Functionalized titania nanotube composite membranes for high temperature proton exchange membrane fuel cells

In this study, functionalized titania nanotubes (F-TiO₂-NT) were synthesized by using 3-mercaptopropyl-tri-methoxysilane (MPTMS) as a sulfonic acid functionalization agent. These F-TiO₂-NT were investigated for potential application in high temperature hydrogen polymer electrolyte membrane fuel cells (PEMFCs), specifically as an additive to the proton exchange membrane. Fourier transform infrared spectroscopy (FT-IR) and X-ray photoelectron spectroscopy (XPS) results confirmed that the sulfonic acid groups were successfully grafted onto the titania nanotubes (TiO₂-NT). F-TiO₂-NT showed a much higher conductivity than non-functionalized titania nanotubes. At 80 °C, the conductivity of F-TiO₂-NT was 0.08 S/cm, superior to that of 0.0011 S/cm for the non-functionalized TiO₂-NT. The F-TiO₂-NT/Nafion composite membrane shows good proton conductivity at high temperature and low humidity, where at 120 °C and 30% relative humidity, the proton conductivity of the composite membrane is 0.067 S/cm, a great improvement over 0.012 S/cm for a recast Nafion membrane. Based on the results of this study, F-TiO₂-NT has great potential for membrane applications in high temperature PEMFCs.     

Composite membranes based on a sulfonated poly(arylene ether sulfone) and proton-conducting hybrid silica particles for high temperature PEMFCs

The organic-inorganic composite membranes are prepared by inserting poly(styrene sulfonate)-grafted silica particles into a polymer matrix of sulfonated poly(arylene ether sulfone) copolymer. The first step consisted in using atom transfer radical polymerization method to prepare surface-modified silica particles grafted with sodium 4-styrenesulfonate, referred to as PSS-g-SiO₂. Ion exchange capacities up to 2.4 meq/g are obtained for these modified silica particles. In a second step, a sulfonated poly(arylene ether sulfone) copolymer is synthesized via nucleophilic step polymerization of sulfonated 4,4′-dichlorodiphenyl sulfone, 4,4′-dichlorodiphenyl sulfone and phenolphthalin monomers in the presence of potassium carbonate. The copolymer is blended with various amounts of silica particles to form organic-inorganic composite membranes. Esterification reaction is carried out between silica particles and the sulfonated polymer chains by thermal treatment in the presence of sodium hypophosphite, which catalyzed the esterification reaction. The water uptake, proton conductivity, and thermal decomposition temperature of the membranes are measured. All composite membranes show better water uptake and proton conductivity than the unmodified membrane. Moreover, the membranes are tested in a commercial single cell at 80 °C and 120 °C in humidified H₂/air under different relative humidity conditions. The composite membrane containing 10%(w/w) of PSS-g-SiO₂ particles, which have ester bonds between polymer chains and silica particles, showed the best performance of 690 mA cm⁻² at 0.6 V, 120 °C and 30 %RH, even higher than the commercial Nafion^® 112 membrane.     

Pore network modeling of cathode catalyst layer of proton exchange membrane fuel cell

In this paper, a pore network model is developed to investigate the coupled transport and reaction processes in the cathode catalyst layer (CCL) of proton exchange membrane fuel cell (PEMFC). The developed model is validated by comparing the predicted polarization curve with the experimental data, and the parametric studies are carried out to elucidate the effects of CCL design parameters. With the decrease of the CCL thickness and the Nafion content, the cell voltage reduces at the low current density but increases when the current density is higher. The cell performance is also improved by increasing the proton conductivity of the Nafion film in the CCL. As compared to the CCL of uniformly distributed Nafion, the CCL with the Nafion volume decreasing along the thickness direction exhibits better performance at the high current density.     

Dynamic characteristics and mitigations of hydrogen starvations in proton exchange membrane fuel cells during start-ups

Hydrogen starvation during a start-up process in proton exchange membrane (PEM) fuel cells could result in drastic local current density variations, reverse cell voltage and irreversible cell damages. In this work, variations of local current densities and temperatures are measured in situ under both potentiostatic and galvanostatic modes. Experimental results show that when the cell starts up under potentiostatic mode with hydrogen starvation, current density undershoots occur in the downstream; while under the galvanostatic mode, local current density in the downstream almost drops to zero, but the current density near the outlet remains almost constant. The phenomenon of near constant current density near the outlet leads to a novel approach to alleviate hydrogen starvations - a hydrogen reservoir is added at the anode outlet. Experimental results show that the exit hydrogen reservoir can significantly reduce the zero current region and alleviate hydrogen starvations. A non-dimensional current-density variation coefficient is proposed to measure the magnitude of local current density changes during starvations. Experimental results show that the exit hydrogen reservoir can significantly reduce the current-density variations coefficient over the entire flow channel, indicating that adding an exit reservoir is an effective approach in mitigating hydrogen starvations.     

Modeling of high temperature proton exchange membrane fuel cells with novel sulfonated polybenzimidazole membranes

In this study, a three-dimensional, steady-state, non-isothermal numerical model of high temperature proton exchange membrane fuel cells (HT-PEMFCs) operating with novel sulfonated polybenzimidazole (SPBI) membranes is developed. The proton conductivity of the phosphoric acid doped SPBI membranes with different degrees of sulfonation is correlated based on experimental data. The predicted conductivity of SPBI membranes and cell performance agree reasonably with published experimental data. It is shown that a better cell performance is obtained for the SPBI membrane with a higher level of phosphoric acid doping. Higher operating temperature or pressure is also beneficial for the cell performance. Electrochemical reaction rates under the ribs of the bipolar plates are larger than the values under the flow channels, indicating the importance and dominance of the charge transport over the mass transport.     

Dynamic behavior of liquid water transport in a tapered channel of a proton exchange membrane fuel cell cathode

A numerical model of a proton exchange membrane fuel cell (PEMFC) cathode with a tapered channel design has been developed in order to examine the dynamic behavior of liquid water transport. Three-dimensional, transient simulations employing the level-set method (available in COMSOL 3.5a, a commercial finite element method software) have been used to explicitly track the liquid-gas interface. A liquid water droplet suspended in the center of the channel, 2 mm from the channel entrance, is subjected to airflow in the bulk of the channel. Three different cases have been studied: 1) hydrophobic bottom wall representing the gas diffusion layer and hydrophilic channel top and side walls, 2) all walls are partially wetted i.e. having a contact angle of 90°, 3) a hydrophilic bottom wall and hydrophobic top and side walls. The results show that tapering the channel downstream helps in water exhaust due to increased airflow velocity. A bottom wall, although hydrophilic, results in fast removal of water droplet as compared to partially wetted and hydrophobic bottom surfaces.     

Dodecylamine-modified carbon supports for cathode in proton exchange membrane fuel cells

In this investigation, hydrophobic dodecylamine-modified carbon supports are prepared for proton exchange membrane fuel cells by organic synthesis. Well-dispersed Pt-Ru nanoparticles, with a narrow size distribution, are then deposited on the dodecylamine-modified carbon supports by methanol reduction to serve as cathodic catalysts. These dodecylamine-modified catalysts are separately mixed with either a commercial catalyst or unmodified catalyst to provide hydrophobic channels to convey the reaction gas to the active sites in the catalyst layer. The best cathode composite catalyst, containing 20-40 wt% of modified-catalyst, gives approximate 30% increase in the maximum power density, comparing to E-TEK catalyst (125 mW cm⁻²). The increase in the maximum power density is attributed to higher activity and lower resistance. This result is discussed in the context of AC-impedance and proton conductivity analysis.     

Non-kinetic losses caused by electrochemical carbon corrosion in PEM fuel cells

This paper presented non-kinetic losses in PEM fuel cells under an accelerated stress test of catalyst support. A cathode with carbon-supported Pt catalyst was prepared and characterized during potential hold at 1.2 V vs. SHE in a PEM fuel cell. Irreversible losses caused by carbon corrosion were evaluated using a variety of electrochemical characterizations including cyclic voltammetry, linear sweep voltammetry, electrochemical impedance spectroscopy, and polarization technique. Ohmic losses at the cathode during potential hold were determined using its capacitive responses. Concentration losses in the PEM fuel cell were analyzed in terms of Tafel behavior and thin film/flooded-agglomerate dynamics.     

质子交换膜燃料电池可靠性分析

可靠性是质子交换膜燃料电池(PEMFC)的重要指标，文中定性分析了PEMFC组成元件、装配工艺和工作过程的可靠性。提出了提高PEMFC可靠性的措施和可靠性的设计原则。     

A novel Ba0.6Sr0.4Co0.9Nb0.1O3−δ cathode for protonic solid-oxide fuel cells

Ba_0.6Sr_0.4Co_0.9Nb_0.1O_3−δ (BSCN), originated from SrCo_0.9Nb_0.1O_3−δ (SCN), is investigated as a cathode material in a protonic solid-oxide fuel cell (SOFC-H⁺) with a BaZr_0.1Ce_0.7Y_0.2O₃ (BZCY) electrolyte. The surface-exchange and bulk-diffusion properties, phase reaction with the electrolyte, electrochemical activity for oxygen reduction, and performance in the real fuel cell condition of SCN and BSCN electrodes are comparatively studied by conductivity relaxation, XRD, EIS and I-V polarization characterizations. Much better performance is found for BSCN than SCN. Furthermore, water has a positive effect on oxygen reduction over BSCN while it has the opposite effect with SCN. A peak power density of 630 mW cm⁻² at 700 °C is achieved for a thin-film BZCY electrolyte cell with a BSCN cathode compared to only 287 mW cm⁻² for a similar cell with an SCN cathode. The results highly recommend BSCN as a potential cathode material for protonic SOFCs.     

Dynamic modeling of a high-temperature proton exchange membrane fuel cell with a fuel processor

A dynamic model of a high-temperature proton exchange membrane fuel cell with a fuel processor is developed in this study. In the model, a fuel processing system, a fuel cell stack, and an exhaust gas burner are modeled and integrated. The model can predict the characteristics of the overall system and each component at the steady and transient states. Specifically, a unit fuel cell model is discretized in a simplified quasi-three-dimensional geometry; therefore, the model can rapidly predict the distribution of fuel cell characteristics. Various operating conditions such as the steam-to-carbon ratio, oxygen-to-carbon ratio, and autothermal reforming inlet temperature are varied and investigated in this study. In addition, the dynamic characteristics exhibited during the transient state are investigated, and an efficiency controller is developed and implemented in the model to maintain the electrical efficiency. The simulation results demonstrate that the steam-to-carbon ratio and the oxygen-to-carbon ratio affect the electrical and system efficiency and that controlling the fuel flow rate maintains the electrical efficiency in the transient state. The model may be a useful tool for investigating the characteristics of the overall system as well as for developing optimal control strategies for enhancing the system performance.     

Enhanced long-term durability of proton exchange membrane fuel cell cathode by employing Pt/TiO2/C catalysts

Pt/TiO₂/C catalysts are employed as the cathode catalysts for proton exchange membrane fuel cell (PEMFC). The comparative studies on the Pt/C and Pt/TiO₂/C catalysts are conducted with the physical and electrochemical techniques.After the accelerating aging test (AAT), the remaining electrochemical active surface area (EAS) of the Pt/TiO₂/C catalysts is 75.6%, which is larger than that of the Pt/C catalysts (42.6%). The apparent exchange current density () of the oxygen reduction reaction (ORR) at the Pt/C catalysts decreases from 3.02 × 10⁻⁹ to 1.32 × 10⁻¹¹ A cm⁻² after the AAT. And the value of of the ORR at the Pt/TiO₂/C catalysts is 2.88 × 10⁻⁹ A cm⁻² before the AAT and 2.51 × 10⁻⁹ A cm⁻² after the AAT. Furthermore, the output performance degradation of the PEMFC using the Pt/TiO₂/C cathode catalysts is also less than that using the Pt/C catalysts. The particle size of the Pt/C catalysts increases significantly from 5.3 to 26.5 nm after the AAT. The mean particle size of the Pt/TiO₂/C catalysts is 7.3 nm before the AAT and 9.2 nm after the AAT. It can be concluded that the long-term durability of the Pt/TiO₂/C catalysts in a PEMFC is much better than that of the Pt/C catalysts.     

From microstructure to the development of water and major reaction sites inside the catalyst layer of the cathode of a proton exchange membrane fuel cell

Microstructures of various sizes and shapes are fabricated on the surface of the catalyst layer (CL) of the cathode of a PEMFC, adjacent to the micro porous layer (MPL). Three major experimental results are: (1) performance is improved by up to 60% and the percentage of the increase is the same as that of the increase in interface area of CL and MPL; (2) the cell suffers no significant performance loss when Pt loading of the cathode is reduced from 1 to 0.25 mg cm⁻² and; (3) transient responses in periodical linear sweep tests show an obvious performance “jump” for all the cathodes with microstructures when approaching steady state, but none for others. Based on observations, a proposal related to the development of water and, consequently, the major reaction sites in the CL is made: there is a general water “surface” inside the CL. Major electrochemical reactions occur above (on the MPL side) of this surface and within a limited height. The surface will “move” from the membrane toward the MPL as more water is produced. The vapor generation rate (current load) relative to the removal rate of the rest of the cell components will determine the steady state position of this water surface.     

Study of the cell reversal process of large area proton exchange membrane fuel cells under fuel starvation

In this research, the fuel starvation phenomena in a single proton exchange membrane fuel cell (PEMFC) are investigated experimentally. The response characteristics of a single cell under the different degrees of fuel starvation are explored. The key parameters (cell voltage, current distribution, cathode and anode potentials, and local interfacial potentials between anode and membrane, etc.) are measured in situ with a specially constructed segmented fuel cell. Experimental results show that during the cell reversal process due to the fuel starvation, the current distribution is extremely uneven, the local high interfacial potential is suffered near the anode outlet, hydrogen and water are oxidized simultaneously in the different regions at the anode, and the carbon corrosion is proved to occur at the anode by analyzing the anode exhaust gas. When the fuel starvation becomes severer, the water electrolysis current gets larger, the local interfacial potential turns higher, and the carbon corrosion near the anode outlet gets more significant. The local interfacial potential near the anode outlet increases from ca. 1.8 to 2.6 V when the hydrogen stoichiometry decreases from 0.91 to 0.55. The producing rate of the carbon dioxide also increases from 18 to 20 ml min⁻¹.