Polarization effects in electrolyte/electrode-supported solid oxide fuel cells

The effects of activation, ohmic and concentration polarization on the overall polarization in solid oxide fuel cells are presented. A complete analysis was conducted based on thermodynamic principles for the calculation of cell voltage. Treating the fuel cell as a control volume, the irreversibility term in a steady flow thermodynamic system was related to the overall polarization. The entropy production was calculated and related to the lost work of the fuel cell, while the heat loss from the cell was determined from the entropy balance. To generalize the cell voltage–current density expression, the Butler–Volmer model was used in the calculation of activation polarization and both ordinary and Knudsen diffusions were considered in the calculation of concentration polarization. The overall cell resistance was deduced from the generalized cell voltage–current density expression. The concentration resistance at the anode can be minimized by humidifying the hydrogen with an appropriate amount of water, depending on the thickness of the anode used. Comparison of polarization effects on the cell performance between the electrolyte-supported and anode-supported cells showed that the latter would give a better cell performance.     

5kW氢-空质子交换膜燃料电池堆的模型研究

采用机理模型和经验模型相结合的建模方法建立了一个5kw质子交换膜燃料电池堆实际装置的电化学模型。利用电池堆与单电池之间的内在关系,首先给出了单电池的数学描述,进而建立燃料电池堆的数学模型,其中包括热力学平衡电势、活化极化电势和欧姆极化电势等表达式,以及单电池内阻的经验公式。由于难以得到机理方程中的某些关键参数,因此采用实验设计,获得燃料电池堆的实验数据,运用线性回归的数学方法获得机理模型中活化极化电势方程中的相关参数,通过模拟研究和模型验证,所建立的模型可以较准确的描述燃料电池的极化曲线,预估出燃料电池的输出电压。     

多层水系流延和共烧法制备具有阳极功能层固体氧化物燃料电池

采用多层水系流延和共烧方法制备具有阳极功能层的单电池。阳极基底、阳极功能层、电解质层和阴极层分别为Ni-YSZ、Ni-ScSZ、YSZ和LSM-ScSZ。在H2/空气气氛中,分别在700、750、800℃测试具有阳极功能层的单电池,其最大功率密度分别为：0.30、0.55W/cm2和0.8W/cm2;其对应的电池欧姆电阻（R0）分别为0.39、0.30cm2和0.19cm2。电池的极化电阻则分别为1.28、0.91cm2和0.62cm2。采用相同工艺制备无阳极功能层的单电池,其在700、750、800℃的最大功率密度分别为0.21、0.31W/cm2和0.56W/cm2,对应的R0为0.41、0.39cm2和0.28cm2。电池的极化电阻为1.40、1.27cm2和0.91cm2。这说明采用的多层水系流延和共烧法制备的固体氧化物燃料电池的阳极功能层能有效减小电池的活化极化,从而提高单电池的电化学性能。     

Simulation of influences of layer thicknesses in an alkaline fuel cell

A computational simulation was conducted by using a one-dimensional isothermal model for an alkaline fuel cell (AFC) single cell to investigate influences of the thicknesses of the separator, catalyst layer, and gas-diffusion layer in an AFC. The cell polarizations were predicted at various thicknesses and their influences were also analysed. Thickening the separator layer decreased the limiting current density and increased the slope of the ohmic polarization region. Investigation on the thickness of the anode catalyst layer showed that the optimum thickness varied between 0.04–0.15 mm according to the cell voltage. The thickness of the cathode catalyst layer significantly influenced the cell performance. Also, a limitation of thickness effect in the cathode catalyst layer was observed. This limitation was considered to be caused by the mass transfer resistance of the electrolyte.     

Fabrication of one-step co-fired proton-conducting solid oxide fuel cells with the assistance of microwave sintering

A microwave sintering technique is reported for fabricating co-sintered proton-conducting solid oxide fuel cells. With this method, high-quality ceramic electrolyte membranes can be prepared at 1100?°C, thus enabling the fabrication of entire cells in a single step. The microwave sintering method not only enhances electrolyte densification but also improves the cathode/electrolyte interface, which is critical for improving fuel cell performance. The power output of the co-sintered cell prepared under the microwave conditions (up to 449?mW?cm^?2 at 700?°C) was significantly higher than that of the cell fabricated using the traditional co-sintering method (approximately 292?mW?cm^?2 at the same temperature). Electrochemical analysis revealed that the enhanced electrolyte density and the improved cathode/electrolyte interface achieved by using the microwave sintering technique decrease both the ohmic resistance and the polarisation resistance of the cell, leading to good fuel cell performance.     

Improving electrochemical performance of (Cu,Sm)CeO2 anode with anchored Cu nanoparticles for direct utilization of natural gas in solid oxide fuel cells

Development of solid oxide fuel cell (SOFC) anode with high resistance to coking and sulfur poisoning is highly desirable for the direct application of natural gas in SOFC. Herein, a (Cu, Sm)CeO₂ anode with anchored Cu nanoparticles has been prepared. Most of Cu nanoparticles particle size ranges from 20 to 50 nm, which can increase the conductivity and catalytic activity of the anode. The Cu/CSCO10 supported cell exhibits a maximum power density of 404.6 mW/cm² at 600 °C when dry methane is used as fuel while its ohmic resistance is only 0.39 Ω cm². The single SOFC shows good stability when H₂S content in the fuel is less than 150 ppm. Up to 900 h of continuous stable operation with simulated natural gas and commercial natural gas as fuel prove the advantages and application potential of this anode.     

H2/Cl2 fuel cell for co-generation of electricity and HCl

A H₂/Cl₂ fuel cell system with an aqueous electrolyte and gas diffusion electrodes has been investigated and the effects of electrolyte concentration and temperature on the open circuit voltage (OCV) and cell performance have been evaluated. Furthermore, the kinetics and long-term stability of Pt as electrocatalyst have been studied under various conditions. In addition, the long-term stability of Rh electrocatalyst has been evaluated. The OCV obtained showed that the Cl₂ reduction is more reversible than O₂ reduction. The ohmic drop was determining the cell performance at high current densities. An output power of about 0.5 W cm^–2 was achieved with this system.     

Robust and reliable solid oxide fuel cells with modified thin film YSZ electrolyte

Reduce electrolyte thickness can improve solid oxide fuel cell (SOFC) performance. However, thinner electrolyte often contains prominent defects and flaws, which may decrease the yield and increase operation risk. This work proposes a method to modify the thin film YSZ electrolyte, to improve cell reliability and durability. The as-sintered anode supported half-cell with screen printed YSZ electrolyte was immersed in precursor solution of Y(NO₃)₃·6H₂O and Zr(NO₃)₄·5H₂O, and being treated under hydrothermal condition of 150°C for 12 h. As a result, the modified cells show slight increase in the OCV values. Furthermore, the hydrothermal modification effectively promotes interface sintering between YSZ electrolyte and GDC barrier layer, yielding a smaller ohmic resistance of .142 Ω·cm² (a decrease of ∼11%) and a higher peak power density of .964 W/cm² (an increase of ∼18%) at 750°C, than pristine cell. Moreover, the modified cell operates stably over 300 h, while the pristine cell presents large and irregular voltage fluctuations. This work suggests that the hydrothermal modification is an effective and promisingly industrial applicable method for thin film electrolyte recovery in SOFCs.     

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.     

Influence of hot-pressing temperature on the performance of PEMFC and catalytic activity

The influence of hot-pressing temperature on catalytic activity and the performance of proton exchange membrane fuel cells (PEMFCs) was investigated using current–voltage (I–V) polarization, electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV). EIS provided detailed information on the contribution, from high to low frequencies, of internal impedance (R_s), interfacial impedance (R_if) and reaction impedance (R_rxn). The ohmic resistance of the cell (R_Ω) was estimated from the I–V diagram for comparison with the R_s and R_if impedances. The R_if is useful for diagnosing catalytic activity and interpreting PEMFC performance. A cell was hot-pressed at 125 °C (near the transition temperature of Nafion), an optimum temperature for lowest ohmic resistance and total impedance in response to a maximal catalyst-specific activity.     

Fabrication and performance of PEN SOFCs with proton-conducting electrolyte

A positive-electrolyte-negative (PEN) assembly solid oxide fuel cell (SOFC) with a thin electrolyte film for intermediate temperature operation was fabricated. Instead of the traditional screen-printing method, both anode and cathode catalysts were pressed simultaneously and formed with the fabrication of nano-composite electrolyte by press method. This design offered some advantageous configurations that diminished ohmic resistance between electrolyte and electrodes. It also increased the proton-conducting rate and improved the performance of SOFCs due to the reduction of membrane thickness and good contact between electrolyte and electrodes. The fabricated PEN cell generated electricity between 600°C and 680°C using H₂S as fuel feed and air as oxidant. Maximum power densities 40 mW·cm⁻² and 130 mW·cm⁻² for the PEN configuration with a Mo-Ni-S-based composite anode, nano-composite electrolyte (Li₂SO₄+Al₂O₃) film and a NiO-based composite cathode were achieved at 600°C and 680°C, respectively.     

电流中断法在线测定直接甲醇燃料电池的欧姆阻抗

Electrochemical impedance spectroscopy (EIS) is widely used in fuel cell impedance analysis. However, for ohmic resistance (RΩ), EIS has some disadvantages such as long test period and complex data analysis with equivalent circuits. Therefore, the current interruption method is explored to measure the value of RΩ in direct methanol fuel cells (DMFC) at different temperatures and current densities. It is found that RΩ decreases as temperature increase, and decreases initially and then increases as current density increases. These results are consistent with those measured by the EIS technique. In most cases, the ohmic resistances with current interruption (RiR) are larger than those with EIS (REIS), but the difference is small, in the range from –0.848% to 5.337%. The errors of RiR at high current densities are less than those of REIS. Our results show that the RiR data are reliable and easy to obtain in the measurement of ohmic resistance in DMFC.     

The internal resistance of a microbial fuel cell and its dependence on cell design and operating conditions

The internal resistance R_int of a mediator-less microbial fuel cell (MFC) has been determined as a function of cell voltage using electrochemical impedance spectroscopy (EIS) for a MFC with and without Shewanella oneidensis MR-1. The same tests were performed for a MFC containing small stainless steel (SS) balls in the anode compartment with a graphite feeder electrode as in a packed bed cell. It has been found that R_int decreased with decreasing cell voltage as the increasing current flow decreases the polarization resistance of the anode and the cathode. The ohmic components of R_int played a very minor role. In the presence of MR-1 R_int was lower by a factor of about 100 than R_int of the MFC with buffer and lactate as anolyte. R_int was also significantly lower for the anode containing SS balls with buffer and lactate as anolyte. For the MFC containing SS balls in the anode compartment no significant further decrease of R_int could be obtained when MR-1 was added to the anolyte since in this case the polarization resistance of the anode was lower than that of the cathode. Similar trends were observed in the cell voltage (V)-current (I) curves that were obtained using potentiodynamic sweeps and the power (P)-V curves that were calculated from the V-I curves.     

Optimization of physical parameters of solid oxide fuel cell electrode using electrochemical model

To enhance the performance of anode-supported solid oxide fuel cell (SOFC), an electrochemical model has been developed in this study. The Butler-Volmer equation, Ohm’s law and dusty-gas model are incorporated to predict the activation, ohmic and concentration overpotentials, respectively. The optimal cell microstructure and operating parameters for the best current-voltage (J-V) characteristics have been sought from the information of the exchange current density and gas diffusion coefficients. As the cell temperature rises, the activation and ohmic overpotentials decrease, whereas the concentration overpotential increases due to the considerable reduction of gas density at the elevated temperature despite the increased diffusion coefficient. Also, increasing the hydrogen molar fraction and operating pressure can further augment the maximum cell output. Since there exists an optimum electrode pore size and porosity for maximum cell power density, the graded electrode has newly been designed to effectively reduce both the activation and concentration overpotentials. The results exhibit 70% improved cell performance than the case with a non-graded electrode. This electrochemical model will be useful to simply understand overpotential features and devise the strategy for optimal cell design in SOFC systems.     

Effect of composition on the performance of cermet electrodes. Experimental and theoretical approach

This work aims to analyse the behaviour of cermet electrodes as a function of their composition, i.e. the ratio between ionic and electronic conducting particles. This is an important parameter to be considered to obtain maximum performance from this type of electrode, which is currently under study for application in oxygen sensors and solid oxide fuel cells. Experimental results of overall electrode resistance, including both ohmic and activation polarisation effects, have been obtained through electrochemical impedance spectroscopy measurements of Pt/YSZ electrodes in air. The results compare favourably with the theoretical predictions for several compositions above the percolation threshold of the electronic conductor. For this reason, the model is a useful tool for the design of optimised cermet electrodes; in particular, the experimental data show that maximum performance is attained for compositions very close to the percolation threshold of the electronic conductor, and this is in very good agreement with the modelling results.     

Non-isothermal effects of single or double serpentine proton exchange membrane fuel cells

Mathematical models on transport processes and reactions in proton exchange membrane (PEM) fuel cell generally assume an isothermal cell behavior for sake of simplicity. This work aims at exploring how a non-isothermal cell body affects the performance of PEM fuel cells with single and double serpentine cathode flow fields, considering the effects of flow channel cross-sectional areas. Low thermal conductivities of porous layers in the cell and low heat transfer coefficients at the surface of current collectors, as commonly adopted in cell design, increase the cell temperature. High cell temperature evaporates fast the liquid water, hence reducing the cathode flooding; however, the yielded low membrane water content reduces proton transport rate, thereby increasing ohmic resistance of membrane. An optimal cell temperature is presented to maximize the cell performance.     

Mathematical modeling of solid oxide fuel cells at high fuel utilization based on diffusion equivalent circuit model

Mass transfer and electrochemical phenomena in the membrane electrode assembly (MEA) are the core components for modeling of solid‐oxide fuel cell (SOFC). The general MEA model is simply governed with the Stefan‐Maxwell equation for multicomponent gas diffusion, Ohm's law for the charge transfer and the current‐overpotential equation for the polarization calculation. However, it has obvious discrepancy at high‐fuel utilization or high‐current density. An advanced MEA model is introduced based on the diffusion equivalent circuit model. The main purpose is to correct the real‐gas concentrations at the triple‐phase boundary by assuming that the resistance of surface diffusion is in series with that of the gaseous bulk diffusion. Thus, it can obtain good prediction of cell performance in a wide range by avoiding the decrement of effective gas diffusivity via unreasonable increment of the electrode tortuosity in the general MEA model. The mathematical model has been validated in the cases of H₂? H₂O, CO? CO₂ and H₂? CO fuel system. © 2009 American Institute of Chemical Engineers AIChE J, 2010     

Moisture Effect on La0.8Sr0.2MnO3 and La0.6Sr0.4Co0.2Fe0.8O3 Cathode Behaviors in Solid Oxide Fuel Cells

Moisture effect on cathode behaviors is a major issue for solid oxide fuel cells servicing under severe high temperature environments. This work studies the effect of dry air and moist air on La_0.8Sr_0.2MnO₃ (LSM821) and La_0.6Sr_0.4Co_0.2Fe_0.8O₃ (LSCF6428) cathodes at 800 °C by investigating the interfacial reaction and degradation through an AISI 441 interconnect/LSM821 (LSCF6428) electrode/yttria‐stabilized zirconia (YSZ) electrolyte tri‐layer structure. Under the same processing condition, the grain size of the LSCF6428 cathode is smaller than that of the LSM821 cathode. Ohmic resistance and polarization resistance of the cathodes are analyzed by deconvoluting the electrochemical impedance spectroscopy (EIS) results. The LSCF6428 cathode has much smaller resistance than the LSM821 cathode. Moisture produces a larger effect on the ohmic resistance and polarization resistance of the LSM821 cathode than on those of the LSCF6428 cathode. More chromium diffuses from the interconnect to the cathode for both LSM821 and LSCF6428 electrodes thermally treated in moist air. Based on the structure, elemental distribution, and EIS analysis, the interaction mechanisms between the electrodes and the AISI 441 alloy interconnect are proposed.     

Chronoamperometry of prussian blue films on ITO electrodes: ohmic drop and film thickness effect

The chronoamperograms associated with the reduction of prussian blue films deposited onto indium tin oxide (ITO) electrodes to the Everitt’s salt form, are influenced by the ohmic drop effect. These chronoamperometric curves have been simulated by means of a numerical finite difference model which is able to explain their shape and their dependence on the thickness of the film and on the uncompensated resistance. An analytical expression which describes the dependence of current against time at initial times considering the ohmic drop effect has also been proved when applied to these chronoamperometric curves at short times.     

Effects of Nafion impregnation on performances of PEMFC electrodes

The effect of Nafion loading on the electrode polarization characteristics of a conventional proton exchange membrane (PEM) fuel cell electrode has been investigated in terms of both H₂/O₂ and H₂/air performance. Correlation of Nafion loading with the activation polarization characteristics shows an initial increase of activity upto a loading of 1.3 mg/cm² followed by a more gradual change with maxima at 1.9 mg/cm² for both oxygen and air. This trend correlated well with the decrease in charge transfer resistance and increase in the electrochemically active surface area. The contributions to the linear ohmic polarization region of both the H₂/O₂ and H₂/air performance are predominantly from ionic resistance as well as diffusional contributions in the catalyst layer. Among all the polarization losses those due to mass transport were the highest. Fits using a thin film agglomerate model showed a rapid increase in the film thickness with Nafion loading in the pores of the carbon of the catalyst layer followed by an equilibrium of 800 Å thickness at a Nafion loading of 1.9 mg/cm². Further additions caused deeper penetration of this Nafion film into the catalyst layer increasing the diffusional pathways for the reactant gases. These results correlate well with the mass transport characteristics in O₂ and air as well as morphological characterization of the electrode based on SEM and pore volume distributions.