Low Humidity Operation Research Articles

(Digital Presentation) Correlative Evaluation Between Water Concentration and Voltage Fluctuation of PEFC Under Low-Humidity and Load-Change Operations Based on Laser Spectroscopy

Low-humidity and load-change operations of polymer electrolyte fuel cells (PEFCs) often tend to induce the excessive voltage drop called “undershoot”. Especially when the load current rapidly increases in steps, the strong electro-osmotic drag dehydrates the electrolyte membrane and deteriorates the proton conductivity, resulting in a temporal increase in ohmic loss. To identify a factor for causing undershoot, it is necessary to precisely grasp the transient water behavior within PEFCs operated under load-change conditions. In the previous works, several researchers vigorously explored the water vapor distribution along flow fields of working cells with micro gas chromatography (μGC) [1]. Although GC techniques provide more accurate species concentrations, it takes a long time (at least two minutes) to collect a gas sample and identify a species. As an alternative approach, the authors have developed a single-ended fiber-optic sensor to measure the in-situ concentration of water vapor within narrow channels of fuel cells at high speed and with high accuracy based on tunable diode laser absorption spectroscopy (TDLAS) [2]. TDLAS is one of the high-speed and high-sensitive absorption spectroscopy techniques which detect the absorbance of a chemical species and identify its concentration using infrared diode laser. This study became able to monitor the transient variation of water vapor concentration in the anode channel of a PEFC during low-humidity and load-change operations using the TDLAS-based fiber-optic sensor and discussed the correlation between the water behavior and the voltage fluctuation. Furthermore, the effects of load current and compression pressure were also investigated.Figs. 1(a) and 1(b) show the dynamic responses of cell voltage and the transient water concentrations in the anode channel of the operating PEFC assembled under two different compression pressures of 1.0 and 2.0 MPa. The electrode area of the experimental cell is 50 mm2. Two separators on both sides have a single-straight channel (1.0 mm width, 1.0 mm depth, 6.0 mm length). Hydrogen (stoichiometry: 2.3@1.0A/cm2, 30%RH) and oxygen (stoichiometry: 9.1@1.0A/cm2, 30%RH) were fed to the anode and cathode channel in the co-flow arrangement. The fiber-optic probe for water detection was positioned at the fractional location of x/L=0.5 on the anode side and the modulated beam emitted from a DFB laser diode (wavelength: 1392.5 nm) was directly injected into the channel. The cell temperature is 70oC. The repetitive current cycling was performed between 0.2 A/cm2 for 23 s and 0.6 A/cm2 for 23 s. In the case of the compression pressure of 1.0 MPa, when the current density increases steeply from 0.2 to 0.6 A/cm2, the strong electro-osmotic drag temporarily reduces the water content on the anode side of the electrolyte membrane and increases the ohmic loss, resulting in the voltage undershoot. After a step increase in the current density, the temporal drop of water vapor concentration in the anode channel can also be observed. When the compression pressure is increased to 2.0 MPa, the porosities of the diffusion media are reduced and the water concentration becomes high especially under the anode rib. Thus, even if the current density is increased stepwise after the repeated current cycles, the water shortage in the anode and the undershoot are not caused. It was noted that the strong compression effectively improves the water retaining on the anode side under low-humidity operations and alleviate the voltage undershoot.

Numerical simulation of gas–heat–water distribution characteristics of Ammonia–Hydrogen fuel cell

Improving the operating conditions can enhance the output performance of PEMFCs based on Ammonia decomposition gas as fuel (Ammonia–Hydrogen fuel cell). Therefore, in this study, a 3-dimensional non-isothermal numerical model was established to study the gas–heat–water distribution of Ammonia–Hydrogen fuel cell under different operating conditions, and the performance improved by operating conditions is analyzed through regional quantification, and then optimize the operating conditions for improving performance. The research results show that increasing the working pressure, gas humidity and nitrogen adsorption can improve the output performance of the fuel cell effectively. Under low-pressure operating conditions, increasing the unit pressure can increases the output performance by 80%; under low-humidity operating conditions, increasing the relative humidity by 0.15 can increases the output performance by 8%; under low-hydrogen content operating conditions, increasing the mole fraction of hydrogen by 5% through nitrogen adsorption can increases the output performance by 10%. Along the flow direction, the molar concentration distribution characteristics of reactant gas is opposite to the change trend of membrane water content, which is closely related to the reaction rate; the overall change trend of heat is consistent with the water content of the membrane, but the hot spot value at the outlet has a downward trend with the strengthening of the reaction conditions and the continuous increase of the load. When operating conditions changes, the variation trend of the regional total output is different, and the maximum value of the total output varies from the operating conditions. Optimizing the operating conditions can make it possible to achieve the output performance based on pure hydrogen as fuel.

Perpendicularly stacked array of PTFE nanofibers as a reinforcement for highly durable composite membrane in proton exchange membrane fuel cells

The configuration of reinforced composite membrane (RCM), composed of porous polytetrafluoroethylene (PTFE) as a mechanical reinforcement and perfluorosulfonic acid (PFSA) as a proton conductive polymer, has gained a large interest due to its promisingly high performance for polymer electrolyte membrane (PEM) fuel cells. However, the inaccessible polymeric nanocomposites in preparing RCMs are still faced with critical challenges associated with immiscible interactions between hydrophilic sulfonate groups in PFSA and the hydrophobic nanoporous PTFE matrix. Herein, we report a well-refined and facile fabrication strategy for producing a cross-aligned PTFE (CA-PTFE) framework. The electric-field guided electrospinning enables the creation of unique micron-scale, grid-type PTFE matrix, which is synthesized by annealing of electrospun conjugated polymers, resulting in the removal of carrier polymer and the formation of continuous fibrious structure via fusion of PTFE particles. The CA-PTFE RCM embodying uniformly impregnated PFSA in a grid-type PTFE matrix, facilitates hydration of the membranes, with minimal swelling and efficient diffusion of protons through concentrated sulfonate groups. The CA-PTFE RCM adopted cell showed outstanding fuel cell currents during both low and high humidity operation, with a current density of 1.38 A cm−2 at 0.6 V and maximum power density of 0.85 W cm−2 under RH 100% condition. Furthermore, the CA-PTFE RCM was able to achieve a long-lasting single-cell operation with a significantly low hydrogen crossover (less than 5 mA cm−2 at 0.4 V) even after 21,000 wet/dry cycles, which surpasses the standard of membrane durability for transportation application. The rational design of fibrous PTFE reinforcements opens up new engineering opportunities for the future development of high-stability PEM fuel cells.

Structurally modulated and functionalized carbon nanotubes as potential filler for Nafion matrix toward improved power output and durability in proton exchange membrane fuel cells operating at reduced relative humidity

Development of a high-performance and durable proton exchange membrane (PEM) for low relative humidity (RH) operation is imperative for further commercialization of hydrogen-air fuel cell (HAFC). In this work, we reported the synthesize of carbon nanotubes-unzipped carbon nanotubes (CNTs-UCNTs) hybrid via calcination process with ammonium fluoride. Further, the 4-benzenediazonium sulfonate was used to sulfonate both CNTs and UCNTs to form sulfonated CNTs-sulfonated UCNTs (SCNTs-SUCNTs) hybrid. The unzipping enlarged the surface area of CNTs-UCNTs hybrid and facilitated the accommodation of a large density of sulfonic acid (-SO 3 H) groups during sulfonation. Thus, SCNTs-SUCNTs hybrid incorporation with the host Nafion matrix results a uniform composite membrane with high density of –SO 3 H groups per unit volume. It positively impacts thermal, mechanical and oxidative stabilities, and physicochemical performances of composite membrane. Besides, Nafion/SCNTs-SUCNTs composite membrane afforded credible proton conductivity, current output, power output, and durability, which are 0.015 S cm −1 , 1.000 A cm −2 , 0.343 W cm −2 , and 0.15 mV h −1 , respectively when operating the HAFC at 60 °C under 20% RH. In contrast, bare Nafion, Nafion/CNTs, and Nafion-212 membranes exhibited inferior proton conductivity, current output, power output, and durability under identical HAFC operating conditions. Efficient proton transport ability of Nafion composite membrane given by SCNTs-SUCNTs hybrid is beneficial for HAFC operation under low relative humidity. • Sulfonation of UCNTs beneficial to Nafion composite membrane to improve physiochemical and thermomechanical properties. • Nafion/SCNTs-SUCNTs showed enhanced proton conductivity compared to pristine Nafion and Nafion-212 membranes under low RH. • The power density and durability of Nafion/SCNTs-SUCNTs membrane are few folds better than pristine Nafion.

Structural architectures of polymer proton exchange membranes suitable for high-temperature fuel cell applications

High-temperature proton exchange membrane (HT-PEM) fuel cells offer more advantages than low-temperature PEM fuel cells. The ideal characteristics of HT-PEMs are high conductivities, low-humidity operation conditions, adequate mechanical properties, and competitive costs. Various molecular moieties, such as benzimidazole, benzo-thiazole, imide, and ether ether ketone, have been introduced to polymer chain backbones to satisfy the application requirements for HT-PEMs. The most common sulfonated polymers based on the main chain backbones have been employed to improve the rties. Side group/chain engineering, includ crosslinking, has been widely applied to HT-PEMs to further improve their proton conductivity, thermal stability, and mechanical properties. Currently, phosphoric acid-doped polybenzimidazole is the most successful polymer material for application in HT-PEMs. The compositing/blending modification methods of polymers are effective in obtaining high PA-doping levels and superior mechanical properties. In this review, the current progress of various membrane materials used for HT-PEMs is summarized. The synthesis and performance characteristics of polymers containing specific moieties in the chain backbones applied to HT-PEMs are discussed systemically. Various modification approaches and their deficiencies associated with HT-PEMs are analyzed and clarified. Prospects and future challenges are also presented.

Ti2Zr2O8 nanotube as an additive in the fuel cell membrane and catalyst layer for improved low humidity operation

The development of high-performance, low-cost and durable membrane electrode assembly (MEA) materials receives significant attention from the fuel cell community. A promising MEA design approach introduces a radical scavenging Ti 2 Zr 2 O 8 nanotubes additive (TZONT) in the polymer electrolyte membrane and cathode catalyst layer for fuel cells operating under dry conditions is described. The modified MEA operating at 20% relative humidity (RH) showed ~18% increase in the peak power density than its counterpart assembled with only a modified membrane. A fluoride emission rate (FER) analysis reveals that the TZONT additive in the Nafion ionomer protects it from radical attack and, thus, lowers the FER value to about 200-times the commercial Nafion-212 membrane. Finally, we revealed a promising approach to lower the Pt loading in the cathode catalytic layer without compromising its intrinsic activity in a relatively cost-competitive manner. • The radical scavenging effect of Ti 2 Zr 2 O 8 in Nafion matrix is reported. • Composite membranes show excellent mechanical strength and chemical stability. • Observed 200-times reduction of FER contributed by Ti 2 Zr 2 O 8 additive. • Promising approach for Pt reduction in the PEMFC cathode catalytic layer.

Composite short-side-chain PFSA electrolyte membranes containing selectively modified halloysite nanotubes (HNTs)

Aquivion membrane displays improved properties as compared to Nafion membrane, partly due to shorter side chains. However, some improvements are still necessary for proton exchange membrane fuel cell to operate at low relative humidity. To overcome this drawback, the addition of clay nanoparticle into the Aquivion matrix can be considered. In this study, different composite membranes have been prepared mixing short-side-chain PFSA (perfluorosulfonic acid) Aquivion and selectively modified halloysite nanotubes for PEMFC low relative humidity operation. Halloysites were grafted with fluorinated groups, sulfonated groups, or perfluoro-sulfonated groups on inner or outer surface of the tubes. The obtained composite membranes showed improved properties, especially higher water uptake associated with reduced swelling and better mechanical strength compared to pristine Aquivion membrane and commercially available Nafion HP used as reference. The best performance in this study was obtained with Aquivion loaded with 5 wt% of pretreated perfluoro-sulfonated halloysite. The composite membrane, referred to as Aq/pHNT-SF5, displayed the largest water uptake and proton conductivity among the panel of membranes tested. The chemical stability was not affected by the presence of halloysite in the Aquivion matrix.Graphical abstract

High-Performance Fuel Cell Operable at 120 °C Using Polyphenlyene Ionomer Membranes with Improved Interfacial Compatibility.

While the performance and durability of proton exchange membrane fuel cells (PEMFCs) have been considerably improved over the last decade, high-temperature operation (above 100 °C) is still an issue. We designed a sulfonated polyphenylene containing tetrafluorophenylene groups (SPP-QP-f) for high-temperature and low-humidity operation of PEMFCs. Compared to state-of-the-art perfluorinated PEMs and the previous polyphenylene ionomer membrane with no fluorine-containing groups, the SPP-QP-f membrane exhibited superior proton conductivity under all testing conditions (80-120 °C, 20-95% RH). Because of the improved interfacial compatibility with the catalyst layers, the SPP-QP-f membrane induced high cathode catalytic activity. These attractive properties of the SPP-QP-f membrane resulted in high fuel cell performance (390 mW cm-2 maximum power density) at 120 °C and 30% RH. The durability was confirmed under accelerated degradation conditions (100 °C, 30% RH) for 1000 h.

Direct Measurement of Proton Conductivity in the Catalyst Layer of PEFC

I ntroduction Since the commercialization of FCVs, PEFCs have attracted attention. For further improvement of the efficiency, raising the operating temperature is focused as one possible solution. We have been studying higher temperature and low humidity operation, and have found that decrease in ionomer’s proton conductivity enhances degradation of the cathode and there is a correlation between the proton conductivity of ionomers in the cathode and PEFC’s durability [1, 2]. Even though the proton conductivity within the cathode is important, such proton conductivity has not been directly measured. Therefore, in this study, we investigated a method of directly measuring proton conductivity in the cathode layer. Experimental By referring to a common method of making MEAs, electrocatalyst layers were prepared by spraying the slurry containing 46%Pt/KB (TEC10E50E) and three different ionomers (Nafion, AquivionD72, and AquivionD83) on the Nafion membrane (Nafion 212). The I/C was kept 0.72 and 0.76 for Nafion and Aquivion, respectively. Then, another Nafion membrane was placed further on the sprayed electrocatalyst layer. The size of electrocatalyst layer and Nafion membranes were 30 mm×10 mm and 35 mm×14 mm, respectively, as shown in Fig. 1(a). Here, the catalyst layer was smaller than the membrane for the catalyst layer not to contact with Pt electrode of the holder shown in Fig. 1(b) even after hot-pressing. Two carbon papers adhered by carbon paste were put between the membrane and Pt electrode. With this holder, impedance was measured in the nitrogen or hydrogen atmosphere by applying DC or AC (10 mV, 0.1 mA, 32 MHz~10 Hz). Measurements were performed under 80oC-RH100% and 105oC-RH57% as a common operating condition and a low humidity condition, respectively. Results and discussion The proton conductivity was calculated based on the impedance. Measuring impedance in nitrogen atmosphere with AC, the movement of electrons within the electrocatalyst layer cannot be excluded completely. If Nafion ionomer within the electrocatalyst layer forms a condensed thin film, the thickness will be 1.25 μm based on the mass and density of Nafion ionomer, resulting in 0.0025 Ω. However, in reality, Nafion ionomer spreads out through the catalyst layer and results in higher resistance. In this study, the obtained resistance for the catalyst layer with Nafion ionomer was 0.031 Ω, which was more than ten times larger. Larger resistance is reasonable at this moment, but again there is high possibility of including electron conductivity within the catalyst layer. Therefore, in the next step, we tried to apply DC. Since the reaction with low overvoltage was desired for this measurements, DC was applied under hydrogen atmosphere where the reaction is reversible and its overvoltage is small. We’ll report the results obtained by applying DC under the hydrogen atmosphere.

Modifying the Catalyst Layer Using Polyvinyl Alcohol for the Performance Improvement of Proton Exchange Membrane Fuel Cells under Low Humidity Operations

A proton exchange membrane fuel cell (PEMFC) system for the application of unmanned aerial vehicles is equipped without humidifiers and the cathode channels of the stack are open to the environment due to limited weight available for power sources. As a result, the PEMFC is operated under low humidity conditions, causing membrane dehydration, low performance, and degradation. To keep the generated water within the fuel cell to humidify the membrane, in this study, polyvinyl alcohol (PVA) is employed in the fabrication of membrane electrode assemblies (MEAs). The effect of PVA content, either sprayed on the gas diffusion layer (GDL) or mixed in the catalyst layer (CL), on the MEA performance is compared under various humidity conditions. The results show that MEA performance is increased with the addition of PVA either on the GDL or in the CL, especially for non-humidified anode conditions. The result suggested that 0.03% PVA in the anode CL and 0.1% PVA on the GDL can improve the MEA performance by approximately 30%, under conditions of a non-humidified anode and a room-temperature-humidified cathode. However, MEAs with PVA in the anode CL show better durability than those with PVA on the GDL according to measurement with electrochemical impedance spectroscopy.

Acid-Doped Hydrogels for Robust Ion Transport in Electrochemical Catalyst Layers

Polymer-electrolyte membranes (PEMs) are ubiquitous to state-of-the-art electrochemical devices. Polymer electrolyte materials have been historically limited to a small class of perfluorinated sulfonic-acid ionomers (PFSAs). Structural changes in PFSA ionomers observed under low humidity and confined length scales (<100’s of nanometers) limit proton conduction and mass transport in the catalyst layer, restricting current density in state-of-the-art devices. Heterogeneous catalyst layer morphology and complex formulation-performance relationships make these limitations difficult to assess. We propose an alternative class of polymer electrolyte based on a thermally crosslinked poly(acrylic acid)-poly(vinyl alcohol) scaffold doped with sulfuric acid, with ion-exchange capacity (IEC) tuned via the dopant concentration. We achieved IEC over 10 meq g-1, well above the upper-limit for PFSAs, and proton conductivity as high as 350 mS cm-1. Notably, proton conductivity was robust across the length (<100 nm film thickness) and humidity scales relevant to electrochemical device operation. The doped-hydrogel electrolyte was employed in a microelectrode vapor-fed water electrolyzer and achieved current densities up to 8 orders of magnitude higher than Nafion under low humidity operation. In this poster, I will present a facile method for fabricating electrolyte-doped hydrogel thin-films with tunable IEC. Furthermore, I will discuss transport properties of these materials across the range of humidity and thicknesses relevant for electrochemical catalyst layers. Finally, I will discuss the use of an electrochemical model system to isolate and systematically study ionomer performance under relevant operating conditions, providing a crucial link between material properties and performance in-operando. The results presented here underscore the potential value of tailoring materials chemistry to specific requirements of individual device components in membrane-electrode assemblies.

A scaled-up proton exchange membrane fuel cell with enhanced performance and durability

A proton exchange membrane fuel cell with enhanced performance and durability has been developed by using graphene for the microporous layer on the cathode side, and short-side-chain ionomer in the catalyst layers and electrolyte membrane layer. The cell components are analyzed for their detailed morphological, microstructural, physical, and electrochemical characteristics, and the performance and durability are investigated following the established protocol of accelerated stress tests for a scaled-up cell with an active area of 45 cm2 at 75 °C and 35 kPag, and a range of reactant flow rate and relative humidity conditions. The results indicate that the graphene flakes are arranged in the in-plane direction of the cell, and coupled with the improved proton conductivity and water retention of the short-side chain ionomer and membrane, the cell exhibits excellent performance and durability, and is highly tolerant to the variations in the operating conditions. Before and after 30,000 cycles of the accelerated stress tests, the maximum power densities are around 1.13 W/cm2 and 0.75 W/cm2 for the high-flow fully humidified reactants, and about 0.77 W/cm2 and 0.50 W/cm2 for the low-flow and low-humidity conditions, respectively. The excellent performance and durability at low relative humidity operation are significantly important for practical applications.

Asymmetric electrode ionomer for low relative humidity operation of anion exchange membrane fuel cells

An asymmetric anion exchange membrane fuel cell enables high performance and operational durability under low RH conditions.

Effect of Polytetrafluoroethylene Particles in Cathode Catalyst Layer Based on Carbon Nanotube for Polymer Electrolyte Membrane Fuel Cells

Abstract Improvement of the utilization efficiency of Pt by improving the mass diffusion is key especially for carbon nanotube (CNT)-based electrocatalyst layers (CL) of polymer electrolyte membrane fuel cells (PEMFC). Here, we introduced hydrophobic spherical polytetrafluoroethylene (PTFE) particles into the CNT-based CL for the first time to improve the diffusion of oxygen and water. Catalyst-coated gas diffusion layers (CCG) were prepared by vacuum filtration method and membrane electrode assembles were prepared by hot pressing of CCGs with Nafion membrane. By incorporating PTFE particles specifically into the cathode CL, a large increase of power density was obtained mainly due to the improvement of hydrophobicity. Addition of the PTFE was effective especially during low humidity operations. Since the filtration process is simple and useful especially for CNT-based electrocatalysts, our findings are quite beneficial to realize CNT-based PEMFC, which is promising especially for the next generation PEMFCs operated under high-temperature and low humidity.

Overpotential Analysis of Low Loading Pt/C Electrodes at Low Humidity Conditions

In 2014, the transportation sector accounted for 35.3% of all greenhouse gas emissions in the State of Connecticut [1]. A significant amount of greenhouse gases emitted can be primarily attributed to the use of fossil fuels in passenger vehicles and other light-duty trucks. In an effort to reduce the amount of transportation-related emissions, there is a need to produce low- and zero-emission vehicles, such as fuel cell vehicles. Compared to vehicles with traditional internal combustion engines which emit greenhouse gases, fuel cell vehicles only emit water vapor. Through research and development of fuel cell and electrolyzer technologies, fuel cell vehicles have the potential to significantly reduce the greenhouse gas emissions related to the transportation sector. In order to improve the viability of fuel cell vehicles, fuel cell performance must be optimized at low relative humidity conditions while keeping production costs low. One such method for improving the fuel cell performance at low relative humidity is examined by changing the ionomer used in the fuel cell electrodes. Research has been shown that reducing the ionomer side chain length as well as reducing the ionomer equivalent weight may be beneficial to low humidity operation [2]. While low humidity operation is important, fuel cell production costs must also be kept to a minimum in order to offer these zero-emission vehicles to a broader market. The primary method that will be used in this work to reduce the cost of fuel cell production is Reactive Spray Deposition Technology (RSDT). RSDT is a one-step flame-based process which can directly spray a Pt/C electrode onto a membrane using without additional drying steps. Previous work by Yu et al examined the use of RSDT to create low Pt loading electrodes for fuel cell applications [3]. With the cost-effective RSDT method chosen for the development of the fuel cells, the RSDT-produced catalyst coated membranes (CCMs) will be tested in 100% relative humidity conditions as a baseline as well as reduced humidity conditions used in fuel cell vehicle applications. While it is important to improve low humidity fuel cell performance, it is important to have a better understanding of how low humidity directly impacts the electrochemical mechanism. This work will analyze collected polarization and impedance data using a six-step overpotential analysis technique used by Yu et al [3, 4]. This overpotential analysis looks to separate the contributions of different polarization sources to have a better understanding of what is impacting the cell performance. This technique used at low humidity should allow us to have a better understanding about low humidity operation and how to improve fuel cell production by minimizing losses. The work presented will incorporate the RSDT process to make the change in ionomer with the target of improving and better understanding the performance of the fuel cells at low humidity conditions. Along with utilizing the six levels of overpotential analysis technique, we can determine how the changes to the ionomer and membrane as well as the carbon support all affect the polarization sources which will allow to better optimize the fuel cells moving and improve the future viability of fuel cell vehicles.

Evaluation of Dry-Wet Transition in PEFC Under Load Changes Based on Laser Spectroscopy Using Fiber-Optic Probe

The low-humidity operations of a polymer electrolyte fuel cell (PEFC) unfortunately induce the membrane dehydration and deteriorate the proton conductivity through the membrane. Especially, when the output current rapidly rises in steps under load-change conditions, the remarkable decrease of the water concentration on the anode side temporarily occurs and it causes the sudden drop of cell voltage called the “undershoot”. To alleviate this issue, it’s necessary to grasp the dry-wet transition in a PEFC operated under low-humidity and load-change conditions. In the previous studies, the transient water distribution along flow fields of operating PEFCs was vigorously investigated using gas chromatography (GC) [1]. Although GC techniques provide more accurate gas concentrations in flow channels, it takes a long time (at least two minutes) to collect a gas sample and identify a species. As an alternative approach, several researchers have applied tunable diode laser absorption spectroscopy (TDLAS) to the measurement of the water transport in working PEFCs [2,3]. TDLAS is one of the high-speed and high-sensitive absorption spectroscopy techniques which detect the absorbance of a chemical species and identify its concentration using infrared diode laser. Since the laser beam is directly injected into gas channels of PEFCs, the in-situ monitoring of water vapor concentration can be performed on operational fuel cells with a high time resolution of millisecond. However, it’s difficult to sufficiently secure the laser beam path within millimeter-scale narrow channels. In addition, the injection of laser beam into narrow channels tends to generate optical fringe noise due to the multiple reflection. To further enhance the sensitivity and accuracy of gas sensing, it’s necessary to improve the optical system of TDLAS inside fuel cell channels. The authors developed a new TDLAS-based gas sensing system with a single-ended fiber-optic probe to highly accurately measure the water vapor distribution in narrow flow channels of fuel cells at high speed. Fig. 1 shows the schematic diagram of the gas sensor developed in this study. The small-size single-ended probe with pitch and catch fibers enables the in-situ laser diagnostics within millimeter-scale narrow channels. In this measurement system, wavelength modulation spectroscopy (WMS) technique is adopted for high-sensitive gas detection. Furthermore, multiple detection signals are synthesized to cancel the fringe noise and improve the S/N ratio. The fiber-optic probe is directly inserted into the serpentine flow field (241 mm length, 1.0 mm width, 1.0 mm depth) of the experimental fuel cell, and the triangular modulated beam emitted from a DFB laser diode (Wavelength: 1392.5 nm) is injected into a channel through the single-mode pitch fiber. The back-reflection lights from the electrode surface are captured by six multi-mode catch fibers and converted electric signals by six photodiodes (PDs). After six signals are synthesized, the composite signal is transmitted to a lock-in amplifier and the second-harmonic (2f) spectra are obtained by phase-sensitive detection. The mole fraction of water vapor can be estimated from the peak height ratio of 2f and 4f spectra. This gas sensor was applied to evaluate the dry-wet transition in the anode channel of an operating PEFC under low-humidity and load change conditions. Fig. 2 presents the dynamic response of cell voltage and the transient water concentrations along the anode channel of a PEFC during the low-humidity and current-cycling operation. The repetitive current cycling was performed between 0.2 A/cm2 for 23 s, and 0.6 A/cm2 for 23 s. The inlet gas humidifications of the anode and cathode are 45 and 0% RH, respectively. When the current density is rapidly increased from 0.2 to 0.6 A/cm2, the large undershoot of cell voltage is observed because the water content at the anode side of the membrane is suddenly decreased by the strong electro-osmotic effect and the proton conductivity is deteriorated. It can also be seen that the water vapor concentration in the anode channel temporarily and largely drops in the downstream section (x/L=0.5-0.8). After the voltage undershoot, the cell voltage is gradually recovered because the back-diffusion of product water hydrates the membrane from the cathode to anode and reduces its ohmic resistance. The water concentration in the anode channel is gradually increased by the back-diffusion effect. It was found that the TDLAS-based gas sensing technique enables to detect the instantaneous fluctuation of water concentration in flow channels of PEFCs during load-change operations.

Materials, technological status, and fundamentals of PEM fuel cells – A review

PEM (Polymer Electrolyte Membrane) fuel cells have the potential to reduce our energy use, pollutant emissions, and dependence on fossil fuels. In the past decade, significant advances have been achieved for commercializing the technology. For example, several PEM fuel cell buses are currently rated at the technical readiness stage of full-scale validation in realistic driving environments and have met or closely met the ultimate 25,000-h target set by the U.S. Department of Energy. So far, Toyota has sold more than 4000 Mirai PEM fuel cell vehicles (FCVs). Over 30 hydrogen gas stations are being operated throughout the U.S. and over 60 in Germany. In this review, we cover the material, design, fundamental, and manufacturing aspects of PEM fuel cells with a focus on the portable, automobile, airplane, and space applications that require careful consideration in system design and materials. The technological status and challenges faced by PEM fuel cells toward their commercialization in these applications are described and explained. Fundamental issues that are key to fuel cell design, operational control, and material development, such as water and thermal management, dynamic operation, cold start, channel two-phase flow, and low-humidity operation, are discussed. Fuels and fuel tanks pertinent to PEM fuel cells are briefly evaluated. The objective of this review is three fold: (1) to present the latest status of PEM fuel cell technology development and applications in the portable and transportation power through an overview of the state of the art and most recent technological advances; (2) to describe materials and water/thermal transport management for fuel cell design and operational control; and (3) to outline major challenges in the technology development and the needs for fundamental research for the near future and prior to fuel cell world-wide deployment.

Physicochemical properties of Aquivion/fluorine grafted sepiolite electrolyte membranes for use in PEMFC

This study proposes a novel sepiolite-based Aquivion electrolyte membrane which could be operated at low relative humidity. In the study, it was discovered that the functionalization of sepiolite with fluorinated groups (i.e., -C7F15), named SEP-F, helps its dispersion in the composite membrane, compared with the use of natural sepiolite. The Aquivion/SEP-F composite membrane showed increased water uptake, thermo-mechanical and chemical stability as well as proton conductivity and decreased swelling compared with commercially available Nafion HP and pristine Aquivion. Their behavior in single cell MEA testing conditions was also assessed. Aquivion/SEP-F composite membrane can be an interesting alternative for low relative humidity operation of PEMFC (proton exchange membrane fuel cell).

Modeling of PEM fuel cell with thin MEA under low humidity operating condition

Compared to conventional proton exchange membrane (PEM) fuel cell with thick membrane electrode assembly, today’s automotive PEM fuel cell adopts thin MEA with thin catalyst coated membrane (CCM). The objective of this modeling study is to shed light on PEM fuel cell with thin MEA under low humidity operation condition. The effects of cathode catalyst oxygen reduction reaction activity and local O2 transport resistance on automotive PEM fuel cell high current operation are elucidated. The local O2 transport resistance is found to have large impacts on limiting current density. The effects of cathode humidifier elimination, operating pressure, and stoichiometry ratio on PEM fuel cell performance are also discussed. The thin membrane in thin MEA has much lower ohmic resistance compared with thick membrane, and it is easy to be hydrated under low humidity condition. But H2 crossover is stronger for thin membrane and needs to be watched out when thinning the membrane. In addition, a 50 cm2 PEM fuel cell with counter-cross flow field is simulated for demonstration of medium scale PEM fuel cell modeling. Under 0.65 V and 50%/0% humidity operating conditions, the 50 cm2 PEM fuel cell is predicted to supply average current density of 2.51 A/cm2.

Mesoporous Silica Nanospheres Impregnated with 12-Phosphotungstic Acid as Inorganic Filler of Nafion Membrane for Proton Exchange Membrane Fuel Cells.

In this work, a novel nanocomposite material, hollow mesoporous silica nanosphere impregnated with 12-phosphotungstic acid, briefed as HMSN-I, was synthesized by a vacuum-assisted impregnation method. The HMSN-I was used as an inorganic filler to synthesize Nafion-based composite membranes for the high temperature and low humidity operation of proton exchange membrane fuel cells (PEMFCs). The Nafion/HMSN-I composite membrane showed much higher conductivity than the pristine Nafion membrane under the identical conditions. The cell with the Nafion/HMSN-I composite membrane demonstrated significantly enhanced cell performance and stability in high temperature and low humidity environment. The mechanism can be ascribed to the enhanced proton conductivity and water retention ability of the composite membrane, which is very promising for the development of high temperature and low humidity PEMFCs.