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About

Here I provided a short-guide of experimental acquisition data. I summarized the main things you need to take into account but gcarrascohuertas is not responsible for the bad use of this guide and its consequences over materials and people involved. Please, be careful and document all steps you perform.

Electrochemical setup

First you need following materials in order to perform experiments:

  • Nitrogen and Oxygen gas cylinder with appropriate security system (e.g. https://www.airliquide.com )

  • Rotating disk electrode equipment (e.g.RDE Metrohm )

  • Three-electrode jacket cell with two-way gas purge (gas-inflow and gas-outflow) (Scheme)

  • Laboratory water chiller connected to jacket cell at 298 K.

  • Potentiostat which can perform Cyclic voltammetry, Chronoamperometry and Electrochemical impedance spectroscopy (e.g. Autolab PGSTAT302N - High Performance )

  • PC with electrochemical acquisition software (e.g. NOVA software)

  • Three-electrode cell

    • Platinum mesh as counter electrode (CE)
    • Ag/AgCl/KCl (3.5M) as reference electrode (RE)
    • Your working electrode (WE). Here I used a Carbon Super P slurry deposited over a polished rotating-glassy carbon electrode (GC, 4 mm diameter, Metrohm). Carbon slurry consists of 5 mg (carbon material after going through a 150 microns sieve), ethanol (170 µl, Merck) and Nafion 117 (5 wt%, 47.5 µl, Merck). Slurry was sonicated for 5 minutes and then was drop-coated (7 µl of the slurry) over a rotating-disk tip (try to fulfill all the glassy carbon active area).

  • Electrolyte

    • Here you can use different electrolytes depeding on your WE material (e.g. 0.1 M KOH, 0.5 M H2SO4 , 0.1 M HClO4, etc..). In the example provided here contained in folder carbon_superP 0.1 M KOH was used.

Electrochemical techniques

Electrochemical techniques performed are cyclic voltammograms with a staircase profile (CV), Chronoamperometry (CA) and Electrochemical impedance spectroscopy (EIS).

  • Cyclic voltammograms with a staircase profile (CV) parameters: Potential windows have to be ranged in the optimum potential windows of your electrode-electrolyte system. Here I used a potential window which varies from 0.1 to -0.9 V vs. VRef repeated 3 times at 10 mV/s fixed scan rate. Upper vertex and Lower vertex were set to 0.11 V and -0.01 V vs. VRef . Step was set to -0.00244 V

  • Chronoamperometry (CA) parameters: This analysis was performed only for 1600 rpm in a O2 saturated electrolyte.

  • Electrochemical impedance spectroscopy (EIS): First activate the cell and measure OCP with 10 seconds duration, interval time of 0.1, time to average 5 seconds and dE/dt limit of 1E-06 V/s. Second step is to apply 10 mV perturbation vs. OCP measured previously. Third step is to wait 5 seconds. Four step is to perform EIS measurement with following parameters: (First applied frequency: 1E+05 Hz, last applied frequency 0.1 Hz, Number of points of freq per decade 10, Amplitude 10 and wave type sine with internal correction.

Electrochemical measurements

In order to follow recomendatios of Kocha et.al., 2017 , first the electrolyte is thermostated at 298 K and then purged with high purity Nitrogen for at least 30 min before each N2 measurements at 0 rpm, 1000 rpm and 2000 rpm. Once nitrogen measurements are achieved electrolyte is purged with Oxygen for at least 30 min prior to O2 measurements at 0 rpm, 250 rpm, 500 rpm, 750 rpm, 1000 rpm , 1200 rpm, 1400 rpm, 1600 rpm, 1800 rpm and 2000 rpm. For purging you can use a large stainless steel needle inserted into a septum (see scheme).

WARNING: Save experimental ORR-results files as following examples in order to avoid problems with cronological order: - In case of nitrogen analysis: N2_2000rpm for 2000 rpm analysis - In case of oxygen analysis: O2_0250rpm for 250 rpm analysis

Experimental NOVA software routine

Once you finish setting-up all the electrochemical cell components. Create two experimental sequences (one for nitrogen and one for oxygen) with the parameteres displayed in the screenshots attached in this project.

WARNING: Due to NOVA software is licensed I can not upload sequence (.nox file). If you want NOVA software sequence for Nitrogen also Oxygen measurements (.nox files) do no hesitate to contact me at : [email protected]

Data treatment. First step: Reference data to RHE

All tools cretaed in this project are refered to RHE as reference scale potential . In order to obtain referenced potential to RHE, first see :

Applying E(RHE) equation:

E(RHE) = E(Ag/AgCl) + 0.059*(pH) + Eo(Ag/AgCl)

with following parameters for our electrode used in this project (Ag/AgCl , 3.5M KCl):

  • Eo(Ag/AgCl) = 0.1976 V at 298 K.
  • E(Ag/AgCl) = Working potential = Ag/AgCl (3.5M KCl) +0.205 V.
  • pH = pH of solution , in our case was 12.8 for KOH 0.1 M.

Data treatment. Second step: Proccesing ORR curves

Data from nitrogen 0 rpm analysis will be substrated to oxygen 0 rpm analysis (O2_0rpm - N2_0rpm)

Data from nitrogen 1000 rpm analysis will be substrated to oxygen 250 , 500 , 750 and 1000 rpm analysis:

  • O2_250rpm - N2_1000rpm
  • O2_500rpm - N2_1000rpm
  • O2_750rpm - N2_1000rpm
  • O2_1000rpm - N2_1000rpm

Data from nitrogen 2000 rpm analysis will be substrated to oxygen 1200 , 1400 , 1600 and 2000 rpm analysis:

  • O2_1200rpm - N2_2000rpm
  • O2_1400rpm - N2_2000rpm
  • O2_1600rpm - N2_2000rpm
  • O2_2000rpm - N2_2000rpm

Data treatment. Third step: Koutecky-Levich Analysis

Before you obtain results from data read this website for undestand Koutecky-Levich Analysis:

https://pineresearch.com/shop/kb/theory/hydrodynamic-electrochemistry/koutecky-levich-analysis/

The Koutecký–Levich equation models the measured electric current at an electrode from an electrochemical reaction in relation to the kinetic activity and the mass transport of reactants. The Koutecky-Levich (K-L) analyses of the RDE data derived from the limiting current measured at various potentials. The number of electrons transferred per O2 molecule (n) was calculated from these plots using the K-L equation.

Following parameters were used in data contained in folder carbon_superP:

  • Scan rate (ν): 0.01004 cm2/s
  • Concentration of O2 at 298 K (C-02): 1.39e-3 mol L-1 (KOH 0.1 M at 298 K)
  • Diffusion constant of O2 (D-O2): 1.9e-5 cm2/s (KOH 0.1 M at 298 K)
  • Faraday constant : 96485 C / mol