the potential at which you observe the sharp increase in reduction current is known as onset potential....keeping in mind you should select the starting point for ORR. all the best

Best regards,

Piyush Kumar sonkar

For a particular electrode in a specific electrolyte medium, onset potential should be constant...

It is the potential where all the thermodynamic and kinetic barriers become downhill. This is constant for a particular electrode. So if you measure ORR of Pt and when I measure it we should get the same results. However this also depends on how you and I prepare the electrodes ( Different crystal facets will have different onset potentials or sometimes even doesnt catalyze the reaction. So Pt(100) and Pt (211) might give different onset potentials. If you prepare Nickel nanoparticles or electropolished smooth Nickel, they may have different onset potentials. Onset potential is usually considered as one of the important indication for the catalytic activity along with the exchange current density in electrocatalysis. However It is usually poorly defined term some people just look manually where the current starts to increase, Some draw tangent lines from the rising current and baseline current and look at the intersect

Electrochemical instabilities[1,2] might occur near the Potential for ORR that might make difficult the measurement of the onset potential.  EIS[2,3] techniques and/or rotating disk electrode methods[4,6] might help you for the measurement of the onset potential.

1.  The theory of electrochemical instabilities   http://ac.els-cdn.com/0013468692850805/1-s2.0-0013468692850805-main.pdf?_tid=dc76cffa-3eec-11e7-852f-00000aab0f6c&acdnat=1495457449_439b75e72277474aed93cbe2e3599fb4

2.  Application of Impedance Spectroscopy to Electrochemical Instabilities   http://link.springer.com/chapter/10.1007/978-3-642-27673-6_3

3.  An impedance model for analysis of EIS of polymer electrolyte fuel cells under platinum oxidation and hydrogen peroxide formation in the cathode   http://ac.els-cdn.com/S1572665716300789/1-s2.0-S1572665716300789-main.pdf?_tid=4116a8ee-3ef3-11e7-a9e8-00000aacb35d&acdnat=1495460194_9422ae8418ff600bb0adea0c9f720621

4.  Benchmarking the oxygen reduction reaction activity of Pt-based catalysts using standardized rotating disk electrode methods   http://ac.els-cdn.com/S0360319915020996/1-s2.0-S0360319915020996-main.pdf?_tid=910b28ba-3eea-11e7-a2a0-00000aab0f01&acdnat=1495456463_b931a51a63b71d187608ca358630064d

5.  O2 Reduction on an Ink‐Type Rotating Disk Electrode Using Pt Supported on High‐Area Carbons   http://jes.ecsdl.org/content/145/11/3713.full.pdf+html

6   pH matters: The influence of the catalyst ink on the oxygen reduction activity determined in thin film rotating disk electrode measurements   http://www.sciencedirect.com/science/article/pii/S0378775317304627/pdfft?md5=501891ab26eabaddef8b09a3f09702fc&pid=1-s2.0-S0378775317304627-main.pdf

Thank you All;

Linear Sweep Voltammetry (LSV) for onset potential, this value variable due to the selection of starting point. the oxidation peak potential of differential pulse voltammetry (DPV) was used for constant value of onset potential (Journal of Electroanalytical Chemistry 634 (2009) 49–58). This method was recommended by peer reviewers in my  paper, which can help you for the measurement of the onset potential.

Intrinsically it's a ill-defined concept. (Ref: https://www.springer.com/us/book/9783642295508)

The ref is a electrochemical dictionary. Look up onset potential in it pls.

You can't find it's position in thermal-dynamics, at all. Because of the maxwell-botzmann distribution, a chemical system doesn't just "start a reaction" when it‘s given the activation energy. It changes continuously, as the Butler-Volmer equation describes.

Also the derivation of the onset potential is kind of BS as well. Fitting an intrinsically non-linear line with a straight line is simply arbitrary.

However, it's intuitive, it's easy to use and easy to get, and it kind makes sense if you don't think about it. So people still use it.

It's a coincidence that the same electrode have the same onset potential. It's linked to the linearity of derivative.

... the ORR onset potential is determined by the point, at which the current for oxygen reduction is first observed (Wang, 2015, 10.1016/j.jpowsour.2015.07.107)

It is not well-defined in the literature. Authors use different ways to measure the onset potential. It is the point where the current starts to raise from "zero" or a tangent could be drawn to obtain the onset potential. Usually the former is employed in most of the articles. If it is a molecular catalyst (homogeneous or heterogeneous), E1/2 is a good reference point.

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