Compressively strained Triple-layer IrO6/MnO6 Octahedral Nanosheets for Oxygen Evolution in Acid Media

doi:10.21203/rs.3.rs-1531422/v1

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Compressively strained Triple-layer IrO₆/MnO₆ Octahedral Nanosheets for Oxygen Evolution in Acid Media

https://doi.org/10.21203/rs.3.rs-1531422/v1

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Oxygen electrochemistry plays a key role in renewable energy technologies such as fuel cells and electrolyzers, but its slow kinetics limits the performance and commercialization of such devices. Here, a novel triple-layer IrO₆/MnO₆ nanosheet was developed by a simple hydrothermal method. During the substitution of Mn by Ir, lattice compressive strain is introduced into the IrO₆/MnO₆ nanosheets, thereby tuning their electronic structure to suit the favorite oxygen evolution reaction (OER) activity. This compressively strained nanosheet catalyst exhibits an excellent mass activity of 7882 A g^− 1 at an overpotential of 270 mV in 0.5 M H₂SO₄, and reaches 10, 50 and 100 mA cm^− 2 at overpotentials of only 62 mV, 197 mV and 234 mV, respectively, ranking the highest OER activity among the state-of-the-art values in acid media. The catalyst exhibits a remarkable stability even at 300 mA cm^− 2 in 0.5 M H₂SO₄. Using the nanosheets as OER catalyst and NiMo alloy as hydrogen evolution reaction catalyst, a two-electrode electrolyzer achieves 10 mA cm^− 2 with only a cell voltage of 1.46 V for overall water splitting in 0.5 M H₂SO₄. This strategy enables the material with high feasibility for practical applications on hydrogen production.

Compressive strain

Oxygen evolution reaction

Nanosheets

Acid

Iridium

Electrochemical water splitting to produce clean and recyclable hydrogen fuels provides a desirable solution to solve the current energy shortage and environmental pollution issues^1–3. Water splitting involves oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). OER is significantly more challenging than HER, because OER involves multiple electron transfer and is kinetically sluggish^4–7. Compared with alkaline electrolysis, oxygen production in acid media has the advantages of simplicity, high-current density, and high-pressure compatibility^8–10. At present, high-performance catalysts for OER in acid media are mostly limited to noble metal oxides, such as IrO₂¹¹. However, their scarcity, low mass activities and large overpotentials at high current densities limit the economically large-scale electrochemical water splitting.

These difficulties mainly arise from tuning the electronic structure of IrO_x in stable Ir-based oxides and exposing them efficiently to electrolytes with lower Ir content. For instance, 6H-SrIrO₃ exhibited a high OER activity under acidic conditions due to the presence of IrO₆ octahedra, which were proved to be the active sites for OER¹². However, in order to maintain the stable 6H-SrIrO₃ perovskite structure, a large amount of electrochemically inactive bulk SrO₆ octahedra is required. As a result, it would be very difficult to reduce the Ir content while avoiding the instability of structure. Seitz et al. propose an efficient method to expose active sites and tune electronic structures via Sr leaching on SrIrO₃ surfaces¹³. The electronic structure adjustment is validated through the increased valence state of Ir. Although the high valence stated IrO_x on the surface accounts for the high activities, the low activity of the inner substrate reduces the overall mass activity. Therefore, fully exposing IrO₆ octahedra and tuning its electronic structure would be greatly beneficial for OER activity. Strain is proved to be an effective way to enhance the OER activity by distorting the crystal structure to tailor the electronic structures. For instance, torsion strain through grain boundary engineering and atomic replacement synergistically tuned the adsorption energy of oxygen intermediates and boosted the OER activity¹⁴. Therefore, few layers of strained IrO₆ octahedra would be idea catalysts for realizing fully exposed IrO₆ octahedra and modified the electronic structure for efficient and stable OER.

In this study, we adopted a facile hydrothermal approach to fabricate a new layered Mn_1 − xIr_xO₂ nanosheet electrocatalyst with compressively strained IrO₆/MnO₆ octahedra (sl-Mn_1 − xIr_xO₂). The obtained sl-Mn_0.98Ir_0.02O₂ catalyst exhibits the highest iridium mass activity (7882 A g^− 1 at an overpotential of 270 mV) and only needs an ultra-low overpotential of 62 mV to achieve a current density of 10 mA cm^− 2. More notably, a two-electrode electrolyzer with sl-Mn_0.98Ir_0.02O₂ as the OER catalyst and NiMo alloy as the HER catalyst for overall water splitting in 0.5 M H₂SO₄ reaches a current density of 10 mA cm^− 2 at a cell voltage of only 1.46 V and shows excellent stability.

For an OER process, the binding strength of three reaction intermediates (OH*, O* and OOH*) to active sites determines the energetics^15–17. ΔG_O* – ΔG_OH* and ΔG_OOH* are used as appropriate descriptors for OER activity^16,18,19. We developed a new layered catalyst by inserting trace Ir to δ-MnO₂ and meanwhile introducing lattice compressive strain, which was defined as sl-Mn_1 − xIr_xO₂ (Supplementary Table 1). All the calculated structural models are indicated in Fig. 1a. Figure 1b shows a constructed contour plot based on the calculated results. It is found that bulk MnO₂ with corner-shared octahedra has a high ΔG_OOH*, meaning a low binding energy. With the formation of layered structure (δ-MnO₂), ΔG_OOH* decreases, and ΔG_O* – ΔG_OH* increases, leading to both descriptors moving towards the ideal zone. The layered Mn_1 − xIr_xO₂ (l-Mn_1 − xIr_xO₂) gives a theoretically lower overpotential of 230 mV, due to the slightly increased OOH* absorption energy on electronic structure modulated IrO_x in l-Mn_1 − xIr_xO₂. With the introduction of lattice compressive strain into l-Mn_1 − xIr_xO₂, the obtained sl-Mn_1 − xIr_xO₂ exhibits appropriate ΔG_O* – ΔG_OH* and ΔG_OOH*, and a lowest overpotential of 150 mV. Remarkably, the free energy of ΔG2 (Supplementary Table 1) for sl-Mn_1 − xIr_xO₂ is 1.22 eV, which is very close to the theoretical value of 1.23 eV, indicating an optimized electronic structure under lattice compressive strain. Continuous projected density of states (PDOS) (Supplementary Fig. 1) indicates that d (ɛ_d) and p (ɛ_p) band centers in sl-Mn_1 − xIr_xO₂ moves closer to Fermi level and O 2p compared to l-Mn_1 − xIr_xO₂ without strain. Further theoretical electronic structure analysis reveals that lattice compressive strain reduces the catalytic energy barriers, facilitates the electron transfer, and thereby favors the enhancement in OER activity.

To experimentally validate the calculated results, the compressively strained Mn_1 − xIr_xO₂ ultra-thin nanosheets (sl-Mn_1 − xIr_xO₂) were synthesized through a simple one-step hydrothermal reduction method (Scheme 1, Supporting Information). XRD patterns of sl-Mn_1 − xIr_xO₂ (Fig. 2a and Supplementary Fig. 2a) show the identical peak positions as those of δ-MnO₂, suggesting it strongly (001) facet oriented.

Scanning electron microscope (SEM) images of sl-Mn_1 − xIr_xO₂ show nanosheets arranged vertically on the surface of carbon paper (Fig. 2c and Supplementary Fig. 2b). Transmission electron microscope (TEM) measurement (Fig. 2d) of sl-Mn_0.98Ir_0.02O₂ shows a curved nanosheet morphology. The sharp decrease of the contact angle (θ) value (Supplementary Fig. 3) indicates that the obtained sl-Mn_1 − xIr_xO₂ sample is beneficial to the rapid mass diffusion of electrolyte on the catalyst surface^20,21. The thickness of the nanosheets determined by atomic force microscopy (AFM) is about 4–4.5 nm (Fig. 2e), corresponding to only three monolayers of IrO₆/MnO₆ octahedral dimers. The ultra-thin nanosheet morphology enables full exposure active sites of IrO₆/MnO₆^22,23. The analyses of elemental composition (Supplementary Tables 2 and 3, Supplementary Fig. 4) reveal that Mn, Ir and O elements uniformly disperse in the synthesized nanosheets without any aggregation.

Aberration corrected high-angle annular dark-field scanning transmission electron microscope (HAADF-STEM) was used to characterize the lattice compressive strain. For comparison, a strain-free sample (l-Mn_0.98Ir_0.02O₂) was synthesized by annealing the sl-Mn_0.98Ir_0.02O₂ sample from room temperature to 400 °C at 1.0 °C min^− 1 to eliminate the strain. Figures 2f and 2g show clear lattice fringes for l-Mn_0.98Ir_0.02O₂ and sl-Mn_0.98Ir_0.02O₂, respectively, indicating a single crystal structure without phase separation. For the annealed sample, l-Mn_0.98Ir_0.02O₂, the integrated pixel intensity indicates that the O-O lattice spacing is 0.244 nm (Fig. 2h), which is consistent with that of δ-MnO₂²⁴ and suggests no compressive strain in it. Given that the bond angle of metal-O-metal is around 100° in a typical MnO₆ octahedron, the metal-O bond length for l-Mn_0.98Ir_0.02O₂ is about 0.198 nm. However, for sl-Mn_0.98Ir_0.02O₂, the O-O lattice spacing shrinks to 0.228 nm (Fig. 2i), and the Mn/Ir-O bond length is shortened to about 0.183 nm. Combing the IrO₆ octahedral unit cell is larger than that of the MnO₆ octahedral, it can be concluded that the incorporation of trace amount of Ir into Mn-O lattice induces lattice compressive strain.

The electronic structures were further explored by X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) measurements. Wavelet transform of EXAFS (WT-EXAFS) (Fig. 3a, b) of Ir L₃-edge related to Ir–O bond for sl-Mn_0.98Ir_0.02O₂ shows an increased intensity of 6.0 at 5.0 Å⁻¹ compared to 3.0 for IrO_2, demonstrating higher valence states of Ir in sl-Mn_0.98Ir_0.02O₂. The higher valence states of Ir in sl-Mn_0.98Ir_0.02O₂ is further verified by higher intensity of Ir L₃-edge white line of sl-Mn_0.98Ir_0.02O₂ than that of IrO₂ (Fig. 3c)^25–27. The fitting results for Fourier transform of EXAFS (FT-EXAFS) spectra (Fig. 3d) exhibit that Ir-O bond in sl-Mn_0.98Ir_0.02O₂ is covalently contracted to 1.96 Å from 1.99 Å of IrO₂ (Supplementary Table 4), leading to 1.51% compressive strain. This result is also evidenced by X-ray photoelectron spectroscopy (XPS) analysis. As shown in Fig. 3g, the peak positions corresponding to Ir 4f_7/2 and Ir 4f_5/2 in sl-Mn_0.98Ir_0.02O₂ move to higher binding energies by 0.20 eV compared to those of IrO₂^25,28. High valence stated and electrophilic Ir favors water molecule attack for promoting the formation of O-O bonds during OER catalysis^29–32.

Mn K-edge XANES spectrum of sl-Mn_0.98Ir_0.02O₂ (Fig. 3e) is quite different from that of δ-MnO₂, implying a varied coordination environment of Mn in sl-Mn_0.98Ir_0.02O₂. The absorption energy of sl-Mn_0.98Ir_0.02O₂ between 6540 and 6560 eV shifts slightly to lower energy than that of δ-MnO₂, indicating that the average valence state of Mn in sl-Mn_0.98Ir_0.02O₂ is less than 4^+ 33−35. FT-EXAFS spectra (Fig. 3f) display sl-Mn_0.98Ir_0.02O₂ have 3 identical characteristic peaks from 1.0 to 3.5 Å as δ-MnO₂, assigned to Mn-O (~ 1.50 Å), Mn-Mn edge-sharing (~ 2.5 Å) and Mn-Mn corner-sharing (~ 3.0 Å) scattering shells, respectively. However, further fitting analysis (Supplementary Table 4) shows that Mn − Mn edge-sharing bond length of sl-Mn_0.98Ir_0.02O₂ shrinks to 2.86 Å from 2.89 Å of δ-MnO₂, resulting in .03 % lattice compressive strain.

The OER electrochemical performance of the compressively strained sl-Mn_0.98Ir_0.02O₂ was measured using a proton exchange membrane separated electrochemical cell with Pt plate as the counter electrode and Hg/Hg₂SO₄ as the reference electrode. IrO₂ and δ-MnO₂ were utilized for comparison. The current densities of sl-Mn_1-xIr_xO₂ (x = 0.01, 0.02) are more than a dozen orders of magnitude higher than that of IrO₂ and MnO₂ at the same voltage (Fig. 4a). Especially, sl-Mn_1-0.98Ir_0.02O₂ can reach a current density of 176.2 mA cm^-2 at an overpotential of 270 mV, while IrO₂ and δ-MnO₂ only can achieve 4.0 and 0.2 mA cm^-2, respectively, at the same bias. sl-Mn_0.98Ir_0.02O₂ with the lowest Tafel slope of 18.1 mV dec^− 1(Fig. 4b) exhibits the best OER kinetics³⁶.

In contrast, l-Mn_1-xIr_xO₂ without strain shows inferior OER activity to sl-Mn_1-xIr_xO₂, although it still has much higher OER activity than IrO₂. In particular, the OER activity of sl-Mn_0.98Ir_0.02O₂ is 1.6 times higher than that of l-Mn_0.98Ir_0.02O₂ at an overpotential of 270 mV. Meanwhile, the overpotentials for sl-Mn_0.98Ir_0.02O₂ to reach 10, 50 and 100 mA cm^-2 are 62, 197 and 234 mV, respectively, much lower than those for l-Mn_0.98Ir_0.02O₂ (105, 227 and 266 mV, respectively) (Fig. 4d). These results demonstrate that the compressive strain has a positive effect on OER activity.

The charge transfer was further analyzed by electrochemical impedance spectroscopy (EIS). Nyquist plots (Fig. 4c) reveal that sl-Mn_0.98Ir_0.02O₂ have smaller charge transfer resistance (R_ct = 1.954 Ω) compared to l-Mn_0.98Ir_0.02O₂, δ-MnO₂, IrO₂, suggesting that the integration of compressive strain accelerates charge transfer. The compressive stain can push the d/p band centers closer to the Fermi level. As a result, the holes in the O 2p orbitals will more easily transfer to the catalyst surface to facilitate the charge transfer, resulting in the improvement of OER activity. Remarkably, sl-Mn_0.98Ir_0.02O₂ exhibits the highest OER activity at the lower Ir loading amount among the state-of-the-art electrocatalysts (Fig. 4e, Supplementary Table 5).

Next, we evaluate the mass activity of Ir, which refers to the current per unit mass of Ir and is an important aspect for the catalytic performance of acidic OER electrocatalysts. At 270 mV, the mass activity of IrO₂ is only 9 A g^− 1, while that of sl-Mn_0.98Ir_0.02O₂ is up to 7882 A g^− 1, outperforming the state-of-the-art OER electrocatalysts (Fig. 4f) ^37–43.

The stability of sl-Mn_0.98Ir_0.02O₂ was evaluated in acidic condition with the current density of 10 mA cm^− 2 with IrO₂ and δ-MnO₂ for comparison (Fig. 4g). No attenuation of the electrocatalytic activity is observed after the sl-Mn_0.98Ir_0.02O₂ catalyst runs for 48 h. In contrast, the OER activity of IrO₂ declines obviously even after 32 h. Then we continue to run this catalyst at a high current density of 100 mA cm^− 2 for another 48 h, and at a higher current density of 300 mA cm^− 2 for extra 72 h. Remarkably, this catalyst can still maintain over 95% activity after more than 168 h (7 days) durability test, indicating excellent stability.

In view of excellent OER activity of sl-Mn_0.98Ir_0.02O₂ in this work and high HER activity of NiMo alloy in the previous work^44–46, a two-electrode system with sl-Mn_0.98Ir_0.02O₂ as the anode and NiMo alloy as the cathode (sl-Mn_0.98Ir_0.02O₂ ‖ NiMo) was assembled for overall water splitting in 0.5 M H₂SO₄ with a scan rate of 5 mV s^− 1. IrO₂ and NiMo alloy electrolytic cell (IrO₂ ‖ NiMo) was also evaluated for comparison. As expected, sl-Mn_0.98Ir_0.02O₂ ‖ NiMo electrolyzer only needs 1.46 V to reach a current density of 10 mA cm^− 2 (Fig. 5a, inset: optical photo of two-electrode electrolyzer), much lower than IrO₂ ‖ NiMo (1.57 V), suggesting outstanding water splitting catalytic activity of the sl-Mn_0.98Ir_0.02O₂ ‖ NiMo electrolyzer. Furthermore, the sl-Mn_0.98Ir_0.02O₂ ‖ NiMo electrolytic cell is very stable for the overall water splitting in 0.5 M H₂SO₄ (Fig. 5b). However, IrO₂ ‖ NiMo fluctuates evidently. Notably, the performance of the sl-Mn_0.98Ir_0.02O₂ ‖ NiMo system is superior to the current series of representative water splitting catalysts (Supplementary Table 6), which makes it the most active water splitting catalyst under acidic conditions.

We developed a compressively strained electrocatalyst, sl-Mn_1-xIr_xO₂ ultra-thin nanosheets, for OER in acid media through a simple hydrothermal method. Especially, sl-Mn_0.98Ir_0.02O₂ exhibits excellent performance for OER with a small Tafel slope of 18.1 mV dec^− 1, a high mass activity with 7882 A g^-1 at an overpotential of 270 mV, low overpotential of 62 and 234 mV under the benchmark current density of 10 and 100 mA cm^-2, respectively. The fabricated catalyst also exhibits good stability even at high current density of 300 mA cm^-2 for practical application, which is superior to currently reported catalysts. The remarkable performance is ascribed to the full exposure of compressively strained IrO₆/MnO₆ octahedral dimers. Moreover, when sl-Mn_0.98Ir_0.02O₂ is used as OER catalyst, and NiMo alloy is used as HER catalyst for two-electrode overall water splitting, the cell shows high electrocatalytic activity, which can reach 10 mA cm^-2 at 1.46 V.

Acknowledgements

This work was financially supported by NSF project (No. 51202281, 21872174, and U1932148). The work was also partially supported by Hunan Key Technology R&D Program (2022GK2045, 2020WK2002), International Science and Technology Cooperation Program (Grant Nos. 2017YFE0127800 and 2018YFE0203400), Hunan Provincial Natural Science Foundation of China (No. 2020JJ2041), Changsha Municipal National Science Foundation (kq2014120) and the foundation of Hunan Environmental Protection Department. The authors also acknowledge the support from the High-Performance Computing Center of Central South University.

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Compressively strained Triple-layer IrO6/MnO6 Octahedral Nanosheets for Oxygen Evolution in Acid Media

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Compressively strained Triple-layer IrO₆/MnO₆ Octahedral Nanosheets for Oxygen Evolution in Acid Media

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