Design,fabrication and performance analysis of a 200 W PEM fuel cell short stack

A PEM fuel cell short stack of 200 W capacity, with an active area of 100 cm² has been designed and fabricated in-house. The status of unit cell performance was 0.55 W cm⁻². Based on the unit cell technology, a short stack has been developed. The proper design of uniform flow distribution, cooling plate and compressed end plate were important to achieve the best performance of the short stack. The performance of four cells stack was analyzed in static and dynamic modes. In the static mode of polarization curve, the stack has peak power density of 0.55 W cm⁻² (220 W) at 0.5 V per cell, when the voltage was scanning from low to high voltage (1.5–3.5 V), and resulted in minimum water flooding inside the stack. In this study a series of dynamic loadings were tested to simulate the vehicle acceleration. The fuel cell performances respond to dynamic loading influenced by the hydrogen/air stoichiometric, back pressure, and dynamic-loading time. It was needed high hydrogen stoichiometric and back pressure to maintain high dynamic performance. In the long-time stable power testing, the stack was difficult to maintain at high performance, due to the water flooding at high output power. An adjusting cathode back-pressure method for purging water was proposed to prevent the water flooding at flow channels and maintain the stable output power at 170 W (0.42 W cm⁻²).     

Quantification of water in hydrophobic and hydrophilic flow channels subjected to gas purging via neutron imaging

Water removal from an idle fuel cell is an important issue for start-up/shutdown down under cold temperature conditions. In our study, we performed an in-situ neutron imaging for a PEM fuel cell with bipolar plates, treated with a super-hydrophobic and super-hydrophilic coating on the flow channels. The coatings were applied to the channels but not on the landings in contact with the GDL. The cells were run at a constant voltage prior to shutdown, then sets of neutron images were taken with purge velocities varied from 1 m s⁻¹ to 4 m s⁻¹, in intervals of 1 m s⁻¹. It was found that changing the wettability of the flow channels can improve the dynamics of water removal during purging. The super-hydrophilic and super-hydrophobic coating had better performance in removing water on the landings and in the channels, respectively. Based on our test cells, we used the amount of water remaining as a metric and found no significant improvement by purging the cell at velocities greater than 3 m s⁻¹.     

Improvement of water management in polymer electrolyte membrane fuel cell thanks to cathode cracks

The role of cathodic structure on water management was investigated for planar micro-air-breathing polymer electrolyte membrane fuel cells (PEMFCs). The electrical results demonstrate the possibility to decrease, with the same structure, both cell drying and cell flooding according to the environmental and operation conditions. Thanks to a simultaneous study of internal resistance and scanning electronic microscope (SEM) images, we demonstrate the advantageous influence of the presence of crack in cathodic catalytic layer on water management. On the one hand, the gold layer used as cathodic current collector is in contact with the electrolyte in the cracked zones which allows water maintenance within the electrolyte. It allows to decrease the cell drying and thus strongly increase the electrical performances. For cells operated in a 10% relative humidity atmosphere at 30 °C and at a potential of 0.5 V, the current density increases from 28 mA cm⁻² to 188 mA cm⁻² (+570%) for the cell with a cathodic cracked network. On the other hand, the reduction in oxygen barrier diffusion due to the cathodic cracks allows to improve oxygen diffusion. In flooding state, the current densities were higher for a cell with a cracked network. For cells operating in a 70% relative humidity atmosphere at 30 °C and at a potential of 0.2 V, a current density increase from 394 mA cm⁻² to 456 mA cm⁻² (16%) was noted for the cell with a cathodic cracked network. Microscopic observations allowed us to visualize water droplets growth mechanism in cathodic cracks. It was observed that the water comes out of the crack sides and partially saturates the cracks before emerging on cathodic collector. These results demonstrate that cathode structuration is a key parameter that plays a major role in the water management of PEMFCs.     

Exploring the effects of symmetrical and asymmetrical relative humidity on the performance of H2/air PEM fuel cell at different temperatures

This article is dedicated to study the interlinked effects of symmetric relative humidity (RH), and asymmetric RH on the performance of H₂/air PEM fuel cell at different temperatures. The symmetric and asymmetric RH were achieved by setting the cathode relative humidity (RHC) and anode relative humidity (RHA) as equal and unequal values, respectively. The cell performance was evaluated by collecting polarization curves of the cell at different RH, RHC and RHA and at different cell temperatures (T_cell). The polarization curves along with the measured internal cell resistance (membrane resistance) were discussed in the light of the present fuel cell theory. The results showed that symmetric relative humidity has different impacts depending on the cell temperature. While at RH of 35% the cell can show considerable performance at T_cell = 70 °C, it is not so at T_cell = 90 °C. At T_cell = 70 °C, the cell potential increases with RH at lower and medium current densities but decreases with RH at higher currents. This was attributed to the different controlling processes at higher and lower current densities. This trend at 70 °C is completely destroyed at 90 °C. Operating our PEM fuel cell at dry H₂ gas conditions (RHA = 0%) is not detrimental as operating the cell at dry Air (O₂) conditions (RHC = 0%). At RHA = 0% and humidified air, water transport by back diffusion from the cathode to the anode at the employed experimental conditions can support reasonable rehydration of the membrane and catalysts. At RHA = 0, a possible minimum RHC for considerable cell operation is temperature dependent. At RHC = 0 conditions, the cell can operate only at RHA = 100% with a loss that depends on T_cell. It was found that the internal cell resistance depends on RH, RHA, RHC and T_cell and it is consistent with the observed cell performance.     

Cross-linked miscible blend membranes of sulfonated poly(arylene ether sulfone) and sulfonated polyimide for polymer electrolyte fuel cell applications

Cross-linked miscible blend (CMB) membranes were prepared from sulfonated poly(arylene ether sulfone) (SPAES) and sulfonated polynaphthalimide (SPI). They were transparent and insoluble in solvents. They showed the intermediate properties between SPAES and SPI concerning mechanical strength, water uptake, membrane swelling and proton conductivity. As for membrane swelling and proton conductivity, SPAES was almost isotropic, whereas SPI was highly anisotropic. CMB membranes were moderately anisotropic and had the advantages of the smaller in-plane membrane swelling and the larger through-plane conductivity compared to SPAES and SPI, respectively. Polymer electrolyte fuel cell performance of CMB2 membrane with an equal weight ratio of SPAES/SPI and an ion exchange capacity (IEC) of 1.74 meq g⁻¹ was investigated, compared to SPI membrane (R1) with a slightly higher IEC of 1.86 meq g⁻¹. At 90 °C, 0.1 MPa and relatively high humidification of 82/68% RH or 0.2 MPa and low humidification of 50-30% RH, CMB2 showed the reasonably high cell performances. At 110 °C and 50-33% RH, the cell performance was fairly high only at a high pressure of 0.3 MPa, but low at 0.2-0.15 MPa. At these conditions, the cell performance was better for CMB2 than for R1 due to the more effective back-diffusion of water formed at cathode into membrane. CMB2 showed the fairly high PEFC durability at 110 °C.     

In and ex situ characterization of an anion-exchange membrane for alkaline direct methanol fuel cell (ADMFC)

Anion exchange membrane fumasep^® FAA-2 was characterized with ex and in situ methods in order to estimate the membranes’ suitability as an electrolyte for an alkaline direct methanol fuel cell (ADMFC). The interactions of this membrane with water, hydroxyl ions and methanol were studied with both calorimetry and NMR and compared with the widely used proton exchange membrane Nafion^® 115. The results indicate that FAA-2 has a tighter structure and more homogeneous distribution of ionic groups in contrast to the clustered structure of Nafion, moreover, the diffusion of OH⁻ ions through this membrane is clearly slower compared to water molecules. The permeability of methanol through the FAA-2 membrane was found to be an order of magnitude lower than for Nafion. Fuel cell experiments in 1 mol dm⁻³ methanol with FAA-2 resulted in OCV of 0.58 V and maximum power density of 0.32 mW cm⁻². However, even higher current densities were obtained with highly concentrated fuels. This implies that less water is needed for fuel dilution, thereby decreasing the mass of the fuel cell system. In addition, electrochemical impedance spectroscopy for the ADMFC was used to determine ohmic resistance of the cell facilitating the further membrane development.     

About internal currents during start-up in proton exchange membrane fuel cell

This paper reports about an experimental and numerical study of the internal currents that occur during fuel cell start-up under open-circuit conditions. The internal currents were measured in a segmented cell specifically designed for this purpose and it was found that they could reach values higher than 1 A cm⁻². They result from the potential that appears at the inlet of the anode compartment while hydrogen pushes oxygen, air, or possibly nitrogen that was introduced for purging water toward the outlet. For a short time, a fraction of the channels is filled with hydrogen while the other part is still occupied by the gas initially present. The model presented in the paper demonstrates that the occurrence of internal currents can be explained mostly by capacitive effects. Carbon oxidation occurs probably simultaneously but its contribution to the internal currents is by all appearances negligible. The model also explains the transient voltage rise (over the steady state open circuit voltage) that is sometimes observed experimentally shortly after the fuel cell start-up.     

Cross-linked poly(vinyl alcohol) and poly(styrene sulfonic acid-co-maleic anhydride)-based semi-interpenetrating network as proton-conducting membranes for direct methanol fuel cells

A series of semi-interpenetrating network (SIPN) membranes was synthesized by using poly(vinyl alcohol) (PVA) with sulfosuccinic acid (SSA) as a cross-linking agent and poly(styrene sulfonic acid-co-maleic acid) (PSSA-MA) as a proton source for direct methanol fuel cell (DMFC) application. A bridge of SSA between PVA molecules not only reinforced the network but also provided extra proton-conducting paths. PSSA-MA chains trapped in the network were the major proton conduction path of the membrane. The SIPN membranes with 80% PSSA-MA (SIPN-80) exhibited a higher proton conductivity value of 2.59 × 10⁻² S cm⁻¹ and very low methanol permeability (4.1 × 10⁻⁷ cm² s⁻¹). More specifically, the SIPN membranes exhibited very high selectivity (proton conductivity/methanol permeability). Membrane characteristics such as water uptake, proton conductivity and methanol permeability were evaluated to determine the effect of PVA molecular weights. The SIPN membranes with higher PVA molecular weight were also evaluated using methanol and oxygen gas in a single cell fuel cell at various temperatures. Power density value of over 100 mW cm⁻² was obtained for SIPN membrane-based membrane electrode assembly at 80 °C and using commercial binary alloy anode catalysts and 2 M methanol.     

Fast proton-conducting glass membrane based on porous phosphosilicate and perfluorosulfonic acid polymer

A proton-conducting glass membrane based on porous phosphosilicate and perfluorosulfonic acid polymer was prepared via a modified sol-gel approach. The morphology, pore structure, water uptake property, proton conductivity and fuel cell performance of the membrane were investigated in this work. The hybrid glass membrane showed extremely high proton conductivity of 0.1 S cm⁻¹ in humid atmosphere. In the H₂/O₂ fuel cell measurement, an open circuit potential (OCV) of 0.94 V and a maximum output power density of 42.6 mW cm⁻² was obtained at 25 °C.     

Solvent processible, high-performance partially fluorinated copoly(arylene ether) alkaline ionomers for alkaline electrodes

A solvent processable, low water uptake, partially fluorinated copoly(arylene ether) functionalized with pendant quaternary ammonium groups (QAPAE) was synthesized and uses as the ionomer in alkaline electrodes on fuel cells. The quaternized polymers containing fluorinated biphenyl groups were synthesized via chloromethylation of copoly(arylene ether) followed by amination with trimethylamine. The resulting ionomers were very soluble in polar, aprotic solvents. Highly aminated ionomers had conductivities approaching 10 mS cm⁻¹ at room temperature. Compared to previous ionomers based on quaternized poly(arylene ether sulfone) (QAPSF) with similar ion exchange capacity (IEC), the water uptake of QAPAE was significantly less due to the hydrophobic octafluoro-biphenyl groups in the backbone. The performance of the fuel cell electrodes made with the QAPAE ionomers was evaluated as the cathode on a hybrid AEM/PEM fuel cell. The QAPAE alkaline ionomer electrode with IEC = 1.22 meq g⁻¹ had superior performance to the electrodes prepared with QAPSF, IEC = 1.21 meq g⁻¹ at 25 and 60 °C in a H₂/O₂ fuel cell. The peak power densities at 60 °C were 315 mW cm⁻² for QAPAE electrodes and 215 mW cm⁻² for QAPSF electrodes.     

Coated 316L stainless steel with CrxN film as bipolar plate for PEMFC prepared by pulsed bias arc ion plating

Three different kinds of Cr_xN films on 316L stainless steels were prepared by pulsed bias arc ion plating as bipolar plates for proton exchange membrane fuel cell (PEMFC). The interfacial contact resistance, corrosion resistance and surface energy of the bipolar plate samples were investigated. Among the three samples, the 316L stainless steel coated with Cr_0.49N_0.51 → Cr_0.43N_0.57 gradient film (sample 2) exhibited the best-integrated performance. The contact resistance between sample 2 and Toray carbon paper was 6.9–10.0 mΩ cm² under 0.8–1.2 MPa. The bipolar plate sample also showed improved corrosion resistance in simulated PEMFC environments. Either in the reduction environment or in the oxidation environment 25 °C and 70 °C, the corrosion current densities of sample 2 were about one to two orders of magnitude lower than those of the base metal. In addition, the open circuit corrosion potential of sample 2 was also the highest in 0.5 M H₂SO₄ + 5 ppm F⁻ solution at 25 °C. The treated bipolar plate had high surface energy; and the contact angle of sample 2 with water was about 90°, which is beneficial for water management in fuel cell.     

A graphite-granule membrane-less tubular air-cathode microbial fuel cell for power generation under continuously operational conditions

An air-cathode microbial fuel cell (MFC) is an efficient and sustainable MFC configuration for recovering electrical energy from organic substances. In this paper, we developed a graphite-granule anode, tubular air-cathode MFC (GTMFC) capable of continuous electricity generation from glucose-based substrates. This GTMFC produced a maximum volumetric power of 50.2 W m⁻³ at current density of 216 A m⁻³ (R_EX = 22 Ω). Electrochemistry impedance spectroscopy (EIS) measurements demonstrated an overall internal resistance of 27 Ω, consisting of ohmic resistance of R_ohm = 13.8 Ω (51.1%), a charge-transfer resistance of R_c = 6.1 Ω (22.6%) and a diffusion resistance of R_d = 7.2 Ω (26.3%). Power generation with respect to initial chemical oxygen demand (COD) concentration was described well by an exponential saturation model. Recirculation was to found to have a significant effect on electrochemical performance at low COD concentrations, while such effect was absent at high COD concentrations. This study suggests a feasible and simple method to reduce internal resistance and improve power generation of sustainable air-cathode MFCs.     

New insights in the polarization resistance of anode-supported solid oxide fuel cells with La0.6Sr0.4Co0.2Fe0.8O3 cathodes

In this study, the polarization resistance of anode-supported solid oxide fuel cells (SOFC) with La_0.6Sr_0.4Co_0.2Fe_0.8O₃ (LSCF) cathodes was investigated by I-V sweep and electrochemical impedance spectroscopy under a series of operating voltages and cathode environments (i.e. stagnant air, flowing air, and flowing oxygen) at temperatures from 550 °C to 750 °C. In flowing oxygen, the polarization resistance of the fuel cell decreased considerably with the applied current density. A linear relationship was observed between the ohmic-free over-potential and the logarithm of the current density of the fuel cell at all the measuring temperatures. In stagnant or flowing air, an arc related to the molecular oxygen diffusion through the majority species (molecular nitrogen) present in the pores of the cathode was identified at high temperatures and high current densities. The magnitude of this arc increased linearly with the applied current density due to the decreased oxygen partial pressure at the interface of the cathode and the electrolyte. It is found that the performance of the fuel cell in air is mainly determined by the oxygen diffusion process. Elimination of this process by flowing pure oxygen to the cathode improved the cell performance significantly. At 750 °C, for a fuel cell with a laser-deposited Sm_0.2Ce_0.8O_1.9 (SDC) interlayer, an extraordinarily high power density of 2.6 W cm⁻² at 0.7 V was achieved in flowing oxygen, as a result of reduced ohmic and polarization resistance of the fuel cell, which were 0.06 Ω cm² and 0.03 Ω cm², respectively. The results indicate that microstructural optimization of the LSCF cathode or adoption of a new cell design which can mitigate the oxygen diffusion limitation in the cathode might enhance cell performance significantly.     

Direct methane solid oxide fuel cell working by gradual internal steam reforming: Analysis of operation

A solid oxide fuel cell was designed to be operated with pure hydrocarbons, without additive or carrier gas, in order to bring technological simplifications, cost reductions and to extend the fuel flexibility limits. The cell was built-up from a conventional cell (LSM/YSZ/Ni-YSZ), to which was added a Ir-CeO₂ catalyst layer at the anode side and an original current collecting system. The cell was first operated with steam in gradual internal reforming (GIR) conditions (R = [H₂O]/[CH₄] < 1) with carrier gas at the anode. The optimal operating parameters were determined in terms of flow rates, cell potential, and fuel utilisation. The cell was finally operated with pure dry methane at 900 °C at 0.6 V yielding current density of about 0.1 A cm⁻² at max power for 120 h. Small but abrupt deterioration of the performances was observed, but no carbon deposition. Electrical and chemical analysis of this degradation are provided.At total, the fuel cell was operated for more than 200 h in pure dry methane, demonstrating that gradual internal reforming actually occurred efficiently in the anode compartment, which make possible operation without reforming agent such as H₂O or CO₂ for other hydrocarbon fuels.     

Characterization of direct methanol fuel cell (DMFC) applications with H2SO4 modified chitosan membrane

Chitosan (Chs) flakes were prepared from chitin materials that were extracted from the exoskeleton of Cape rock lobsters in South Africa. The Chs flakes were prepared into membranes and the Chs membranes were modified by cross-linking with H₂SO₄. The cross-linked Chs membranes were characterized for the application in direct methanol fuel cells. The Chs membrane characteristics such as water uptake, thermal stability, proton resistance and methanol permeability were compared to that of high performance conventional Nafion 117 membranes. Under the temperature range studied 20-60 °C, the membrane water uptake for Chs was found to be higher than that of Nafion. Thermal analysis revealed that Chs membranes could withstand temperature as high as 230 °C whereas Nafion 117 membranes were stable to 320 °C under nitrogen. Nafion 117 membranes were found to exhibit high proton resistance of 284 s cm⁻¹ than Chs membranes of 204 s cm⁻¹. The proton fluxes across the membranes were 2.73 mol cm⁻² s⁻¹ for Chs- and 1.12 mol cm⁻² s⁻¹ Nafion membranes. Methanol (MeOH) permeability through Chs membrane was less, 1.4 × 10⁻⁶ cm² s⁻¹ for Chs membranes and 3.9 × 10⁻⁶ cm² s⁻¹ for Nafion 117 membranes at 20 °C. Chs and Nafion membranes were fabricated into membrane electrode assemblies (MAE) and their performances measure in a free-breathing commercial single cell DMFC. The Nafion membranes showed a better performance as the power density determined for Nafion membranes of 0.0075 W cm⁻² was 2.7 times higher than in the case of Chs MEA.     

Hygrothermal effects on properties of highly conductive epoxy/graphite composites for applications as bipolar plates

The hygrothermal effects on mechanical, thermal, and electrical properties of highly conductive graphite-based epoxy composites were investigated. The highly conductive graphite-based epoxy composites were found to be suitable for applications as bipolar plates in proton exchange membrane (PEM) fuel cells. The hygrothermal aging experiments were designed to simulate the service conditions in PEM fuel cells. Specifically, the composite specimens were immersed in boiling water, aqueous sulphuric acid solution, and aqueous solution of hydrogen peroxide. The water uptake, changes in surface appearance and dimensions, glass transition behavior and thermal stability, and electrical and mechanical properties were evaluated. The water uptake at short time increased linearly with the square root of time as in linear Fickian diffusion. The presence of graphite significantly reduced both the rate and extent of water uptake. No discernible changes in specimen dimensions, surface appearance, and morphology of the composites were observed. The electrical conductivity and mechanical properties remained almost unchanged. The wet specimens showed slight reduction of glass transition temperature (T_g) due to plasticization of epoxy networks by absorbed water, while the re-dried specimens showed small increase of T_g. The composites maintained high electrical conductivity of about 300–500 S cm⁻¹ and good mechanical properties and showed thermal stability up to 350 °C.     

Experimental investigation of PEM fuel cell aging under current cycling using segmented fuel cell

The effect of inhomogeneous humidification on the local aging of the membrane electrode assemblies (MEAs) was investigated using segmented fuel cells under current cycling conditions. The active area of the MEA was divided into 8 segments along the flow channels. The effects of the local humidification of the MEA were measured by the high frequency resistance (HFR). It was found that the HFR of the first segment was largest due to the lower humidification. The concentration of water and water vapor was higher downstream, the MEA was humidified, and consequently the value of HFR also decreased. The fuel cells were operated at 60% RH, cell temperature 60 °C for uniform cell performance, with current cycling between 700 and 70 mA/cm². The polarization curve showed less differentiation between each segment after 450 cycles and 150 hours of the current loading test. The MEA was exposed to cycling expansion and contraction during the drying and humidification processes, respectively. The results suggest that the membrane might experience mechanical degradation in downstream segments. In addition, the slope of the cyclic voltammetry (CV) curves increased from region 6 to region 8. The surface area of the catalyst also decreased due to the downstream water flooding, which might block the mass transfer and result in fuel starvation, carbon corrosion and catalyst degradation.     

Covalent-ionically cross-linked polyetheretherketone proton exchange membrane for direct methanol fuel cell

In this paper, the proton exchange membrane prepared by covalent-ionically cross-linking water soluble sulfonated-sulfinated poly(oxa-p-phenylene-3,3-phthalido-p-phenylene-oxa-p-phenylene-oxy-phenylene) (SsPEEK-WC) is reported. Compared with covalent cross-linked PEEK-WC membrane, this covalent-ionically cross-linked PEEK-WC membrane exhibits extremely reduced water uptake and methanol permeability, but just slightly sacrificed proton conductivity. The proton conductivity of the covalent-ionically cross-linked PEEK-WC membrane reaches to 2.1 × 10⁻² S cm⁻¹ at room temperature and 4.1 × 10⁻² S cm⁻¹ at 80 °C. The methanol permeability is 1.3 × 10⁻⁷ cm² s⁻¹, 10 times lower than that of Nafion^® 117 membrane. The results suggest that the covalent-ionically cross-linked PEEK-WC membrane is a promising candidate for direct methanol fuel cell because of low methanol permeability and adequate proton conductivity.     

Initialization of a methane-fueled single-chamber solid-oxide fuel cell with NiO + SDC anode and BSCF + SDC cathode

The initialization of an anode-supported single-chamber solid-oxide fuel cell, with NiO + Sm_0.2Ce_0.8O_1.9 anode and Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ + Sm_0.2Ce_0.8O_1.9 cathode, was investigated. The initialization process had significant impact on the observed performance of the fuel cell. The in situ reduction of the anode by a methane–air mixture failed. Although pure methane did reduce the nickel oxide, it also resulted in severe carbon coking over the anode and serious distortion of the fuel cell. In situ initialization by hydrogen led to simultaneous reduction of both the anode and cathode; however, the cell still delivered a maximum power density of ∼350 mW cm⁻², attributed to the re-formation of the BSCF phase under the methane–air atmosphere at high temperatures. The ex situ reduction method appeared to be the most promising. The activated fuel cell showed a peak power density of ∼570 mW cm⁻² at a furnace temperature of 600 °C, with the main polarization resistance contributed from the electrolyte.     

Quaternized cardo polyetherketone anion exchange membrane for direct methanol alkaline fuel cells

Quaternized cardo polyetherketone (QPEK-C) membranes for alkaline fuel cells were prepared via chloromethylation, quaternization and alkalization of cardo polyetherketone (PEK-C). The chemical reaction for PEK-C modification was confirmed by nuclear magnetic resonance (¹H NMR) and energy-dispersive X-ray spectroscopy (EDAX). The QPEK-C membrane was characterized by X-ray photoelectron spectroscopy (XPS) and thermo gravimetric analysis (TG). The ion-exchange content (IEC), water and methanol uptakes, methanol permeability and conductivity of the QPEK-C membranes were measured to evaluate their applicability in alkaline methanol fuel cells. The ionic conductivity of the QPEK-C membrane varied from (1.6 to 5.1) × 10⁻³ S cm⁻² over the temperature range 20-60 °C. The QPEK-C membrane showed excellent methanol resistance. When the concentration of methanol was 4 M, the methanol permeability was less than 10⁻⁹ cm² s⁻¹ at 30 °C.