Ceria-reduced graphene oxide as a co-catalyst for oxygen reduction in low platinum loading acidic fuel cells

Hydrogen has become, especially during the recent years, one of the protagonists on the continuous race towards sustainable and clean energy sources. Following this trend, proton exchange membrane fuel cells (PEMFCs) are electrochemical devices able to directly convert chemical energy into electricity oxidizing hydrogen in the anode and reducing oxygen in the cathode with a total reaction 𝐻2(𝑔)+ 1/2 O2 H2O (𝑔/𝑙), producing only water as by-product and free electrons collected by an external circuit to produce energy. The very high efficiency of these systems (~60%) compared to combustion systems (~30%) encourages their study and their development. Anyways, PEMFCs have some problems to overcame, the main one is related to the low kinetics of the oxygen reduction reaction (ORR) that occurs on the cathode side of the cell and that requires the employment of a catalyst to enhance the kinetics and, at the same time, reduce the overpotential of the process. The best choice in terms of catalytic activity for this type of fuel cells is Platinum, but it is very expensive (~$28.95 per gram) due to its low abundance and it suffers, like other noble metals, for easy passivation and contamination. For this reason, the demand for a cheaper catalyst alternative is of paramount importance. Here, 𝐶𝑒𝑂2/𝑟𝐺𝑂 (Ceria-reduced-Graphene Oxide) is proposed as a promising additive to the commercial Platinum over carbon (Pt/C) catalyst, aimed to reduce the Platinum loading and enhance the catalyst efficiency. 𝐶𝑒𝑂2 has attracted a lot of interest as additive in catalytic fields for its redox properties consisting on the ability to easily change oxidation state passing from Ce4+ to Ce3+, respectively CeO2 to Ce2O3 structure creating oxygen vacancies in 3+ oxidation state to get crystal neutrality. Oxygen vacancies in non stoichiometric ceria act as catalytic centers where the reduction of the oxygen can take place. Moreover, thanks to its hygroscopicity, 𝐶𝑒𝑂2 can give a help in the cell water management and can reduce flooding effects. This non stoichiometric compound acts also as an oxygen radical scavenger, thanks to their redox conversion consuming unwanted peroxides of the complex ORR. Despite all these good qualities, it is known that cerium ions may become soluble in water, migrating from the electrode towards the membrane in which they can form ionic interaction with the anionic sulfonic groups of the acidic polymer, thus reducing the proton conductivity [1]. Reduced graphene oxide (𝑟𝐺𝑂) is found to be a good choice to get a better stability of the ceria and, according to its reduction degree, it can promote good electrical conductivity and provide high surface area for a more efficient catalyst activity. Herein, a composed ceria-reduced graphene oxide is proposed as a co-catalyst to the standard Pt/C to lower the amount of platinum and reduce the overall cost of this type of systems. Finally, the role of this co-catalyst in boosting the ORR is exhibited.