Corrosion is one of the major threats to a fuel cell. Unlike, for example, a car—where the danger is a slowly corroding hole in a metal part—the real danger in a fuel cell is the release of metal ions during corrosion. These ions can poison both the catalyst layer and the membrane. If metallic corrosion occurs inside the fuel cell, you can see the cell's degradation in real time.
In our case, an error during the gold plating of the flow fields caused parts of the copper cladding to be exposed to the inside of the OFC. Have you ever seen copper exposed to nature? It changes from that usual bronze finish to a dark green—verdigris. The same thing happened to one of our OFC models. Just look at the flow field: the gold layer is peeled away and the copper is exposed. The copper layer itself still looked reasonable—but the effect on the cell was not.
How do we know that copper ions are toxic to MEA? We dissected the dead OFC and examined individual components using X-ray fluorescence analysis (XRF). We observed high copper contamination of the MEA at both ends of the active zone, while high concentrations of nickel ions were observed more towards the middle. When we compared this to the image of the gold plated flow area, it was clear that the gold plating had very rough surface irregularities consistent with the copper contamination seen in the MEA.
And nickel—where did it come from? The assembly instructions provide a clue: in gold plating on copper, a nickel barrier layer is used to prevent the gold from spreading to the metal base, which would render the gold layer ineffective.
What conclusions can we draw from this experience? Metal ions are certainly toxic to a fuel cell—and have the potential to ruin your day if you're a fuel cell enthusiast. Therefore, always pay close attention to the quality of the surface finish and the polishing processes used. And if you examine other types of metal flow fields, such as stainless steel, the metal ions released by slow corrosion can be one of the reasons that cause the slow decay of the fuel cell.