Describe the features that one desires when selecting a fuel cell electrode material. Describe and illustrate both features of electrode structure and properties as well as interactions between the electrodes and other fuel cell components (e. g., reactant gases, electrolyte, product gases, interconnects). Identify some specific electrode materials and why they work well for a (b) PEMFC and (c) SOFC, respectively.

(a) The necessary features are: 1) Intimate interconnectivity of all three phases (gas phase, ion, and electron conductors) (2) Fine interconnected microstructure to produce many triple phase boundaries (3) Similar thermal expansion coefficients for all solid phases (4) Lack of chemical reactivity amongst the phases.

(b) Proton exchange membrane fuel cells (PEMFC) usually make good electrode-electrolyte interfaces by using a composite of three materials; carbon particles, which each support (hold in place) many platinum nano-particles, and Nafion®.  The carbon conducts electrons to/from each of the catalyst particles, while Nafion® conducts ions to/from the same catalyst sites.  Sufficient porosity for access of the gas phase is also required.  Fine microstructure must be introduced with intimate connections of all four phases (incl. catalyst) to produce lots of TPB and fully connected paths for each phase (electron, ion, and gas) must be included to make each TPB “active.

(c)  Solid oxide fuel cells (SOFC) usually make good electrode-electrolyte interfaces by using a composite of two materials; one for conducting electrons and one for conducting ions, or by using mixed ionic electronic conducting materials.  Also use of some of the electrolyte phase at the interface (usually YSZ) can help with matching thermal expansion and increasing the mechanical strength of the interface.  Fine microstructure must be introduced to produce lots of TPB and fully connected paths for each phase (electron, ion, and gas) must be included to make each TPB “active.

Explanation:

In the selection of specific fuel material, it is vital to consider some characteristics of the interface between the electrode and electrolyte such as the interconnectivity and thermal expansion coefficient among all phases. The SOFC normally uses a two-material composite to ensure the good interface between the electrode and electrolyte while the PEMFC uses a three-material composite for effective interface between the electrode and electrolyte.