A High-Entropy Oxide as High-Activity Electrocatalyst for Water Oxidation

Abstract

High-entropy materials are an emerging pathway in the development of high-activity (electro)catalysts because of the inherent tunability and coexistence of multiple potential active sites, which may lead to earth-abundant catalyst materials for energy-efficient electrochemical energy storage. In this report, we identify how the multication composition in high-entropy perovskite oxides (HEO) contributes to high catalytic activity for the oxygen evolution reaction (OER), i.e., the key kinetically limiting half-reaction in several electrochemical energy conversion technologies, including green hydrogen generation. We compare the activity of the (001) facet of LaCr_0.2Mn_0.2Fe_0.2Co_0.2Ni_0.2O_3-δ with the parent compounds (single B-site in the ABO₃ perovskite). While the single B-site perovskites roughly follow the expected volcano-type activity trends, the HEO clearly outperforms all of its parent compounds with 17 to 680 times higher currents at a fixed overpotential. As all samples were grown as an epitaxial layer, our results indicate an intrinsic composition-function relationship, avoiding the effects of complex geometries or unknown surface composition. In-depth X-ray photoemission studies reveal a synergistic effect of simultaneous oxidation and reduction of different transition metal cations during the adsorption of reaction intermediates. The surprisingly high OER activity demonstrates that HEOs are a highly attractive, earth-abundant material class for high-activity OER electrocatalysts, possibly allowing the activity to be fine-tuned beyond the scaling limits of mono- or bimetallic oxides.

Keywords: green hydrogen; high-entropy oxides; oxygen evolution reaction; perovskite oxide catalysts; scaling reactions; water electrolysis.

Figure 1

(a) Schematic illustration of the P-HEO electrocatalyst for water electrolysis. (b) The epitaxial layer is deposited via PLD. The PLD target is synthesized via sintering of P-HEO powder using reverse co-precipitation followed by calcination (see Methods section). The insets show optical micrographs of the powder and the sintered target and an X-ray diffractogram of the target, which confirm the single-phase perovskite structure, as detailed in our previous work.

Figure 2

(a) Atomic force microscopy morphology of an 11 nm P-HEO deposited on a Nb:SrTiO₃ substrate. (b,c) HRXRD patterns of the same P-HEO film, indicating epitaxial growth of the HEO thin film in the (001) orientation of the perovskite structure.

Figure 3

(a) STEM micrograph of a P-HEO film on an SrTiO₃ substrate. The top surface was protected with Au/Al₂O₃, giving rise to clustered features above the film surface. (b) FFT of (a). (c) Magnified view of TEM micrographs of the P-HEO film. (d) EDX chemical composition maps of the film and the substrate.

Figure 4

(a) Cyclic voltammetry of P-HEO and its parent compounds. The plots show the average of anodic and cathodic scans in the second consecutive cycle. (b) Comparison of the specific OER activities (current density at an overpotential of 450 mV, i.e., 1.68 V vs RHE). The underlying volcano is a guide for theeye, inspired by the predicted activity volcano from ref (50).

Figure 5

(a) Valence band spectra of P-HEO and its parent perovskite oxides. The intensity was normalized to the area of the binding energy region −2 to 8 eV. O 2p states (peaks A and B, blue) and TM 3d states (peak C, red) are indicated schematically for the examples of LaCrO₃ (low degree of covalence) and P-HEO (high degree of covalence). Raw data are shown as open dots; lines are a guide for the eye obtained by 5-point adjacent average smoothing. The valence band maximum is indicated via the zero photoemission intensity intercept of a linear regression fit of the low-binding-energy edge of the valence band spectra. (b) O 2p nonbonding state binding energy and OER activity for P-HEO and parent perovskite oxides. The error bars represent the possible maximum deviation of the consecutive measurements (top panel) or estimated relative errors from triplicate CV measurements (bottom panel). Note that the parent compounds are ordered by rather than atomic number of the TM. (c) Schematic energy band diagram for LaNiO₃, LaCrO₃ and P-HEO. The Fermi level (E_F) is labeled by dashed lines in (a) and (c), and the valence band maximum is schematically shown as linear extrapolation of the leading edge of the VB.

Figure 6

(a) Cr 2p core level of P-HEO during APXPS experiments. (b) Cr 2p core level of P-HEO with different mean escape depths d. Cr 2p core level of LaCrO₃ is shown for reference. Spectra collected in UHV after annealing in O₂. (c) TM 3p core levels of P-HEO during APXPS experiments. (d) VB spectra of P-HEO during APXPS experiments. Raw data in (a), (b), and (d) are shown as open dots; lines are a guide for the eye obtained by 5-point adjacent average smoothing. (e) Hypothetical reaction mechanism involving three reaction intermediates (RI) and surface diffusion between a strongly binding site (red) and a weakly binding site (blue).