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Perovskite With Tunable Active-Sites Oxidation State by High-Valence W for Enhanced Oxygen Evolution Reaction

Perovskite oxides have been established as a promising kind of catalyst for alkaline oxygen evolution reactions (OER), because of their regulated non-precious metal components. However, the surface lattice is amorphous during the reaction, which gradually decreases the intrinsic activity and stability of catalysts. Herein, the precisely control tungsten atoms substituted perovskite oxides (Pr0.5Ba0.5Co1-xWxO3-δ) nanowires were developed by electrostatic spinning. The activity and Tafel slope were both dependent on the W content in a volcano-like fashion, and the optimized Pr0.5Ba0.5Co0.8W0.2O3-δ exhibits both excellent activity and superior stability compared with other reported perovskite oxides. Due to the outermost vacant orbitals of W6+, the electronic structure of cobalt sites could be efficiently optimized. Meanwhile, the stronger W-O bond could also significantly improve the stability of latticed oxide atoms to impede the generation of surface amorphous layers, which shows good application value in alkaline water splitting.


INTRODUCTION
Electrochemical catalysis is considered as an efficient and promising strategy to convert and store sustainable energy (Chu and Majumdar, 2012). The oxygen evolution reaction (OER) is the core process and the efficiency-determining step of many electrochemical systems, such as water splitting, CO 2 electroreduction, and rechargeable metal-air batteries (Rao et al., 2020;Li et al., 2021). The kinetic sluggishness of the OER process is the major bottleneck for improving electrolytic efficiency . The design of superior electrocatalysts has attracted significant attention . Moreover, actual large-scale applications create the demand that a lot of work should be completed to find out economically viable non-precious metal catalysts with excellent OER performance Garcia et al., 2019).
Perovskite oxides (ABO 3 ) are widely used as electrocatalysts for OER considering the abundant elemental compositions, tunable structure, and low cost (Grimaud et al., 2013) (Arandiyan et al., 2021). However, their low conductivity and less active sites restrict further development (Li et al., 2020b). Strategies to solve these problems include increasing intrinsic activity, exposing more active sites, enhancing electrical conductivity, and so on . Component engineering could adjust the physical and chemical structure of perovskites to improve catalytic activity , which is very attractive for perovskite materials with good element compatibility . For example, Sr 2+ doping is found to enhance the concentration of Co 4+ on the surface of the oxide and promote the catalytic activity for OER with double perovskite oxides PrBa 1-x Sr x Co 2 O 5+δ (Wu_ et al., 2016). The two-component (Fe and Mn) controlled La 0.6 Sr 0.4 Co 0.8 Fe 0.1 Mn 0.1 O 3-δ produces a higher surface oxygen vacancy (Vo) concentration and a faster oxygen ion diffusion coefficient, strengthening the participation of lattice oxygen in OER and significantly improving the activity (Tang et al., 2021). Numerous studies have shown that component optimization is feasible and promising, while most of them emphasize the improvement of activity, but ignore the regulation of stability (Chen et al., 2018).
The reasonable design of perovskite electrocatalysts should consider the optimization of both activity and stability to meet the actual application requirements (Yan et al., 2017;Song et al., 2020). Although the participation of lattice oxygen in the reaction (lattice oxygen oxidation mechanism, LOM) can break the theoretical limit of the conventional adsorbate evolution mechanism (AEM) and greatly reduce the reaction overpotential (Grimaud et al., 2017), it also leads to surface amorphization which reduces the catalytic stability, such as Ba 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3-δ , SrCo 0.8 Fe 0.2 O 3-δ , Pr 0.5 Ba 0.5 CoO 3-δ , (Shao and Haile, 2004;Da et al., 2019). Therefore, research into regulating the electronic structure of active sites and preventing surface amorphization remains necessary for designing perovskite electrocatalysts (Pi et al., 2021).
Herein, we introduced high-valence W metal cations to optimize the OER performance of Pr 0.5 Ba 0.5 CoO 3 (named PrBaCo). Highvalence cations with strong electron attraction could enhance the binding ability of A or B sites to lattice oxygen to prevent surface amorphization and effectively adjust the electronic state of the active site to regulate the activity simultaneously. The catalyst with 20% W replacement exhibits superior OER activity. Moreover, reinforced covalency of the Co-O bond not only adjusts the activity but also improves the stability by inhibiting the oxidation of lattice oxygen.

RESULTS AND DISCUSSIONS
A series of W-substituted perovskite oxides ( Figure 1A) were synthesized by precisely controlling the W content through an electrospinning method. As displayed in Figure 1B, X-ray diffraction (XRD) patterns indicated that all the catalysts (Pr 0.5 Ba 0.5 Co 1-x W x O 3-δ , x 0.05, 0.1, 0.2, 0.3) good crystalline phases with the W ions successfully replaced Co ions. The diffraction peaks of Pr 0.5 Ba 0.5 Co 0.8 W 0.2 (PrBaCo 0.8 W 0.2 ) are observed at 32.8 and 26.5, corresponding to planes (310) and (201), being consistent with the PDF#26-0144. It is also then taken as an example to study the morphology and structure information. Scanning electronic microscopy (SEM) images reveal that one-dimensional nanowire is the dominant product ( Figures  2A,B). Transmission electron microscopy (TEM) images are further used to give detailed morphology information. It is observed that the resulting solid product is a thin nanowire and the diameter of the nanowire is about 50 nm ( Figures 2C,D). The corresponding highresolution TEM (HRTEM) images show that the Pr 0.5 Ba 0.5 Co 0.8 W 0.2 sample displays a single-crystalline nature and the lattice spacing is 0.29 nm, which are clearly consistent with the XRD characterization ( Figure 2E). Elemental distributions were then detected by a STEM-EDX mapping, where Pr (orange), Ba (indigo), Co (purple), W (yellow), and O (red) are uniformly distributed throughout the whole nanowire ( Figure 2F).
Information on the chemical states of the electrocatalysts was examined by X-ray photoelectron spectroscopy (XPS) (Wu et al., 2019). The XPS full spectrum was provided in Supplementary Figure  S1. The fine-scanned Co 2p XPS spectra of the PrBaCo, PrBaCo 0.95 W 0.05 , PrBaCo 0.9 W 0.1, and PrBaCo 0.8 W 0.2 were given in Figure 3A, in which the peaks of Co 2p 3/2 and Co 2p 1/2 were located at around 780 and 797 eV, respectively . The two fitted peaks for Co 2p 3/2 are Co 3+ (ca. 779.5 eV) and Co 2+ (ca. 780.8 eV) in PrBaCo. For W-substituted PrBaCo, the newly fitted peaks (778.3 eV) increased with the increasing W content ( Figure 3B), corresponding to the Co 0 . Taking into account the prone to surface remodeling during the OER reaction (Diaz-Morales et al., 2016), cyclic voltammetry (CV) testing was carried out to obtain a more realistic catalytic surface (Supplementary Figure S2). After CV for 100 cycles, the Co 0 peak disappeared and the content of Co 2+ reached up to 66% (Zhou et al., 2019), which was originated from the formation of cobalt oxyhydroxide (CoOOH) as active sites for enhanced OER ( Figure 3C) (Da et al., 2019). At the lower potential, the active sites (Co) with lower valence state are more conducive to being preoxidated and the reconstruction of the intermediate Co-OOH structure are carried out (Xiao et al., 2020).
XPS of O 1s was also applied to further study the surface states for these perovskite catalysts (Supplementary Figure S3). In the  Figure  S3B). Adsorption of the active site to the hydroxyl is a key step in the OER process (Teng et al., 2020), and the hydroxyl-rich surface is undoubtedly beneficial to increase the reaction rate (Xia et al., 2020). The disappearance of highly oxidative oxygen suggests that it is difficult for lattice oxygen to participate in the OER process and become oxidized, which would prevent surface amorphization. The OER performance was evaluated in 1 M KOH aqueous solution at 25°C with the use of a standard three-electrode system. As shown in Figure 4A, all of the W-substituted (>10%) catalysts exhibit better OER activity in comparison to PrBaCo. When the current density reaches 10 mA/cm 2 , the potential is only 1.55 V for PrBaCo 0.8 W 0.2 , while it is 1.62 V for PrBaCo, exhibiting excellent competitiveness in the field of alkaline OER based on perovskite oxides (Supplementary Table S1). Interestingly, the electrocatalytic activity is dependent on the W content in a volcano-like fashion ( Figure 4B), and PrBaCo 0.8 W 0.2 is located near the apex of the volcano curve, demonstrating the optimal catalytic activity. To understand the mechanism for the improvement of catalytic activity, the Tafel slope was studied for the reaction kinetics ( Figure 4C). Consistent with the trend of electrocatalytic activity, W-substituted perovskite oxides also show the volcano-like Tafel slope (Supplementary Figure S4). The Tafel slope of optimized PrBaCo 0.8 W 0.2 is 63.88 mV dec −1 , much smaller than that of PrBaCo (79.33 mV dec −1 ) and exhibiting the faster kinetics rate. Furthermore, electrochemical impedance spectroscopy (EIS) was conducted to reveal the reaction kinetics occurring at the electrolyte/electrode interface, considering the charge transfer resistance (Rct) is closely related to the OER process. As seen from Figure 4D, the Rct are 68 Ω, 85 Ω, 65Ω, 55Ω, 300Ω for PrBaCo, PrBaCo 0.95 W 0.05 , PrBaCo 0.9 W 0.1 , PrBaCo 0.8 W 0.2 , and PrBaCo 0.7 W 0.3 , respectively. Among them, PrBaCo 0.8 W 0.2 also exhibits optimal charge transfer capacity during the OER process. The information from Tafel slope and Rct suggests the improved OER reaction kinetics is due to the enhanced electron transfer through introducing appropriate W. In order to obtain the real reaction area, CV testing was used at different scan rates in the non-Faradaic potential region to acquire the doublelayer capacitances (C dl ) for PrBaCo and PrBaCo 0.8 W 0.2 (Supplementary Figure S5 and Figure 4E). The C dl of PrBaCo 0.8 W 0.2 (19.04 mF cm 2 ) is about three times larger than that of PrBaCo (6.43 mF cm 2 ). Larger C dl represents a larger electrochemically active area (ECSA) that exposes more active sites, which contributes to the enhancement of electrocatalytic activity. Moreover, LSV polarization curve was normalized by the ECSA to further evaluate the improvement of the intrinsic activity of PrBaCo 0.8 W 0.2 in Figure 4E.
The stability of perovskite oxide is one of the most important properties for alkaline OER, and chronopotentiometry was undertaken to assess it. As displayed in Figure 4F, the optimized PrBaCo 0.8 W 0.2 exhibited a long-term catalytic stability (>10000s) Frontiers in Chemistry | www.frontiersin.org January 2022 | Volume 9 | Article 809111 compared with the initial PrBaCo. Additionally, the structural change of PrBaCo 0.8 W 0.2 after the stability test was investigated by TEM images in Figure 5. It is worth noting that the one-dimensional nanostructure was well retained and there were only two to three amorphous layers on the surface of the catalyst, indicating the suppressed surface amorphous for good structural stability. All test results demonstrate that the OER performance of perovskite oxides could be productively adjusted through component engineering of W-substitution. The substitution of high-valence W cations could effectively increase the covalent between active sites and oxygen, prevent surface amorphization, and boost stability and activity.

CONCLUSION
In summary, tungsten atoms substituted into perovskite oxide (Pr0.5Ba0.5Co1-xWxO3-δ) nanowires were developed by electrostatic spinning. The activity and Tafel slope were both dependent on the W content in a volcano-like fashion, and the optimized Pr 0.5 Ba 0.5 Co 0.8 W 0.2 O 3-δ exhibits both excellent activity (overpotential 325 mV at 10 mA cm 2 ) and superior stability (10 mA cm 2 for >10,000 s) compared with other reported perovskite oxides. The W atoms in the B site of the perovskite oxide could tune the local coordination environment for the lower valence state of Co into the active site (CoOOH) availably for enhancing the intrinsic activity. Meanwhile, it could significantly improve the lattice stability by reinforcing the covalency of the Co-O bond to impede the surface amorphous phenomenon, showing good application value in alkaline water splitting.

Preparation of Catalysts
To synthesis the perovskite catalysts, Pr(NO 3 ) 3 .6H2O (0.1 mmol), C4H6BaO4 (0.1 mmol), Co(NO 3 ) 2 .6H 2 O (0.2x mmol), and (NH 4 ) 6 H 2 W 12 O 40 ·XH 2 O (x/12 mmol), were dissolved in a solution containing DMF (5 ml) and PVP (0.8 g). Next, the solution was stirred continuously for 12 h at a speed of 500 r/min at room temperature. The obtained pick solution was transferred to an injection syringe, and then electrospun using a DC voltage of 18 kV. Finally, the resulting solid product was collected and calcined at 700°C for 6 h in air. The heating rate was 1°C/min.

Electrochemical Measurements
Electrochemical measurements for OER were performed in 1 M KOH aqueous solution at 25°C on a standard three-electrode system on CHI760E electrochemical workstation (CH Instrument, United States). A glassy carbon electrode (5 mm in diameter) was used as the working electrode, and a large surface area Platinum mesh (1 × 1 cm) and a saturated calomel electrode (SCE) were used as the counter and reference electrode, respectively. All potentials measured were calibrated vs. RHE using the following equation: The catalyst (4 mg) was dispersed in ethanol (2 ml) and ultrasonicated for 15 min, followed by adding 100 µl Nafion solution (5 wt%, Sigma Aldrich, United States). The working electrode was coated with 10 µL of the good dispersity liquid and dried naturally. All potentials were converted to the reversible hydrogen electrode (RHE) to unequivocally compensate for the pH changes.

DATA AVAILABILITY STATEMENT
The original contributions presented in the study are included in the article/Supplementary Material, further inquiries can be directed to the corresponding author.

AUTHOR CONTRIBUTIONS
JY, MX, and CZ have done the experimental work. JY has written the manuscript. DC designed the experiment and revised the manuscript, FD directed the experiment.

FUNDING
This work was supported by the Natural Science Foundation of Shandong Province (ZR2020QB120).