Operational characteristics of a passive air-breathing direct methanol fuel cell under various structural conditions

The operational characteristics of a small-scale passive air-breathing direct methanol fuel cell (PAB-DMFC) are comprehensively investigated under both steady-state and dynamic conditions. As the most important operating parameter, methanol concentration has significant effects on the cell performance. For different methanol concentrations (e.g., 0.5 and 8 M), the structural adaptations are particularly discussed. The results show that the structural factors are closely related to the influence degree of methanol concentration. In this study, the characteristics of the open circuit voltage (OCV) under various structural and methanol-concentration conditions are presented. Besides, the effects of other operating conditions such as running time, forced air convection and refueling action on the cell performance are also evaluated. In addition, a series of dynamic operations of the PAB-DMFC are conducted under different load cycles. Accordingly, the transient phenomena such as voltage undershoot and overshoot are explored. A fundamental principle for evaluating the operational characteristics of a PAB-DMFC is to simultaneously take into account the mass transfer requirements such as reactant delivery, product removal, methanol/water crossover control and so on.     

Ultra-low catalyst loading cathode electrode for anion-exchange membrane fuel cells

A new cathode architecture for anion-exchange membrane fuel cells (AEMFCs) is proposed and fabricated by direct deposition of palladium (Pd) particles onto the surface of the micro-porous layer (MPL) that is interfaced with a backing layer. The MPL is composed of carbon nanotubes while the backing layer is made of a carbon paper. The sputter-deposited electrode with a worm-like shape not only extends the electrochemical active surface area, but also facilitates the oxygen transport. This new cathode, albeit with a Pd loading as low as 0.035 mg cm⁻², enables the peak power density of an AEM direct ethanol fuel cell to be as high as 88 mW cm⁻² (at 60 °C), which is even higher than that using a conventional cathode with a 15-times higher Pd loading. The significance of the present work lies in the fact that the new sputter-deposited electrode is more suitable for fuel-electrolyte-fed fuel cells than the conventional electrode designed for proton-exchange membrane fuel cells (PEMFCs).     

Synthesis and characterization of a new anion exchange membrane by a green and facile route

A new anion exchange membrane was synthesized by a green and facile route---chemically grafting polybenzimidazole (PBI) membrane with BrCH₂CH₃. The obtained membrane was characterized by means of ex-situ and in-situ tests (EDX, FTIR, AC impedance, single cell test and so on). The results suggested that the group of -CH₂CH₃ was successfully grafted onto N atom within PBI backbone, while with Br⁻ as counter ion. The corresponding ionic conductivity and ethanol permeability were about 0.022 Scm⁻¹ and 5.24 × 10⁻⁸ cm²s⁻¹, respectively. Finally, single cell test suggested that air-breathing alkaline direct ethanol fuel cell with quaternized PBI membrane can deliver a peak power density of about 11 mWcm⁻² even at ambient temperature of 13 °C, which was better than those of air-breathing alkaline direct methanol fuel cell in literatures. In addition, a possible reaction mechanism was also proposed and discussed.     

Performance of air-breathing direct methanol fuel cell with anion-exchange membrane

This report details development of an air-breathing direct methanol alkaline fuel cell with an anion-exchange membrane. The commercially available anion-exchange membrane used in the fuel cell was first electrochemically characterized by measuring its ionic conductivity, and showed a promising result of 1.0 × 10⁻¹ S cm⁻¹ in a 5 M KOH solution. A laboratory-scale direct methanol fuel cell using the alkaline membrane was then assembled to demonstrate the feasibility of the system. A high open-circuit voltage of 700 mV was obtained for the air-breathing alkaline membrane direct methanol fuel cell (AMDMFC), a result about 100 mV higher than that obtained for the air-breathing DMFC using a proton exchange membrane. Polarization measurement revealed that the power densities for the AMDMFC are strongly dependent on the methanol concentration and reach a maximum value of 12.8 mW cm⁻² at 0.3 V with a 7 M methanol concentration. A durability test for the air-breathing AMDMFC was performed in chronoamperometry mode (0.3 V), and the decay rate was approximately 0.056 mA cm⁻² h⁻¹ over 160 h of operation. The cell area resistance for the air-breathing AMDMFC was around 1.3 Ω cm² in the open-circuit voltage (OCV) mode and then is stably supported around 0.8 Ω cm² in constant voltage (0.3 V) mode.     

A high-performance integrated electrode for anion-exchange membrane direct ethanol fuel cells

We propose a new anode electrode structure that is composed of a nickel foam layer with thin catalyst films coated onto the skeleton of the foam. This innovative design of the anode electrode enables the integration of the catalyst and diffusion layers, thereby extending the electrochemical active surface area and facilitating the transport of species. The experimental results indicate that the use of the integrated electrode in an anion-exchange membrane direct ethanol fuel cell can significantly improve the cell performance as compared with the use of the conventional electrode that has separated catalyst and diffusion layers; a peak power density of 130 mW cm⁻² and a maximum current density of 1060 mA cm⁻² are achieved at 80 °C.     

Understanding the performance degradation of anion-exchange membrane direct ethanol fuel cells

It has recently been demonstrated that anion-exchange membrane direct ethanol fuel cells (AEM DEFCs) can yield a high power density. The operating stability and durability of this type of fuel cell is, however, a concern. In this work, we report the durability test of an AEM DEFC that is composed of a Pd/C anode, an A201 membrane, and a Fe-Co cathode and show that the major voltage loss occurs in the initial discharge stage, but the loss becomes smaller and more stable with the discharge time. It is also found that the irreversible degradation rate of the fuel cell is around 0.02 mV h⁻¹, which is similar to the degradation rate of conventional acid direct methanol fuel cells (DMFCs). The experimental results also reveal that the performance loss of the AEM DEFC is mainly attributed to the anode degradation, while the performance of the cathode and the membrane remains relatively stable. The TEM results indicate that the particle size of the anode catalyst increases from 2.3 to 3.5 nm after the long-term discharge, which reduces the electrochemical active surface area and hence causes a decrease in the anode performance.     

Cathode flooding behaviour in alkaline direct ethanol fuel cells

In an alkaline fuel cell, such as a direct ethanol fuel cell (DEFC), owing to the fact that water is consumed as a reactant at the cathode and the electro-osmotic drag (EOD) moves water from the cathode to the anode, a conventional conception is that the cathode flooding is unlikely. In this work, however, it is shown experimentally that cathode flooding also occurs in an alkaline DEFC, primarily because of the fact that the diffusion flux from the anode to the cathode outweighs the total water flux due to both the oxygen reduction reaction and EOD. More interestingly, rather than in acid electrolyte based fuel cells where the cathode flooding occurs at a high (limiting) current, in an alkaline DEFC the cathode flooding occurs at an intermediate current.     

Comparison of different types of membrane in alkaline direct ethanol fuel cells

It has been understood that the use of cation-exchange membranes (CEM) and alkali-doped polybenizimidazole membranes (APM) in alkaline direct ethanol fuel cells (DEFC) with an added base in the fuel exhibits performance similar to the use of anion-exchange membranes (AEM). The present work is to assess the suitability of the three types of membrane to alkaline DEFCs by measuring and comparing the membrane properties including the ionic conductivity, the species permeability, as well as the thermal and mechanical properties. The comparison shows that: (i) the AEM is still the most promising membrane for the alkaline DEFC, although the thermal stability needs to be further enhanced; (ii) before solving the problem of the poor thermal stability of AEMs, the CEM is another choice for the alkaline DEFC running at high temperatures (<90 °C); and (iii) the APM can also be applied to the alkaline DEFC operating at high temperatures, but its mechanical property needs to be substantially enhanced and the species permeability needs to be dramatically decreased.     

Durability study of KOH doped polybenzimidazole membrane for air-breathing alkaline direct ethanol fuel cell

Recently, KOH doped polybenzimidazole (PBI/KOH) membrane has been reported as polymer electrolyte membrane for alkaline direct alcohol fuel cell (ADAFC), but little is known about its durability for ADAFC application. In this paper, the durability of PBI/KOH membrane for air-breathing alkaline direct ethanol fuel cell (ADEFC) is evaluated by means of ex situ and in situ tests. In the case of ex situ durability test, the ionic conductivity of PBI/KOH degrades from initial 0.023 S cm⁻¹ to 0.01 S cm⁻¹ after 100 h, and the degrading rate was 1.3 × 10⁻⁴ S cm⁻¹ h⁻¹. As for in situ test, Pt-free air-breathing ADEFC with PBI/KOH membrane can output a peak power density of 16 mW cm⁻² at 60 °C. Moreover, it can stably operate for 336 h above 0.3 V. In addition, the interaction between KOH and PBI matrix is also explored by density functional theory study.     

Effect of cathode micro-porous layer on performance of anion-exchange membrane direct ethanol fuel cells

The effect of a cathode micro-porous layer that is composed of carbon powder or carbon nanotubes on cell performance is investigated. Polarization curves, together with the respective anode and cathode potentials, are measured. The results show that the cathode potential can be significantly improved with adding a hydrophobic micro-porous layer between the cathode catalyst layer and the gas-diffusion layer. The increased performance with the cathode micro-porous layer is mainly attributed to the fact that the cathode water flooding can be alleviated as a result of the reduced water crossover, which consequently facilitates the transport of oxygen to the catalyst layer. It is also found that a crack-free micro-porous layer made of carbon nanotubes gives a much higher cathode potential compared with a micro-porous layer composed of carbon powder.     

Anion-exchange membranes composed of quaternized-chitosan derivatives for alkaline fuel cells

A series of quaternized-chitosan derivatives (QCDs) with various degrees of quaternization was synthesized using glycidyltrimethylammonium chloride as a main quaternized reagent. These QCDs were then processed into hydroxide—form quaternary ammonium salts with aqueous potassium hydroxide solutions. The resultant hydroxide—form QCD gels were further crosslinked into anion-exchange membranes using ethylene glycol diglycidyl ether. The crosslinking density, crystallinity, swelling index, ion exchange capacity, ionic conductivity and thermal stability of the crosslinked membranes were subsequently investigated. It was found that properties of crosslinked membranes were modulated mainly by the degree of quaternization and crosslinking density of membranes. Some membranes exhibited promising characteristics and had the potential for applications in alkaline polymer electrolyte fuel cells in considering their integrative properties.     

Measurements of water uptake and transport properties in anion-exchange membranes

This paper reports on the measured water uptake and water transport properties through an anion-exchange membrane, including the water diffusivity, the electro-osmotic drag (EOD) coefficient, and the mass-transfer coefficient of water at the cathode catalyst layer/membrane interface. The water uptake of the membrane in equilibrium with liquid water and water vapor at different values of relative humidity is measured at 30, 40 and 60 °C. The measured water uptake at each measured temperature is correlated in terms of the relative humidity. Like in Nafion^® membranes, the Schroeder’s paradox phenomenon is also found to exist in the anion-exchange membrane. The water diffusivity of the anion-exchange membrane shows the same order of 10⁻¹⁰ m² s⁻¹ as that of the Nafion^® membrane. The measured mass-transfer coefficient of water at the cathode catalyst layer/membrane interface falls in the range of 1.0 × 10⁻⁶ to 1.0 × 10⁻⁵ m s⁻¹. The EOD coefficients measured at 30 °C and 40 °C are, respectively, 2.3 and 3.6.     

Comparative study between platinum supported on carbon and non-noble metal cathode catalyst in alkaline direct ethanol fuel cell (ADEFC)

In this study, we reported a comparison study between the performance of the commercial non-noble metal cathode electrode (made by Hypermec™ K14 catalyst - provided by Acta S.p.A.) and cathode electrode containing 10 wt% Pt/C, in the alkaline direct ethanol fuel cell (ADEFC) under different conditions.Further electrochemical investigations have been done by RDE and driven mode cell to compare the intrinsic activity and selectivity of 10 wt% Pt/C and Hypermec™ K14 transition metals cathode catalysts. It is worthwhile to point out that Hypermec™ K14 cathode catalyst shows a remarkable selectivity to oxygen reduction and it has a superior intrinsic activity in oxygen reduction reaction (ORR) especially in terms of volumetric current density (A/cm³). Test results of active DEFC made by non-noble cathode catalyst showed superior performance compared to the cell made by 10 wt% Pt/C cathode catalysts in terms of power density and OCV at 60 °C and ambient pressure. This result is related to the higher ORR kinetic of non-noble metal cathode catalyst in alkaline media.     

Alkaline anion-exchange polymer membrane with grid-plug microstructure for hydrogen fuel cell application

Porous polysulfone membrane, prepared by a phase-inversion technique, is filled with (3-acrylamidopropyl)trimethylammonium chloride and N,N′-methylenebisacrylamide via interfacial diffusion. The impregnated membrane is then subjected to UV-irradiation for polymerizing monomers that are entrapped in pore channels of the membrane. This in-situ polymerization engenders a grid-plug microstructure, where the grid is polysulfone and the plugs are an ion (OH⁻) conducting phase. As the plugs are extensively interconnected and non-tortuous throughout the membrane matrix, the ion-conducting phase sustains a power density as high as 55 mW cm⁻² at 60 °C. Thermal analysis indicates that the pore-filling condition affects the packing density of the plugs that in turn, impacts on ion transport flux.     

Performance of an alkaline direct ethanol fuel cell with hydrogen peroxide as oxidant

An alkaline direct ethanol fuel cell (DEFC) with hydrogen peroxide as the oxidant is developed and tested. The present fuel cell consists of a non-platinum anode, an anion exchange membrane, and a non-platinum cathode. It is demonstrated that the peak power density of the fuel cell is 130 mW cm⁻² at 60 °C (160 mW cm⁻² at 80 °C), which is 44% higher than that of the same fuel cell setup but with oxygen as the oxidant. The improved performance as compared with the fuel cell with oxygen as the oxidant is mainly attributed to the superior electrochemical kinetics of the hydrogen peroxide reduction reaction and the reduced ohmic loss associated with the liquid oxidant.     

Two-dimensional two-phase mass transport model for methanol and water crossover in air-breathing direct methanol fuel cells

A two-dimensional two-phase mass transport model has been developed to predict methanol and water crossover in a semi-passive direct methanol fuel cell with an air-breathing cathode. The mass transport in the catalyst layer and the discontinuity in liquid saturation at the interface between the diffusion layer and catalyst layer are particularly considered. The modeling results agree well with the experimental data of a home-assembled cell. Further studies on the typical two-phase flow and mass transport distributions including species, pressure and liquid saturation in the membrane electrode assembly are investigated. Finally, the methanol crossover flux, the net water transport coefficient, the water crossover flux, and the total water flux at the cathode as well as their contributors are predicted with the present model. The numerical results indicate that diffusion predominates the methanol crossover at low current densities, while electro-osmosis is the dominator at high current densities. The total water flux at the cathode is originated primarily from the water generated by the oxidation reaction of the permeated methanol at low current densities, while the water crossover flux is the main source of the total water flux at high current densities.     

Hydrogen generation from solid NaBH4 with catalytic solution for planar air-breathing proton exchange membrane fuel cells

In the pursuit of the development of alternative mobile power sources with high energy densities, this study elucidated a new hydrogen generation approach from solid NaBH₄ using a new catalyst, sodium hydrogen carbonate (NaHCO₃), which was placed in a small and compact cartridge. A planar air-breathing PEMFC system fitted with the cartridge has been investigated for testing hydrogen generation from NaBH₄ and NaHCO₃. NaHCO₃ allowed the hydrogen cartridge to control hydrogen generation and to improve the power density, fuel efficiency, energy efficiency, and cell response. The cell performance of solid NaBH₄ air-breathing PEMFC system strongly depended on the operating conditions: the feeding rates and concentrations of catalytic solutions for NaBH₄ hydrolysis. In various concentrations (5 - 12 wt %) of NaHCO₃ aqueous solutions, 10 wt % NaHCO₃ aqueous solution exhibited the highest maximum power density of 128 mW cm⁻² at 0.7 V, which was estimated to be a Faradic efficiency of 78.4% and an energy efficiency of 46.3%. The data illustrated that NaHCO₃ was an effective catalyst for hydrogen generation with the solid NaBH₄, which is considered as a hydrogen carrier for air-breathing micro PEMFCs operated without auxiliary hydrogen controller or devices.     

In situ detection of anode flooding of a PEM fuel cell

This paper proposes an early detection scheme of anode flooding in a PEM fuel cell. Through experimental testing of an eight-cell hydrogen-fueled polymer electrolyte stack it is shown that anode flooding can be detected prior to a rapid voltage decline. The proposed detection scheme requires no additional costly instrumentation and uses the existing voltage scan cards.     

Transient and steady-state analysis of catalyst poisoning and mixed potential formation in direct methanol fuel cells

The present dynamic model is developed to investigate the coupled reaction mechanisms in a DMFC and therein associated voltage losses in the catalyst layers. The model describes a complete five-layer membrane electrode assembly (MEA), with gas diffusion layers, catalyst layers and membrane. The analysis of the performance losses are mainly focused on the electrochemical processes. The model accounts for the crossover of both, methanol from anode to cathode and oxygen from cathode to anode. The reactant crossover results in parasitic internal currents that are finally responsible for high overpotentials in both electrodes, so-called mixed potentials. A simplified and general reaction mechanism for the methanol oxidation reaction (MOR) was selected, that accounts for the coverage of active sites by intermediate species occurring during the MOR. The simulation of the anode potential relaxation after current interruption shows an undershoot behavior like it was measured in the experiment [1]. The model gives an explanation of this phenomenon by the transients of reactant crossover in combination with the change of CO and OH coverages on Pt and Ru, respectively.     

Mathematical modeling of alkaline direct ethanol fuel cells

A one-dimensional model is developed for alkaline direct ethanol fuel cells (DEFC) by considering the complicated physicochemical processes, including mass transport, charge transport, and electrochemical reactions. The model is validated against experiments and shows good agreement with the literature data. The model is then used to investigate the effects of various operating and structural design parameters on the cell performance. Numerical results show that the cell performance increases with increasing the ethanol concentration from 1.0 M to 3.0 M and with increasing the OH⁻ concentration from 1.0 M to 5.0 M. The model is further applied to the study of the effect of the design of the anode diffusion layer (DL) on the performance; it is shown that the cell performance improves when the porosity of the DL is increased and the thickness of the DL is decreased.