Fabrication and Characterization of Ni/ScSZ Cermet Anodes for Intermediate Temperature SOFCs

Solid oxide fuel cells (SOFCs) are of increasing interest as low emission, high efficiency, energy conversion devices for the production of electricity, and in some cases heat, from a wide range of fuels. In general, a porous cermet of nickel/ytrria-stabilized-zirconia (Ni/YSZ) is used as an anode with a dense electrolyte of ytrria stabilized zirconia (YSZ), enabling operation at temperatures above around 750 oC. Nevertheless, operating at high temperature leads to various problems such as metal corrosion, electrode sintering, and unwanted interfacial diffusion in the cell. In this regard, cermet anodes such as nickel/samarium-doped-ceria (Ni/SDC) and nickel/gadolinium-doped-ceria (Ni/CGO) have been proposed as alternative anodes to be operated with SDC and CGO electrolytes at intermediate temperatures (< 700 oC), respectively. However, less attention has been given to nickel/scandia-stabilized-zirconia (Ni/ScSZ) cermet anodes as alternatives for intermediate temperature SOFCs, despite the fact that the ScSZ electrolyte exhibits a higher ionic conductivity compared to other zirconia electrolytes, which may offer some advantages, especially at lower temperature. Furthermore, Ni/ScSZ anodes have shown improved tolerance towards carbon deposition and sulphur poisoning in addition to improved durability when compared to Ni/YSZ anodes. To date there have only been limited studies into the relationship between the materials used, the processing conditions, and the properties and performance of Ni/ScSZ anodes. This thesis is therefore aimed at (i) studying the optimum fabrication conditions and properties of Ni/ScSZ anodes, (ii) investigating the effect of ingredients such as binder content, solvent type and solid content in the ink formulation on the rheological properties of NiO/ScSZ inks and their applicability for screen-printing and (iii) relating the rheological properties of NiO/ScSZ inks to the performance and properties of the resultant anode films. A large part of the work is focussed on the fabrication and rheological properties of NiO/ScSZ screen-printing inks. The properties are linked to the particle network strength within the formulated inks and relate to the microstructure, mechanical strength, electrical performance and electrochemical performance of the resultant anode films. Overall, Ni/ScSZ anodes having 40 vol% Ni were found to be optimum in terms of both electronic conductivity and electrode polarization resistance. The anode exhibited improved tolerance towards carbon deposition compared to Ni/YSZ at intermediate temperature (700 oC). The effects of binder and solid content on the rheological properties of NiO/ScSZ screen-printing inks were studied by evaluating the thixotropic properties, yield stress and viscoelastic properties of the inks. The study indicated improved thixotropic properties, yield stress and particle network strength within the inks as the binder and solid content increased. These improved properties can be related to better particle bridging within the inks. A percentage ink recovery of 40 to 65 % was determined sufficient for the production of quality films with minimum defects. From the study, inks having 26 vol% solid with 3 wt% binder or 28-30 vol% solid with 2 wt% binder were determined as optimum for screen-printing using squeegee load, squeegee length, printing speed, snap-off and screen type of 6 kg, 5 in, 0.02 m/s, 2 mm and 325, respectively. Furthermore, films fabricated using these inks revealed improved particle connectivity, higher electronic conductivity, lower electrode polarization resistance and improved mechanical hardness. The solvent type was determined to have only a small impact compared to the binder and solid contents in the inks

Pengoptimuman proses penyemperitan gentian karbon terkisar dan polipropilena bagi komposit polimer pengalir

Proses penyemperitan merupakan salah satu proses pra-pencampuran yang dapat membantu meningkatkan tahap serakan bahan pengalir dalam komposit polimer pengalir (CPC). Tahap keberaliran elektrik dilihat tidak begitu memuaskan walaupun telah melalui proses serakan melalui pengacuan mekanik. Kajian ini dijalankan bagi mengoptimumkan proses penyemperitan bahan gentian karbon terkisar (MCF) dan polipropilena (PP) iaitu suhu penyemperitan dan halaju putaran melalui kaedah reka bentuk eksperimen (Taguchi). Susunan orthogonal Taguchi L9 digunakan bagi menentukan aras yang paling optimum serta menjalankan analisis varian bagi memperoleh nilai keberaliran elektrik yang paling baik. Pengoptimuman parameter pada suhu penyemperitan 210ºC hingga 250ºC dan halaju putaran 50 hingga 90 rpm menggunakan komposisi bahan sebanyak 80 % bt. MCF dan 20 % bt. PP dengan tahap keberaliran elektrik meningkat pada tahap maksimum 3.67 S/cm. Pengoptimuman parameter ini menunjukkan bahawa reka bentuk eksperimen yang terhasil mampu menghasilkan nilai keberaliran elektrik yang tinggi serta mempunyai sifat mekanik yang baik

THERMAL EXPANSION BEHAVIOR OF THE Ba0.2Sr0.8Co0.8Fe0.2O3−δ (BSCF) WITH Sm0.2Ce0.8O1.9

Nanostructured perovskite oxides of Ba0.2Sr0.8Co0.8Fe0.2O3−δ (BSCF) were synthesized through the co-precipitation method. The thermal decomposition, phase formation and thermal expansion behavior of BSCF were characterized by thermogravimetric analysis, X-ray diffraction (XRD), and dilatometry, respectively. XRD peaks were indexed to a cubic perovskite structure with a Pm3m (221) space group. All the combined oxides produced the desired perovskite-phase BSCF. The microstructures were characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The TEM analysis showed that BSCF powders had uniform nanoparticle sizes and high homogeneity. The cross-sectional SEM micrograph of BSCF exhibited a continuous and no delaminated layer from the electrolyte-supported cell. The thermal expansion coefficient (TEC) of BSCF was 16.2×10-6 K-1 at a temperature range of 600°C to 800°C. Additional experiments showed that the TEC of BSCF is comparable to that of Sm0.2Ce0.8O1.9 (SDC) within the same temperature range. The results demonstrate that BSFC is a promising cathode material for intermediate-temperature solid-oxide fuel cells

Enhanced Oxygen Reduction Reaction of LSCF Cathode Material Added with NiO for IT-SOFC

La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) is one of the mixed ionic electronic conductors that could be feasibly used in an intermediate temperature solid oxide fuel cell (IT-SOFC). In this study, LSCF and NiO were prepared using a modified Pechini method and calcined at three different temperatures ranging from 600 °C to 900 °C. The prepared LSCF was added with 5% NiO (denoted as LSCF-NiO) as cathode material. The physical and electrical properties of the prepared cathode were investigated. X-ray diffraction data revealed that at calcination temperatures of 600 °C–900 °C, NiO and LSCF maintained their phases and conformed the cubic structure for NiO and orthorhombic structure for LSCF. The calcination temperature showed significant influence on the particle size of the prepared LSCF-NiO, as depicted by scanning electron microscopy (SEM), and all the powders reached a nanoscale size. The SEM cross section of LSCF-NiO layer on gadolinium-doped cerium electrolyte showed an acceptable percentage of cathode porosity and good adhesivity at cathode/electrolyte interface. Energy dispersive X-ray analysis further verified the purity of the samples. Brunauer–Emmett–Teller surface area analysis was conducted, and the results revealed a trend of decreased surface area with an increase in calcining temperature. At an operating temperature of 800 °C, the electrochemical impedance spectroscopic results showed that LSCF-NiO 800 had a low Rp of 0.07 Ω cm2, and its Ea was found to be 159.5 kJ/mol, indicating that LSCF-NiO 800 is fit to be used as cathode material in IT-SOFC application

Challenges in Fabricating Solid Oxide Fuel Cell Stacks for Portable Applications: A Short Review

Despite being the most efficient and quiet operation type of fuel cells, solid oxide fuel cells (SOFCs) deal with several constraints in terms of fabrication cost, material selection and durability issues due to their high operating temperature. The high operating temperature of SOFCs limits their stationary and large-scale applications. Moreover, these constraints restrict the commercialization of portable SOFCs. Therefore, the operation temperature of SOFCs must be reduced to overcome the aforementioned problems. However, this task is challenging because the operation temperature mainly affects the material preparation and the stack design to produce the electrical power needed for small-scale applications. This paper provides an overview of the challenges faced by each component such as the materials, the design of stack, fabrication cost and related research in fabricating high power SOFC stacks

Towards the 3D Modelling of the Effective Conductivity of Solid Oxide Fuel Cell Electrodes - Validation against experimental measurements and prediction of electrochemical performance

© 2015 Elsevier Ltd. All rights reserved.The effective conductivity of thick-film solid oxide fuel cell (SOFC) electrodes plays a key role in their performance. It determines the ability of the electrode to transport charge to/from reaction sites to the current collector and electrolyte. In this paper, the validity of the recently proposed 3D resistor network model for the prediction of effective conductivity, the ResNet model, is investigated by comparison to experimental data. The 3D microstructures of Ni/10ScSZ anodes are reconstructed using tomography through the focused ion beam and scanning electron microscopy (FIB-SEM) technique. This is used as geometric input to the ResNet model to predict the effective conductivities, which are then compared against the experimentally measured values on the same electrodes. Good agreement is observed, supporting the validity of the ResNet model for predicting the effective conductivity of SOFC electrodes. The ResNet model is then combined with the volume-of-fluid (VOF) method to integrate the description of the local conductivity (electronic and ionic) in the prediction of electrochemical performance. The results show that the electrochemical performance is in particular sensitive to the ionic conductivity of the electrode microstructure, highlighting the importance of an accurate description of the local ionic conductivity

Fabrication of high-quality electrode films for solid oxide fuel cell by screen printing: A review on important processing parameters

Summary Solid oxide fuel cell (SOFC) is known as the most efficient fuel cell, with an efficiency of 60% in converting fuel to electricity and up to 80% in fuel to energy conversion (including heat). A SOFC consists of three primary components, namely, anode, electrolyte and cathode. Given the demand for reducing the operating temperature below 800 C, not only thin electrolytes have become a necessity for their ability to reduce ohmic losses but also high-quality porous electrode (anode and cathode) films for their ability to accelerate electrochemical reactions with fuels. In this context, screen printing is known for its capability to form high-quality porous electrode films in a cost-effective manner. In addition, screen printing offers fabrication-related parameters that can be easily manipulated to produce different film qualities depending on the requirements which have been explored in various applications. However, screen printing is only utilised in SOFC application as a fabrication tool to produce electrode films, neglecting the effects of its fabrication-related parameters on electrode performance, as indicated by the limited number of related works. Despite limited resources, this study aims to review the fabrication-related parameters in producing SOFC electrodes through screen printing and their effects on electrochemical performance. The parameters at different stages (ie, prior, during and post printing), including ink formation, printing numbers and sintering, are extensively reviewed. To the best of our knowledge, this study is not only the first review that discusses the effects of screen-printing fabrication-related parameters on electrode potentials but also offers suggestions on future directions regarding these parameters towards the improvement of SOFC performance. Novelty Statement Among all thin-film fabrication methods, screen printing is known for its capability to form homogenous-porous SOFC electrode films; however, the processing parameters of screen printing at three primary stages (prior, during, and postprinting) are rarely explored. As such, the current paper highlights important parameters in screen printing, such as ink rheology, printing number, and sintering, to contribute to the understanding of the influence of these parameters on SOFC electrode film quality and their effects on electrochemical performance

ELECTRICAL CONDUCTIVITY MODELS OF DIE CONFIGURATION FOR POLYPROPYLENE-REINFORCED MILLED CARBON FIBRE

Extrusion is one of the pre-mixing processes that produce a highly conductive polymer composite material when the appropriate die geometry is applied. However, the development of a conductivity model is still in the preliminary stage because the current model being used is unable to accurately predict the electrical conductivity. A generally effective media model and a modified fibre contact model were adapted in this study. To obtain a good agreement between the predicted model and the experimental data, the FCM model was modified by adapting the extrusion parameters, including the shear rate and rotational speed, into the equation. This was known as the MFCM with shear rate. This modified MFCM with shear rate showed a good correlation with the r-square of 0.99, as evidenced by the minimized gap between the predicted and experimental data. The MCF/PP composite produced using the rod die had the highest electrical conductivity of 3.7 S/cm as the fibre was aligned in the converging dies than in the sheet dies, which had an electrical conductivity of 1.3 S/cm

A short review on the modeling of solid-oxide fuel cells by using computational fluid dynamics: assumptions and boundary conditions

Performance tests are vital for the development of solid-oxide fuel cells (SOFCs) and can help determine the potential of developed SOFCs. However, the challenges in performing these tests such as cost, time, and safety limit the development of SOFCs. Computational fluid dynamics (CFD) can be used to numerically predict the performance of developed SOFCs. CFD methods enable the exploration of many design and operational parameters that are difficult to assess experimentally. This paper focuses on the assumptions and boundary conditions used to model the SOFC stack by using a CFD method. Through the discussions, we briefly explain several assumptions that are commonly found in SOFC modeling. These assumptions are important because they can influence the modeling processes and parameters required for simulations. The boundary conditions required for SOFC modeling are then described to provide an overview on how SOFC operations are incorporated into the model and simulations. Our results can help elucidate the significance of assumptions and boundary conditions used in the CFD modeling of SOFCsÂ