A solid oxide fuel cell (or SOFC) is an electrochemical conversion device that produces electricity directly from oxidizing a fuel. Fuel cells are characterized by their electrolyte material; the SOFC has a solid oxide or ceramic electrolyte.
Advantages of this class of fuel cells include high combined heat and power efficiency, long-term stability, fuel flexibility, low emissions, and relatively low cost. The largest disadvantage is the high operating temperature, which results in longer start-up times and mechanical and chemical compatibility issues.
Introduction Solid oxide fuel cells are a class of fuel cells characterized by using a solid oxide material as the electrolyte. SOFCs use a solid oxide electrolyte to conduct negative oxygen ions from the cathode to the anode. The electrochemical oxidation of the hydrogen, carbon monoxide or other organic intermediates by oxygen ions thus occurs on the anode side. More recently, proton-conducting SOFCs (PC-SOFC) have been developed to transport protons instead of oxygen ions through the electrolyte with the advantage of running at lower temperatures than traditional SOFCs.
They operate at very high temperatures, typically between 600 and 1,000 °C. At these temperatures, SOFCs do not require expensive platinum group metals catalysts, as is currently necessary for lower temperature fuel cells such as PEMFCs, and are not vulnerable to carbon monoxide catalyst poisoning. However, vulnerability to sulfur poisoning has been widely observed, and the sulfur must be removed before entering the cell. For lower-quality fuels, such as gasified biomass, coal, or biogas, fuel processing becomes increasingly complex and, consequently, more expensive. The gasification process, which transforms the raw material into a gaseous state suitable for fuel cells, can generate significant quantities of compounds like methane and toluene, as well as larger polyaromatic and short-chain hydrocarbon compounds. These substances can lead to carbon buildup in SOFCs. The expenses associated with reforming and desulfurization are comparable in magnitude to the cost of the fuel cell itself. These factors become especially critical for systems with lower power output or greater portability requirements.
Solid oxide fuel cells have a wide variety of applications, from using them as auxiliary power units in vehicles to stationary power generation with outputs from 100 W to 2 MW. A proposed maritime application is a 300 kW SOFC auxiliary generator on a 174,000 m³ LNG carrier planned by Mitsui O.S.K. Lines (MOL) and Samsung Heavy Industries (SHI). In 2009, the Australian company, Ceramic Fuel Cells, successfully achieved an efficiency of an SOFC device up to the previously theoretical mark of 60%.Ceramic fuel cells achieves world-best 60% efficiency for its electricity generator units . Ceramic Fuel Cells Limited. 19 February 2009Electricity from wood through the combination of gasification and solid oxide fuel cells, Ph.D. Thesis by Florian Nagel, Swiss Federal Institute of Technology Zurich, 2008 The higher operating temperature makes SOFCs suitable for application with heat engine energy recovery devices or combined heat and power, further increasing overall fuel efficiency.
Because of these high temperatures, light hydrocarbon fuels, such as methane, propane, and butane, can be internally reformed within the anode. SOFCs can also be fueled by externally reforming heavier hydrocarbons, such as gasoline, diesel, jet fuel (JP-8) or biofuels. Such reformates are mixtures of hydrogen, carbon monoxide, carbon dioxide, steam and methane, formed by reacting the hydrocarbon fuels with air or steam in a device upstream of the SOFC anode. SOFC power systems can increase efficiency by using the heat from the exothermic electrochemical oxidation within the fuel cell for an endothermic steam reforming process. Solid fuels, such as coal and biomass, may also be gasified to form syngas suitable for fueling SOFCs in integrated gasification fuel cell power cycles.
Thermal expansion demands a uniform and well-regulated heating process at startup. SOFC stacks with planar geometry require an hour to heat to operating temperature. Micro-tubular fuel cell design geometries promise much faster start-up times, typically in the order of minutes.
Unlike most other types of fuel cells, SOFCs can have multiple geometries. The planar fuel cell design geometry is the typical sandwich-type geometry employed by most types of fuel cells, where the electrolyte is sandwiched between the electrodes. SOFCs can also be made in tubular geometries where either air or fuel is passed through the inside of the tube, and the other gas is passed along the outside of the tube. The tubular design is advantageous because it is much easier to seal air from the fuel. The performance of the planar design is currently better than the performance of the tubular design, however, because the planar design has a lower resistance comparatively. Other geometries of SOFCs include modified planar fuel cell designs (MPC or MPSOFC), where a wave-like structure replaces the traditional flat configuration of the planar cell. Such designs are highly promising because they share the advantages of both planar cells (low resistance) and tubular cells.
Operation
A solid oxide fuel cell is made up of four layers, three of which are ceramics (hence the name). A single cell consisting of these four layers stacked together is typically only a few millimeters thick. Hundreds of these cells are then connected in series to form what most people refer to as an "SOFC stack". The ceramics used in SOFCs do not become electrically and ionically active until they reach very high temperatures. As a consequence, the stacks have to run at temperatures ranging from 500 to 1,000 °C. Reduction of oxygen into oxygen ions occurs at the cathode. These ions can then diffuse through the solid oxide electrolyte to the anode where they can electrochemically oxidize the fuel. In this reaction, water byproduct and two electrons are given off. These electrons then flow through an external circuit where they can do work. The cycle then repeats as those electrons enter the cathode material again.
Balance of plant Most of the downtime of an SOFC is caused by the mechanical balance of the plant (from components like the air preheater, prereformer, afterburner, water heat exchanger, and anode tail gas oxidizer) and the electrical balance of the plant (including the power electronics, hydrogen sulfide sensor and fans). By using internal reforming (converting methane into hydrogen internally), lower cooling requirements are needed, helping decrease the complexity and costs of the balance of plant.
Anode The ceramic anode layer must be very porous to allow the fuel to flow towards the electrolyte. Consequently, granular matter is often selected for anode fabrication procedures. Like the cathode, it must conduct electrons. A capacity for ionic conductivity is an additional advantage. The anode's smaller polarization losses allows a comparatively thicker layer to provide the cell's mechanical support.
The electrochemical function of the anode is to facilitate the oxidation of a fuel, typically hydrogen, to produce heat, water and electricity. Where a light hydrocarbon (e.g. methane) is used, the anode can also act as a catalyst for steam reformation of the fuel. This provides another operational benefit to the fuel cell stack because the reforming reaction is endothermic, which cools the stack internally.
The most common material used is a cermet made of nickel mixed with the ceramic material used for the electrolyte in that particular cell, typically YSZ (yttria stabilized zirconia). Anodes can be produced by tape casting, using slurry prepared by sonication. This low-impact mixing allows for the effective combination of NiO-YSZ slurries in about 30 minutes, much faster than conventional ball milling (72 hours). The nanomaterial-based catalysts help stop the grain growth of nickel. Larger grains of nickel would reduce the contact area that ions can be conducted through, which would lower the cell's efficiency. Perovskite materials (mixed ionic/electronic conducting ceramics) 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 a larger activation energy.
Chemical Reaction
H2 + O2- —> H2O + 2e-
However, a few disadvantages are associated with YSZ as an anode material. Ni coarsening, carbon deposition, reduction-oxidation instability, and sulfur poisoning are the main obstacles limiting the long-term stability of Ni-YSZ. Ni coarsening refers to the evolution of Ni particles in YSZ, which grows larger in grain size, decreasing the surface area for the catalytic reaction. Carbon deposition occurs when carbon atoms, formed by hydrocarbon pyrolysis or CO disproportionation, deposit on the Ni catalytic surface. Carbon deposition becomes important, especially when hydrocarbon fuels, i.e., methane and syngas, are used. The high operating temperature of SOFC and the oxidizing environment facilitate the oxidation of Ni catalyst through the reaction Ni + O2 = NiO. The oxidation reaction of Ni reduces the electrocatalytic activity and conductivity. Moreover, the density difference between Ni and NiO causes volume change on the anode surface, potentially leading to mechanical failure. Sulfur poisoning arises when fuel such as natural gas, gasoline, or diesel is used. Again, due to the high affinity between sulfur compounds (H2S, (CH3)2S) and the metal catalyst, even the smallest impurities of sulfur compounds in the feed stream could deactivate the Ni catalyst on the YSZ surface.
Current research is focused on reducing or replacing Ni content in the anode to improve long-term performance. The modified Ni-YSZ containing other materials, including CeO2, Y2O3, La2O3, MgO, TiO2, Ru, Co, etc., have been invented to resist sulfur poisoning, but the improvement is limited due to the rapid initial degradation. Copper-based cermet anode is considered a solution to carbon deposition because it is inert to carbon and stable under typical SOFC oxygen partial pressures (pO2). Cu-Co bimetallic anodes show a great resistance to carbon deposition after exposure to pure CH4 at 800C. Cu-CeO2-YSZ exhibits a higher electrochemical oxidation rate over Ni-YSZ when running on CO and syngas. It can achieve even higher performance using CO than H2, after adding a cobalt co-catalyst. Oxide anodes, including zirconia-based fluorite and perovskites, are also used to replace Ni-ceramic anodes for carbon resistance. Chromite, i.e., La0.8Sr0.2Cr0.5Mn0.5O3 (LSCM), is used as anodes and exhibited comparable performance against Ni–YSZ cermet anodes. LSCM is further improved by impregnating Cu and sputtering Pt as the current collector.
Electrolyte The electrolyte is a dense layer of ceramic that conducts oxygen ions. Its electronic conductivity must be kept as low as possible to prevent losses from leakage currents. The high operating temperatures of SOFCs allow the kinetics of oxygen ion transport to be sufficient for good performance. However, as the operating temperature approaches the lower limit for SOFCs at around the electrolyte begins to have large ionic transport resistances and affect the performance. Popular electrolyte materials include yttria-stabilized zirconia (YSZ) (often the 8% form 8YSZ), scandia stabilized zirconia (ScSZ) (usually 9 mol% Sc2O3 – 9ScSZ) and gadolinium doped ceria (GDC). The electrolyte material has a crucial influence on the cell performance. Detrimental reactions between YSZ electrolytes and modern cathodes such as lanthanum strontium cobalt ferrite (LSCF) have been found, and can be prevented by thin (<100 nm) ceria diffusion barriers.