Fabrication of MEA with hydrocarbon based membranes using low temperature decal method for DMFC

The membrane electrode assembly (MEA) with hydrocarbon (HC) based membranes made by a low temperature decal method has been investigated for the direct methanol fuel cells (DMFCs). The conventional low temperature decal (LTD) transfer method (comprised of three layers; viz., carbon, Nafion bonded electrodes and outer ionomer layers over the decal Teflon substrates) meant for the MEAs made of Nafion type membranes is suitably modified to use with hydrocarbon (HC) based membranes. The modification of conventional LTD method is effected by means of modulating the three-layered structure and optimizing other parameters to facilitate complete transfer of catalyst layers onto the HC membranes. The MEAs prepared by the modified LTD method have yielded 21 % higher DMFC performance compared to that of the MEAs produced by conventional LTD method. The structure and electrochemical properties of the MEAs have been analyzed by the field-emission scanning electron microscopy (FE-SEM) and the electrochemical impedance spectroscopy (EIS).     

Sulfonated poly(ether ether ketone) as an ionomer for direct methanol fuel cell electrodes

Sulfonated poly(ether ether ketone) has been investigated as an ionomer in the catalyst layer for direct methanol fuel cells (DMFC). The performance in DMFC, electrochemical active area (by cyclic voltammetry), and limiting capacitance (by impedance spectroscopy) have been evaluated as a function of the ion exchange capacity (IEC) and content (wt.%) of the SPEEK ionomer in the catalyst layer. The optimum IEC value and SPEEK ionomer content in the electrodes are found to be, respectively, 1.33 meq. g⁻¹ and 20 wt.%. The membrane-electrode assemblies (MEA) fabricated with SPEEK membrane and SPEEK ionomer in the electrodes are found to exhibit superior performance in DMFC compared to that fabricated with Nafion ionomer due to lower interfacial resistance in the MEA as well as larger electrochemical active area. The MEAs with SPEEK membrane and SPEEK ionomer also exhibit better performance than that with Nafion 115 membrane and Nafion ionomer due to lower methanol crossover and better electrode kinetics.     

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.     

An efficient decal transfer method using a roll-press to fabricate membrane electrode assemblies for direct methanol fuel cells

This study has focused on the development of a roll-press based decal transfer method to fabricate membrane electrode assemblies (MEAs) for direct methanol fuel cells (DMFCs). This method exhibits an outstanding transfer rate of catalyst layers from substrates to the membrane, despite hot-pressing at a considerably lower pressure and for a much shorter duration than the flat-press based conventional decal method. The MEA produced by a roll-press (R-MEA) delivers an excellent single-cell performance with power densities more than 30% higher than that fabricated using a flat-press (F-MEA). The new method considerably improves catalyst active sites in both electrodes and renders a high cathode porosity. The superior pore structure of the cathode makes the R-MEA more efficient in terms of performance and operation stability under lower air stoichiometries. Moreover, MEAs can be prepared in a continuous mode using this new method due to the unique design of the roll-press. All these advantages demonstrate the superiority of this method over the conventional flat-press decal method and make it suitable for use in the commercial manufacturing of MEAs for direct methanol fuel cells.     

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.     

Effect of ionomer content and relative humidity on polymer electrolyte membrane fuel cell (PEMFC) performance of membrane-electrode assemblies (MEAs) prepared by decal transfer method

Membrane-electrode assemblies (MEAs) were fabricated by the decal transfer method with various Nafion ionomer contents (10–40 wt%) and their single cell performance and electrochemical characteristics were examined in atmospheric air at relative humidities of 25–95%. At high humidity (95%), the MEA performance was the highest with a cathode ionomer content of 30 and 20 wt% at 0.6 and 0.4 V, respectively. The optimum ionomer content of the decal MEAs increased with decreasing humidity, because of the change in the oxygen transport rate (water flooding) and number of active sites (ionic resistance). The concentration overpotential gradually increased with relative humidity up to about 0.4 V at 0.8 A/cm², which was not considered in previous studies using pressurized air and oxygen. The combined effect of the electrochemical active surface area and ionic resistance of the cathodes on the activation overpotential was also investigated, focusing on intermediate and low humidity levels, using a newly developed impedance analysis method.     

Influence of anion ionomer content and silver cathode catalyst on the performance of alkaline membrane electrode assemblies (MEAs) for direct methanol fuel cells (DMFCs)

Alkaline membrane electrode assemblies (MEAs) were fabricated by a dry spraying method in order to evaluate and improve their performance. I–V tests indicated that the performance of alkaline direct methanol fuel cells (DMFCs) deeply depends on the ionomer contents of MEAs. MEA with 45.4% mass ionomer content showed the highest performance when non-alkaline (MeOH (1 M)) and alkaline (MeOH (1 M), NaOH (0.5 M)) fuels were used. When alkaline fuel was used, the anode and cathode performances of MEAs were also measured. The ionomer content has been shown to contribute ohmic polarization of the anode and diffusion polarization of the cathode. Furthermore, the performance of MEA with an Ag cathode catalyst was characterized. The Ag cathode catalyst was demonstrated to be a promising alternative to a Pt cathode catalyst because of its tolerance for methanol crossover.     

Performance of a direct methanol fuel cell (DMFC) at low temperature: Cathode optimization

The effects of Pt loading, Nafion content in the cathode and membrane–electrode assembly (MEA) preparation techniques (CCS_cathode/CCS_anode and CCM_cathode/CCS_anode) on the performance of MEAs for direct methanol fuel cells (DMFC) were studied. The MEA performance was analyzed with polarization curves, electrochemical impedance spectroscopy and scanning electron microscopy data. It was shown, that the cathode prepared by the catalyst coated membrane (CCM) method forms a mainly microporous and mesoporous structure, whereas the catalyst coated substrate (CCS) method generates macroporosity together with micropores and mesopores. The power density of the CCM_cathode/CCS_anode typed MEAs strongly depends on the CCM-cathode composition: Pt loading and Nafion content in the cathode. Nafion (10.7 wt.%) was found to be an optimum for DMFC performance, and at this composition, the power density gradually increased with the Pt loading up to 6.0 mg cm⁻². At higher Nafion contents, a significant mass transfer limitation at high Pt loadings occurs. Comparing the CCM and CCS methods of the cathode fabrication, the latter revealed a higher power density, which reached 104 mW cm⁻² at 0.4 V and 70 °C owing to the lack of significant mass transfer limitations. This behavior indicates that in addition to Pt loading and Nafion content, the cathode pore structure is critical to DMFC MEA performance.     

Effects of hot pressing conditions on the performance of Nafion membranes coated by ink-jet printing of Pt/MWCNTs electrocatalyst for PEMFCs

In our previous work, a hydrothermal method was employed to prepare Pt/MWCNTs nanocomposites with 20 wt.% Pt, a low mean Pt nanoparticles size (2.8 nm) and a specific surface area of 99 m² g⁻¹. In this work, the membrane electrode assemblies (MEAs) with hydrothermally synthesized Pt/MWCNTs nanocatalysts were fabricated by catalyst-coated membrane (CCM) method. For this purpose, a commercial HP inkjet printer was used to deposit Pt/MWCNTs ink (as catalyst ink) directly on to the substrate (Nafion membrane or decal substrate) with a loading of 0.2 mg cm⁻² Pt for both the anode and cathode. The effects of hot-pressing conditions on the performance of MEAs were investigated through Taguchi design of experiments method using temperature (100 and 130 °C), pressure (800 and 1000 psi) and time (3 and 5 min) as effective experimental parameters. The compression ratios of MEAs were determined by testing the thicknesses before and after hot-pressing process. The performance of MEAs was characterized by the polarization curves and cyclic voltammetry (CV) and the surface morphologies of the electrodes were observed by scanning electron microscopy (SEM). The results showed that the most appropriate hot-pressing conditions were 800 psi, 100 °C, and 3 min. Electrochemical analysis and physical property examination revealed that the MEA fabricated by CCM method has a better performance compared to the one prepared by conventional decal transfer (DT) method.     

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.     

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.     

The effects of Nafion ionomer content in PEMFC MEAs prepared by a catalyst-coated membrane (CCM) spraying method

We analyzed the effects of ionomer content on the proton exchange membrane fuel cell (PEMFC) performance of membrane electrode assemblies (MEAs) fabricated by a catalyst-coated membrane (CCM) spraying method in partially humidified atmospheric air and hydrogen. When high loading Pt/C catalysts (45.5 wt.%) were used, we observed that catalytic activity was not directly proportional to electrochemical active surface area (EAS). This suggests that ionic conductivity through ionomers in catalyst layers is also an important factor affecting MEA performance. In addition, the effects of mass transport were experimentally evaluated by manipulating the air stoichiometry ratio at the cathodes. MEA performance was more sensitive to flow rates under conditions of higher ionomer content. Due to the combined effect of EAS, ionic conductivity, and mass transfer characteristics (all of which varied according to the ionomer content), an MEA with 30 wt.% ionomer content at the cathode (25 wt.% at the anode) was shown to yield the best performance.     

Stable operation of air-blowing direct methanol fuel cells with high performance

A membrane electrode assembly (MEA) that is a combination of a catalyst-coated membrane (CCM) for the anode and a catalyst-coated substrate (CCS) for the cathode is studied under air-blower conditions for direct methanol fuel cells (DMFCs). Compared with MEAs prepared by only the CCS method, the performance of DMFC MEAs employing the combination method is significantly improved by 30% with less methanol crossover. This feature can be attributed to an enhanced electrode|membrane interface in the anode side and significantly higher catalyst efficiency. Furthermore, DMFC MEAs designed by the combination method retain high power density without any degradation, while the CCM-type cell shows a downward tendency in electrochemical performance under air-blower conditions. This may be due to MEAs with CCM have a much more difficult structure of catalytic active sites in the cathode to eliminate the water produced by electrochemical reaction. In addition, DMFCs produced via combination methods exhibit a lower water crossover flux than CCS alternatives, due to the comparatively dense structure of the CCM anode. Hence, DMFCs with a combination MEA structure demonstrate the feasibility of a small fuel cell system employing the low noise of a fan, instead of a noisy and large capacity air pump, for portable electronic devices.     

Direct methanol fuel cells: The effect of electrode fabrication procedure on MEAs structural properties and cell performance

In the present paper, the effect of electrode preparation procedure on the structural properties of membrane electrode assembly (MEA) and consequently on the performance of direct methanol fuel cells (DMFCs) was investigated. Commercial PtRu black anode catalyst and Pt black cathode catalyst were characterized by XRD in their initial form and in their intermediate and final states after each step involved in catalyst-coated membrane electrode preparation procedure by a decal transfer method (DTM). XRD results demonstrated that the DTM process has a significant effect on the catalyst structural properties, especially on the particle size of Pt black cathode catalyst. It is also discussed that among all the steps involved in the electrode fabrication procedure, catalyst ink preparation and high temperature transfer process are key factors affecting the particle size of Pt black catalyst. Furthermore, it was found that the maximum power density of the single DMFC using a MEA fabricated by the DTM, when air is used as oxidant, is more than two times greater than that of the cell using conventionally prepared MEA, and more than three times greater when pure oxygen is used as oxidant. This could be attributed to the easier mass transportation due to the thinner catalyst layer and the better contact between the catalyst layer and the electrolyte membrane in the former case, even if, according to in situ CO stripping voltammetry results in the fuel cell anode environment, the surface composition of PtRu anode has been changed.     

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.     

The effect of the MEA preparation procedure on both ethanol crossover and DEFC performance

In the present work, the changes of Nafion^®-115 membrane porosity in the presence of ethanol aqueous solutions of different concentrations were determined by weighing vacuum-dried and ethanol solution-equilibrated membranes. It was found that membrane porosity increases as ethanol concentration increases. Membrane electrode assemblies (MEAs) have been prepared by following both the conventional and the decal transfer method. The ethanol crossover through these two MEAs was electrochemically quantified by a voltammetric method. A 10 h stability test of direct ethanol fuel cell (DEFC) at a current density of 50 mA cm⁻² was carried out. It was found that the electrode preparation procedure has an obvious effect on ethanol crossover and direct ethanol fuel cell's performance and stability. The single DEFC test results showed that about 15 and 34% of the original peak power density was lost after 10 h of life test for the MEAs prepared by the decal transfer method and the conventional method, respectively. Electrochemical impedance spectrum (EIS) results of the MEAs showed that, in the case of the membrane electrode assembly prepared by the following decal transfer method, the internal cell resistance was almost the same, 0.236 Ω cm² before the life test and 0.239 Ω cm² after 10 h of life test, while the respective values for the membrane electrode assembly by the conventional method are 0.289 and 0.435 Ω cm². It is supposed that the improved cell performance with MEA by the decal transfer method could be resorted to both a better contact between the catalyst layer and the electrolyte membrane and higher catalyst utilization. Furthermore, based on the experimental results, the increased internal cell resistance and the degraded single DEFC performance could be attributed to the delamination of the catalyst layer from the electrolyte membrane.     

Effects of platinum loading on the performance of proton exchange membrane fuel cells with high ionomer content in catalyst layers

The effect of Pt loading on the performance of proton exchange membrane fuel cells with atmospheric air feed was evaluated at various relative humidities. The membrane electrode assemblies (MEAs) were fabricated by decal methods with high Nafion ionomer content (30 and 40 wt.%). When the Pt loading was decreased, the performance of the MEAs with an ionomer content of 30 wt.% gradually decreased, mainly due to the insufficient active Pt surfaces with low proton conductivity. With a higher ionomer content of 40 wt.%, the activation overpotential was not significantly increased by the decrease in Pt loading, and the concentration overpotential could be largely reduced by decreasing the Pt loading to 0.25 mg/cm². When the Pt loading was further decreased to 0.15 mg/cm², even though the flooding became more severe, the cell performance at 0.6 V and intermediate relative humidity of 55% was about 71.6%, compared to the MEA with a high Pt loading of 0.35 mg/cm² (ionomer content: 30 wt.%). The cell performance could be further enhanced by decreasing the ionomer content in the anode to enhance the water back diffusion.     

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.     

Optimization of catalyst-coated membranes for enhancing performance in proton exchange membrane electrolyzer cells

To achieve large-scale application of proton exchange membrane electrolyzer cells (PEMECs) for hydrogen production, it is highly desirable to reduce the manufacturing cost while enhancing cell performance. In the PEMPECs, a catalyst-coated membrane (CCM) is the vital component where electrochemical reactions and mass transport mainly occur. The fabrication methods and catalyst layer (CL) structure can significantly affect the cell performance. Herein, for the first time, a comparative study of CCM fabrications with decal transfer and direct spray deposition methods have been conducted by both ex-situ materials characterization and in-situ performance testing in PEMECs. It is found CCMs that are fabricated with a direct spray deposition method display enhanced cell performance compared to CCMs fabricated with a decal transfer method, mainly due to the largely reduced ohmic resistance and improved mass transport. More importantly, cell performance can be greatly enhanced by simply regulating the Nafion ionomer content at the anode CL. The optimal Nafion ionomer content of 10 wt% gives the best cell performance at 80 °C with a low cell voltage of 1.887 V at 2 A cm^?2, outperforming the commercial CCM and most other previous publications. Our study provides a valuable guidance for fabrication and optimization of CCMs with significantly enhanced performance and reduced cost for practical application of the PEMECs.     

Application of N-doped carbon nanotube-supported Pt-Ru as electrocatalyst layer in passive direct methanol fuel cell

In this research, nitrogen-doped carbon nanotubes (N-CNT) were prepared through the low-temperature thermal method and used as the support material for the bimetallic catalyst PtRu and Pt nanoparticles. A passive single-cell direct methanol fuel cell (DMFC) was designed and fabricated to investigate and compare the performance of three discrete membrane electrode assemblies (MEA) with carbon black (CB), CNT, and N-CNT as the catalyst support, respectively. Adding N to the structure of CNTs remarkably improves the physical and electrochemical characteristics of the catalyst. More active sites and stronger interaction between support and metal particles lead to the formation of smaller metal clusters and higher surface area as well as superior electrochemical activity. Compared to PtRu/CB and PtRu/CNT, PtRu/N-CNT illustrate 32% and 12% higher surface area, 3 and 1.9 times higher MOR activity, and 62% and 18% higher power output (26.1 mW/cm²), respectively. Moreover, it is revealed that PtRu/N-CNT has long-term stability in the MOR. The research work presented in this paper exhibits the outstanding performance of Pt and PtRu supported on N-CNT in a passive single-cell DMFC.