Anode Paste Research Articles

Facile preparation of patterned anode substrate for solid oxide fuel cells by direct-writing 3D printing technology

3D printing has revolutionized solid oxide fuel cell (SOFC) by introducing complicated structure and new functionalities that cannot be realized using traditional preparation process. This work provides guide and insights into the application of direct-writing 3D printing to fabricate the patterned electrolyte|electrode interface for the anode-supported SOFC using 3 mol % yttria-stabilized zirconia-nickel oxide (3YSZ-NiO) as the anode substrate material. The parameters that affected the shaping and performance of SOFC including the rheological properties of 3YSZ-NiO anode paste, the printing parameters, and the heat treatment process of the pre-sintered anode substrate were evaluated. The prepared 3YSZ-NiO paste exhibited a typical shear-thinning characteristic, “yielding” solid-like behavior, and time- and temperature-independent stability performance, which ensured the paste suitable for extrusion while allowing the shape of the printed body to be maintained after extrusion during the direct-writing printing process. The as-prepared anode paste, combined with the obtained values of 0.1 MPa, 100 mm s−1, 100 rpm, and 0.3 mm for printing parameters of the extrusion pressure, the printing speed, the screw speed, and the nozzle diameter, respectively, as well as the devised heat treatment program for the pre-sintered 3YSZ-NiO anode substrate, successfully produced defect-free corrugated anode substrates. The cell made from the as-mentioned anode substrate showed an open circuit voltage of 1.02 V and a maximum power density of 619.44 mW cm−2 at 850 °C, which was comparable to the other identical cells fabricated by traditional and other 3D printing processes. Using the 3D printing process shown here, the preparation time of patterned anode substrate can be reduced and simplified.

Intelligent Online Inspection of the Paste Quality of Prebaked Carbon Anodes Using an Anomaly Detection Algorithm

Prebaked carbon anodes are a critical consumable in the aluminum electrolysis industry. Prebaked carbon anode paste is the intermediate product of the prebaked carbon anode, and its quality significantly impacts the prebaked carbon anode. Therefore, inspecting the quality of the prebaked carbon anode paste is essential. Currently, the quality inspection of the paste still relies on laboratory analysis or manual experience. A laboratory inspection cannot obtain results in real time, while manual inspection poses potential risks. To address these issues, an online intelligent inspection method for prebaked carbon anode paste based on an anomaly detection algorithm was proposed. Firstly, we acquired the temperature of the paste and the power of the kneading motor. Secondly, we transformed these time-series data into images using the Gramian Angular Field (GAF) technique and joined them to create the paste anomaly detection dataset. Thirdly, we trained a matched anomaly detection model based on the PatchCore algorithm. Finally, we compared two advanced models: HaloAE and TSRD. PatchCore performs best on our dataset with an AUC-ROC score of 0.9943, followed by HaloAE (0.9906) and TSRD (0.9811). Our proposed method enables on-time intelligent inspection of prebaked carbon anode paste quality. This eliminates the need for manual inspection, reduces labor requirements, and ensures worker safety.

Effect of Fabrication Processes on Electrochemical Performance of Metal-Supported Solid Oxide Fuel Cells

Metal supported solid oxide fuel cells (MSCs) which have the robust porous metal substrate are expected for the use in some mobilities. MSCs are prepared by various fabrication processes such as sputtering and screen printing. When the electrolyte layer is fabricated by the wet processes, the electrolyte must be sintered with metal substrate at a high temperature. However, it is difficult to obtain the dense electrolyte on porous metal substrate because sintering temperatures of an electrolyte ceramic and a metal material used in substrates are different. In this study, MSCs were fabricated on the sintered porous metal substrates by screen printing and spin coating with high-temperature sintering in reduced atmosphere. The electrochemical performance of MSCs strongly depended on the process conditions. MSCs were prepared by the process which is similar to the process for anode supported cell fabrication. Anode paste including nickel oxide and scandia stabilized zirconia with a weight ratio of 60/40 was coated on the green compact of substrate by screen printing. After drying, the electrolyte layer is coated on it by same procedure. Samples were heat-treated around 400 ºC for 1 h to degrease the organic compounds included in cells. These are finally sintered at 1100 – 1250 ºC in reduced atmosphere. The relationship between the electrochemical performance and the structural property were investigated.In order to obtain dense electrolyte, the shrinkage behavior of porous metal substrates is quite important. Large shrink of substrate can assist the densification of electrolyte in the sintering process. It was found that the degrease temperature affected the shrinkage behavior in sintering process. It was considered that the oxidation of metal particles in substrate affected on the shrinkage. When the sintering temperature were high to obtain the dense electrolyte, Fe and Cr diffused from the substrate to the anode layer. This metal elements diffusion strongly deteriorated the performance of the anode. Additionally, gas atmosphere also affected the metal elements diffusion. When the MSCs are prepared by wet process with high temperature sintering, controlling the sintering behavior of all components by adjusting the process parameter was quite important to low polarization resistance.

Influence of Atmosphere and Temperature on Polycyclic Aromatic Hydrocarbon Emissions from Green Anode Paste Baking.

Coal tar pitch, a well-known source of polycyclic aromatic hydrocarbons (PAHs), is used as a binder of petroleum coke in prebaked anodes used for electrolysis of aluminum. Anodes are baked up to 1100 °C over a 20-day period, where flue gas containing PAHs and volatile organic compounds (VOCs) are treated using techniques such as regenerative thermal oxidation, quenching, and washing. Conditions during baking facilitate incomplete combustion of PAHs, and due to the various structures and properties of PAHs, the effect of temperature up to 750 °C and various atmospheres during pyrolysis and combustion were tested. PAH emissions from green anode paste (GAP) dominate in the temperature interval of 251-500 °C, where PAH species of 4-6 rings make up the majority of the emission profile. During pyrolysis in argon atmosphere, a total of 1645 μg EPA-16 PAHs are emitted per gram of GAP. Adding 5 and 10% CO2 to the inert atmosphere does not seem to affect the PAH emission level significantly, at 1547 and 1666 μg/g, respectively. When adding oxygen, concentrations decreased to 569 μg/g and 417 μg/g for 5% and 10% O2, respectively, corresponding to a 65% and 75% decrease in emission.

Characterization of the compaction and frictional behaviors of the green anode paste at various temperatures

The aim of this work was to investigate the effect of the green anode paste temperature on its compaction behavior and on its frictional behavior with a steel mold during the anode forming process. An experimental campaign composed of monotonic compaction tests and friction tests was carried out at temperatures ranging from 130 °C to 170 °C. The compaction tests were performed with an instrumented thin-walled mold to characterize the axial and radial behaviors of the anode paste. The results highlight that the nonlinear behavior of the anode paste is significantly affected by temperature. Friction tests were performed under various normal loads to evaluate the static and kinetic friction coefficients of the paste/steel at different interface temperatures. The results bring a new insight into the behavior of the anode paste and pave the way for the development of a temperature-dependent material model of the anode paste compaction process.

Consumption of Anode Paste for Aluminum Production

Consumption of Anode Paste for Aluminum Production

Pre-baked anode structure and properties as a function of the Blaine number

Anode paste for pre-baked (PB) anodes represents a composite material based on calcined coke and coal tar pitch (used as a binder). A coke aggregate mix includes 3 to 5 fractions with a particle size of 12 mm down to 75 μm, and even less. Grain fractions larger than 0.2 mm form a sort of skeleton of the binder matrix and are prepared by crushing and screening. For filling the space between the grains, a dust fraction with particles less than 0.2 mm is used. The dust fraction constitutes up to 50% of the aggregate and can contain up to 70% of particles with a size of <75 μm. The dust fraction is prepared in a fine grinding circuit of ball mills. The dust fineness is determined based on the Blaine number (BN). The BN correlates with both Specific Surface Area (SSA) and Particle Size Distribution (PSD). High BN dust may account for up to 90% of the total aggregate surface area. If dust is too fine, it leads to a higher surface area of coke particles, which causes both higher particle reactivity and higher pitch demand. The above might result in higher carbon consumption. Therefore, it is important to keep a balance in terms of the particle size of the dust fraction. This paper discusses the PB anode structure and properties as a function of the BN. Lab anodes containing dust fractions with a BN of 2120, 2880, 3600, 3950 and 4700 were used for analysis. The paper presents obtained correlations between the BN and the physical, chemical and mechanical properties of PB anodes. An optimal BN of 3600 to 4200 was determined based on porosity tests of anode blocks with different BNs using mercury porosimetry; the above range can now be recommended for PB anode production.

Anode paste consumption for aluminum production

In Russia, most aluminum smelters are equipped with cells with self-baking anodes featuring an urgent problem of lowering the anode paste consumption, since the share of anode materials in the cost of aluminum varies from 8 to 20 %. To solve this problem, it is necessary to determine the anode paste demand. The method for calculating the specific anode paste consumption used at aluminum smelters has a poor accuracy. The paper discusses the main errors of this method, shows the stages of anode paste consumption calculation, assesses the adequacy of calculations and gives recommendations for method improvement. It is shown that in general the considered method adequately reflects carbon consumption processes, but the final result of calculations may differ significantly from the actual one. The values taken constant to simplify the calculation, in fact, may vary during the electrolysis, which leads to a significant change in the final result of calculations. For example, an increase in the CO2 fraction from 0.45 to 0.5 leads to a decrease in the anode paste consumption by 15.3 kg/tonAl. At the same time, it is known that the composition of anode gases changes sharply as the anode effect occurs: the CO2 fraction decreases, and the CO fraction increases. In summer, at high ambient temperatures, the proportion of both vaporized pitch and anodes with an increased surface temperature rise. As the latter changes to 0.25, the consumption increases by 6.6 kg/tonAl. The same applies to air oxidation. The number of depressurized cells may increase with the subsequent increase in the carbon consumption. It is necessary to pay attention to factors affecting the anode quality. Incorrectly selected particle size distribution or worn equipment may significantly degrade the anode quality and lead to an increase in carbon consumption. It is necessary to make adjustments to the calculation in order to consider the peculiarities of carbon monoxide formation properly.

Effect of Particle Size Distributions and Shapes on the Failure Behavior of Dry Coke Aggregates.

Carbon anodes participate in chemical reactions to reduce alumina in the Hall–Héroult process, of which coke aggregates make up a major part. The failure analysis of coke aggregates not only leads to a better understanding of the deformation mechanisms of anode paste under compressive loading but also can identify potential causes of structural defects in carbon anodes, such as horizontal cracks. The coke aggregates are composed of particles with different size distributions and shapes, which may strongly affect the failure behavior of the anode during compaction. In this paper, the effects of particle size distributions and shapes on the mechanical behavior and the failure of coke aggregates are investigated using the discrete element method modeling technique. The numerical results reveal that, although the mechanical behavior of coke mixtures is generally dependent on larger particles, the presence of fine particles in the coke aggregates reduces fluctuations in the stress–strain diagram. In addition, the rolling resistance model is employed as a parameter representing the effect of particle shape. It is shown that the rolling resistance model can be an alternative to the overlapped spheres model, which has a higher computational cost than the rolling resistance model. The second-order work criterion is used to evaluate the stability of the coke aggregates, the results of which indicate that the addition of fine particles as well as increasing the rolling resistance between the particles increases the stability range of the coke aggregates. Moreover, by using the analysis of micro-strain contour evaluations during the compaction process, it is shown that, both by adding fine particles to the coke mixture and by increasing the rolling resistance between the particles, the possibility of creating a compression band in the coke aggregates is reduced. Since the presence of the compaction bands in the anode paste creates an area prone to horizontal crack generations, the results of this study could lead to the production of carbon anodes with fewer structural defects.

Modeling of Thermo-Chemo-Mechanical Properties of Anode Mixture during the Baking Process.

In the Hall–Héroult process, prebaked carbon anodes are utilized to produce primary aluminium. The quality of the anode plays a crucial role in the efficiency of electrowinning primary aluminium. In the production of anodes, the anode baking is considered as the stage most frequently causing anode problems. During the baking process, the anode undergoes complex physicochemical transformations. Moreover, the anode at a lower position, imposed by loading pressures from upper anodes, will creep during this process. Thus, the production of high-quality anodes demands efficient control of their baking process. This paper aims to investigate the thermo-chemo-mechanical properties of the anode paste mixture at high temperatures. These properties include kinetic parameters of pitch pyrolysis such as the activation energy and the pre-exponential factor, the thermal expansion coefficient (TEC) and relevant mechanical parameters related to the elastic, the viscoelastic and the viscoplastic behaviours of the anode. For this purpose, experiments consisting of the thermogravimetric analysis, the dilatometry and the creep test were carried out. Based on the obtained results, the forementioned parameters were identified. Relevant mechanical parameters were expressed as a function of a new variable, called the shrinking index, which is related to the volatile released in open and closed pores of the anode. This variable would be used to highlight the chemo-mechanical coupling effect of the anode mixture. New insights into the phenomena such as the expansion due to the increase of the pore pressure and the chemical shrinkage of the anode during the baking process were also gained in this work. These investigations pave the way for modeling the thermo-chemo-poromechanical behaviour of the anode during the baking process.

The Study of Destructible Annealed Anode Paste in the Current CO<sub>2</sub>

The article is devoted to the study of destructibility of the annealed anode paste of different composition in the CO2 current. It is shown that the destructibility of CO2 current of petroleum coke is much less than that of pitch coke. The comparison of the qualitative characteristics of the anode paste on the basis of pitch coke and petroleum coke revealed the advantage of petroleum coke, due to its low sodium content. The average destructibility of the anode paste on the basis of different grades of cokes in the CO2 current is determined. The conducted semi-industrial tests were in good agreement with the laboratory studies.

Physical Property Evolution of the Anode Mixture during the Baking Process.

The Hall-Héroult process uses prebaked carbon anodes as electrodes. The anode’s quality plays a crucial role in the efficiency of the aluminium production process. During the baking process, the anode undergoes complex physicochemical transformations. Thus, the production of high-quality anodes depends, among others, on the efficient control of their baking process. This paper aims to investigate the evolution of some physical properties of the anode paste mixture during the baking process. These properties include the mass loss fraction, real and apparent densities, the ratio of apparent volume, the permeability, and porosities. For this purpose, experiments consisting of thermogravimetric analysis, dilatometry, air permeability, and helium-pycnometric measurements were carried out. The anode permeability at high temperatures was linked to the air permeability through a permeability correlator due to experimental limitations. Moreover, the real density at high temperatures was estimated by combining real densities of the coal tar pitch and coke aggregates. Different porosities, such as the open porosity and the closed porosity related to the pitch binder, were estimated by taking the permeability at high temperatures into account. In this context, the effect of the permeability correlator, which was introduced to link the permeability at high temperatures to the air permeability, was investigated through a sensitivity analysis. These results allow an estimation of the shrinking index, a new variable introduced to reflect the baking level of the anode mixture, which is linked to the volatile that is released in both open and closed pores. Afterwards, the pore pressure inside closed pores in the coal tar pitch was estimated. The obtained results highlight some new insights related to the baking process of the anode mixture. Moreover, they pave the way for better modeling of the thermo-chemo-mechanical behavior of anodes at high temperatures.

Development of composite electrode materials based on nickel oxide for additive manufacturing of fuel cells

In this work, a new composition of a paste based on nanosized nickel oxide (NiO) for jet-based 3D printing has been developed and investigated. The optimal parameters of anode paste with necessary viscosity and NiO loading have been found for printing by using custom made laboratory 3D printer. Experiments have been carried out to print two-and three-dimensional test objects using the developed ceramic paste; printing modes have been investigated to fabricate objects of a given shape and size with laser sintering of the deposited layers. Experiments have been conducted for the selection of optimal temperature conditions for the post-processing of the printed samples.

The use of aerodynamic turbo classifier in the production of calcined coke dust fraction

Anode paste for aluminum reduction cells represents a composite material consisting of calcined coke and coal tar pitch. A coke aggregate mix includes 3 to 7 fractions with a particles size of 12 mm down to 63 μm, and even less. Grain fractions larger than 0.2 mm are prepared by crushing and screening. Grain fractions form a sort of skeleton, which makes the aggregate more stable. For filling the space between the grains, a dust fraction with particles less than 0.2 mm is used. The dust fraction comprises up to 50% of the aggregate and can contain up to 75% of particles with a size of <63 μm. The dust fraction is prepared in a fine grinding circuit of ball mills. The separation (or classification) of particles from a gas-dispersed flow, which is generated in the ball mill, takes place in classifiers of various designs. Modern air classification systems use aerodynamic turbo classifiers providing for a wide range of classification. This paper discusses the use of turbo classifiers in the process of preparing dust for the anode paste used in aluminum reduction cells. It contains the results of simulating flow separation boundaries based on different classification conditions: air flow, angle of rotation of guide vanes and turbine RPM. Particle size distributions (PSDs) were obtained for those products that result from separating the gas-dispersed flow. They were obtained empirically, under shop floor conditions, at turbine RPMs of 150 and 200 rpm. Based on the said data, separation curves were plotted and 50-percent separation boundaries were calculated: 55 μm at 150 rpm and 35 μm at 250 rpm.

Analysis of the Anode Paste Charge Composition

Anode pastes are widely used in the production of ferroalloys and primary aluminum and in other processes in the metal and chemical industries. In consideration that their manufacturing involves dosing and mixing, the properties and performance of the commercial product are determined by the type of initial charge components and their ratio. This study analyzed the composition of the initial charge mixtures of two anode pastes. At the initial stage, the solid carbon filler was separated from the pitch binder. The filler separated was classified by the components based on the difference in their physical properties and then divided into size classes. A comprehensive thermal analysis of the initial samples and the filler fractions was performed to evaluate the thermal transformations in the samples and to determine the filler–binder ratio in the charge. The extraction results of individual pitch fractions in the corresponding solvents were considered in the analysis. The charge mixture for two anode paste samples was analyzed to determine the possibility of using this approach. This approach can be developed further for possible applications in metal industry enterprises and in laboratory studies.

Microextrusion printing for increasing electrode–electrolyte interface in anode-supported solid oxide fuel cells

Microextrusion printing is employed for increasing the interfacial area between the electrode and electrolyte of anode-supported solid oxide fuel cells (SOFCs) to enhance their electrochemical performance. A homogeneous anode paste is prepared as an extrusion material and its rheological properties are measured. Then, the printing resolution (line width of the printed paste) is evaluated by extruding the paste under various printing conditions to determine the appropriate stage speed and flow rate for cell fabrication. A mathematical model is also developed to predict the printing resolution from the printing parameters, and the experimental data of the line width agree well with the predicted values, particularly when the contact angle between the paste and substrate is considered. By varying a paste coverage area, two types of anode-supported SOFCs having different interfacial area enlargement ratios are prepared by extruding the anode paste onto the flat anode substrates. The electrochemical performance of the cell is improved by increasing the interfacial area; the current density at the terminal voltage of 0.7 V at 600 °C is 0.17 A cm−2 in a flat cell, whereas 0.32 A cm−2 in a patterned cell whose enlargement factor is 1.14 compared with that of the flat cell.

Study of Microextrusion Printing for Enlarging Electrode–Electrolyte Interfacial Area in Anode-Supported SOFCs

Anode ridge structures are fabricated by printing anode pastes having different rheological properties to increase the electrode–electrolyte interfacial area in anode-supported solid oxide fuel cells. Homogeneous anode pastes are prepared by varying solid loadings, then their rheological characteristics are evaluated. It is confirmed that the shear stress at the same shear rate increases with increasing solid loading. The prepared pastes are printed onto flat anode substrates and then the width and height of the printed pastes are observed before and after drying and after sintering. As increasing solid loading in the paste, not only the width and height become smaller and larger, respectively, but also the shrinkage rate of the aspect ratio becomes smaller. Consequently, a large extension of the interfacial area of about 285% is obtained by printing the paste with the solid loading of 80%.

Through Interface Optimization to New Generation of Robust Electrolyte Supported Cell with High Power Density

Although significant enhancement of cell performance has been achieved in recent years, there is still considerable opportunity for further improvement through the optimization of materials, interfaces and microstructure of electrodes combined with engineering focus on volume manufacturing processes, reproducibility and cost reduction. The developments of cells follow different directions in parallel. The first one represents G6 cell with new multilayer anode based on Ni/GDC cermet, which fuel electrode interfaces as well as chemical composition are improved for reducing activity against CH4 reforming using different catalytic active additives. The ASR values of G6 cell measured without additional cathode contact layer (CCL) are decreased up to 25 % in comparison to state-of-the-art G3 cell electrodes.The second development direction represents cell with the stateof the-art G3 cell electrodes, but with additional adhesion layer between substrate and air electrode.Third direction considers optimization of the cathode and anode pastes manufacturing technology with further increase of the three phase boundary length

Understanding the Role of Polymeric Binders in Water Based Lithium Ion Electrodes

Polymers find application in Lithium ion Batteries (LiB) as binders, providing cohesion in the dry electrode layer as well as adhesion to the current collector. They may also be added as a thickener to control the flow and hence the processing behavior of the paste. Furthermore, polymeric binders can act as dispersing agents for active material or carbon black particles, thus strongly determining the component distribution in dry electrodes. Despite the vast amount of research activities in the field of LiB, the contribution of the binder to the cell performance still remains elusive. Therefore, the adsorption of carboxymethylcellulose (CMC) on LiFePO4 and graphite particles as active material was investigated. The flow behavior of the electrode pastes as well as the electrical conductivity, mechanical properties and microstructure of corresponding dry electrodes were thoroughly characterized. Rheological measurements were carried out to study paste flow behavior as a function of CMC and solid particle concentration. The electrical conductivity of thin electrode layers was measured using the four point resistivity test. The adhesion force between electrode layer and current collector was determined via 90°-peel test. Thick electrode layers were prepared to investigate the cohesion in the electrode employing compression, bending and shear tests. Finally, the microstructure of the dry electrodes was characterize by means of scanning electron microscopy (SEM). Yield stress and viscosity exhibit a pronounced minimum with increasing CMC concentration for both the cathode and anode pastes, indicating that CMC acts as dispersing agent at low concentrations and as a thickener at higher concentration when the equilibrium adsorption concentration is exceeded. Furthermore, the degree of substitution of CMC have no significant influence on the slurry viscosity, whereas higher molecular weights and CMC concentrations lead to significantly higher viscosities. Electrical conductivity of thin electrode layers increases with increasing CMC concentration and yields a pronounced maximum when the equilibrium adsorption concentration is covered, suggesting a critical polymer concentration at which electrode components ideally distribute. In addition, mechanical tests show no clear dependence of the adhesion strength on CMC concentration, irrespective of the degree of substitution or molecular weight, i.e. it does not contribute to the adhesive strength. In contrast, the cohesive strength of the electrode layers increases with higher polymer concentration and molecular weight, but decreases with increasing degree of substitution. Micrographs of the electrode layers show an alignment of the particles for electrodes based on high molecular weight CMC. Preliminary measurements exhibit higher adhesion strength when adding styrene butadiene rubber (SBR), whereas cohesion strength and electrical conductivity significantly decreases. This demonstrates that the optimum CMC concentration has to be chosen according to the required specifications with respect to cohesion and electrical performance.

Influence of Different Acids on Si/C Anode Pastes pH Values and the Corresponding Electrochemical Performance in Li Ion Cells

Silicon particles are applied as anode active material in Li ion batteries to achieve higher energy densities. However, the usage of silicon particles today, comes along with increased aging and safety issues. The lithium uptake by the silicon particles during the charging process is accompanied by huge volume expansion (over 280%).1 Hence, the electrochemical performance suffers from ongoing capacity fading with each cycle due to contact loss between active material particles and the current collector. 2,3 To counter these issues different approaches have been done in literature including the reduction of the silicon particle size, the application of different binders, the use of silicon nanotubes, nanowires or core-shell particles.4,5,6 Acidic pretreatment is also well-known in the framework of surface functionalization of silicon particles. 7,8 In this work phosphoric, sulfuric, salt and oxalic acid were used to achieve better adhesion between active material particles and current collector. It also improves the electrochemical performance by lowering the discharge capacity fading. The pH value of the anode paste was varied to 3.6, 2.8 and 1.4, respectively, to investigate the effect on the adhesion between the silicon particles. To accelerate the reaction between the silicon particles and the appropriate acid the reaction conditions were optimized. The improvement in the electrochemical performance was coherent to the reaction progress, the period individually depended on the used acid. For all four acid types the application resulted in a decrease of discharge capacity fading. An increase of the anode paste viscosity could be observed during the reaction time. Phosphoric acid turned out to be an appropriate acid for the application in the anode paste as it reached the highest capacity retention after 100 cycles at 1.0C and the best reproducibility. Oxalic acid showed the worst performance as a high ratio of parasitic reactions could be observed in the first charge cycle at 0.1C. With lower pH value bigger agglomerations between silicon particles were shown in SEM images simultaneously deteriorating the performance. The best performance was achieved with a pH value of 3.6 for every acid type.