Enhancement of Activity Low-Pt-Content O2-Reduction Catalysts through Formation of Hybrid Systems with Sub-Stoichiometric Cerium Oxide Nanostructures
Authors: Kostuch Aldona , Kulesza Pawel J. , Wadas Anna , Dembinska Beata , Rutkowska Iwona A. , Zdunek Kinga , Negro Enrico , Di Noto Vito , Vezzu Keti 
Published: July 2022

Platinum is a main catalyst for the electroreduction of oxygen, a reaction of primary importance to the technology of low-temperature fuel cells. Due to the high cost of platinum, there is a need to significantly lower its loadings at interfaces. However, under such condition, O2-reduction reaction (ORR) often proceeds at a less positive potential, and produces higher amounts of undesirable H2O2-intermediate. Recent studies have demonstrated that the rational design of a catalytic system is based on nanostructures of platinum, or carbon-supported platinum (Pt/C), and metal oxide (MOx). By combining the catalytic properties of both components, such hybrid Pt-MOx catalysts are capable of tuning of the activity and durability of Pt. Due to strong interactions of MOx with Pt catalytic nanoparticles leading to the improved activity and durability during ORR, certain nanostructured metal oxides are considered as co-catalysts or active components of supports. The existence of specific interactions between MOx and noble metal (Pt) nanoparticles should improve the stability and activity of the metal catalytic sites due to modification of the Pt electronic structure and diminishing of the oxo (OH) species adsorption on Pt surface, thus promoting centers for the adsorption of oxygen and the cleavage of O=O bonds. The interactions mentioned above would also facilitate dispersion of Pt, inhibit their detachment and further aggregation, and, consequently, prevent or decrease their degradation during the fuel cell operation [1].
The catalytic activity of substoichiometric ceria, CeOx (where x < 2) additive results from its unique structure, presence of oxygen vacancies, and other features resulting from the 4f-electronic configuration of cerium. While the oxygen defects are likely to serve as active oxygen adsorption sites, the mixed-valent CeIII/CeIV redox sites should permit the electron shuffling within the lattice of oxygen vacancies and enhancing the ORR activity. The formation of oxygen-defects is accompanied by the localization of electrons left behind in Ce 4f states, thus leading to the formation of CeIII species capable of elongating and reducing the O–O bond strength of the adsorbed O2 molecule; consequently, by increasing the relative ratio of CeIII-to-CeIV, the ORR electrocatalytic activity can be improved [1]. Therefore, in the present study, we consider the intentionally reduced CeOx nanostructured components through subjecting them to high-temperature pretreatment in the presence of argon gas admixed with hydrogen. It is reasonable to expect that, the increased population of CeIII sites on the surfaces of the pretreated ceria particles would stabilize the neighboring active Pt-metal centers, exhibit reductive interactions toward Pt-oxo species, and to improve durability of the catalytic materials. Thus the boundary formed between the Pt-metal and CeOx-metal oxide should facilitate inhibition of the Pt oxide formation by the CeOx layer. Furthermore, our recent studies clearly show that, in the presence of ceria, the oxidative degradation of carbon carriers is also largely decreased. Finally, cerium oxide is known to act as the oxidative scavenger for free radicals such as hydroxyl (HO•) and hydroperoxyl (HOO•), which, once generated, would otherwise lead to the formation of undesirable hydrogen peroxide. Sub-stoichiometric CeOx is capable of rapidly switching between CeIII and CeIV oxidation states, thus inducing the decomposition of both radicals and peroxides. The above observation seems to be very helpful when it comes to designing highly active and durable ORR catalysts containing low platinum loadings.
[1] A.Kostuch, I.A. Rutkowska, B. Dembinska, A. Wadas, E. Negro, K. Vezzù, V. Di Noto, P.J. Kulesza, Molecules 26 (2021) 5147.

Acknowledgements: This work was supported by the National Science Center (Poland) under Opus Project (2018/29/B/ST5/02627) and under auspices of the European Union EIT Raw Materials ALPE 19247 Project (Specific Grant Agreement No. EIT/RAW MATERIALS/SGA2020/1).