Detection of Free Oxygen Reduction Reaction Intermediates Generated at Pt Nanoparticles By Electron Paramagnetic Resonance Spectroscopy
Authors: den Hartog Stephan Alexander , Samanipour Mohammad , Ching Hong Yue Vincent , Van Doorslaer Sabine , Breugelmans Tom , Hubin Annick , Ustarroz Jon 
Published: February 2020

In the search for clean energy conversion systems, PEM fuel cells have attracted a lot of attention over the last decades. Nanostructured electrocatalysts such as porous platinum nanoparticles (NPs) are considered one of the most promising advances to lower the cost of these fuel cells and make them more commercially attractive. The efficiency of these catalysts is limited by their activity towards the oxygen reduction reaction (ORR), and most research is thus focused on understanding reaction mechanisms and improving the activity. The mechanism is well understood for alkaline conditions, but there is still some debate for that under acidic conditions. It has been shown indirectly[1],[2] that radical intermediates formed during the reaction can desorb from the catalyst surface, but direct proof is still absent. More importantly, these free radicals are suspected to be responsible for the degradation of the carbon support, leading to the deactivation of the electrocatalyst NPs[3].

We propose an approach to detect and identify the free radicals produced during the ORR using electron paramagnetic resonance (EPR) spectroscopy. EPR spectroscopy combined with spin trapping has been shown to be useful for detecting radicals produced in electrochemical reactions[4],[5]. This method relies on binding the free oxygen radicals to the spin trap DMPO, forming the stable radical complex DMPO-OH•. The DMPO-OH complex can be detected using an EPR spectrometer. Custom-made glassy carbon RDE’s with a large geometrical surface area have been designed and were loaded with Pt nanoparticles via double pulse electrodeposition. The large surface area is necessary in order to produce sufficiently large amounts of free radicals to be detectable in the EPR spectrometer.

A DMPO-OH• signal is formed and increases in strength for the duration of the ORR experiments. Furthermore, we observe a direct correlation between the charge produced during the experiment and the strength of the DMPO-OH• signal. This approach, capable of detecting the free oxygen radicals produced during the ORR, is employed to shed light on the ORR mechanism at carbon electrodes loaded with Pt NPs in acidic conditions.

References

[1] J. M. Noël, Y. Yu, and M. V. Mirkin, “Dissolution of Pt at moderately negative potentials during oxygen reduction in water and organic media”, Langmuir, vol. 29, no. 5, pp. 1346–1350, 2013.

[2] J. M. Noël, A. Latus, C. Lagrost, E. Volanschi, and P. Hapiot, “Evidence for OH radical production during electrocatalysis of oxygen reduction on pt surfaces: Consequences and application”, J. Am. Chem. Soc., vol. 134, no. 5, pp. 2835–2841, 2012.

[3] J. Ustarroz et al., “Mobility and Poisoning of Mass-Selected Platinum Nanoclusters during the Oxygen Reduction Reaction”, ACS Catal., 2018.

[4] S. Neukermans et al, “A continuous in-situ EPR electrochemical reactor as a rapid in-depth mechanistic screening tool for electrocatalysis”, Electrochemistry Communications, Vol. 97, pp. 42-45, 2018.

[5] M. Kitazawa, A. Y. Nosaka, and Y. Nosaka, “Radical formation in polymer electrolyte fuel cell components as studied by ESR spectroscopy,” J. Appl. Electrochem., vol. 38, no. 4, pp. 491–496, 2008.