Ceramic fuel cells, also known as solid oxide fuel cells provide an eco-friendly conversion system to produce electric energy (from chemical energy). They offer several advantages over the traditional power generation system, such as high conversion efficiency, low emissions of gases such as carbon dioxide, carbon monoxide, sulphur dioxide and a lot more.
Most fuel cells are composed of an anode and cathode, an electrolyte (generally liquid). In ceramic fuel cells, this liquid electrolyte is replaced with ceramic membrane/ solid oxide material. The absence of liquid electrolyte in the solid oxide fuel permits the free shape of the geometry. Due to this, they are manufactured in different shapes and sizes. Moreover, the high-temperature resistance of ceramic cells allows its use in the gas turbines coupled with electro generators. The most commonly used ceramic membrane is Zirconia that has high thermal stability with a high-temperature range.
Working
CFC contains four or three layers of ceramics( hence called ceramic fuel cells) Single-cell comprising these layers are then connected in series to form a stack. These ceramic layers don’t get electrically active until they reach a very high temperature. In the ceramic fuel cell cathode, oxygen reduces to oxygen ions. These ions then diffuse through the electrolyte to the anode to electrochemically oxidize the fuel in the cell. In response to this, two electrons and a byproduct of water are given. These electrons then flow into the external circuit to do the remaining work. This cycle then repeats.
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| Ceramic fuel cell |
Cathode
In the cathode, the reduction of oxygen takes place. The cathode must have good compatibility with the connecting components, high electron conductivity, good porosity to allow diffusion for electrochemical reduction. Cathode materials are divided into two groups- a) Pure electronic conductors b) MIEC materials.
In pure electronic conductors, the oxygen reduction process occurs at the three-phase boundary of the cathode. MIEC cathodes, on the other hand, reduce oxygen all over the surface of the cathode and transport oxygen in bulk to the electrolyte. Using MIEC, the cathode performance can improve significantly.
Electrolyte
The electrolyte in a fuel cell is responsible for conducting oxygen ions, without which no current can flow through the cell. There are broadly three types of electrolytes: Anionic, Protonic, mixed ionic. Most of the high-temperature fuel cells such as SOFC/CFC employ oxygen ion conduction.
The oxygen ions in ceramic fuel cells move in the direction of the electric field throughout the crystal lattice structure. Therefore the conduction takes place due to the presence of vacancies. In other words, the crystal lattice must have unoccupied sites for oxygen ion conduction.
The most popular ceramic material electrolyte used in high-temperature fuel cells are Zirconia. This most investigated electrolyte exhibits three polymorphs in different temperature ranges: Mono-clinic( room temperature), tetragonal( at 1170°C) cubic phases (Above 2370 °C).
Both the tetragonal and cubic phase structures are not stable at low temperatures. Therefore, doping is required to stabilize them at room temperature. The dopants mainly added to Zirconia are Mgo, Cao, Y2O3 and SC2O3. All of these exhibit high solubility with Zr crystal lattice, but the most widely used are Y2O3 and SC2O3.
The dopant concentration also plays an important role in the conductivity of the doped Zirconia. Several studies suggested that adding Y2O3 to 8 molecules per cent significantly increases the conductivity of the Zirconia. (Y2O3)0.08(ZrO2)0.92 has higher ionic conductivity and stability due to which it is widely used as an electrolyte material in the CFC/SOFC. Moreover, the components used in this are inexpensive and found in abundance.
The scandia doped Zr has high ionic conductivity but faces thermal ageing at extreme temperatures. The high conductivity might allow its usage at an intermediate temperature range. However, its limiting factors must be taken into consideration. The electrolyte characteristics, such as conductivity and width determine the operating temperature range of CFC/SOFC. The yttrium based Zirconia performs well at temperatures above 850°C. But for intermediate temperature( between 600-800°C) doped LaGaO3 and CeO2 have shown better results.
The ceria has a large iconic radius which makes its magnitude approximately one order greater than YSZ (for a small temperature range). Moreover, ceria possesses a fluorite structure at room temperature up to its melting point, unlike its counterpart. Therefore the only function of doping is to increase the conductivity of the electrolyte through the formation of vacancies.
Doped lanthanum gallate has also been studied to use as an electrolyte in CFC/SOFC. The lanthanum was partially replaced by Ca, Ba, Sm and Sr and gallate by Mg, In and Al. These compositions especially, Mg and Sr offer high conductivity both in reduction and oxidation environments. However, there are some limitations to these compositions. They reduce the stability of the ceramic and sometimes leads to the formation of new phases.
Anode
At the anode, the electrochemical oxidation of the fuel takes place. For satisfactory performance, it must have- high electrical conductivity, compatibility with interconnects ( like electrolyte), high porosity and good conductive phases. The Ni/YSZ cermet (Yttria stabilized zirconia) is the most common anode material employed in developing ceramic fuel cells. Their widespread use is due to their chemical stability and low cost.
The Ni( Nickel) has high catalytic activity and relatively low cost as compared to other catalysts such as Cu and Co. The volume ratio of Ni to YSZ varies from 35:65 to 55:45 and the selected ratio determines the conductivity of the material.
There are two mechanisms through which cermet conductivity occurs: iconic and electronic. The conductivity is iconic if the ni concentration is below 30% and electronic if above 30%. The anode performance mainly depends upon the thickness, Ni particles connectivity, grain size distribution, the initial particle size of Ni and YSZ particles.
The Ni/YSZ has a few disadvantages that limit its long term stability such as, carbon deposition and sulphur poisoning. However, research has been going on to decrease/ replace the use of Ni in the anode to improve the performance of the cermet. The modified Ni/YSZ use materials such as CeO2, Y2O3, La2O3, Mgo that resists sulphur poisoning and provide solutions for carbon deposition.
Conclusion:
At last, ceramic fuel cells are offering clean power generation options to businesses and technologies. And deep research has been going on in this technology to take it to another level. Several new materials have been tested to increase the stability and cost-effectiveness of ceramic fuel cells and older components are being replaced. In short, it is a fuel cell technology that is both advanced and cost-effective.
