Why the ring current is not increasing with respect to rotation in rrde of oxygen reduction
Answers
1. Diffusion limited current is scaling with the geometric surface if the loading is high enough. See Levich equation
2. The onset potential is changing with real surface area and therefore with the loading. Tafel Volmer equation.
Comparing different catalysts can however be tricky.What do you really want to compare?
The observed activity is a function of real surface area and specific activity. So when you compare different catalysts you want to compare the specific activity. This quantity might be hard to obtain if you cannot measure the real surface. so instead the mass activity might be a good substitute. To get a proper value you have to measure the activity for a wide range of cat. loadings as you can only claim to have the correct value if the activity is linearly changing with loading. But, keep in mind that different catalysts might have different surface area relative their mass so, comparing mass activity is like comparing apples and oranges.
Why you get a lower diffusion limit current can be because you either have a not fully covered surface or the film is so thick you see membrane effects. Thus, vary the loading to see if this was the issue.
Further, you might have a saturation problem. Thus, ALWAYS measure at different rotation rates and extracting the Levich B factor using a Levich-Koutechy plot. The B factor should be independent of the sample.
Good luck.