Reliability and accuracy of measured overpotential in a three-electrode fuel cell system

Numerical simulation was conducted to study the potential and current density distributions at the active electrode surface of a solid oxide fuel cell. The effects of electrode deviation, electrolyte thickness and electrode polarization resistance on the measurement error were investigated. For a coaxial anode/electrolyte/cathode system where the radius of the anode is greater than that of cathode, the cathode overpotential is overestimated while the anode overpotential is underestimated. Although the current interruption method or impedance spectroscopy can be employed to compensate/correct the error for a symmetric electrode configuration, it is not useful when dealing with the asymmetric electrode system. For the purpose of characterizing the respective overpotentials in a fuel cell, the cell configuration has to be carefully designed to minimize the measurement error, in particular the selection of the electrolyte thickness, which may cause significant error. For the anode-support single fuel cell, it is difficult to distinguish the polarization between the anode and cathode with reference to a reference electrode. However, numerical results can offer an approximate idea about the source/cause of the measurement error and provide design criteria for the fuel cell to improve the reliability and accuracy of the measurement technique.     

Realising a reference electrode in a polymer electrolyte fuel cell by laser ablation

This work presents a new concept for realising a reference electrode configuration in a PEM fuel cell by means of laser ablation. The laser beam is used to evaporate a small part of the electrode of a catalyst-coated membrane (CCM) to isolate the reference electrode from the active catalyst layer. This method enables the simultaneous ablation of the electrodes on both sides of the CCM because the membrane is transparent for the laser beam. Therefore, a smooth electrode edge without electrode misalignment can be realised. A test fuel cell was constructed which together with the ablated CCM enables the separation of the total cell losses during operation into the cathode, anode and membrane overpotentials in PEFC as well as in DMFC mode. The methanol tolerance of a selenium-modified ruthenium-based catalyst (RuSe _x ) was investigated under real fuel cell conditions by measuring polarisation curves, electrochemical impedance spectroscopy (EIS) and current interrupt measurements (CI).     

Design of a reference electrode for high-temperature PEM fuel cells

In this work, we present the design of an external reference electrode for high-temperature PEM fuel cells. The connection between the reference electrode with one of the fuel cell electrodes is realized by an ionic connector. Using the same material for the ionic connection as for the fuel cell membrane gives us the advantage to reach temperatures above 100 °C without destroying the reference electrode. This configuration allows for the separation of the anode and cathode overpotential in a working fuel cell system. In addition to the electrode overpotentials in normal hydrogen/air operation, the influence of CO and CO + H₂O in the anode feed on the fuel cell potentials was investigated. When CO poisons the anode catalyst, not only the anode potential increased, but also the cathode overpotential, due to fewer protons reaching the cathode. By the use of synthetic reformate containing hydrogen, carbon monoxide and water on the anode, fuel cell voltage oscillations were observed at high constant current densities. The reference electrode measurements showed that the fuel cell oscillations were only related to reactions on the anode side influencing the anode overpotential. The cathode potential, in contrast, was only negligibly affected by the oscillations under the applied conditions.     

Carbon supported and unsupported Pt–Ru anodes for liquid feed direct methanol fuel cells

A comparative study of the use of supported and unsupported catalysts for direct methanol fuel cells has been performed. The effect of catalyst loading, fuel concentration and temperature dependence on the anode, cathode and full fuel cell performance was determined in a fuel cell equipped with a reversible hydrogen reference electrode. Although the measured specific activities of supported catalysts were as much as 3-fold greater than the unsupported catalysts, membrane electrode assemblies prepared with supported catalysts showed no improvement with loadings above 0.5 mg/cm². Fuel cells utilizing 0.46 mg/cm² supported catalyst electrodes performed as well as unsupported catalyst electrodes with 2 mg/cm². The temperature dependence and methanol concentration dependence studies both suggest increased methanol permeation through the thinner supported catalyst layers relative to the unsupported catalyst layers.     

Accelerated durability test of DMFC electrodes by electrochemical potential cycling

Durability of direct methanol fuel cell electrodes was evaluated by electrochemical potential cycling and we observed the degradation phenomena during the performance decay. An individual potential measurement of anode and cathode with built-in reversible hydrogen electrode revealed that the anode and cathode performance contributions are almost of the same order of magnitude to the entire performance loss, although the anode degradation is relatively bigger, due to the dominating effect of ruthenium dissolution, corresponding loss of electrocatalytic activity. On the contrary, it was apparent that the electrochemical active surface area of Pt cathode decreased significantly with potential cycling under methanol crossover condition, which is not clearly reflected on the performance loss due to the initial decrease of interfacial resistance between membrane and cathode catalyst layer. Impedance studies could reinforce the current–voltage polarization by more comprehensive information.     

An experimental study on the placement of reference electrodes in alkaline polymer electrolyte membrane fuel cells

The ability to accurately measure separate in situ anode and cathode overpotentials and impedance responses is still a source of debate when investigating fuel cells of planar configuration containing <100 μm thickness solid electrolytes and when using the common three-electrode arrangement. The results obtained in this study indicate that the overpotentials and impedances of the anode and cathode can be successfully measured when using two spatially separated reference electrodes and when the cathode and anode of alkaline membrane electrode assemblies (for alkaline polymer electrolyte membrane fuel cells) are precisely and optimally misaligned. The frequency dependent response between the two reference electrodes is attributed to the membrane response and the “crosstalk” between anode and cathode.     

Using electrochemical impedance spectroscopy to compensate for errors when measuring polarisation curves during three-electrode measurements of solid oxide fuel cell electrodes

The use of three-electrode techniques involving an independent reference electrode is invaluable in determining the overpotential losses at solid oxide fuel cell (SOFC) electrodes. However, there are numerous barriers to achieve the accurate measurement of such overpotentials in an SOFC. Furthermore electrochemical impedance spectroscopy (EIS) is commonly used to analyse the processes occurring on SOFC electrodes, and there has been considerable work in establishing viable three-electrode techniques for EIS experiments under open circuit conditions. However, the three-electrode techniques currently developed for EIS experiments are not well suited for conditions of high load, or changing gas compositions; either intentionally or under diffusion limiting conditions. This paper reports a solution for commonly used pellet cells, which mitigates these problems. The paper presents a method using EIS to correct for errors when measuring the working electrode overpotential during polarisation arising from a shift in the electrolyte current distribution from the primary to the secondary current distribution under load. This technique enables meaningful overpotentials to be calculated using experimentally simple cell geometries under conditions where they cannot normally be accurately measured.     

Alcohol Crossover Behavior in Direct Alcohol Fuel Cells (DAFCs) System

The alcohols (methanol, ethanol, and 1‐propanol) crossover behavior of through fuel cell membrane electrode assembly (MEA) in direct alcohol fuel cell (DAFC) system was studied. We divided five different factors which affect alcohol crossover behavior through MEA to analyze alcohol crossover behavior. Those are membrane effect, physical blocking effect of anode, alcohol oxidation effect of anode electrocatalysts, physical blocking effect of cathode, and alcohol oxidation effect of cathode. Among these five factors, the four factors caused by two different electrodes (anode and cathode) were evaluated by fabricating various types of MEA. In the case of alcohols through membrane without any electrode was increased when the cell temperature was raised from room temperature to 100 °C, but it was decreased above the cell temperature of 100 °C. Among the electrode effects on alcohol crossover rate, physical blocking effect of electrodes played dominant role below 100 °C. However alcohol oxidation effects of electrodes was predominant above the 100 °C.     

Measurement of single electrode potentials and impedances in hydrogen and direct methanol PEM fuel cells

A commercial proton exchange membrane fuel cell has been fitted with a simple dynamic hydrogen reference electrode (DHE). Single electrode potentials and impedances measured with hydrogen and methanol as the fuel have been critically evaluated. It has been shown that the anode overpotential and impedance can be very significant in hydrogen cells operated at ambient temperature, due to dehydration of the anode. The DHE provides a powerful way of monitoring the hydration state of the membrane and electrodes, so that operating conditions can be adjusted to optimise water management. Individual electrode potentials and impedances are even more important in methanol cells, and can be conveniently measured with the DHE.     

Iron(III) tetramethoxyphenylporphyrin(FeTMPP) as methanol tolerant electrocatalyst for oxygen reduction in direct methanol fuel cells

Carbon supported iron (III) tetramethoxyphenylporphyrin (FeTMPP) heat treated at 800°C under argon atmosphere was used as catalysts for the electroreduction of oxygen in direct methanol polybenzimidazole (PBI) polymer electrolyte fuel cells that were operated at 150°C. The electrode structure was optimized in terms of the composition of PTFE, polymer electrolyte and carbon-supported FeTMPP catalyst loading. The effect of methanol permeation from anode to cathode on performance of the FeTMPP electrodes was examined using spectroscopic techniques, such as on line mass spectroscopy (MS), on line Fourier transform infrared (FTIR) spectroscopy and conventional polarization curve measurements under fuel cell operating condition. The results show that carbon supported FeTMPP heat treated at 800°C is methanol tolerant and active catalyst for the oxygen reduction in a direct methanol PBI fuel cell. The best cathode performance under optimal condition corresponded to a potent ial reached of 0.6V vs RHE at a current density of 900 mAcm–2.     

自呼吸式直接甲醇燃料电池性能及其传质特性

针对有效面积为1 cm2的自呼吸式直接甲醇燃料电池（direct methanol fuel cell,DMFC）单电池,阳极采用燃料罐供液,将阴极侧集流体和夹具设计为一体式结构,并用自制的七合一膜电极组件对其进行测试,讨论了催化剂类型、扩散层材料、集流体结构等因素对其性能的影响,分析了电池内部的传质特性,优化了电池特别是其在中高电流密度条件下的性能。实验结果表明：采用Pt黑、Pt-Ru黑催化剂制作的自呼吸式DMFC能强化反应物的传质;采用碳布制作的膜电极更倾向于获得更高的极限电流密度;低电流密度时,因甲醇渗透电池电压随着甲醇浓度的增加而降低,但在中高电流密度下,电池性能随甲醇浓度的增大先升高后降低;平行集流体有利于阴阳极生成物的排出和反应物的传质,因此易获得较高的电池性能。     

Pt/laponite clay-modified gold electrode for direct methanol fuel cells

This work employs a novel technique in which laponite clay-modified gold electrodes are used as the anode for direct methanol fuel cells. The platinum/laponite clay (Pt/Clay) films on indium tin oxide electrode were characterized by using scanning electron microscope and energy-dispersive X-ray spectroscopy. Various contents of laponite clay (0.1, 0.5, 1.0, and 2.0?wt%) with constant platinum (Pt) catalyst content on modified gold electrodes were studied as an anode catalyst for methanol oxidation. The catalyst poisoning was observed as a function of time. The 1.0?wt% Pt/Clay-modified gold electrode shows the highest activity for methanol oxidation, 27.73?% higher than Pt only modified gold electrode at 2.5?min. The peak current of 1?% Pt/Clay-modified gold electrode is 3.50?% higher than the peak current of Pt only modified gold electrode at 57.5?min. The higher content of Pt/Clay-modified gold electrode shows strong resistance to catalyst poisoning. The Pt/Clay-modified gold electrode is a new and promising electrode for a direct methanol fuel cell and can replace existing commercial catalysts.     

Portable Size DMFC‐Stack

A small, low temperature, direct methanol fuel cell stack for portable applications has been developed. Several flow field designs were investigated with respect to stable operation and high performance. Due to carbon dioxide and water production on the anode and cathode, respectively, methanol and oxygen access to the electrodes is hindered. During single cell operation the effect of both carbon dioxide evolution and water production on the current output was observed. The difference between parallel and serial feeding of both fuel and oxidant to the DMFC stack was also investigated. It was found that it is very important to remove reaction products from the active cell surface in order to ensure stable stack operation at low temperatures. The maximal power realised with the 12‐cell direct methanol fuel cell stack was 30 W.     

Effect of methanol crossover on the cathode behavior of a DMFC: A half-cell investigation

A half-cell consisting of a normal direct methanol fuel cell (DMFC) cathode and a membrane that contacts with an electrolyte solution was developed to investigate the effect of methanol crossover on the cathode behavior. Open circuit potentials, cyclic voltammetry profiles, polarization curves and electrochemical impedance spectroscopy (EIS), resulting from the oxygen reduction reaction (ORR) with/without the effect of methanol oxidation reaction (MOR), were measured. The transient measurements indicated that both current and open circuit potential of the electrode exhibited significant oscillations when the anodic MOR was superposed on the cathodic ORR, which explain the instabilities that may be encountered in the practical DMFC operation. The steady-state results confirmed that the presence of methanol at the cathode led to a significant poisoning effect on the ORR, especially when the DMFC operates at higher methanol concentrations and discharges at lower potentials. More importantly, the half-cell was proved to be ideal for the EIS study of DMFC electrodes because the system not only facilitates an accurate potential control but also reflects the actual mass transport process that occurs in practical DMFCs.     

Modeling and experimental validation of overpotentials of a direct ethanol fuel cell

Direct ethanol fuel cell (DEFC) is a promising power source for future use in portable electronic equipments. In general, the power density obtained in DEFC is lower than that of direct methanol fuel cell. In the present study, various losses in DEFC are estimated by performing experiments with the prepared membrane electrode (MEA) to obtain current–voltage characteristics and comparing it with the prediction of mathematical model. MEA for the DEFC is prepared using Pt–Ru (40:20 by wt.%)/C as anode catalyst, Pt–black as cathode catalyst with 1 mg/cm² of loadings and cast Nafion^® (SE5112, DuPont) ionomer as proton exchange membrane. The mathematical model for DEFC is developed considering different overpotentials. The activation overpotential term is formulated considering ethanol electrooxidation mechanism proposed in literature and Butler–Volmer equation. The ohmic overpotential is modeled based on proton conductivity of Nafion^® membrane and ohmic losses at the electrodes, current collectors and electrode–current collector interfaces. The concentration overpotential is formulated using Fick's law, modified Butler–Volmer equation and transport process through electrodes and electrocatalyst layers. The experiment data on current–voltage characteristics is predicted by the model with reasonable agreement and the influence of ethanol concentration and temperature on the performance of DEFC is captured by the model.     

Bifunctional electrodes for an integrated water-electrolysis and hydrogen-oxygen fuel cell with a solid polymer electrolyte

An alternative concept of an integrated water electrolysis/hydrogen-hydrogen fuel cell using metal electrocatalysts and a solid polymer electrolyte is described. Instead of operating both electrodes as hydrogen and oxygen electrodes respectively the electrodes are used as oxidation and reduction electrodes in both modes of operation. A more suitable selection of electrocatalysts and an improved cell design are possible; both can increase the efficiency of the cell considerably. New results on the electrocatalytic activity of various noble-metal containing catalysts with respect to both oxygen evolution and hydrogen oxidation in a proton exchange membrane-cell at 80°C are reported. Kinetic data derived from Tafel plots of the oxygen evolution polarization curves agree closely with those of experiments with aqueous sulphuric acid electrodes. This agreement allows the determination of kinetic parameters for electrocatalysts difficult to prepare in solid smooth electrodes but easy to be made into porous deposits. Polarization curves of the hydrogen oxidation reaction clearly indicate a relative activity rating of the studied catalysts. In cycling tests the lifetime stability of the new bifunctional oxidation electrode was determined. Polarization data obtained under these conditions agree with those obtained in earlier experiments where electrodes were exposed to only one type of oxidation reaction. During a test of 10 cycles (30 min of electrolyser and 30 min of fuel cell mode each) no changes in the electrode potential were observed. With the conventional cell design employing a hydrogen and an oxygen electrode both catalyzed with platinum and a current density of 100 mA cm^–2 a storage efficiency of 50% was calculated; with the alternative concept of oxidation and reduction electrodes and selected oxidation catalysts this was improved to 57%. With further improvements these efficiencies seem possible even at current densities of 500 mA cm^–2.     

Optimization of cathode catalyst layer for direct methanol fuel cells: Part I. Experimental investigation

The cathode catalyst layer (CL) in direct methanol fuel cells (DMFCs) has been optimized through a balance of ionomer and porosity distributions, both playing important roles in affecting proton conduction and oxygen transport through a thick CL of DMFC. The effects of fabrication procedure, ionomer content, and Pt distribution on the microstructure and performance of a cathode CL under low air flowrate are investigated. Electrochemical methods, including electrochemical impedance, cyclic votammetry and polarization curves, are used in conjunction with surface morphology characterization to correlate electrochemical characteristics with CL microstructure. CLs in the form of catalyst-coated membrane (CCM) have higher cell open circuit voltages (OCVs) and higher limiting current density; while catalyzed-diffusion-media (CDM) CLs display better performance in the moderate current density region. The CL with a composite structure, consisting both CCM and CDM, shows better performance in both kinetic and mass-transport limitation region, due to a suitable ionomer distribution across the CL. This composite cathode is further evaluated in a full DMFC and the cathode performance loss due to methanol crossover is discussed.     

Cell Configurations for Performance Evaluation in Planar Solid Oxide Fuel Cells

Cell configurations with asymmetric and symmetric electrode geometries and different reference electrode positions were investigated on 50 mm×50 mm planar solid oxide fuel cells (SOFC). The reliability and accuracy of the polarization performance of individual electrodes were studied with respect to the electrode geometry and the reference electrode position. The results indicate that a centrally located reference electrode creates inactive electrolyte regions in the center of the cell, pushing the equipotential lines close to the electrode–electrolyte interface region and thus introducing error in the measurement of polarization performance. The potential of reference electrodes located at the corner of the electrode coating was not stable due to the steam build-up in the reference electrode region. Cells with a symmetric electrode geometry arrangement and reference electrodes located at the side of the working electrodes, away from the receiving end of the fuel and oxidant gases, were found to be suitable for performance evaluation in planar SOFC.     

Dynamic Simulation of Oxygen Transport Rates in Highly Ordered Electrodes for Proton Exchange Membrane Fuel Cells

Highly ordered Pt electrode has been recognized as an important technology for reducing Pt usage in fuel cells due to its improved oxygen transport capability. However, ordered Pt electrode can lead to the decrease in roughness of electrode, which in turn makes it unclear whether the improved oxygen transport can offset the decreased roughness of ordered electrode. Herein, we theoretically investigate the oxygen distribution, generated current, and minimum Pt loading of ordered Pt electrode based on kinetic model of oxygen transport. The results reveal that ordered Pt electrodes do not exhibit concentration polarization with the electrode thickness up to 100 μm. For ordered Pt electrode with diameter of nanorod of 60 nm, the limited current density reaches 110.2 A cm⁻² that is much higher than that for conventional electrode without considering Ohmic loss and mass transport loss outside electrode. To generate a current of 1.5 A cm⁻² at 0.67 V for fuel cell, the minimum Pt loading of cathode in PEMFC reaches 0.029 mg cm⁻² assuming that the electrocatalyst nanorods contain 1 nm Pt layer at the outmost surface.     

Cell performances of direct methanol fuel cells with grafted membranes

Cell performances were evaluated with grafted polymer membranes as an electrolyte for a direct methanol fuel cell (DMFC). The membranes were prepared using a poly(ethylene-tetrafluoroethylene), or ETFE, film. The base polymer film was added to sulfonic groups using γ-radiation activated grafting technique as ion-exchange groups. These membranes had more suitable properties for DMFCs, i.e. higher electric conductivity and lower methanol permeability than perfluorinated ionomer membrane (Nafion). Nevertheless, the cell performance with the grafted membrane was inferior to that with Nafion. The analysis of electrode potentials vs. reversible hydrogen electrode showed larger activation overpotential for both the electrodes on the grafted membranes. We concluded that this is due to poor bonding of the catalyst layers to the grafted membranes.