Direct Methanol Fuel Cell Anode Research Articles

Electrochemical behavior of surface treated metal bipolar plates used in passive direct methanol fuel cell

Corrosion resistance performance of SS316L treated by passivation solution was investigated in a simulated environment of the passive direct methanol fuel cell (DMFC). Electrochemical impedance spectroscopic (EIS) test showed that polarization resistance of untreated and treated SS316L were 1191 Ω cm 2 and 9335 Ω cm 2, respectively. The above result agreed with the Tafel slope analysis of potentiodynamic polarization curves. Comparing the untreated and treated SS316L in the simulated environment of DMFC anode working conditions, it was observed that the corrosion current density of treated SS316L as estimated by 4000 s potentiostatic test reduced from 38.7 μA cm −2 to 0.297 μA cm −2, meanwhile, the current densities of untreated and treated SS316L in cathode working conditions were 3.87 μA cm −2 and 0.223 μA cm −2, respectively. It indicated that the treated SS316L should be suitable in both anode and cathode environment of passive DMFCs. The treated SS316L bipolar plates have been assembled in a passive single fuel cell. A peak power density of 1.18 mW cm −2 was achieved with 1 M methanol at ambient temperature.

Optimum Pt and Ru atomic composition of carbon-supported Pt–Ru alloy electrocatalyst for methanol oxidation studied by the polygonal barrel-sputtering method

The optimum Pt and Ru atomic composition of a carbon-supported Pt–Ru alloy (Pt–Ru/C) used in a practical direct methanol fuel cell (DMFC) anode was investigated. The samples were prepared by the polygonal barrel-sputtering method. Based on the physical properties of the prepared Pt–Ru/C samples, the Pt–Ru alloy was found to be deposited on a carbon support. The microscopic characterization showed that the deposited alloy forms nanoparticles, of which the atomic ratios of Pt and Ru (Pt:Ru ratios) are uniform and are in accordance with the overall Pt:Ru ratios of the samples. The formation of the Pt–Ru alloy is also supported by the electrochemical characterization. Based on these results, methanol oxidation on the Pt–Ru/C samples was measured by cyclic voltammetry and chronoamperometry. The results indicated that the methanol oxidation activities of the prepared samples depended on the Pt:Ru ratios, of which the optimum Pt:Ru ratio is 58:42 at.% at 25 °C and 50:50 at.% at 40 and 60 °C. This temperature dependence of the optimum Pt:Ru ratio is well explained by the relationship between the methanol oxidation reaction process and the temperature, which is reflected in the rate-determining steps considered from the activation energies. It should be noted that at 25–60 °C, the Pt–Ru/C with Pt:Ru = 50:50 at.% prepared by our sputtering method has the higher methanol oxidation activity than that of a commercially available sample with the identical overall Pt:Ru ratio. Consequently, the polygonal barrel-sputtering method is useful to prepare the practical DMFC anode catalysts with the high methanol oxidation activity.

Methanol oxidation catalysis and substructure of PtRu/C bimetallic nanoparticles synthesized by a radiolytic process

Nanoparticle catalysts of PtRu/C for the direct methanol fuel cell anode were synthesized by a radiolytic process. Bimetallic substructures were controlled by varying irradiation dose rate and by addition of NH 4OH or NaH 2PO 2. Material characterization was performed with the transmission electron microscopy, the X-ray diffraction, the X-ray fluorescence spectroscopy and the X-ray absorption fine structure techniques. Methanol oxidation activity was evaluated by the linear sweep voltammetry. We concluded that the structure of the radiolytically synthesized catalysts has a Pt-rich core/Ru-rich shell structure or incomplete alloy structure. A correlation between the substructures and catalytic activities was found by using a pairing factor defined from coordination numbers determined by the extend X-ray absorption fine structure analysis, which indicates the validity of the bifunctional mechanism in the PtRu nanoparticle system. This radiolytic process is promising for synthesizing advanced PtRu/C catalysts with well-mixed bimetallic substructures enhancing methanol oxidation.

Improved Electrochemical Durability of PtRuAu/C Catalyst Synthesized by Radiolytic Process

ABSTRACTNanoparticle catalyst of PtRuAu/C for direct methanol fuel cell anodes was synthesized by a radiolytic process. Its methanol oxidation activity and the electrochemical durability were evaluated by using the linear sweep voltammetry and the cyclic voltammety. The Au addition significantly improved the durability in comparison with PtRu/C catalyst without losing its high activity. The atomic structure was characterized with techniques of the transmission electron microscopy, the X-ray diffraction, the X-ray fluorescence spectroscopy and the X-ray absorption fine structure. These results implied that the arrangement of Pt and Ru atoms in the PtRuAu/C has no significant difference from that without Au, possessing a structure of Pt rich core and PtRu alloy shell. We concluded that the improvement in durability could originate from these PtRu nanoparticles decorated with Au, but not from particles with high Au contents.

High pressure anode operation of direct methanol fuel cells for carbon dioxide management

Experiments with independent pressurization of the direct methanol fuel cell anode and cathode allow for the observation of DMFC operation with carbon dioxide gas formation suppressed. Results indicate that the limiting current density is strongly related to the applied pressure, and, therefore, to the presence of CO 2 in the liquid phase. An additional experiment where CO 2 is allowed to accumulate in recycled anode fuel solution over a period of time and is then stripped from solution using nitrogen gas indicates that the presence of CO 2 in anode fuel solution at any pressure contributes to significant decreases in power and current density. Because CO 2 bubbles are ubiquitous in direct methanol fuel cells, this finding is key to the optimization of these systems.

Electrochemical preparation of Pt-based ternary alloy catalyst for direct methanol fuel cell anode

The CoPtRu catalyst was prepared with electrochemical methods on carbon paper. The preparation of Co particles on the carbon paper was performed through an electrodeposition process by varying the deposition potential and time. After Co electrodeposition, Pt and Ru galvanic displacements were carried out by controlling displacement time. The bulk and surface composition of the catalysts were analyzed by using inductively coupled plasma (ICP) mass spectroscopy and X-ray photoelectron spectroscopy (XPS), respectively. It was proved that the CoPtRu catalyst was successfully synthesized using the electrochemical process. In this study, the electrochemically prepared catalysts showed superior catalytic activity for methanol oxidation and tolerance to CO poisoning compared to a commercial PtRu/C catalyst (E-tek).

Total harmonic distortion analysis for direct methanol fuel cell anode

In this study, total harmonic distortion (THD) analysis is put forward to illustrate nonlinear behavior of direct methanol fuel cell (DMFC) anode. THD simulations by means of different methanol oxidation kinetics as well as its experimental validation are both carried out. It is shown that the THD model adopting a three-step methanol oxidation mechanism with Kauranen–Frumkin/Temkin kinetics can be used to illustrate the THD variation for DMFC anode qualitatively. The experimental THD response at the frequency range from 0.063 Hz to 0.4 Hz is identified as the reflection of the nonlinearity variation of those kinetic steps involving intermediates in the methanol oxidation. In such a frequency domain, THD value decrease monotonously with decreasing methanol concentration, which notices its accessibility on methanol concentration detection.

Preparation of Anodes for DMFC by Co-Sputtering of Platinum and Ruthenium

Anodes used for direct methanol fuel cells (DMFCs) were fabricated by magnetron sputtering process. A house-made plasma sputtering system was used to deposit Pt and PtRu onto un-catalyzed gas diffusion layers (GDLs) at different radio frequency (RF) powers and sputtering-gas pressures. The thin film catalyst layers were characterized by X-ray diffraction, energy dispersive X-ray analysis, and X-ray photoelectron spectroscopy. The sputtered anodes were assembled in custom-made membrane electrode assemblies (MEAs) with a commercial cathode and the electrical performance of the single cell were tested under passive methanol supply, ambient temperature and air-breathing conditions. The electrochemical performance of the anodes prepared with PtRu alloy was compared with a reference anode sputtered with Pt only. X-ray diffraction and X-ray photoelectron spectroscopy revealed that platinum and ruthenium existed as a form of alloy. The cell polarization measurements showed that all the PtRu alloy catalysts had better electrochemical performance than the Pt1 catalyst, and the Pt0.43Ru0.57 catalyst achieved the best performance.

The influence of sodium ion as a potential fuel impurity on the direct methanol fuel cells

Na + is a likely intrinsic impurity in water and is a sort of common cation impurity in the direct methanol fuel cells (DMFCs). In this paper, the effect of Na + on the DMFC electrochemical response is studied by adding Na + into the methanol water solution fed in the anode of DMFC. The dynamic variation of cell voltage results shows that the DMFC performance degraded by the presence of Na + impurity, and the higher concentration of Na + impurity, the higher poisoning rate is observed. In the meantime, an external reference electrode is used to measure the potential and impedance of the cathode and anode. It is found that the dramatic decrease of the cell voltage is mainly ascribed to the increase of the cathode overpotential which is caused by Na + exchange with protons in the cathode catalyst layer. The electrochemical impedance measurements suggest that the lack of available protons and low oxygen concentration at the cathode catalytic sites contributed to this degradation. Furthermore, the recovery strategy is introduced and it is found that the poisoned MEA could be partly recovered by immersing in 0.5 M H 2SO 4 solution for 4 h.

Evaluation of a compression molded composite bipolar plate for direct methanol fuel cell

A polymer–graphite composite bipolar plate of direct methanol fuel cell (DMFC) was fabricated by a compression molding method. The electrical conductivity and electrochemical behavior of the composite material under DMFC operating conditions were evaluated. The results show that the composite bipolar plate has a good electrical conductivity. Moreover, the through-plane conductivity of the composite material is higher than the in-plane one, which is ascribed to the anisotropic property of the composite bipolar plate resulted from the compression molding process. Corrosion tests show that the stable current density is below 10 μA cm −2 under both anode and cathode conditions of DMFC. The discharge test of the DMFC single cell also presents a satisfactory result.

Pseudo-half-cell measurements on symmetrical catalyst-coated membranes and their relevance for optimizing DMFC anodes

The preparation of catalyst-coated membranes (CCMs) with two anodic catalyst layers (60% PtRu/C as catalyst) using a decal technique and their characterization by pseudo-half-cell measurements using both sides of the CCMs by simply turning the test cell around, allows the characterization of quasi-identical CCMs with a much smaller experimental uncertainty than observed for the classical direct methanol fuel cell (DMFC) testing of membrane electrode assemblies under similar working conditions (5 mV vs. 12 mV at a current density of 140 mA cm−2). With this new sensitive tool, we study the influence of the dispersing technology and the Nafion content on the performance of DMFC anodes. While the ionomer content shows a broad optimum between 20 and 40%, the dispersing technology does not have a strong impact on the fuel cell performance under the experimental conditions of this study, but influences strongly the stability of the catalyst slurries and the homogeneity of the electrode coatings.

Two-phase hydrodynamics in a miniature direct methanol fuel cell

We present controlled experiments on a miniature direct methanol fuel cell (DMFC) to study the effects of methanol flow rate, current density, and void fraction on pressure drop across the DMFC anode. We also present an experimental technique to measure void fraction, liquid slug length, and velocity of the two-phase slug flow exiting the DMFC. For our channel geometry in which the diameter of the largest inscribed sphere ( a) is 500 μm, pressure drop scales with the number of gas slugs in the channel, surface tension, and a. This scaling demonstrates the importance of capillary forces in determining the hydrodynamic characteristics of the DMFC anode. This work is aimed at aiding the design of fuel pumps and anode flow channels for miniature DMFC systems.

Mordenite-incorporated PVA–PSSA membranes as electrolytes for DMFCs

Composite membranes with mordenite (MOR) incorporated in poly vinyl alcohol (PVA)–polystyrene sulfonic acid (PSSA) blend tailored with varying degree of sulfonation are reported. Such a membrane comprises a dispersed phase of mordenite and a continuous phase of the polymer that help tuning the flow of methanol and water across it. The membranes on prolonged testing in a direct methanol fuel cell (DMFC) exhibit mitigated methanol cross-over from anode to the cathode. The membranes have been tested for their sorption behaviour, ion-exchange capacity, electrochemical selectivity and mechanical strength as also characterized by Fourier transform infrared spectroscopy and thermogravimetric analysis. Water release kinetics has been measured by magnetic resonance imaging (NMR imaging) and is found to be in agreement with the sorption data. Similarly, methanol release kinetics studied by volume-localized NMR spectroscopy (point resolved spectroscopy, PRESS) clearly demonstrates that the dispersion of mordenite in PVA–PSSA retards the methanol release kinetics considerably. A peak power-density of 74 mW/cm 2 is achieved for the DMFC using a PVA–PSSA membrane electrolyte with 50% degree of sulfonation and 10 wt.% dispersed mordenite phase. A methanol cross-over current as low as 7.5 mA/cm 2 with 2 M methanol feed at the DMFC anode is observed while using the optimized composite membrane as electrolyte in the DMFC, which is about 60% and 46% lower than Nafion-117 and PVA–PSSA membranes, respectively, when tested under identical conditions.

Electrocatalysts for methanol oxidation with ultra low content of Pt and Ru

We have developed efficient electrocatalysts for methanol oxidation using new synthetic method facilitating deposition of Pt–Ru very thin nanoplatelets on carbon nanoparticles The method involves oxidation of carbon support, adsorption of Pb 2+, its reduction and galvanic displacement of Pb 0 by Pt and/or Ru. The Pt mass activity of this catalyst is about 10 times higher than that of the commercial Pt–Ru/C. The catalyst with the 1:1 Pt/Ru ratio displayed the highest methanol oxidation activity per surface Pt atom. Our results demonstrate the new synthetic method that yields the catalyst with potential for solving the problem of high Pt loading in direct methanol fuel cell anodes.

Effect of anode current collector on the performance of passive direct methanol fuel cells

The effect of anode current collector on the performance of passive direct methanol fuel cell (DMFC) was investigated in this paper. The results revealed that the anode of passive DMFC with perforated current collector was poor at removing the produced CO2 bubbles that blocked the access of fuel to the active sites and thus degraded the cell performance. Moreover, the performances of the passive DMFCs with different parallel current collectors and different methanol concentrations at different temperatures were also tested and compared. The results indicated that the anode parallel current collector with a larger open ratio exhibited the best performance at higher temperatures and lower methanol solution concentrations due to enhanced mass transfer of methanol from the methanol solution reservoir to the gas diffusion layer. However, the passive DMFC with a smaller open ratio of the parallel current collector exhibited the best performance at lower temperatures and higher methanol solution concentrations due to the lower methanol crossover rate. Copyright © 2009 John Wiley & Sons, Ltd.

The effect of oxygen pretreatment on the morphological and electrochemical characteristics of a direct methanol fuel cell anode of different structures

Oxygen pretreatments on two types of carbon supports used in the anode of a direct methanol fuel cell (DMFC) was carried out to generate oxygen functional groups (OFGs) on their surfaces. These carbon supports, conventional carbon blacks and a special mesoporous carbon structure designated as CMK-1, were then deposited with platinum and ruthenium particles by a wet-incipient method. Transmission electron spectroscopy and X-ray diffraction spectroscopy were conducted to distinguish the surface morphologies of the prepared catalysts. In addition, the prepared catalysts were mixed with a liquid Nafion solution and pasted onto carbon paper to generate samples for electrochemical analyses in methanol solutions. Test results showed that the OFG treatment to the two types of carbon supports seemed to help in catalyst dispersion and in particle size reduction. The OFG-treated samples exhibited comparatively better catalytic activity for methanol oxidation and CO tolerance. In addition, the CMK-1 carbon support, whether OFG-treated or not, tended to favor the formation of a crystal structure by the catalyst that could alleviate the CO adsorption phenomenon.

DMFC electrode preparation, performance and proton conductivity measurements

A method that involves stenciling electrodes using dry powders for fuel cells is described and compared to anodes and cathodes prepared by the traditional spraying method using catalyst inks. Methods to determine the proton conductivity of the DMFC anode layer are also discussed. The stenciling method allows for the preparation of highly reproducible membrane electrode assemblies (MEAs) utilizing little waste material. MEAs can be prepared in a controlled manner using the stenciling technique. The resulting morphology of the as-prepared electrodes is observed to be dependent on the preparation method, while the thickness of the once hot-pressed catalyst layers appears to be independent of the preparation method. Stenciled anodes of the same catalyst loading were found to show a lower proton resistance (Rp) than sprayed anodes. However, the lower Rp value was not sufficient to result in a measurable increase in the performance of a direct methanol fuel cell (DMFC); as in fact, the average steady-state DMFC performance was found to be the same using sprayed or stenciled electrodes. The DMFC performance was found to be strongly dependent on the Nafion content and large increases in the Nafion content were needed to increase the DMFC performance measurably. Even though thick electrodes were prepared in this work, the Rp values of the stenciled anodes were found to be comparable to results reported in the literature for much thinner electrodes made using high metal catalyst loadings on carbon. This observation is most probably due to the higher Nafion content used in this work.

Model and Experimental Study of Hydrodynamic Coupling between a Fuel Pump and a Direct Methanol Fuel Cell

Controlled experiments on a direct methanol fuel cell (DMFC) elucidate the effects of methanol flow rate and current density on pressure drop across the DMFC anode. A novel experimental technique is developed to measure void fraction, liquid slug length, and velocity of the slug/annular two phase flow exiting a DMFC anode. For the explored channel geometry, hydraulic diameter (Dh) of 600 microns, pressure drop scales with the number of slug/bubble interfaces, surface tension, and Dh. We present an empirical correlation of more than 160 measurements to aid in predicting pressure drop for DMFCs of similar scale. This work will aid in the design of fuel pumps for miniature DMFC systems.

Evaluation of the Microstructure and the Elementary Steps from the Impedance of DMFC Anode

The microstructure and the elementary steps of the direct methanol fuel cell (DMFC) anode are investigated using an advanced equivalent circuit. A serial circuit of two parallel circuits of resistance and capacitance, which is in parallel with a Faradaic impedance, is introduced to simulate the anodic reaction. Three processes with different anode polarization dependencies are clearly reproduced by the simulation using this equivalent circuit, and they are considered to be interfacial resistance representing the transport of ions through the electrode pores, electrooxidation reaction of the anode, and another process parallel to the reaction intermediate. The anode polarization dependence of the interfacial resistance and the change in the distortion was better explained by the modified equivalent circuit than by the conventional equivalent circuit using a constant phase element (CPE) with unfixed exponent. The relationship of the interfacial resistance with catalyst loading or electrode thickness was clarified using this equivalent circuit.

Water and methanol crossover in direct methanol fuel cells—Effect of anode diffusion media

Various anode diffusion media have been experimentally studied to reduce water crossover in a direct methanol fuel cell (DMFC). A two-phase water transport model was also employed to theoretically study their effects on water transport and saturation level in a DMFC anode. It is found that wettability of the anode microporous layer (MPL) has a dramatic effect on water crossover or the water transport coefficient ( α) through the membrane. Under different current densities, the MEA with a hydrophobic anode MPL has consistently low α, several times smaller than those with a hydrophilic MPL or without an anode MPL. Methanol transport in the anode is found to be not influenced by a hydrophobic anode MPL but inhibited by a hydrophilic one. Constant-current discharge shows that the MEA with hydrophobic anode MPL displays much smaller voltage fluctuation than that with the hydrophilic one. A modeling study of anode water transport reveals that the liquid saturation in the anode is lowered significantly with the increase of anode MPL contact angle, which is thus identified as a key parameter to minimize water crossover in a DMFC.