Oxidation State and Oxygen-Vacancy-Induced Work Function Controls Bifunctional Oxygen Electrocatalytic Activity

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/cm2 BET, 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 (|Δi OER(η = 0.25 V)‑ORR(η = −0.45 V) specific|: pristine = 0.25 mA/cm2 BET, Air-700 °C = 0.20 mA/cm2 BET, physical mixture = 0.037 mA/cm2 BET). 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.

Engineering Crystallinity and Oxygen Vacancies of Co(II) Oxide Nanosheets for High Performance and Robust Rechargeable Zn–Air Batteries

“…At the same time, lowering the oxygen partial pressure tends to lower the μ O , hence reducing the formation energy of oxygen vacancies. [ 27 ]…”

Steric Hindrance‐ and Work Function‐Promoted High Performance for Electrochemical CO Methanation on Antisite Defects of MoS2 and WS2

“…Work function ( Φ ) regulation is useful for tailoring the catalytic activity, where low Φ value usually means high activity [18] . In experiments, work function regulation can be achieved by changing the coordination environment of active sites, such as structural defects introduction, [18d] new interactions addition, [18c,e] and heteroatoms doping [18f,g] . For double S vacancy of TMDs (DV‐TMDs), three under‐coordinated transition metal (uc‐TM) atoms constitute an active site as shown in Figure 2a, and their coordination environment can be changed by tensile strain and heteroatoms doping, while the bowl structure is retained.…”

Novel engineering of ruthenium‐based electrocatalysts for acidic water oxidation: A mini review

“…Generally, as shown in Figure 7(A), pyrochlore oxides can be denoted as A 2 B 2 O 6 O′ or [A 2 O′][B 2 O 6 ], which consists of a network of corner‐sharing BO 6 octahedra with interstitial sites being occupied by O′ and A atoms (A 2 O′) 109 . The Ru‐based pyrochlore oxides, in which the B sites are dominated by Ru atoms have also drawn a great deal of notice because of the lowered usage of Ru, while maintaining the excellent catalytic activity and stability toward OER 49,50,52–54,110–113 . For examples, Kim et al 49 used a sol–gel method to synthesize Y 2 Ru 2 O 7−δ, in which all diffraction of Y 2 Ru 2 O 7−δ was consistent with cubic phase ( Fd3m ) pyrochlore‐type Y 2 Ru 2 O 7 (YRO) without any unpurified phase.…”