Chemistry of Fuel Cells
General Chemistry of a Fuel Cell
Fuel cells work via an electrochemical reaction that converts the chemical
energy stored in a fuel directly into electricity. There are a variety
of types of fuel cells which utilize different electrochemical reactions
(see Types of Fuel Cells) but the general
process is always the same. Fuel is oxidized at the anode, electrons flow
through an external circuit to do electrical work, and then fuel is reduced
at the cathode. Most research today focuses on the hydrogen-oxygen fuel
cell that employs the following half-reactions:
Cathode half reaction: O2 + 4H+ + 4e- -> 2H2O
Overall reaction: 2H2 + O2 -> 2H2O
This simple reaction is the appeal of hydrogen-oxygen fuel cells; by combining hydrogen gas and oxygen in a non-combustion process, only electricity, heat and water are produced. This is the exactly the reverse of the popular chemistry demonstration where an applied electric current splits water into its two constituent gasses (see Electrolysis of Water). The reactions take place over precious metal catalysts, usually platinum or an alloy of platinum, palladium, or ruthenium, and the protons then travel through a special proton exchange membrane. (Fuel Cell Catalysts)
An ideal hydrogen-oxygen fuel cell produces an open circuit potential of 1.229 volts. Real fuel cells, however, face three main types of resistances that reduce performance. Depending on the resistance of the external load applied to the fuel cell, the cell voltage and current varries as shown in the graph to the right. Each type of resistance can be the predominant loss factor depending on the cell voltage and current density.
The activation barrier is the major factor contributing to inefficiency when operating a cell at high voltage and low current density. Approximately 200 mV of potential are lost because the reactions take more energy to catalyze than in the ideal case, especially reduction of oxygen at the cathode. Researchers are looking for better catalysts and more efficient ways of utilizing the catalyst in order to reduce the activation barrier.
In the heart of the fuel cell's operating range, voltage drops as current density increases according to Ohm's Law (V=I/R), where the resistance in this case is that of the physical cell and circuit.
At very high current densities, the required fuel and waste water flow rates exceed the capabilities of the cell's physical structure, and mass transport losses quickly degrade performance. Mass transport is analogous to a traffic jam inside the cell.
The power output of a fuel cell (or any electrical device) is equal to
the voltage multiplied by the current, and is measured in watts. Therefore,
the more current that can be produced at a given voltage, the more power
the cell can deliver.
The Resistive Losses in a Fuel Cell
