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Rational Manipulation of IrO2 Lattice Strain on #-MnO2 Nanorods as a Highly Efficient Water Splitting Catalyst Wei Sun, Zhenhua Zhou, Waqas Qamar Zaman, Li-mei Cao, and Ji Yang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b12775 • Publication Date (Web): 17 Nov 2017 Downloaded from http://pubs.acs.org on November 18, 2017

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ACS Applied Materials & Interfaces

Rational Manipulation of IrO2 Lattice Strain on α-MnO2 Nanorods as a Highly Efficient Water Splitting Catalyst Wei Sun, Zhenhua Zhou, Waqas Qamar Zaman, Li-mei Cao and Ji Yang State Environmental Protection Key Laboratory of Environmental Risk Assessment and Control on Chemical Processes, School of Resources and Environmental Engineering, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, P.R. China.

ABSTRACT: Developing more efficient and stable oxygen evolution reaction (OER) catalysts is critical for future energy conversion and storage technologies. We demonstrate that inducing a lattice strain in IrO2 crystal structure due to interface lattice mismatch enables an enhancement of the OER catalytic activity. The lattice strain is obtained by the direct growth of IrO2 nanoparticles on a specially exposed surface of α-MnO2 nanorods via a simple two-step hydrothermal synthesis. Interestingly, the prepared hydride OER activity increases with a lower IrO2 grown mass, which offers an opportunity to reduce the usage of precious iridium and ultimately obtains a specific mass activity of 3.7 times than IrO2 prepared under the same condition and exhibits equivalent stability. The lattice mismatch in the underlying interface induces the formation of lattice strain in IrO2 rather than the charge transfer between the materials. The lattice strain changes are in good agreement with the order of the OER activity. Our experimental results indicate that using the special exposed surface substrates or tuning the supporting morphology structure can manipulate the catalyst materials lattice strain for the design of more efficient OER catalysts.

Key words: IrO2; α-MnO2; Lattice Strain; Interface Mismatch; OER

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Introduction Electrochemical water splitting has attracted increasing attention due to its ability to convert renewable power sources such as solar and wind for storage in the high-power-density chemical energy of hydrogen (H2)1, which is a major energy source for fuel cell vehicles. A promising technology for collecting H2 is polymer electrolyte membrane (PEM) electrolysis2-3. However, the noble anode catalyst iridium dioxide (IrO2) is currently the only material that can be used in an anode reaction due to its excellent OER activity and stability4, which is one of the primary challenges to expanding the use of this technology. Lowering the loading mass of IrO2 from 1.5 mg cm-2 to 0.6 mg cm-2 can reduce the cost of the materials by as much as 60%3. This requires the specific mass activity to be increased by at least a factor of 2.5. The improvement of the intrinsic OER activity of IrO2 is an efficient approach for reducing the mass loading. Unfortunately, the complexity of the OER mechanism, which involves four protons and coupled electrons in the transfer reaction, hinders most progress in overcoming this sluggish kinetics process5-6. The critical step to address this challenge is to design and develop a new and efficient IrO2 catalyst. In addition to the macroscopic properties of materials that contribute to the catalytic activity, the electronic structure and crystal structure play a more fundamental role in determining the reactivity7-8. Inducing a lattice strain of materials towards to a particular direction can modify the physical properties and catalytic performance9-11. For example, the non-ferroelectric SrTiO3 perovskite can become ferroelectric at room temperature when lattice strain is created in thin films12. More closely related to OER, the compressive strain induced in a Pt-rich shell in the Pt-based bimetallic alloys oxygen reduction (ORR) catalysts allows the optimization of the band structure and results in a high ORR activity13. The presence of lattice strain in multi-elemental alloys or metal oxides plays modifies the interatomic distances11, 13 or MO6 coordination symmetry distortions. Manipulating the lattice strain in alloys or metal oxides is a very effective way to tune the electronic structure of materials. Thus, it inspired us to design new IrO2-based OER catalyst from the point of view of tuning the lattice strain. ACS Paragon Plus Environment

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Moreover, understanding the role of the lattice strain that has been largely neglected in previous design strategies is essential for controlling and developing more efficient water-splitting catalysts. The appropriate selection of substrates and preparation methods enables the lattice strain to be induced in catalytic components. Growing the catalysts on an appropriate substrate is a valid way to tune their catalytic activity and reduce their usage; a famous construct is a range of core-shell structure materials14-15, which can be applied in a broad range of fields. The uncommon crystallographic directions of nano-substrate not only induce an intriguing geometric growth of catalyst particles but can also tailor the amount of surface strain at the interface, thus giving rise to an alteration of bandstructure16-17. Additionally, the three-dimensional constraints of the substrate play a role in capping agents to decrease the catalyst particle size18. The strength of the induced lattice strain may be closely related to the obtained particle size. The properties of larger particles are more similar to those of the bulk material. Thus, to avoid this adverse situation, the appropriate selection of substrates and preparation methods is particularly important. In this study, we demonstrate that the lattice strain of IrO2 been induced by an efficient strategy of the direct growth of IrO2 nanoparticles (NPs) on α-MnO2 nanorods. The interface lattice mismatch between IrO2 and α-MnO2 is responsible for the formation of lattice strain. Electrochemical experiments show that the hybrid not only exhibits high catalytic activity for OER but also largely reduces the mass of IrO2 to approximately 55% relative to pure IrO2 as well as exhibits robust stability. The specific mass activity of the developed catalyst is 3.7 times that of pure IrO2 catalyst, thus meeting the requirement of 2.5 times. Here, α-MnO2 serves as the substrate due to its high regular morphology 2D nanorods19-21 (basically nanorods, no irregular particles) and a single exposure surface (most are (200) planes). The prepared hydride shows an intriguing feature that the obtained OER activity depends on the grown IrO2 mass, with a smaller mass giving rise to a higher observed OER activity. More importantly, the change tendency in the lattice strain of IrO2 NPs is in good agreement with the order of the obtained OER activity values, thus confirming that lattice strain plays an integral role in determining the OER activity. ACS Paragon Plus Environment


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Results and Discussions Crystal Structure and Morphology

Scheme 1. Schematic illustration showing the preparation process of IrO2@ α-MnO2. Scheme 1 provides a general illustration of the IrO2@α-MnO2 preparation process. The two-step hydrothermal synthesis is employed here, with the preparation of the nanorod α-MnO2 followed by the direct growth of IrO2 NPs on the surface via a heterogeneous nucleation process. Detailed preparations are shown in Supporting Information. Given the confining effect of the substrate on the size of the growing particle, the iridium precursor/α-MnO2 mass ratio may affect the size of the growing particles. To obtain a precise understanding of this confinement effect, we prepared a series of catalysts with varying IrO2/α-MnO2 mass ratios (Table S1 list the IrO2 mass fraction in different prepared catalysts). Their crystal structure diffraction patterns clearly and quantitatively display this trend (Figure 1). The colored rejoins (I, II and III) clearly show that IrO2 diffraction intensity increasing with a greater content of the iridium precursor, and this phenomenon is clearly observed in the peak intensity changes of IrO2-(110) and α-MnO2-(310) (in rejoin II). Notably, the peaks positions of α-MnO2 do not show any shift. The extended X-ray


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adsorption fine structure (EXAFS) spectra of Mn-K edge (Figure S1) also confirm that the αMnO2 substrate maintains its crystal structure well, in agreement with the XRD results. Furthermore, the XPS surface components reveal the presence of a compositional gradient between iridium and magnesium on the surface (Figure S2).

Figure 1. XRD patterns of prepared materials. The α-MnO2 substrate and pure IrO2 are employed as references. The red vertical line shows the α-MnO2 standard diffraction peaks, and the blue line corresponds to IrO2. The diffraction peak varies with the composition of the material, marked by different colors. To observe and monitor the changes in the shape and particle size, we performed transition electron microscopy (TEM). The prepared α-MnO2 shows a nanorod shape (Figure S3), which is consistent with previous studies19-20, and we find that the vast majority of exposed surface are (200) with the spacing of 0.49 nm (Figure S3b). HRTEM (Figure 2) images show the morphology properties of the prepared IrO2@α-MnO2 materials. For pure IrO2 nanoparticles (NPs), the size range is 15-30 nm. In contrast, the NP sizes for IrO2 grown on α-MnO2 are far smaller than those of the pure samples in all prepared catalysts, with sizes below 5 nm or even less than 2 nm. However, having too many Ir precursors will cause some of the IrO2 to form standalone particles. This phenomenon is observed for both IrO2@Mn-1 and 2 (Figure S4). When the IrO2 mass is reduced to 59%, there are no standalone IrO2 particles. Moreover, TEM


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images also reveal a fairly uniform distribution of the IrO2 NPs on the α-MnO2 surface, especially for IrO2@Mn-3,4 and 5. The TEM images (Figure S5) show that a small amount of larger IrO2 particles is also present for IrO2@Mn-3,4 and 5. These large particles show novel polyhedron shapes such as roofed pentahedrons and symmetrical decahedrons. This is mainly because the special exposure surface of the α-MnO2 induce the intriguing particle growth direction of IrO2. The observed morphological differences indicate that the α-MnO2 nanorods can serve as a useful support to mediate the growth of other functional nanomaterials.

Figure 2. HRTEM image of (a) pure IrO2 NPs. (b)-(f) correspond to IrO2@Mn-1, IrO2@Mn-2, IrO2@Mn-3, IrO2@Mn-4 and IrO2@Mn-5, respectively.

Electrochemical Properties and OER Activity


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Figure 3. (a) ECSAs variation trend in different compositional IrO2@α-MnO2. (b) OER polarization curves of prepared catalyst. The inset is the required overpotential to achieve 10 mA cm-2. All measurements are performed in 0.1 M HClO4 under a scan rate of 10 mV s-1. The iR loss from the solution resistance is corrected. Catalyst loading mass ~0.2 mg cm-2, electrode area 0.25 cm-2. (c) Nyquist plots of prerecorded catalysts at 1.25 V vs. SCE. (d) Variation trend of TOF with the different compositions. All currents are selected at the overpotential of 323 mV, which corresponds to IrO2 with a 10 mA cm-2 overpotential. To evaluate the OER catalytic activity of the prepared IrO2@α-MnO2 materials, films of IrO2@α-MnO2 with different mass ratios are prepared on etched and clean Ti plate for cyclic voltammetry (CV) in 0.1 M HClO4 solution (pH~1). Figure S6 shows their CV curves within the potential range of 0.0-1.5 V (vs. RHE), with no obvious presence of water oxidation. A redox reaction occurred in the 0.6-0.95 V range for all prepared catalysts and consistent with the previous studies, which revealed22-24 that this reaction is mainly due to the surface coupling of Ir4++e→Ir3+. A significant difference between IrO2 and IrO2@α-MnO2 in the potential range of


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1.1-1.3 V is that the pure IrO2 and high-content IrO2 samples (IrO2@Mn-1 and 2) only exhibit capacitive currents, while a distinct peak is observed in IrO2@Mn-4 and 5 and shows scan rate dependent current increases (Figure S7a and 7b). This characteristic peak can also be observed in pure α-MnO2 (Figure S7c) and is attributed to the intrinsic tunnel structure with a larger tunnel size (Figure S7d) that can adsorb water molecules to produce an adsorption current25. This will result in the catalysis of water oxidation of IrO2@α-MnO2 at lower potential with respect to pure IrO2, thus lowering the onset potential (Figure S8) for water oxidation. The CV measurements were also used to evaluate the electrochemically active surface areas (ECSAs) that are closely related to the surface accessible active sites26-27. The ECSAs are relevant to electrochemical double layer capacitance, and the potential window (here, 0.1-0.3 V) with no apparent faradic response can be used to calculate the value. Figure 3a shows that ECSAs are normalized by the IrO2 mass, and a tendency of decreasing ECSA with lower IrO2 grown mass is observed; for example, IrO2@Mn-5 gives only 55-65 m2 g-1 per unit mass IrO2, while pure IrO2 gives the value of 97 m2 g-1. It stands to reason that more tiny particles may be expected to have higher ECSAs due to their larger surface area. In fact, capacitance depends on many factors28-29 such as the materials, structure and surface roughness. Thus, directly relate ECSAs to particle size is rather challenging. The OER polarizations are shown in Figure 3b, from which we can see that IrO2@Mn-5 exhibits the highest current and that the lower the IrO2 mass is, the higher the OER activity is. The overpotential required to achieve 10 mA cm-2 is regarded as a useful metric for evaluating the OER activity of a catalyst due to its relevance to solar fuel synthesis30. The insert in Figure 3b shows that IrO2@Mn-5 only requires 275 mV, whereas IrO2 needs 323 mV. Electrochemical impedance spectroscopy (EIS) (Figure 3c) reveals that charge transport efficiency increases with


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lower IrO2 grown mass, with the smallest value obtained for IrO2@Mn-5. Their OER kinetics also confirm this tendency, which shown in Figure S-9. The IrO2@Mn-5 has the lowest slope of 59 mV dec-1 and the slope value is increasing with grown more IrO2 mass and ultimately close to pure IrO2 performance. Thus, the OER activity of IrO2@α-MnO2 strongly depends on the mass ratio between the IrO2 and α-MnO2 which suggests that the modification effect of α-MnO2 substrate plays critical role in determining the hybrid catalytic performance. As discussed above, the modification effect of α-MnO2 substrate strongly depends on the mass ratio between the IrO2 and α-MnO2. To further understand this modification effect, the turnover frequency (TOF) that represents the catalytic activity of a unit site31 is employed to evaluate the intrinsic activity. Examination of Figure 3d shows that α-MnO2 substrate can efficiently promote the intrinsic OER activity of IrO2; the smaller the grown mass is, the higher the TOF is, and that of IrO2@Mn-5 is appropriately 3.8 times that of pure IrO2. This implies that the intrinsic catalytic activity of the Ir sites is enhanced substantially when it is fully grown on the α-MnO2 substrate. To fully meet the requirement of reducing the cost by 60% in catalyst usage, the mass activity must be improved by at least 2.5 times. Figure 4a shows the specific mass activity of IrO2 loaded on the Ti plate, from where we can see that the mass activity of IrO2@Mn-5 is 3.7 times higher than that of IrO2. This fully meets the desired reduction in the cost without sacrificing the activity. In addition to the OER activity, we focus on the catalyst stability, which often faces great challenges. Figure 4b shows the chronopotentiometric curves, from which we can see that IrO2@Mn-5 has the highest OER activity and exhibits good stability. The potential is raised by 2.07% under 5h test, while IrO2 gives 2.2% potential change after 5 h. That result means that the α-MnO2 substrate has not yet dissolved and that the surface IrO2 protects the α-MnO2 from corrosion under strong oxygen evolution conditions. However, when we further reduce the IrO2


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grown mass (~36% mass fraction), the OER activity is improved further, but the stability is unsatisfactory (Figure S10).

Figure 4. (a) Specific mass activity at overpotential of 323 mV for the different compositional materials. Catalytic components are based on IrO2. (b) Chronopotentiometric curves of IrO2@Mn-5 and pure IrO2 under 10 mA cm-2. The inset shows their potential loss. Control of Lattice Strain on OER Activity Our experiments results reveal that the strategy for direct growth of IrO2 particles on α-MnO2 substrate is very effective. Previous studies on IrO2-based mixed oxides either as a homogenous solid solution or as separate phases, such as binary or ternary mixed oxides (RuO232-33, SnO234-35, Sb-SnO236 and Ta2O537-38), show that an enhancement of OER activity or stability can be


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obtained. However, our work is quite different from these mixed oxides due to the growth of extremely small particles (most less than 5 nm, some less than 2 nm), and the intrinsic activity depends on the mass ratio of IrO2/α-MnO2. For IrO2@Mn-5, the tiny particles are quite similar to nanoclusters. More notably, the underlying lattice is bipartile due to the presence of a lattice mismatch at their interface (as shown in Figure 4a), making the IrO2 crystal structure different from that of bulk particles. This catalyst-support interaction may induce an intriguing electronic structure of IrO2 that may responsible for the OER catalytic activity. The XRD patterns show diffraction peaks for a mixture of IrO2 and α-MnO2, and the intensity is limited. We used X-ray adsorption spectroscopy (XAS) to understand the role of catalyst-support interactions in determining the IrO2 OER activity. Figure 5a shows the EXAFS spectra of IrO2 and IrO2@Mn-5. Three typical peaks are observed corresponding to the nearest scattering path of the Ir-O shell, the c axis of the Ir-Ir shell and the Ir-Ir shell (atomic distance defined l) from the body-centered scattering path. These are depicted in the crystal diagram shown in the inset of the figure. In a tetragonal system (IrO2 belongs to this system), there is a relation between the length of l and c axis (cell parameters) given in follow equation:

l2 =

1 2 1 2 a + c 2 4

where a is the a-axis length and c is the c-axis length. We can thus obtain the c/a ratio, which is an important parameters for describing the crystal structure (IrO2 is the rutile structure).


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Figure 5. (a) k2-normalized Ir-LIII edge EXAFS of IrO2@Mn-5 and IrO2. Phase correction are applied, and the k-range was 2-14 Å. The different scattering paths are indicated in a crystal structure diagram of IrO2 unit cell. (b) The c/a ratio varies with IrO2 mass fraction. The cell parameters are extracted from the IFEFFIT fitting. (c) The relationship between OER activity and lattice strain of IrO2. OER activity is expressed by the intrinsic TOF. (d) Diagram of process of α-MnO2 substrate induces the lattice strain of IrO2. The lattice mismatch at their interface results in the modifications of the crystal structure, which exhibits a compression in the c-axis; these modifications are responsible for the variations of c/a ratio of IrO2 on the α-MnO2. It also means that the IrO6 geometry transfers from the D2h symmetry to a relatively elongated structure. We use the IFEFFIT calculation package to fit the two Ir-Ir shell peaks and thereby obtain the cell parameters. Figure S11 depicts the fitting results. As mentioned above, the c/a ratio is a critical parameter for the rutile structure because it determines the tetragonal distortion of the IrO6 octahedron39. The six Ir-O bonds of the IrO6 octahedron are not equal, four elongated planar Ir-O bonds (1.9986 Å) contribute to the c-axis, and two compressed apical Ir-O bonds (1.9604 Å) determine the a-axis, leading to a compressed IrO6 octahedron (D2h symmetry). Therefore, a larger c/a ratio corresponds to a more compressed IrO6 octahedral structure. Figure 5b shows the variation of the c/a ratio of different IrO2 grown mass samples. It can be clearly seen that a smaller grown mass corresponds to lower c/a ratio. It is apparent that the strategy of direct growth of IrO2 on the substrate can modify the IrO2 crystal structure and depends on the growing


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mass. To precisely estimate the influence of the crystal structure on the OER activity, we use the lattice strain to determine their relationship. The lattice strain is defined as follows:

LS =

(c / a − c0 / a0 ) × 100 c0 / a0

where c0/a0 is the value for pure IrO2. Figure 5c shows that there is an explicit linear dependence of OER activity on the lattice strain, with a larger lattice strain giving higher TOF values. Although IrO2 and α-MnO2 are both tetragonal, their space groups and cell parameters are different; the former is P42/mnm(136), with dimensions of 4.5051×4.5051×3.1586 Å3, and the latter is I4/m(87), with dimensions of 9.785×9.785×2.863 Å3. Moreover, the special exposed (200) face of α-MnO2 determines the lattice mismatch of the underlying interface. Thus, it will induce the lattice strain of IrO2, resulting in a slight increase of the a-axis and a decrease of the c-axis. This leads to a compressive stress along the c-axis throughout the crystal structure. The reduced c/a ratio means that the Ir-O bonds have been changed. The apical Ir-O bonds are elongated, and the planar Ir-O bonds are compressed, which will relieve the D2h symmetry and lead to elongated IrO6 geometry relative to the original compression shape. This process is illustrated in Figure 5d. Notably, the smaller size IrO2 NPs are more susceptible. As determined by the TEM images, the grown IrO2 particles size is decreasing with lowing mass loading. It naturally will lead to more prominent lattice contract, which gives rise to a larger lattice strain. This is also the reason that IrO2@Mn-5 shows the highest OER activity, in which almost all IrO2 particles are grown on the support, and no separate IrO2 particles are formed. The results of our OER experiments are in good agreement with this hypothesis


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Figure 6. (a) Ir-LIII edge XANES of prepared IrO2@α-MnO2 and pure IrO2. The inset shows their second derivative of normalized µ(E) and their adsorption edge. (b) Ir valence states determined by the adsorption edge energy. (c) Relation between Ir valence states and IrO2 mass fraction. (d) Relation between Ir-4f7/2 binding energy and IrO2 mass fraction. The inset illustrates the uncertainty in charge transfer between IrO2 NPs and α-MnO2 substrate. Growth of IrO2 NPs on this unique nanorod support not only directly induces the lattice strain of IrO2 but may also give rise to charge transfer at their interface40-41 due to the difference in the surface charges of the two materials. This phenomenon was observed in IrOx particles on antimony-doped tin oxide (ATO) giving rise to lower Ir oxidation states (~3.2)42. We combine the XANES of the Ir-LIII edge (allowed by dipole selection rules) and XPS to analyze whether a charge transfer is present. The “white line” intensity of the Ir-LIII edge in XANES describes the 5d orbital electrons’ occupation states because with greater electron occupation, the intensity decreases43. As shown in Figure 6a, we can see that the “white line” intensity in all of the prepared IrO2@Mn catalysts is lower than that for the pure IrO2, but no linear relationship with


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the IrO2 mass fraction is observed. Additionally, the adsorption edge determined by the first derivative of XANES (Figure 6a insert) is often used to evaluate the oxidation states44 (Figure S12 depicts the relationship between the adsorption edge and Ir oxidation states). Figure 6b shows that the valence states of the grown IrO2 NPs on α-MnO2 are below 4+, indicates that a charge transfer from the α-MnO2 substrate to IrO2. However, this charge transfer capability is not inversely proportional to the grown mass (shown in Figure 6c). In addition to the XAS results, this charge transfer is also verified by qualitatively described binding energy of Ir-4f7/2 in the Ir4f core level XPS spectra (Figure 6d and fitting diagram shown in Figure S13). Such a charge donation from the α-MnO2 substrate to the IrO2 particles on the α-MnO2 substrate surface will result in a higher Mn oxidation state. The adsorption edge positions in IrO2@α-MnO2 are lower than those in pure α-MnO2 (Figure S14, Mn-K edge XANES), which means that a lower Mn oxidation state is present, contradicting the above assumption. Figure S15 shows the Mn-2p core level XPS spectra, from which we can see that the positions of Mn-2p remain almost constant. This may occur because the XPS detects the uncovered α-MnO2 surface. Given that the IrO2@Mn-5 is one of the most representative of all prepared samples, but both XAS and XPS show an abnormal characterization in the oxidation states. From the point of view of the electronic structure, the α-MnO2 is a semiconductor with the band gap of 1.33 eV45, while IrO2 is a conductor for which the Fermi level easily crosses over its t2g orbitals46. In terms of electrons occupation at near the Fermi level, IrO2 is more likely to transfer electrons to the α-MnO2 substrate as observed in Mn-XANES. However, the valence states of Ir in XAS and XPS give an opposite result. Therefore, the charge transfer between α-MnO2 and IrO2 requires further investigations. We have reason to believe that the charge transfer in support-catalyst interactions


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may not contribute to the lattice strain. The occurrence of lattice strain results primarily from the interface lattice mismatch.

Conclusion As demonstrated by our experiments, the strategy of growing IrO2 NPs directly on the MnO2 surface has

via

the

heterogeneous

nucleation

process

of

the

iridium

precursor

proven to be immensely successful. The prepared IrO2@Mn-5 not only exhibits a

satisfactory specific mass activity that is 3.7 times higher than that of pure IrO2 but also shows good stability. The creation of lattice strain in IrO2 NPs on the α-MnO2 surface is result of the lattice mismatch in the underlying interface. Our evidence indicates that an increase in lattice strain means a lower c/a ratio of IrO2, which is beneficial for enhancing its OER activity. This is mainly due to the direct relationship of the c/a ratio to the IrO6 octahedral geometry, which is in turn closely related to the electronic structure changes. The consistency of the changes between the lattice strain and OER activity indicates that the lattice strain may play an essential role in controlling the IrO2 catalytic activity. Our findings highlight that it is possible to enhance the OER activity by inducing lattice strain in the IrO2 crystal structure. ASSOCIATED CONTENT Supporting Information. Materials preparations and characterizations are presented in Supporting Information. Figures include TEM images, XPS-Ir and Mn, Mn-K EXAFS spectra and materials electrochemical properties are also supplied. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION


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Corresponding Author * Corresponding Author: [email protected]. Author Contributions Wei Sun design and analysis the experiments data. Zhenhua Zhou, Waqas Qamar Zaman and Limei Cao contribute some experiments. Ji Yang guide experimental investigation. ACKNOWLEDGMENT This work is financially support by the National Natural Science Fundation of China (51778229). We would like to thank beamline BL14W1 (Shanghai Synchrotron Radiation Facility) for providing the beam time. REFERENCES 1. Walter, M. G.; Warren, E. L.; McKone, J. R.; Boettcher, S. W.; Mi, Q.; Santori, E. A.; Lewis, N. S., Solar Water Splitting Cells. Chem. Rev. 2010, 110, 6446-6473. 2. Rozain, C.; Millet, P., Electrochemical Characterization of Polymer Electrolyte Membrane Water Electrolysis Cells. Electrochim. Acta 2014, 131, 160-167. 3. Carmo, M.; Fritz, D. L.; Mergel, J.; Stolten, D., A Comprehensive Review on Pem Water Electrolysis. Int. J. Hydrogen Energy 2013, 38, 4901-4934. 4. Paidar, M.; Fateev, V.; Bouzek, K., Membrane Electrolysis − History, Current Status and Perspective. Electrochim. Acta 2016, 209, 737-756. 5. Lewis, N. S.; Nocera, D. G., Powering the planet: Chemical Challenges in Solar Energy Utilization. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 15729-15735. 6. Katsounaros, I.; Cherevko, S.; Zeradjanin, A. R.; Mayrhofer, K. J., Oxygen Electrochemistry as a Cornerstone for Sustainable Energy Conversion. Angew. Chem. Int. Ed. 2014, 53, 102-121. 7. Norskov, J. K.; Bligaard, T.; Rossmeisl, J.; Christensen, C. H., Towards the Computational Design of Solid Catalysts. Nat. Chem. 2009, 1, 37-46. 8. Suntivich, J.; Gasteiger, H. A.; Yabuuchi, N.; Nakanishi, H.; Goodenough, J. B.; ShaoHorn, Y., Design Principles for Oxygen-Reduction Activity on Perovskite Oxide Catalysts for Fuel Cells and Metal–Air Batteries. Nat. Chem. 2011, 3, 546-550. 9. Xie, C.; Budnick, J.; Hines, W.; Wells, B.; Woicik, J., Strain-Induced Change in Local Structure and its Effect on the Ferromagnetic Properties of La0.5Sr0.5Co0.3 Thin Films. Appl. Phys. Lett. 2008, 93, 182507.


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Interface lattice mismatch induces lattice strain of IrO2 by direct growth on nanorods α-MnO2 for highly efficient water splitting.


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Lattice Strain on Î±-MnO - ACS Publications - American

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