Model for the Degradation Performance of a Single‐Cell Direct Methanol Fuel Cell under Varying Operational Conditions

The effect of varying operating parameters on the degradation of a single‐cell direct methanol fuel cell (DMFC) with serpentine flow channels was investigated. Fuel cell internal temperature, methanol concentration, and air and methanol flow rates were varied in experimental tests and fuel cell performance was chronologically recorded. A DMFC semi‐empirical performance model was developed to predict the polarization curves of the DMFC and validated at different operating conditions. Performance degradation was observed and modeled over time by a linear regression model. Unlike previous studies, the cumulative exposure of the operating factors to the fuel cell was considered in the degradation analysis. The degradation model shows the cell voltage generation capacity does not significantly degrade. However, the Tafel slope of the cell changes with cumulative exposure to methanol concentration and air flow, and the ohmic resistance changes with cumulative exposure to temperature, methanol and air flow.     

The role of under-rib convection in mass transport of methanol through the serpentine flow field and its neighboring porous layer in a DMFC

Numerical simulation of a direct methanol fuel cell (DMFC) operating under discharging conditions is challenged by the difficulties in modeling of complicated liquid-gas two-phase flows and coupled electrochemical kinetics. Under open-circuit conditions, the net electrochemical reactions in the DMFC anode cease, but, owing to the methanol concentration difference between the anode and cathode, the mass transport of methanol remains, creating a mass transport process of methanol in a single-phase liquid flow with no electrochemical reactions in the DMFC anode. Consequently, an accurate simulation of mass transport of methanol under such open-circuit conditions becomes possible. In this work, we performed a 3D numerical simulation of mass transport of methanol in the DMFC anode under open-circuit conditions and obtained the mass flux of methanol through the porous layer for different values of permeability. We also measured the mass flux of methanol permeation from the anode flow field to the cathode under open-circuit conditions. The comparison between the numerical and measured mass flux of methanol made it possible to in situ determine the permeability of the typical commercial porous layer. Using this in situ determined permeability, we then investigated numerically the effect of methanol feed rates on mass transport and found that the in-plane under-rib convection plays an important role, even at low methanol feed rate, to make the reactant evenly distributed over the entire catalyst layer.     

Gas management in flow field design using 3D direct methanol fuel cell model under high stoichiometric feed

This study presents a 3D CFD model for modeling gas evolution in anode channels of a DMFC under high stoichiometric feed. The improved two-phase model includes a new submodel for mass source and interphase transfer in anode channels. Case studies of typical flow field designs such as parallel and serpentine flow fields illustrate applications of the CFD model. Simulation results reveal that gas management of typical flow fields is ineffective under certain operating conditions. The CFD-based simulations are used to visualize and to analyze the gas evolution and flow patterns in anode channels. The developed CFD model is useful in flow field design for improving gas management in DMFC.     

Model-based analysis of the feasibility envelope for autonomous operation of a portable direct methanol fuel-cell system

Portable power systems based on direct methanol fuel cells (DMFC) have to provide power in various environmental conditions: it is advantageous for such a power-supply system to be autonomous, i.e. able to operate without water refills for the methanol solution. It is shown that system autonomy depends mainly on environmental humidity, condenser temperature and air excess ratio: this result is valid in general for any DMFC, as cell parameters have only a marginal role. The environmental conditions in which a portable DMFC system may be autonomous are considered, delineating a feasibility envelope. Two methods are proposed to extend this envelope: operating with a diluted methanol reservoir, which improves the autonomy of the system only marginally and at the cost of a large loss in energy density, and system pressurisation, which delivers a more significant improvement in autonomy properties, but at the cost of system efficiency and simplicity.     

Influence of Contamination with Inorganic Impurities on the Durability of a 1 kW DMFC System

IEK‐3 at Forschungszentrum Jülich successfully replaced the battery tray of a horizontal order picker (ECE 220, Jungheinrich) with a direct methanol fuel cell (DMFC) hybrid system in the kilowatt class. The DMFC combined with a lithium‐ion battery forms the hybrid drive for electrically driven horizontal order pickers. Such vehicles are used to transport and collect goods for orders in warehouses. The order picker is fueled by pure methanol to achieve a maximum range with the smallest possible space for the battery tray. An advantage of such energy systems is that there is no need for the relatively complicated and time‐consuming recharging of the conventional lead‐acid batteries, nor are spare batteries required for multishift operation. This study focuses on the influence of contamination with inorganic impurities on the durability of the 1 kW DMFC system V3.3‐1. The data from a long‐term test will be presented, in which the DMFC system was subjected to a realistic dynamic load profile for 3,000 h. The impurities identified in the post‐mortem analysis of the membrane electrode assembly will be outlined as well the influence of selected impurities, the sources of these impurities and how the DMFC V3.3‐2 system was modified on the basis of these findings.     

Operation characteristics of portable direct methanol fuel cell stack at sub-zero temperatures using hydrocarbon membrane and high concentration methanol

Cold start and operation of a direct methanol fuel cell (DMFC) are investigated at sub-zero temperatures by using a 10-cell stack. The stack is manufactured with a hydrocarbon membrane to minimize the methanol crossover problem, which can be caused by use of high concentration methanol solutions. The stack is heated up for the cold start and operation only by heat of the exothermic reactions without any heating device and additional insulation means, to examine operation characteristics of the DMFC stack at low temperatures. The concentration of methanol solutions is selected in the range of 3-8 M, considering the freezing points of the solution for corresponding operation temperatures (−5 to −15 °C). Although the DMFC stack undergoes a sharp voltage drop and a significant performance decrease at the initial stage of the frozen condition, the self-heating DMFC are successfully operated at −5 and −10 °C in both constant current or constant voltage modes. The cold start-up time also is nearly independent of the operating modes. In contrast, the stack at −15 °C is barely started up only by a constant voltage mode with some voltage fluctuation. The DMFC stack after the cold operation exhibits the performance loss of about 45%. Such performance loss is mainly caused by degradation of the electrocatalysts.     

直接甲醇燃料电池溶胶-凝胶流动相的制备与表征

对直接甲醇燃料电池溶胶-凝胶流动相的制备工艺进行了分析,采用溶胶-凝胶法以正硅酸甲酯为前驱体制备出了溶胶-凝胶流动相.分别以溶胶-凝胶流动相和液体流动相为燃料对比研究了直接甲醇燃料电池的放电性能,测定了溶胶-凝胶流动相在Nation117膜中的甲醇渗透率,研究了溶胶-凝胶流动相的质子电导率.实验结果表明,使用溶胶-凝胶...     

Enhanced transport properties in polymer electrolyte composite membranes with graphene oxide sheets

Polymer electrolyte membranes have been widely investigated for high performance fuel cells. Here, we report the synthesis of ionic conductive Nafion/graphene oxide (GO) composite membranes for application in direct methanol fuel cells. GOs interact with both the non-polar backbone and the polar ionic clusters of Nafion because of their amphiphilic characteristics attributable to hydrophobic conjugation and hydrophilic functional groups. Accordingly, GO sheets serve to modify the microstructures of two domains of Nafion. In particular, the transport properties of Nafion are favorably manipulated by the incorporation of GO. This modulated the ionic channels of Nafion and decrease methanol crossover while preserving ionic conductivity. Furthermore, strong interfacial interactions due to the insertion of GO nanofillers into the Nafion matrix improve the thermal and mechanical properties of the material. In particular, we exploit Nafion/GO composite membrane as electrolyte material for direct methanol fuel cell (DMFC) in order to resolve current issue of methanol crossover. This composite membrane-based DMFC compared to the Nafion 112-based DMFC remarkably enhanced cell performance, especially in severe operating conditions.     

Novel approach to membraneless direct methanol fuel cells using advanced 3D anodes

A conventional membrane electrode assembly (MEA) for a direct methanol fuel cell (DMFC) consists of a polymer electrolyte membrane (PEM) compressed between an anode and cathode electrode. Limitations with this conventional design include: cost, fuel crossover, membrane degradation or contamination, ohmic losses and reduced active triple phase boundary (TPB) sites for catalyst located away from the electrode/membrane interface. In this work, ex situ and in situ characterization of a novel electrode assembly based on a membraneless architecture and advanced 3D anodes was investigated. The approach was shown to be fuel independent and scaleable to a conventional bi-polar fuel cell arrangement. The membraneless configuration exhibits comparable performance to a conventional ambient (25 °C, 1 atm) air-breathing DMFC. However, it has the additional advantages of a simplified design, the elimination of the membrane (a significant component expense) and enhanced fuel and catalyst utilization through the extension of the active catalyst zone.     

DMFC at low air flow operation: Study of parasitic hydrogen generation

In this paper, the effect of hydrogen generation in direct methanol fuel cells (DMFC) is described. Under certain operating conditions hydrogen generation occurs in DMFC causing an additional methanol consumption and a decrease of the cell voltage.For the present experiments a segmented cell with an active area of 244 cm² is used. The cell has 196 segments which are regularly distributed on the whole area. By this experimental setup hydrogen generation was found in regions with insufficient air supply. Hydrogen generation was analyzed by systematically applying different air flow rates and detecting the local current densities. The theory for hydrogen generation is confirmed by the results obtained from the segmented cell. A correlation between open circuit voltage (OCV), air flow rate and hydrogen generation was observed. Furthermore, half-cell measurements with different methanol concentrations were performed and used for analyzing the processes during hydrogen generation.The work clearly indicates the importance of sufficient cathode air supply for DMFC. Starved cathode areas not only do not contribute to the overall current generation but in addition reduce the power and efficiency by the parasitic generation of hydrogen.     

Operating Temperature Dependency on Performance Degradation of Direct Methanol Fuel Cells

The effect of operating temperature on performance degradation of direct methanol fuel cell (DMFCs) is examined to disclose the main parameter of the degradation mechanism and the degradation pattern in the membrane electrode assemblies (MEAs). The DMFC MEA degradation phenomenon is explained through the use of various electrochemical/physicochemical tools, such as electrochemical impedance spectroscopy, electrode polarization, methanol stripping voltametry, field emission‐scanning electron microscopy, X‐ray diffraction, inductively coupled plasma‐atomic emission spectroscopy, and X‐ray photoelectron spectroscopy analysis. The operation of DMFC under high temperature accelerates the degradation process of the DMFC. The higher degradation rate under high temperature DMFC operation is mainly attributed to the formation of membrane pinhole with interfacial delamination and cathode degradation. A high operating temperature may result in more considerable thermal and mechanical stress of the polymeric membrane continuously due to frequent dry–wet cycling mode and substantial uneven distribution of water between the anode and the cathode during a long period of DMFC operation. On the other hand, the electrochemical surface area deterioration by Pt coarsening and ionomers loss is not directly related to the larger DMFC performance decay at high temperature.     

Prediction of methanol and water fluxes through a direct methanol fuel cell polymer electrolyte membrane

Development of a direct methanol fuel cell (DMFC) mass flux model, using conventional transport theory, is presented and used to predict the fluid phase superficial velocity, methanol and water molar fluxes, and the chemical species (methanol and water) dimensionless concentration profiles in the polymer electrolyte membrane, Nafion^® 117, of a DMFC. Implementation of these equations is illustrated to generate the numerical data as functions of the variables such as the pressure difference across the membrane, methanol concentration at the cell anode, temperature, and position in the membrane.     

Operation Characteristic Analysis of a Direct Methanol Fuel Cell System Using the Methanol Sensor‐less Control Method

The application of methanol sensor‐less control in a direct methanol fuel cell (DMFC) system eliminates most of the problems encountered when using a methanol sensor and is one of the major solutions currently used in commercial DMFCs. This study focuses on analyzing the effect of the operating characteristics of a DMFC system on its performance under the methanol sensor‐less control as developed by Institute of Nuclear Energy Research (INER). Notably, the influence of the dispersion of the methanol injected on the behavior of the system is investigated systematically. In addition, the mechanism of the methanol sensor‐less control is investigated by varying factors such as the timing of the injection of methanol, the cathode flow rate, and the anode inlet temperature. These results not only provide insight into the mechanism of methanol sensor‐less control but can also aid in the improvement and application of DMFC systems in portable and low‐power transportation.     

Effect of operating conditions on energy efficiency for a small passive direct methanol fuel cell

Energy conversion efficiency was studied in a direct methanol fuel cell (DMFC) with an air-breathing cathode using Nafion 117 as electrolyte membrane. The effect of operating conditions, such as methanol concentration, discharge voltage and temperature, on Faradic and energy conversion efficiencies was analyzed under constant voltage discharge with quantitative amount of fuel. Both of Faradic and energy conversion efficiencies decrease significantly with increasing methanol concentration and environmental temperature. The Faradic conversion efficiency can be as high as 94.8%, and the energy conversion efficiency can be as high as 23.9% if the environmental temperature is low enough (10 °C) under constant voltage discharge at 0.6 V with 3 M methanol for a DMFC bi-cell. Although higher temperature and higher methanol concentration can achieve higher discharge power, it will result in considerable losses of Faradic and energy conversion efficiencies for using Nafion electrolyte membrane. Development of alternative highly conductive membranes with significantly lower methanol crossover is necessary to avoid loss of Faradic conversion efficiency with temperature and with fuel concentration.     

直接甲醇燃料电池技术及应用

本文回顾了直接甲醇燃料电池（ＤＭＦＣ）的研究开发历史,系统阐述了ＤＭＦＣ系统中电催化剂选择与设计基本原则、电解质膜材料与甲醇渗透的关系。分析了电池工作温度、工作压力和甲醇进料方式对ＤＭＦＣ电化学性能的影响。     

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

A 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.     

Membranen für Polymerelektrolyt‐Brennstoffzellen

Membranes for Polymer Electrolyte Fuel Cells The polymer electrolyte membrane is the heart of hydrogen fuelled Polymer Electrolyte Fuel Cells (PEFC) and methanol fuelled Direct Methanol Fuel Cells (DMFC). Membranes of sulfonated fluoropolymers are already commercially available. Important goals for research and development are for PEFCs an increased operating temperature without the need for additional humidification and for DMFC the reduction of methanol transport through the membrane. The use of non‐fluorinated polymers aims at a reduction in membrane cost and environmental hazards. Membranes already in industrial product development are considered as well as novel membrane concepts in fundamental research.     

Three-dimensional two-phase mass transport model for direct methanol fuel cells

A three-dimensional (3D) steady-state model for liquid feed direct methanol fuel cells (DMFC) is presented in this paper. This 3D mass transport model is formed by integrating five sub-models, including a modified drift-flux model for the anode flow field, a two-phase mass transport model for the porous anode, a single-phase model for the polymer electrolyte membrane, a two-phase mass transport model for the porous cathode, and a homogeneous mist-flow model for the cathode flow field. The two-phase mass transport models take account the effect of non-equilibrium evaporation/ condensation at the gas-liquid interface. A 3D computer code is then developed based on the integrated model. After being validated against the experimental data reported in the literature, the code was used to investigate numerically transport behaviors at the DMFC anode and their effects on cell performance.     

Optimization of operating parameters of a direct methanol fuel cell and physico-chemical investigation of catalyst–electrolyte interface

A physico-chemical investigation of catalyst–Nafion® electrolyte interface of a direct methanol fuel cell (DMFC), based on a Pt–Ru/C anode catalyst, was carried out by XRD, SEM-EDAX and TEM. No interaction between catalyst and electrolyte was detected and no significant interconnected network of Nafion micelles inside the composite catalyst layer was observed. The influence of some operating parameters on the performance of the DMFC was investigated. Optimal conditions were 2 M methanol, 5 atm cathode pressure and 2–3 atm anode pressure. Power densities of 110 and 160 mW cm⁻² were obtained for operation with air and oxygen, respectively, at temperatures of 95–100°C and with 1 mg cm⁻² Pt loading.     

1D Model for Coupled Simulation of Steam Cracker Convection Section with Improved Evaporation Model

The radiation and convection section of a steam cracker are thermally coupled. Optimization and design requires a coupled simulation of both sections. In this work a 1D model for the convection section, CONVEC‐1D, is developed. Several models for the different heat transfer phenomena are implemented and evaluated. For flow boiling, an empirical and a mechanistic model are developed and compared for both single‐ and multicomponent hydrocarbon feeds. The latter is performing best over a wide range of operating conditions, taking into account the different two‐phase flow regimes. The coupled iterative procedure is demonstrated for an n‐pentane steam cracker convection section.