Electrode Structure

Electrode structures were introduced in Chapter 5. One of the distinguishing features of a fuel-cell electrode is that three phases are present. First, there is a solid phase that is electronically conductive to supply or remove electrons. Second, an electrolyte phase that conducts ions is needed. The electrolyte may be solid or liquid. So far, this arrangement is similar to that of a battery. The difference is that the reactants and products flow to and from the electrode to allow continuous operation. What’s more, the reactants and products are distinct from the electrolyte, and invariably a gas phase is present in practical devices. Thus, contact between three phases is needed to carry out the electrochemical reactions. This region of contact is known as a triple-phase boundary (TPB). Of course, the intersection of three phases in three dimensions defines a line. As we have learned from Chapter 5 on porous electrodes, one key to good performance is to achieve a high surface area for reaction. In fact, from their inception, this precise challenge for fuel cells was identified by Grove (he is credited with the discovery of the fuel cell in 1839), which he referred to as the “notable surface action.” Let’s look at two examples of fuel-cell electrode structures that are widely used to get around the difficulties of bringing three phases together while providing adequate surface area for the reactions.

Let’s first examine the SOFC. Shown on the left in Figure 9.7 is the line that defines the intersection of the three phases. The reaction is limited to a very small region (TPB) around the line, and only very small currents can be drawn from this cell.

A common approach for solid oxide fuel cells is to make a porous electrode that is both ionically and electronically conductive. This region is called a mixed ionic–electronic conductor (MIEC). More specifically, it is a composite of two materials, but still porous so that the reactant gas can enter the region. Typically, the effective conductivities for the electronic and ionic phases are within a couple orders of magnitude of each other. With this composite material, the reaction zone goes from a line to a three-dimensional region where the reactions can occur throughout the region, which is electrochemically active.

The second example is used in cells with liquid electrolytes (Figure 9.8):

On the left, imagine we have a planar electrode partially immersed in aqueous KOH. The region indicated at the surface of the liquid is the only place where all three phases are present. Because there is some small solubility of oxygen in the electrolyte, the reaction zone is not strictly just the line; even so, the performance of such a system would be poor. The flooded-agglomerate model, introduced in Chapter 5, describes the approach to overcome this limitation by taking advantage of the solubility of the gases. On the right, small agglomerates of carbon and catalyst are seen that form an electrode structure. The particles are in intimate contact ensuring electronic conductivity, and at the same time a porous structure is formed to allow gas access. The microporosity of the agglomerate is filled with electrolyte to form a continuous ionic path through the electrode. The agglomerates are made small enough so that oxygen can dissolve and diffuse through the agglomerate, extending the reaction zone through the diameter of the agglomerate.

About 100 years after Grove’s discovery, Francis Bacon took up the hydrogen/oxygen fuel cell. Bacon, the father of the modern fuel cell, made seminal contributions to improving the electrode structure. Previously, the electrochemical reactions had been limited to a small interfacial area, resulting in low limiting currents because of the poor access of gases to the catalysts. Bacon developed alkaline fuel-cell electrodes with controlled porosity and almost immediately obtained current densities in excess of 10 kA·m⁻² at 0.6 V at temperatures around 230 °C. This concept was subsequently extended to phosphoric acid fuel cells (PAFCs). The theme of controlled porosity, highlighted in Chapter 5, has proved to be critical for the development of all fuel cells.