Decrease In Proton Conductivity Research Articles

Mechanically strengthened polybenzimidazole membrane via a two-step crosslinking strategy for high-temperature proton exchange membrane fuel cell

To simultaneously increase the mechanical strength and proton conductivity of Phosphoric acid-doped polybenzimidazole (PA-PBI), a facile two-step crosslinking approach is proposed to prepare the imidazole-rich crosslinked polybenzimidazole membranes for high-temperature proton exchange membrane fuel cells (HT-PEMFCs) in this work. Unlike the classic crosslinking approaches that usually enhance mechanical strength of membrane but lead to a decrease of proton conductivity, the mechanical strength of two-step crosslinked membrane improves from 3.93 MPa to 9.62 MPa and proton conductivity increases from 91 mS cm−1 to 183 mS cm−1 at 200 °C compared with pristine OPBI membrane. The promotion should be ascribed to the introduction of PA-affinity sites on crosslinkers and the formation of homogeneous crosslinking networks. Meanwhile, due to the crosslinked networks, the oxidative and dimensional stability of crosslinked membranes are also promoted. In addition, the two-step cross-linking method can avoid the self-reaction between cross-linking agents, thereby obtaining a more uniform cross-linking network, which is of great significance for the use of fuel cells. When the membranes are used in a fuel cells, they exhibited a maximum power density of 448.8 mW cm−2 at 160 °C (H2/Air, without humidification.).

Enhancement of comprehensive performance of high-temperature and low-humidity proton exchange membranes: Crosslinking of trifunctional proton conductor with branched polybenzimidazole

In order to improve the proton conductivity and antioxidant stability of proton exchange membranes (PEMs) at wide relative humidity (RH), composite membranes cross-linked with tribranched polybenzimidazole (TaPBI) and poly(azido-1-propanesulfonic acid −2,2′- methylsulfonic acid −5,5′-bis-benzimidazole) polyphosphazenes (PSBI) (TaPBI-PSBI) are prepared. TaPBI, as a branched polybenzimidazole, exhibits a large free volume. The trifunctional proton conductor PSBI-Na incorporates abundant flexible acidic groups and basic sites to enhance proton conductivity, and cross-linking groups to increase its binding strength with PBI. By casting and cross-linking, the proton conductivity, mechanical properties, and oxidation resistance of the TaPBI-PSBI membranes are significantly improved. The generation of cross-linked structures in the membranes is demonstrated by FT-IR. SEM shows that the surface of composite membrane is smooth and free of through holes. The elements are uniformly distributed. The TEM shows microphase separation occurs in the TaPBI-PSBI membrane, facilitating the formation of the continuous proton channels. The proton conductivity of TaPBI-PSBI (40) composite membrane reaches 0.170, 0.081, 0.059, and 0.037 S cm−1 at 180 °C, 100%, 50%, 30%, and 0% RH, respectively. The composite membrane is demonstrated the outstanding stability with the small decrease of proton conductivity and mass after durability testing of 96 h. The composite membrane exhibits high comprehensive performances as HT-PEM.

Silane-crosslinked polybenzimidazole with different hydroxyl content for high-temperature proton exchange membrane

High temperature proton exchange membrane fuel cells (HT-PEMFCs) can operate over 100 °C without humidification, which avoids the catalyst poisoning and enhances the efficiency of the power conversion. However, the classic phosphoric acid (PA) doped polybenzimidazole (PBI) still remains the challenge in the trade-off between mechanical strength and proton conductivity. The γ-(2,3-epoxypropoxy) propyltrimethoxysilane (KH560), as a crosslinking agent, is widely used to improve the mechanical properties of membranes. Although it can enhance the mechanical strength of PBI membranes, it sacrifices the hydrogen of imidazole groups, resulting in a decrease in proton conductivity. In this present work, silane-crosslinked PBI proton exchange membranes (PEMs) were synthesized by the reaction between the pyridine-bridged PBI containing different hydroxyl (-OH) content with KH560. Through comprehensive structural characterization of the polymer and model compound, it has been determined that this crosslinking method does not compromise the N–H acidic sites on the imidazole ring. The crosslinking agent KH560 provides additional hydroxyl groups to PBI and participates in the construction of hydrogen bonding networks after the ring-opening reaction. It was found that the addition of pyridine and hydroxyl groups could promote the phosphoric acid uptake, and the appropriate degree of silane crosslinking could maintain the PBI membranes with reasonable mechanical properties. Proton conductivity and HT-PEMFCs performance of the obtained crosslinked films were further enhanced with the dimensional stability, thermal stability and mechanical properties. Among them, CPyOPBI–OH–20 presented the best overall performance, exhibiting the highest proton conductivity of 0.095 S cm−1 and the maximum power density of 767 mW cm−2 at 180 °C under the anhydrous condition, which was 4.2 times that of OPBI (182 mW cm−2).

Preparation and Evaluation of Nafion/CeO2 Composite Electrolyte Membranes for Polymer Electrolyte Fuel Cells

Polymer electrolyte fuel cells (PEFCs) are increasingly applied to the vehicles (FCVs) toward the suppression of carbon dioxide emission. The larger FCVs requires the high power density to exceed the presumed temperature limit of 80 ℃, causing some issues of material durability and system performances. In particular, a commercial electrolyte membrane of Nafion occurs the decrease of proton conductivity at the high temperatures under an atmospheric pressure. To overcome this problem, our research group has developed a composite electrolyte membrane consisting of Nafion and Ta-TiO2 nanoparticles with a chain-like network structure. The composite electrolyte membrane improves a proton conductivity in a wide temperature range over 100 ℃ by the effect of hydrophilic surface of the oxide. In this study, the other candidate materials of CeO2, nanoparticles with highly hydrophilic surface rather than that of Ta-TiO2 was selected to try to improve the proton conductivity of the composite electrolyte membranes.The hydrophilic CeO2 nanoparticles with a chain-like network structure were prepared using flame oxide-synthesis method. The hydrophilic CeO2 nanoparticles were dispersed in a dispersion solvent in desired ratio. The dispersion was then added to 20 wt.% Nafion dispersion solution (DE2020 CS, Chemours Co.) neutralized with potassium hydroxide. The resulting dispersion was coated onto a die coater and dried using a hot air. The CeO2/Nafion composite membrane was then treated in high purified water with a sulfuric acid solution. The transmission electron microscopy (TEM) and water adsorption, and four-probe alternating current method was conducted to detect the microstructure, water uptake and proton conductivity of the composite membrane.TEM images confirmed that the CeO2 nanoparticles were highly dispersed in the composite membrane. The proton conductivity of the 3 wt.% CeO2 containing composite membrane was 2.1 times higher than that of commercial Nafion membrane (NRE211, Chemours Co.) at 80 ℃, 20% RH, and was 1.5 times higher than that at 80 ℃, 80% RH (Fig. 1). The proton conductivity of the 3 wt.% CeO2 containing composite membrane was found to be higher than that of the Nafion monolayer in each water uptake (Fig. 2), indicating that the morphology of the membrane might be changed by the adding of CeO2 oxide. Further research will be needed to examine the proton conductivity mechanism of this composite membrane, and fuel cell evaluations using this composite membrane. Figure 1

Efficiency Analysis of Fuel Cell Components with Ionic Poly-Arylether Composite Membrane.

We use polyethylene glycol as an additive to explore how the hydrogen bonding of this additive changes the properties of SA8 blended sulfonated polyetheretherketone (SPEEK) composite films. We mixed a 5%wt polyethylene glycol solution into a 12.5%wt SA8 solution, and then prepared a film with a total weight of 40 g at a ratio of 1:99. The SA8 (PEG) solution was prepared and then mixed with 5%wt SPEEK solution, and a film-forming solution with a total weight of 8g in different mixing ratios was created. Polyethylene glycol (PEG) was mixed into the sulfonated polyarylether polymer SA8 to form physical cross-linking. Therefore, the sulfonated polyether ether ketone SPEEK was mixed in, and it exhibited good thermal stability and dimensional stability. However, there was some decrease in proton conductivity as the proportion of SPEEK increased. Although SPEEK mixed with sulfonated polymer reduces the proton conductivity, the physical cross-linking of PEG can improve the proton conductivity of the composite membrane, and adding SPEEK can not only solve the problem of the high sulfonation film swelling phenomenon, it can also improve the dimensional stability of the film through the hydrogen bonding force of PEG and obtain a composite film with excellent properties.

Modeling of Relative Humidity-Dependent Impedance of Polymer Electrolyte Membrane Fuel Cells

1. IntroductionPolymer electrolyte membrane fuel cells (PEMFCs) are highly efficient devices that utilize hydrogen energy. The large overpotential of PEMFCs, particularly under low relative humidity (R.H.) conditions [1], is a challenge. Equivalent circuit modeling is an effective technique for impedance analysis, in which circuit elements are used to simulate electrode reactions [2]. The transmission line model (TLM) is often used for porous electrodes including PEMFCs [3]. In this study, a TLM was constructed considering the resistance distribution in the cathode catalyst layer (CCL), and the dependence of the impedance on R.H. was investigated.2. ModelingFigure 1 shows a TLM. The proton potential Xl was set as a parameter for each triple phase boundary (TPB), where an oxygen reduction reaction (ORR) occurs. At a certain TPB, Equation 1 holds based on Kirchhoff's current law [4]. Rion is the proton conduction resistance; Rct is the resistance of charge transfer in the ORR; Tct and P are parameters of the constant phase element. In this study, Rct was made a function of proton potential using Equation 2, where i0 is the exchange current density of the cathode and n is the number of exchanged electrons. The potential X0 of the TPB on a Nafion membrane was specified as the boundary condition. Subsequently, the model impedance was calculated by varying the frequency f from 106 to 0.1 Hz.Using the proposed model, a simulation was performed by varying Rion to be similar to the measured impedance spectra in the next section. For comparison, a simulation was performed under the same conditions using a conventional TLM in which Rct is a constant value.3. ExperimentalA membrane electrode assembly (MEA) was prepared by spraying a catalyst ink with an ionomer/carbon weight ratio of 0.92 onto a Nafion membrane. For both the cathode and anode, the electrode area was 1 cm2 and the Pt loading was 0.4 mgpt cm-2. Power generation tests were conducted at a cell temperature of 80 °C. Gases flowing at 200 sccm were supplied to the cathode at an oxygen partial pressure of 0.2 atm and to the anode at a hydrogen partial pressure of 0.4 atm. The impedance spectra of the MEA were measured under various R.H. conditions by electrochemical impedance spectroscopy [5] in the potentiostatic mode at 0.7 V. The distribution of relaxation times (DRT) analysis [6] was conducted on the impedance spectra.4. Results and discussionFigure 2 shows the experimental and simulation results. The left and right columns show the impedance spectra and the DRT analysis results, respectively.Based on the analysis of DRT peaks by Heinzmann et al. [7], the peak in the mid-frequency range, PM, is associated with the resistance of charge transfer in the ORR, whereas the peak group in the high-frequency range, PH, is associated with the resistance of proton conduction. The PH of the DRT of the measured impedance increases as R.H. decreases. This indicates that a decrease in R.H. promotes a decrease in proton conductivity. The increase in Rion in the simulation represents a decrease in proton conductivity, which increases the PH as shown in both simulation results. As the proton conductivity decreases, the PM of the DRTs of the measured impedance and the calculated impedance using the proposed model increases. However, the PM of the DRT of the calculated impedance using the conventional model does not change. This indicates that the proposed model can accurately simulate the actual phenomena in the CCL. The reason for the increase in PM caused by the decrease in proton conductivity could be that the local charge transfer resistance increases by a decrease in the proton potential at each TPB.5. ConclusionsA TLM was constructed to predict the dependence of impedance on R.H. Experimental results showed that an unsatisfactory proton conductivity resulted in an increase in the charge transfer resistance. The results of the simulation with varying proton conduction resistances using the proposed model are consistent with the experimental trend. In future studies, circuit parameters should be appropriately determined by fitting the model to the measured impedance spectra.

Nano-Graphene Layer from Facile, Scalable and Eco-Friendly Liquid Phase Exfoliation Strategy as Effective Barrier Layer for High-Performance and Durable Direct Liquid Alcohol Fuel Cells.

Graphene, in spite of exceptional physio-chemical properties, still faces great limitations in its use and industrial scale-up as highly selective membranes (enhanced ratio of proton conductivity to fuel cross-over) in liquid alcohol fuel cells (LAFCs), due to complexity and high cost of prevailing production methods. To resolve these issues, a facile, low-cost and eco-friendly approach of liquid phase exfoliation (bath sonication) of graphite to obtain graphene and spray depositing the prepared graphene flakes, above anode catalyst layer (near the membrane in the membrane electrode assembly (MEA)) as barrier layer at different weight percentages relative to the base membrane Nafion 115 was utilized in this work. The 5 wt.% nano-graphene layer raises 1 M methanol/oxygen fuel cell power density by 38% to 91 mW·cm−2, compared to standard membrane electrode assembly (MEA) performance of 63 mW·cm−2, owing to less methanol crossover with mild decrease in proton conductivity, showing negligible voltage decays over 20 h of operation at 50 mA·cm−2. Overall, this work opens three prominent favorable prospects: exploring the usage of nano-materials prepared by liquid phase exfoliation approach, their effective usage in ion-transport membrane region of MEA and enhancing fuel cell power performance.

Sandwich‐like Nafion composite membrane with ultrathin ceria barriers for durable fuel cells

Improving the durability of the membrane electrode assembly (MEA) while obtaining high performance is required to further commercialize polymer electrolyte membrane fuel cells (PEMFCs). The durability of PEMFCs is improved by incorporating radical scavengers, such as CeO2 (ceria), into the MEA, especially the membrane. However, nanosized ceria particles are generally mixed with ionomers and are cast on substrates to fabricate composite membranes. In such a case, controlling their morphology and avoiding particle agglomeration is difficult. Herein, we report a novel method for constructing a robust membrane by incorporating ultrathin ceria barriers into the outermost sides of a commercial Nafion membrane to effectively alleviate radical attacks while ensuring the high uniformity and controllability of ceria layers. The improved durability of the composite membrane is confirmed via ex situ Fenton's test and in situ operation of a fuel cell. Moreover, we observe that the amount of fluoride ion emission and the loss of proton conductivity of the membranes decrease as the CeO2 density increases. The MEA comprises a modified membrane with CeO2 barriers that show proper areal density. It demonstrates excellent durability under accelerated environmental conditions (open circuit voltage test) and acceptable initial performance with an insignificant decrease in proton conductivity.

Dynamic Mechanical Fatigue Behavior of Polymer Electrolyte Membranes for Fuel Cell Electric Vehicles Using a Gas Pressure-Loaded Blister.

This study reports on an innovative press-loaded blister hybrid system equipped with gas-chromatography (PBS-GC) that is designed to evaluate the mechanical fatigue of two representative types of commercial Nafion membranes under relevant PEMFC operating conditions (e.g., simultaneously controlling temperature and humidity). The influences of various applied pressures (50 kPa, 100 kPa, etc.) and blistering gas types (hydrogen, oxygen, etc.) on the mechanical resistance loss are systematically investigated. The results evidently indicate that hydrogen gas is a more effective blistering gas for inducing dynamic mechanical losses of PEM. The changes in proton conductivity are also measured before and after hydrogen gas pressure-loaded blistering. After performing the mechanical aging test, a decrease in proton conductivity was confirmed, which was also interpreted using small angle X-ray scattering (SAXS) analysis. Finally, an accelerated dynamic mechanical aging test is performed using the homemade PBS-GC system, where the hydrogen permeability rate increases significantly when the membrane is pressure-loaded blistering for 10 min, suggesting notable mechanical fatigue of the PEM. In summary, this PBS-GC system developed in-house clearly demonstrates its capability of screening and characterizing various membrane candidates in a relatively short period of time (<1.5 h at 50 kPa versus 200 h).

Significant improving H+/VO2+ permselectivity of Nafion membrane by modification with PDDA in aqueous isopropanol

The modification of Nafion® membranes with poly(diallyldimethylammonium chloride) (PDDA) in aqueous solutions of isopropanol has been investigated. A remarkable decrease in vanadyl ion permeability (by 3.5 orders of magnitude), accompanied by a decrease in proton conductivity sixfold at most, was found for Nafion membranes modified in 30–35 wt% isopropanol solutions providing a high swelling of the membranes. The Bradford reagent has been used for the detection of PDDA on the membrane surface.

Photogated proton conductivity of ZIF-8 membranes co-modified with graphene quantum dots and polystyrene sulfonate

Smart proton conductive metal-organic framework (MOF) membranes with dynamic remote control over proton conduction show high potential for use in advanced applications, such as sensors and bioprocesses. Here, we report a photoswitchable proton conductive ZIF-8 membrane by coencapsulating polystyrene sulfonate and graphene quantum dots into a ZIF-8 matrix (GQDs-PSS@ZIF-8) via a solid-confined conversion process. The proton conductivity of the GQDs-PSS@ZIF-8 membrane is 6.3 times higher than that of pristine ZIF-8 and can be reversibly switched by light due to photoluminescence quenching and the photothermal conversion effect, which converts light into heat. The local increase in temperature allows water molecules to escape from the porous channels, which cuts off the proton transport pathways and results in a decrease in proton conductivity. The proton conductivity is restored when the light is off owing to regaining water molecules, which act as proton carriers, from the surroundings. The GQDs-PSS@ZIF-8 membrane responds efficiently to light and exhibits an ON/OFF ratio of 12.8. This photogated proton conduction in MOFs has potential for the development and application of MOF-based protonic solids in advanced photoelectric devices.

Light-Responsive Proton Conductor: Record High Gain of Proton Conductivity Achieved by Photoinduced Electron-Transfer Strategy

Light-Responsive Proton Conductor: Record High Gain of Proton Conductivity Achieved by Photoinduced Electron-Transfer Strategy

Active electrospun nanofibers as an effective reinforcement for highly conducting and durable proton exchange membranes

Mechanical reinforcement of proton exchange membranes is a great challenge allowing the reduction of their thickness, with the advantages of lower resistance, improved water transport, decreased fuel crossover and high durability, which are crucial for fuel cells. We describe herein a new class of reinforced membranes based on nanofibers of polysulfone (PSU) functionalized with 4-heptyl-1,2,3-triazole (PSUT) likely interacting via hydrogen bonds or ionic cross-linking with a short-side-chain type perfluorosulfonic acid (Aquivion®) matrix. PSUT with two functionalization degrees was electrospun into webs which were impregnated with Aquivion® to afford composite membranes that presented higher dimensional stability in water, increased mechanical strength and Young modulus in comparison to reinforced membranes of non-functionalized PSU and pristine ionomer membrane, without any decrease in proton conductivity. Membrane-electrode assemblies incorporating an Aquivion® membrane reinforced with PSUT exhibited 5 times superior durability than those with a pristine Aquivion® membrane without any radical scavenger. These features may be ascribed to the specific interaction between basic fibers and acidic ionomer. The incorporation of webs of active fibers in ionomer membranes is an effective strategy of reinforcement leading to high performance and increased durability, which can be extended to other kinds of ion exchange membranes and devices.

Sulfonated Poly(ether ether ketone) Doped with Ammonium Ionic Liquids and Nano-Silicon Dioxide for Polymer Electrolyte Membranes

Nano-silicon dioxide (SiO2) was incorporated into the matrix of sulfonated poly(ether ether ketone) (SPEEK)/ammonium ionic liquid (AIL) membranes by solution casting, with the aim of reducing ionic liquid leaching and any consequent decrease in proton conductivity. Fourier transform infrared (FTIR) spectra results indicate incorporation of the SPEEK matrix with AIL and nano-SiO2. Scanning electron microscopy (SEM) and X–ray diffraction (XRD) images revealed that incorporation of nano-SiO2 make the ternary composite membranes more flexible. The maximum ion exchange capacity (IEC) value was 3.25 meq/g, and the net structure formed by the nano-SiO2 not only retarded AIL leaching, but also increased the proton conductivity of the composite membranes. AIL leaching from the membranes was between 20% and 30%, which was lower than that of the SPEEK/AIL membrane. The maximum proton conductivity for the SPEEK/AIL/SiO2 membranes reached 10.73 mS/cm at 393 K.

Nanocomposite proton exchange membranes based on phosphotungstic acid immobilized by polydopamine-coated halloysite nanotubes

In this work, sulfonated poly(ether ether ketone) (SPEEK) nanocomposite membranes were prepared with high loading of phosphotungstic acid (HPW) immobilized by polydopamine coated halloysite nanotubes (DHNTs). XPS and FTIR analysis revealed the formation of acid-base interactions between HPW and DHNTs in the nanocomposite membranes, indicating the successful immobilization of HPW on the surface of high aspect-ratio DHNTs, which helped shorten the distance of proton hopping and create more continuous proton transport pathways along the high aspect-ratio nanotubes. The activation energy for the proton transfer in the nanocomposite membranes was greatly reduced and membrane proton conductivity was significantly enhanced. Compared with the SPEEK control membrane, the nanocomposite membrane with 42.9 wt% HPW loading achieved 114% increase in proton conductivity, up from 0.048 S cm−1 to 0.117 S cm−1, and the membrane water uptake remained almost the same. Under the long term water immersion test, the SPEEK/DHNTs/HPW nanocomposite membrane exhibited much slower decrease in proton conductivity than the SPEEK/HPW membrane without DHNTs, indicating the strong HPW-binding effect from DHNTs on slowing down the leakage of HPW.

Solution-blended sulfonated polyphenylene and branched poly(arylene ether sulfone): Synthesis, state of water, surface energy, proton transport, and fuel cell performance

A poly(arylene ether sulfone) (PAES) was synthesized with a sulfonated poly(phenyl sulfone) side-chain (sB) to create a branched sBPAES ionomer. This sBPAES ionomer was solution blended with sulfonated polyphenylene (sPP). The sulfonated side-chain dramatically improved miscibility between these distinctly different ionomers. This functional group facilitated their solution-casting into robust films that were flexible and tough. Ionomer film proton conductivity, water uptake, density, state-of-water, contact angle, surface energy, wide angle x-ray scattering, and hydrogen fuel-cell function were studied based upon blend concentration. Results showed that sPP blended with 5 wt% sBPAES had a relative free water content that increased by 97% as compared to unblended sPP. Ionomer blends containing 10 wt% sBPAES reduced the film's water uptake by 37.5% and only caused a 3.3% decrease in proton conductivity from 97.5 to 94.3 mS/cm. The relative free water concentration improved within the blend up to 10 wt% sBPAES. These improvements were attributed to changes in ion clustering noted using wide angle x-ray scattering. The film's apparent water contact angle revealed that introducing 10 wt % sBPAES into sPP lowered its surface energy from 27.2 mJ/m2 to 24.3 mJ/m2. This led to a higher membrane-electrode interfacial resistance for these ionomer-blended films. An optimal ionomer-blend containing 5 wt% sBPAES had a H2/O2 peak-power output of 630 mW/cm2 at 1490 mA/cm2, which was greater than its unmodified sPP version of 570 mW/cm2 and 1220 mA/cm2.

Transport properties of proton conductive Y‐doped BaHfO3 and Ca or Sr‐substituted Y‐doped BaZrO3

AbstractAn electrolyte in fuel cells requires not only high ionic conductivity, but also high transport numbers of ionic conduction. Although Y‐doped BaZrO3 is regarded to be the most promising candidate as the electrolyte in protonic ceramic fuel cells (PCFCs), significant hole conduction generates in wet oxygen at high temperatures. With the aim to increase the transport number of ionic conduction, in this work, Sr and Ca were introduced to partially substitute Ba in BaZr0.8Y0.2O3‐δ. The results revealed that a single cubic perovskite phase was obtained for Ba0.95Ca0.05Zr0.8Y0.2O3‐δ and Ba1‐xSrxZr0.8Y0.2O3‐δ (x = 0.05, 0.10, 0.15, 0.20 or 0.40). However, replacing Ba with Sr resulted in almost no increase in the transport number of ionic conduction in wet oxygen atmosphere, but drastic decrease in proton conductivity at all replacement levels. In addition, Ba0.95Ca0.05Zr0.8Y0.2O3‐δ shows no meaningful change in the transport number of ionic conduction, compared with BaZr0.8Y0.2O3‐δ. Incorporating Ca or Sr into the Ba‐site of BaZr0.8Y0.2O3‐δ appears to impart no positive influence on electrochemical properties. These interesting results also indicate that the hole conductivity decreases with the decrease in proton conductivity, and will aid to consider the hole conduction mechanism. BaHfO3 doped with 10 and 20 mol% Y was also prepared. A bimodal microstructure was observed for BaHf0.9Y0.1O3‐δ, whereas BaHf0.8Y0.2O3‐δ shows uniform grain size after sintering at 1600°C for 24 hours. The transport numbers of ionic conduction and bulk conductivity in such Y‐doped BaHfO3 samples are close to those of BaZrO3 doped with the same amount of Y.

Estimation of Cation Contamination Level in Polymer Electrolyte Membrane Fuel Cells By Electrochemical Impedance Spectroscopy

One of the fundamental hurdles for further commercialization of fuel cell vehicles (FCV) using polymer electrolyte membrane fuel cells (PEMFC) is their upper limitation of operating temperature. Operating temperature of current PEMFC is much lower than that of internal-combustion engines and it makes FCV require huge radiators for high power operation. Raising operating temperature is beneficial for downsizing and cost reduction for FCV system because it downsizes not only cooling system but also PEMFC stack. One of the root causes of this limitation is the decrease in proton conductivity of the membrane during long term operation. In order to avoid the decrease in proton conductivity, reducing the contamination of metal cations in the membrane is an effective way. Metal cations, which contaminate into the membrane during operation, reduce the proton conductivity of the membrane due to decrease in the proton path in the membrane. Proton conductivity loss by contamination of metal cations is critical for the high temperature performance. In order to reduce cation contamination into the membrane, it is necessary to understand its mechanism. An in-situ measurement for the amount of metal cations should be helpful to understand the phenomena. For this reason, we propose an in-situ quantitative measurement for cation contamination ratio in PEMFC. Electrochemical impedance spectroscopy (EIS) was adapted to the measurement because cation transport inside the membrane was expected to be detectable with EIS. Experimental results showed that EIS spectrum under a certain condition reflects metal cation contamination ratio strongly, which was proposed to be an indicator of cation contamination ratio. The mechanism of EIS spectrum change is explained theoretically by a numerical simulation using a simple model of cation and water transport. Detail of the measurement will be shown in the conference. Therefore this EIS measurement is proposed for the analysis of metal cation contamination mechanisms in PEMFC. Further study will elucidate the impact of catalyst layer, membrane and metal cation species against the EIS spectrum.

Comprehensive Analysis of Trends and Emerging Technologies in All Types of Fuel Cells Based on a Computational Method

Fuel cells have been attracting significant attention recently as highly efficient and eco-friendly energy generators. Here, we have comprehensively reviewed all types of fuel cells using computational analysis based on a citation network that detects emerging technologies objectively and provides interdisciplinary data to compare trends. This comparison shows that the technologies of solid oxide fuel cells (SOFCs) and electrolytes in polymer electrolyte fuel cells (PEFCs) are at the mature stage, whereas those of biofuel cells (BFCs) and catalysts in PEFCs are currently garnering attention. It does not mean, however, that the challenges of SOFCs and PEFC electrolytes have been overcome. SOFCs need to be operated at lower temperatures, approximately 500 °C. Electrolytes in PEFCs still suffer from a severe decrease in proton conductivity at low relative humidity and from their high cost. Catalysts in PEFCs are becoming attractive as means to reduce the platinum catalyst cost. The emerging technologies in PEFC catalysts are mainly heteroatom-doped graphene/carbon nanotubes for metal-free catalysts and supports for iron- or cobalt-based catalysts. BFCs have also received attention for wastewater treatment and as miniaturized energy sources. Of particular interest in BFCs are membrane reactors in microbial fuel cells and membrane-less enzymatic biofuel cells.

Oxygen Transport Effects of Cobalt Cation Contamination of Ionomer Thin Films in Proton Exchange Membrane Fuel Cells

Proton exchange membrane fuel cells (PEMFCs) are a promising automotive energy conversion technology for reducing the dependence on petroleum and reducing greenhouse gas emissions. However, the high cost of the platinum (Pt) electrocatalyst responsible for increasing the rate of the oxygen (O2) reduction reaction (ORR) in the cathode is one of the primary inhibitors to this technology’s mass market penetration. In order for PEMFCs to become cost competitive with internal combustion engine technologies, the electrocatalyst cost must be significantly reduced. To address this issue, the use of Pt alloys such as Pt3Co has been explored. Alloys such as Pt3Co have been shown to have higher activities than pure Pt electrocatalysts,(1) reducing the mass of the expensive Pt. However, the utilization of Pt3Co poses contamination risks to the ionomers used in PEMFCs due to cobalt’s (Co) propensity to dissolve out of the catalyst, migrate to the ionomer, and replace protons on the sulfonic acid sites(1). Cai et al.(2) demonstrated the effects of this risk through the testing of membrane electrode assemblies (MEAs) containing Co doped ionomers, which showed a decrease in proton conductivity through the ionomer. In addition, Kongkanand et al.(3) have shown that voltage losses at high current densities for low-Pt loaded electrodes can be attributed to localized O2 transport resistances in the thin films of ionomer covering the electrocatalyst surface. Thus, the effects of dissolved Co on the local O2 transport through these thin films of ionomer are of particular interest. The purpose of this work is to experimentally characterize the effects of Co contamination on the localized O2 transport properties of ionomer thin films. To study these effects, we modified an existing fuel cell to allow for the testing of gas (O2) transport through ionomer thin films in a well-controlled environment, similar to the setup used by Liu et al.(4) The setup allows for the films’ transport resistance to be measured under non-polarized (uniform Co distribution) and interface free conditions, in order to isolate the effect of the Co contamination. In addition, the setup allows for the control of the ionomer sample’s temperature and relative humidity, to evaluate the effects of each on the O2 transport resistance of the doped thin films. To test the effects of differing levels of Co cation contamination during PEMFC operation, we take O2 transport resistance measurements for various amounts of controlled Co ion doping. Our preliminary results have indicated significant increases in O2 transport resistances for high Co contamination levels, consistent with our hypotheses derived from the results mentioned above(2, 3). This work was partially supported by the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy under grant DE-EE0007271. Figure 1