Fuel Cell Working Principle
A fuel cell is an electro-chemical device in which the chemical energy of fuel is continuously converted into electric energy. This conversion of energy takes place at constant pressure and temperature.
To explain, fuel cell working principle, here we shall consider a hydrogen (H2) — oxygen (02) called Hydrox fuel cell. The main components of a fuel cell are:
- A fuel electrode (anode)
- An oxidant electrode (cathode)
- An electrolyte (a solution of H2SO4 for acidic fuel cell and KOH for alkali fuel cells)
- Additional components are container, separators, sealings, fuel and oxidant supply etc.
The basic feature of a fuel cell is that the fuel and the oxidant are combined in the form of ions then in form of neutral molecules.
Figure shows the schematic diagram of a fuel cell using hydrogen as fuel and oxygen as oxidant and alkaline solution of KOH as electrolyte.
It is called alkaline fuel cell (AFC). It consists of two permeable Nickel electrodes immersed in an electrolyte of good conductivity.
An electrolyte may be an alkaline solution of KOH as shown or acidic solution of H2SO4 called respectively as alkali fuel cells and acidic fuel cells.
The porous fuel electrode is anode (negative pole) and the other porous oxidant electrode is cathode (positive pole). These electrodes are separated by a porous gas barrier called separator (not shown in Figure).
The anode is supplied H2 gas as fuel at a certain pressure and the cathode is supplied 02, as oxidant at a pressure. These gases pass through the respective electrodes and bubble around through the electrolyte solution.
The pores provide an opportunity to gases, electrodes and electrolyte to come in contact for their electrochemical reactions. The electrodes are connected through an external circuit as shown in Figure.
The electro-chemical reactions are generally slow and a catalyst is required in the electrodes to accelerate the reaction. Platinum is the best catalyst but costly. In general, less expensive catalysts like nickel and silver are used according to application and design.
H2— O2 Fuel Cell Chemical Reactions
The hydrogen gas is ionized at anode and it produces a free electron and H+ ions. Every hydrogen molecule brought to electrode surface is dissociated into two H atoms by catalytic property of electrode.
These enter into electrolyte solution as hydrogen ions leaving behind two electrons which pass through the external circuit to the cathode (positive electrode). The reaction at anode is as follows:
Anode: H2 → 2H+ + 2e
The oxygen supplied to cathode (positive electrode) reacts with water of electrolyte and the electrons transmitted to it to produce hydroxyl (OH–) ions. Thus,
Cathode: ½O2 + H2O + 2e → 2OH–
These hydroxyl ions migrate from cathode to anode through electrolyte. The hydrogen and hydroxyl ions then combine in the electrolyte to produce water i.e.
2H+ + 2OH– → 2H2O
Above equation shows that OH ions produced at one electrode (cathode) are involved in a reaction at the other electrode (anode). By adding the above three equations, the overall process is chemical reaction of H2, and 02, gases to form water i.e.
Overall reaction: H2 + ½O2 → H2O
This is the net reaction of a fuel cell in which hydrogen and oxygen is supplied and the water, electrical energy and heat is produced.
Chemical Reaction with Acidic Electrolyte (H2SO4)
Anode: H2 → 2H+ + 2e
Cathode: ½O2 + 2H+ + 2e → H2O
Overall reaction: H2 + ½O2 → H2O
Thus the chemical reaction is similar to alkaline electrolyte.
H2 – O2 fuel cells are efficient in operation. The operating temperatures are generally in the range of 100oC to 200oC which reduces to 1.15 at 200oC. The actual value of EMF decreases with current and at rated value of current the EMF lies between 0.7V to 0.8V.
The rated voltage and current of fuel cells can be increased by using number of cells in series parallel combination. The expected life of a fuel cell is ten thousand hours.
Losses in Fuel Cell
As the fuel cell is subjected to load i.e. current is supplied, the voltage of the cell drops considerably with increase in load. The voltage – current characteristics of a fuel cell is shown in Figure.
The electrical energy generated by the fuel cell depends on the free Gibbs energy and not on the heat energy for overall cell reaction.
The maximum efficiency of a H2 — O2 fuel cell is about 83% at 25°C and 1 atmospheric pressure while the actual efficiency is in the range of about 50 to 60 percent.
The actual value of EMF generated at about 180°C is 1.12 volts. This loss of voltage at no load is called activation loss of fuel cell (E —Vb) as shown in Figure by curve (a – b). These losses are associated with the activity of cell i.e. its ability to dissociate and derive the chemical reaction at low temperature.
The voltage of the cell further drops as the current is drawn from the cell when subjected to load as shown by the curve (b — c). These losses represent the ohmic or resistance losses as a result of electrical resistance of the cell to the current. These losses (Vc — Vb) are called resistance losses of fuel cell.
At moderate currents the voltage Vc after activation and resistance losses of a (H2 — O2) fuel cell is in the range of 0.7 to 0.8 volts.
Beyond the point ‘c’ the EMF of cell drops suddenly due to mass transport processes in the cell. These losses occur when the cell’s ability reduces to maintain adequate concentrations of H2 and O2 in it due to high current demand.
Due to the reasons explained above, a fuel cell is normally operated in the region of the curve (b — c) of Figure. It can be further observed that voltage drop across the cell increases with the increase in temperature. Therefore in practice, a fuel cell is operated at higher end of its temperature range.
All the above losses of fuel cell combined together produce heat in the fuel cell. Thus, it becomes necessary to cool the fuel cell continuously to remove the heat generated so that the cell works efficiently.
Thanks for reading about ‘fuel cell working principle’.