Operation of a proton exchange membrane fuel cell under non-humidified conditions using a membrane–electrode assemblies with composite membrane and electrode

Composite membranes with hydrophilic substances can retain water and allow the operation of proton exchange membrane fuel cells (PEMFCs) under non-humidified conditions. In this work, thin Nafion composite membranes with silica are prepared to operate a PEMFC with dry fuel and oxidant. In addition, the role of silica in the catalyst layer as a water retainer is studied. In particular, the anode and the cathode are modified separately to elucidate the effect of silica. The incorporation of silica in the membrane and the catalyst layer enhances single-cell performance under non-humidified operation. The cell performance of membrane–electrode assemblies using the composite membrane and electrode is higher than that of a MEA using commercial Nafion 111 membrane under non-humidified conditions.     

Novel multilayer Nafion/SPI/Nafion composite membrane for PEMFCs

A novel multilayer membrane for the proton exchange membrane fuel cell (PEMFC) was developed. Nafion was dispersed uniformly onto both sides of the sulfonated polyimide (SPI) membrane. The Nafion/SPI/Nafion composite membrane was prepared by immersing the SPI into the Nafion-containing casting solution. Through immersing both membranes into the Fenton solution at 80 °C for 0.5 h for an accelerated ex situ test, chromatographic analysis of the water evacuated from the cathode and the anode of the cells and a durability test of a single proton exchange membrane fuel cells, it was proved that the stability of the composite membrane has been greatly improved by adding the Nafion layer compared with the SPI membrane. The fuel cell performance with the SPI and Nafion/SPI/Nafion membranes was similar to the performance with the commercial product Nafion^® NRE-212 membrane at 80 °C.     

Effect of ammonium ion distribution on Nafion conductivity

Nafion^® conductivity in a proton exchange membrane fuel cell (PEMFC) with the fuel stream containing ammonia is mainly affected by the ammonium ion composition and operating conditions. In this study, the effect of ammonium ion distribution on Nafion conductivity was investigated for the first time. The conductivities of two kinds of contaminated membranes having uniform and non-uniform ammonium ion distributions were studied. To simulate a membrane with a well-defined ammonium ion concentration profile, three individual Nafion membranes containing known amounts of ammonium ions were physically stacked together. The uniform and non-uniform cases represented membranes having three layers with the same yN⁺H₄ or step changes in concentration, respectively. Under fuel cell operations, the conductivities of non-uniformly poisoned membranes were ca. 1.07-1.86 times larger than those of uniformly poisoned membranes, depending on humidity, contamination level, and ammonium ion distribution. Consequently, the performance prediction of a cationic-poisoned PEMFC needs to consider any concentration gradients that may exist in MEA. The liquid-phase conductivities of composite membranes were also studied and the results show that conductivity measurements performed in deionized water are not representative of what exists under fuel cell conditions due to rapid redistribution of ions in the Nafion via the liquid phase.     

Parallel serpentine-baffle flow field design for water management in a proton exchange membrane fuel cell

In a proton exchange membrane fuel cell (PEMFC) water management is one of the critical issues to be addressed. Although the membrane requires humidification for high proton conductivity, water in excess decreases the cell performance by flooding. In this paper an improved strategy for water management in a fuel cell operating with low water content is proposed using a parallel serpentine-baffle flow field plate (PSBFFP) design compared to the parallel serpentine flow field plate (PSFFP). The water management in a fuel cell is closely connected to the temperature control in the fuel cell and gases humidifier. The PSBFFP and the PSFFP were evaluated comparatively under three different humidity conditions and their influence on the PEMFC prototype performance was monitored by determining the current density–voltage and current density–power curves. Under low humidification conditions the PEMFC prototype presented better performance when fitted with the PSBFFP since it retains water in the flow field channels.     

Investigation of water transport through membrane in a PEM fuel cell by water balance experiments

Water balance in a polymer electrolyte membrane fuel cell (PEMFC) was investigated by measurements of the net drag coefficient under various conditions. The effects of water balance in the PEMFC on the cell performance were also investigated at different operating conditions. Experimental results reveal that the net drag coefficient of water through the membrane depended on current density and humidification of feed gases. It was found that the net drag coefficient (net number of water molecules transported per proton) ranged from −0.02 to 0.93, and was dependent on the operating conditions, the current load and the level of humidification. It was also found that the humidity of both anode and cathode inlet gases had a significant effect on the fuel cell performance. The resistance of the working fuel cell showed that the membrane resistance increased as the feed gas relative humidity (RH) decreased. The diffusion of water across Nafion membranes was also investigated by experimental water flux measurements. The electro-osmotic drag coefficient was evaluated from the experimental results of water balance and diffusion water flux measurements. The value of electro-osmotic drag coefficient, ranging from 1.5 to 2.6 under various operating conditions, was in agreement with literature values. The electro-osmotic drag coefficient, the net flux of water through the membrane and the effective drag as a function of operating conditions will also provide validation data for the fuel cell modeling and simulation efforts.     

Understanding of temperature-dependent performance of short-side-chain perfluorosulfonic acid electrolyte and reinforced composite membrane

The short-side-chain (SSC) perfluorosulfonic acid (PFSA) membranes are important candidates as membrane electrolytes applied for high temperature or low relative humidity (RH) proton exchange membrane fuel cells. In this paper, the fuel cell performance, proton conductivity, proton mobility, and water vapor absorption of SSC PFSA electrolytes and the reinforced SSC PFSA/PTFE composite membrane are investigated with respect to temperature. The pristine SSC PFSA membrane and reinforced SSC composite membrane show better fuel cell performance and proton conductivity, especially at high temperature and low relative humidity conditions, compared to the long-side-chain (LSC) Nafion membrane. Under the same condition, the proton mobility of SSC PFSA membranes is lower than that of the LSC PFSA membrane. The water vapor uptake values for Nafion 211 membrane, pristine SSC PFSA membrane and SSC PFSA/PTFE composite membrane are 9.62, 11.13, and 11.53 respectively at 40 °C and they increase to 9.89, 12.55 and 13.09 respectively at 120 °C. The high water content of SSC PFSA membrane makes it maintain high performance even at elevated temperatures.     

Preparation and characterization of phosphotungstic acid-derived salt/Nafion nanocomposite membranes for proton exchange membrane fuel cells

Nafion/Cs_2.5H_0.5PW₁₂O₄₀ nanocomposite membranes are prepared and characterized as alternate materials for PEMFC operation at high temperature/low humidity. The Cs_2.5H_0.5PW₁₂O₄₀ solid acid particles (hereafter CsPWA) have the high surface area, the high hygroscopic property and the ability to generate proton in the presence of water molecules. The results of prepared membranes at three levels (0, 10 and 15%) indicate that the CsPWA particles have influence on the water content, ion exchange capacity, thermal properties (TGA and DSC), proton conductivity and PEM fuel cell performance. Particles agglomeration and Nafion active sites (sulfonic groups) covering are seen in the nanocomposite membranes. The conductivity of nanocomposite membranes at high temperatures (110 and 120 °C) is higher than plain Nafion and may be related to the additional water within the nanocomposite membrane and/or the additional surface functional site provide by CsPWA. The fuel cell responses show that in the fully hydrated state and at the higher current densities, the prepared MEAs with nanocomposite membranes possess better response compared with the plain Nafion. In partially hydrated cell, at both low and high current densities, the superior performance of the MEA prepared by nanocomposite membranes is observed.     

Poly(ethylene glycol) modified activated carbon for high performance proton exchange membrane fuel cells

A high water retention membrane is developed by co-assembling poly(ethylene glycol) (PEG) grafted activated carbon (AC-PEG) with Nafion. The AC-PEG is prepared via a sol–gel process. The use of PEG as a transporting medium in AC-PEG shows a largely improved water retention ability, a higher proton conductivity and a reduced swelling ratio, making it well suited for proton exchange membrane fuel cells (PEMFCs). Further, the composite membranes show improved mechanical properties at high temperature, thus ensuring the structural stability of membranes during the fuel cell operation. Compositional optimized AC-PEG/Nafion composite membrane (15 wt% compared to Nafion) demonstrates a better performance than the commercially available counterpart, Nafion 212, in fuel cell measurements. To identify the key factor of the improved performance, current interrupt technique is used to quantitatively verify the changes of resistance under different relative humidity environment.     

High temperature operation of PEMFC: A novel approach using MEA with silica in catalyst layer

Composite membranes with inorganic substances can retain water and allow the operation of polymer electrolyte membrane fuel cells (PEMFCs) at high temperature under low humidity. In this work, the single cell was operated at high temperature using silica–Nafion composite membrane in addition with silica in catalyst layer. The cell was operated at various temperatures under different relative humidity conditions. We observed that the single cell performance decreased steeply as the cell temperature increased. The role of silica in the catalyst layer at high temperature operation was studied by varying the silica content in the catalyst layers. There was a gradual decrease in cell performance when the silica content increased in catalyst layer. The single cell performance of membrane electrode assemblies (MEAs) with composite membrane and electrode was higher than that of MEA with commercial Nafion 112 membrane for high temperature operation.     

The development of PTFE/Nafion/TEOS membranes for application in moderate and high temperature proton exchange membrane fuel cells

PTFE/Nafion (PN) and PTFE/Nafion/TEOS (PNS) membranes were fabricated for the application of moderate and high temperature proton exchange membrane fuel cells (PEMFCs), respectively. Membrane electrode assemblies (MEAs) were fabricated by PTFE/Nafion (and PTFE/Nafion/TEOS) membranes with commercially available low and high temperature gas diffusion electrodes (GDEs). The effects of relative humidity, operation temperature, and back pressure on the performance and durability test of the as-prepared MEAs were investigated. Incorporating TEOS into a PNS membrane and adding another layer of carbon onto a GDE would result in low membrane conductivity and low fuel cell performance respectively. However, in this work it is shown that HT-PNS MEAs demonstrate a higher performance than LT-PN MEAs in severe conditions - high temperature (118 °C) and low humidity (25% RH). The TEOS and additional carbon layer function as water retaining agents which are especially important for high temperature and low humidity conditions. The HT-PNS MEA showed good stability in a 50 h fuel cell test at high temperature, moderate relative humidity (50% RH) and back pressure of 14.7 psi.     

Parametric model of an intermediate temperature PEMFC

Proton exchange membrane fuel cells operating with Nafion^® membranes have encountered numerous problems associated with water management and CO poisoning because of their low temperature of operation. Higher temperature membranes have been investigated, one such membrane being polybenzimidazole (PBI). This paper presents a parametric model, which predicts the polarization performance of an intermediate temperature proton exchange membrane fuel cell (PEMFC). It also investigates the effects of porous media characteristics on fuel cell performance. Results show that for intermediate temperature fuel cells, mass limitation effects are absent as long as the catalyst regions are sufficiently permeable. It is predicted that the greatest scope for improving PBI PEMFC performance is increasing the membrane conductivity and improving the catalyst performance, as it interfaces with the PBI membrane.     

Characterization of electrosprayed Nafion films

Nafion has long served as a benchmark material when evaluating the performance of proton conducting membranes used in PEM fuel cells. Traditionally, the membranes are either extruded from dry polymer or cast from a liquid polymer solvent solution. Recently, a different technique for fabricating membranes has been developed. The new approach exploits electrostatic spraying, or electrospraying, to deposit PEM films, with the longer-term goal of fabricating membrane-electrode assemblies using this technique. The focus of this paper is to compare the proton transport and physical properties of electrosprayed membranes with those of extruded Nafion.Audio frequency complex impedance studies of the electrosprayed membranes were conducted at a variety of temperatures and pressures over a wide range of membrane water contents. The results are compared with similar data for Nafion 117 and for membranes cast from the electrosprayed solutions. Water uptake, dimensional changes, and electrical conductivity measurements indicate that extruded, cast, and electrosprayed Nafion films are similar, with the exception that the electrosprayed Nafion absorbs as much as 15 wt.% water more than the other two membranes with only a slight increase in conductivity. The activation volumes for electrosprayed Nafion are also consistent with those for Nafion 117 and concur with proposed mechanisms for proton transport in Nafion membranes. As in Nafion 117, a dielectric loss peak appeared in the electrosprayed Nafion at low temperatures after heating in vacuum at 380 K, further demonstrating the similarity between the two materials.     

Nanocomposite membranes of surface-sulfonated titanate and Nafion® for direct methanol fuel cells

Various thiol and sultone groups were grafted onto the surface of titanate nanosheets to render organic sulfonic acid (HSO₃–) functionality. The nanocomposite membranes were cast together with Nafion^® using these materials as inorganic fillers. Nanocomposite membranes containing surface-sulfonated titanates showed higher proton conductivity than composite membranes containing untreated TiO₂ P25 particles. They showed better mechanical and thermal stability than Nafion alone. The methanol permeability of nanocomposite membranes decreased with increasing the content of the sulfonated titanate in the nanocomposite membranes. The relative permeability of methanol through these composite membranes with 2 and 5 M methanol solutions was reduced by up to 38 and 26%, respectively, relative to pristine Nafion 115 membranes. The membrane electrode assembly using Nafion/sulfonated titanate nanocomposite membranes exhibited up to 57% higher power density than the assembly containing a pristine Nafion membrane under typical operating conditions of direct methanol fuel cells.     

Nafion/polyaniline composite membranes specifically designed to allow proton exchange membrane fuel cells operation at low humidity

A Nafion and polyaniline composite membrane (designated Nafion/PANI) was fabricated using an in situ chemical polymerization method. The composite membrane showed a proton conductivity that was superior to that obtained with Nafion^® 112 at low humidity (e.g. RH = 60%). Water uptake measurements revealed similarities between the Nafion^® 112 and Nafion/PANI membranes at different humidities. The high conductivity of the Nafion/PANI membrane at low humidity is hypothesized to be due to the existence of the extended conjugated bonds in the polyaniline; proton transfer is facilitated via the conjugated bonds in lower humidity environments allowing retention of the relatively high conductivity. Correspondingly, the performance of a single cell fuel cell containing the Nafion/PANI composite membrane is improved compared to a Nafion^® 112-containing cell under low humidity conditions. This is important for portable fuel cells, which are required to operate without external humidification.     

Chemical durability of thin pore-filling membrane in open-circuit voltage hold test

Long-term chemical stability of proton exchange membranes in polymer electrolyte fuel cells (PEFCs) is an important issue for widespread commercialization. Here, we report on the chemical stability of a membrane-electrode assembly with a 7 μm thick pore-filling membrane (porous substrate filled with high ion exchange capacity perfluorosulfonic acid (PFSA) polymer) using an open-circuit voltage hold test. The very thin pore-filling membrane shows comparable chemical durability to Nafion 211. Interestingly, the pore-filling membrane shows a different degradation behavior from Nafion 211 due to the use of chemically and mechanically stable porous substrate, with no thickness change and little amounts of fluorine leakages are observed in the pore-filling membrane compared to membrane thinning and large amounts of fluorine leakage in Nafion 211. The thin pore-filling membrane shows promise for application in PEFCs, as it balances high fuel cell performance at high temperature and low relative humidity with high chemical durability.     

Sulfonated polyimide/PTFE reinforced membrane for PEMFCs

A sulfonated polyimide (SPI)/PTFE reinforced membrane was synthesized by impregnating PTFE (porous polytetrafluoroethylene) membrane with a SPI/DMSO solution. The resulting composite membrane was mechanically durable and quite thin relative to traditional perfluorosulfonated ionomer membranes (PFSI). We expect the PTFE to restrict the swelling and dimensional change of the SPI when it is immersed into water. And the reinforcement with PTFE can increase the hydrolysis stability of the SPI due to the low swelling and low dimensional change. From ex-situ testing and a short term fuel cell “life” test, we conclude that the PTFE reinforced sulfonated polyimide had a higher hydrolytic stability than pure sulfonated polyimide. The thin SPI/PTFE membrane showed comparable fuel cell performance with the commercial NRE-212 membrane with H₂/O₂ at 80 °C under fully humidified conditions. The chemically modified membrane with Nafion layer showed good stability in a 120 h fuel cell test.     

Novel Nafion composite membranes with mesoporous silica nanospheres as inorganic fillers

Novel Nafion composite proton exchange membranes are prepared using mesoporous MCM-41 silica nanospheres as inorganic fillers. The novelty of this study lies in the structural design of inorganic silica fillers: the nanosized and monodisperse spherical morphology of fillers facilitates the preparation of homogenous composite membranes, whilst the superior water adsorption of the mesostructure in fillers consigns enhanced water retention properties to the polymer membranes. Scanning electron microscopy images of the composite membranes indicate that well-dispersed silica nanospheres are embedded in the Nafion matrix, but a large amount of added fillers (3 wt.%) causes some agglomeration of the nanospheres. Compared with the Nafion cast membrane, the composite membranes offer improved thermal stability, enhanced water retention properties, and reduced methanol crossover. Despite the enhancement of water retention, the composite membranes still exhibit a proton conductivity reduction of 10–40% compared with pristine Nafion. This is likely due to the incorporation of much less conductive silica fillers than Nafion. The composite membrane containing 1 wt.% of fillers displays the best cell performance in direct methanol fuel cell tests; it gives a maximum power density of 21.8 mW cm⁻², i.e., ∼20% higher than the Nafion cast membrane. This is attributed to its similar conductivity to Nafion, and its markedly reduced methanol crossover, namely, ∼1.2 times lower.     

Fabrication and evaluation of Nafion nanocomposite membrane based on ZrO2–TiO2 binary nanoparticles as fuel cell MEA

Synthesis and characterization of nanocomposite membranes for proton exchange membrane fuel cell (PEMFC) operating at different temperatures and humidity were investigated in this study. Recast Nafion composite membrane with ZrO₂ and TiO₂ nanoparticles with 75 nm in mean size diameter, prepared for PEM fuel cells. Nafion/TiO₂ composite membranes have been also fabricated by in-situ sol–gel method. However, fine particles of the ZrO₂ were synthesized and Nafion/ZrO₂ composite membrane were produced by blending a 5% (w/w) Nafion-water dispersion with the inorganic compound. All nanocomposite membranes demonstrated higher water retention in comparison with unmodified membranes. Proton conductivity increased with increasing ZrO₂ content while TiO₂ additive (with mean size of 25 nm) enhanced water retention. Subsequently, structures of the membranes were investigated by Atomic Force Microscopy (AFM), Scanning Electron Microscopy (SEM) as well as X-Ray Diffraction (XRD). In addition, water uptake and proton conductivity of the modified membranes were also measured. The nanocomposite membrane was tested in a 25 cm² commercial single cell at the temperature range of 80–110 °C and in humidified H₂/O₂ under different relative humidity (RH) conditions. The membrane electrode assembly (MEA) prepared from Nafion/TiO₂, ZrO₂ presented highest PEM fuel cell performance in respect of I–V polarization under condition of 110 °C, 0.6 V and 30% RH and 1 atm.     

Study of morphological characteristics on hydrophilicity‐enhanced SiO2/Nafion composite membranes by using multimode atomic force microscopy

Air‐breathing proton exchange membrane fuel cells (AB‐PEMFCs) have a great potential for commercialization owing to their simple mechanical configuration and low cost compared with traditional proton exchange membrane fuel cells (PEMFCs). However, AB‐PEMFCs perform worse than traditional PEMFCs owing to the omission of the humidifier and a poor air supply system. In this study, hygroscopic metal oxide materials with good water absorption characteristics were employed in a Nafion membrane without humidification to compensate for the lack of performance owing to low proton conductivity. Among the various metal oxide materials, mesoporous structured silica has been synthesized with Nafion to increase the water content in nonhumidified conditions. The local morphological variation and surface charge distribution on the pristine Nafion and SiO₂/Nafion composite membranes were analyzed by using multimode atomic force microscopy and force distance analyses. Several remarkable results were revealed, including considerable morphological changes and a locally separated water cluster network structure.