Influence of the Oxygen Partial Pressure on the Oxygen Reduction Reaction Kinetics for the Low Platinum Loading Catalysts
Authors: Jäger Rutha , Ereth Kasatkin Piia , Härk Eneli , Kanarbik Rait , Kallio Tanja , Jiang Hua , Joost Urmas , Aruväli Jaan , Paiste Päärn , Kurig Heisi , Tallo Indrek , Lust Enn 
Published: February 2020

The performance of fuel cells is first and foremost defined by the kinetics of the oxygen reduction reaction (ORR). The electrocatalysts have to conform among the others with the following requirements set by the Department of Energy (DOE) like: reduced precious metal loading, increased activity, improved durability/stability, and increased tolerance to air, fuel and system-derived impurities [1]. One possibility to ensure the excellent performance of the electrode is using the electrocatalyst with the small size platinum nanoparticles deposited onto the hierarchically porous carbon support. In this way, the reactant gas (oxygen or air) can be uniformly distributed over the catalyst surface. Many authors reported the influence of the oxygen/air pressure on the kinetics of ORR and demonstrated that the effect of pressure on the oxygen solubility is more significant in dilute electrolyte solutions compared to the concentrated ones [2-4].
Therefore, in this work the electrocatalytic activity toward ORR for 8.3wt%Pt-C, 12.4wt% Pt-C and 20wt%Pt-Vulcan was analysed in pure oxygen and synthetic air saturated 0.1M KOH electrolyte solutions at atmospheric pressure.
Platinum nanoparticles were deposited (8.3 and 12.4wt%) onto the molybdenium carbide derived carbon support [5] using the sodium borohydride method. The commercially available 20wt%Pt-Vulcan catalyst was used as a reference system for comparison [6]. The several physical and electrochemical characterization techniques like the low temperature nitrogen sorption, scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, inductively coupled plasma mass spectrometry, x-ray diffraction analysis, cyclic voltammetry and rotating disc electrode methods were applied.
The results clearly indicate that increasing partial pressure of the oxygen significantly influences the ORR kinetics, due to improved solubility of the oxygen in alkaline electrolyte solution. As expected, the limiting diffusion current density values for various Pt catalysts in oxygen saturated solution found to be approximately 5 times higher than that determined for the same catalysts in the syntethic air conditions. This result is in good agreement with literature data [3].
For detailed electrochemical analysis the Tafel-like plots were constructed. The results established indicate that the ORR mechanism remains unchanged and four electron reduction mechanism is valid for both catalysts, e.q. for the synthesised and commercial one.
It should be emphasized that electrochemical activity toward ORR for 8.3wt% Pt-C catalyst was higher than that for 20wt%Pt-Vulcan in 0.1M KOH solution at fixed oxygen partial pressure conditions.

References:

[1] https://www1.eere.energy.gov/hydrogenandfuelcells/.../fuel_cells.pdf
[2] M. Chatenet, M. Aurousseau, R. Durand, and F.Andolfatto, Journal of The Electrochemical Society, 150(3), D47 (2003).
[3] A. Parthasarathy, S. Srinivasan, A.J. Appleby, and C.R. Martin, Journal of The Electrochemical Society 139(10), 2856 (1992).
[4] W.-Y. Yan, S.-L. Zheng, W. Jin, Z. Peng, S.-N. Wanga, H. Du, and Y. Zhang, Journal of Electroanalytical Chemistry, 741, 100 (2015).
[5] A. Jänes, T. Thomberg, H. Kurig, and E. Lust, Carbon, 47, 23 (2009).
[6] E. Härk, R. Jäger, and E. Lust, Electrocatalysis, 6, 242 (2015).

Acknowledgments

This project has received funding from The Estonian Ministry of Education and Research (institutional research project IUT20-13, personal research grant PUT55) and European Regional Development Fund (The Centres of Excellence TK117 and TK141).	The performance of fuel cells is first and foremost defined by the kinetics of the oxygen reduction reaction (ORR). The electrocatalysts have to conform among the others with the following requirements set by the Department of Energy (DOE) like: reduced precious metal loading, increased activity, improved durability/stability, and increased tolerance to air, fuel and system-derived impurities [1]. One possibility to ensure the excellent performance of the electrode is using the electrocatalyst with the small size platinum nanoparticles deposited onto the hierarchically porous carbon support. In this way, the reactant gas (oxygen or air) can be uniformly distributed over the catalyst surface. Many authors reported the influence of the oxygen/air pressure on the kinetics of ORR and demonstrated that the effect of pressure on the oxygen solubility is more significant in dilute electrolyte solutions compared to the concentrated ones [2-4].
Therefore, in this work the electrocatalytic activity toward ORR for 8.3wt%Pt-C, 12.4wt% Pt-C and 20wt%Pt-Vulcan was analysed in pure oxygen and synthetic air saturated 0.1M KOH electrolyte solutions at atmospheric pressure.
Platinum nanoparticles were deposited (8.3 and 12.4wt%) onto the molybdenium carbide derived carbon support [5] using the sodium borohydride method. The commercially available 20wt%Pt-Vulcan catalyst was used as a reference system for comparison [6]. The several physical and electrochemical characterization techniques like the low temperature nitrogen sorption, scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, inductively coupled plasma mass spectrometry, x-ray diffraction analysis, cyclic voltammetry and rotating disc electrode methods were applied.
The results clearly indicate that increasing partial pressure of the oxygen significantly influences the ORR kinetics, due to improved solubility of the oxygen in alkaline electrolyte solution. As expected, the limiting diffusion current density values for various Pt catalysts in oxygen saturated solution found to be approximately 5 times higher than that determined for the same catalysts in the syntethic air conditions. This result is in good agreement with literature data [3].
For detailed electrochemical analysis the Tafel-like plots were constructed. The results established indicate that the ORR mechanism remains unchanged and four electron reduction mechanism is valid for both catalysts, e.q. for the synthesised and commercial one.
It should be emphasized that electrochemical activity toward ORR for 8.3wt% Pt-C catalyst was higher than that for 20wt%Pt-Vulcan in 0.1M KOH solution at fixed oxygen partial pressure conditions.

References:

[1] https://www1.eere.energy.gov/hydrogenandfuelcells/.../fuel_cells.pdf
[2] M. Chatenet, M. Aurousseau, R. Durand, and F.Andolfatto, Journal of The Electrochemical Society, 150(3), D47 (2003).
[3] A. Parthasarathy, S. Srinivasan, A.J. Appleby, and C.R. Martin, Journal of The Electrochemical Society 139(10), 2856 (1992).
[4] W.-Y. Yan, S.-L. Zheng, W. Jin, Z. Peng, S.-N. Wanga, H. Du, and Y. Zhang, Journal of Electroanalytical Chemistry, 741, 100 (2015).
[5] A. Jänes, T. Thomberg, H. Kurig, and E. Lust, Carbon, 47, 23 (2009).
[6] E. Härk, R. Jäger, and E. Lust, Electrocatalysis, 6, 242 (2015).

Acknowledgments

This project has received funding from The Estonian Ministry of Education and Research (institutional research project IUT20-13, personal research grant PUT55) and European Regional Development Fund (The Centres of Excellence TK117 and TK141).