membrane fuel cells fabricated with different catalyst loadings (20, 40 and 60 wt.% on a carbon support) was examined. The membrane electrode assembly (MEA) of the catalyst coated membrane (CCM) type was fabricated without a hot-pressing process using a spray coating method with a Pt loading of 0.2 mg cm⁻². The surface was examined using scanning electron microscopy. The catalysts with different loadings were characterized by X-ray diffraction and cyclic voltammetry. The single cell performance with the fabricated MEAs was evaluated and electrochemical impedance spectroscopy was used to characterize the fuel cell. The best performance of 742 mA cm⁻² at a cell voltage of 0.6 V was obtained using 40 wt.% Pt/C in both the anode and cathode.     

Ionomer content in the catalyst layer of polymer electrolyte membrane fuel cell (PEMFC): Effects on diffusion and performance

The percolating paths of the carbons and electrolytes in a cathode catalyst layer (CCL) could be successfully visualized in three-dimensions in order to investigate both the electronic and ionic connectivity by modeling a three-dimensional (3-D), meso-scale CCL of a polymer electrolyte membrane fuel cell (PEMFC). The effective Knudsen diffusion coefficients could also be obtained by computing pore tortuosity values. Electrochemical simulation studies were carried out by feeding air at 70 °C. Low platinum (Pt) loading (0.1 mg cm⁻²) catalysts with ionomer contents ranging from 14 to 50% were studied. The performance of a PEMFC electrode was affected by the ionomer content which is optimal at about 33%. In this case, both electronic and ionic connectivity produced the broadest active surface area of the Pt catalyst. The polarization drop tendency was in good agreement with the experiment, and this percolation study could successfully explain the existence of an optimum amount of ionomer.