Tunable Internal and Surface Structures of the Bifunctional Oxygen Perovskite Catalysts

Abstract

candidates, whose performance is nearly identical to IrO 2 at a catalytic activity. [ 5 ] Any transition-metal-oxide perovskite that has an e g -fi lling (σ * -orbital occupation) close to 1 shows the maximum ORR activity, on the base that the B-site transition-metal-oxygen covalency between metal-3d and oxygen-2p works as a criterion in determining catalytic activity. [ 2,6 ] As a mixed electronic-and ionic-conductor (MEIC), Ba 0.5 Sr 0.5 Co x Fe 1− x O 3−δ (BSCF) is a well-known perovskite material that keeps a wide range of oxygen stoichiometry (3-δ) under different conditions such as temperature, oxygen partial pressures (pO 2 ), and B-site cation ratio (Co/Fe). [ 7,8 ] The structural/chemical fl exibilities and high oxygen nonstoichiometry (δ), while maintaining the cubic symmetry of crystal structure, enables it to be applied as a bifunctional oxide electrocatalyst in diverse renewable energy fi elds, e.g., metal-air batteries, oxygen membrane, and solid oxide fuel cell (SOFC). [9][10][11][12][13] Upon the base of the fl exible nonstoichiometry in the perovskite structure, a single chemical composition perovskite can be facilely modifi ed to meet the required criterion of the OER and ORR catalytic performance. This, in turn, implies that even a good OER/ORR perovskite catalyst can be digressed into a poor catalyst via internal changes of oxygen vacancy concentration, mixed valence state of each transition metal ion, etc., and vice versa. As for the ORR catalytic performance, which is in general evaluated by ORR activity as "on-set potential" that signifi es the facile rate of reaction initiation in the beginning of electrochemical activities, [ 5,6 ] though, the peroxide formation rate (HO 2 − %), limiting kinetic current ( i d ) and electron transfer number ( n ) also should be counted. [ 4,[14][15][16] They contribute signifi cantly to transport limiting current which occurs at the three phase boundaries (gas, solid catalyst, and liquid electrolyte) during the ORR process of the full-cell test. In this study, we focus on how the limiting kinetic currents ( i d ) of the electrochemical performance are affected by the changes of internal and surface structures in BSCF series catalysts.When the defect chemistry of BSCF series is manipulated by monitoring oxygen nonstoichiometry (δ), not only the valence of B-site cation and unit cell volume expansion change, [ 7,8,17 ] but also the surface morphology and chemistry are expected to vary under different heat treatment conditions. [ 8,14,18 ] Particularly, optimizing surface structure via engineering the defect Perovskite oxide ceramics attracts signifi cant attention as a strong candidate of bifunctional oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) electrocatalyst for the metal-air batteries. Numerous approaches to the viability of bifunctional perovskite electrocatalyst represent that the electrochemical performance is highly correlated with defect chemistry, surface structure, and overall polycrystalline perovskite structure. By making ...

Perovskite materials in energy storage and conversion

“…Upon heat treatment of BSCF in an oxygen atmosphere with an oxygenation process, not only did the crystallinity of the overall cubic perovskite structure increase but also the inherent heterogeneous surface‐layer spinel phase between the amorphous layer and cubic perovskite bulk was removed, which contributes to the enhancement of the ORR activity of the pristine BSCF catalyst. Later, they also investigated the heat‐treatment effect of Ba 0.5 Sr 0.5 Co x Fe 1 − x O 3 − δ ( x = 0.2 and 0.8) perovskites in the Ar atmosphere at 950 °C [Ar‐BSCF and Ar‐Ba 0.5 Sr 0.5 Co 0.2 Fe 0.8 O 3 − δ (BSCF5528)] on the surface structure and electrocatalytic performance . Upon heat treatment under an Ar atmosphere, the amorphous thickness layer in BSCF increases rapidly with decreased ORR activity, while there is little change in BSCF5528 with significantly enhanced ORR performance.…”

“…Owing to their low cost, high efficiency and flexible structures, perovskite oxides have recently drawn tremendous attention as promising alternatives to high‐efficiency OER electrocatalysis in alkaline media . Most notably, BSCF, which was formerly known as a high‐performance perovskite in oxygen permeation membranes and SOFCs, has been identified by Shao‐Horn and co‐workers as a highly active OER catalyst that even outperforms the gold‐standard IrO 2 in terms of intrinsic activity .…”

Design of Efficient Bifunctional Oxygen Reduction/Evolution Electrocatalyst: Recent Advances and Perspectives

“…[49,50] Perovskite oxides (ABO 3 ) are also widely investigated as bifunctional ORR/OER catalyst in alkaline electrolytes. [21,[51][52][53][54][55][56][57][58] Their compositions and properties can be finely tuned by partially substituting A and B with other metal components. Typically, A-site mainly influences on the capability of oxygen adsorption, and B-site mainly influences on the activity of the adsorbed oxygen.…”

A Composite Catalyst Based on Perovskites for Overall Water Splitting in Alkaline Conditions

“…However, the conventional hydrogen production using a steam reforming process with fossil resources promotes fossil fuel depletion and CO 2 emissions. [5,6] Therefore, tremendous efforts have been devoted toward developing active catalysts to accelerate their chemical kinetics. [3,4] Nonetheless, its commercial application is still hindered by the sluggish reaction kinetics of oxygen evolution reaction (OER) and hydrogen evolution reaction (HER).…”