For an OER process, the binding strength of three reaction intermediates (OH*, O* and OOH*) to active sites determines the energetics15–17. ΔGO* – ΔGOH* and ΔGOOH* are used as appropriate descriptors for OER activity16,18,19. We developed a new layered catalyst by inserting trace Ir to δ-MnO2 and meanwhile introducing lattice compressive strain, which was defined as sl-Mn1 − xIrxO2 (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 ΔGOOH*, meaning a low binding energy. With the formation of layered structure (δ-MnO2), ΔGOOH* decreases, and ΔGO* – ΔGOH* increases, leading to both descriptors moving towards the ideal zone. The layered Mn1 − xIrxO2 (l-Mn1 − xIrxO2) gives a theoretically lower overpotential of 230 mV, due to the slightly increased OOH* absorption energy on electronic structure modulated IrOx in l-Mn1 − xIrxO2. With the introduction of lattice compressive strain into l-Mn1 − xIrxO2, the obtained sl-Mn1 − xIrxO2 exhibits appropriate ΔGO* – ΔGOH* and ΔGOOH*, and a lowest overpotential of 150 mV. Remarkably, the free energy of ΔG2 (Supplementary Table 1) for sl-Mn1 − xIrxO2 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 (ɛd) and p (ɛp) band centers in sl-Mn1 − xIrxO2 moves closer to Fermi level and O 2p compared to l-Mn1 − xIrxO2 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 Mn1 − xIrxO2 ultra-thin nanosheets (sl-Mn1 − xIrxO2) were synthesized through a simple one-step hydrothermal reduction method (Scheme 1, Supporting Information). XRD patterns of sl-Mn1 − xIrxO2 (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-Mn1 − xIrxO2 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-Mn0.98Ir0.02O2 shows a curved nanosheet morphology. The sharp decrease of the contact angle (θ) value (Supplementary Fig. 3) indicates that the obtained sl-Mn1 − xIrxO2 sample is beneficial to the rapid mass diffusion of electrolyte on the catalyst surface20,21. 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/MnO622,23. 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-Mn0.98Ir0.02O2) was synthesized by annealing the sl-Mn0.98Ir0.02O2 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-Mn0.98Ir0.02O2 and sl-Mn0.98Ir0.02O2, respectively, indicating a single crystal structure without phase separation. For the annealed sample, l-Mn0.98Ir0.02O2, 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-Mn0.98Ir0.02O2 is about 0.198 nm. However, for sl-Mn0.98Ir0.02O2, 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-Mn0.98Ir0.02O2 shows an increased intensity of 6.0 at 5.0 Å−1 compared to 3.0 for IrO2, demonstrating higher valence states of Ir in sl-Mn0.98Ir0.02O2. The higher valence states of Ir in sl-Mn0.98Ir0.02O2 is further verified by higher intensity of Ir L3-edge white line of sl-Mn0.98Ir0.02O2 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-Mn0.98Ir0.02O2 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 4f7/2 and Ir 4f5/2 in sl-Mn0.98Ir0.02O2 move to higher binding energies by 0.20 eV compared to those of IrO225,28. High valence stated and electrophilic Ir favors water molecule attack for promoting the formation of O-O bonds during OER catalysis29–32.
Mn K-edge XANES spectrum of sl-Mn0.98Ir0.02O2 (Fig. 3e) is quite different from that of δ-MnO2, implying a varied coordination environment of Mn in sl-Mn0.98Ir0.02O2. The absorption energy of sl-Mn0.98Ir0.02O2 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-Mn0.98Ir0.02O2 is less than 4+ 33−35. FT-EXAFS spectra (Fig. 3f) display sl-Mn0.98Ir0.02O2 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-Mn0.98Ir0.02O2 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-Mn0.98Ir0.02O2 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-Mn1-xIrxO2 (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-Mn1-0.98Ir0.02O2 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-Mn0.98Ir0.02O2 with the lowest Tafel slope of 18.1 mV dec− 1(Fig. 4b) exhibits the best OER kinetics36.
In contrast, l-Mn1-xIrxO2 without strain shows inferior OER activity to sl-Mn1-xIrxO2, although it still has much higher OER activity than IrO2. In particular, the OER activity of sl-Mn0.98Ir0.02O2 is 1.6 times higher than that of l-Mn0.98Ir0.02O2 at an overpotential of 270 mV. Meanwhile, the overpotentials for sl-Mn0.98Ir0.02O2 to reach 10, 50 and 100 mA cm-2 are 62, 197 and 234 mV, respectively, much lower than those for l-Mn0.98Ir0.02O2 (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-Mn0.98Ir0.02O2 have smaller charge transfer resistance (Rct = 1.954 Ω) compared to l-Mn0.98Ir0.02O2, 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-Mn0.98Ir0.02O2 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-Mn0.98Ir0.02O2 is up to 7882 A g− 1, outperforming the state-of-the-art OER electrocatalysts (Fig. 4f) 37–43.
The stability of sl-Mn0.98Ir0.02O2 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-Mn0.98Ir0.02O2 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-Mn0.98Ir0.02O2 in this work and high HER activity of NiMo alloy in the previous work44–46, a two-electrode system with sl-Mn0.98Ir0.02O2 as the anode and NiMo alloy as the cathode (sl-Mn0.98Ir0.02O2 ‖ 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-Mn0.98Ir0.02O2 ‖ 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-Mn0.98Ir0.02O2 ‖ NiMo electrolyzer. Furthermore, the sl-Mn0.98Ir0.02O2 ‖ 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-Mn0.98Ir0.02O2 ‖ 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.