Hi,
Why the catalyst (NiMn in 1M KOH) shows such a high Tafel value for HER? How I can interpret and justify in relation to overpotential.
Kindly see the attachment.
"Tafel value?" Do you mean Tafel slope? You did no describe how you obtained your data. Your measurements are in a very narrow range of potentials.
Firstly for the Tafel Plot you need to take the Log base 10 of the Current on the x-axis. Also Plot you provided appeared to have a negative Tafel slope that curve is reflected over the x-axis. As for the magnitude 277mV/dec is some what large comparing to the theoretical value around 120mV/dec for OHad, but it is not unrealistic. Certain catalyst structures that are too compacted could cause reaction to be diffusion limited, composition choices that certain metals are better for absorbing certain targeted molecules than others, and also if the system has external factors such as excess or type of ionomers, and string speed or no string at all from the RDE could all attribute to the increase of the Tafel slope. The Tafel slope provide you the information about the kinetics of your reaction. Also are you sure it's for HER? your solvent is alkaline, I believe it gives you values about OH absorption.
I agree with the others too, the information you provided are too limited. It's hard to know that exactly is happing without the structure of your catalyst and the type of reaction you are carrying out.
Hope it helps!
Thank you Chuqing Liu for your reply. I have used spinel Ni/MnO-Ni foam as a catalyst for hydrogen evolution reaction.
Hi Tahazal, in the case of Nanomaterial Foam, the Tafel Slope with a magnitude of 277mV/dec is reasonable due to the intricate networking system which impede the physical molecular diffusion. Assuming there is no influence from the solvents you are using, stirring factors and scanning rate, I would recommend two ways to improve at the catalyst level.
1. Physically: making the foam less aggregated, in the case of networks, make it more separate out, if were wires making them shorter. Although, this method might decrease the ECSA and CV results a little, but it would increase the kinetics of your reaction, faster diffusion.
2. Chemically: if Ni/Mn-O-Ni is your catalyst of choice try to vary the composition a little (e.g. increase your NI or Mn content), this is harder to do. I would first read over the literatures on Ni & Mn for HER, see what elemental ratio other people use. Of course you might have to test many combinations of compositions.
Lastly, concentration of your electrolyte, stirring speed (if u are using RDE), scan rate, temperature will all have fact, but probably want to be consistent with other literatures.
Best!
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