Oxidation State and Oxygen-Vacancy-Induced Work Function Controls Bifunctional Oxygen Electrocatalytic Activity
Pralay Gayen, Sulay Saha, Kaustava Bhattacharyya, Vijay Ramani
Abstract
Developing highly active oxygen evolution and reduction reaction (OER/ORR) bifunctional electrocatalysts is key to multiple technologies, including regenerative fuel cells and metal-air batteries. To this end, we have investigated structure–activity relationships in Pb2Ru2O7–x having pyrochlore structure by tuning the structural oxygen vacancy (Ovac) and metal oxidation states. Increase in Ovac with temperature boosts the ORR activity by facilitating molecular oxygen dissociation via decrease in work function. Ovac formation is accompanied by lowering of the Ru(V)/Ru(IV) ratio due to charge-compensation which leads to decreased OER activity. Air-annealing of Pb2Ru2O7–x accelerates the formation of Ovac in comparison to Ar-annealing since atmospheric oxygen facilitates the reduction and phase-segregation of Ru from Pb2Ru2O7–x as RuO2. A maximum bifunctionality index and specific bifunctionality index of 0.69 V and 274.0 μA/cm2BET, respectively, are observed for pristine Pb2Ru2O7–x. However, the activity is skewed toward OER for pristine Pb2Ru2O7–x, creating an asymmetric bifunctional property which is not desirable for practical applications. To reduce the asymmetric behavior, pristine and air-annealed Pb2Ru2O7–x samples at 700 °C are physically mixed which yields a higher symmetric OER/ORR activity (|ΔiOER(η = 0.25 V)-ORR(η = −0.45 V)specific|: pristine = 0.25 mA/cm2BET, Air-700 °C = 0.20 mA/cm2BET, physical mixture = 0.037 mA/cm2BET). The inverse OER/ORR relationship in Pb2Ru2O7–x is attributed to the presence of an optimal ratio of 0.75 for Ru(V)/Ru(IV) and Ovac/Olattice, which provides symmetric bifunctional activity essential in electrochemical devices. An increase in Ru(V)/Ru(IV) ratio in pristine Pb2Ru2O7–x with no detectable Ru dissolution in the electrolyte observed subsequent to a 5-h OER hold-test, confirming high stability.