Influence of anisotropic bending stiffness of gas diffusion layers on the electrochemical performances of polymer electrolyte membrane fuel cells

The effects of gas diffusion layer’s (GDL’s) anisotropic bending stiffness on the electrochemical performances of polymer electrolyte membrane fuel cells have been investigated for carbon fiber-felt and -paper GDLs. The bending stiffness values of all GDLs in the machine direction are higher than those in the cross-machine direction. We have prepared GDL sheet samples such that the machine direction of GDL roll is aligned with the major flow field direction of a metallic bipolar plate at angles of 0° (parallel: ‘0° GDL’) and 90° (perpendicular: ‘90° GDL’). The I–V performances of all the 5-cell stacks with 90° GDLs are higher than those with 0° GDLs. All the 5-cell stacks with 90° GDLs show lower values of high-frequency resistance (HFR) than those with 0° GDLs. However, the gas pressure differences at both anode and cathode of 5-cell stacks with 90° GDLs appear to be similar to or slightly lower than those with 0° GDLs, making the lower HFR as a dominant factor for the improved I–V performances. This may result from the reduced intrusion of 90° GDLs into gas channels than 0° GDLs as observed by less thickness reduction under compression of 90° GDLs. A 45° GDL (skew alignment) also shows better performances than the 0° GDL.     

Correlation between anisotropic bending stiffness of GDL and land/channel width ratio of polymer electrolyte membrane fuel cells

A correlation between anisotropic bending stiffness of a gas diffusion layer (GDL) and land/channel width ratios of metallic bipolar plates (MBPs) in polymer electrolyte membrane fuel cells has been systematically investigated. I–V performances of the fuel cells with 90° GDLs, whose directions of higher stiffness are perpendicular to the direction of the major flow field, are generally higher than those with 0° GDLs, whose directions of higher stiffness are parallel with the direction of the major flow field. However, the differences of I–V performances and high-frequency resistance values between 0° and 90° GDL cells gradually decrease with increasing land/channel width ratio, because of the reduced anisotropic stiffness effects of the GDLs due to the better support by the MBPs with wider lands. The cross-sectional images of GDLs upon compression indicate that the 0° GDL appears to be more deformed and intruded into channel than the 90° GDL under the narrowest lands, whereas both 0° and 90° GDLs show very little intrusion and deformation under the widest lands. The results clearly explain why some MBPs (i.e., narrower lands) exhibit strong effects of GDL’s anisotropic stiffness on cell performances, whereas other MBPs (i.e., wider lands) do not experience such effects.     

Effects of anisotropic bending stiffness of gas diffusion layer on the MEA degradation of polymer electrolyte membrane fuel cells by wet/dry gas

The effects of anisotropic bending stiffness of a gas diffusion layer (GDL) on membrane electrode assembly (MEA) degradation were investigated. We prepared GDLs with a fiber direction perpendicular to the major flow (i.e., “90° GDL”) and with a fiber direction parallel to the major flow (i.e., “0° GDL”). To analyze the mechanical durability as a function of GDL anisotropy, we examined cell performances such as the I–V characteristics and impedances and the hydrogen crossover characteristics during wet/dry cycles. The results showed that the 90° GDL fuel cell is superior to the 0° GDL fuel cell in terms of higher I–V performance, lower resistance at high frequency, and lower hydrogen crossover through the MEA. Mechanical degradation of the 0° GDL was investigated using scanning electron microscopy (SEM).     

Externally reformed solid oxide fuel cell–micro-gas turbine (SOFC–MGT) hybrid systems fueled by methanol and di-methyl-ether (DME)

Solid oxide fuel cell–micro-gas turbine (SOFC–MGT) hybrid power plants integrate a solid oxide fuel cell and a micro-gas turbine and can achieve efficiencies of over 60% even for small power outputs (200–500 kW). The SOFC–MGT systems currently developed are fueled with natural gas, which is reformed inside the same stack, but the use of alternative fuels can be an interesting option. In particular, as the reforming temperature of methanol and di-methyl-ether (DME) (200–350 °C) is significantly lower than that of natural gas (700–900 °C), the reformer can be sited outside the stack. External reforming in SOFC–MGT plants fueled by methanol and DME enhances efficiency due to improved exhaust heat recovery and higher voltage produced by the greater hydrogen partial pressure at the anode inlet. The study carried out in this paper shows that the main operating parameters of the fuel reforming section (temperature and steam-to-carbon ratio (SCR)) must be carefully chosen to optimise the hybrid plant performance. For the stoichiometric SCR values, the optimum reforming temperature for the methanol fueled hybrid plant is approximately 240 °C, giving efficiencies of about 67–68% with a SOFC temperature of 900 °C (the efficiency is about 72–73% at 1000 °C). Similarly, for DME the optimum reforming temperature is approximately 280 °C with efficiencies of 65% at 900 °C (69% at 1000 °C). Higher SCRs impair stack performance. As too small SCRs can lead to carbon formation, practical SCR values are around one for methanol and 1.5–2 for DME.     

Freezing characteristics of supercooled water in gas diffusion layer of proton exchange membrane fuel cells

The freezing characteristics of supercooled water in a gas diffusion layer (GDL), which are the bases for the cold start-up of proton exchange membrane fuel cells (PEMFCs), were investigated. An experimental apparatus for noncontact temperature measurement and observation systems was developed. GDL and GDL with a microporous layer (MPL) were prepared, and freezing experiments using a water-containing GDL under various cooling rates were performed with variations in polytetrafluoroethylene (PTFE) content and water saturation. Furthermore, based on the experimental results, the freezing initiation probability was theoretically investigated to elucidate the freezing characteristics. Results showed that, with increasing supercooling of water in GDL, the freezing probability of water increased abruptly. The effect of saturation showed a different trend depending on PTFE addition. For the GDL without PTFE, the freezing initiations occurred at approximately 6 °C of supercooling degree, and the probability approached 1.0 at approximately 9.5–11.5 °C, with saturation dependency. In contrast, for both GDL and GDL + MPL containing PTFE, the initiation temperature characteristics were relatively similar, which were approximately 8–12 °C, regardless of the saturation and PTFE content. In these cases, the ice-nucleating activity of water in the GDL was possibly stronger than that in the MPL.     

Laser-induced micro-bubbles in cells

We report experimental study and detection of laser-induced micro-bubbles in individual live cells and in confined micro-volumes of solutions. Using pulsed laser radiation (532 nm, 10 ns) as the source of local heating we detected micro-bubbles in light-absorbing media that were caused at least by two mechanisms—heating of the media above (more than 300 °C) critical temperature (referred as ‘hot’ bubbles) and action of rarefaction pressure waves (referred as ‘cold’ bubbles) at much lower temperatures (30-150 °C). Bubble generation thresholds, probabilities of bubble forming and bubble lifetimes were experimentally studied with two photothermal (PT) methods—thermal lensing and PT-time-resolved imaging for human red blood cells, homogeneous solutions of haemoglobin, neutral red dye and for suspension of polystyrene nano-particles.     

An experimental investigation into the CO2 gasification of deactivated activated-carbon catalyst used for methane decomposition to produce hydrogen

A series of experiments was conducted to study the CO₂ gasification of a deactivated palm-shell-based activated-carbon (ACPS) catalyst used for the thermocatalytic decomposition of methane to produce hydrogen. This catalyst becomes deactivated due to the accumulation of carbon deposits during the methane-decomposition process. The CO₂ gasification was carried out at 850, 900, 950 or 1000 °C to study the deactivated ACPS, which was used at methane-decomposition temperatures of 850 or 950 °C. A series of six methane-decomposition cycles at 950 °C alternating with five gasification cycles using CO₂ at 900, 950 and 1000 °C was also carried out to evaluate the stability of the catalyst. The experiments were conducted using a thermobalance by monitoring the change in mass of the catalyst with time, i.e., the mass gain during methane decomposition or the mass loss during CO₂ gasification. Gasification of the virgin and deactivated ACPS showed strong temperature dependence, with the half and complete gasification times having an exponential dependence on temperature. The gasification reactivity at different conversions was higher for the virgin ACPS and increased with increases in the decomposition temperatures used for deactivation of the ACPS. The activation energies of virgin ACPS and ACPS deactivated at a decomposition temperature of 850 °C decreased with an increase in conversion, while they increased for the ACPS deactivated at a decomposition temperature of 950 °C; the activation energies varied between 81 and 163 kJ/mol. The gasification reactivity changed with methane conversion, showing maximum values for both the virgin and deactivated ACPS at a decomposition temperature of 950 °C. The initial gasification reactivity of the catalyst decreased after three gasification cycles at 1000 °C, while no significant change was observed with gasification cycles at 950 or 900 °C.     

High-temperature mechanical properties of a solid oxide fuel cell glass sealant in sintered forms

High-temperature mechanical properties of a silicate-based glass sealant (GC-9) for planar solid oxide fuel cell have been studied in sintered forms. Ring-on-ring biaxial flexural tests are carried out at room temperature to 800 °C for the sintered GC-9 glass. The results are also compared with those in cast bulk forms. From the force-displacement curves, the glass transition temperature (T_g) of the non-aged, sintered GC-9 glass is estimated to be between 700 °C and 750 °C, while that of the aged one is between 750 °C and 800 °C. Due to a crack healing effect of the residual glass at high temperature, the flexural strength of the sintered GC-9 glass at temperature of 650 °C to T_g point is greater than that at room temperature. At temperature above T_g, the flexural strength and stiffness are considerably reduced to a level lower than the room-temperature one. The sintered GC-9 glass with pores and crystalline phases has a flexural strength lower than the cast bulk one at temperature of 650 °C and below. Due to a greater extent of crystallization, the flexural strength and stiffness of the sintered GC-9 glass are greater than those of the cast bulk one at 700-800 °C.     

Effect of temperature uncertainty on polymer electrolyte fuel cell performance

The temperature of operation is a key parameter in determining the performance and durability of a polymer electrolyte fuel cell (PEFC). Controlling temperature and understanding its distribution and dynamic response is vital for effective operation and design of better systems. The sensitivity to temperature means that uncertainty in this parameter leads to variable response and can mask other factors affecting performance. It is important to be able to determine the impact of temperature uncertainly and quantify how much PEFC operation is influenced under different operating conditions. Here, a simple lumped mathematical model is used to describe PEFC performance under temperature uncertainty. An analytical approach gives a measure of the sensitivity of performance to temperature at different nominal operating temperatures and electrical loadings. Whereas a statistical approach, using Monte Carlo stochastic sampling, provides a ‘probability map’ of PEFC polarisation behaviour. As such, a polarisation ‘area’ or ‘band’ is considered as opposed to a polarisation ‘curve’. Results show that temperature variation has the greatest effect at higher currents and lower nominal operating temperatures. Thermal imaging of a commercial air-cooled stack is included to illustrate the temporal and spatial temperature variation experienced in real systems.     

Water transport characteristics in the gas diffusion media of proton exchange membrane fuel cell - Role of the microporous layer

Water transport through the gas diffusion media of a proton exchange membrane fuel cell (PEMFC) was investigated with a focus on the role of the microporous layer (MPL) coated on the cathode gas diffusion layer (GDL). The capillary pressure of the MPL and GDL, which plays a significant role in water transport, is derived as a function of liquid saturation using a pore size distribution (PSD) model. PSD functions are derived with parameters that are determined by fitting to the measured total PSD data. Computed relations between capillary pressure and liquid saturation for a GDL and a double-layered GDL (GDL + MPL) show good agreement with the experimental data and proposed empirical functions. To investigate the role of the MPL, the relationship between the water withdrawal pressure and liquid saturation are derived for a double-layered GDL. Water transport rates and cell voltages were obtained for various feed gas humidity using a two-dimensional cell model, and are compared with the experimental results. The calculated results for the net drag with application of the capillary pressure derived from the PSD model show good agreement with the experimental values. Furthermore, the results show that the effect of the MPL on the cell output voltage is significant in the range of high humidity operation.     

Feasibility study on power ultrasound for regeneration of silica gel—A potential desiccant used in air-conditioning system

A new regeneration method using power ultrasound was put forward to overcome the limitations of silica gel in air-conditioning applications, such as high regeneration temperature and low regeneration efficiency. The technical feasibility of the new method was validated experimentally and demonstrated in detail from different sides. The experiments were performed under different regeneration temperatures, i.e. 45 °C, 55 °C, 65 °C and 75 °C. The power and frequency of ultrasound applied in this experimental study was set as 40 W and 26 kHz, respectively. The three indicators, including the regeneration degree (RD), enhanced rate of regeneration (ER) and energy-saving rate (ESR), were suggested to evaluate the effect of power ultrasound in the regeneration. The Crank’s diffusion model was used for the calculation of the moisture diffusivity in silica gel, and the Arrhenius equation for the determination of energy activation of moisture desorption on silica gel. The analysis results prove that the introduction of high-intensity ultrasound to the regeneration of silica gel can help to improve the regeneration efficiency and reduce regeneration energy. The benefits should owe to the special ‘heating effect’ and ‘micro-vibration effect’ caused by power ultrasound that can enhance the moisture diffusivity in silica gel and lower the activation energy of moisture desorption on silica gel.     

Performance and cost of automotive fuel cell systems with ultra-low platinum loadings

An automotive polymer-electrolyte fuel cell (PEFC) system with ultra-low platinum loading (0.15 mg-Pt cm⁻²) has been analyzed to determine the relationship between its design-point efficiency and the system efficiency at part loads, efficiency over drive cycles, stack and system costs, and heat rejection. The membrane electrode assemblies in the reference PEFC stack use nanostructured, thin-film ternary catalysts supported on organic whiskers and a modified perfluorosulfonic acid membrane. The analyses show that the stack Pt content can be reduced by 50% and the projected high-volume manufacturing cost by >45% for the stack and by 25% for the system, if the design-point system efficiency is lowered from 50% to 40%. The resulting penalties in performance are a <1% reduction in the system peak efficiency; a 2-4% decrease in the system efficiency on the urban, highway, and LA92 drive cycles; and a 6.3% decrease in the fuel economy of the modeled hybrid fuel-cell vehicle on the combined cycle used by EPA for emission and fuel economy certification. The stack heat load, however, increases by 50% at full power (80 kW_e) but by only 23% at the continuous power (61.5 kW_e) needed to propel the vehicle on a 6.5% grade at 55 mph. The reduced platinum and system cost advantages of further lowering the design-point efficiency from 40% to 35% are marginal. The analyses indicate that thermal management in the lower efficiency systems is very challenging and that the radiator becomes bulky if the stack temperature cannot be allowed to increase to 90-95 °C under driving conditions where heat rejection is difficult.     

Development of dual purpose greenhouse coupled with north wall utilization for higher economic gains

A dual purpose greenhouse was designed, developed and tested for simultaneously producing full crop on the greenhouse floor and nursery plants on the existing north wall (in optimized stacks) for higher economic gains. Using solar geometry, it was computed that the optimum ratio between the height of two consecutive stacks (H) and width of the tray (W) is 1.07 at 31°N. The optimally designed nursery trays were mounted in two stacks on the north wall in such a way that at solar noon no fraction of solar radiation leaves through it and also does not cast any shadow of the upper stack onto the lower stack and on the plants grown on the floor. Nursery plants of tomato crop were raised on these two stacks having three trays of length (L) as 160 cm and width (W) as 67 cm size each with H as 72 cm. During the four winter months, the small greenhouse of 600 cm × 400 cm size can be used commercially to produce about 45,000 nursery plants by completing five cycles. This can be done in addition to the normal crop raised on the greenhouse floor and significant extra revenue can be generated. These returns can further be increased at higher latitudes because at constant H, width of the tray (W) increases with increase in latitude. Economic analysis showed that for tomato crop, currently the cost of greenhouse construction (without north wall stacks) can be recovered after five years time but with simultaneous nursery raising on the north wall, the whole investment cost (greenhouse and stacks) can be recovered in less than two year’s time and thereafter it becomes a profitable venture. Optimum number of stacks and other stack parameters for realistic north wall heights of 210 cm, 260 cm and 310 cm have been computed and discussed at 30°N, 35°N, 40°N, 45°N and 50°N latitudes.     

Thermocatalytic decomposition of methane for hydrogen production using activated carbon catalyst: Regeneration and characterization studies

A series of experiments was conducted to study the deactivation and regeneration of activated carbon catalyst used for methane thermocatalytic decomposition to produce hydrogen. The catalyst becomes deactivated due to carbon deposition and six decomposition cycles of methane at temperatures of 850 and 950 °C, and five cycles of regeneration by using CO₂ at temperatures of 900, 950 and 1000 °C were carried out to evaluate the stability of the catalyst. The experiment was conducted by using a thermobalance by monitoring the mass gain during decomposition or the mass lost during the regeneration with time. The initial activity and the ultimate mass gain of the catalyst decreased after each regeneration cycle at both reaction temperatures of 850 and 950 °C, but the amount is smaller under the more severe regenerating conditions. For the reaction at 950 °C, comparison between the first and sixth reaction cycles shows that the initial activity decreased by 69, 51 and 42%, while the ultimate mass gain decreased by 62%, 36% and 16% when CO₂ gasification carried out at 900, 950 and 1000 °C respectively. Temperature -programmed oxidation profiles for the deactivated catalyst at reaction temperature of 950 °C and after several cycles showed two peaks which are attributed to different carbon characteristics, while one peak was obtained when the experiment was carried out at 850 °C. In conclusion, conducting methane decomposition at 950 °C and regeneration at 1000 °C showed the lowest decrease in the mass gain with reaction cycles.     

Mechanisms of spray formation and combustion from a multi-hole injector with E85 and gasoline

The spray formation and combustion characteristics of gasoline and E85 (85% ethanol, 15% gasoline) have been investigated using a multi-hole injector with asymmetric nozzle-hole arrangement. Experiments were carried out in a quiescent optical chamber using high-speed shadowgraphy (9 kHz) to characterise the spray sensitivity to both injector temperature and ambient pressure in the range of 20-120 °C and 0.5, 1.0 bar. Spray-tip penetrations and ‘umbrella’ spray cone angles were calculated for all conditions. Phase Doppler Anemometry was also used to measure droplet sizes in the core of one of the spray plumes, 25 mm below the injector tip. To study the effect of fuel properties on vaporisation and mixture preparation under realistic operating conditions, a separate set of experiments was carried out in a direct-injection spark-ignition optical engine. The engine was run at 1500 RPM under cold and fully warmed-up conditions (20 °C and 90 °C) at part load and full load (0.5 and 1.0 bar intake pressure). Floodlit laser Mie-scattering images of the sprays on two orthogonal planes corresponding to the swirl and tumble planes of in-cylinder flow motion were acquired to study the full injection event and post-injection mixing stage. These were used to make comparisons with the static chamber sprays and to quantify the liquid-to-vapour phase evaporation process for both fuels by calculating the projected ‘footprint’ of the sprays at different conditions. Analysis of the macroscopic structure and turbulent primary break-up properties of the sprays was undertaken in light of jet exit conditions described in terms of non-dimensional numbers. The effects on stoichiometric combustion were investigated by imaging the natural flame chemiluminescence through the engine’s piston crown (swirl plane) and by post-processing to derive flame growth rates and trajectories of flame motion.     

Discharge properties of all-solid sodium–sulfur battery using poly (ethylene oxide) electrolyte

An all-solid sodium/sulfur battery using poly (ethylene oxide) (PEO) polymer electrolyte are prepared and tested at 90 °C. Each battery is composed of a solid sulfur electrode, a sodium metal electrode, and a solid PEO polymer electrolyte. During the first discharge, the battery shows plateau potentials at 2.27 and at 1.76 V. The first discharge capacity is 505 mAh g⁻¹ sulfur at 90 °C. The capacity drastically decreases by repeated on charge–discharge cycling but remains at 166 mAh g⁻¹ sulfur after 10 cycles. The latter value is higher than that reported for a Na/poly (vinylidene difluoride)/S battery at room temperature.     

One thousand-hour long term characteristics of a propane-fueled solid oxide fuel cell hot zone

One thousand-hour continuous test of a propane-fueled portable solid oxide fuel cell (SOFC) based hot zone has been successfully performed in order to assess the degradation characteristics of its performance. Comparing the different operating modes, the degradation rate based on constant current mode was three times lower than that based on constant voltage mode. The stack power output initially increased 3.7% during the first 34 h probably due to electrode activation processes improving cell performance under polarization during the early stage of operation, and then gradually decreased. It has been clearly illustrated that operating condition of constant current is more beneficial to the long term performance test. Further, based on thermodynamics analysis, the electromotive force of nickel oxidation is 13.2 V for the stack voltage at the stack temperature of 740 °C. From the initial current-power curve data, it can be derived that if the hot zone durability test was performed at constant current of 9 A from the beginning, the stack degradation rate would be 15% per 1000 h. The 1000-h durability test and analysis can better understand how to run longer term stability on the hot zone and guide the optimization of hot zone operating conditions.     

A numerical study of flow crossover between adjacent flow channels in a proton exchange membrane fuel cell with serpentine flow field

The focus of this paper is to study the flow crossover between two adjacent flow channels in a proton exchange membrane (PEM) fuel cell with serpentine flow field design in bipolar plates. The effect of gas diffusion layer (GDL) deformation on the flow crossover due to the compression in a fuel cell assembly process is particularly investigated. A three-dimensional structural mechanics model is created to study the GDL deformation under the assembly compression. A three-dimensional PEM fuel cell numerical model is developed in the aforementioned deformed domain to study the flow crossover between the adjacent channels in the presence of the GDL intrusion. The models are solved in COMSOL Multiphysics—a finite element-based commercial software package. The pressure, velocity, oxygen mass fraction and local current density distribution are presented. A parametric study is conducted to quantitatively investigate the effect of the GDL’s transport related parameters such as porosity and permeability on the flow crossover between the adjacent flow channels. The polarization curves are also examined with and without the assembly compression considered. It is found that the compression effect is evident in the high current density region. Without considering the assembly compression, the fuel cell model tends to over-predict the fuel cell’s performance. The proposed method to simulate the crossover with the deformed computational domain is more accurate in predicting the overall performance.     

A novel approach to determine the in-plane thermal conductivity of gas diffusion layers in proton exchange membrane fuel cells

Heat transfer through the gas diffusion layer (GDL) is a key process in the design and operation of a proton exchange membrane (PEM) fuel cell. The analysis of this process requires determination of the effective thermal conductivity. This transport property differs significantly in the through-plane and in-plane directions due to the anisotropic micro-structure of the GDL.A novel test bed that allows separation of in-plane effective thermal conductivity and thermal contact resistance in GDLs is described in this paper. Measurements are performed using Toray carbon paper TGP-H-120 samples with varying polytetrafluoroethylene (PTFE) content at a mean temperature of 65-70 °C. The measurements are complemented by a compact analytical model that achieves good agreement with experimental data. The in-plane effective thermal conductivity is found to remain approximately constant, k ≈ 17.5 W m⁻¹ K⁻¹, over a wide range of PTFE content, and its value is about 12 times higher than that for through-plane conductivity.     

Characterization of internal wetting in polymer electrolyte membrane gas diffusion layers

Capillary pressure vs. saturation (P_C(S_L)) curves are fundamental to understanding liquid water transport and flooding in PEM gas diffusion layers (GDLs). P_C(S_L) curves convolute the influence of GDL pore geometry and internal contact angles at the three-phase liquid/solid/gas boundary. Even simple GDL materials are a spatially non-uniform mixture of carbon fiber and binder, making a Gaussian distribution of contact angles likely, based on the Cassie–Baxter equation. For a given Gaussian contact angle distribution with mean (θ_Mean) and standard deviation (σ), a realistic P_C(S_L) curve can be computed using a bundle of capillaries model and GDL pore size distribution data. As expected, computed P_C(S_L) curves show that θ_Mean sets the overall hydrophilic (θ_Mean < 90°) or hydrophobic (θ_Mean > 90°) character of the GDL (i.e., liquid saturation level at a given capillary pressure), and σ affects the slope of the P_C(S_L) curve. The capillary bundle model also can be used with (θ_Mean, σ) as unknown parameters that are best-fit to experimentally acquired P_C(S_L) and pore size distribution data to find (θ_Mean, σ) values for actual GDL materials. To test this, pore size distribution data was acquired for Toray TGP-H-090 along with hysteretic liquid and gas intrusion capillary pressure curve data. High quality best-fits were found between the model and combined datasets, with GDL liquid intrusion showing fairly neutral internal surface wetting properties (θ_Mean = 92° and σ = 10°) whereas gas intrusion displayed a hydrophilic character (θ_Mean = 52° and σ = 8°). External liquid advancing and receding contact angles were also measured on this same material and they also showed major hysteresis. The new methods described here open the door for better understanding of the link between GDL material processing and the wetting properties that affect flooding.