Effect of Nafion aggregation in the anode catalytic layer on the performance of a direct formic acid fuel cell

The effect of Nafion ionomer aggregation within the anode catalytic layer for a direct formic acid fuel cell (DFAFC) has been investigated. By simple heat treatment, the aggregation states of Nafion ionomers in aqueous solution can be tuned. Nafion agglomerate sizes in the solution decrease and aggregate size distribution becomes narrow with the increase in heat-treatment temperature. At a heat-treatment temperature of ca. 80 °C, nearly monodispersed Nafion ionomers corresponding to an aggregate size of ca. 25 nm in the solution are observed. The use of small Nafion ionomer agglomerates in the Nafion solution for anode catalytic layer significantly improves the performance of the passive DFAFCs. Impedance analysis indicates that the increased performance of the passive DFAFC with the anode using Nafion solution pretreated at elevated temperatures could be attributed to the decrease in charge-transfer resistance of the anode reaction. The decrease in Nafion aggregation within the catalyst ink leads to an increase in Nafion ionomer utilization within the catalyst layer and an improvement in catalyst utilization; thus enabling us to decrease Nafion loading within the anode catalytic layer but with slight improvement in DFAFC's performance.     

Improved mass transfer using a pore former in cathode catalyst layer in the direct methanol fuel cell

The cathode catalyst layer in direct methanol fuel cells (DMFCs) was prepared using polystyrene beads as a pore former. Field emission scanning electron microscopy showed that the catalyst layer with the pore former contained pores with a uniform shape and size. Mercury intrusion porosimetry showed that the pore former increased the volume of secondary pores in the catalyst layer. The electrochemical properties of the membrane electrode assembly (MEA) were evaluated by current–voltage polarization measurements, electrochemical impedance spectroscopy and cyclic voltammetry. These results suggest that the catalyst layer with the pore former reduces the mass transfer resistance and improves the cell performance by approximately 50% through modification of its morphology.     

Enhancement of direct methanol fuel cell performance through the inclusion of zirconium phosphate

Nafion/zirconium hydrogen phosphate (ZrP) composite membranes containing 2.5 wt.% ZrP (NZ-2.5) or 5 wt.% ZrP (NZ-5) were prepared to improve the performance of a direct methanol fuel cell (DMFC). The influence of ZrP content on the Nafion matrix is assessed through characterization techniques, such as Thermogravimetric Analysis (TGA), X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), Electrochemical Impedance Spectroscopy (EIS), and water uptake measurement. Performance testings of the DMFCs based on these composite membranes as well as commercial Nafion^® 115 membrane were performed using a computer aided fuel cell test station for different values of cell temperature (40 °C, 60 °C, 80 °C, and 100 °C) and methanol concentration (0.75 M, 1.00 M, and 1.50 M). Characterization studies indicated that incorporation of ZrP into polymer matrix enhanced the water uptake and proton conductivity values of Nafion membrane. The results of the performance tests showed that the Membrane Electrode Assembly (MEA) having NZ-2.5 provided the highest performance with the peak power density of 551.52 W/m² at 100 °C and 1.00 M. Then, the performances of the MEAs having the same NZ-2.5 membrane but different cathode catalysts were investigated by fabricating two different MEAs using cathode catalysts made of Pt/C–ZrP and Pt/C (HiSPEC^® 9100). According to the results of these experiments, the MEA having NZ-2.5 membrane and Pt/C (HiSPEC^® 9100) cathode catalyst containing 10 wt.% of ZrP exhibited the highest performance with the peak power density of 620.88 W/m² at 100 °C and 1.00 M. In addition, short-term stability tests were conducted for all the MEAs. The results of the stability tests revealed that introduction of ZrP to commercial (HiSPEC^® 9100) cathode catalyst improves its stability characteristics.     

Nafion/polyaniline/silica composite membranes for direct methanol fuel cell application

This work investigates the characterization and performance of polyaniline and silica modified Nafion membranes. The aniline monomers are synthesized in situ to form a polyaniline film, whilst silica is embedded into the Nafion matrix by the polycondensation of tetraethylorthosilicate. The physicochemical properties are studied by means of X-ray diffraction and Fourier transform infrared techniques and show that the polyaniline layer is formed on the Nafion surface and improves the structural properties of Nafion in methanol solution. Nafion loses its crystallinity once exposed to water and ethanol, whilst the polyaniline modification allows crystallinity to be maintained under similar conditions. By contrast, the proton conductivities of polyaniline modified membranes are 3–5-fold lower than that of Nafion. On a positive note, methanol crossover is reduced by over two orders of magnitude, as verified by crossover limiting current analysis. The polyaniline modification allows the membrane to become less hydrophilic, which explains the lower proton conductivity. No major advantages are observed by embedding silica into the Nafion matrix. The performance of a membrane electrode assembly (MEA) using commercial catalysts and polyaniline modified membranes in a cell gives a peak power of 8 mW cm⁻² at 20 °C with 2 M methanol and air feeding. This performance correlates to half that of MEAs using Nafion, though the membrane modification leads to a robust material that may allow operation at high methanol concentration.     

The effect of Nafion content in a graphitized carbon nanofiber-based anode for the direct methanol fuel cell

The performance and stability of a direct methanol fuel cell (DMFC) with membrane electrode assemblies (MEA) using different Nafion^® contents (30, 50 and 70 wt% or MEA30, MEA50 and MEA70, respectively) and graphitized carbon nanofiber (GNF) supported PtRu catalyst at the anode was investigated by a constant current measurement of 9 days (230 h) in a DMFC and characterization with various techniques before and after this measurement. Of the pristine MEAs, MEA50 reached the highest power and current densities. During the 9-day measurement at a constant current, the performance of MEA30 decreased the most (−124 μV h⁻¹), while the MEA50 was almost stable (−11 μV h⁻¹) and performance of MEA70 improved (+115 μV h⁻¹). After the measurement, the MEA50 remained the best MEA in terms of performance. The optimum anode Nafion content for commercial Vulcan carbon black supported PtRu catalysts is between 20 and 40 wt%, so the GNF-supported catalyst requires more Nafion to reach its peak power. This difference is explained by the tubular geometry of the catalyst support, which requires more Nafion to form a penetrating proton conductive network than the spherical Vulcan. Mass transfer limitations are mitigated by the porous 3D structure of the GNF catalyst layer and possible changes in the compact Nafion filled catalyst layers during constant current production.     

High performance membrane electrode assemblies by optimization of coating process and catalyst layer structure in direct methanol fuel cells

High performance membrane electrode assemblies (MEAs) for direct methanol fuel cells (DMFCs) are developed by changing the coating process, optimizing the structure of the catalyst layer, adding a pore forming agent to the cathode catalyst layer, and adjusting the hot-pressing conditions, such as pressure and temperature. The effects of these MEA fabrication methods on the DMFC performance are examined using a range of physicochemical and electrochemical analysis tools, such as FE-SEM, electrochemical impedance spectroscopy (EIS), polarization curves, and differential scanning calorimetry (DSC) of the membrane. EIS and polarization curve analysis show that an increase in the thickness and porosity of the cathode catalyst layer plays a key role in improving the cell performance with reduced cathode reaction resistance, whereas the MEA preparation methods have no significant effects on the anode impedance. In addition, the addition of magnesium sulfate as a pore former reduces the cathode reaction transfer resistance by approximately 30 wt%, resulting in improved cell performance.     

Double microporous layer cathode for membrane electrode assembly of passive direct methanol fuel cells

A novel membrane electrode assembly (MEA) is described that utilizes a double microporous layer (MPL) structure in the cathode of a passive direct methanol fuel cell (DMFC). The double MPL cathode uses Ketjen Black carbon as an inner-MPL and Vulcan XC-72R carbon as an outer-MPL. Experimental results indicate that this double MPL structure at the cathode provides not only a higher oxygen transfer rate, but enables more effective back diffusion of water; thus, leading to an improved power density and stability of the passive DMFC. The maximum power density of an MEA with a double MPL cathode was observed to be ca. 33.0 mW cm⁻², which is found to be a substantial improvement over that for a passive DMFC with a conventional MEA. A. C. impedance analysis suggests that the increased performance of a DMFC with the double MPL cathode might be attributable to a decreased charge transfer resistance for the cathode oxygen reduction reaction.     

The influences of multi-walled carbon nanotube addition to the anode on the performance of direct methanol fuel cells

Different amounts of multi-walled carbon nanotubes (MWCNTs) are added to anode catalyst layer in the membrane electrode assemblies (MEAs) of direct methanol fuel cells (DMFCs). The MEA with 0.5 wt.% carbon nanotubes (CNTs) shows the best performance in DMFC. In the protonic conductivity tests, a 0.5 wt.% amount of MWCNTs results in the highest protonic conductivity. SEM and TEM observations show that a continuous and uniform distribution of Nafion ionomer layer is formed on the MWCNT surface. Therefore, the dispersed MWCNTs in the catalyst layer are considered to be helpful for developing the pathways of protons transport.     

Mass balance research for high electrochemical performance direct methanol fuel cells with reduced methanol crossover at various operating conditions

Mass balance research in direct methanol fuel cells (DMFCs) provides a more practical method in characterizing the mass transport phenomena in a membrane electrode assembly (MEA). This method can be used to measure methanol utilization efficiency, water transport coefficient (WTC), and methanol to electricity conversion rate of a MEA in DMFCs. First, the vital design parameters of a MEA are recognized for achieving high methanol utilization efficiency with increased power density. In particular, the structural adjustment of anode diffusion layer by adding microporous layer (MPL) is a very effective way to decrease WTC with reduced methanol crossover due to the mass transfer limitation in the anode. On the other hand, the cathode MPL in the MEA design can contribute in decreasing methanol crossover. The change of structure of cathode diffusion layer is also found to be a very effective way in improving power density. In contrast, the WTC of DMFC MEAs remains virtually constant in the range of 3.4 and 3.6 irrespective of the change of the cathode GDL. The influence of operating condition on the methanol utilization efficiency, WTC, and methanol to electricity conversion rate is also presented and it is found that these mass balance properties are strongly affected by temperature, current density, methanol concentration, and the stoichiometry of fuel and air.     

Long-term durability test for direct methanol fuel cell made of hydrocarbon membrane

A long-term durability test has been conducted for a direct methanol fuel cell (DMFC) using the commercial hydrocarbon membrane and Nafion ionomer bonded electrodes for 500 h. Membrane electrode assembly (MEA) made by a decal method has experienced a performance degradation about 34% after 500 h operation. Cross-sectional analysis of the MEA shows that the poor interfacial contact between the catalyst layers and membrane in the MEA has further deteriorated after the durability test. Therefore, the internal resistance of a cell measured by electrochemical impedance spectroscopy (EIS) has considerably increased. The delamination at the interfaces is mainly attributed to incompatibility between polymeric materials used in the MEA. Furthermore, X-ray diffraction (XRD) analysis reveals that the catalyst particles have grown; thereby decreasing the electrochemical surface area. Electron probe micro analysis (EPMA) shows a small amount of Ru crossover from anode to cathode; and its effect on the performance degradation has been analyzed.     

Monolithic micro-direct methanol fuel cell in polydimethylsiloxane with microfluidic channel-integrated Nafion strip

We demonstrate a monolithic polymer electrolyte membrane fuel cell by integrating a narrow (200 μm) Nafion strip in a molded polydimethylsiloxane (PDMS) structure. We propose two designs, based on two 200 μm-wide and two 80 μm-wide parallel microfluidic channels, sandwiching the Nafion strip, respectively. Clamping the PDMS/Nafion assembly with a glass chip that has catalyst-covered Au electrodes, results in a leak-tight fuel cell with stable electrical output. Using 1 M CH₃OH in 0.5 M H₂SO₄ solution as fuel in the anodic channel, we compare the performance of (I) O₂-saturated 0.5 M H₂SO₄ and (II) 0.01 M H₂O₂ in 0.5 M H₂SO₄ oxidant solutions in the cathodic channel. For the 200 μm channel width, the fuel cell has a maximum power density of 0.5 mW cm⁻² and 1.5 mW cm⁻² at room temperature, for oxidants I and II, respectively, with fuel and oxidant flow rates in the 50-160 μL min⁻¹ range. A maximum power density of 3.0 mW cm⁻² is obtained, using oxidant II for the chip with 80 μm-wide channel, due to an improved design that reduces oxidant and fuel depletion effects near the electrodes.     

Characterization of electrode structures and the related performance of direct methanol fuel cells

The performance of membrane electrode assemblies (MEAs) in fuel cells is substantially affected by the structures of the electrodes. An increase of about 25% in power performance was achieved merely by controlling the pressure of hot press in the MEA fabrication process for a given Pt loading, instead of by employing pore formers and heat treatment-a widely accepted method-to modify the structures of the electrode. The microstructures of the different hot-pressed electrodes were examined by transmission electron microscopy, scanning electron microscopy, and small angle X-ray scattering to assess the effect of the pressure on the structures of the electrodes. Based on experimental observations, the improved performance of the MEA is attributed to the porosity of the cathode electrode, in which a network of macrofissures and sub-microfissures allows air to penetrate the electrode. Emphasis is also placed on the relationship between the total porosity of the electrodes and the MEA performance. Results of this study demonstrate that the specific power density nearly doubles when the total porosity increased from 57% to 76%. Also, the MEAs mounted in an air-breathing DMFC small pack were fabricated in-house to supply power for a mobile phone.     

Performance evaluation of passive direct methanol fuel cell with methanol vapour supplied through a flow channel

This work examines the effect of fuel delivery configuration on the performance of a passive air-breathing direct methanol fuel cell (DMFC). The performance of a single cell is evaluated while the methanol vapour is supplied through a flow channel from a methanol reservoir connected to the anode. The oxygen is supplied from the ambient air to the cathode via natural convection. The fuel cell employs parallel channel configurations or open chamber configurations for methanol vapour feeding. The opening ratio of the flow channel and the flow channel configuration is changed. The opening ratio is defined as that between the area of the inlet port and the area of the outlet port. The chamber configuration is preferred for optimum fuel feeding. The best performance of the fuel cell is obtained when the opening ratio is 0.8 in the chamber configuration. Under these conditions, the peak power is 10.2 mW cm⁻² at room temperature and ambient pressure. Consequently, passive DMFCs using methanol vapour require sufficient methanol vapour feeding through the flow channel at the anode for best performance. The mediocre performance of a passive DMFC with a channel configuration is attributed to the low differential pressure and insufficient supply of methanol vapour.     

Enhanced performance of supercritical CO2 treated Nafion 212 membranes for direct methanol fuel cells

Supercritical carbon dioxide (Sc-CO₂) thermal treatment to enhance performances of both Nafion 212 (NR212) commercial membranes with H-form and Na-form for direct methanol fuel cells (DMFCs) is described. XRD measurements show that the crystallinity of H-form NR212 membranes increases with increasing the treated temperature in the Sc-CO₂ system, however, the crystallinity of Na-form NR212 membranes decreases with increasing the treated temperature. Since the bigger crystallites formed after the Sc-CO₂ treatments, it improves the mechanical strength and dimensional stability of the Sc-CO₂ treated NR212 membranes with H-form and Na-form. Compared with the as-received NR212 membranes, all the Sc-CO₂ treated NR212 membranes show higher proton conductivity and better capacity of barrier to methanol crossover. From Fenton test, it can be found that the Sc-CO₂ treated NR212 membranes have better chemical stability than that of NR212 membranes. Therefore, NR212 membranes treated by the Sc-CO₂ method may be promising candidate electrolytes for DMFC applications.     

Effects of MEA preparation on the performance of a direct methanol fuel cell

The effects of membrane electrode assemblies (MEAs) fabrication methods (spraying and scraping methods) and the hot-pressing pretreatment of anode electrodes on the performance of direct methanol fuel cells (DMFCs) were investigated. The MEA prepared with scraped anode catalyst layer without the hot-pressing pretreatment showed the highest power density of 67 mW cm⁻² at 80 °C and ambient pressure. The scraping method proved to be a little more profitable for improving the cell performance than the spraying method. Atomic force microscopy (AFM) analysis revealed relatively smooth surface of the scraped anode catalyst layer compared with that of sprayed anode catalyst layer. Scanning electron microscopy (SEM) images showed that a suitable number of cracks which were uniformly distributed on the surface of scraped catalyst layer formed a porous structure. It was demonstrated that the surface structure and roughness of the anode catalyst layer had less effect on the performance of the anode electrode in a DMFC. The hot-pressing pretreatment of the anode electrode decreased the performance of the MEA due to the difficulty for electrons and mass transport in the anode electrode, namely the increase of internal cell resistance.     

An impedance study for the anode micro-porous layer in an operating direct methanol fuel cell

This study presents the benefit to an operating direct methanol fuel cell (DMFC) by coating a micro-porous layer (MPL) on the surface of anode gas diffusion layer (GDL). Taking the membrane electrode assembly (MEA) with and without the anodic MPL structure into account, the performances of the two types of MEA are evaluated by measuring the polarization curves together with the specific power density at a constant current density. Regarding the cell performances, the comparisons between the average power performances of the two different MEAs at low and high current density, various methanol concentrations and air flow rates are carried out by using the electrochemical impedance spectroscopy (EIS) technique. In contrast to conventional half cell EIS measurements, both the anode and cathode impedance spectra are measured in real-time during the discharge regime of the DMFC. As comparing each anode and cathode EIS between the two different MEAs, the influences of the anodic MPL on the anode and cathode reactions are systematically discussed and analyzed. Furthermore, the results are used to infer complete and reasonable interpretations of the combined effects caused by the anodic MPL on the full cell impedance, which correspond with the practical cell performance.     

Parametric investigation of a high-yield decal technique to fabricate membrane electrode assemblies for direct methanol fuel cells

This study comprehensively investigates various technical aspects of a roll-press-based decal process that is used to fabricate membrane electrode assemblies (MEAs) for direct methanol fuel cells (DMFCs). Decal transfer yield, flexibility of processing conditions and electrochemical performance of MEAs are taken into account for monitoring the productiveness of the current method. A complete transfer of both electrodes is achieved even under a pressure as low as 1.0 MPa, which is 8–35 times lower than that of conventional decal processes. This method permits use of a H⁺-form Nafion membrane in a wide press temperature domain ranging from 140 to 180 °C without the occurrence of degradation problems that are generally encountered in the conventional decal processes. The effective hot-pressing time is successfully shortened to only 2–5 s, which is far less than those of the conventional decal processes (3–10 min). The structure of cathode catalyst layer is optimized by regulating the ionomer amount. The decal MEA prepared under optimal conditions delivers a peak power density of 115 mW cm⁻² at 60 °C, which is substantially high in a DMFC operation. Superior throughput and flexibility of processing conditions over a wide range make the current method appropriate for use in the mass-production of MEAs.     

Numerical study of the effect of the GDL structure on water crossover in a direct methanol fuel cell

A two-dimensional two-phase non-isothermal mass transport model is developed to numerically investigate the behavior of water transport through the membrane electrode assembly (MEA) of a direct methanol fuel cell. The model enables the visualization of the distribution of the liquid saturation through the MEA and the analysis of the distinct effects of the three water transport mechanisms: diffusion, convection and electro-osmotic drag, on the water-crossover flux through the membrane. A parametric study is then performed to examine the effects of the structure design of the gas diffusion layer (GDL) on water crossover. The results indicate that the flow-channel rib coverage on the GDL surface and the deformation of the GDL can cause an uneven distribution of the water-crossover flux along the in-plane direction, especially at higher current densities. It is also found that both the contact angle and the permeability of the cathode GDL can significantly influence the water-crossover flux. The water-crossover flux can be reduced by improving the hydrophobicity of the cathode GDL.     

On the electrochemical impedance spectroscopy of direct methanol fuel cell

The direct methanol fuel cell (DMFC) was operated under a variety of current densities to monitor the electrochemical impedance spectroscopy (EIS) for understanding its reaction mechanism. Based on the EIS analysis, the impedance of the cell reaction is divided into three components, two of them are current dependent and the remainder is current independent. Through detailed exploration of the impedance components, the high-frequency impedance was attributed to interfacial behavior, the medium-frequency impedance to electrochemical reactions, and the low-frequency impedance to the adsorption/relaxation of CO. Based on EIS analysis, a qualitative model is proposed to delineate the reaction mechanisms of DMFC, which is confirmed quantitatively by one set of equivalent circuit elements. The experimental data are satisfactorily consistent with the results simulated from the proposed model.     

Effects of porous and dense electrode structures of membrane electrode assembly on durability of direct methanol fuel cells

This study addresses how durability of direct methanol fuel cells (DMFCs) is involved with the electrode structures of membrane electrode assembly (MEA) with different porosity and microstructures. The different electrode structures of the MEAs (porous (MEA-1) and dense (MEA-2) electrode structure) bring about the difference in the reaction kinetics associated with the electrochemical active surface area (ECSA) and in mass transport on the electrodes. The dense electrode structures of the MEA-2 cause the continual non-uniformity of the mass transport-related phenomena at the cathode, and thereby the catalysts of the MEA-2 experience much severer particle growth and agglomeration to decrease ECSA and activity of the catalysts. During the long-term operation, the decay rate of the MEA-2 was faster by more than three times compared to the MEA-1 with the relatively porous electrode structures. These results show that an electrode structure of a MEA is an important factor to govern durability of DMFCs.