Operation of the Alkaline Direct Formate Fuel Cell in the Absence of Added Hydroxide

The direct formate fuel cell (DFFC) has recently been demonstrated as a viable alkaline direct liquid fuel cell (DLFC) that does not require addition of hydroxide to the fuel stream for operation. In this work, we report that the DFFC can produce significant power at low temperatures without added hydroxide, especially when compared with other alkaline DLFCs powered by alcohols. Using oxygen at the cathode, the DFFC powered by 1 M HCOOK achieves a maximum power density of 106 mW cm^–2 at 50 °C and 64 mW cm^–2 at 23 °C. Using air at the cathode, the same DFFC achieves a maximum power density of 76 mW cm^–2 at 50 °C and 27 mW cm^–2 at 23 °C. These power densities were achieved without addition of hydroxide to the fuel stream. Constant current operation demonstrates that the maximum power density can be maintained at least for several hours of operation. Finally, we use electrochemical analysis to demonstrate that the formate oxidation reaction is not dependent on pH between 9 and 14, which permits the use of formate fuel without added hydroxide in the DFFC. An alkaline DLFC that does not require added hydroxide is promising for safe and practical operation.     

Temperature and Humidity Control of a Micro PEM Fuel Cell Stack

This paper deals with the control of a miniaturised fuel cell system. A single air blower is used to control both heat and water management of the fuel cell. As the number of manipulated variables is smaller than the number of control variables, classical control algorithms are not applicable. To find a suitable controller, a system model is developed that shows the qualitatively same behaviour as the experimental setup. The dynamic behaviour of the model and the influence of the blower are studied by phase portraits. A control algorithm is then conceived by qualitative analysis of the phase portraits and tested in simulations.     

Influence of Methanol Crossover on the Fuel Utilization of Passive Direct Methanol Fuel Cell

The effect of methanol crossover on the fuel utilization of a passive direct methanol fuel cell (DMFC) was reported. The results revealed that the Faradaic efficiency decreased from 46.9 to 17.4% when methanol concentration increased from 1.0 to 8.0 mol L^–1 at the lower current density 11.1 mA cm^–2. However, the Faradaic efficiency increased from 14.7 to 31.3% when methanol concentration increased from 1.0 to 8.0 mol L^–1 at a higher current density of 44.4 mA cm^–2. On the other hand, although the amount of methanol was increased, the Faradaic efficiency did not change, obviously due to the uniform methanol crossover and methanol diffusion at the same methanol concentration and constant current.     

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。     

A Fuel‐Flexible Alkaline Direct Liquid Fuel Cell

We constructed a fuel‐flexible fuel cell consisting of an alkaline anion exchange membrane, palladium anode, and platinum cathode. When an alcohol fuel was used with potassium hydroxide added to the fuel stream and oxygen was the oxidant, the following maximum power densities were achieved at 60 °C: ethanol (128 mW cm⁻²), 1‐propanol (101 mW cm⁻²), 2‐propanol (40 mW cm⁻²), ethylene glycol (117 mW cm⁻²), glycerol (78 mW cm⁻²), and propylene glycol (75 mW cm⁻²). We also observed a maximum power density of 302 mW cm⁻² when potassium formate was used as the fuel under the same conditions. However, when potassium hydroxide was removed from the fuel stream, the maximum power density with ethanol decreased to 9 mW cm⁻² (using oxygen as oxidant), while with formate it only decreased to 120 mW cm⁻² (using air as the oxidant). Variations in the performance of each fuel are discussed. This fuel‐flexible fuel cell configuration is promising for a number of alcohol fuels. It is especially promising with potassium formate, since it does not require hydroxide added to the fuel stream for efficient operation.     

Empirical Model Equations for the Direct Methanol Fuel Cell DMFCs

Empirical model equations, proposed for polymer electrolyte fuel cells, are used to predict the cell voltage vs. current density response of a liquid feed direct methanol fuel cell. The model equations are validated against experimental data for a small-scale fuel cell over a wide range of methanol concentration and temperatures. A new empirical equation is presented which is able to predict the voltage response of liquid feed direct methanol fuel cells over a wide range of operating conditions and even in the case of very low current densities caused by, for example, the use of dilute methanol solutions or low cell temperatures.     

Alkaline Fuel Cells

Alkaline fuel cells may become an important element in pollution free energy conversion. In the literature most papers in the field of low temperature fuel cells are concerned with polymer electrolyte fuel cells. However, there are still a lot of research groups and companies working on alkaline fuel cells. The advantages and disadvantages of AFCs are presented. The main technical problems are solved. It can be concluded, due to work carried out at DLR, that carbon dioxide poisoning of electrodes from rolled DLR electrodes does not take place. These investigations demonstrate that AFCs are highly efficient cells for portable and stationary applications.     

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.     

Modeling Methanol Crossover by Diffusion and Electro-Osmosis in a Flowing Electrolyte Direct Methanol Fuel Cell

A CFD model is created to analyze methanol transport in a flowing electrolyte direct methanol fuel cell (FE-DMFC) by solving the 3D advection-diffusion equation, with consideration of electro-osmosis. The average methanol flux at the anode and cathode surfaces is simulated and compared to equivalent direct methanol fuel cells. Methanol crossover is defined as methanol flux at the cathode surface, and the results reveal that methanol crossover can be drastically reduced by the flowing electrolyte. The performance of the FE-DMFC at peak power current density is evaluated, and diffusion is shown to be the dominant contribution, although electro-osmosis increases with current density. The power consumption of the electrolyte pump is shown to be negligible compared to the cell power output. This indicates that thin electrolyte channels with high flow rates could further improve the efficiency.     

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 Porosity in Catalyst Layers on Direct Methanol Fuel Cell Performances

Effects of porosity of catalyst layers (CLs) on direct methanol fuel cell (DMFC) performances are investigated using silicon dioxide (SiO₂) particles as a pore former. The pore size and volume of CLs are controlled by changing the size and content of SiO₂. As the size of pore formed by removal of SiO₂ increases, DMFC performances are enhanced. The augmentation in performances can be explained by facilitation of fuel transport to catalyst particles, increase of utilization efficiency of catalysts, diminishment in methanol crossover, reduction in activation loss and facilitation of water discharging out of CLs of cathode due to the controlled porosity in CLs. The enhanced fuel transport, accessibility of fuels to Pt catalyst surface, is proved by the active areas of Pt catalyst. In addition to the active area of Pt catalyst, porous CLs exhibit a decline in methanol crossover, leading to increase of open circuit voltage (OCV). The porous CLs also show improvements in activation loss due to high porosity, causing enhancement in DMFC performances. In aspect of pore volume contribution to cathode performance, the SiO₂ content is optimized. Based on the DMFC performances, it can be suggested that the optimum conditions of SiO₂ are 500 nm in size and 20 wt.% in content. The porosity effect on both electrodes appears as follows: the pores in cathode are more effective on DMFC performances (55.5%) than those of anodes (44.5%) based on the maximum power of DMFC, indicating that the pores in CLs facilitate removal of water from electrodes.     

A Pumpless Methanol Feeding Method for Application in Direct Methanol Fuel Cell Systems

This paper presents a simple and reliable pumpless methanol feeding (PLMF) method for application in direct methanol fuel cell (DMFC) systems. The primary feature and advantage of the PLMF is as follows: it employs an approach that allows the cathode gas pressure to be connected with a fuel container for supplying the methanol fuel into the anode fuel loop, instead of using any feeding pump or other specially designed apparatuses. The PLMF has been used in a portable 25 W DMFC system and realised feeding methanol in real time for meeting the requirements of the system. The PLMF method not only is suitable for the DMFC system, but also can be used in other liquid‐feeding fuel cell systems.     

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.     

Mechanical Response of a Large Fuel Cell Stack to Impact: A Numerical Analysis

Large fuel cell stacks usually undergo dynamic load (vibration and impact) during packing, transportation, and serving time, in particular for those used in the automobiles. This may cause the decay in the performance, and even structure damage of the stack. We numerically analyze the mechanical response of a large fuel cell stack clamped by steel belts to a violent impact in the present paper. It is found that the location of the clamping belts has a great effect on the anti‐impact performance of the stack. The results also indicate that the cells near the endplates have a worse anti‐impact performance than those far from the endplates. When subjected to a large impact in the direction parallel with the cells, the stack may give rise to interface slippage between cells, showing a downward bowing phenomenon. The relative slippage between cells is affected by the clamping force, impact acceleration and the friction coefficient between cells. A large interface friction coefficient and a reasonable high clamping force are expected in order to increase the anti‐impact performance.     

Analysis of Leakages in a Solid Oxide Fuel Cell Stack in a System Environment

A solid oxide fuel cell (SOFC) stack can exhibit both anodic and cathodic leakages, i.e. a fuel leak from the anode side and an air leak from the cathode side of the stack, respectively. This study describes the results of an in‐situ leakage analysis conducted for a planar SOFC stack during 2000 hours of operation in an actual system environment. The leakages are quantified experimentally at nominal system operating conditions by conducting composition analysis and flow metering of gases for both fuel and air subsystems. Based on the calculated atomic hydrogen‐to‐carbon ratio of the fuel and air gases, it is found that the fuel leakages are mostly selective by nature: the leaking fuel gas does not have the same composition as the fuel system gas. A simple diffusive leakage model, based on the leakage being driven by concentration differences weighted by diffusion coefficients, is applied to quantify the amount of leakages. The leakage model provides a good correspondence with the experimental results of the gas analysis.     

Investigation of Ni Foam Effect for Direct Borohydride Fuel Cell

Ni foam has been used as a substrate for the anode electrocatalyst in our previous works. In this study, the effect of nickel foam as an anode electrode in direct borohydride cells has been investigated under steady state/steady‐flow and uniform state/uniform‐flow systems, since nickel has catalytic property. Cathode catalyst used has been 0.3 mg cm^–2 on PTFE‐treated Toray carbon paper. The results have showed that power densities have increased by increasing the temperature. Peak power densities of 5.01 and 9.55 mW cm^–2 have been achieved at 25 and 60 °C, respectively, for 1.5 mol dm^–3 NaBH₄. On the other hand, the electrochemical performance has not been significantly different by the sodium borohydride concentration; only a small increase of power density has been observed in steady state/steady‐flow system, and only a small decrease of fuel utilization ratio has been obtained in uniform state/uniform‐flow systems.     

Evaluation of a Non‐sealed Solid Oxide Fuel Cell Stack with Cells Embedded in Plane Configuration

A non‐sealed solid oxide fuel cell stack with cells embedded in plane configuration was fabricated and operated successfully in a box‐like stainless‐steel chamber. For a two‐cell stack, it demonstrated an open circuit voltage (OCV) of 2.13 V and a maximum power output of 569 mW at the flow rate of 67 sccm CH₄ and 33 sccm O₂. A fuel utilization of 4.16% was obtained. The cell performance was dominated by two different mechanisms, the polarization of the cathode at low current and the concentration polarization of the anode at high current. Finally, a scaled‐up stack with six cells in series generated an OCV of 6.4 V and a maximum power output of 8.18 W.     

Sulphonated Tetramethyl Poly(ether ether ketone)/Epoxy/Sulphonated Phenol Novolac Semi‐IPN Membranes for Direct Methanol Fuel Cells

The sulphonated phenol novolac (PNBS) which was used as a curing agent of epoxy was synthesised from phenol novolac (PN) and 1, 4‐butane sultone and confirmed by FTIR and ¹H NMR. The degree of sulphonation (DS) in PNBS was calculated by ¹H NMR. The semi‐IPN membranes composed of sulphonated tetramethyl poly(ether ether ketone) (STMPEEK) (the value of ion exchange capacity is 2.01 meq g^–1), epoxy (TMBP) and PNBS were successfully prepared. The semi‐IPN membranes showed high thermal properties which were measured by differential scanning calorimeter (DSC) and thermogravimetric analyses (TGA). With the introduction of the cross‐linked TMBP/PNBS, the mechanical properties, dimensional stability, methanol resistance and oxidative stability of the membranes were improved in comparison to the pristine STMPEEK membrane. Although the proton conductivities of the semi‐IPN membranes were lower than those of the pristine STMPEEK membrane, the higher selectivity defined as the ratio of the proton conductivity to methanol permeability was obtained from the STMPEEK/TMBP/PNBS‐14 semi‐IPN membrane. The results indicated that the semi‐IPN membranes could be promising candidates for usage as proton exchange membranes in direct methanol fuel cells (DMFCs).     

Enhancement of the Passive Direct Methanol Fuel Cells Performance by Modification of the Cathode Microporous Layer Using Carbon Nanofibers

Mass transfer is a key parameter affecting the performance of the passive direct methanol fuel cells (DMFCs), which work under natural convection. In this study, effect of carbon nanofibers (CNFs) addition to the cathode microporous layer (MPL) on the performance of the passive DMFCs was investigated. The results indicated that CNFs content has a significant influence on both of the mass transport and the electrochemical surface area (ECSA). Interestingly, addition of the CNFs (20 wt.%) leads to increase the power density of the passive DMFC to 160% compared to pristine carbon black MPL. At low current density, the CNFs content has no influence on the performance, while at high current density the maximum performance can be obtained at 20 wt.% CNFs then the performance decreases with further increase in the CNFs content. Although the highest catalyst utilization is observed at 40 wt.% CNFs, a maximum power density of 36 mW cm^–2 can be obtained at 20 wt.% CNFs and this is related to the significant effect of the mass transfer resistance under the passive operation conditions. Overall, addition of CNFs to the MPL can be considered an effective strategy to modify the passive DMFCs performance.     

Residential Solid Oxide Fuel Cell Generator Fuelled by Ethanol: Cell,Stack and System Modelling with a Preliminary Experiment

The flexibility and feasibility of a 5 kW SOFC generator designed for natural gas (NG) and fuelled by a non‐conventional liquid fuel such as ethanol is analysed. A complete generator model is implemented to predict and determine the main criticalities when ethanol fuel is adoperated. The main balance‐of‐plant (BoP) units considered are the reformer, the recirculation system based on an ejector, the tubular cells bundles constituting the stack unit, the after‐burner zone and the air blower. The electrical and global efficiencies achieved at nominal operating conditions show how ethanol maintains generator performance good, while only slightly reducing the system AC efficiency from 48% (achieved by NG) to 45%. The effectiveness and flexibility of the recirculation system when changing the fuel is also verified since a safe steam‐to‐carbon ratio (STCR) is established after the fuel is switched from NG ethanol. The stack thermal management is analysed in detail and related to the system performances, showing how a high endothermic fuel reforming reaction is required to maintain the overall system efficiency. A preliminary experiment with ethanol feeding the Siemens generator is finally presented. The system response to the new fuel is monitored by several measured parameters and the system regulation is explained.