Estimation of the Rate Constants for the 2x2-Electron ORR by Combining the Voltammograms of O2 and H2O2 Reduction Reactions
Authors: Muthukrishnan Azhagumuthu , Nabae Yuta 
Published: February 2020

1. Introduction

After the major discovery of the platinum based electro-catalysts for the cathodic reduction of the oxygen in the polymer electrolyte membrane fuel cells, commercialization of PEM fuel cells begins for automobile applications. However, it is necessary to replace the platinum-based electro-catalysts due to the high cost and scarcity by using other cheap and abundant non-precious metal catalysts. Fe-containing nitrogen-doped carbon catalysts (Fe-N-C) have been demonstrated as the potential alternative for the Pt-based catalysts for the last few years. However the poor understanding of the active sites of Fe-N-C catalysts prevents the development of highly active durable catalysts. Our recent studies[1] of the ORR on the Fe-N-C and N-C catalyst demonstrates that the ORR predominantly proceeds via the step wise reduction. Though the conventional Damjanovic model[2] (as shown in Scheme 1) concludes the direct 4-electron ORR, it actually overestimates the k
1 and underestimates the k
2 and k
3. Since the metal free catalysts are very active towards the reduction of O2 to H2O2, the role of the Fe on the reduction of H2O2 to H2O is studied and the estimated rate constants for the reduction of H2O2 to H2O will be used to estimate the real current for direct 4-electron ORR (I
1), 2-electron ORR to H2O2 (I
2) and 2-electron H2O2 reduction to H2O (I
3).

2. Methodology

The precursor synthesis has been already discussed in the previous papers[3,4]. Briefly, the stoichiometric amount of pyromellitic acid dianhydride and 4,4’-oxydianiline was polymerized in the presence of tris(acetylacetonato) iron (III) (Fe(acac)3) yielding polyimides/Fe(acac)3 (the size of the polymer is controlled by maintaining low temperature during polymerization). The precursor was subjected to the multi-step pyrolysis reported earlier and the resulting catalyst is designated as Fe/PI. The Fe/PI-modified GC electrode with the loading density of 100 µg cm-2, carbon cloth and reversible hydrogen electrode was used as working, counter and reference electrodes, respectively. Both oxygen reduction reaction (ORR) and H2O2 reduction reaction was measured using rotating disk and rotating ring (Pt)-disk electrode voltammetry in 0.5 M H2SO4 solution. The H2O2 reduction rate constants (k
3) with potential were estimated using Koutecký‒Levich plot. From the ORR voltammograms, the value of I
1
, I2
and I
3
were determined by the following equations;

I
2=0.5*{I
r/N+ [(Ir/N)2(1+4k3/(Zω0.5)]0.5} (1)
I
3=0.5*{-I
r/N+ [(Ir/N)2(1+4k3/(Zω0.5)]0.5} (2)
I
1=I
d - [(Ir/N)2(1+4k3/(Zω0.5)]0.5 (3)
where I
d and I
r represents disk and ring current of the oxygen reduction reaction, respectively, k
3 represents the rate constant of the H2O2 reduction reaction and ω refers the rotational speed of the ORR voltammogram. The value of the Z can be estimated from the kinematic viscosity (v) and diffusion coefficient (D) of H2O2 in 0.5 M H2SO4 (Z=0.62D
2/3
v
-1/6)

3. Results and Discussion

The H2O2-reduction steady-state voltammograms indicates that the Fe/PI catalyst is more active for both H2O2 oxidation and reduction indicated by the sharp intersection on the potential axis around 0.8 V vs. RHE. This simple method will be helpful separate the currents of many ORR voltammograms to describe the kinetics because this does not requires any parameters to be plotted against the rotation speed. The I
2 and I
3 values estimated using this method is significantly improved than the conventional Damjanovic model currents estimated form the rate constants (Figure 1).

4. Conclusion

The newly developed methodology successfully separated I
1, I
2 and I
3 for ORR over Fe/N/C cathode catalysts.

5. Acknowledgements

This work was financially supported by the New Energy and Industrial Technology Development Organization (NEDO), Japan.

6. References

[1] A. Muthukrishnan, Y. Nabae, T. Okajima, T. Ohsaka, ACS Catalysis, 5 (2015) 5194-5202.
[2] A. Damjanovic, M.A. Genshaw, J.O.M. Bockris, J. Chem. Phys., 45 (1966) 4057-4059.
[3] Y. Nabae, Y. Kuang, M. Chokai, T. Ichihara, A. Isoda, T. Hayakawa, T. Aoki, J. Mater. Chem. A, 2 (2014) 11561-11564.
[4] Y. Nabae et al., Sci. Rep., 6 (2016) 23276.

Figure 1