Triple phase boundary

A triple phase boundary (TPB) is a region of contact between three different phases. This concept is particularly important in the field of fuel cells, where the three phases are an electrolyte, an electrode, and a gaseous fuel. The electrochemical reactions that fuel cells use to produce electricity occur in the presence of these three phases, so the triple phase boundaries can be thought of as the active areas of the cell.

The oxygen reduction reaction that occurs at a solid oxide fuel cell's cathode, can be written as follows:

O
2(gas) + 4e⁻(electrode) → 2O2−
(electrolyte)

Different mechanisms bring these reactants to a TPB to carry out this reaction.^[1] The kinetics of this reaction is one of the limiting factors in cell performance, so increasing the TPB density will increase the reaction rate, and thus increase cell performance.^[2] Analogously, TPB density will also influence the kinetics of the oxidation reaction that occurs between oxygen ions and fuel on the anode side of the cell. Transport to and from each TPB will also affect kinetics, so optimization of the pathways to get reactants and products to the active area is also an important consideration. Researchers working with fuel cells are increasingly using 3D imaging techniques like FIB-SEM to measure TPB density as a way of characterizing cell activity.^[3] Recently, processing techniques such as infiltration have been shown to substantially increase TPB density, leading to higher efficiency and, potentially, more commercially viable SOFCs.^[4]

See also

• Glossary of fuel cell terms

References