Direct Methanol Fuel Cell Cathode Research Articles

Simulation of mass transfer in a passive direct methanol fuel cell cathode with perforated current collector

The current collector offers passages for mass transport and is one of the key components of the passive direct methanol fuel cell (DMFC). The effect of perforated current collector design on mass transport is studied based on a three-dimensional (3D), unsteady-state, two-phase mass transport model of a passive DMFC cathode. The model is implemented via the mixture multiphase model, which solves the continuity and momentum equations for the mixture and the volume fraction equation for the secondary phases. Numerical results indicate that the distributions of oxygen in both cathode catalyst layer (CCL) and cathode diffusion layer (CDL) are non-uniform because of the effect of the perforated current collector plate (CCP) structure. Liquid water produced by an electrochemical reaction in the CCL accumulates significantly at the bottom. Clearly, the distribution of liquid water in the cathode catalyst and diffusion layers is affected by gravity. The size of the circular holes and the distance between them are taken into account to investigate the effect of the CCP structure. Small uniformly arrayed circular holes in the entire active area of CCP are advantageous to the transfer of gas and liquid water in the cathode side of a passive DMFC.

Carbon Dioxide Transport in Nafion 1100 EW Membrane and in a Direct Methanol Fuel Cell

The CO2 transport in the proton form Nafion membranes (E.W. 1100) under various hydration and temperature conditions were studied by using an IR based CO2 detector to monitor both the transient and steady state CO2 permeate flux through the membranes. A time-lag method was used to extract the CO2 diffusivity and solubility from the CO2 permeation-time curve. It was found that the CO2 diffusivity in a dry membrane is very low, but it increases rapidly with the membrane-hydration level. A high CO2 solubility was observed in dry membranes as compared to those in humidified membranes, for which the CO2 solubility remains nearly invariant to the membrane hydration level at a given temperature. By comparing the measured data with those calculated for the CO2 transport solely in the water channels, it is concluded that the CO2 transport in Nafion membranes is dominated by its transport in the water channels. CO2 and methanol crossovers in a direct methanol fuel cell (DMFC) configuration were also examined. By operating the DMFC configuration in methanol electrolysis mode, one can measure the CO2 crossover rate, which can be used to correct the methanol crossover measurements from measuring CO2 flux in the DMFC cathode effluent stream, and to derive the local CO2 partial pressure at the DMFC anode catalyst layer.

Novel cathode catalyst for DMFC: Study of the density of states of oxygen adsorption using density functional theory

Platinum (Pt) is the most common cathode catalyst in Direct Methanol Fuel Cells (DMFC). However, Pt involved both in the oxidation reaction of methanol and the reduction of oxygen on the cathode side, which limits the performance of DMFCs. Therefore, this study investigates cobalt phthalocyanine (CoPc) as a potential DMFC cathode catalyst capable of inhibiting the reactivity of methanol oxidation. This study investigates the reaction mechanism and adsorption energy of oxygen molecules on Cobalt Phthalocyanine (CoPc) using the Density Functional Theory (DFT). The basis sets of semi-core pseudopotential (DSPP) and effective core potential are used to compare the geometric optimization of the Cobalt Phthalocyanine structure. The adsorption strength of molecular oxygen on the Cobalt metal center of Phthalocyanine is investigated using Partial Density of States and compared with that of Iron Phthalocyanine (FePc). Finally, the results show that the adsorption energy of carbon monoxide and methanol on the Cobalt and Iron metal centers of the Phthalocyanine complex yield low energy adsorption on CoPc. It was observed that the adsorption energy when two metal sites of CoPc and supported CoPc on tungsten atom are involved yields high energy adsorption and bond length of molecular oxygen that would lead to oxygen bond dissociation. Therefore, this study concludes that CoPc has the potential to replace Pt due to its high tolerance to methanol and carbon monoxide oxidation.

Synthesis of Pd3Co1@Pt/C Core‐Shell Catalysts for Methanol‐Tolerant Cathodes of Direct Methanol Fuel Cells

A composite Pd-based electrocatalyst consisting of a surface layer of Pt (5 wt.%) supported on a core Pd3Co1 alloy (95 wt.%) and dispersed as nanoparticles on a carbon black support (50 wt.% metal content) was prepared by using a sulphite-complex route. The structure, composition, morphology, and surface properties of the catalyst were investigated by XRD, XRF, TEM, XPS and low-energy ion scattering spectroscopy (LE-ISS). The catalyst showed an enrichment of Pt on the surface and a smaller content of Co in the outermost layers. These characteristics allow a decrease the Pt content in direct methanol fuel cell cathode electrodes (from 1 to 0.06 mg cm(-2)) without significant decay in performance, due also to a better tolerance to methanol permeated through the polymer electrolyte membrane.

PtPdCo ternary electrocatalyst for methanol tolerant oxygen reduction reaction in direct methanol fuel cell

Carbon-supported PtPdCo/C ternary electrocatalysts were prepared by using the sodium borohydride method for use as a cathode catalyst in direct methanol fuel cells (DMFCs). The electrocatalyst particles with a size of 2–3nm were uniformly dispersed on carbon supports. PtPdCo/C showed a similar performance compared to commercial Pt/C in the oxygen reduction reaction (ORR) tests conducted with a rotating disk electrode (RDE). On the other hand, PtPdCo/C showed higher methanol tolerance than Pt/C in acidic media with methanol. In the single-cell tests, the performance of the PtPdCo/C electrocatalyst was approximately 50% higher than that of Pt/C owing to its enhanced methanol tolerance. In the long-term operation test with the single-cell, the maximum power density of PtPdCo/C decreased only by 14% from its initial value. These results indicate that the PtPdCo/C catalyst is potentially an alternative electrocatalyst for the cathode in DMFCs.

Polarization and Electrocatalyst Selection for Polybenzimidazole Direct Methanol Fuel Cells

High temperature direct methanol fuel cells (DMFCs) using polybenzimidazole (PBI) membranes could improve the energy density of portable power sources. This study examines the polarization of vapor phase PBI DMFCs constructed with commercial membranes manufactured by a sol-gel method. The polarization of the high temperature DMFCs is compared to similar low temperature membrane electrode assemblies (MEAs) using Nafion® membranes. The results showed that the cathode of the PBI DMFC had higher kinetic losses that are likely due to phosphate poisoning of the Pt electrocatalyst. At the tested conditions, the membrane conductivity of the PBI MEAs was comparable to the Nafion® MEA even with no humidification. Higher cell temperatures significantly improved PBI DMFC performance for Pt electrocatalyst electrodes. In full cell tests, the PBI DMFC MEAs had higher performance than Nafion® MEAs with similar catalyst loadings. The Pt and PtRu catalysts were tested for methanol oxidation and oxygen reduction activity by a rotating disk electrode (RDE) under 0.5 M H2SO4 and 0.5 M H3PO4. The combination of the polarization and RDE results for the PBI and Nafion® DMFCs suggest that Pt is a more active electrocatalyst for methanol oxidation in PBI than in Nafion®.

The features of a direct methanol fuel cell cathode impedance due to methanol crossover: Modeling and experiment

We report a model for impedance of the cathode catalyst layer (CCL) of a DMFC. The model employs a non-Tafel rate of the methanol oxidation reaction (MOR) in the cathode and it takes into account diffusive transport of oxygen and methanol. Due to large MOR overpotential on the cathode side, the rate of this reaction is determined by the potential-independent methanol adsorption on the catalyst surface. According to the model, at all methanol concentrations, the presence of methanol in the CCL affects the low-frequency part of the spectrum. In addition, at high methanol concentration, a MOR-dominated domain (virtual anode) forms at the membrane interface. This domain manifests itself as a weakly pronounced second high-frequency arc. The model predictions are compared to the measured in-house and literature cathode spectra.

Mathematical Modeling of a DMFC Cathode with a Double Catalyst Layered Structure Membrane Electrode Assemblies

Membrane electrode assemblies (MEA) is the key component of a direct methanol fuel cell (DMFC) and its structure and fabrication technology influenced the performance of DMFC a lot. A novel structured MEA including a hydrophilic inner thin catalyst layer and a traditional outer catalyst layer, provide higher performance. To optimize the combination of the two catalysts layer a mathematical model based on Tafel type kinetics and semi-empirical mass transport coefficient was applied. The simulation of cathode overpotential results showed a DMFC with a 5μm thick inner Pt Blk catalyst layer and an 8μm thick outer 40wt%Pt/C catalyst layer as cathode electrode was the best.

Water Evolution in Direct Methanol Fuel Cell Cathodes Studied by Synchrotron X‐Ray Radiography

AbstractWater evolution, distribution, and removal in the cathodes of a running direct methanol fuel cell were investigated by means of synchrotron X‐ray radiography. Radiographs with a spatial resolution of around 5 μm were taken every 5 s. Special cell designs allowing for through‐plane and in‐plane viewing were developed, featuring two mirror‐symmetrical flow field structures consisting of one channel with the through‐plane design. Evolution and discharge of water droplets and the occurrence of water accumulations in selected regions of the channels were investigated. These measurements revealed a nonuniform distribution of water in the channels. Both irregular and periodic formation of water droplets were observed. In‐plane measurements revealed, that the droplets evolve between adjacent carbon fiber bundles of the gas diffusion layer. The water distribution within the channel cross‐section fits very well to the pressure difference between cathode channel inlet and outlet. The quick discharge of water droplets causes sudden decreases of the pressure difference up to 4.5 mbar.

Three-dimensional transient modeling and analysis of two-phase mass transfer in air-breathing cathode of a fuel cell

Two-phase transport is essential for water and thermal management of passive direct methanol fuel cells (DMFCs). This paper develops a three-dimensional (3D), transient, isothermal and two-phase mixture model and investigates the mass transfer characteristic in the air-breathing cathode of a direct methanol fuel cell. The velocity and volume fraction of species inside the fuel cell along with the process of electrochemical reaction are studied. The effects of several key parameters such as porosity of porous medium, current density and cell operation pressure are investigated to discuss mass transfer characteristic in the passive DMFC cathode. The numerical simulation results indicate that cathode catalyst layer porosity has definite effects on oxygen transfer and removal of water. It is also found that an adequate current density and appropriate cathode pressure could facilitate a more quickly removal of water from the cathode to the ambient.

First principles study of oxygen adsorption on Se-modified Ru nanoparticles

We present here the results of our density-functional-theory-based calculations of the electronic and geometric structures and energetics of Se and O adsorption on Ru 93- and 105-atom nanoparticles. These studies have been inspired by the fact that Se/Ru nanoparticles are considered promising electrocatalysts for the oxygen reduction reaction (ORR) on direct methanol fuel cell cathodes and the oxygen binding energy is a descriptor for the catalyst activity toward this reaction. We find the character of chemical bonding of Se on a flat nanoparticle facet to be ionic, similar to that obtained earlier for the Se/Ru(0001) surface, while in the case of a low-coordinated Ru configuration there is an indication of some covalent contribution to the bonding leading to an increase in Se binding energy. Se and O co-adsorbed on the flat facet both accept electronic charge from Ru, whereas the adsorption on low-coordinated sites causes more complicated valence charge redistribution. The Se modification of the Ru particles leads to weakening of the oxygen bonding to the particles. However, overall, O binding energies are found to be higher for the particles than for Se/Ru(0001). The high reactivity of the Se/Ru nanoparticles found in this work is not favorable for ORR. We thus expect that larger particles with well-developed flat facets will be more efficient ORR catalysts than small nanoparticles with a large fraction of under-coordinated adsorption sites.

A model for mixed potential in direct methanol fuel cell cathode and a novel cell design

A model for performance of the cathode side of a direct methanol fuel cell at small currents is developed. In our recent model [13], a direct catalytic methanol–oxygen reaction on the cathode side was assumed. In this work, we assume an electrochemical mechanism of methanol oxidation reaction (MOR) in the cathode. The model predicts 600mV-potential loss at OCV, which is close to the experimental data. This result gives confidence on the electrochemical mechanism of MOR in the cathode and it suggests a novel three-electrode DMFC design. The third auxiliary electrode is inserted between the anode and the cathode, and the carbon phase of this electrode is connected to the cathode through a small resistor. The auxiliary electrode is expected to effectively convert the flux of methanol in the membrane into the useful current.

Evaluation of porous carbon substrates as catalyst supports for the cathode of direct methanolfuel cells

This paper presents an investigation into the effect of the cathode catalyst substrate morphology on the performance of supported catalysts for direct methanol fuel cells (DMFC). Two porous carbons were synthesized by a sacrificial templating method, one using disordered, macroporous silica diatomaceous earth, the other using ordered, mesoporous SBA-15. The Pt particles deposited onto the synthesized mesoporous substrate and commercial Vulcan XC-72 were shown to have fairly uniform dispersion, with particle size in the range 2–3 nm, and high catalyst surface areas. In situ single cell DMFC testing showed that the Pt catalyst supported on carbon based on diatomaceous earth (Pt/C–Celatom) exhibited the lowest cell impedance, which may be attributed to the large pores, which facilitate oxygen transport, and to the low porosity, which enhances protonic and electronic conductivity of the catalyst layer. In DMFC polarization tests, the Pt/C–Celatom also exhibited the highest power output of the tested catalysts, with a peak power density 28% better than the current commercial standard, Vulcan XC-72.

A model for mixed potential in direct methanol fuel cell cathode

We report a model for performance of the cathode catalyst layer (CCL) in a DMFC. Analytical solutions for the CCL polarization voltage and for the specific voltage loss due to methanol crossover (mixed potential) are obtained. The methanol–oxygen reaction in the CCL decreases the local oxygen concentration in the electrode, thereby raising the local overpotential and increasing the oxygen flux in the CCL. For typical parameters, the mixed potential peak of 150–200 mV is reached at the current density of 10–50 mA cm −2. A key parameter of crossover, which determines the value of mixed potential can be measured using the half-cell impedance spectroscopy.

Functional DMFC Cathode Catalysts and Supports Based on Niobium Oxide Phase

Composite electrocatalysts consisting of platinum nanophase supported on carbon black decorated with niobium oxide phase were prepared by a two step precipitation/reduction procedure. The intrinsic catalytic activity of these materials was evaluated for the oxygen electroreduction reaction, measured in the absence and presence of methanol. It was found that the NbxOy-composite materials are more hydrophilic and thus initially have lower intrinsic catalytic activity for oxygen reduction. However, materials containing NbxOy offer better overall platinum mass utilization, achieved through much better dispersion of the platinum nanophase. Such advantageous dispersion is observed even for high metal loadings. Another advantage of these materials is minimal effect of poisoning with the intermediates formed during methanol oxidation that results in the enhanced methanol tolerance in direct methanol fuel cells (DMFC).

Synthesis and evaluation of carbon nanotube-supported RuSe catalyst for direct methanol fuel cell cathode

Ruthenium selenide (RuSe) supported on a carbon nanotube (CNT) material, i.e., RuSe/CNT, with a controlled composition (Ru:Se = 1:0.2) was synthesized using a modified polyol method as a model catalyst for direct methanol fuel cell (DMFC) cathode. The prepared electrocatalyst was physically characterized by means of Transmission Electron Microscopy (TEM), X-ray Diffraction (XRD) Spectroscopy and X-ray Photoelectron Spectroscopy (XPS), and its activity for oxygen reduction reaction (ORR) was examined using Linear-Sweep Voltammetry (LSV). In addition, the methanol tolerance was characterized using Electrochemical Impedance Spectroscopy (EIS). It was found that the prepared RuSe/CNT catalyst has good catalyst morphology, uniform and small particle size, and controllable catalyst composition. After subjecting to a proper heat treatment at 400 °C, the electrocatalyst exhibits a good oxygen reduction activity with high methanol tolerance. From both LSV and XPS analyses, it was concluded that a high Se3d5/2 content plays an important role for oxygen reduction on RuSe/CNT. The EIS characterization also identified the presence of reaction intermediates during the oxygen reduction process. Based on the test results, the mechanisms underlying the dual function of the RuSe/CNT catalyst are proposed. The prepared catalyst was further evaluated for its potential application to DMFC. At 70 °C, the single-cell DMFC integrated with RuSe/CNT exhibited a performance much better than that incorporated with Pt/C counterpart when operated with a high-concentration (i.e., 6 M) methanol fuel. However, substantial improvements are still needed for practical applications.

Combinatorial discovery of new methanol-tolerant non-noble metal cathode electrocatalysts for direct methanol fuel cells

Combinatorial synthesis and screening were used to identify methanol-tolerant non-platinum cathode electrocatalysts for use in direct methanol fuel cells (DMFCs). Oxygen reduction consumes protons at the surface of DMFC cathode catalysts. In combinatorial screening, this pH change allows one to differentiate active catalysts using fluorescent acid-base indicators. Combinatorial libraries of carbon-supported catalyst compositions containing Ru, Mo, W, Sn, and Se were screened. Ternary and quaternary compositions containing Ru, Sn, Mo, Se were more active than the "standard" Alonso-Vante catalyst, Ru(3)Mo(0.08)Se(2), when tested in liquid-feed DMFCs. Physical characterization of the most active catalysts by powder X-ray diffraction, gas adsorption, and X-ray photoelectron spectroscopy revealed that the predominant crystalline phase was hexagonal close-packed (hcp) ruthenium, and showed a surface mostly covered with oxide. The best new catalyst, Ru(7.0)Sn(1.0)Se(1.0), was significantly more active than Ru(3)Se(2)Mo(0.08), even though the latter contained smaller particles.

Electrochemical Evaluation of Porous Non‐Platinum Oxygen Reduction Catalysts for Polymer Electrolyte Fuel Cells

AbstractA porous non‐platinum electrocatalyst for the oxygen reduction reaction (ORR), obtained by pyrolysing a cobalt porphyrin precursor, was evaluated by electrochemical means. The reactivity of the non‐platinum ORR catalyst was investigated with a rotating disc electrode (RDE) experimental set up. RDE data were collected in an acidic electrolyte containing N2, O2, CO and under mixed reactant O2/methanol conditions. The electrochemical performance of such‐obtained non‐platinum catalyst is discussed and compared to platinum‐based ORR catalysts. Based on the results collected here, we are able to propose and test possible proton exchange fuel cell (PEFC) operating conditions where non‐platinum ORR catalysts can be utilised. Direct methanol fuel cell (DMFC) data demonstrating a superior performance of the non‐platinum catalyst relative to platinum black, often perceived as the state‐of‐the‐art oxygen–reduction catalyst for the DMFC cathode is presented.

Numerical analysis of the electrochemical impedance spectra of the cathode of direct methanol fuel cells

A mathematical model is developed to simulate the electrochemical impedance spectra (EIS) of the cathode of a direct methanol fuel cell (DMFC) based on the electrode kinetics and mass transports. Successful simulation of the impedance spectra confirms the usefulness of the model as a diagnostic tool for interpreting the impedance characteristics of the cathode. Numerically, the capacitive semicircle in the impedance pattern is ascribed to the charge transfer process and the inductive semicircle is mainly due to the CO adsorption relaxation. Results show that the impedance pattern is strongly dependent on the electrode potential, which can be used as a criterion for judging the relative effect of the methanol permeation on the cathode. Another capacitive semicircle appears and the charge transfer resistance is changed when the oxygen transport is limited. The effects of the methanol permeation on the impedance pattern are also delineated, indicating that the methanol permeation often leads to larger oxygen transport impedance and the charge transfer resistance of the DMFC cathode depends on the methanol permeation rate.

Investigations of flow field designs in direct methanol fuel cell

An experimental and simulation research had been performed to investigate the performance as well as the flow distribution in the cathode flow field in the case of direct methanol fuel cells (DMFCs). The gas was well distributed in serpentine flow field, whereas stagnation of the gas was observed in parallel flow field. These would contribute to the cell performance greatly due to mass transfer effect when the cells start operating. In addition, the durability test of DMFC was drastically affected in parallel flow field due to poor ability to drain flooded water produced electrochemically at cathode and crossover from anode. In addition, pressure drops of different flow fields were also investigated to evaluate their contribution and feasibility as an economic application for DMFC. DMFC with serpentine flow field featuring higher pressure difference resulted in a larger parasitic energy demand. However, the optimal flow field designs are needed to balance the performance and pressure loss to achieve a uniform fluid distribution and simultaneously minimize energy demand for mass transport. Consequently, flow field with grid pattern appears to be the optimal design for the DMFC cathode.