Based on my own experience, the empirical value of diffusion limited current density of 5.8 mA cm–2 should be associated with the rotating rate of 1600 rpm.

For the current Pt/C catalysts, it is commonly  lower than the above value. However, for Pt alloys, this value may be larger. Furthermore, if the composite catalyst has higher ionic conductivity (usually this is offered by impregnated Nafion resin), it may also be higher than 5.8 mA cm–2. In addition, if co-catalysts are used, the diffusion limited current density may also be slightly higher.

You may compare Pt/C and Pt/TiO2. The much lower electrical conductivity of TiO2 as the support is expected to decrease the diffusion limited current density as confirmed by http://pubs.acs.org/doi/suppl/10.1021/ja904810h/suppl_file/ja904810h_si_001.pdf

Particle size and loading of the catalyst on the substrate affect the diffusion limited currents. Some catalysts reveal lower currents in case of incomplete coverage of GC substrate. Some Pt/C particles showed higher diffusion limited current due to surface roughness and thickness approaching the dimensions of the diffusion layer.

Hi Jiangshui and Hatem,

Thanks for your comments. About the rotating rate you are right and I was also talking about the same (1600 rpm).

You properly named a number of important parameters for diffusion limited currents like catalyst type and ionic conductivity of the ionomer, particle size of the catalyst and loading. Works of Gasteiger and Markovic, linked below, explain well these effects. 

But about the support conductivity I am wondering if the comparison of carbon and TiO2 could be wise. Since they are not only different in terms of conductivity, but also many other factors such as pore structure, catalyst support interactions, ionic diffusivity and may be, even the arrangement and quality of the film on the GCE. Therefore, probably, conductivity is not the only difference of these two and can hardly be known as the only reason of big change in diffusion limited current density. I have also seen similar works reporting lower values for metal oxide supports. But, a work reporting on supports of the same family (like carbon) with similar catalyst and loading and so on, but with different electrical conductivity qualities, is what I feel can best evidence such effect.

A nice idea can be checking similar catalyst loaded similarly on reduced graphene oxides (RGOs) with different degrees of reduction.

Article Activity benchmarks and requirements for Pt, Pt-Alloy, and n...

Article Measurement of oxygen reduction activities via the rotating ...

I totally agree with your points.

It would be interesting to read papers to justify "A nice idea can be checking similar catalyst loaded similarly on reduced graphene oxides (RGOs) with different degrees of reduction."

Concerning the influence of ionic conduction network (which is usually afforded by Nafion resin), I would like to give a very good example, which I noticed long time ago. You would see that they used the same support but different ionic conduction network: the use of ionic liquid vs. without ionic liquid. The former shows slight higher value than 5.8 mA cm–2! In fact, this example could also confirm that if Pt is replaced by its alloys, the value of diffusion-limited current densities could also be changed. See the paper titled "Oxygen reduction in nanoporous metal–ionic

liquid composite electrocatalysts" DOI: 10.1038/NMAT2878

http://www.nature.com/nmat/journal/v9/n11/full/nmat2878.html

Fig. 2.

Thanks again Jiangshui for the comment and the paper.

Probably all the factors and parameters cited, and many others you can imagine as relevant, would affect, either positively or negatively, the ORR difusion currents. The most difficult question would be opposite: which parameters do not affect it?

Dear Juan,

Thanks for your contribution in the discussion. Of course you are right about the variety of influential parameters. However, my main intention behind this question is to know the effect of the support’s electrical conductivity. This one has rarely been discussed directly in the literature. Do you have any idea or experience about that?

Mazdak

Dear Mazdak,

I have experience with ORR and RDE separately, but not direct experience with ORR on RDE. Anyway, mi "feeling" on your especific demand is that the conductivity of a well designed Pt/C cathode should not limit such a low current density... That's possibly the reason why this aspect is rarely discussed in the literature.

Dear Juan,

Thanks again for your answer.

I have done some conductivity measurements on a couple of carbon species (powder, compressed) for other applications and the values I could measure for activated carbon black (Norit) was in the range of 1 mS/m under 5 MPa pressure. You see that this is not a promising value for conductivity. Other species like CNTs (home-made) had a conductivity of almost 350 S/m, close to pure graphite (400 S/m). Also reduced graphene oxide was in the range 1 S/m that although better than non-electrochemical grade of A.C.B, is still low. Of course, those CBs specially prepared and treated for catalyst support of fuel cells should have a better conductivity, but I don’t expect a huge improvement for this type of carbon.

Referring to the question, do you think that even with conductivities similar to those mentioned above (the situation could be even worse in RDE, since there is not mechanical pressure on the carbon-catalyst layer on GCE) and also considering the wide range of difference showed (CNTs were almost x350 K more conductive than A.C.B) we shouldn't expect any difference in the current density level of diffusion limited region.

With so low conductivities you could indeed be limited by electric conduction instead of diffusion of disolved oxygen.

Ok Juan, Thanks

Hi everybody,

I am back with a complement to the same question.

Imagine that we have a support that is so poorly conductive that can actually limit the current level in the diffusion controlled region. In such case, being aware of the V–I relation under a purely resistive condition following the Ohm's law (V/I=R), shall we expect a slop formation in the plateau or there will be other parameters modifying this and keeping the plateau still horizontal? In case we expect a ramp due to ohmic resistance of the support, should it be uphill or downhill? For better understanding of the problem, check the attached file and see which condition (a, b or c) best fits your expectation.

Dear all

You can have a look for some ideas:

10.1002/cphc.200900817

10.1149/2.0011815jes

Cheers

Carlos

Ali Feizabadi You have given the same answer 351 times:

https://www.researchgate.net/profile/Ali-Feizabadi/answers

The answer is a link to one of your papers…

Are all your answers pertinent to the questions? Consider the ethics of what you're doing. At least you do not have recommendations for each answer...