Solid Polymer Electrolytes: Polymer Science meets Electrochemistry
Fuel cells convert the available energy of a fuel and oxidant into electrical power. Improvements in fuel cell technology combined with environmental concerns about conventional power generation have sparked widespread interest in polymer electrolyte fuel cells for portable, transportation and stationary applications. A schematic description of their operation is shown below for the H2/O2 system. The advantages of solid polymer fuel cells (SPE FC) are high power density, the absence of corrosive liquid electrolytes, the relative simplicity of stack designs (no moving parts), the sturdiness of the system, and the relatively low operating temperatures and operating pressure.
In SPE fuel cells, the central component is a polymeric membrane that provides an ionic path between the anode and the cathode of the galvanic cell and serves to separate the two reactant gases. The properties of the membrane are paramount to the successful operation and commercialization of fuel cells.
In the corollary case of solid polymer electrolyzers, electrical energy is converted into fuel for storage and future use. The advantages of solid polymer electrolyzers (SPE EL) are similar to those for fuel cells, the absence of corrosive liquid electrolytes, the relative simplicity of stack designs, and the sturdiness of the system.
Solid polymer electrolyte fuel cells and electrolyzers come in two forms, acidic and alkaline. Acidic systems rely on proton exchange polymer membranes (PEMs) to transport protons from one electrode to the other to support electrochemical reactions. Alkaline systems rely on anion exchange polymer membranes (AEMs) to transport hydroxide ions from one electrode to support electrochemical reactions. The most notable difference between PEM and AEM technology is the pH at which reactions are carried out, which dictates the nature of the catalyst that can be used. PEM technology is more mature, but requires expensive Pt-group metal catalysts; AEM technology has the potential to utilize much less expensive catalysts but the highly caustic conditions impinge on the stability of the membrane.
Fuel Cells vs. Electrolyzers, Acidic vs. Alkaline