We are working with the oxygen reduction reaction on N-doped carbon nanofibers. We are facing problems in obtaining a plateau current, see the figure. We are wondering what factors can affect the plateau current? Is it possible that we could have a problem with diffusion within the material since the carbon nanofibers have more micropores?
You may consider this issue this way. For example, the mixed kinetics-diffusion control region of your Pt/C is between 0.6 and 1.0 V, and some more active catalysts such as PtNi between 0.8 and 1.0 V. The higher the catalytic activity, the higher onset potential and narrower the kinetics-diffusion control region. You may consider your carbon catalyst is not so active, thus the onset potential may be so negative within the hydrogen evolution region that you cannot observe.
In order to observe a plateau, the rate limiting step should be the diffusion of oxygen not the kinetic of ORR. In you case look like the catalyst is not that active and the rate limiting step is the kinetic of ORR. To confirm that you have to test the catalyst performance on rotating disc electrode to take out the diffusion limitations. If you cannot get a plateau then definitely the catalyst is not performing very well.
I agree with the Sofia for the explanation of you results. However, you can do a few things to improve you results.
1. Try to increase catalyst loading on the electrode surface. This can improve your onset potential as-well as plateau current. For detailed explanation you can consult following publications as we have similar results. Journal of Power Sources 220 (2012) 306-316; Carbon 64 (2013) 472-486.
2. Try changing the sweep rate. The effect of sweep rate depends on porosity of your samples. For more details see same publications.
3. Factors also include nitrogen doping level. By increasing the amount of nitrogen in the surface to about 10 at. % you can improve the activity. If you can alter nitrogen surface groups by post synthetic treatment this could be the way to go.
Bare in mind that increasing the nitrogen content the catalyst conductivity will decrease.
Basically, increasing the active surface area may increase the current intensity. If the active surface area is high enough, you will observe the plateau current even the catalyst is not so active. On the other hand, decreasing the oxygen concentration (such as lower rotation speed of the disc electrode) in the solution, you also may observe the plateau current. There is a balance between your active sites and oxygen amount.
Theoretically, increasing the catalyst loading may not change your onset potential that depends on the reaction kinetics.
I agree with Dr. Chunhua Cui. As the KL equation shows that, if ik is significantly larger than il, thus it is mass-controlled process；Vice versa, if il is significantly larger than ik,
it is kinetic-controlled process. Diffusion limited current is proportional to oxygen concentration and kinetic current is proportional to active sites' densities. So the balance between diffusion limited current and kinetic current in fact is the balance between oxygen concentration and active sites' densities.
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