Solid Oxide Fuel Cells (SOFC)
Certain solid ceramic oxides (e.g. zirconium dioxide) are able to conduct electricity at high temperatures, therefore such ceramic oxides can be used as electrolytes in such fuel cells. This material is able to conduct O— ions at high temperatures. Therefore these cells operate at high temperatures in the range of 650°C to 1000°C. Due to high temperature operation no catalyst is required.
The negative electrode (anode) is made of porous nickel and positive electrode (cathode) uses metal oxide.
The schematic diagram of solid oxide fuel cell is shown in Figure A using hydrogen as fuel and oxygen as oxidant.
At the cathode the O2 molecules are split into oxygen ions with addition of two electrons. These O ions migrate to anode through electrolyte and combine with one hydrogen molecules with formations of H2O and two electrons. Thus the chemical reactions are :
At anode : H2 + O— = H2O + 2e
At cathode : 1/2 O2 + 2e = O—
Overall reaction : H2 + 1/2 O2 = H2O + heat
The electrons move through the external circuit to produce power. The output voltage is about 0.65V at about 800°C. A tubular type of SOFC system has been developed which operates at high temperature of about 900°C – 1000°C.
Like molten carbonate fuel cell, the heat of discharge can be utilized for process heat or additional power generation.
Other fuels for SOFC system :
Apart from H2, other sources of fuel for SOFC system could be methanol, ammonia and hydrazine.
Methanol can be catalytically reformed with steam at 200°C to yield the mixture of H2 and CO2 gas. Liquid NH3 – air fuel cell can be catalytically converted into H2 and N2. The electrolyte used is KOH solution.
Another source of fuel as hydrazine (N2H4) and air as oxidant can be used. Disadvantage of N2H4 is that it is toxic and highly reactive. This uses porous nickel as anode and silver deposit on porous material as cathode. The chemical reactions are :
At anode : N2H4 + 4OH– = N2
+ 4H2O + 4e
At cathode : O2 + 2H2O + 4e = 4OH–
Overall reaction : N2H4 + O2 =
N2 + 4H2O
Thus in this cell nitrogen, water vapour and heat is produced as by-product with generation of power.