High-Valence Oxides for High Performance Oxygen Evolution Electrocatalysis

Abstract

Valence tuning of transition metal oxides is an effective approach to design high-performance catalysts, particularly for the oxygen evolution reaction (OER) that underpins solar/electric water splitting and metal-air batteries. Recently, high-valence oxides (HVOs) are reported to show superior OER performance, in association with the fundamental dynamics of charge transfer and the evolution of the intermediates. Particularly considered are the adsorbate evolution mechanism (AEM) and the lattice oxygen-mediated mechanism (LOM). High-valence states enhance the OER performance mainly by optimizing the e_g -orbital filling, promoting the charge transfer between the metal d band and oxygen p band. Moreover, HVOs usually show an elevated O 2p band, which triggers the lattice oxygen as the redox center and enacts the efficient LOM pathway to break the "scaling" limitation of AEM. In addition, oxygen vacancies, induced by the overall charge-neutrality, also promote the direct oxygen coupling in LOM. However, the synthesis of HVOs suffers from relatively large thermodynamic barrier, which makes their preparation difficult. Hence, the synthesis strategies of the HVOs are discussed to guide further design of the HVO electrocatalysts. Finally, further challenges and perspectives are outlined for potential applications in energy conversion and storage.

Keywords: electrocatalysis; high-valence oxides; oxygen evolution reaction; valence tuning.

Figure 1

Design and synthesis strategies of high‐valence oxides as OER electrocatalysts.

Figure 2

a) Schematic of the OER pathways for oxides in acidic (red route) and alkaline (blue route) conditions. The black line indicates the adsorbates evolution mechanism (AEM), while the green line indicates lattice oxygen‐mediated mechanism (LOM). b) Example of the linear scaling relation of binding energies between the HO∗ and HOO∗ intermediates for molecular OER catalysts. Reproduced under the terms of the Creative Commons Attribution 4.0 International License.^{[ 26 ]} Copyright 2019, The Authors, published by Springer Nature.

Figure 3

a) OER catalytic activity, defined by the overpotential at 50 µA cm⁻² _ox, and the occupancy of the e _g‐symmetry electron of the transition metal (B in ABO₃). Reproduced with permission.^{[ 14 ]} Copyright 2011, American Association for the Advancement of Science. b) Mass activities and BET surface area‐normalized intrinsic activities of catalysts at η = 0.37 V derived from activity curves. c) Intrinsic activity versus e _g electron filling of PBSCF‐0, III. Reproduced under the terms of the Creative Commons Attribution 4.0 International License.^{[ 42 ]} Copyright 2017, The Authors, published by Springer Nature. d) The OER activity increased with conductivity and e _g electrons filling status optimized by hydrogen treatment. Reproduced with permission.^{[ 43 ]} Copyright 2015, Wiley‐VCH. Inset: Jahn–Teller distortion promoted the forming of oxygen defects, resulting in optimal Mn e _g filling state and better electrical conductivity. e) Specific activities (current density at 1.6 V vs RHE) for SFO, CFO, CCFO, LMO, BSCF, and RuO₂. Reproduced under the terms of the Creative Commons Attribution 4.0 International License.^{[ 9 ]} Copyright 2015, The Authors, published by Springer Nature. f) Electronic spin states of the octahedral site Co ions of ACoO₃ (A = Ca, Sr) and Co₃O₄. Reproduced with permission.^{[ 10 ]} Copyright 2019, American Association for the Advancement of Science.

Figure 4

a) The energy of free ions in vacuum determined by their ionization energy/electron affinity; the on‐site Madelung potential of ions shifts these energies in the crystal lattice. b) Asymmetric covalent mixing between M 3d and O 2p orbitals form σ‐ and π‐bonding and antibonding orbitals (known as the “crystal field” interactions), with illustration of the M 3d and O 2p atomic orbitals—for octahedral coordination around a transition metal, the M 3d orbitals are split into e _g and t _2g states. c) Schematic diagram of the one‐electron band structure showing states with partial transition metal character (orange) and oxygen character (blue). Often, the three oxygen bands are shown as a single broad band indicated by the dashed curve. Reproduced with permission.^{[ 24 ]} Copyright 2015, Royal Society of Chemistry.

Figure 5

a) Simulated Co L _2,3 XAS spectra of hex‐BSCF. b) Measured O‐K XAS spectra of hex‐BSC, hex‐BSCF, and several reference materials. Reproduced with permission.^{[ 45 ]} Copyright 2019, Wiley‐VCH. c) Valence states of Zn, Fe, and Co as a function of composition x in ZnFe_xCo_2−xO₄ oxides. d) OER activity versus potential at 25 µA cm⁻² _ox, as a function of “covalency” (the N–V parameter, see text). e) Computed partial electronic density of states (PDOS) of Zn_V–Fe–ZnCo₂O₄. Reproduced with permission.^{[ 46 ]} Copyright 2018, Wiley‐VCH.

Figure 6

Schematic of the transition of the electronic structure (from semiconducting to metallic) via enhanced valence.

Figure 7

a) Comparison of Ni L‐edge XANES spectra of NiCo@NiCoO NTAs and NiCoFe@NiCoFeO NTAs to those of NiO (Ni²⁺ _HS, O_h) and LaNiO₃ (Ni³⁺ _LS, O_h). b) XPS for Ni 2p. The shaded regions in (b) show the peak convolution areas from Ni species of different valence states. Ni/Co/Fe = 1:1:0.5. c) Calculated partial density of states (PDOSs) of bulk NiCoO and NiCoFeO. PDOSs above and below zero represent spin‐up and spin‐down states, respectively. The vertical dotted line represents Fermi energy level EF (set to zero). Reproduced with permission.^{[ 47 ]} Copyright 2019, American Chemical Society. d) VB XPS spectra and e) O K‐edge XAS of LSFO‐x (x = 0, 0.1, 0.33, 0.67, and 0.8). The VBM shows a gradual shift toward EF. The VBM is determined by linear extrapolation of the leading edge of the VBM to zero baseline intensity. e) The O K‐edge XAS show the development of a hole state at 528 eV. f) VBM shift values relative to the LFO VBM and electrical conductivity as a function of x. g) Experimentally measured occupied and unoccupied DOS near EF for LFO and LSFO‐0.8; the energy level is relative to the vacuum level (vs Vac.). Reproduced with permission.^{[ 48 ]} Copyright 2020, Royal Society of Chemistry.

Figure 8

a) Theoretical limiting potential plot of ΔOOH* and ΔOH*. b) OER volcano plot for metal oxides. Reproduced with permission.^{[ 52 ]} Copyright 2017, American Association for the Advancement of Science. c) The classical four‐steps OER mechanism of proton‐coupled electron transfer (PCET), for which the rate‐determining step is often found to involve the formation of an *OOH intermediate, assisted by increasing the covalence of the M—O bond. d) Schematic of the lattice oxygen‐mediated mechanism. Reproduced under the terms of the Creative Commons Attribution 4.0 International License.^{[ 53 ]} Copyright 2022, The Authors, published by Springer Nature.

Figure 9

a) Triggering the anionic redox process in a solid. Schematic of the transition metal ligand (MX) band structure made from antibonding MX* states (described as d band), non‐bonding purely ligand X states (described as p band) and bonding MX states (this band is very low in energy and not involved in the redox reaction, it is therefore not represented for the sake of clarity). Reproduced with permission.^{[ 55 ]} Copyright 2016, Springer Nature. b,c) ³⁴O₂/³²O₂ ratios and ³⁶O₂ signal, where the straight lines correspond to the natural abundance of ¹⁸O of 0.2%. The arrows indicate forward and backward scans. Reproduced with permission.^{[ 8 ]} Copyright 2017, Springer Nature. d) Free energy diagram of OER cycling at Fe–Ni dual‐site on (FeCoCrNi)OOH model. e) The determined ΔG of RLS via LOM and AEM pathway in different models. f) The detected MS signals of generated oxygen molecule using ¹⁸O isotope‐labeled catalysts. The signals are normalized through initializing the intensity of ¹⁶O₂ as 1000 a.u. Reproduced under the terms of the Creative Commons Attribution 4.0 International License.^{[ 54 ]} Copyright 2020, The Authors, published by Springer Nature.

Figure 10

a) Square reaction scheme for deprotonation from the oxide surface (S) showing the sequential and concerted proton‐coupled electron transfer reactions. Reproduced with permission.^{[ 56 ]} Copyright 2016, Elsevier. b) CV of ZnFe_0.1Co_1.6O₄ scanned in O₂‐saturated KOH (pH = 12.5–14) at a scan rate of 10 mV s⁻¹. Reproduced with permission.^{[ 46 ]} Copyright 2018, Wiley‐VCH. c) CV measurements from O₂‐saturated 0.03 m KOH (pH 12.5) to 1 m KOH (pH 14) recorded at 10 mV s⁻¹. Reproduced with permission.^{[ 8 ]} Copyright 2017, Springer Nature. The pH‐dependent OER behavior of Na_0.67CoO₂. d) Zeta potential of the catalysts. e) CV measurements of Na_0.67CoO₂ in O₂‐saturated KOH with pH 12.5–14. Inset shows the enlarged CV part from 1.3 to 1.6 V. Reproduced under the terms of PNAS license.^{[ 36 ]} Copyright 2019, National Academy of Sciences.

Figure 11

a) Correlation of oxygen evolution activity with the vacancy parameter d. The vacancy parameter is indicative of the underlying electronic structure where vacancies are generated when there is significant Co 3d and O 2p band overlap. b) Correlation of oxygen evolution activity with the oxygen ion diffusion rate, indicating that increased surface exchange kinetics trend with increased OER activity. Error bars represent standard deviation of triplicate measurements. c) O vacancy formation energy of the surface versus O vacancy formation enthalpy in the bulk. Reproduced under the terms of the Creative Commons Attribution 4.0 International License.^{[ 19 ]} Copyright 2016, The Authors, published by Springer Nature. d) OER performance (i.e., overpotential at 5 mA cm⁻²) versus the shortest O—O bond length in Na_xCoO₂ and Li_xCoO₂ with different Na and Li contents, respectively. Reproduced under the terms of PNAS license.^{[ 36 ]} Copyright 2019, National Academy of Sciences.

Figure 12

a,b) High‐resolution transmission electron microscopy (HRTEM) and fast Fourier transforms (FFTs) before and after OER tests for Hg₂Ru₂O₇ (100 cycles). c) The linear sweep voltammograms (LSVs) of Hg₂Ru₂O₇ at the scan rate of 10 mV s⁻¹ for 1, 10, 50, and 100 cycles in 0.1 m KOH solution. Reproduced under the terms of the Creative Commons Attribution License.^{[ 11 ]} Copyright 2017, The Authors. d) evolution of the iR‐corrected potential at 0.5 mA cm⁻² _oxide versus the O p‐band center relative to E_F (eV) of (Ln_0.5Ba_0.5)CoO_3–δ with Ln = Pr, Sm, Gd, and Ho, for LaCoO₃ (LCO), La_0.4Sr_0.6CoO_3–δ (LSC46), Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3–δ (BSCF82), Ba_0.5Sr_0.5Co_0.4Fe_0.6O_3–δ (BSCF46) and SrCo_0.8Fe_0.2O_3–δ (SCF82). Reproduced with permission.^{[ 13 ]} Copyright 2013, Springer Nature. HRTEM of e) as‐prepared Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3‐δ (BSCF82) powder and f) BSCF82 electrodes after 185 cycles (inset of (e) and (f): their corresponding FFTs). Reproduced with permission.^{[ 59 ]} Copyright 2012, American Chemical Society. g) Schematic diagrams of rigid band models for RuO₂ and W_0.2Er_0.1Ru_0.7O_2−δ−1 in acidic OER. Reproduced under the terms of the Creative Commons Attribution 4.0 International License.^{[ 60 ]} Copyright 2020, The Authors, published by Springer Nature. h) Stability tests of Ni‐RuO₂, RuO₂ and Com‐RuO₂. Reproduced with permission.^{[ 61 ]} Copyright 2022, Springer Nature. i) HRTEM and fast Fourier transform (FFT) images of as‐cast, after 1st and 1000 OER cycles for BaIr_0.8Mn_0.2O₃. The boundaries between the crystalline layer and the amorphous layer are divided by yellow dotted lines. Reproduced with permission.^{[ 62 ]} Copyright 2022, Royal Society of Chemistry.

Figure 13

Schematic of preparation methods for high‐valence oxides: a) element doping; b) high pressure and high temperature via diamond anvil; and c) electrochemical/chemical delithiation/desodiation/depotassiation.

Figure 14

Crystal structures of common HVOs for OER, including perovskites, spinels, pyrochlores, rutiles, and LDHs.

Figure 15

Design of high‐valence perovskite and spinel catalysts for OER. a) Schematic of high‐valence perovskite structure with A/B‐site doping. b) Cyclic voltammetry (CV) of LSNO films measured in O₂ saturated 0.1 m KOH at a scan rate of 10 mV s⁻¹, normalized by the specific area with voltage corrected for the electrolyte resistance. c) La 3d and Ni 2p XPS measured in situ for the LSNO film series. All spectra are shifted so the associated O 1s peaks fall at 530.0 eV. Inset: Ni 2p1/2 spectra for which the peak shift with x is clearly seen. Reproduced under the terms of the Creative Commons Attribution License.^{[ 88 ]} Copyright 2019, The Authors, published by Wiley‐VCH. d) Summary of the doping strategies in high‐valence spinel structure. Reproduced with permission.^{[ 102 ]} Copyright 2020, Royal Society of Chemistry. e) OER CVs, f) Computed pDOS, and g) Charge deviation of octahedral Co of ZnCo₂O₄, Li_0.5Zn_0.5Co₂O₄ (LZCO) and Li_0.5Zn_0.5Fe_0.125Co_1.875O₄ (LZCFO). Reproduced with permission.^{[ 64 ]} Copyright 2019, Wiley‐VCH.

Figure 16

a) High‐valence pyrochlore (A₂B₂O₇) structures synthesized by different site doping. Reproduced under the terms of the Creative Commons Attribution License.^{[ 118 ]} Copyright 2019, The Authors, published by the Royal Society. b) CVs and corresponding TOFs (inset) of porous Y₂[Ru_1.6Y_0.4]O_7−δ , Y₂Ru₂O_7−δ and RuO₂ electrocatalysts. c) Normalized Ru K‐edge XANES spectra with absorption energy (E ₀) of porous Y₂[Ru_1.6Y_0.4]O_7−δ and Y₂Ru₂O_7−δ with Ru foil and RuO₂ as references. d) Ru oxidation state as a function of E ₀. Reproduced with permission.^{[ 107 ]} Copyright 2018, Wiley‐VCH. e) The schematic diagram of high‐valence LDHs structure via different metal doping. f) Polarization curves of the non‐noble metal catalysts (NiFeV LDHs, NiFe LDHs, Ni(OH)₂, and NiV LDHs) and the commercial RuO₂/C catalyst. g) Total density of states (TDOS) curves of NiFeV LDHs and NiFe LDHs, the narrower bandgap of NiFeV LDHs indicates a more conductive structure. Reproduced with permission.^{[ 69 ]} Copyright 2018, Wiley‐VCH. h) High‐valence rutile structure synthesized by doping other metal ions. Reproduced with permission.^{[ 119 ]} Copyright 2014, Elsevier. i) LSVs of Cr_0.6Ru_0.4O₂ (550) and commercial RuO₂ for the first and 10 000th cycle. Inset shows the comparison of overpotentials for Cr_0.6Ru_0.4O₂ (550) and RuO₂ at the current density of 10 mA cm⁻². For RuO₂ after 10 000 cycles, the overpotential is corresponded to 3.5 mA cm⁻². j) Normalized Ru K‐edge XANES spectra and k) Fourier transformed EXAFS spectra of Cr_0.6Ru_0.4O₂ (550), Ru foil and commercial RuO₂. Reproduced under the terms of the Creative Commons Attribution 4.0 International License.^{[ 79 ]} Copyright 2019, The Authors, published by Springer Nature.