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• W2897816629 abstract "•The p-blocking centers in perovskites can be modified via F-anion doping•The F-anions help uplift the O p band center and activate the lattice O•The proton- and electron-transfer step has been promoted via F-anion doping•The desorption energy associated with Co–OO* bond has been reduced Water splitting is regarded as an eco-friendly way to convert and store clean and sustainable energy. The development of active, affordable, and earth-abundant electrocatalysts for water-splitting catalysis has been the subject of intense research for decades. Perovskite oxides have attracted broad attention as one group of oxygen evolution reaction and hydrogen evolution reaction catalysts with promising performance. However, the current demand calls for higher activity and stability than what state-of-the-art perovskite oxides provide. This article describes a promising anion substitution method for regulating the lattice O activity in perovskite oxides from a performance and design perspective. Furthermore, it discusses the fundamental concepts in electrocatalysis for the rational design of perovskite oxides through anion substitution and minutely inspects the current understanding and its impact on performance trends and future directions of perovskite electrocatalysts. Activation of the redox reaction is of prime importance for improving water-splitting reactions over transition-metal oxides with the crystal family of ABO3 perovskites. Nowadays, much effort is being devoted to rationalizing the redox activities and searching for optimal compositions by changing the cations in A and B sites. Here, we show that different from general strategy, modification of the p-blocking centers through F-anion doping can provide opportunities to make a significant improvement in catalytic performance. Using this approach, we introduce an effective perovskite catalyst, La0.5Ba0.25Sr0.25CoO2.9–δF0.1, for water splitting. Through ab initio modeling, we show that F-anions help uplift the O p band center and activate the redox capability of the lattice O for electrocatalysis, which suggests the potential mechanisms for designing new materials. Additionally, our findings move beyond the conventional charge transfer steps on transition-metal sites and emphasize the importance of the lattice O redox mediation for electrocatalysis. Activation of the redox reaction is of prime importance for improving water-splitting reactions over transition-metal oxides with the crystal family of ABO3 perovskites. Nowadays, much effort is being devoted to rationalizing the redox activities and searching for optimal compositions by changing the cations in A and B sites. Here, we show that different from general strategy, modification of the p-blocking centers through F-anion doping can provide opportunities to make a significant improvement in catalytic performance. Using this approach, we introduce an effective perovskite catalyst, La0.5Ba0.25Sr0.25CoO2.9–δF0.1, for water splitting. Through ab initio modeling, we show that F-anions help uplift the O p band center and activate the redox capability of the lattice O for electrocatalysis, which suggests the potential mechanisms for designing new materials. Additionally, our findings move beyond the conventional charge transfer steps on transition-metal sites and emphasize the importance of the lattice O redox mediation for electrocatalysis. Advances in renewable energy strongly depend on mature, reliable technologies for energy storage and conversion. Water electrolysis (2H2O → 2H2 + O2) has been an important research area for conversions of intermittent energy sources, such as sunlight and wind, into versatile and easily controllable forms of energy.1Jia J. Seitz L.C. Benck J.D. Huo Y. Chen Y. Ng J.W.D. Bilir T. Harris J.S. Jaramillo T.F. Solar water splitting by photovoltaic-electrolysis with a solar-to-hydrogen efficiency over 30%.Nat. Commun. 2016; 7: 13237Crossref PubMed Scopus (456) Google Scholar, 2Torella J.P. Gagliardi C.J. Chen J.S. Bediako D.K. Colón B. Way J.C. Silver P.A. Nocera D.G. Efficient solar-to-fuels production from a hybrid microbial–water-splitting catalyst system.Proc. Natl. Acad. Sci. USA. 2015; 112: 2337-2342Crossref PubMed Scopus (261) Google Scholar, 3Cabán-Acevedo M. Stone M.L. Schmidt J.R. Thomas J.G. Ding Q. Chang H.-C. Tsai M.-L. He J.-H. Jin S. Efficient hydrogen evolution catalysis using ternary pyrite-type cobalt phosphosulphide.Nat. Mater. 2015; 14: 1245-1251Crossref PubMed Scopus (1005) Google Scholar, 4Gu J. Yan Y. Young J.L. Steirer K.X. Neale N.R. Turner J.A. Water reduction by a p-GaInP2 photoelectrode stabilized by an amorphous TiO2 coating and a molecular cobalt catalyst.Nat. Mater. 2015; 15: 456-460Crossref PubMed Scopus (194) Google Scholar, 5Chen M. Liu Y. Li C. Li A. Chang X. Liu W. Sun Y. Wang T. Gong J. Spatial control of cocatalysts and elimination of interfacial defects towards efficient and robust CIGS photocathodes for solar water splitting.Energy Environ. Sci. 2018; 11: 2025-2034Crossref Google Scholar One of the greatest challenges that keep this technology from undergoing large-scale utilization is the lack of suitable electrocatalysts. Pt and RuO2 are the state-of-the-art electrode components for catalyzing the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER), respectively, although both suffer from high cost, rare reserve, and poor durability.6Wang P. Zhang X. Zhang J. Wan S. Guo S. Lu G. Yao J. Huang X. Precise tuning in platinum-nickel/nickel sulfide interface nanowires for synergistic hydrogen evolution catalysis.Nat. Commun. 2017; 8: 14580Crossref PubMed Scopus (568) Google Scholar, 7Subbaraman R. Tripkovic D. Chang K.-C. Strmcnik D. Paulikas A.P. Hirunsit P. Chan M. Greeley J. Stamenkovic V. Markovic N.M. Trends in activity for the water electrolyser reactions on 3d M(Ni,Co,Fe,Mn) hydr(oxy)oxide catalysts.Nat. Mater. 2012; 11: 550-557Crossref PubMed Scopus (2045) Google Scholar, 8Seitz L.C. Dickens C.F. Nishio K. Hikita Y. Montoya J. Doyle A. Kirk C. Vojvodic A. Hwang H.Y. Norskov J.K. Jaramillo T.F. A highly active and stable IrO2/SrIrO3 catalyst for the oxygen evolution reaction.Science. 2016; 353: 1011-1014Crossref PubMed Scopus (1207) Google Scholar Therefore, the discovery of new electrocatalytic materials, as well as reliable methods for evaluating the electrocatalytic performance of these materials, are crucial for the development of a practical water electrolyzer.9McCrory C.C.L. Jung S. Peters J.C. Jaramillo T.F. Benchmarking heterogeneous electrocatalysts for the oxygen evolution reaction.J. Am. Chem. Soc. 2013; 135: 16977-16987Crossref PubMed Scopus (4303) Google Scholar, 10McCrory C.C.L. Jung S. Ferrer I.M. Chatman S.M. Peters J.C. Jaramillo T.F. Benchmarking hydrogen evolving reaction and oxygen evolving reaction electrocatalysts for solar water splitting devices.J. Am. Chem. Soc. 2015; 137: 4347-4357Crossref PubMed Scopus (2598) Google Scholar As one of the most promising transition-metal compounds, perovskite transition-metal oxides (ABO3) have been studied extensively to promote the kinetics of the OER and HER in alkaline solution, but attempts to enhance the performance are far from successful.11Suntivich J. May K.J. Gasteiger H.A. Goodenough J.B. Shao-Horn Y. A perovskite oxide optimized for oxygen evolution catalysis from molecular orbital principles.Science. 2011; 334: 1383-1385Crossref PubMed Scopus (3535) Google Scholar, 12Xu X. Chen Y. Zhou W. Zhu Z. Su C. Liu M. Shao Z. A perovskite electrocatalyst for efficient hydrogen evolution reaction.Adv. Mater. 2016; 28: 6442-6448Crossref PubMed Scopus (346) Google Scholar, 13Zhu Y. Zhou W. Zhong Y. Bu Y. Chen X. Zhong Q. Liu M. Shao Z. A perovskite nanorod as bifunctional electrocatalyst for overall water splitting.Adv. Energy Mater. 2017; 7: 1602122Crossref Scopus (304) Google Scholar The current understanding of the keys that govern the perovskite electrocatalysis of OER include the surface binding strength of oxygen-related intermediates, the B-O covalence, and the eg (σ*) occupancy of transition metals, all of which involve the redox of transition-metal oxides.11Suntivich J. May K.J. Gasteiger H.A. Goodenough J.B. Shao-Horn Y. A perovskite oxide optimized for oxygen evolution catalysis from molecular orbital principles.Science. 2011; 334: 1383-1385Crossref PubMed Scopus (3535) Google Scholar, 14Chen G. Zhou W. Guan D. Sunarso J. Zhu Y. Hu X. Zhang W. Shao Z. Two orders of magnitude enhancement in oxygen evolution reactivity on amorphous Ba0.5Sr0.5Co0.8Fe0.2O3-δ nanofilms with tunable oxidation state.Sci. Adv. 2017; 3: e1603206Crossref PubMed Scopus (150) Google Scholar Similarly, the exploration of the underlying function mechanism regarding the HER activity of perovskite has also been centered on the redox chemistry of BO6 octahedron.12Xu X. Chen Y. Zhou W. Zhu Z. Su C. Liu M. Shao Z. A perovskite electrocatalyst for efficient hydrogen evolution reaction.Adv. Mater. 2016; 28: 6442-6448Crossref PubMed Scopus (346) Google Scholar, 13Zhu Y. Zhou W. Zhong Y. Bu Y. Chen X. Zhong Q. Liu M. Shao Z. A perovskite nanorod as bifunctional electrocatalyst for overall water splitting.Adv. Energy Mater. 2017; 7: 1602122Crossref Scopus (304) Google Scholar, 15Hua B. Li M. Zhang Y.-Q. Sun Y.-F. Luo J.-L. All-in-one perovskite catalyst: smart controls of architecture and composition toward enhanced oxygen/hydrogen evolution reactions.Adv. Energy Mater. 2017; 7: 1700666Crossref Scopus (99) Google Scholar With these findings, one would envisage that activating the redox kinetics of BO6 octahedrons, i.e., the lattice O and oxygen vacancy (VO··) in the ABO3 structure, would afford an exciting opportunity for developing effective catalysts for water electrolysis applications. Earlier studies have demonstrated that by making use of the exceptional capability of the ABO3 structure, perovskite could accommodate dopants of different size and charge in A and B sites, thus effectively tailoring the nonstoichiometry, electronic structure, crystal structure, and so forth. Using this methodology, many successful applications in fuel cells, metal-air batteries, and water electrolysis have been achieved.16Jung J.-I. Jeong H.Y. Kim M.G. Nam G. Park J. Cho J. Fabrication of Ba0.5Sr0.5Co0.8Fe0.2O3-δ catalysts with enhanced electrochemical performance by removing an inherent heterogeneous surface film layer.Adv. Mater. 2015; 27: 266-271Crossref PubMed Scopus (101) Google Scholar, 17Chen D. Chen C. Baiyee Z.M. Shao Z. Ciucci F. Nonstoichiometric oxides as low-cost and highly-efficient oxygen reduction/evolution catalysts for low-temperature electrochemical devices.Chem. Rev. 2015; 115: 9869-9921Crossref PubMed Scopus (678) Google Scholar, 18Irvine J.T.S. Neagu D. Verbraeken M.C. Chatzichristodoulou C. Graves C. Mogensen M.B. Evolution of the electrochemical interface in high-temperature fuel cells and electrolysers.Nat. Energy. 2016; 1: 15014Crossref Scopus (456) Google Scholar, 19Hua B. Zhang Y.-Q. Yan N. Li M. Sun Y.-F. Chen J. Li J. Luo J.-L. The excellence of both worlds: developing effective double perovskite oxide catalyst of oxygen reduction reaction for room and elevated temperature applications.Adv. Funct. Mater. 2016; 26: 4106-4112Crossref Scopus (93) Google Scholar Alternatively, modification of the p-blocking centers (i.e., the valence electrons are located in the p orbital), where the common approach is to introduce VO··, can provide exciting new opportunities to tune their properties. In contrast to the traditional strategies (i.e., introductions of cations and VO··), we put forward a unique way of F-anion substitution to regulate the p-blocking centers for water-splitting electrocatalysis, as revealed by our ab initio modeling and trial run. We attempt to rationalize the higher activities by considering the redox chemistry of the p-blocking center (i.e., lattice O) along with their strong hybridization with B-site transition metals. Using this approach, we report a perovskite oxyfluoride catalyst, La0.5Ba0.25Sr0.25CoO2.9–δF0.1 (LBSCOF), with an enhanced electrocatalytic activity toward the alkaline water electrolysis. The proper size mismatch between La3+, Ba2+, and Sr2+ leads to a crystal structure exhibiting desired cubic symmetry, 15Hua B. Li M. Zhang Y.-Q. Sun Y.-F. Luo J.-L. All-in-one perovskite catalyst: smart controls of architecture and composition toward enhanced oxygen/hydrogen evolution reactions.Adv. Energy Mater. 2017; 7: 1700666Crossref Scopus (99) Google Scholar, 20Jung J.-I. Jeong H.Y. Lee J.-S. Kim M.G. Cho J. A bifunctional perovskite catalyst for oxygen reduction and evolution.Angew. Chem. Int. Ed. 2014; 53: 4582-4586Crossref PubMed Scopus (282) Google Scholar, 21Kim J. Choi S. Jun A. Jeong H.Y. Shin J. Kim G. Chemically stable perovskites as cathode materials for solid oxide fuel cells: La-Doped Ba0.5Sr0.5Co0.8Fe0.2O3-δ.ChemSusChem. 2014; 7: 1669-1675Crossref PubMed Scopus (68) Google Scholar and CoO6 octahedrons have been well established as catalytically active centers in perovskites, so we choose La0.5Ba0.25Sr0.25CoO3–δ (LBSCO) as the model system to exemplify our strategy. In view of these findings, we propose the possible function mechanisms that are fundamentally beyond the current understandings drawn from A- and B-site doping and can shed light on the influences of the F-anion over the catalytic performance. The F-anion doping approach is simple and universal, and begins at atomic levels, so it can provide a new design guide for perovskite oxides. We start with the first-principle calculation based on density functional theory (DFT) to enlighten the rational design of perovskite materials with F-anion substitution. The LBSCOF oxyfluoride and LBSCO oxide belong to the space group Pm–3m (vide infra) because of the three-way hybrid A-site doping. To simplify the simulation, we computed the typical cubic symmetry units, i.e., La0.5Ba0.5CoO2.9–δF0.1/La0.5Ba0.5CoO3–δ (LBCOF/LBCO) and La0.5Sr0.5CoO2.9–δF0.1/La0.5Sr0.5CoO3–δ (LSCOF/LSCO), in the perovskite structure for the proof-of-concept purpose. Because of the strong hybridization of Co 3d electrons with the O 2p orbitals and F 2p orbitals, an intricate interplay of different degrees of freedom as well as a complex electronic structure occur in perovskite oxyfluoride, making the investigations of the eg filling and the antibonding molecular orbitals impossible (Figure 1A). The introduction of F-anion induces the presence of multiple, distinct Co and O sites, producing a square pyramidal symmetry (C4V) and an additional type of octahedral (i.e., Oh′) symmetry, as illustrated in Figure 1A. Each of them has different orbital symmetries and doping dependencies, leading to a complex reconfiguration of the molecular orbitals in the eg and t2g (π*) orbitals. Here, the catalytic activity is rationalized in line with the O 2p band center relative to the Fermi level (EO2p–EF) of perovskites. Unlike the previous work,22Grimaud A. May K.J. Carlton C.E. Lee Y.-L. Risch M. Hong W.T. Zhou J. Shao-Horn Y. Double perovskites as a family of highly active catalysts for oxygen evolution in alkaline solution.Nat. Commun. 2013; 4: 2439Crossref PubMed Scopus (1007) Google Scholar VO·· is considered in the simulation since this study involves strong reduction reaction. Figure 1B depicts the projected density of states (PDOS) of O 2p in a series of modular structures, and the O 2p band centers are marked. It is observed that as each F-anion replaces an O atom in the supercell, it traps the electrons in the neighboring atoms (Figure S1). Therefore, the O 2p band centers have been uplifted upon the incorporation of F-anions for all the control groups. Previous studies have demonstrated a proportional relationship between the value of EO2p–EF and the OER catalytic activity,22Grimaud A. May K.J. Carlton C.E. Lee Y.-L. Risch M. Hong W.T. Zhou J. Shao-Horn Y. Double perovskites as a family of highly active catalysts for oxygen evolution in alkaline solution.Nat. Commun. 2013; 4: 2439Crossref PubMed Scopus (1007) Google Scholar, 23Grimaud A. Diaz-Morales O. Han B. Hong W.T. Lee Y.-L. Giordano L. Stoerzinger K.A. Koper M.T.M. Shao-Horn Y. Activating lattice oxygen redox reactions in metal oxides to catalyse oxygen evolution.Nat. Chem. 2017; 9: 457-465Crossref PubMed Scopus (1012) Google Scholar which, in turn, implies that F-doped perovskite would boost the OER. Albeit many studies have illustrated the positive role of VO·· in the (electro)catalysis,24Du J. Zhang T. Cheng F. Chu W. Wu Z. Chen J. Nonstoichiometric perovskite CaMnO3-δ for oxygen electrocatalysis with high activity.Inorg. Chem. 2014; 53: 9106-9114Crossref PubMed Scopus (183) Google Scholar, 25Wang J. Gao Y. Chen D. Liu J. Zhang Z. Shao Z. Ciucci F. Water splitting with an enhanced bifunctional double perovskite.ACS Catal. 2018; 8: 364-371Crossref Scopus (155) Google Scholar it is worth noting that the VO·· gives rise to the negative shifts of the O 2p band centers. As F-anion helps decrease the number of VO··,26Zhang Z. Zhu Y. Zhong Y. Zhou W. Shao Z. Anion doping: a new strategy for developing high-performance perovskite-type cathode materials of solid oxide fuel cells.Adv. Energy Mater. 2017; 7: 1700242Crossref Scopus (157) Google Scholar the O 2p center moves up toward the positive direction. The surface of the perovskite should be fully oxidized in KOH at high anodic potentials (OER regions) so that the influence of VO·· is ignorable. However, high cathodic potentials in HER will generate a considerable amount of VO··. The unexpected role of VO·· guides our search for the impact of F-anions on the lattice O activity. We estimated the activity of lattice O by calculating the formation energy of mobile oxygen species (O*) in the selected perovskites.27Yoo S. Jun A. Ju Y.-W. Odkhuu D. Hyodo J. Jeong H.Y. Park N. Shin J. Ishihara T. Kim G. Development of double-perovskite compounds as cathode materials for low-temperature solid oxide fuel cells.Angew. Chem. Int. Ed. 2014; 53: 13064-13067Crossref PubMed Scopus (169) Google Scholar As aligned in Figure 1C, it is found that the formation energies of mobile O* are negative for all the modules, and the magnitude increases monotonically from F-free perovskites to F-doped perovskites. F-anion has the strongest electronegativity among all the elements, so it will weaken the covalence in the Co–O bond, leading to accelerated oxygen mobility and oxygen exchange kinetics. As such, one would conceive of the increased activity of the lattice O upon F-anion substitution. Accordingly, the F-anion substitution allows for the activation of lattice O forming mobile O*, and helps move the O 2p band closer to the Femi level. In addition, remarkable trapping of electrons occurs at the F-anion centers, as the localized electron difference revealed, which might significantly impact the charge compensation process between the perovskites and reaction intermediates (see the following Gibbs free energy calculation). These features collectively regulate the p-blocking centers in perovskites and thus promote the water-splitting reaction, as will be shown later on. Using the classical solid-state reaction (SSR) process, we optimized the synthesis temperature of LBSCOF oxyfluoride and LBSCO oxide (Figure S2). Pyrolysis above critical phase formation temperature (e.g., 1,050°C and 1,070°C for LBSCOF and LBSCO, respectively) is required to produce materials with an identical crystalline structure, while overheating (1,070°C for LBSCOF) will lead to the second phase precipitation. The temperature-dependent X-ray diffraction (XRD) results imply that F-anions can accelerate the atom diffusion within the bulks, and thus lower the synthesis temperature. X-ray photoelectron spectroscopy (XPS) and energy dispersive X-ray spectroscopy (EDX) were adopted to ensure the incorporation of F atoms. Figure 2A collects the XPS signal in the F1s region. LBSCOF presents a pronounced response at ∼685 eV, assignable to the metal-fluoride bonding features.28Xu C. Sun B. Gustafsson T. Edstrom K. Brandell D. Hahlin M. Interface layer formation in solid polymer electrolyte lithium batteries: an XPS study.J. Mater. Chem. A. 2014; 2: 7256-7264Crossref Google Scholar, 29Iandolo B. Wickman B. Zoric I. Hellman A. The rise of hematite: origin and strategies to reduce the high onset potential for the oxygen evolution reaction.J. Mater. Chem. A. 2015; 3: 16896-16912Crossref Google Scholar Also, both scanning electron microscopy (SEM)-EDX (Figure S3) and scanning transmission electron microscopy (STEM)-EDX (Figures 2B and S4) display the presence of F in the final product. These chemical mappings indicate that La, Ba, Sr, Co, O, and F atoms are distributed uniformly in LBSCOF (1,050°C) without obvious segregations of SrO, BaO, and Co3O4, for example, the common impurities in perovskite. Similarly, a slightly higher firing temperature (1,070°C) enables the even dispersions of La, Ba, Sr, Co, and O in LBSCO (Figure S4). The crystal structure was identified through XRD Rietveld refinement and high-resolution TEM (HRTEM). The Rietveld refinement indicates that both LBSCO and LBSCOF indeed share the same crystal phase that belongs to the space group Pm–3m (Figure 2C and Table S1), in good agreement with similar compounds, such as Ba0.5Sr0.5Co0.8Fe0.2O3–δ (BSCF) and Lax(Ba0.4Sr0.4Ca0.2)1–xCo0.8Fe0.2O3–δ (LBSCCF) series.23Grimaud A. Diaz-Morales O. Han B. Hong W.T. Lee Y.-L. Giordano L. Stoerzinger K.A. Koper M.T.M. Shao-Horn Y. Activating lattice oxygen redox reactions in metal oxides to catalyse oxygen evolution.Nat. Chem. 2017; 9: 457-465Crossref PubMed Scopus (1012) Google Scholar, 15Hua B. Li M. Zhang Y.-Q. Sun Y.-F. Luo J.-L. All-in-one perovskite catalyst: smart controls of architecture and composition toward enhanced oxygen/hydrogen evolution reactions.Adv. Energy Mater. 2017; 7: 1700666Crossref Scopus (99) Google Scholar Because F– has a smaller size than O2−, it is reasonable to find that the lattice constant decreases from 3.8548 Å (LBSCO) to 3.8470 Å (LBSCOF) upon incorporation of F. The precise crystal structure can be directly examined by HRTEM imaging. Figure 2D projects an HRTEM image for an LBSCOF particle exhibiting a typical body center cubic structure. The fast Fourier transform pattern orientates at [011] direction, and the d-spacing (1.924 Å) measured from the proceeding picture is very close to the d-spacings of (200)LBSCOF plane. Thus, the formation of the cubic structural LBSCOF is confirmed. In addition to the crystal form, the produced LBSCO and LBSCOF particles share similar morphologies and specific surface areas (Figures S5 and S6). As a consequence, they can be excluded from the governing factors for the improvement of the electrocatalytic property, as discussed in the following sections. The rational design of F-anion substitution is first confirmed by the enhanced HER performance. Linear sweep voltammetry (LSV) curves and chronoamperometric measurements were carried out to acquire the polarization curves and Tafel slopes of each catalyst toward HER on a rotating disc electrode (RDE). During the polarization tests, the RDEs were kept rotating at 2,500 rpm to continuously remove the H2 bubbles. Figures 3A and 3B summarize the HER catalytic activities of various catalysts in Ar-saturated 1 M KOH at 2,500 rpm. All the catalysts respond aggressively toward hydrogen evolution. In particular, LBSCOF demonstrates much lower overpotentials (η) than LBSCO at identical current densities (Figures 3A and S7A). Although not as powerful as the commercial Pt/C catalyst for HER, LBSCOF perovskite oxyfluoride matches the capabilities of the state-of-the-art perovskite counterparts, i.e., La0.5Ba0.25Sr0.25Co0.8Fe0.2O3–δ (LBSCF) and Pr0.5Ba0.25Sr0.25Co0.8Fe0.2O3–δ (PBSCF) nanofibers (NFs).15Hua B. Li M. Zhang Y.-Q. Sun Y.-F. Luo J.-L. All-in-one perovskite catalyst: smart controls of architecture and composition toward enhanced oxygen/hydrogen evolution reactions.Adv. Energy Mater. 2017; 7: 1700666Crossref Scopus (99) Google Scholar The nanoconfined LBSCF and PBSCF have one-order higher surface areas in comparison with the ball-milled LBSCO powder (Figure S8), so they indeed achieve better HER performance. However, F-anion substitution renders the bulk material to rival these nanomaterials. For example, the η values at 100 mA cm−2 are 256, 252, and 255 mV for LBSCOF, LBSCF, and PBSCF, respectively, suggesting that they have similar reaction barrier for HER. The significantly enhanced reaction rate in the HER of LBSCOF is also revealed by the fact that the Tafel slope is smaller than those of LBSCO, PBSCF, and LBSCF. While LBSCOF (44 mV dec−1) demonstrates a higher Tafel slope than Pt/C (30 mV dec−1) at low current density region (0.3–8 mA cm−2), it is worth noting that the Tafel slope of LBSCOF (90 mV dec−1) is only about half of that of Pt/C (167 mV dec−1) at high current density region (>10 mA cm−2). F-anions enable the bulk perovskite to rival the Pt/C in the fast discharge area, which is considered as a merit for practical use. After achieving the inspiring HER activity, we turned to the water oxidation reaction. Figure 3C plots the polarization curves that display a rapid increase of anodic current in reference to OER. The remarkable effectiveness of F-anion substitution toward OER is affirmed by comparing the polarization curves with the control groups. BSCF is a well-known OER catalyst that exhibits outstanding OER catalytic activity, so it can serve as a benchmark.16Jung J.-I. Jeong H.Y. Kim M.G. Nam G. Park J. Cho J. Fabrication of Ba0.5Sr0.5Co0.8Fe0.2O3-δ catalysts with enhanced electrochemical performance by removing an inherent heterogeneous surface film layer.Adv. Mater. 2015; 27: 266-271Crossref PubMed Scopus (101) Google Scholar, 20Jung J.-I. Jeong H.Y. Lee J.-S. Kim M.G. Cho J. A bifunctional perovskite catalyst for oxygen reduction and evolution.Angew. Chem. Int. Ed. 2014; 53: 4582-4586Crossref PubMed Scopus (282) Google Scholar LBSCO and its parent structure La0.5Sr0.5CoO3–δ (LSC) share similar polarization curves toward OER. At 100 mA cm−2, the working potentials of LBSCO and LSC are 1.792 and 1.802 V, respectively. LBSCOF, by contrast, has a much more negative working potential (1.748 V) at 100 mA cm−2, outperforming the other perovskites in this study (1.801 V at 100 mA cm−2 for BSCF). More electrochemical information on the performance enhancement can be found in Figures S7B and S7C. In the case that we do not see an increase in OER activity going from O2 (1.748 V at 100 mA cm−2) to Ar (1.749 V at 100 mA cm−2) in LBSCOF, the high activity reported here would have a high value in practical applications, including water electrolysis and solar fuel synthesis.30Chang X. Wang T. Gong J. CO2 photo-reduction: insights into CO2 activation and reaction on surfaces of photocatalysts.Energy Environ. Sci. 2016; 9: 2177-2196Crossref Google Scholar These findings would suggest that anion doping is a more efficient way to tune the OER catalytic property of perovskite in relation to the conventional cation doping strategy. Although the performance is less perfect with reference to the commercial RuO2/C hybrid at low working voltages (<1.71 V), we can still prove that LBSCOF could surpass the commercial RuO2/C hybrid whose operating voltage is as high as 1.77 V at 100 mA cm−2. Furthermore, RuO2 is highly unstable at high anodic potential, at which it suffers from deep oxidation by forming RuO4 that dissolves in solution.31Saveleva V.A. Wang L. Luo W. Zafeiratos S. Ulhaq-Bouillet C. Gago A.S. Friedrich K.A. Savinova E.R. Uncovering the stabilization mechanism in bimetallic ruthenium–iridium anodes for proton exchange membrane electrolyzers.J. Phys. Chem. Lett. 2016; 7: 3240-3245Crossref PubMed Scopus (54) Google Scholar In terms of the reaction kinetics (Figure 3D), a series of similar rules were found: F-anion substitution yields a low Tafel slope (113 mV dec−1), the smallest among all the perovskites, and rivals that of the commercial RuO2/C (105 mV dec−1). The RDE tests have led us to confirm the intrinsic bifunctional excellence of F-anion substitution for enhancing the perovskite electrocatalysis of overall water splitting. However, each half reaction responds to the interfacial potential at the corresponding electrode in a real electrolysis system. Whether they can succeed depends on how well they can cooperate with one another. As such, a two-electrode electrolyzer with fluorine-doped tin oxide (FTO) glasses or Ni foams as the catalyst support was adopted to investigate the electrochemical performance of the overall water splitting. Figure 4A compares the polarization curves of three different types of electrode couples for overall water splitting. Water splitting in the state-of-the-art Pt/C(−)/RuO2(+) electrolyzer has the most positive onset potential among the control groups. To achieve an operating current density of 100 mA cm−2, we require a potential difference of ∼1.80 V between the RuO2(+) and the Pt/C(−). With an identical mass loading (0.255 mg cm−2), the potential difference between the two electrodes is ∼1.91 V at 100 mA cm−2 for LBSCOF(±) electrolyzer. Although this performance is inferior to that of the precious-metal-based electrolyzer, the energy barrier could be further reduced with a 2-fold increase in the mass loading. By increasing the mass loading to 0.51 mg cm−2, the LBSCOF(±) electrolyzer rivals the Pt/C(−)/RuO2(+) electrolyzer when the working voltage is above ∼1.87 V. Note that the cost of LBSCOF is hundreds of times cheaper than that of Pt-RuO2. More importantly, such excellent water-splitting activity is comparable with many high-performance water electrolyzers (Tables S2), including those based on the highly active transition-metal compounds such as metal phosphides, metal selenide, and metal hydroxides. A good elect" @default.
• W2897816629 created "2018-10-26" @default.
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• W2897816629 date "2018-12-01" @default.
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• W2897816629 title "Activating p-Blocking Centers in Perovskite for Efficient Water Splitting" @default.
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