Facilitating the Deprotonation of OH to O through Fe⁴⁺‐Induced States in Perovskite LaNiO₃ Enables a Fast Oxygen Evolution Reaction

Abstract

OER, 4OH − → O 2 + 2H 2 O + 4e −). The OER involves a thermodynamically uphill, complex four-electron/proton transfer process, and it is the bottleneck limiting the overall efficiency of water splitting. [2] Although Ru-and Ir-based oxides show very high OER catalytic activity, their scarcity and high-cost hinder large-scale applications. [3] These problems have stimulated a wide search for earth-abundant transition-metal oxides (TM = Fe, Co, and Ni) with multi-valent states of TM cations to accelerate the OER. [1a,4] To design active electrocatalysts, efforts to understand the OER reaction mechanisms and to identify the structural and electronic factors determining the intrinsic catalytic activity are essential. Similar to the d-band center theory established for noble metal catalysis, [5] Nørskov et al. proposed that the OER kinetics on TM oxides is generally governed by the adsorption binding strength of reaction intermediates (*OH, *O, and *OOH) on the surfaces. [2a,6] Therefore, many researchers have attempted to correlate different electronic structure parameters to OER catalytic activity, including the number of electrons in the e g orbitals of TM cations (with e g = 1.2 displaying the highest OER activity), [7] the degree of TM 3d-O 2p hybridization, [8] the energy position of O 2p band center. [9] Following these guidelines, high Aliovalent doping is widely adopted to tune the electronic structure of transition-metal oxides for design of low-cost, active electrocatalysts. Here, using single-crystalline thin films as model electrocatalysts, the structureactivity relationship of Fe states doping in perovskite LaNiO 3 for oxygen evolution reaction (OER) is studied. Fe 4+ state is found to be crucial for enhancing the OER activity of LaNiO 3 , dramatically increasing the activity by six times, while Fe 3+ has negligible effect. Spectroscopic studies and DFT calculations indicate Fe 4+ states enhance the degree of Ni/Fe 3d and O 2p hybridization, and meanwhile produce down-shift of the unoccupied density of states towards lower energies. Such electronic features reduce the energy barrier for interfacial electron transfer for water oxidization by 0.2 eV. Further theoretical calculations and H/D isotope experiments reveal the electronic states associated with Fe 4+-O 2−-Ni 3+ configuration accelerate the deprotonation of *OH to *O (rate-determining step), and thus facilitate fast OER kinetics.

Revealing the Electronic Structure and Optical Properties of CuFeO2 as a p-Type Oxide Semiconductor

“…This comparison suggests that there are many Fe 3d states that contributed at the top of the VB in CuFeO 2 . Because Fe 3+ is in O 2– octahedral coordination with t 2g 3 and e g 2 configuration, it is likely that the Fe e g states dominated at the top of the VB and the Fe t 2g states contribute more at the higher BE region (3–8 eV). , These assignments will be further proved by resPES detailed in the following.…”

Understanding the Electronic Structure Evolution of Epitaxial LaNi1–xFexO3 Thin Films for Water Oxidation

“…Indeed, a range of B-site valences have been reported, and the basic electronic properties of LNFO remain controversial. For example, early studies of LNFO (0.5 ≤ x ≤ 1) powder samples indicated that the Ni (Fe) valence in LNFO is lower (higher) than 3+. , However, more recent studies of LNFO nanorods, nanostructures, and solid-solution thin films made by PLD report that introducing Fe into LNO does not alter the Ni 3+ oxidation state.…”

Engineering Charge Redistribution within Perovskite Oxides for Synergistically Enhanced Overall Water Splitting

“…Single-phase ABO 3 perovskite oxides, where A are generally occupied by rare-earth metals and B are taken up by transition metals, have exhibited enormous application potentials, because of their low cost and structural flexibilities. − More recently, perovskite oxides have been explored as excellent OER electrocatalysts. However, only a few perovskites, such as Pr 0.5 (Ba 0.5 Sr 0.5 ) 0.5 Co 0.8 Fe 0.2 O 3−δ , (Gd 0.5 La 0.5 )­BaCo 2 O 5.5+δ , and SrCo 0.7 Fe 0.25 Mo 0.05 O 3−δ, 18 have recently been applied as HER catalysts, because most single-phase perovskite oxides possess low intrinsic electrical conductivity and activity, hampering the practical application of perovskite oxides for overall water splitting. − To date, strategies such as ion doping, creating vacancies, tuning the strain, and nanostructuring the perovskite oxides have been utilized to regulate the surface properties, crystal structure, and electronic structure of perovskite oxides, thus optimizing the catalytic behavior. ,− In particular, the internal electronic structure is reported to influence the HER and/or OER catalytic activity by modifying the metal–oxygen covalency, oxidation state of B-site metals, interaction between B-site cations and O anion, e g orbital occupancy, O 2p-band center to Fermi level, and d electron number. − In addition, it also could activate the surface lattice oxygen within perovskite oxide to participate in the OER, which may trigger a change from a conventional adsorbate evolution mechanism (AEM) to a lattice oxygen-mediated mechanism (LOM) for OER. − Therefore, engineering the electronic structure within perovskite oxides is highly efficient for improving the catalytic performance for the HER and OER. In addition, the electrical conductivity of electrocatalysts also plays an essential role in suppressing ohmic potential drop and accessing electron transfer during the catalytic process.…”

“…Moreover, the relative amount of surface Fe 4+ of LSFCP-55 fitted by the Fe 2p is 16.8%, more than those of LSF (12.5%) and LSFCP-B (13.4%, shown in Figure a and Figure S7a in the Supporting Information). It is accepted that the occurrence of Fe 4+ could enhance the degree of Fe 3d and O 2p hybridization and accelerate the deprotonation of *OH to *O of OER (rate-determining step), thus improving the OER and HER performance. ,− Furthermore, the Co 2p peaks of LSFCP-55 shift to positive location compared with bulk SCP, implying the reduction of Co oxidation state (Figure b). Furthermore, the coexistence of Co 3+ and Co 4+ as a redox couple in LSFCP-55 is also reported to be beneficial to catalytic activity. ,, Additionally, the O 1s XPS spectra for these above catalysts were collected and further fitted into four peaks, including lattice oxygen species (O 2– ), highly oxidative oxygen species (O 2 2– /O – ), hydroxyl species or surface adsorbed oxygen (O 2 /OH – ), and surface adsorbed water species or carbonates (H 2 O/CO 3 2– ) to research the surface oxygen species.…”