Paper‐Fibres Used as a Pore‐Former for Anode Substrate of Solid Oxide Fuel Cell

Paper‐fibres are studied for use as a pore‐former to produce gas channels in the anode substrates of solid oxide fuel cells (SOFCs). These fibres produce cylindrical pores within the anode substrate, which are different from the pores formed by the conventional pore‐formers such as wheat flour and graphite. The cylindrical pores make it easier to connect each other to form continuous pathways for rapid gas diffusion. Paper‐fibres can create more open porosity than the same amount of flour. The application of the paper‐fibres significantly improves the cell performance by enhancing the gas diffusion process. The anode‐supported YSZ film cells with 5 wt.‐% and 10 wt.‐% paper‐fibres exhibit maximum power densities of 0.72 and 1.06 W cm^–2, respectively, using hydrogen as fuel and ambient air as oxidant at 800 °C.     

Understanding Water Transport in Polymer Electrolyte Fuel Cells Using Coupled Continuum and Pore‐Network Models

Water management remains a critical issue for polymer electrolyte fuel cell performance and durability, especially at lower temperatures and with ultrathin electrodes. To understand and explain experimental observations better, water transport in gas diffusion layers (GDLs) with macroscopically heterogeneous morphologies was simulated using a novel coupling of continuum and pore‐network models. X‐ray computed tomography was used to extract GDL material parameters for use in the pore‐network model. The simulations were conducted to explain experimental observations associated with stacking of anode GDLs, where stacking of the anode GDLs increased the limiting current density. Through imaging, it is shown that the stacked anode GDL exhibited an interfacial region of high porosity. The coupled model shows that this morphology allowed more efficient water movement through the anode and higher temperatures at the cathode compared to the single GDL case. As a result, the cathode exhibited less flooding and hence better low temperature performance with the stacked anode GDL.     

Effects of Anode Microstructure on Mechanical and Electrochemical Properties for Anode‐Supported Microtubular Solid Oxide Fuel Cells

The effects of anode microstructure on mechanical and electrochemical properties were investigated for anode‐supported microtubular solid oxide fuel cells (SOFCs). The anode microstructures can be varied by the change in pore formers. For example, the acrylic resin pore former was burnt more rapidly at lower temperature than the graphite pore former during sintering. The acrylic resin pore former can introduce macropores with a diameter of several micrometers in nickel–yttria‐stabilized zirconia (Ni–YSZ) anode. The walls of the macropores were packed with the nickel and YSZ particles. Although the Ni–YSZ anode microtube using the 10 wt% acrylic resin pore former was compatible with high porosity and mechanical strength, the maximum fuel utilization was limited to 72%. On the other hand, the graphite pore former can produce a relatively uniform distribution of micropores with a diameter of several hundred nanometers. The mechanical strength was reduced with a rise in porosity for the Ni–YSZ microtube using the graphite pore former in comparison with the acrylic resin. However, a high fuel utilization of 93% was realized for the microtubular SOFCs using the 10 wt% graphite pore former in spite of lower porosity than the acrylic resin. The selection of a pore former is important to obtain higher power generation efficiency for anode‐supported microtubular SOFCs.     

造孔剂含量对固体氧化物燃料电池阳极的结构与电性能的影响

通过向阳极添加造孔剂（PMMA）改善阳极的微观结构,研究不同含量的造孔剂（PMMA）对阳极的显微结构、电性能的影响。利用SEM、电化学1二作站等测试手段对单电池的结构和电性能进行了表征。研究结果表明,添加7wt．％的PMMA造孔剂制备的单电池,阳极的孔隙率高,阳极中的气孔分布均匀,结构规整,降低了燃料气的传输阻力,提高了三相反应界面,获得了良好的电性能。以H2＋3％H：0为燃料气,在750℃下单电池的开路电压（OCV）为1．08V、最大功率密度为0．82W／cm2、欧姆阻抗为0．20Ω·cm2、两极阻抗为0．53Ω·cm2。     

The effect of vapor-grown carbon fiber as an additive to the catalyst layer on the performance of a direct methanol fuel cell

Vapor-grown carbon fibers (VGCFs) were added to the anode catalyst layer of a direct methanol fuel cell to improve the cell performance through structural modification of the catalyst layer. The amount of VGCF varied up to 6 wt.% with respect to the weight of the PtRu black catalyst that was used. A catalyst layer with 2 wt.% VGCF loading showed the best cell performance. The electrodes that included the catalyst, VGCF, and gas diffusion layer, were directly examined by electron microscopic analyses. Electrochemical methods, such as cyclic voltammometry and impedence analysis, were applied to investigate the actual role of VGCF in the electrode. The porosity of the catalyst layer was increased by the addition of the fibers. This was clearly observed in pore diameters less than 1 μm. Sub-micron pore diameters are significant as they relate to micro-diffusive transport, compared to the macro-diffusion experienced by the large pores in the GDL. However, improved mass transport was only observed for 2 wt.% VGCF loading, probably due to insufficient optimization of the cell design. Microstructural and electrochemical analyses indicated that the improved performance was mainly ascribed to an increased electrochemically active surface area of the catalyst.     

Innovative strategy for designing proton conducting ceramic tapes and multilayers for energy applications

Aim of this work is to present, for the first time, the use of Dynamic Mechanical Analysis as a tool to characterize the thermo-mechanical behavior of green tapes defining the process conditions for the subsequent lamination step. This method was applied on tapes of protonic conductors, key-materials for different applications in the energy sector, from gas separation membranes to solid oxide fuel cells and electrolyzers. The pore former (rice starch) content was found to considerably affect the thermomechanical behavior (elastic and storage moduli, elongation to break, viscosity) of the tape and therefore the lamination process. The temperature required for a proper lamination increases from 50 up to 75 °C passing from the system without rice starch to the one with the highest pore former amount. This work identifies for the first time an optimal lamination viscosity (10¹⁰ Pa s), regardless the tapes formulation, required for a suitable adhesion among the layers.     

Effect of diffusion-layer porosity on the performance of polymer electrolyte fuel cells

The gas-diffusion layer (GDL) influences the performance of electrodes employed with polymer electrolyte fuel cells (PEFCs). A simple and effective method for incorporating a porous structure in the electrode GDL using sucrose as the pore former is reported. Optimal (50 w/o) incorporation of a pore former in the electrode GDL facilitates the access of the gaseous reactants to the catalyst sites and improves the fuel cell performance. Data obtained from permeability and porosity measurements, single-cell performance, and impedance spectroscopy suggest that an optimal porosity helps mitigating mass-polarization losses in the fuel cell resulting in a substantially enhanced performance.     

Improvement of Ni-Cermet performance of protonic ceramic fuel cells by catalyst

Generally, the NiO composite anode becomes porous after reduction. To infiltrate additional catalysts such as Pd into the NiO-composite anode before reducing NiO to Ni, a porous NiO composite anode for protonic ceramic fuel cells (PCFCs) was fabricated in this study. The porous NiO composite was fabricated by adding graphite as a pore former along with CuO as a sintering agent. The addition of graphite increased the porosity of the NiO composite anode but resulted in poor sinterability, which was addressed by adding CuO as a sintering agent to the NiO composite anode. The Pd catalyst was added to the NiO-composite anode before reducing NiO to Ni. The composite anode for PCFC with three components, namely Ni, protonic ceramics, and a Pd catalyst, was obtained by reducing NiO to Ni during the measurement. The addition of the Pd catalyst improved the anode performance in methane fuel and hydrogen fuel by enhancing the catalytic activity for the electrochemical reaction on the surface.     

Three-dimensional random resistor-network model for solid oxide fuel cell composite electrodes

A three-dimensional reconstruction of solid oxide fuel cell (SOFC) composite electrodes was developed to evaluate the performance and further investigate the effect of microstructure on the performance of SOFC electrodes. Porosity of the electrode is controlled by adding pore former particles (spheres) to the electrode and ignoring them in analysis step. To enhance connectivity between particles and increase the length of triple-phase boundary (TPB), sintering process is mimicked by enlarging particles to certain degree after settling them inside the packing. Geometrical characteristics such as length of TBP and active contact area as well as porosity can easily be calculated using the current model. Electrochemical process is simulated using resistor-network model and complete Butler-Volmer equation is used to deal with charge transfer process on TBP. The model shows that TPBs are not uniformly distributed across the electrode and location of TPBs as well as amount of electrochemical reaction is not uniform. Effects of electrode thickness, particle size ratio, electron and ion conductor conductivities and rate of electrochemical reaction on overall electrochemical performance of electrode are investigated.     

The rapid one-step fabrication of bilayer anode for protonic ceramic fuel cells by phase inversion tape casting

The bilayer anode fabricated by phase inversion tape casting has an excellent microstructure for protonic ceramic fuel cell compared with the dry pressing method. The large diameter and straight hole structure facilitates the fuel gas transportation thus eliminates the concentration polarization loss. But a dense skin layer (70 μm) results in a power density of only 150 mWcm⁻² at 600 ℃. The anode added with 10 wt% corn starch could eliminate the skin layer, but cause a mismatch between electrolyte suspension and anode. To resolve the mismatch between electrolyte and anode, an anode functional layer (AFL) is employed with different content of corn starch (10−40 wt%). Finally, the single cell with optimized bilayer anode has a maximum power density of 574 mWcm⁻² at 650 ℃. This work provides an easy and rapid method for the preparation of planar protonic ceramic fuel cell with satisfactory performance at intermediate temperature range.     

Structure investigations of SOFC anode cermets Part I: Porosity investigations

The number of electrolyte/metal/pore three-phase boundaries in the SOFC anode cermet was determined in a cross-section photo according to the intercepted-segment method and given as the volume concentration of the electrochemically active phase boundary. The apparent geometrical exchange current density of hydrogen oxidation served as a relativizing measure of electrochemical activity. A direct correlation was found between the number of optically visible three-phase boundaries of electrolyte/nickel agglomerates/gas pores' in the unit volume of the electrode and the apparent electrochemical exchange current density. The electrode was also analysed porosimetrically by a standard porosimetry method (MSP) and the porosimetric curves were compared with the optical/electrochemical measurements of the same electrode. This comparison revealed that the sum of the integrated volume of nickel agglomerates and the single nickel particles in contact with the electrolyte give the total amount of the active nickel volume whose surface is directly proportional to the amount of optically visible three-phase boundaries in the unit volume.     

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。     

Fabrication of continuous linear pores in an SOFC anode using unidirectional carbon fibers as sacrificial templates

We describe the manufacturing of a solid-oxide fuel cell anode of NiO−8 mol.% Y₂O₃-stabilized ZrO₂ with micro-scale continuous linear pores (CLPs) and nano-scale interparticle pore structures, achieved through thermal decomposition of unidirectional amorphous carbon fibers. The CLP structure prepared by this sacrificial templating method is characterized by its controllable uniform size, a tortuosity (ie, uniformity) of 1.003, and a coefficient of variation of 0.59. These highly regular CLPs are expected to minimize Knudsen diffusion, resulting in enhanced mass transport of hydrogen gas at the active sites, known as triple-phase boundary sites. Simulations using the Lattice Boltzmann Method (LBM) were used to determine the mass transport in the systems. An optimum diameter of 3 µm and an interparticle pore size of 185 nm was shown to maximize the acceleration of mass transport of H₂ and maintain the number of TPB sites to minimize concentration overpotential. Thus, the proposed porous design can increase the energy efficiency of a solid-oxide fuel cell primarily by reducing the concentration overpotential.     

Rationally structuring proton-conducting solid oxide fuel cell anode with Ni metal catalyst and porous skeleton

In response to the urgent demand for highly active anodes for lower-temperature proton-conducting solid oxide fuel cells (H–SOFCs) and with the aim to explore the function of metal catalysts and porous skeletons, two series anode-supported BaZr_0.1Ce_0.7Y_0.2O_3-δ (BZCY)-based single cells with varying Ni catalysts and pore formers were assembled and evaluated comparably. In the exploration of Ni catalyst variable, the NBZCY65-35-20SS (65 wt% NiO in NiO-BZCY prepared with adding 20 wt% starch) anode possesses the highest performance, for the 1:1 vol ratio of Ni-BZCY could offer the maximum effective triple-phase boundary (TPB) area in the prerequisite of having abundant pores achieved with the 20 wt% pore former as a constant. In addition, both the NBZCY65-35-15SS and NBZCY65-35-25SS anode demonstrate inferior electrochemical properties separately due to the inadequate reducing gas transmission channels and reaction sites. The BZCY cell assembled with NBZCY65-35-20SS reveals an excellent performance, in which the peak power densities (PPDs) were 660, 539, 413, 272 mW cm⁻² and the polarization resistances (R_P) were 0.061, 0.126, 0.28, 0.652 Ω cm² at 700, 650, 600, 550 °C, respectively. NBZCY65-35-20SS, which has both a superior TPB area and a fine porous anode skeleton, is a preferable option for anode-supported H–SOFCs. On the whole, the scientific regulations governing metal catalysis and pore-forming could be beneficial to the architecture of fine H–SOFC anode structures.     

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.     

Modeling and Optimization of Anode‐Supported Solid Oxide Fuel Cells on Cell Parameters via Artificial Neural Network and Genetic Algorithm

An artificial neural network (ANN) and a genetic algorithm (GA) are employed to model and optimize cell parameters to improve the performance of singular, intermediate‐temperature, solid oxide fuel cells (IT‐SOFCs). The ANN model uses a feed‐forward neural network with an error back‐propagation algorithm. The ANN is trained using experimental data as a black‐box without using physical models. The developed model is able to predict the performance of the SOFC. An optimization algorithm is utilized to select the optimal SOFC parameters. The optimal values of four cell parameters (anode support thickness, anode support porosity, electrolyte thickness, and functional layer cathode thickness) are determined by using the GA under different conditions. The results show that these optimum cell parameters deliver the highest maximum power density under different constraints on the anode support thickness, porosity, and electrolyte thickness.     

PMMA含量对水系流延IT-SOFC性能的影响

通过向阳极添加单一分散性的球形造孔剂PMMA改善阳极的微观结构,研究不同含量的PMMA对阳极的孔隙率、显微结构、电性能的影响。文中分别制备了造孔剂(PMMA)含量分别为6wt.%、8wt.%、10wt.%和12wt.%四种阳极材料的单电池,通过测试阳极还原前的开口气孔率分别为17vol.%,22.4vol.%,30.6vol.%和42.1vol.%;单电池的最大功率密度分别为0.66W/cm2、0.78W/cm2、1.15W/cm2和1.01W/cm2;极化电阻分别为1.12Ω.cm2、1.03Ω.cm2、0.88Ω.cm2和1.02Ω.cm2。实验结果表明:以单一分散性的球形PMMA为SOFC阳极材料的造孔剂,其最佳添加量为10wt.%,所制备的单电池可以获得最佳的电化学性能,即以3%H2O+H2为燃料气,750℃下,单电池的开路电压(OCV)为1.01V,最大功率密度为1.15W/cm2,极化电阻为0.88Ω.cm2。     

Enhanced Performance of Solid Oxide Fuel Cell by Manipulating the Orientation of Cylindrical Pores in Anode Substrate

Effect of the orientation of cylindrical pores within an anode has been studied on the performance of anode‐supported solid oxide fuel cell (SOFC). Paper‐fibers are used as pore‐former and highly oriented cylindrical pores are formed within the anode prepared by uniaxial compaction. A thick anode brick is fabricated followed by cutting in different directions to obtain anode substrates with desirable orientation of pores. When the orientation of cylindrical pores is perpendicular to the anode surface, the gas transport is significantly improved so that the reduction rate of the NiO/YSZ anode is considerably accelerated and the cell concentration polarization is minimized. The corresponding single cell exhibits a maximum power density as high as 1.54 W cm^–2 in hydrogen and 0.90 W cm^–2 in nitrogen diluted methane at 800 °C. The result indicates that the output performance of anode‐supported cells could be significantly improved by manipulating the orientation of pores.     

Analysis of Design Parameters in Anode‐Supported Solid Oxide Fuel Cells Using Response Surface Methodology

Achieving high performance from a solid oxide fuel cell (SOFC) requires optimal design based on parametric analysis. In this paper, design parameters, including anode support porosity, thicknesses of electrolyte, anode support, and cathode functional layers of a single, intermediate temperature, anode‐supported planar SOFC, are analyzed. The response surface methodology (RSM) technique based on an artificial neural network (ANN) model is used. The effects of the cell parameters on its performance are calculated to determine the significant design factors and interaction effects. The obtained optimum parameters are adopted to manufacture the single units of an SOFC through tape casting and screen‐printing processes. The cell is tested and its electrochemical characteristics, which show a satisfactory performance, are discussed. The measured maximum power density (MPD) of the fabricated SOFC displays a promising performance of 1.39 W cm^–2. The manufacturing process planned to fabricate the SOFC can be used for industrial production purposes.     

A Direct Ammonia Fuel Cell Using Barium Cerate Proton Conducting Electrolyte Doped With Gadolinium and Praseodymium

The performance characteristics of fuel cells based on proton conducting BaCeO₃ solid electrolyte doubly doped with gadolinium and praseodymium are reported. The amount of praseodymium doping is systematically varied in order to optimize the fuel cell performance. Fuel cells incorporating the optimum amount of praseodymium exhibit power density levels enhanced by a factor of three, compared to those incorporating undoped BaCeO₃. The performance of the fuel cell is essentially the same irrespective of the fuel used. However, the performance of the fuel cell is slightly better in hydrogen than in ammonia. Nevertheless, fuel cells operated in ammonia show a greater decrease in peak power density with decreasing temperature than those operated in hydrogen. This behaviour suggests that alternative anode materials need to be utilized at lower operating temperatures.