2.1. Proton Conductor
SOFCs are different from other types of fuel cell in a lot of aspects. It is a fuel cell that is made up of solid materials only and this solid nature of all SOFC components means that there is nothing that can affect its cell configuration. SOFCs can operate at temperatures as high as 1000°C, which is hotter than most major fuel cells.
In an SOFC, there is a hard ceramic electrolyte in between two electrodes: the anode and the cathode. What happens is that hydrogen is fed into the fuel cell’s anode while oxygen enters the cell via the anode. The oxygen concentration on one side of the electrolyte is drop dramatically due to the burning of the fuel containing hydrogen on the other side of the electrolyte.
The electrode on this surface will allow oxygen ions to leave the electrolyte and react with the fuel which is oxidised, thereby releasing electrons (e-) 2, 3. On the other side of the plate, which is exposed to air, an oxygen concentration gradient is created across the electrolyte, which attracts oxygen ions from the cathode, to the anode. If there is an electrical connection between the cathode and the anode, this allows electrons to flow from the anode to the cathode, where a continuous supply of oxygen ions (O2-) for the electrolyte is maintained, and oxygen ions from cathode to anode, maintaining overall electrical charge balance, thereby generating useful electrical power from the combustion of the fuel. The only by-product of this process is a pure water molecule (H2O) and heat, as shown in Fig. 2. The SOFC reactions include:
i. Anode side:
H2 + O2-?H2O + 2e-, CO + O2-?CO¬2 + 2e-, CH4 + 4O2-?2H¬¬2O + CO2 + 8e- (Fuel containing hydrogen)
ii. Cathode side:
O2 + 4e-?2O2-
A SOFC is mainly composed of two electrodes (the anode and the cathode), and a solid electrolyte. The SOFC, which relies on O2- oxygen ion transport, also works with high purity hydrogen, but it does not rely upon this fuel, which is expensive to produce and difficult to handle 2. The main function of the electrode is to bring about reaction between the fuel or oxygen and the electrolyte, without itself being consumed or corroded. It must also bring into contact the three phases, i.e., the gaseous fuel, the solid electrolyte and the electrode itself.
The anode, used as the negative electrode of the fuel cell, disperses the hydrogen gas equally over its whole surface and conducts the electrons that are freed from hydrogen molecule, to be used as a useful power in the external circuit. The cathode, the positive post of the fuel cell, distributes the oxygen fed to it onto its surface and conducts the electrons back from the external circuit where they can recombine with oxygen ions, passed across the electrolyte, and hydrogen to form water. There are no water molecules generated at the fuel anode side 4. This is one of the benefits of using proton conducting electrolyte in solid oxide fuel cell where fuel circulation is unnecessary and therefore fuel at the anode remains pure 3.
The electrolyte determines the operating temperature of the fuel cell and is used to prevent the two electrodes from coming into electronic contact by blocking the electrons. The electrolyte also functions to let charged ions flow from one electrode toe the other to maintain the overall charge balance. It can either be an oxygen ion conductor or a hydrogen ion (proton) conductor, the major difference between the two types is the side in the fuel cell in which the water is produced: the oxidant side in proton– conductor fuel cells and the fuel side in oxygen-ion-conductor ones.
SOFCs provide several benefits over other type of fuel cells, due to their fuel flexibility, modularity, reliability, low polluting qualities, and its absence of corrosive subtances. SOFCs can operate with a great variety of hydrocarbon fuels. Furthermore, Fabbri reports that because of its high operating temperatures, natural gas fuel can be reformed within the SOFC which can reduce the cost and simplify the fuel cell, making it more efficient because there is no external reformers needed 5.
2.2. Electrolyte Material
The best SOFCs should have a high ionic conductivity, a negligible electronic conductivity, chemical stability, no matter the condition and mechanical integrity 6.
2.2.1. Strontium/Magnesium-Doped Lanthanum Gallate Electrolyte
The perovskite, LaGaO3, can be doped with strontium and magnesium, La1?xSrxGa1?yMgyO3 (LSGM), to produce a material with good low-temperature oxygen-ion conductivity, as reported by Fergus 7. But according to Feng, the use of LSGM as a solid electrolyte in SOFC is problematic because of its low mechanical stability 8, its reaction with other components of the fuel cell 9 and the high costs of gallium 10. Brett also supports that using LSGM is less desirable due to it being more expensive than other electrolytes and it faces durability problems when paired with some electrodes 11. Although, Fergus states that LSGM has a higher conductivity compared to YSZ and ScSZ, which opposes a report done by Malavasi, and similar to or lower than that of CGO and that its conductivity relies on the concentration of dopant 7.
2.2.2. Doped Ceria
Ce-based oxide ion conductors like Gd doped CeO2 (GCO) offer a high ionic conductivity 10. It is reported by Huijsmans that the disadvantage of using of the ceria-based electrolytes is its relatively small width of the electrolytic domain boundary 12. But a report conducted by Fergus states that ceria has a higher conductivity, particularly at low temperatures 7.
Fergus also reports that in atmospheres that have hydrogen or water vapour, doped BaCeO3 has shown to have high proton conductivity. However it is not chemically and mechanically stable because it reacts with acidic gases, like CO2, and moisture. Barium cerate is not ideal as an electrolyte material for fuel cell applications, especially for use with fuels different from pure hydrogen 1.
2.2.3. Stabilised Zirconia
Stabilised zirconia is a good candidate for being the electrolyte in a SOFC. The most common solid electrolyte material used in solid oxide fuel cells is scandia-stabilised zirconia (ScSZ) and yttria-stabilised zirconia (YSZ). Sharma states that the ionic conductivity of stabilized zirconia is dependent on the size and concentration of the dopant 13.
ScSZs are an electrolyte that exhibits the highest conductivity among all electrolytes. But it has several disadvantages like the high cost of scandia 14, insufficient long-term stability and the aging effect caused by annealing at high temperature 15, this makes ScSZs difficult to use as electrolytes in SOFCs. According to Fergus, another issue with ScSZ is that for higher scandia contents (e.g. 10–12 mole %), its cubic phase transforms to a rhombohedral phase at lower temperatures, lowering its conductivity 7.
Yttrium stabilized zirconia (YSZ) have been considered as an alternative electrolyte candidate for SOFC at intermediate temperature in compared to the conventional oxygen ion conducting electrolytes 3. According to Fergus, Yttria is added to stabilise its conductive cubic fluorite phase and to increase the concentration of vacancies, increasing its ionic conductivity 7. Yttria is the preferred stabiliser for zirconia because of its abundance and cost effectiveness compared to other dopants 16. Stambouli states that YSZ exhibits purely oxygen ionic conduction (with no electronic conduction). This means that in Y2O3, for every yttrium, there is only 1.5 oxide ions creating vacancies in the crystal structure where oxide ions are missing. This causes oxide ions from the cathode leap from hole to hole until they reach the anode. 2. In addition to that, yttria doped zirconia poly crystals exhibit excellent strength and fracture toughness, as reported by Sharma 13.
Another alternative electrolyte for SOFCs is doped BaZrO3, which has been found to be chemically more stable than BaCeO3, as stated by Fabbri. But BaZrO3 has a large grain boundary surface due to its poor sinterability, which lowers its electrical conductivity, making it less conductive than barium cerates 1. Therefore, to solve these problems, yttrium-doped BaCeZrO3 (BCZY) is used as electrolyte instead.
BCZY combines the best of both worlds, BZY’s chemical stability and BCY’s proton conductivity. Fuel cell tests done by Fabbri et al showed that the power output of the BaCe0.3Zr 0.5Y0.2 O3 ? ? solid solution increased more than twice with respect to Y-doped barium zirconate, without impairing the chemical stability 1.
2.1. Proton Conductor