I have done LSV for HER. I have applied potential from 0.5 to -0.8 and the steep decrease of current was observed from the potential -0.2V vs RHE and continue up to -0.6V vs RHE and I have got steep decrease of current towards negative scale in micro orders. Now I have to plot for the calculation of Tafel slope. When I am taking log j (current density), I will not get any results as log (-) current can't be evaluated. How can I plot Log j vs Overpotential and how the overpotential is measured?
For an accurate Tafel plot, you need to remove Ohmic overpotential and mass-transport (MT) overpotential to isolate the kinetic contribution to the measured potential. For HER there are typically no MT limitations within your potential region in liquid electrolyte (unless your working electrode is very large and then you get "bubble-trouble"). To remove the Ohmic losses you need to know the solution resistance, which is typically obtained by EIS. The HFR value from EIS can be used to calculate the Ohmic overpotential for each i-V data point: V(ohmic) = i(measured) * HFR. If you don't have EIS capabilities, many P/Gstats can compensate for ohmic overpotential using current-interrupt method such that your measured data is ~95% iR-compensated.
Once you have isolated the kinetic overpotential, you can use the absolute value of current (ignore the - sign from cathodic HER and just work with magnitude of current). This plot of log i vs E(kinetic) should be an accurate Tafel plot.
For an accurate Tafel plot, you need to remove Ohmic overpotential and mass-transport (MT) overpotential to isolate the kinetic contribution to the measured potential. For HER there are typically no MT limitations within your potential region in liquid electrolyte (unless your working electrode is very large and then you get "bubble-trouble"). To remove the Ohmic losses you need to know the solution resistance, which is typically obtained by EIS. The HFR value from EIS can be used to calculate the Ohmic overpotential for each i-V data point: V(ohmic) = i(measured) * HFR. If you don't have EIS capabilities, many P/Gstats can compensate for ohmic overpotential using current-interrupt method such that your measured data is ~95% iR-compensated.
Once you have isolated the kinetic overpotential, you can use the absolute value of current (ignore the - sign from cathodic HER and just work with magnitude of current). This plot of log i vs E(kinetic) should be an accurate Tafel plot.
Thank you Mike. I am doing as per you have suggested. If any problem arises, I will get back to you.
To get accurate Tafel slope, you should correct your LSV curve by IR compensation.
Don't need to take all data from LSV. Take only the data around the onset potential (here -0.2 V vs. RHE) from where steep increase in current was observed. You take data points in the range of -0.1 to -0.3 V .
Plot : potential vs. Log(|J/mA-cm-2|). ignore the sign of current density.
I am attaching a file of LSV containing data for HER of 5% Pt/C. How do I calculate the Overpotential from these data? Will I plot the applied potential vs log J? If so, then what is the difference between applied potential and Overpotential?
Is your measured LSV iR-compensated using current interrupt method? If not, you need to measure and account for resistance between WE & CE.
OCP just tells you the equilibrium potential at your WE wrt RE. Do you have EIS capabilities on your P/GStat? If so, I would recommend running potentiostatic EIS at a 2 or 3 potentials near on-set of HER. The high frequency resistance (HFR) will be the R(s) value in a circle-fit of a Nyquist plot. The HFR should be nearly constant over a few Pstat EIS scans and can be used to compensate for the Ohmic overpotential at each measured point.
For each data point: V(kinetic) = V(measured) - abs i(measured) * HFR.
If you don't have EIS, check to see if the instrument has the iR-compensation capability. It's important to correct for the Ohmic drop at higher currents. For example, the HFR of 0.1 M acid electrolyte is typically on the order of 10 Ohm, so at 1 mA current, you have: (V=iR) 1 mA * 10 Ohm = 10 mV of Ohmic overpotential. At your max current of 7 mA, you could have on the order of 70 mV Ohmic overpotential.
Mike Bates
Sir, I have just started my research. Actually, I need to know the criteria to decide whether a system should run at G/Pstats?
And what would be the advantages and disadvantages in both types of measurements?
I am new to water splitting and electrochemistry. I am trying to get the Tafel slope from Tafel plot. What part of Tafel plot should be used to extract Tafel slope for HER? Papers always mention the straight portion of the plot but it s kind of vague to me. Should it be the region around onset potential? what would be the potential window in that case? Thank you in advance for your answers.
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