A variety of known electrochemical cells fall within a category of cells often referred to as solid polymer electrolyte (SPE) cells. An SPE cell typically employs a membrane of an ion exchange polymer which serves as a physical separator between the anode and cathode while also serving as an electrolyte. SPE cells can be operated as electrolytic cells for the production of electrochemical products or they may be operated as fuel cells for the production of electrical energy. The most well known fuel cells are those which operate with gaseous fuels such as hydrogen and with a gaseous oxidant, usually pure oxygen or oxygen from air, and those fuel cells using direct feed organic fuels such as methanol.
In some SPE cells including many fuel cells, a cation exchange membrane is employed and protons are transported across the membrane as the cell is operated. Such cells are often referred to as proton exchange membrane (PEM) cells. For example, in a cell employing the hydrogen/oxygen couple, hydrogen molecules (fuel) at the anode are oxidized donating electrons to the anode, while at the cathode the oxygen (oxidant) is reduced accepting electrons from the cathode. The H.sup.+ ions (protons) formed at the anode migrate through the membrane to the cathode and combine with oxygen to form water. In many fuel cells, the anode and/or cathode are provided by forming a layer of electrically conductive, catalytically active particles, usually also including a polymeric binder, on the proton exchange membrane and the resulting structure (sometimes also including current collectors) is referred to as a membrane and electrode assembly or MEA.
Membranes made from a cation exchange polymer such as perfluorinated sulfonic acid polymer have been found to be particularly useful for MEA's and electrochemical cells due to good conductivity and good chemical and thermal resistance which provides long service life before replacement. However, increased proton conductivity is desired for some applications, particularly for fuel cells which operate at high current densities.
In fuels cells which employ direct feed organic fuels such as methanol, a problem with known cells has been the so-called crossover of fuel through the membrane. The term "crossover" refers to the undesirable transport of fuel through the membrane from the fuel electrode or anode side to the oxygen electrode or cathode side of the fuel cell. After having been transported across the membrane, the fuel will either evaporate into the circulating oxygen stream or react with the oxygen at the oxygen electrode.
The fuel crossover diminishes cell performance for two primary reasons. Firstly, the transported fuel cannot react electrochemically and, therefore, contributes directly to a loss of fuel efficiency (effectively a fuel leak). Secondly, the transported fuel interacts with the cathode i.e., the oxygen electrode, and lowers its operating potential and hence the overall cell voltage. The reduction of cell voltage lowers specific cell power output and also reduces the overall efficiency. Therefore, it is especially desirable to provide a cation exchange membrane for use in a fuel cell which has a low fuel crossover rate.