How polymer electrolyte membrane fuel cells work
Fuel cells are electro-chemical energy conversion devices which are highly efficient in converting the chemical energy derived from renewable fuel into equivalent electric energy. Significant development and research activities have been focused on the process of developing proton-exchange membrane fuel cells which have a polymer electrolyte membrane, composed of an ion exchange polymer, and is disposed between a cathode (positive electrode) and an anode (negative electrode).
The role of polymer electrolyte membrane is to provide a means for the transport of ions and for separation of the cathode and the anode compartments. More particularly, the proton-exchange membrane fuel cells traditionally have a polymer electrolyte membrane lying between two gas-diffusion electrodes, a cathode and an anode, each often containing a metal catalyst and supported by an electrically conductive material. These gas diffusion electrodes are exposed to the reactant gases, the oxidant gas and the reducing gas. At each of the two junctions or three phase boundaries, an electrochemical reaction occurs, where electrolyte polymer membrane, reactant gas and one of the two electrodes interface.
The electrochemical fuel cells occurring in case of hydrogen fuel cells during its operation at both electrodes i.e. cathode and anode are as following:
Anode: H2→2H++2e−; Cathode: ½O2+2H++2e−→H2O; Overall: H2+½O2→H2O.
During the operation of fuel cell, hydrogen permeates through the anode, interacts with metal catalyst and produces electrons and protons. The electrons are then conducted via an electrically conductive material to the cathode through an external circuit, while the protons are transferred simultaneously via an ionic route to the cathode through a polymer electrolyte membrane.
Oxygen on the other hand permeates to the catalyst sites of the cathode, gains electrons and then reacts with proton forming water. Consequently, the products of the reactions of proton-exchange membrane fuel cells are water, heat and electricity. Current is also conducted simultaneously through electronic and ionic route in the proton-exchange membrane fuel cells. Efficiency of these proton-exchange membrane fuel cells is said to be largely dependent on their ability to minimize both electronic and ionic resistivity.
An important role is played by Polymer electrolyte membranes in proton-exchange membrane fuel cells. The polymer electrolyte membrane has two main functions: (1) it acts as the electrolyte providing ionic communication between the cathode and the anode; and (2) it also serves as a separator for the two reactant gases (e.g., O2 and H2).
In other words, the polymer electrolyte membrane, while being useful as a good proton transfer membrane, must also have low permeability for the reactant gases to avoid cross-over phenomena that reduce performance of the fuel cell. This is especially important in fuel cell applications in which fuel cell are operated at elevated temperatures and the reactant gases are under pressure. If electrons are able to pass through the membrane, the fuel cell is partially or fully shorted out and the produced power is either reduced or even annulled.