Separating the Effects of Band Bending and Covalency in Hybrid Perovskite Oxide Electrocatalyst Bilayers for Water Electrolysis

Abstract

The Co−O covalency in perovskite oxide cobaltites such as La 1−x Sr x CoO 3 is believed to impact the electrocatalytic activity during electrochemical water splitting at the anode where the oxygen evolution reaction (OER) takes place. Additionally, space charge layers through band bending at the interface to the electrolyte may affect the electron transfer into the electrode, complicating the analysis and identification of true OER activity descriptors. Here, we separate the influence of covalency and band bending in hybrid epitaxial bilayer structures of highly OER-active La 0.6 Sr 0.4 CoO 3 and undoped and less-active LaCoO 3 . Ultrathin LaCoO 3 capping layers of 2−8 unit cells on La 0.6 Sr 0.4 CoO 3 show intermediate OER activity between La 0.6 Sr 0.4 CoO 3 and LaCoO 3 evidently caused by the increased surface Co−O covalency compared to single LaCoO 3 as detected by X-ray photoelectron spectroscopy. A Mott−Schottkyanalysis revealed low flat band potentials for different LaCoO 3 capping layer thicknesses, indicating that no limiting extended space charge layer exists under OER conditions as all catalyst bilayer films exhibited hole accumulation at the surface. The combined X-ray photoelectron spectroscopy and Mott−Schottky analysis thus enables us to differentiate between the influence of the covalency and intrinsic space charge layers, which are indistinguishable in a single physical or electrochemical characterization. Our results emphasize the prominent role of transition metal oxygen covalency in perovskite electrocatalysts and introduce a bilayer approach to fine-tune the surface electronic structure.

Diffusion-Mediated Morphological Transformation in Bifunctional Mn2O3/CuO–(VO)3(PO4)2·6H2O for Enhanced Electrochemical Water Splitting

“…The shift in the electron density from the Mn 2 O 3 /CuO semiconductor to the VOP overlayer (as depicted by XPS spectra of V 2p and P 2p) and the lower φ value of VOP confirms the electroactive nature of the VOP molecule. Also, there is the formation of the n–p heterojunction by virtue of favorable band alignments between the Mn 2 O 3 and CuO (confirmed from the M-S plot and DFT calculations) along with the overlayer of 2D VOP having inherent faster charge transfer kinetics, which allows facile water oxidation and reduction reactions. − Thus, in the composite, the formation of a type-II heterojunction between Mn 2 O 3 and CuO results in the availability of more electrons and holes at the electrolyte interface for both the OER and HER. In the literature, similar conclusions were also proposed using the change in Gibb’s free energy and electrochemically active sites calculated using DFT. − The generated holes in the valence band of Mn 2 O 3 transfers to the valence band of CuO due to the favorable band alignment, leading to the accumulation of charge carriers at the semiconductor–electrolyte interface to be effectively utilized for the OER.…”

“…In the presence of CuO in the composite, the formation of an n–p heterojunction allows the material to act as an HER (hydrogen evolution reaction) catalyst. In Mn 2 O 3 /CuO–VOP, the type-II heterojunction between Mn 2 O 3 and CuO allows a facile charge transfer and accumulation due to their favorable band alignments, which are suitable to reduce the water molecules generating hydrogen with the 2D VOP overlayer, accelerating the reaction process. − Thus, the synthesized electrocatalysts were subjected to HER in the alkaline medium by varying the potential from 0.1 to −1 V versus the RHE. Figure S12a represents the LSV curves of all bare counterparts where the obtained overpotential (η 10 ) at cathodic current density of 10 mA/cm 2 for Mn 2 O 3 is 550 (±2) mV and for CuO is 490 (±2) mV.…”

CuxCo3–xO4 Spinel Nanofibers for Selective Oxidation of 5-Hydroxymethylfurfural into Fuel Additives

“…The partial density of states (PDOS) of Cu x Co 3– x O 4 in Figure a–c is calculated in detail to assess the effect of Cu introduction on the electronic structure of Co. The negative and positive values correspond to the antibonding and bonding states of Co–O, respectively . For Co 3 O 4 , the O 2p band mainly overlaps with the Co 3d antibonding state, which indicates that Co 3 O 4 has strong Co–O bonding.…”

Facile Synthesis and Origin of Enhanced Electrochemical Oxygen Evolution Reaction Performance of 2H-Hexagonal Ba2CoMnO6−δ as a New Member in Double Perovskite Oxides

“…Typically, the perovskite oxide (ABO 3 ) structure involves 3d transition metals at the B-site, and these metals are active for OER. − Versatile crystal and electronic structures can be achieved via altering the A-site and/or B-site elements in a perovskite oxide structure. Hence, the perovskite oxide structure offers a great platform for establishing the material’s nature and electrochemical performance relationship. − Several times, it has been reported that the OER activity of perovskite oxides is closely interrelated with the electronic structure of B-site metal, lattice oxygen participation, and oxygen vacancies. − In fact, this implies that any strategy manipulating the B-site element can result in a variation in the OER activity. Although in most studies, oxidation of the B-site element has been varied via substitution of a B-site element, − A-site management strategy-induced OER performance has been less explored.…”