Experimental evaluation and mathematical modeling of a direct alkaline fuel cell

The performance of a direct alkaline fuel cell (AFC) is studied separately using methanol, ethanol and sodium borohydride as fuel. Potassium hydroxide solution was used as an electrolyte. Pt-black and manganese dioxide catalyst were used to prepare the anode and cathode electrodes. Ni mesh was used as current collector. The direct alkaline fuel cell was constructed with the prepared anode and cathode electrodes and Ni mesh. The current density–cell voltage characteristics of the fuel cell were determined by varying load and at different experimental conditions, e.g., electrolyte concentration, fuel concentration and temperature. The fuel cell performance increases initially with the increase in electrolyte (KOH) concentration and then decreases with further increase of the same. The cell performance increases initially and then no appreciable improvement noticed with the increase in fuel concentration. The performance of the fuel cell increases with increase in temperature in general with the exception to NaBH₄ alkaline fuel cell. A mathematical model for the direct alkaline fuel cell is developed based on reaction mechanism available in the literature to predict the cell voltage at a given current density. The model takes into account activation, ohmic, concentration overpotentials and other losses. The model prediction is in fair agreement with the experimental data on current–voltage characteristics and captures the influence of different experimental conditions on current–voltage characteristics.     

Investigation and application of lithium difluoro(oxalate)borate (LiDFOB) as additive to improve the thermal stability of electrolyte for lithium-ion batteries

Lithium difluoro (oxalate) borate (LiDFOB) is used as thermal stabilizing and solid electrolyte interface (SEI) formation additive for lithium-ion battery. The enhancements of electrolyte thermal stability and the SEIs on graphite anode and LiFePO₄ cathode with LiDFOB addition are investigated via a combination of electrochemical methods, nuclear magnetic resonance (NMR), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared-attenuated total reflectance (FTIR-ATR), as well as density functional theory (DFT). It is found that cells with electrolyte containing 5% LiDFOB have better capacity retention than cells without LiDFOB. This improved performance is ascribed to the assistance of LiDFOB in forming better SEIs on anode and cathode and also the enhancement of the thermal stability of the electrolyte. LiDFOB-decomposition products are identified experimentally on the surface of the anode and cathode and supported by theoretical calculations.     

Effect of nanostructured anode functional layer thickness on the solid-oxide fuel cell performance in the intermediate temperature

Effect of anode functional layer thickness on the performance of solid-oxide fuel cells (SOFCs) has been investigated in the intermediate temperatures of 600–650 °C. Three types of cells with different thickness (0, 4, 10 micron) of nanostructured anode functional layer (AFL) consisting of Ni-ScSZ (Scandia stabilized zirconia) are prepared. The SOFCs consist of Ni-3YSZ (3 mol% yttria stabilized zirconia) anode tube support with the AFL, ScSZ electrolyte, and LSCF (lanthanum strontium cobalt ferrite) and GDC (gadolinium doped ceria) mixture cathode. It is shown that the performance of the cell is improved as the thickness of the anode functional layer increases. Power densities of the cell with 10 micron thick AFL at 600 and 650 °C are shown to be 0.22 and 0.27 W/cm² at 0.75 V, respectively. According to impedance spectroscopy, improvement of both ohmic and polarization resistances has been observed by increasing the thickness of the AFL, suggesting that the AFL also acts as a better contact layer between the electrolyte and the anode support, and the effectiveness of the AFL by optimizing the thickness.     

Enhancement of the cathode/electrolyte interface by a sintering-active barrier layer for solid oxide fuel cells

It's a critical issue for the successful commercialization of solid oxide fuel cells (SOFCs) to achieve long-term operation and thermal cycling stability without significant degradation. Current work reports an almost-dense, sintering-active and Sr-blocking cathode/electrolyte interface, fabricated through a cost-competitive and scalable method of modifying porous Gadolinia doped Ceria (GDC) barrier layer by in-situ grown GDC nanoparticles. Result show that the robust interface enables improved durability and thermal cycling stability. The hydrothermal modified anode supported cell performance only deteriorates by ∼0.5% after 20 times thermal cycles between 200 and 750 °C, which is more prominent enhancement than ∼16% for pristine cell. The modified symmetric cell shows smaller cathode polarization resistance and well-attached cathode/electrolyte interfaces after ∼1200 h of operation, including 4 times thermal cycles. While the pristine cell shows more obvious area specific resistances increases and the peeled-off cathode layer. It is discussed and concluded that the hydrothermal modified sintering-active barrier layer has contributed mainly to the construction of robust cathode/electrolyte interface, yielding the improved performance and prolonged operation.     

Performance of tubular Solid Oxide Fuel Cell at reduced temperature and cathode porosity

The effect of decreasing the inlet temperature and the cathode porosity of tubular Solid Oxide Fuel Cell (SOFC) with one air channel and one fuel channel is investigated using Computational Fluid Dynamics (CFD) approach. The CFD model was developed using Fluent SOFC to simulate the electrochemical effects. The cathode and the anode of the cell were resolved in the model and the convection and conduction heat transfer modes were included. The results of the CFD model are presented at inlet temperatures of 700 °C, 600 °C and 500 °C and with cathode porosity of 30%, 20% and 10%. It was found that the Fluent-based SOFC model is an effective tool for analyzing the complex and highly interactive three-dimensional electrical, thermal, and fluid flow fields associated with the SOFCs. It is found that the SOFC can operate in the intermediate temperature range and with low porosity cathodes more efficient than at high temperatures given that the transport properties of the cathode, anode and the electrolyte can be kept the same.     

Analysis of performance losses of direct ethanol fuel cells with the aid of a reference electrode

The performances of direct ethanol fuel cells with different anode catalysts, different ethanol concentrations, and at different operating temperatures have been studied. The performance losses of the cell have been separated into individual electrode performance losses with the aid of a reference electrode, ethanol crossover has been quantified, and CO₂ and acetic acid production have been measured by titration. It has been shown that the cell performance strongly depends on the anode catalyst, ethanol concentration, and operating temperature. It was found that the cathode and anode exhibit different dependences on ethanol concentration and operating temperature. The performance of the cathode is very sensitive to the rate of ethanol crossover. Product analysis provides insights into the mechanisms of electro-oxidation of ethanol.     

A flow channel design procedure for PEM fuel cells with effective water removal

Proper water management in polymer electrolyte membrane (PEM) fuel cells is critical to achieve the potential of PEM fuel cells. Membrane electrolyte requires full hydration in order to function as proton conductor, often achieved by fully humidifying the anode and cathode reactant gas streams. On the other hand, water is also produced in the cell due to electrochemical reaction. The combined effect is that liquid water forms in the cell structure and water flooding deteriorates the cell performance significantly. In the present study, a design procedure has been developed for flow channels on bipolar plates that can effectively remove water from the PEM fuel cells. The main design philosophy is based on the determination of an appropriate pressure drop along the flow channel so that all the liquid water in the cell is evaporated and removed from, or carried out of, the cell by the gas stream in the flow channel. At the same time, the gas stream in the flow channel is maintained fully saturated in order to prevent membrane electrolyte dehydration. Sample flow channels have been designed, manufactured and tested for five different cell sizes of 50, 100, 200, 300 and 441 cm². Similar cell performance has been measured for these five significantly different cell sizes, indicating that scaling of the PEM fuel cells is possible if liquid water flooding or membrane dehydration can be avoided during the cell operation. It is observed that no liquid water flows out of the cell at the anode and cathode channel exits for the present designed cells during the performance tests, and virtually no liquid water content in the cell structure has been measured by the neutron imaging technique. These measurements indicate that the present design procedure can provide flow channels that can effectively remove water in the PEM fuel cell structure.     

Effect of humidity of reactants on the cell performance of PEM fuel cells with parallel and interdigitated flow field designs

This study investigates the effects of the relative humidity (RH) of the reactants on the cell performance and local transport phenomena in proton exchange membrane fuel cells with parallel and interdigitated flow fields. A three-dimensional model was developed taking into account the effect of the liquid water formation on the reactant transport. The results indicate that the reactant RH and the flow field design all significantly affect cell performance. For the same operating conditions and reactant RH, the interdigitated design has better cell performance than the parallel design. With a constant anode RH = 100%, for lower operating voltages, a lower cathode RH reduces cathode flooding and improves cell performance, while for higher operating voltages, a higher cathode RH maintains the membrane hydration to give better cell performance. With a constant cathode RH = 100%, for lower operating voltages, a lower anode RH not only provides more hydrogen to the catalyst layer to participate in the electrochemical reaction, but also increases the difference in the water concentrations between the anode and cathode, which enhances back-diffusion of water from the cathode to the anode, thus reducing cathode flooding to give better performance. However, for higher operating voltages, the cell performance is not dependent on the anode RH.     

Stress field and failure probability analysis for the single cell of planar solid oxide fuel cells

An analytical model is developed to predict the residual thermal stresses in a single cell of solid oxide fuel cells (SOFCs), which consists of a thick porous 8 mol% Y₂O₃ stabilized zirconia/nickel oxide (8YSZ/NiO) anode, a dense 8YSZ electrolyte and a porous lanthanum strontium manganite (LSM) cathode. The simulated stresses in the cell at room temperature, which are resulted from the contraction mismatch of its components, indicate that the major principal stress in the anode is tensile while the electrolyte and cathode are under compressive stresses. The stress in one component decreases with the increase of its thickness when the thicknesses of the other two components are fixed, and the decrease of the tensile stress in the anode will cause the increase of the compressive stresses in both the cathode and the electrolyte, and vice versa. The analysis also reveals that the anode is the part that is most susceptible to fracture since the tensile thermal stress is so high that it reaches to the fracture strength of the anode material. The Weibull statistic is employed to estimate the failure probability of the anode. The simulation results indicate that the anode failure probability decreases with the increase of the anode thickness and the decrease of the electrolyte thickness. To keep the anode failure probability less than 1E−06, the anode thickness should be greater than 0.7 mm for a cell with an electrolyte thickness of 10 μm and a cathode thickness of 20 μm.     

Testing of a cathode fabricated by painting with a brush pen for anode-supported tubular solid oxide fuel cells

We have studied the properties of a cathode fabricated by painting with a brush pen for use with anode-supported tubular solid oxide fuel cells (SOFCs). The porous cathode connects well with the electrolyte. A preliminary examination of a single tubular cell, consisting of a Ni-YSZ anode support tube, a Ni-ScSZ anode functional layer, a ScSZ electrolyte film, and a LSM-ScSZ cathode fabricated by painting with a brush pen, has been carried out, and an improved performance is obtained. The ohmic resistance of the cathode side clearly decreases, falling to a value only 37% of that of the comparable cathode made by dip-coating at 850 °C. The single cell with the painted cathode generates a maximum power density of 405 mW cm⁻² at 850 °C, when operating with humidified hydrogen.     

Water balance simulations of a polymer-electrolyte membrane fuel cell using a two-fluid model

A previously published computational multi-phase model of a polymer-electrolyte membrane fuel cell cathode has been extended in order to account for the anode side and the electrolyte membrane. The model has been applied to study the water balance of a fuel cell during operation under various humidification conditions. It was found that the specific surface area of the electrolyte in the catalyst layers close to the membrane is of critical importance for the overall water balance. Applying a high specific electrolyte surface area close to the membrane (a water-uptake layer) can prevent drying out of the anode and flooding at the cathode while the average membrane water content is only weakly affected. The results also indicate that in contrast to common presumption membrane dehydration may occur at either anode or cathode side, entirely depending on the direction of the net water transport because the predominant transport mechanism is diffusion. Consequently, operating conditions with a high net water transport from anode to cathode should be avoided as it is important to keep the cathode catalyst layer well humidified in order to prevent high protonic losses. Addition of the micro-porous layer did not affect the overall water balance or membrane water content in our study.     

A general electrolyte–electrode-assembly model for the performance characteristics of planar anode-supported solid oxide fuel cells

A general electrode–electrolyte-assembly (EEA) model has been developed, which is valid for different designs of solid oxide fuel cells (SOFCs) operating at different temperatures. In this study, it is applied to analyze the performance characteristics of planar anode-supported SOFCs. One of the novel features of the present model is its treatment of electrodes. An electrode in the present model is composed of two distinct layers referred to as the backing layer and the reaction zone layer. The other important feature of the present model is its flexibility in fuel, having taking into account the reforming and water–gas shift reactions in the anode. The coupled governing equations of species, charge and energy along with the constitutive equations in different layers of the cell are solved using finite volume method. The model can predict all forms of overpotentials and the predicted concentration overpotential is validated with measured data available in literature. It is found that in an anode-supported SOFC, the cathode overpotential is still the largest cell potential loss mechanism, followed by the anode overpotential at low current densities; however, the anode overpotential becomes dominant at high current densities. The cathode and electrolyte overpotentials are not negligible even though their thicknesses are negligible relative to the anode thickness. Even at low fuel utilizations, the anode concentration overpotential becomes significant when chemical reactions (reforming and water–gas shift) in the anode are not considered. A parametric study has also been carried out to examine the effect of various key operating and design parameters on the performance of an anode-supported planar SOFCs.     

A comprehensive,consistent and systematic mathematical model of PEM fuel cells

This paper presents a comprehensive, consistent and systematic mathematical model for PEM fuel cells that can be used as the general formulation for the simulation and analysis of PEM fuel cells. As an illustration, the model is applied to an isothermal, steady state, two-dimensional PEM fuel cell. Water is assumed to be in either the gas phase or as a liquid phase in the pores of the polymer electrolyte. The model includes the transport of gas in the gas flow channels, electrode backing and catalyst layers; the transport of water and hydronium in the polymer electrolyte of the catalyst and polymer electrolyte layers; and the transport of electrical current in the solid phase. Water and ion transport in the polymer electrolyte was modeled using the generalized Stefan–Maxwell equations, based on non-equilibrium thermodynamics. Model simulations show that the bulk, convective gas velocity facilitates hydrogen transport from the gas flow channels to the anode catalyst layers, but inhibits oxygen transport. While some of the water required by the anode is supplied by the water produced in the cathode, the majority of water must be supplied by the anode gas phase, making operation with fully humidified reactants necessary. The length of the gas flow channel has a significant effect on the current production of the PEM fuel cell, with a longer channel length having a lower performance relative to a shorter channel length. This lower performance is caused by a greater variation in water content within the longer channel length.     

Comparison of electrolyte fabrication techniques on the performance of anode supported solid oxide fuel cells

A comparison of three solid oxide electrolyte fabrication processes, namely dip coating, screen printing and tape casting, for planar anode supported solid oxide fuel cells (SOFCs) is presented in this study. The effect of sintering temperature (1325–1400 °C) is also examined. The anode and cathode layers of the anode-supported cells, on the other hand, are fabricated by tape casting and screen printing, respectively. The quality of the electrolytes is evaluated via performance measurements, impedance analyses and microstructural investigations of the cells. It is found that the density of the electrolyte increases with the sintering temperatures for all fabrication methods studied. The results also show that with the process and fabrication parameters considered in this study, both dip coating and screen printing do not yield a desired dense electrolyte structure as proven by open circuit potentials measured and SEM photos. The cells with tape cast electrolytes, on the other hand, provide the highest performances regardless of the electrolyte sintering and cell operating temperatures. The best peak performance of 0.924 W/cm² is obtained from the cell with tape cast electrolyte sintered at 1400 °C. SEM investigations and measured open circuit potentials reveal that almost fully dense electrolyte layer can be obtained with a tape cast electrolyte particularly sintered at temperatures higher than 1350 °C. Impedance analyses indicate that the main reason behind the significantly higher performances is due to not only increased electrolyte density but a decrease in the interface resistance of the anode functional and electrolyte layer is also responsible. This can be explained by the load applied during the lamination step in the fabrication of the tape cast electrolyte, providing better powder compaction and adhesion.     

Electrode structure analysis and surface characterization for lithium-ion cells simulated low-Earth-orbit satellite operation: I. Electrochemical behavior and structure analysis

Lithium-ion cells for satellite applications operate under a special condition, and are expected to behave differently from those for commercial purposes. To understand the performance-degradation mechanism of lithium-ion cells experienced cycle-life testing in a simulated low-Earth-orbit (LEO) satellite operation, we conducted the structure analysis and surface characterization of the aged LiCoO₂ cathode and graphite anode obtained from a lithium-ion cell with 4350-cycle LEO simulation experience. The analysis results were compared with a fresh cell which served as control. This paper provides a review of testing results on electrochemical and structure analysis. The capacity-verification and impedance measure results indicated that the LiCoO₂ cathode, rather than graphite anode, was responsible for the performance degradation of the aged cell. This conclusion was confirmed by the structure analysis. The qualitative analysis of the XRD spectra disclosed that the aged cathode exhibited a much larger structure change than the aged anode. We also detected the lithium ions that were irreversibly reserved in graphite anode in XRD and ⁷Li nuclear magnetic resonance (NMR) analysis of aged graphite anode. These results lead us to deduce that the serious structure change in LiCoO₂ cathode was primarily responsible for the performance degradation of the aged cell.     

Effect of operating parameters and anode diffusion layer on the direct ethanol fuel cell performance

A parametric study was conducted on the performance of direct ethanol fuel cells. The membrane electrode assemblies employed were composed of a Nafion^® 117 membrane, a Pt/C cathode and a PtRu/C anode. The effect of cathode backpressure, cell temperature, ethanol solution flow rate, ethanol concentration, and oxygen flow rate were evaluated by measuring the cell voltage as a function of current density for each set of conditions. The effect of the anode diffusion media was also studied. It was found that the cell performance was enhanced by increasing the cell temperature and the cathode backpressure. On the contrary, the cell performance was virtually independent of oxygen and fuel solution flow rates. Performance variations were encountered only at very low flow rates. The effect of the ethanol concentration on the performance was as expected, mass transport loses observed at low concentrations and kinetic loses at high ethanol concentration due to fuel crossover. The open circuit voltage appeared to be independent of most operating parameters and was only significantly affected by the ethanol concentration. It was also established that the anode diffusion media had an important effect on the cell performance.     

Modeling of methane fed solid oxide fuel cells: Comparison between proton conducting electrolyte and oxygen ion conducting electrolyte

An electrochemical model was developed to study the methane (CH₄) fed solid oxide fuel cell (SOFC) using proton conducting electrolyte (SOFC-H) and oxygen ion conducting electrolyte (SOFC-O). Both the internal methane steam reforming (MSR) and water gas shift (WGS) reactions are considered in the model. Previous study has shown that the CH₄ fed SOFC-H had significantly better performance than the SOFC-O. However, the present study reveals that the actual performance of the CH₄ fed SOFC-H is considerably lower than the SOFC-O, partly due to higher ohmic overpotential of SOFC-H. It is also found that the CH₄ fed SOFC-H has considerably higher cathode concentration overpotential and lower anode concentration overpotential than the SOFC-O. The anode concentration overpotentials of the CH₄ fed SOFC-H and SOFC-O are found to decrease with increasing temperature, which is different from previous analyses on the H₂ fed SOFC. Therefore, high temperature is desirable for increasing the potential of the CH₄ fed SOFC. It is also found that there exist optimal electrode porosities that minimize the electrode total overpotentials. The analyses provided in this paper signify the difference between the CH₄ fed SOFC-H and SOFC-O. The model developed in this paper can be extended to 2D or 3D models to study the performance of practical SOFC systems.     

Solid-state batteries using silver-based glassy materials

A new, fast ion-conducting, silver seleno molybdate (SSM), glassy electrolyte has been synthesized. Solid-state primary cells of the design anode/solid electrolyte/cathode have been fabricated with the highest conducting composition of the SSM glass as the solid electrolyte. The polarization and discharge characteristics of the cells are studied with different compositions of the cathode material. The effect of adding the solid electrolyte and tetraalkylammonium iodide (alkyl = methyl, butyl) to the cathode on the cell performance is investigated. The cell parameters are evaluated and the results are compared with a cell of the type Ag/AgI/(iodine + graphite) (I + C) that is fabricated using polycrystalline AgI as the solid electrolyte.     

Detailed analysis of polymer electrolyte membrane fuel cell with enhanced cross‐flow split serpentine flow field design

Most generally used flow channel designs in polymer electrolyte membrane fuel cells (PEMFCs) are serpentine flow designs as single channels or as multiple channels due to their advantages over parallel flow field designs. But these flow fields have inherent problems of high pressure drop, improper reactant distribution, and poor water management, especially near the U‐bends. The problem of inadequate water evacuation and improper reactant distribution become more severe and these designs become worse at higher current loads (low voltages). In the current work, a detailed performance study of enhanced cross‐flow split serpentine flow field (ECSSFF) design for PEMFC has been conducted using a three‐dimensional (3‐D) multiphase computational fluid dynamic (CFD) model. ECSSFF design is used for cathode part of the cell and parallel flow field on anode part of the cell. The performance of PEMFC with ECSSFF has been compared with the performance of triple serpentine flow design on cathode side by keeping all other parameters and anode side flow field design similar. The performance is evaluated in terms of their polarization curves. A parametric study is carried out by varying operating conditions, viz, cell temperature and inlet humidity on air and fuel side. The ECSSFF has shown superior performance over the triple serpentine design under all these conditions.     

Effects of operating temperatures on performance and pressure drops for a 256 cm proton exchange membrane fuel cell: An experimental study

Cell performance and pressure drop were experimentally investigated for two commercial size 16 cm × 16 cm serpentine flow field proton exchange membrane fuel cells with Core 5621 and Core 57 membrane electrode assemblies at various cell temperatures and humidification temperatures. At cell temperature lower than the humidification temperature, the cell performance improved as the cell temperature increased, while reversely at cell temperature higher than the humidification temperature. At a specified cell temperature, increasing the cathode and/or anode humidification temperature improved the cell performance, and their effects weakened as cell temperature decreased. The effects of the cell and the humidification temperature on the pressure drops were closely related to the reactant feed mode. For the constant stoichiometric flow rate mode, both cathode and anode pressure drops increased as humidification temperature and average current density increased. For the constant mass flow rate mode, both cathode and anode pressure drops increased as humidification temperature increased, while anode pressure drops decreased and cathode pressure drops increased as average current density increased. The optimal cell performance occurred at cell temperature of 65 °C and humidification temperature of 70 °C. The effects of these operating parameters on the cell performance and pressure drop were analyzed based on the catalytic activity, membrane hydration, and cathode flooding.