Solid oxide fuel cell

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A solid oxide fuel cell (or SOFC ) is an electrochemical conversion device that generates electricity directly from fuel oxidation. Fuel cells are characterized by their electrolyte materials; SOFC has a solid oxide or ceramic electrolyte. The advantages of this fuel cell class include high efficiency, long term stability, fuel flexibility, low emissions, and relatively low cost. The biggest disadvantage is the high operating temperature that results in longer start-up times and mechanical and chemical compatibility issues.

Video Solid oxide fuel cell

Introduction

The solid oxide fuel cell is a class of fuel cells characterized by the use of solid oxide materials as electrolytes. SOFCs use solid oxide electrolytes to conduct negative oxygen ions from the cathode to the anode. Electrochemical oxidation of oxygen ions with hydrogen or carbon monoxide occurs on the anode side. Recently, proton-based SOFCs (PC-SOFC) are being developed that transport protons in lieu of oxygen ions through electrolytes with the advantage of being able to run at lower temperatures than traditional SOFCs.

They operate at very high temperatures, typically between 500 and 1,000 ° C. At this temperature, SOFC does not require expensive platinum catalyst materials, such as those currently required for lower temperature fuel cells such as PEMFC, and are not susceptible to poisoning carbon monoxide catalyst. However, susceptibility to sulfur poisoning has been widely observed and sulfur must be discarded before entering the cell through the use of adsorbent beds or other means.

Solid oxide fuel cells have a wide range of applications, ranging from being used as auxiliary power units in vehicles to stationary power plants with outputs of 100 W to 2 MW. In 2009, the Australian company Ceramic Fuel Cells successfully achieved SOFC device efficiency up to the previous theoretical limit of 60%. Higher operating temperatures make SOFC candidates suitable for applications with heat engine recovery devices or combined heat and power, which further improves overall fuel efficiency.

Because of this high temperature, light hydrocarbon fuels, such as methane, propane, and butane can be reformed internally inside the anode. SOFCs can also be triggered by externally reforming heavier hydrocarbons, such as gasoline, diesel, jet fuel (JP-8) or biofuel. Such a reform is a mixture of hydrogen, carbon monoxide, carbon dioxide, vapor, and methane, formed by reacting hydrocarbon fuels with air or vapor in devices upstream of the SOFC anode. The SOFC power system can improve efficiency by using heat released by exothermic electrochemical oxidation in fuel cells for the process of endothermic steam reform. In addition, solid fuels such as coal and biomass can be gasified to form syngas suitable for charging SOFCs in an integrated gasification fuel cell power cycle.

Thermal expansion demands a uniform and well-regulated heating process at startup. The SOFC stack with planar geometry takes one hour to heat up until the temperature is on. Micro-tubular fuel cell design geometry promises faster start time, usually in minute order.

Unlike most other types of fuel cells, SOFCs can have many geometries. Planar fuel cell design geometry is a typical sandwich type geometry that is used by most types of fuel cells, where the electrolyte is sandwiched between the electrodes. SOFC can also be made in tubular geometry where air or fuel is passed through the inside of the tube and other gases are passed along the outside of the tube. The tubular design is advantageous because it is much easier to cover the air from the fuel. The performance of the current planar design is better than the performance of tubular design, however, because the planar design has a comparatively lower resistance. Other SOFC geometries include modifications to the design of planar fuel cells (MPC or MPSOFC), in which wave-like structures replace the traditional flat configuration of the planar cells. Such designs are very promising as they share the advantages of both planar cells (low resistance) and tubular cells.

Maps Solid oxide fuel cell

Operation

The solid oxide fuel cell consists of four layers, three of which are ceramic (hence the name). A single cell consisting of four layers is stacked together usually only a few millimeters thick. Hundreds of these cells are then connected in series to form what most people call "SOFC piles". The ceramic used in SOFC does not become electrically and ionically active until it reaches very high temperatures and as a result, the pile must run at temperatures ranging from 500 to 1,000 ° C. The reduction of oxygen to oxygen ions occurs at the cathode. These ions can then diffuse through a solid oxide electrolyte to the anode where they can oxidize the fuel electrochemically. In this reaction, the by-product water is released as well as two electrons. These electrons then flow through external circuits where they can do the work. The cycle then repeats when the electrons enter the cathode material again.

Plant balance

Most of the SOFC downtime comes from the mechanical balance of plants, air preheater, prereformer, afterburner, water heat exchanger, oxidation of tail anode gas, and electrical balance of plants, power electronics, hydrogen sulphide sensors and fans. Internal reforms lead to a large decline in factory cost balances in designing a full system.

Anode

The ceramic anode layer must be highly porous to allow the fuel to flow into the electrolyte. As a result, granular material is often selected for anode fabrication procedures. Like a cathode, it must perform electrons, with the ionic conductivity of a definite asset. The most commonly used material is a nickel-made sermet mixed with ceramic materials used for electrolytes in certain cells, usually YSZ (yttria stabilized zirconia) nanomaterial-based catalysts, this YSZ part helps stop the growth of nickel grains.. A larger grain of nickel will reduce the ionic contact area, which will lower the efficiency of the cell. Anodes are generally the thickest and strongest layers in each individual cell, because they have the smallest polarization losses, and often the layers that provide mechanical support. Speaking electrochemically, the anode task is to use oxygen ions that diffuse through the electrolyte to oxidize hydrogen fuel. The oxidation reaction between oxygen and hydrogen ions produces heat and water and electricity. If fuel is a mild hydrocarbon, for example, methane, another function of the anode is to act as a catalyst for steam that reforms fuel into hydrogen. This provides other operational benefits for fuel cell stacks because the reform reaction is endothermic, which cools the pile internally. Perovskite materials (electronic ionic/ceramic conductor mixtures) have been shown to produce a power density of 0.6 W/cm2 at 0.7 V at 800 ° C which is possible because they have the ability to overcome larger activation energies.

Electrolytes

Electrolytes are a ceramic solid layer that performs oxygen ions. The electronic conductivity must be kept as low as possible to prevent losses from current leakage. High SOFC operating temperatures allow the oxygen ion transport kinetics to perform well enough. However, as the operating temperature approaches the lower limit for SOFCs around 600 Ã, Â ° C, electrolytes begin to have a large ionic transport resistance and affect performance. Popular electrolyte materials include yttria-stabilized zirconia (YSZ) (often 8% form of 8YSZ), scandia stabilized zirconia (ScSZ) (usually 9Ã, mol% Sc2O3 - 9ScSZ) and gadolinium doped ceria (GDC). Electrolyte materials have an important influence on cell performance. Detrimental reactions between the YSZ electrolyte and the modern cathode such as lanthanum strontium cobalt ferrite (LSCF) have been found, and can be prevented with a thin cheerful diffusion barrier (& lt; 100 nm).

If the oxygen ion conductivity in the SOFC can remain high even at lower temperatures (current target in the study ~ 500 Â ° C), the choice of material for SOFC will be widespread and many existing problems can be potentially solved. Certain processing techniques such as thin film deposition can help solve this problem with existing materials by:

• reduces the travel distance of oxygen ions and electrolyte resistance because the resistance is proportional to the length of the conductor;
• produces less resistive grain structures such as columnar granular structures;
• controls fine grains of micro-crystal nano-crystals to achieve "fine-tuning" electrical properties;
• building composites that have large interface areas as interfaces have been shown to have exceptional electrical properties.

Cathode

Katoda, atau elektroda udara, adalah lapisan berpori tipis pada elektrolit di mana pengurangan oksigen terjadi. Reaksi keseluruhan ditulis dalam Notasi KrÃÆ'¶ger-Vink sebagai berikut:

${\ displaystyle {\ frac {1} {2}} \ mathrm {O_ {2} (g)} 2 \ mathrm {e '} {V} _ { o} ^ {\ bullet \ bullet} \ longrightarrow {O} _ {o} ^ {\ times}}  ÂÂ$

The cathode material shall be, at a minimum, electronically conductive. Currently, lanthanum strontium manganite (NGO) is the preferred cathode material for commercial use due to its compatibility with doped zirconia electrolyte. Mechanically, it has the same thermal expansion coefficient for YSZ and thus limits the buildup of stress due to CTE mismatch. Also, NGOs have a low level of chemical reactivity with YSZ that extends the life of the material. Unfortunately, NGOs are poor conductor ionic, so the active electrochemical reaction is limited to the three-phase limit (TPB) in which electrolytes, air and electrodes meet. NGOs work well as cathodes at high temperatures, but their performance quickly falls when operating temperatures are lowered below 800 ° C. To increase the reaction zone beyond the TPB, potential cathode materials must be capable of performing both electrons and oxygen ions. A composite cathode comprising a YSZ NGO has been used to increase the length of this three phase limit. The ionic/electronic conducting mix (MIEC) ceramics, such as LSCF perovskite, are also being researched for use in medium temperature SOFCs as they are more active and can make up for the increased activation energy of the reaction.

Interconnection

Interconnections can be either metal or ceramic layers that lie between each cell. The goal is to connect each cell in series, so that the electricity generated by each cell can be combined. Because the interconnect is exposed to oxidation and reduces the cell side at high temperatures, it must be very stable. For this reason, ceramics are more successful in the long run than metals as interconnect materials. However, this ceramic interconnect material is very expensive compared to metal. Nickel-based alloys and steels become more promising because of lower temperatures (600-800 Â ° C) SOFC is developed. The material of choice for interconnection in contact with Y8SZ is 95Cr-5Fe alloy. Metal-ceramic composites called 'mirrors' are also being considered, as they have shown thermal stability at high temperatures and excellent electrical conductivity.

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Polarizations

Polarization, or overpotential, is a voltage loss due to imperfections in material, micro, and fuel cell designs. Polarization results from oxygen ion oxygen resistance through electrolytes (iR?), Electrochemical activation resistance in the anode and cathode, and finally polarization of concentration due to the inability of the gas to spread at high levels through the porous anode and cathode (shown as? A for anodes and C for cathode). The cell voltage can be calculated using the following equation:

$Â\; Â{\displaystyle Â\; Â\; Â\; Â\; Â\; Â\; Â\; Â\; ÂÂ\; Â\; Â\; Â\; Â\; Â\; Â\; Â\; ÂVÂ\; Â\; Â\; Â\; Â\; Â\; Â\; ÂÂ\; Â\; Â\; Â\; Â\; Â\; Â\; Â=Â\; Â\; Â\; Â\; Â\; Â\; Â}$ _{                       E                          ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ,                          -                                me           R                                  ?                          -                               ?                           ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂï <½       Â      ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂï <½     Â  <Â>      Â       Â             e                          -                               ?                            Â       Â ·       Â       Â             e                                {\ displaystyle {V} = {E} _ {0} - {iR} _ {\ omega} - {\ eta} _ {cathode} - { \ eta} _ {anode}}  Â}

dimana:

• ${\ displaystyle {E} _ {0}}  ÂÂ$ = Potensi nernst reaktan
• ${\ displaystyle R}  ÂÂ$ = Nilai resistensi setara thÃÆ' © venin dari bagian yang melakukan elektrik sel
• ${\ displaystyle {\ eta} _ {cathode}}  ÂÂ$ = kerugian polarisasi dalam katoda
• ${\ displaystyle {\ eta} _ {anode}}  ÂÂ$ = kerugian polarisasi dalam anoda

In SOFCs, it is often important to focus on ohmic polarization and concentration because high operating temperatures experience little polarization activation. However, because the lower limit of SOFC operating temperature is approximated (~ 600 Â ° C), this polarization becomes important.

The above-mentioned equation is used to determine the SOFC voltage (actually for the fuel cell voltage in general). This approach resulted in a good deal with specific experimental data (which sufficient factors obtained) and a poor deal for apart from the original experimental work parameters. In addition, most of the equations used require the addition of many factors that are difficult or impossible to determine. This greatly complicates the optimization process of SOFC working parameters as well as the selection of architectural design configurations. Due to the circumstances, several other equations are proposed:

$Â\; Â{\displaystyle Â\; Â\; Â\; Â\; Â\; Â\; Â}$ _{          E            S            O     Â¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯             C                          =                                  Â               E                                 m                  a                  x        ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ,       Â      Â     Â      Â         me                                 m                  a                  x        ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ,       Â               ?    Â                ?                                 f        ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ,       Â               ?    Â         Â                        Â 1        ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ,       Â                                                         ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ...                    r                                      1      ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ,     ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ...     ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ...                    r                                      2      ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ,     ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ...                       ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ,                   ?                         Â
      Â 1                -     ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ...                  ?                                       f      ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ,     ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ...        ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ, <<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<                 Â 1                                            {\ displaystyle E_ {SOFC} = {\ frac {E_ {max} -i_ {max} \ cdot \ eta _ {f} \ cdot r_ {1 }} {{\ frac {r_ {1}} {r_ {2}}} \ cdot \ left (1- \ eta _ {f} \ right) 1}}}  Â}

dimana:

• ${\ displaystyle E_ {SOFC}}  ÂÂ$ = tegangan sel
• ${\ displaystyle E_ {max}}  ÂÂ$ = tegangan maksimum yang diberikan oleh persamaan Nernst
• ${\ displaystyle i_ {max}}  ÂÂ$ = kepadatan arus maksimum (untuk aliran bahan bakar yang diberikan)
• ${\ displaystyle \ eta _ {f}}  ÂÂ$ = faktor pemanfaatan bahan bakar
• ${\ displaystyle r_ {1}}  ÂÂ$ = ketahanan spesifik ion elektrolit
• ${\ displaystyle r_ {2}}  ÂÂ$ = tahanan khusus elektrolit.

This method is validated and found suitable for optimization and sensitivity studies in plant-level modeling of various systems with solid oxide fuel cells. With this mathematical description it is possible to take into account various SOFC properties. There are many parameters that affect the working condition of the cell, e.g. electrolyte material, electrolyte thickness, cell temperature, inlet gas composition and outlet at the anode and cathode, and porosity of the electrode, just to name a few. The flow in this system is often calculated using the Navier-stokes equation.

Ohmic polarization

Ohmic loss in SOFC results from ionic conductivity through electrolyte. This is inherently the material property of the crystal structure and the atoms involved. However, to maximize ionic conductivity, several methods can be performed. First, operating at higher temperatures can significantly reduce this ohmic loss. Substitution doping methods to further refine crystal structures and control flaw concentrations may also play an important role in improving the conductivity. Another way to reduce ohmic resistance is to reduce the thickness of the electrolyte layer.

Ionic Conductivity

Ketahanan spesifik ion elektrolit sebagai fungsi suhu dapat dijelaskan oleh hubungan berikut:

${\ displaystyle r_ {1} = {\ frac {\ delta} {\ sigma}}}  ÂÂ$

di mana: ${\ displaystyle \ delta}  ÂÂ$ - ketebalan elektrolit, dan ${\ displaystyle \ sigma}  ÂÂ$ - konduktivitas ionik.

Konduktivitas ionik oksida padat didefinisikan sebagai berikut:

${\ displaystyle \ sigma = \ sigma _ {0} \ cdot e ^ {\ frac {-E} {R \ cdot T}}}  ÂÂ$

di mana: ${\ displaystyle \ sigma _ {0}}  ÂÂ$ dan ${\ displaystyle E}  ÂÂ$ - faktor tergantung pada material elektrolit, ${\ displaystyle T}  ÂÂ$ - suhu elektrolit, dan ${\ displaystyle R}  ÂÂ$ - konstanta gas ideal.

Polarisasi konsentrasi

Concentration polarization is the result of practical limitations on mass transport within the cell and represents a loss of stress due to spatial variation in the concentration of the reactant at the active chemistry site. This situation can be caused when the reactants are consumed by electrochemical reactions faster than those that can diffuse into the porous electrode, and can also be caused by variations in the composition of the bulk flow. The latter is due to the fact that consumption of species that react in the reactant stream causes a decrease in the concentration of the reactants as they move along the cell, leading to a decrease in local potential near the tail end of the cell.

Polarization concentrations occur in both the anode and the cathode. Anodes can be very problematic, because hydrogen oxidation produces steam, which further dilutes the flow of fuel as it flows along the cell. This polarization can be reduced by reducing the fraction of reactant utilization or increasing the porosity of the electrode, but this approach each has a significant design trade-off.

Activation polarization

The activation polarization is the result of the kinetics involved with the electrochemical reaction. Each reaction has a certain activation barrier that must be resolved to continue and this barrier leads to polarization. The activation barrier is the result of many complex electrochemical reaction steps where usually the rate-limiting step is responsible for polarization. The polarization equation shown below is found by solving the Butler-Volmer equation in a high current density regime (in which the cell normally operates), and can be used to estimate the activation polarization:

$Â\; Â{\displaystyle Â\; Â\; Â\; Â\; Â\; Â\; Â}$ _{                       ?                            Â Â}      ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂï <½Â     ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂï <½Â                          =                                           R              T                                                      ?        ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ,           Â  F                                     ÃÆ' -          l          n                   (                           Â         me    Â                      Â          me        ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ,                                 0        ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ,       Â      ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ,                     )                       {\ displaystyle {\ eta} _ {act} = {\ frac {RT} {{\ beta} zF}} \ times ln \ left ({\ frac {i} {{i} _ {0}}} \ right)}  Â

dimana:

• ${\ displaystyle R}  ÂÂ$ = konstanta gas
• ${\ displaystyle {T} _ {0}}  ÂÂ$ = suhu operasi
• ${\ displaystyle {\ beta}}  ÂÂ$ = koefisien transfer elektron
• ${\ displaystyle z}  ÂÂ$ = elektron yang terkait dengan reaksi elektrokimia
• ${\ displaystyle F}  ÂÂ$ = Konstanta Faraday
• ${\ displaystyle i}  ÂÂ$ = mengoperasikan saat ini
• ${\ displaystyle i_ {0}}  ÂÂ$ = menukarkan rapat arus

Polarization can be modified by microstructure optimization. The long Triple Phase Boundary (TPB), which is the length at which the porous, ionic and electronic pathways all meet, is directly related to the electrochemical active length in the cell. The larger the length, the more reactions can occur and thus the less activation polarization. The optimization of TPB length can be done with processing conditions to influence microstructure or by selecting materials to use mixed ionic/electronic conductors to further increase TPB length.

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Target

The DOE target requirement is 40,000 hours of service for stationary fuel cell applications and over 5,000 hours for a transportation system (fuel cell vehicle) at a factory cost of $ 40/kW for a 10 kW coal-based system without additional requirements. The effects of a lifetime (phase stability, thermal expansion compatibility, element migration, conductivity and aging) must be addressed. The 2008 (interim) State Energy Conversion Alliance (interim) for overall degradation per 1,000 hours is 4.0%.

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Research

Research now leads to low temperature SOFC (600 ° C). Low temperature systems can reduce costs by reducing insulation, material, start-up costs and associated degradation. With higher operating temperatures, the temperature gradient increases the thermal stress severity, which affects material costs and system life. The medium temperature system (650-800 Â ° C) will allow the use of cheaper metal materials with better mechanical properties and thermal conductivity. New developments in nano-scale electrolyte structures have been shown to lower operating temperatures to around 350 ° C, which will allow the use of cheaper steel components and elastomers/polymers.

Lowering the operating temperature has the added benefit of increased efficiency. The efficiency of the theoretical fuel cell increases with the decrease in temperature. For example, the efficiency of SOFC using CO as fuel increased from 63% to 81% when lowering the system temperature from 900 ° C to 350 ° C.

Research is also underway to increase the flexibility of SOFCs fuel. While stable operation has been achieved on a variety of hydrocarbon fuels, these cells typically rely on external fuel processing. In the case of natural gas, either external or internal fuel is reformed and sulfur compounds are removed. These processes add to the cost and complexity of the SOFC system. Work is under way at a number of agencies to improve the stability of the anode material for hydrocarbon oxidation and, therefore, relax the requirements for fuel processing and reduce the SOFC balance of the plant costs.

Research also occurs in reducing start-up time to be able to implement SOFCs in mobile applications. This can be achieved in part by lowering the operating temperature, which is the case for proton exchange membrane fuel cells (PEMFCs). Due to their fuel versatility, they can run on partly reformed diesels, and this makes the SOFC attractive as an additional power unit (APU) in refrigerated trucks.

Specifically, Delphi Automotive Systems is developing SOFC which will power additional units in cars and tractor-trailers, while BMW recently suspended similar projects. A high temperature SOFC will generate all the electricity required to allow the engine to become smaller and more efficient. SOFC will run on the same petrol or diesel as the engine and will keep the AC unit and other electrical systems required running when the engine is off when not needed (for example, in stop light or truck stop).

Rolls-Royce is developing solid oxide fuel cells manufactured with screen printing to cheap ceramic materials. Rolls-Royce Fuel Cell Systems Ltd. is developing a gas turbine-driven SOFC turbine gas system for power generation applications in megawatt sequences (eg Futuregen).

3D printing is being explored as a possible manufacturing technique that can be used to make SOFC manufacturing easier by Shah Lab at Northwestern University. This manufacturing technique will allow the SOFC cell structure to become more flexible, which can lead to more efficient design. This process can work in the production of any cell part. The 3D printing process works by combining about 80% ceramic particles with 20% binder and solvent, and then converting the slurry into inks that can be incorporated into 3D printers. Some solvents are very volatile, so the ceramic ink immediately hardened. Not all solvents evaporate, so the ink retains some flexibility before being fired at high temperatures to solidify it. This flexibility allows cells to be fired in a circular shape that will increase the surface area at which electrochemical reactions can occur, which increases cell efficiency. Also, 3D printing techniques allow cell layers to be printed on top of each other rather than having to go through manufacturing steps and stacking them apart. The thickness is easy to control, and the coating can be made in the exact size and shape required, so waste is minimized.

Ceres Power Ltd. has developed a low and low cost (500-600 degrees) SOFC stack using cerium gadolinium oxide (CGO) instead of the current industry standard ceramic, stable yttria zirconia (YSZ), which allows the use of stainless steel to support ceramics.

Solid Cell Inc. has developed a unique and low-cost cell architecture that combines the planar and tubular design properties, along with the Cr-free interconnect mirrors.

The high-temperature electrochemical center (HITEC) at the University of Florida, Gainesville is focused on studying ionic transport, electrocatalytic phenomena and micro-characterization of ion regulators.

SiEnergy Systems, a Harvard spinoff company, has demonstrated the first macro-oxide solid-oxide fuel cell that can operate at 500 degrees.

SOEC

The solid oxide electrolytic cell (SOEC) is a solid oxide fuel cell arranged in regenerative mode for electrolysis of water with solid oxide, or ceramics, electrolytes to produce oxygen and hydrogen gas.

SOEC can also be used to perform CO2 electrolysis to produce CO and oxygen or even co-electrolysis of water and CO2 to produce syngas and oxygen.

ITSOFC

SOFCs operating in medium temperature range (IT), which means between 600 and 800 ° C, are named ITSOFCs. Due to the high rate of degradation and material costs occurring at temperatures over 900 ° C, it is economically more profitable to operate SOFC at lower temperatures. The drive for high performance ITSOFCs today is the topic of research and development. One of the focus areas is the cathode material. It is thought that the oxygen reduction reaction is responsible for many losses in performance so that catalytic catalytic activity is being studied and improved through various techniques, including catalyst impregnation. Research on NdCrO ₃ proves it as a potential cathode material for the ITSOFC cathode because the chemical thermos is stable within the temperature range.

Other focus areas are electrolyte materials. To make SOFC competitive in the market, ITSOFC has always been the focus of research and people are trying to lower operating temperatures by using new alternative materials. However, the efficiency and stability of the material limit its feasibility. One option for new electrolyte materials is ceria-salt composite ceramics (CSC). The two-phase electrode CSC GDC (gadolinium-doped ceria) - and SDC (samaria-doped ceria) -MCO ₃ (M = Li, Na, K, single or carbonate mixture) can achieve a 300- 800 mW * cm ^-2 .

LT-SOFC

Low temperature solid oxide fuel cells (LT-SOFCs), operating lower than 650 ° C, are particularly attractive for future research because of the high current operating temperatures that limit the development and deployment of SOFCs. Low temperature SOFC is more reliable due to smaller thermal discrepancies and easier sealing. In addition, lower temperatures require less insulation and therefore have a lower cost. Costs are lowered further due to wider material options for interconnects and compressive nonglass/ceramic seals. Perhaps most importantly, at lower temperatures, SOFC can be started faster and with less energy, suitable for use in portable and portable applications.

Interestingly, when the temperature decreases, the maximum theoretical fuel cell efficiency increases, in contrast to the Carnot cycle. For example, the maximum theoretical efficiency of SOFC using CO as fuel increased from 63% at 900 ° C to 81% at 350 ° C.

This is a material problem, especially for electrolytes in SOFC. YSZ is the most commonly used electrolyte because of its superior stability, although it does not have the highest conductivity. Currently, YSZ electrolyte thickness is ~ 10 m minimum due to settling method, and this requires temperatures above 700 Â ° C. Therefore, low temperature SOFCs are only possible with higher conductivity electrolytes. Various alternatives that can be successful at low temperatures include gallium-doped ceria (GDC) and bismuth stabilization erbia-cation (ERB). They have superior ionic conductivity at lower temperatures, but these come at the expense of lower thermodynamic stability. The electrolyte of CeO2 becomes electronically conductive and the electrolyte Bi2O3 decomposes into the Bi metal under a reduced fuel environment.

To counter this, the researchers created a functional gradual ceria/bismuth-oxide electrolyte in which the GDC layer on the anode side protects the ESB layer from decomposition while the ESB on the cathode side blocks the leak current through the GDC layer. This causes the potential of near-theoretical open circuit (OPC) with two highly conductive electrolytes, which are not stable enough for the application. This bilayer proved to be stable for 1400 test hours at 500 ° C and showed no indication of interfacial phase formation or thermal mismatch. While this makes steps to lower the operating temperature of SOFC, it also opens the door for future research to try and understand this mechanism.

Researchers at the Georgia Institute of Technology dealt with the instability of BaCeO ₃ differently. They replaced the desired fraction of Ce in BaCeO ₃ with Zr to form a solid solution that exhibits proton conductivity, but also chemical and thermal stability over various conditions relevant to the fuel cell operation. New special composition, Ba (Zr0.1Ce0.7Y0.2) O3-? (BZCY7) featuring the highest ionic conductivity of all known electrolyte materials for SOFC applications. This electrolyte is made by dry pressing powder, which allows the production of crack-free films thinner than 15 m. Implementation of this simple and cost-effective fabrication method can enable significant cost reductions in SOFC fabrication. However, this electrolyte operates at a higher temperature than the bilayer electrolyte model, closer to 600 Ã, Â ° C than 500 Ã, Â ° C.

Currently, given the state of the field for LT-SOFC, advances in electrolytes will reap the most benefits, but research on potential anode and cathode materials will also lead to useful results, and has begun to be discussed more frequently in the literature.

SOFC-GT

The SOFC-GT system is one that consists of solid oxide fuel cells combined with a gas turbine. Such systems have been evaluated by Siemens Westinghouse and Rolls-Royce as a means to achieve higher operating efficiencies by running SOFC under pressure. The SOFC-GT system usually includes anodic and/or cathodic atmospheric recirculation, thus increasing efficiency.

Theoretically, the combination of SOFC and gas turbines can provide overall high efficiency (electricity and thermal). Further combinations of SOFC-GT in a combined configuration of cooling, heat and power (or triggering) (via HVAC) also have the potential to produce higher thermal efficiency in some cases.

In addition, another interesting feature of the introduced hybrid system is the acquisition of 100% CO2 capture at comparable high energy efficiency. Features such as zero CO2 emissions and high energy efficiency make power generation performance important.

DCFC

For the direct use of solid coal fuel without additional gasification and reform processes, direct carbon fuel cells (DCFC) have been developed as promising new concepts of high temperature energy conversion systems. The underlying progress in the development of coal-based DCFCs has been categorized primarily in accordance with the electrolytic materials used, such as solid oxide, liquid carbonate, and liquid hydroxide, as well as hybrid systems comprising solid oxide and liquid carbonate binary electrolytes or from liquid anodes (Fe, Ag , In, Sn, Sb, Pb, Bi, and alloys and metal/metal oxides) solid oxide electroly

Source of the article : Wikipedia