Confined Ir single sites with triggered lattice oxygen redox: Toward boosted and sustained water oxidation catalysis

Abstract

•First demonstration of lattice oxygen redox in Ir single-site catalyst (Ir–MnO2)•Ir–MnO2 exhibited an over 42 times mass activity than that of commercial IrO2•Ir–MnO2 show only 15 mV increasement in overpotential after 650 h durability test Proton exchange membrane water electrolyzers (PEMWEs) represent a promising technology to efficiently convert renewable energy into high-quality hydrogen. However, their scale-up is restricted by the high cost of the state-of-the-art Ir-based catalysts in the anodic oxygen evolution reaction (OER). Developing highly active and stable OER electrocatalysts with low Ir content is thus imperative. In this work, a novel Ir single site embedded in γ-MnO2 (with only 0.87 atom % Ir) is fabricated through a simple thermal decomposition procedure. Benefiting from the locally activated lattice oxygen redox, the as-prepared sample exhibited outstanding electrocatalytic activity and long-term stability, much superior to commercial IrO2. This work demonstrates a new strategy to precisely modulate the physicochemical properties of confined active sites and boost OER activity and stability simultaneously in a single site system. Efficient electrocatalysts for oxygen evolution reaction (OER) in acid are critical to the development of clean energy conversion schemes. Lattice-oxygen-mediated mechanism (LOM) has been developed to boost OER kinetic via triggering lattice oxygen redox. However, the promoted intrinsic activity is compromised by low stability due to bulk structure reconstruction during OER. Here, we demonstrate that a single-site Ir-doping strategy can effectively address this challenge. Attributing to the carefully defined chelation environment of Ir, increased Ir–O covalency and engaged lattice oxygen oxidation have been observed. More importantly, locally triggered LOM introduces no structure evolution during OER. As a result, the constructed catalyst (Ir–MnO2) exhibited over 42 times more mass activity than that of commercial IrO2 as well as over 650 h stability with only a 15 mV increase in overpotential. Our work opens up a feasible strategy to boost OER activity and stability simultaneously. Efficient electrocatalysts for oxygen evolution reaction (OER) in acid are critical to the development of clean energy conversion schemes. Lattice-oxygen-mediated mechanism (LOM) has been developed to boost OER kinetic via triggering lattice oxygen redox. However, the promoted intrinsic activity is compromised by low stability due to bulk structure reconstruction during OER. Here, we demonstrate that a single-site Ir-doping strategy can effectively address this challenge. Attributing to the carefully defined chelation environment of Ir, increased Ir–O covalency and engaged lattice oxygen oxidation have been observed. More importantly, locally triggered LOM introduces no structure evolution during OER. As a result, the constructed catalyst (Ir–MnO2) exhibited over 42 times more mass activity than that of commercial IrO2 as well as over 650 h stability with only a 15 mV increase in overpotential. Our work opens up a feasible strategy to boost OER activity and stability simultaneously. The imperative high usage of iridium catalysts toward water oxidation has plagued scale-up of the proton exchange membrane water electrolysis (PEMWE) technology for decades.1Lagadec M.F. Grimaud A. Water electrolysers with closed and open electrochemical systems.Nat. Mater. 2020; 19: 1140-1150Crossref PubMed Scopus (104) Google Scholar, 2Carmo M. Fritz D.L. Mergel J. Stolten D. A comprehensive review on PEM water electrolysis.Int. J. Hydr. Energy. 2013; 38: 4901-4934Crossref Scopus (2461) Google Scholar, 3Song H.J. Yoon H. Ju B. Kim D.W. Highly efficient perovskite-based electrocatalysts for water oxidation in acidic environments: a mini review.Adv. 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Role of lattice oxygen participation in understanding trends in the oxygen evolution reaction on perovskites.ACS Catal. 2018; 8: 4628-4636Crossref Scopus (165) Google Scholar In this mechanism, lattice oxygen participates in the water oxidation process through its own oxidation. Therefore, to trigger LOM, the O 2p band needs to be upshifted to closely approximate the Fermi level (EF), thereby increasing its orbital overlap with metal d band (M–O bond covalency15Yagi S. Yamada I. Tsukasaki H. Seno A. Murakami M. Fujii H. Chen H. Umezawa N. Abe H. Nishiyama N. Mori S. Covalency-reinforced oxygen evolution reaction catalyst.Nat. Commun. 2015; 6: 8249Crossref PubMed Scopus (294) Google Scholar) and making the redox of lattice oxygen more energetically favorable.16Fabbri E. Schmidt T.J. Oxygen evolution reaction—the enigma in water electrolysis.ACS Catal. 2018; 8: 9765-9774Crossref Scopus (176) Google Scholar,17Hwang J. Rao R.R. Giordano L. Katayama Y. Yu Y. Shao-Horn Y. 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Therefore, higher intrinsic OER activity is generally achieved due to the switching on of LOM.23Huang Z.-F. Song J. Dou S. Li X. Wang J. Wang X. Strategies to break the scaling relation toward enhanced oxygen electrocatalysis.Matter. 2019; 1: 1494-1518Abstract Full Text Full Text PDF Scopus (149) Google Scholar Similar results have been obtained on Co–Zn oxyhydroxide oxygen evolution electrocatalysts as reported by Xu and Wang et al.24Huang Z.-F. Song J. Du Y. Xi S. Dou S. Nsanzimana J.M.V. Wang C. Xu Z.J. Wang X. Chemical and structural origin of lattice oxygen oxidation in Co–Zn oxyhydroxide oxygen evolution electrocatalysts.Nat. Energy. 2019; 4: 329-338Crossref Scopus (528) Google Scholar in alkaline electrolyte. However, the boosted OER kinetic is often counterbalanced by structure destabilization,25Rong X. Parolin J. Kolpak A.M. A fundamental relationship between reaction mechanism and stability in metal oxide catalysts for oxygen evolution.ACS Catal. 2016; 6: 1153-1158Crossref Scopus (250) Google Scholar ascribable to the cationic dissolution of M (in IrMOx) in the anodic acid condition.22Grimaud A. Demortière A. Saubanère M. Dachraoui W. Duchamp M. Doublet M.-L. Tarascon J.-M. Activation of surface oxygen sites on an iridium-based model catalyst for the oxygen evolution reaction.Nat. Energy. 2017; 2: 17002Crossref Scopus (1) Google Scholar Even worse, the dynamic formation of a large number of oxygen vacancies (OV) during OER drives the migration of bulk lattice oxygen to the surface to replenish the surface OV, resulting in bulk phase reconstruction and insufficient durability.22Grimaud A. Demortière A. Saubanère M. Dachraoui W. Duchamp M. Doublet M.-L. Tarascon J.-M. Activation of surface oxygen sites on an iridium-based model catalyst for the oxygen evolution reaction.Nat. 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In other words, dispersing Ir single site into a stable MOx substrate, which possesses favorable lattice parameters to increase Ir–O bond covalency (trigger LOM), as well as a low oxygen bulky diffusion rate to ensure structural stability, might be able to ultimately solve the primary stability bottleneck of catalysts based on LOM. Through this Ir single-site regulation, we might be able to concurrently address the activity and stability issue of Ir-based water oxidation catalysts. Stimulated by the above envision, herein we have achieved, for the first time, the activation of lattice oxygen in Ir single-site catalysts (denoted as Ir–MnO2). Compared with bulk iridium oxides, the atomic isolated Ir sites dispersed in acid stable γ-MnO231Li A. Ooka H. Bonnet N. Hayashi T. Sun Y. Jiang Q. Li C. Han H. Nakamura R. Stable potential windows for long-term Electrocatalysisby manganese oxides Under acidic conditions.Angew. Chem. Int. Ed. Engl. 2019; 58: 5054-5058Crossref PubMed Scopus (98) Google Scholar accommodates the chelation structure of the latter and exhibits much higher bond covalency due to the shortened Ir–O bond length (∼5% shrinkage compared with that in IrO2). Using in situ 18O isotope labeling differential electrochemical mass spectrometry (DEMS), we provide direct experimental evidence that LOM is turned on at the isolated Ir sites. Ir–MnO2 thus presented a promising low overpotential (218 mV @10 mA cm−2 in 0.5 M H2SO4) and quite impressive TOF (7.7 s−1) at an overpotential of 300 mV. Moreover, ascribable to the localized lattice oxygen activation, no increase in bulk cationic and anionic migration and structure reconstruction is evidenced during OER. As such, the catalyst well retained its activity and bulky structure after a 650 h durability test. In conclude, we offer a strategy to turn on the LOM of Ir catalysts without compromising its structural stability, while the intrinsic activity and utilization of Ir are simultaneously optimized. We choose γ-MnO2 (Figure S1) as the supporting matrix of Ir single sites due to its excellent stability under OER condition.31Li A. Ooka H. Bonnet N. Hayashi T. Sun Y. Jiang Q. Li C. Han H. Nakamura R. Stable potential windows for long-term Electrocatalysisby manganese oxides Under acidic conditions.Angew. Chem. Int. Ed. Engl. 2019; 58: 5054-5058Crossref PubMed Scopus (98) Google Scholar To prepare γ-MnO2 and a series of Ir doped samples at different Ir doping levels, thermal decomposition of Mn(NO3)2 and its mixture with H2IrCl6 was carried out (Note S1). Within this synthetic method, up to 5.13 wt % Ir can be obtained in the catalysts without affecting the structure of γ-MnO2 (Note S2). Due to the compositional and structural similarity of the samples at different doping levels, the 5.13 wt % Ir doped MnO2, hereafter denoted as Ir–MnO2, is selected for detailed elaboration. The physical properties of γ-MnO2 before and after Ir doping (Ir–MnO2) were first characterized and systemically compared (Figures 1A and S2–S8). High-resolution transmission electron microscope (HRTEM) images (Figures 1B and 1C) confirm the crystalline structure of γ-MnO2 after Ir doping, with inter-growth of pyrolusite and ramsdellite phases unambiguously identified (Figure S1).32Schilling O. Dahn J.R. Fits of the [gamma]-MnO2 structure model to disordered manganese dioxides.J. Appl. Crystallogr. 1998; 3: 396-406Crossref Scopus (17) Google Scholar While no Ir or IrO2 particles were observed in Ir–MnO2 through HRTEM (Figure 1B and S3), inductively coupled plasma-optical emission spectrometry (ICP-OES; Table S1), energy dispersive spectroscopy (EDS; Figure S5), elemental mapping (Figure 1D), and X-ray photoelectron spectroscopy (XPS; Figure S6) all suggest the presence of Ir in the sample, corroborating the Ir atomic doping in γ-MnO2. As the ICP-OES (Table S1) and surface sensitive XPS (Figure S6) characterization reveal similar Ir doping levels, we confirm that Ir is doped into the bulk of γ-MnO2 homogeneously, rather than sitting on the surface. Aberration corrected high-angle annular dark-field scanning transmission electron microscope (HAADF-STEM; Figures 1E and 1F) result clearly verifies the atomic isolation of Ir single atoms in the obtained Ir–MnO2 (Figure S7). The Ir atoms, notable as the scattered bright dots in the lattice, are located at exactly the same columns of Mn atoms (Figures 1F, 1G, and S9), suggesting the Ir atoms replace the Mn sites in the lattice of γ-MnO2.33Wang Q. Huang X. Zhao Z.L. Wang M. Xiang B. Li J. Feng Z. Xu H. Gu M. Ultrahigh-loading of Ir single atoms on NiO matrix to dramatically enhance oxygen evolution reaction.J. Am. Chem. Soc. 2020; 142: 7425-7433Crossref PubMed Scopus (183) Google Scholar X−ray diffraction (XRD) patterns (Figure S8A) of all prepared samples presented typical diffraction peaks of γ-MnO2.31Li A. Ooka H. Bonnet N. Hayashi T. Sun Y. Jiang Q. Li C. Han H. Nakamura R. Stable potential windows for long-term Electrocatalysisby manganese oxides Under acidic conditions.Angew. Chem. Int. Ed. Engl. 2019; 58: 5054-5058Crossref PubMed Scopus (98) Google Scholar The 2θ values shifted slightly toward smaller angles after Ir doping, ascribable to the lattice expansion induced by ionic radii difference between iridium (0.625 Å) and manganese (0.530 Å)34Shannon R.D. Revised Effective ionic-radii and systematic studies of interatomic distances in halides and chalogenids.Acta Cryst. 1976; 32: 751-767Crossref Scopus (52148) Google Scholar (Figure S8B), which also indicates that Ir should substitute Mn rather than O with larger ionic radii (1.36 Å).35Bisht A. Zhang P. Shivakumara C. Sharma S. Pt-doped and Pt-supported La1–xSrxCoO3: comparative activity of Pt4+and Pt0 toward the CO poisoning effect in formic acid and methanol electro-oxidation.J. Phys. Chem. C. 2015; 25: 14126-14134Crossref Scopus (31) Google Scholar Using Rietveld refinement, the XRD data were satisfactorily refined (Figure S10). Specifically, as the structure of Ir–MnO2 resembles that of γ-MnO2 based on the results from XRD Rietveld refinements (Table S2; Figure S11), it can be considered that Ir and Mn occupy the same position in the Ir–MnO2.36Nikam R. Rayaprol S. Mukherjee S. Kaushik S.D. Goyal P.S. Babu P.D. Radha S. Siruguri V. Structure and magnetic properties of Mn doped α-Fe2O3.Phys. B. 2019; 574: 411663Crossref Scopus (8) Google Scholar We further employed XPS and X−ray absorption fine structure (XAFS) to study the chemical state and coordination environment of these atomically dispersed Ir. First, Ir 4f XPS spectrum (Figure 2A) exhibits a 0.35 eV positive shift in binding energy (65.50 eV 4f5/2) with regard to commercial IrO2 (65.15 eV 4f5/2), suggesting an increased oxidation state of Ir.37Chen Y. Li H. Wang J. Du Y. Xi S. Sun Y. Sherburne M. Ager 3rd, J.W. Fisher A.C. Xu Z.J. Exceptionally active iridium evolved from a pseudo-cubic perovskite for oxygen evolution in acid.Nat. 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Catal. 2018; 1: 841-851Crossref Scopus (242) Google Scholar in comparison to IrO2, which is also supported by the positive shift in the second derivative of XANES (Figure 2B, insert). The average extrapolated valence of Ir is calculated as +4.75 (Figure S13),20Nong H.N. Reier T. Oh H.-S. Gliech M. Paciok P. Vu T.H.T. Teschner D. Heggen M. Petkov V. Schlögl R. et al.A unique oxygen ligand environment facilitates water oxidation in hole-doped IrNiOx core–shell electrocatalysts.Nat. Catal. 2018; 1: 841-851Crossref Scopus (242) Google Scholar,39Choy J.-H. Kim D.-K. Hwang S.-H. Demazeau G. Jung D.-Y. XANES and EXAFS studies on the Ir-O bond covalency in ionic iridium perovskites.J. Am. Chem. Soc. 1995; 117: 8557-8566Crossref Scopus (110) Google Scholar ascribable to the confinement of Ir in γ-MnO2 and the local Mn–O–Ir–O–Mn chelation structure (Figure S14 and Note S3, more details are provided in the DFT calculation section). The influence of the Mn–O–Ir–O–Mn chelation environment on local Ir–O bonding feature is further revealed by analyzing the extended X−ray absorption fine structure (EXAFS) data (Figures 2C, S15, and S16). Phase uncorrected Fourier transform EXAFS (FT-EXAFS) results demonstrate an approximately 5% Ir–O bond length shrinkage in Ir–MnO2 (1.57 Å) compared with IrO2 (1.65 Å), suggesting the covalency contraction between Ir and O.39Choy J.-H. Kim D.-K. Hwang S.-H. Demazeau G. Jung D.-Y. XANES and EXAFS studies on the Ir-O bond covalency in ionic iridium perovskites.J. Am. Chem. Soc. 1995; 117: 8557-8566Crossref Scopus (110) Google Scholar,40Görlin M. Chernev P. Ferreira de Araújo J. Reier T. Dresp S. Paul B. Krähnert R. Dau H. Strasser P. Oxygen evolution reaction dynamics, faradaic charge efficiency, and the active metal redox states of Ni–Fe oxide water splitting electrocatalysts.J. Am. Chem. Soc. 2016; 138: 5603-5614Crossref PubMed Scopus (654) Google Scholar This difference originates from the distinction in second shell structures between Ir–MnO2 and IrO2, with Ir–Mn and Ir–Ir scattering paths (represents Ir–O–Mn and Ir–O–Ir in the crystalline structure) observed at 2.607 and 2.914 Å (without phase correction Figures 2C and S15, Table S3), respectively. In other words, γ-MnO2 provides a completely different chelation environment to Ir single sites, and the accommodation of Ir into the former leads to regulated Ir–O bonding strength that is inclined to triggering on LOM.41Shi Z. Wang X. Ge J. Liu C. Xing W. Fundamental understanding of the acidic oxygen evolution reaction: mechanism study and state-of-the-art catalysts.Nanoscale. 2020; 12: 13249-13275Crossref PubMed Google Scholar Through EXAFS wavelet transforms (WTs) analysis (Figure S16), k-space information pertaining to the coordination environment of Ir further verifies the second sphere difference between Ir–MnO2 and IrO2. The lack of intensity maximum at 8.5 Å−1 corroborates the absence of Ir–Ir coordination in Ir–MnO2. It is worth noting that the EXANS and EXAFS spectra are almost identical between Ir–MnO2 with the lowest and highest Ir content (Figure S17). On the contrary to the significantly modulated Ir, the Mn 2p XPS spectra, 2s XPS spectra, and Mn K-edge XAFS (Figures 2E, 2F, S18, and S19) analysis show no obvious difference in either electronic structure or local coordination environment with or without Ir, primarily due to the low doping content of Ir (0.87 atom %). Besides, the electric conductivity was determined to be 5.38 and 5.41 S cm−1 for γ-MnO2 and Ir–MnO2, respectively (Figure S20A), attributable to the similarity in the overall electronic structure of γ-MnO2 and Ir–MnO2, which is consistent with the total density of states (DOS) based on DFT calculation (Figure S20B). Therefore, it is safe to conclude that while the Ir–O covalency contraction is significant at the Ir single site, the γ-MnO2 bulk structure is hardly influenced by Ir doping. The electrocatalytic properties for OER of Ir–MnO2 and the control samples are studied in 0.5 M H2SO4 (see detail in supplemental information). First, before any electrochemical test, Ir–MnO2 is pretreated in 0.5 M H2SO4 at 10 mA cm−2 for 2 h to remove the possible impurity phases (Note S4). Second, Ir–MnO2 represents far superior OER activity to that of commercial IrO2 and γ-MnO2 (Figure 3A). Excitingly, the overpotential to reach current density at 10 mA cm−2 is only 218 mV on Ir–MnO2, which is 132 mV and 253 mV lower than those of commercial IrO2 and γ-MnO2, respectively (Figure 3A, inserted). The mass activity and TOF of Ir–MnO2 (based on the results from gas chromatography, Figure S21) is even more promising, i.e., reaching 766 A gIr−1 and 7.7 s−1, respectively, at 1.53 V (versus RHE). This corresponds to 42.56 (18 A gIr−1) and 350 (0.022 s−1) times enhancement in comparison to the state-of-the-art commercial IrO2. Furthermore, Ir–MnO2 outperforms those of the best catalysts previously reported in literature, in terms of both mass activity and specific activity (Figure 3B; Table S4).4Chen J. Cui P. Zhao G. Rui K. Lao M. Chen Y. Zheng X. Jiang Y. Pan H. Dou S.X. Sun W. Low-coordinated iridium oxide confined on graphitic carbon nitride for highly efficient oxygen evolution.Angew. Chem. Int. Ed. 2019; 36: 12540-12544Crossref Scopus (117) Google Scholar,20Nong H.N. 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Mater. 2018; 28: 1704796Crossref Scopus (146) Google Scholar This implies that the performance enhancement can not only be attributed to the single-site dispersion of Ir but also to the optimization in electronic structure and possibly the shift in reaction mechanism, as discussed later. Third, the Tafel plots of Ir–MnO2 demonstrates a slope of 59.61 mV dec−1 (Figure 3C), smaller than that of IrO2 (78.62 mV dec−1) and far less than that of γ-MnO2 (167.39 mV dec−1). It is quite clear t