Nafion/sulfated β-cyclodextrin composite membranes for direct methanol fuel cells

Proton-conducting composite membranes based on H⁺-form sulfated β-cyclodextrin (sb-CD) in a Nafion matrix are prepared via the solution-casting method and their methanol permeabilities, proton conductivities, proton diffusion coefficients and cell performances are measured. The methanol permeabilities of the composite membranes increase very slightly with increases in their sb-CD content. As a result of adding sb-CD with its many sulfonic acid groups into the Nafion matrix, the proton conductivities of the composite membranes increase with increases in their sb-CD content. The methanol permeability and proton conductivity results are used to show that the best selectivity of the membranes is that of the NC5 membrane (‘NCx’ denotes a Nafion/sb-CD composite membrane containing x wt.% sb-CD). The proton diffusion coefficients are measured with ¹H pulsed field gradient nuclear magnetic resonance (PFG-NMR) and found to increase with increase in the sb-CD content in the order NC5 > NC3 > NC1 > NC0. Thus the presence of sb-CD in the Nafion membranes increases the proton diffusion coefficients as well as the proton conductivities, ionic cluster size, water uptakes and the ion-exchange capacities (IECs). A maximum power density of 58 mW cm⁻² is obtained for the NC5 membrane. The combination of these effects should lead to an improvement in the performance of direct methanol fuel cells prepared with Nafion/sb-CD composite membranes.     

Characterization and performance evaluation of Pt–Ru electrocatalysts supported on different carbon materials for direct methanol fuel cells

The paper addresses the effect of the carbon support on the microstructure and performance of Pt–Ru-based anodes for direct methanol fuel cells (DMFC), based on the study of four electrodes with a carbon black functionalized with HNO₃, a mesoporous carbon (CMK-3), a physical mixture of TiO₂ and carbon black and a reference carbon thermally treated in helium atmosphere (HeTT). It is shown that CMK-3 hinders the growth of the electrocatalyst nanoparticles (2.7 nm) and improves their distribution on the support surface, whereas the oxidized surfaces of HNO₃ carbon and TiO₂+carbon lead to larger (4–4.5 nm), agglomerated particles, and the lowest electrochemical active areas (54 and 26 m² g⁻¹, in contrast with 90 m² g⁻¹ for CMK-3), as determined from CO stripping experiments. However, HNO₃ and TiO₂ are characterized by the lowest CO oxidation potential (0.4 V vs. RHE), thus suggesting higher CO tolerance for the se electrodes. Tests in DMFC configuration show that the three modified electrodes have clearly better performance than the reference HeTT. The highest power density attained with electrodes supported on carbon treated with HNO₃ (65 mW cm⁻²/300 mA cm⁻² at 90 °C) and the equally interesting performance of the TiO₂-based electrodes (53 mW cm⁻²/300 mA cm⁻²), is a strong indication of the positive effect of the presence of oxygenated groups on the methanol oxidation reaction. The results are interpreted in order to identify separate microstructural (electrocatalyst particle size, porosity) and compositional (oxygenated surface groups, presence of oxide phase) effects on the electrode performance.     

Binary Pt–Pd and ternary Pt–Pd–Ru nanoelectrocatalysts for direct methanol fuel cells

Nano-sized binary and ternary alloys are synthesized by polyol process on Vulcan XC72-R support. Nanostructured binary Pt–Pd/C catalysts are prepared either by co-deposition or by depositing on each other. Ternary Pt–Pd–Ru/C catalysts are prepared by co-deposition. The structural characteristics of the nanocatalysts are examined by TEM and XRD. Their electrocatalytic activity toward methanol oxidation and CO stripping curves were measured by electrochemical measurements and compared with that of commercial Pt/C catalyst. The results show that the binary nanocatalyst prepared by depositing the Pt precursor colloids on Pd-Vulcan XC-72R are more active toward methanol oxidation than that of the co-deposited binary alloy nanocatalyst. The co-deposited ternary Pt–Pd–Ru/C nanocatalyst based membrane electrodes assembly shows higher power density compared to the binary nanocatalysts as well as commercial Pt/C catalyst in direct methanol fuel cell. Significantly higher catalytic activity of the nanocatalysts toward methanol oxidation compared to that of the commercial Pt/C is believed to be due to lower level of catalyst poisoning.     

Integrated carbon composite bipolar plate for polymer–electrolyte membrane fuel cells

The electrical resistance of bipolar plates for polymer–electrolyte membrane fuel cells (PEMFCs) should be very low to conduct the electricity generated with minimum electrical loss. The resistance of a bipolar plate consists of the bulk material resistance and the interfacial contact resistance when two such plates are contacted to provide channels for fuel and air (oxygen) supplies.     

Application of FTIR in direct methanol fuel cells – Quantitative analysis of PTFE in gas diffusion layers

Gas diffusion layers (GDLs) of direct methanol fuel cells (DMFCs), consisting of a microporous layer (MPL) and a back layer (BL), influence the cell performance and stability significantly due to the critical function that the GDL undertook, i.e., distribution of reactants and removal of the products in electrodes. The hydrophilic/hydrophobic properties of the GDLs are required to tailor to the transport/transfer of reactants/products depending on a specific electrode reaction. One important way to adjust the hydrophobic/hydrophilic properties of GDLs is to vary PTFE content in GDLs. In this paper, we employ infrared spectroscopy technique, specifically, diffuse reflection (DR) method and attenuation total reflection (ATR) method, to determine the PTFE content in both MPLs and BLs quantitatively by comparing the measured C-F intensity with the pre-calibrated standard plots. Compared to the ATR method, the DR method takes advantages of sensitivity, wide range and precision. By the DR method, we succeed in observing that PTFE in MPLs migrates to BLs, consistent with the corresponding EDX results for a sample experienced 600 h lifetime test, suggesting DR method an effective approach to determine quantitatively the hydrophilic/hydrophobic properties of both MPLs and BLs.     

Performance of organic–inorganic hybrid anion-exchange membranes for alkaline direct methanol fuel cells

A series of organic–inorganic membranes were prepared through sol–gel reaction of quaternized poly(vinyl alcohol) (QAPVA) with different contents of tetraethoxysilanes (TEOS) for alkaline direct methanol fuel cells. These hybrid membranes are characterized by FTIR, X-ray diffraction (XRD), scanning electron microscopy/energy-dispersive X-ray analysis (SEM/EDXA) and thermo gravimetric analysis (TGA). The ion exchange content (IEC), water content, methanol permeability and conductivity of the hybrid membranes were measured to evaluate their applicability in fuel cells. It was found that the addition of silica enhanced the thermal stability and reduced the methanol permeability of the hybrid membranes. The hybrid membrane M-5, for which the silica content was 5 wt%, showed the lowest methanol permeability and the highest ion conductivity among the three hybrid membranes. The ratio of conductivity to methanol permeability of the membrane M-5 indicated that it had a high potential for alkaline direct methanol fuel cell applications.     

CO tolerant PtRu–MoOx nanoparticles supported on carbon nanofibers for direct methanol fuel cells

Novel nanostructured catalysts based on PtRu–MoO_x nanoparticles supported on carbon nanofibers have been investigated for CO and methanol electrooxidation. Carbon nanofibers are prepared by thermocatalytic decomposition of methane (NF), and functionalized with HNO₃ (NF.F). Electrocatalysts are obtained using a two-step procedure: (1) Pt and Ru are incorporated on the carbon substrates (Vulcan XC 72R, NF and NF.F), and (2) Mo is loaded on the PtRu/C samples. Differential electrochemical mass spectrometry (DEMS) analyses establish that the incorporation of Mo increases significantly the CO tolerance than respective binary counterparts. The nature of the carbon support affects considerably the stabilization of MoO_x nanoparticles and also the performance in methanol electrooxidation. Accordingly, a significant increase of methanol oxidation is obtained in PtRu–MoO_x nanoparticles supported on non-functionalized carbon nanofiber, in parallel with a large reduction of the Pt amount in comparison with binary counterparts and commercial catalyst.     

Nafion/Pd–SiO2 nanofiber composite membranes for direct methanol fuel cell applications

One of the major challenges for direct methanol fuel cells is the problem of methanol crossover. With the aim of solving this problem without adverse effects on the membrane conductivity, Nafion/Palladium–silica nanofiber (N/Pd–SiO₂) composite membranes with various fiber loadings were prepared by a solution casting method. The silica-supported palladium nanofibers had diameters ranging from 100 nm to 200 nm and were synthesized by a facile electro-spinning method. The thermal properties, ionic exchange capacities, water uptake, proton conductivities, methanol permeabilities, chemical structures, and micro-structural morphologies were determined for the prepared membranes. It was found that the transport properties of the membranes were affected by the fiber loading. All of the composite membranes showed higher water uptake and ion exchange capacities compared to commercial Nafion 117 and proved to be thermally stable for use as proton exchange membranes. The composite membranes with optimum fiber content (3 wt%) showed an improved proton conductivity of 0.1292 S cm⁻¹ and a reduced methanol permeability of 8.36 × 10⁻⁷ cm² s⁻¹. In single cell tests, it was observed that, the maximum power density measured with composite membrane is higher than those of commercial Nafion 117.     

Electrodeposition of Pt–Ru and Pt–Ru–Ni nanoclusters on multi-walled carbon nanotubes for direct methanol fuel cell

A full-electrochemical method is developed to deposit three dimension structure (3D) flowerlike platinum-ruthenium (PtRu) and platinum-ruthenium-nickel (PtRuNi) alloy nanoparticle clusters on multi-walled carbon nanotubes (MWCNTs) through a three-step process. The structure and elemental composition of the PtRu/MWCNTs and PtRuNi/MWCNTs catalysts are characterized by transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDX), X-ray polycrystalline diffraction (XRD), IRIS advantage inductively coupled plasma atomic emission spectroscopy (ICP-AES), and X-ray photoelectron spectroscopy (XPS). The presence of Pt(0), Ru(0), Ni(0), Ni(OH)₂, NiOOH, RuO₂ and NiO is deduced from XPS data. Electrocatalytic properties of the resulting PtRu/MWCNTs and PtRuNi/MWCNTs nanocomposites for oxygen reduction reaction (ORR) and methanol oxidation reaction (MOR) are investigated. Compared with the Pt/MWCNTs, PtNi/MWCNTs and PtRu/MWCNTs electrodes, an enhanced electrocatalytic activity and an appreciably improved resistance to CO poisoning are observed for the PtRuNi/MWCNTs electrode, which are attributed to the synergetic effect of bifunctional catalysis, three dimension structure, and oxygen functional groups which generated after electrochemical activation treatment on MWCNTs surface. The effect of electrodeposition conditions for the metal complexes on the composition and performance of the alloy nanoparticle clusters is also investigated. The optimized ratios for PtRu and PtRuNi alloy nanoparticle clusters are 8:2 and 8:1:1, respectively, in this experiment condition. The PtRuNi catalyst thus prepared exhibits excellent performance in the direct methanol fuel cells (DMFCs). The enhanced activity of the catalyst is surely throwing some light on the research and development of effective DMFCs catalysts.     

Enhanced electrochemical performance and durability for direct CH4–CO2 solid oxide fuel cells with an on-cell reforming layer

Coking is a major issue with the traditional Ni-based anodes when directly oxidizing CH₄ in solid oxide fuel cells (SOFCs). Dry reforming to convert CH₄–CO₂ into CO–H₂ syngas before entering Ni-based anode may potentially be an effective and economical method to address the coking problem. Consequently, an on-cell reforming layer outside the Ni-based anode is expected to offer a unique solution for direct CH₄–CO₂ SOFCs without coking. In this study, Ni-GDC anode-supported cells with and without a Sr₂Co_0.4Fe_1.2Mo_0.4O_6-δ (SCFM) layer outside the anode support have been fabricated and evaluated using either H₂ or CH₄–CO₂ as fuel. Both types of cells show excellent electrochemical performance when H₂ is used as fuel, and the SCFM layer has negligible impact on the cell performance. When CH₄–CO₂ is used as fuel, however, the electrochemical performance and durability of the cells with the SCFM layer are much better than those without the SCFM layer outside the Ni-GDC anode, indicating that the SCFM layer can efficiently perform dry reforming. This unique on-cell dry reforming design enables direct CH₄–CO₂ solid oxide fuel cells and offers a very promising route for energy storage and conversion.     

Bipolar polymer electrolyte interfaces for hydrogen–oxygen and direct borohydride fuel cells

Direct borohydride fuel cells (DBFCs) using liquid hydrogen peroxide as the oxidant are safe and attractive low temperature power sources for unmanned underwater vehicles (UUVs) as they have excellent energy and power density and do not feature compressed gases or a flammable fuel stream. One challenge to this system is the disparate pH environment between the anolyte fuel and catholyte oxidant streams. Herein, a bipolar interface membrane electrode assembly (BIMEA) is demonstrated for maintaining pH control of the anolyte and catholyte compartments of the fuel cell. The prepared DBFC with the BIMEA yielded a promising peak power density of 110 mW cm⁻². This study also investigated the same BIMEA for a hydrogen–oxygen fuel cell (H₂–O₂ FC). The type of gas diffusion layer used and the gas feed relative humidity were found to impact fuel cell performance. Finally, a BIMEA featuring a silver electrocatalyst at the cathode in a H₂–O₂ FC was successfully demonstrated.     

Nano-sized Fe2O3–SO4 solid superacid composite Nafion membranes for direct methanol fuel cells

In this paper, Fe₂O₃–SO₄²⁻/Nafion^® composite membranes were prepared by a solution casting method. The physico-chemical properties of composite membranes were characterized by X-ray diffraction (XRD), SEM–EDX and thermogravimetric analysis (TGA). The water uptake ability, proton conductivity, and methanol permeability of the composite membranes were evaluated and compared with the recast Nafion^® membrane. The results showed that the proton conductivity and the water uptake of the composite membranes were slightly higher than that of the recast Nafion^® membrane. The composite membrane containing 5 wt.% Fe₂O₃–SO₄^2- showed superior ability to suppress methanol crossover, and it further improved the direct methanol fuel cell (DMFC) performances with both 1 M and 5 M methanol feeding, compared with the recast Nafion^® membrane. The preliminary 30 h lifetime test of the DMFC with the composite membrane with 5% Fe₂O₃–SO₄²⁻ indicated that the composite membrane is stable working at the real DMFC operating conditions at least during the test. These results suggest the applicability of the composite membranes in DMFCs.     

A mechanical–electrical finite element method model for predicting contact resistance between bipolar plate and gas diffusion layer in PEM fuel cells

Contact resistance between the bipolar plate (BPP) and the gas diffusion layer (GDL) plays a significant role on the power loss in a proton exchange membrane (PEM) fuel cell. There are two types of contact behavior at the interface of the BPP and GDL, which are the mechanical one and the electrical one. Furthermore, the electrical contact behavior is dependent on the mechanical one. Thus, prediction of the contact resistance is a coupled mechanical–electrical problem. The current FEM models for contact resistance estimation can only simulate the mechanical contact behavior and moreover they are based on the assumption that the contact surface is equipotential, which is not the case in a real BPP/GDL assembly due to the round corner and margin of the BPP.     

Effect of carbon substrate materials as a Pt–Ru catalyst support on the performance of direct methanol fuel cells

The support effect of carbon nanotubes (CNTs) for direct methanol fuel cell (DMFC) was studied using CNTs with and without defect preparation, carbon black, and fishbone-type CNTs. The Pt–Ru/defect-free CNTs afforded the highest catalytic activity of methanol oxidation reaction (MOR) in rotating disk electrode experiments and the highest performance as the anode catalysts in DMFC single cell tests with the one-half platinum loading compared to Pt–Ru/VulcanXC-72R. CO stripping voltammograms with Pt–Ru/defect-free CNTs also revealed the lowest CO oxidation potential among other Pt–Ru catalysts using different carbon support. It is thus considered that the carbon substrates significantly affect the CO oxidation activity of anode electrocatalysts in DMFC. This is ascribed to the geometrical effect that the flat interface between CNTs and metal catalysts has a unique feature, at which the electron transfer occurs, and this interface would modify the catalytic properties of Pt–Ru particles.     

Clay as a dispersant in the catalyst layer for zinc–air fuel cells

This study proposes a four-layer membrane electrode assembly (MEA) consisting of air-electrode, proton exchange membrane, Zn-electrode with KOH or NaCl aqueous electrolyte and a steel supporter, for use in Zn–air fuel cells. Montmorillonite clay was used to disperse carbon black (CB) and MnO₂ catalyst to improve the performance of the air-electrode. The microstructures of the air-electrode and cell characteristics were investigated by field emission scanning electron microscopy (FE-SEM), optical microscopy (OM) and an electrochemical analyzer. The experimental results indicate that the four-layer MEA for Zn–air fuel cells reached a power density of 6 mW cm⁻² (at 10 mA cm⁻²) without electrolyte leakage from the cells. The open circuit voltage (OCV) and current density were improved by adding clay to the air-electrode as clay can minimize CB aggregation. In the polarization test, the OCV value (1.40 V) reached approximately 90% of the standard potential (1.65 V) and remained steadily over 48 h. These experimental results demonstrate the four-layer MEA can replace conventional Zn–air fuel cells that utilize aqueous electrolyte.     

A one-dimensional,two-phase model for direct methanol fuel cells – Part I: Model development and parametric study

A one-dimensional, steady-state, two-phase direct methanol fuel cell (DMFC) model is developed to precisely investigate complex physiochemical phenomena inside DMFCs. In this model, two-phase species transport through the porous components of a DMFC is formulated based on Maxwell–Stefan multi-component diffusion equations, while capillary-induced liquid flow in the porous media is described by Darcy's equation. In addition, the model fully accounts for water and methanol crossover through the membrane, which is driven by the effects of electro-osmotic drag, diffusion, and the hydraulic pressure gradient. The developed model is validated against readily available experimental data in the literature. Then, a parametric study is carried out to investigate the effects of the operating temperature, methanol feed concentration, and properties of the backing layer. The results of the numerical simulation clarify the detailed influence of these key designs and operating parameters on the methanol crossover rate as well as cell performance and efficiency. The results emphasize that the material properties and design of the anode backing layer play a critical role in the use of highly concentrated methanol fuel in DMFCs. The present study forms a theoretical background for optimizing the DMFC's components and operating conditions.     

Polyoxometallate-stabilized Pt–Ru catalysts on multiwalled carbon nanotubes: Influence of preparation conditions on the performance of direct methanol fuel cells

A novel catalyst, polyoxometallate-stabilized platinum–ruthenium alloy nanoparticles supported on multiwalled carbon nanotubes (Pt–Ru–PMo₁₂-MWNTs), was synthesized by a microwave-assisted polyol process. The effects of microwave reaction time, microwave reaction power, and pH value of the reaction solution on the electrocatalytic properties of Pt–Ru–PMo₁₂-MWNTs catalysts were also investigated. The polyoxometallate (PMo₁₂) formed a self-assembled monolayer on the surface of the Pt/Ru nanoparticles and MWNTs, which effectively prevented the agglomeration of Pt, Ru nanoparticles and MWNTs, due to the electrostatic repulsive interactions between the negatively charged PMo₁₂ monolayers. Energy dispersive spectroscopy examination and electrochemical measurements showed that the loading content of Pt/Ru and their electrochemical activity vary with the synthesis conditions, such as pH, reaction time, and microwave power. It was found that the a Pt–Ru–PMo₁₂-MWNTs electrocatalyst with high Pt loading content, small crystallite size, and good electrocatalytic activity could be synthesized using a long reaction time, intermediate microwave power, and a pH value of 7. The electrocatalysts obtained were characterized using X-ray diffraction, and scanning and transmission electron microscopy. Their electrocatalytic properties were also investigated by using the cyclic voltammetry technique.     

40SiO2–40P2O5–20ZrO2 sol-gel infiltrated sSEBS membranes with improved methanol crossover and cell performance for direct methanol fuel cell applications

Membranes commonly used in direct methanol fuel cell (DMFC) are expensive and show a great permeability to methanol which reduces fuel utilization and leads to mixed potential at the cathode. In this work, sulfonated styrene-ethylene-butylene-styrene (sSEBS) modified membranes with zirconia silica phosphate sol-gel phase are developed and studied in order to evaluate their potential use in DMFC applications. The synthesized hybrid membranes and sSEBS are subjected to an exhaustive physicochemical characterization by liquid uptake, ion exchange capacity, atomic force microscopy, X-ray photoelectron spectroscopy and dynamic mechanical and thermogravimetric analyses. Likewise, the potential use of the prepared membranes in DMFC is evaluated by means of electrochemical characterizations in single cell, determining the limiting methanol crossover current densities, proton conductivities and DMFC performances. The hybrid membranes show lower water and methanol uptakes, higher stiffness, water retention capability, upper power density and lower methanol crossover than sSEBS and Nafion 112.     

A new nickel–ceria composite for direct-methane solid oxide fuel cells

Various Ni–La_xCe_1−xO_y composites were synthesized and their catalytic activity, catalytic stability and carbon deposition properties for steam reforming of methane were investigated. Among the catalysts, Ni–La_0.1Ce_0.9O_y showed the highest catalytic performance and also the best coking resistance. The Ni–La_xCe_1−xO_y catalysts with a higher Ni content were further sintered at 1400 °C and investigated as anodes of solid oxide fuel cells for operating on methane fuel. The Ni–La_0.1Ce_0.9O_y anode presented the best catalytic activity and coking resistance in the various Ni–La_xCe_1−xO_y catalysts with different ceria contents. In addition, the Ni–La_0.1Ce_0.9O_y also showed improved coking resistance over a Ni–SDC cermet anode due to its improved surface acidity. A fuel cell with a Ni–La_0.1Ce_0.9O_y anode and a catalyst yielded a peak power density of 850 mW cm⁻² at 650 °C while operating on a CH₄–H₂O gas mixture, which was only slightly lower than that obtained while operating on hydrogen fuel. No obvious carbon deposition or nickel aggregation was observed on the Ni–La_0.1Ce_0.9O_y anode after the operation on methane. Such remarkable performances suggest that nickel and La-doped CeO₂ composites are attractive anodes for direct hydrocarbon SOFCs and might also be used as catalysts for the steam reforming of hydrocarbons.