Gas crossover and membrane degradation in polymer electrolyte fuel cells

Hydrogen gas crossover measurements and durability tests of a single cell under open-circuit conditions were carried out to investigate membrane degradation in polymer electrolyte fuel cells (PEFCs). The limiting current density for hydrogen crossover was approximately 0.8 mA cm⁻² at 80 °C under atmospheric pressure, and gas crossover increased with an increase in cell temperature, humidity and hydrogen gas pressure. Under open-circuit conditions, the perfluorinated ionomer electrolyte membrane deteriorated significantly although no net electrochemical reactions took place at the cathode and anode. The mechanism for membrane degradation was discussed in terms of heat generation and hydrogen peroxide formation upon gas crossover and the resulting catalytic combustion, and it was concluded that the latter is the primary reason, in which hydrogen peroxide is most probably formed by gas crossover of oxygen and the resulting catalytic combustion at the anode side. In addition, it was inferred that reactive oxygen radicals (HO and HO₂) were formed in the presence of minor impurities such as Fe²⁺ and Cu²⁺ ions, which could accelerate the membrane degradation.     

Investigation of electrode composition of polymer fuel cells by electrochemical impedance spectroscopy

In order to optimize the electrode composition and performance of Polymer Fuel Cells and to reduce the production cost of membrane electrode assemblies (MEAs), different MEAs using different catalyst powders, carbon supported and unsupported catalysts with different proton conducting electrolyte powder (Nafion) content were produced by using a dry powder spraying technique developed at German Aerospace Research Center (DLR, Deutsches Zentrum fuer Luft- und Raumfahrt). The electrochemical characterization was performed by recording current-voltage curves and electrochemical impedance spectra (EIS) in the galvanostatic mode of operation at 500 mA cm⁻². The evaluation of the measured impedance spectra with an adequate equivalent circuit shows that the cathode of the fuel cell is very sensitive to the electrode composition whereas the contribution of the anode is very small and invariant to the electrode composition. Furthermore, it could be shown for the first time using electrolyte powder in the electrodes that the charge transfer of the cathode decreasing monotonically with increasing electrolyte content in the cathode. These findings suggest that with increasing electrolyte content in the electrodes, in particular in the cathode, the utilization degree of the catalyst increasing linearly with increasing electrolyte content in the electrode.     

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.     

Durability of a novel sulfonated polyimide membrane in polymer electrolyte fuel cell operation

A novel sulfonated polyimide membrane containing triazole groups (SPI-8) was subjected to long-term fuel cell operation. Excellent durability of the SPI-8 membrane was confirmed by single cell operation for 5000 h at 80 °C. Open circuit voltage and hydrogen crossover through the membrane showed only minor changes during cell operation, indicating a lack of catastrophic damage for the SPI-8 membrane. It was found by post-test analyses of the membrane that the ion exchange capacity (IEC) decreased only slightly, but the molecular weight decreased to 1/10, resulting in a loss of mechanical strength. It was concluded that the major degradation mode of the sulfonated polyimide membrane involves the ring-opening of the imide linkages via hydrolysis, while a certain degree of side chain degradation occurs as a result of oxidative attack by radical species.     

Progress in non-precious metal oxide-based cathode for polymer electrolyte fuel cells

The cathode catalysts for polymer electrolyte fuel cells should have high stability as well as excellent catalytic activity for oxygen reduction reaction (ORR). Group 4 and 5 metal oxide-based compounds have been evaluated as a cathode from the viewpoint of their high catalytic activity and high stability. Although group 4 and 5 metal oxides have high stability even in acidic and oxidative atmosphere, they are almost insulator and have poor ORR activity because they have a large band-gap. It is necessary to modify the surface of the oxides to improve the ORR activity. We have tried the surface modification methods of oxides into four methods: (1) formation of complex oxide layer containing active sites, (2) substitutional doping of nitrogen, (3) introduction of surface oxygen defect and (4) partial oxidation of carbonitrides. These modifications were effective to improve the ORR activity of the oxides. The solubility of the oxide-based catalysts in 0.1 mol dm⁻³ at 30 °C under atmospheric condition was mostly smaller than that of platinum black, indicating that the oxide-based catalysts had sufficient stability compare to the platinum. The onset potential of various oxide-based cathodes for the ORR in 0.1 mol dm⁻³ at 30 °C achieved over 0.9 V vs. a reversible hydrogen electrode.     

Simulation of a polymer electrolyte fuel cell electrode

A detailed one dimensional dynamic model of a gas diffusion electrode as part of a complete fuel cell model is presented. Various effects of parameter changes are considered. Comparison of experimental results and simulation is performed and a new approach to simulation of a complete current voltage curve is discussed.     

Effect of diffusion-layer porosity on the performance of polymer electrolyte fuel cells

The gas-diffusion layer (GDL) influences the performance of electrodes employed with polymer electrolyte fuel cells (PEFCs). A simple and effective method for incorporating a porous structure in the electrode GDL using sucrose as the pore former is reported. Optimal (50 w/o) incorporation of a pore former in the electrode GDL facilitates the access of the gaseous reactants to the catalyst sites and improves the fuel cell performance. Data obtained from permeability and porosity measurements, single-cell performance, and impedance spectroscopy suggest that an optimal porosity helps mitigating mass-polarization losses in the fuel cell resulting in a substantially enhanced performance.     

Two-phase transport and the role of micro-porous layer in polymer electrolyte fuel cells

Two-phase transport of reactants and products constitutes an important limit in performance of polymer electrolyte fuel cells (PEFC). Particularly, at high current densities and/or low gas flow rates, product water condenses in open pores of the cathode gas diffusion layer (GDL) and limits the effective oxygen transport to the active catalyst sites. Furthermore, liquid water covers some of the active catalytic surface, rendering them inactive for electrochemical reaction. Traditionally, these two-phase transport processes in the GDL are modeled using so-called unsaturated flow theory (UFT), in which a uniform gas-phase pressure is assumed across the entire porous layer, thereby ignoring the gas-phase flow counter to capillarity-induced liquid motion. In this work, using multi-phase mixture (M²) formalism, the constant gas pressure assumption is relaxed and the effects of counter gas-flow are studied and found to be a new oxygen transport mechanism. Further, we analyze the multi-layer diffusion media, composed of two or more layers of porous materials having different pore sizes and/or wetting characteristics. Particularly, the effects of porosity, thickness and wettability of a micro-porous layer (MPL) on the two-phase transport in PEFC are elucidated.     

Three-dimensional multiphase modeling of cold start processes in polymer electrolyte membrane fuel cells

Startup from subzero temperatures, referred to as “cold start”, has been one of the technical challenges hindering the commercialization of polymer electrolyte membrane fuel cell (PEMFC). In this study, a mathematical model has been developed to simulate the cold start processes in a PEMFC. The present three-dimensional multiphase model uniquely includes the water freezing in the membrane electrolyte, the non-equilibrium mass transfer between the water in the ionomer and the water (vapour, liquid and ice) in the pore region of the catalyst layer (CL), and the water freezing and melting in the CL and gas diffusion layer (GDL). Both the failed and successful cold start processes have been numerically investigated. Numerical results indicate that increasing the ionomer fraction in the cathode CL has more significant effects than increasing the thickness of the membrane layer in reducing the amount of ice formation, and the ohmic heat is the largest heating source at low cell voltages. It is observed that water freezes first in the cathode CL under the land, and ice melts first in the CLs under the flow channel, the melted water in the anode is also removed faster than in the cathode.     

Pd-Co carbon-nitride electrocatalysts for polymer electrolyte fuel cells

In this paper is described the preparation of new platinum-free Pd-Co carbon-nitride electrocatalysts (Pd-Co-CNs) for application in low-temperature fuel cells. Two groups of materials with formula K_n[Pd_xCo_yC_zN_lH_m] were synthesized, which are grouped in two ensembles: the first is characterized by a molar ratio y/x > 1 (I), and the second by y/x < 1 (II). K_n[Pd_xCo_yC_zN_lH_m] materials were prepared through a two-step synthesis protocol. The effect of the Pd/Co molar ratio and of the temperature of the thermal treatments on the structure and properties of the products were studied extensively by thermogravimetry, scanning electron microscopy, and vibrational (FT-IR and micro-Raman) and XPS spectroscopy. Vibrational studies revealed that I and II systems consist of two polymorphs of α- and graphitic-like carbon-nitride nanomaterials. The electrochemical activity towards the oxygen reduction reaction (ORR) and the hydrogen oxidation reaction (HOR) was measured by cyclic voltammetry measurements with thin-film rotating disk electrode (CV-TF-RDE). The electrochemical performance of Pd-Co-CNs of group I obtained at t_f ≥ 700 °C resulted higher than that measured for a platinum-based commercial electrocatalyst in terms of both the activity towards the ORR and the HOR and of the resistance towards the poisoning effect of methanol towards the ORR.     

Surface conductivity and stability of metallic bipolar plate materials for polymer electrolyte fuel cells

Different types of commercial stainless steels, Ni-based alloys and nitride-coated steels were evaluated as metallic bipolar plate in terms of interfacial contact resistance (ICR) and corrosion resistance in conditions typical of PEFC anode and cathode environments. Results show that stainless steel have a high ICR and undergo corrosion in both anode and cathode. Moreover, although Ni-based alloys showed an ICR comparable with graphite, their behaviour was not satisfactory in corrosive acidic medium. Only nitride-coated stainless steel demonstrated to have low ICR and very good corrosion resistance.     

Characteristics of membrane humidifiers for polymer electrolyte membrane fuel cells

Water management plays an important role in obtaining high performance from a polymer electrolyte membrane fuel cell (PEMFC). To reduce the volume and energy consumption of widely-used bubble humidifiers, membrane humidifiers were fabricated by using an ultrafiltration (UF) membrane and Nafion membranes. The performance of the membrane humidifiers was examined as a function of gas flow rate and operating temperature. A single cell was operated using the UF membrane humidifiers exhibiting almost the same performance with that employing bubble humidifiers.     

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.     

Silicate-based polymer-nanocomposite membranes for polymer electrolyte membrane fuel cells

Proton-exchange membrane fuel cells have emerged as a promising emission free technology to fulfill the existing power requirements of the 21st century. Nafion^® is the most widely accepted and commercialized membrane to date and possesses excellent electrochemical properties below 80 °C, under highly humidified conditions. However, a decrease in the proton conductivity of Nafion^® above 80 °C and lower humidity along with high membrane cost has prompted the development of new membranes and techniques. Addition of inorganic fillers, especially silicate-based nanomaterials, to the polymer membrane was utilized to partially overcome the aforementioned limitations. This is because of the lower cost, easy availability, high hydrophilicity and higher thermal stability of the inorganic silicates. Addition of silicates to the polymer membrane has also improved the mechanical, thermal and barrier properties, along with water uptake of the composite membranes, resulting in superior performance at higher temperature compared to that of the virgin membrane. However, the degrees of dispersion and interaction between the organic polymer and inorganic silicates play vital roles in improving the key properties of the membranes. Hence, different techniques and solvent media were used to improve the degrees of nanofiller dispersion and the physico-chemical properties of the membranes. This review focuses mainly on the techniques of silicate-based nanocomposite fabrication and the resulting impact on the membrane properties.     

Scanning electrochemical microscopy for the investigation of localized degradation processes in coated metals

Scanning electrochemical microscopy has been employed to obtain spatially-resolved information regarding surface topology, water uptake and blister formation at intact coatings, as well as the onset and progress of corrosion reactions within coating defects. Topographical lines and maps as well as chronoamperometric plots were measured during operation in the feedback mode. Next, the release of metal ions at the anodic sites and the consumption of oxygen at the cathodic sites developed in holidays could be monitored during operation in the generator–collector mode. Furthermore, the surface topography of defective coatings was imaged by using the redox competition mode.     

The effect of cathodic water on performance of a polymer electrolyte fuel cell

A simple analytical model of water transport in the polymer electrolyte fuel cell is developed. Nonlinear membrane resistance and voltage loss due to incomplete membrane humidification are calculated. Both values depend on parameter r, the ratio of mass transport coefficients of water in the membrane and in the backing layer. Simple equation for cell performance curve, which incorporates the effect of cathodic water is constructed. Depending of the value of r, the cell may operate in one of the two regimes. When r ≥ 1, incomplete membrane humidification simply reduces cell voltage; the limiting current density is determined by oxygen transport in the backing layer (oxygen-limiting regime). If r < 1, limiting current density is determined by membrane drying (water-limiting regime). In that case there exists optimal current density, which provides minimal membrane resistance. It is shown that membrane drying may lead to parasitic “in-plane” proton current.     

Microfabricated polymer electrolyte membrane fuel cells with low catalyst loadings

Miniaturized fuel cells as compact power sources fabricated in Pyrex glass using standard polymer electrolyte membrane (PEM) and electrode materials are presented. Photolithographic patterned and wet chemically etched serpentine flow channels of 1 mm in width and 250  m in depth transport the fuels to the cell of 1.44 cm² active electrode area. Feeding H₂/O₂ a maximum power density of 149 mW cm⁻² is attained at a very low Pt loading of 0.054 mg cm⁻², ambient pressure, and room temperature. Operated with methanol and oxygen about 9 mW cm⁻² are achieved at ambient pressure, 60 °C, and 1 mg cm⁻² PtRu/Pt (anode/cathode) loading. A planar two-cell stack to demonstrate and investigate the assembly of a fuel cell system on Pyrex wafers has successfully been fabricated.     

Corrosion behavior of austenitic stainless steels as a function of pH for use as bipolar plates in polymer electrolyte membrane fuel cells

Stainless steels (types 304 and 310S) were employed as bipolar plates for polymer electrolyte membrane fuel cells. For the cell operation, the decayed cell voltage was approximately 22 mV for the type 310S stainless steel after 1000 h operation, while that for type 304 stainless steel was about 46 mV. Corrosion products appeared on the cathode side bipolar plate for the type 304 stainless steel, while trace of corrosion was barely detected for type 310S stainless steel. In order to follow the pH on the bipolar plates during fuel cell operation, polarization tests were carried out for the type 310S stainless steel in synthetic solutions (0.05 M SO₄²⁻ (pH 1.2-5.5) + 2 ppm F⁻) as a function of pH (1.2-5.5) at 353 K. We also examined the contact resistance between the stainless steel and carbon diffusion layer before and after polarization. X-ray photoelectron spectroscopic (XPS) analyses were carried out for comparison of the surface states of the steels after the polarization tests and cell operation. In the synthetic solutions with lower pHs (≤3.3), the films were thinner and were mainly composed by enriched with chromium oxide. Whereas, they mainly consisted of relatively thick iron oxide when the solution pH was higher (≥4.3). XPS analyses for the bipolar plate of type 310S stainless steel on cathode side after cell operation demonstrated pH gradient on the plate, that is, the thicker iron-rich surfaces presented relatively higher pH from the gas inlet to center area, and the thinner chromium-rich surface appeared with lower pH around the gas outlet.     

Development and electrochemical studies of gas diffusion electrodes for polymer electrolyte fuel cells

Electrochemical studies on low catalyst loading gas diffusion electrodes for polymer electrolyte fuel cells are reported. The best performance is obtained with an electrode formed from 20 wt% Pt/C, 0.4 mg Pt cm^–2 and 1.1 mg Nafion^® cm^–2 in the catalyst layer and 15% PTFE in a diffusion layer of 50 µm thickness, for both the cathode and the anode. However, it is also observed that the platinum requirement can be diminished to values close to 0.2 mg Pt cm^–2 in the cathode and 0.1 mg pt cm^–2 in the anode, without appreciably affecting the good characteristics of the fuel cell response. The experimental fuel cell data were analysed using theoretical models of the electrode structure and of the fuel cell system. It is seen that most of the electrode systems present limiting currents and some also show linear diffusion components arising from diffusion limitations in the gas channels and/or in the thin film of electrolyte covering the catalyst particles.