In order to study the structural and performance advantages, the OER measurements of IrO2(101) monolayer, IrO2 NP, C-IrO2 (Alfa Aesar) and C-Ir black were initially carried out in a three-electrode configuration using 0.1 M HClO4 as the aqueous electrolyte. All reference electrodes were calibrated before OER tests (Supplementary Fig. 13). As plotted in Fig. 5a, IrO2(101) monolayer displays the lowest overpotential of 227 mV at a current density of 10 mA cmgeo− 2, compared to 307, 311, and 301 mV of IrO2 NP, C-IrO2 and C-Ir black (Supplementary Fig. 14a). Even at 50 mA cmgeo−2, IrO2(101) monolayer maintains an ultralow overpotential of 272 mV, nearly 200 mV lower than its nanoparticle counterparts (Supplementary Fig. 14b). The most active linear sweep voltammetry (LSV) curve confirms that IrO2(101) facet plays the most vital role in optimizing the OER performance over other facets. Tafel slope is another important parameter to probe the response of IrO2 catalyst to the interfacial charge transfer towards the working potential. As depicted in Fig. 5b, IrO2(101) monolayer has the lowest Tafel slope of 44.73 mV dec− 1 within the potential range of 1.45–1.47 V. For comparison, higher Tafel slopes are obtained by IrO2 NP, C-IrO2 and C-Ir black, with delivering 83.06, 70.28 and 80.97 mV dec− 1, respectively. The OER kinetic on the (101) facet is much faster than those of nanoparticle counterparts, confirming that the 2D single-facet monolayer structure is more favorable for OER pathway37. The kinetics of OER processes on IrO2(101) monolayer was further evaluated by EIS measurement (Supplementary Fig. 15 and Supplementary Table 6). Fitting EIS curves reveals that IrO2(101) monolayer has the smallest charge transfer resistance (RCT) of 10.32 ohm, compared to those measured for IrO2 NP (109.60 ohm), C-IrO2 (88.49 ohm), and C-Ir/C (18.06 ohm). We attribute the lowest high frequency resistance to the maximized charge transfer process resulted from the monolayer thickness and highly oriented facet, which is the main origin of the superior OER performance. To identify the origin of monolayer structural advantages in mass transfer, we quantified the surface superaerophobicity of IrO2 catalysts by assessing the adhesive force between bubbles and the surface (Supplementary Fig. 16). Interestingly, the adhesive force measured on the monolayer IrO2(101) surface was found to be merely 17.7 µN, featuring excellent superaerophobicity in electrolyte. Employing IrO2 NP and C-IrO2 as contrastive samples, the values of adhesive force were measured to be 52.4 µN and 32.0 µN, respectively. Therefore, the bubble releases from the MEA surface of monolayer IrO2(101) seems to be easier in the solution, which can be attributed to the exceptionally low contact region between the bubbles and the uniform monolayer surface and thus low adhesive force38, 39. Above results demonstrate that the fine construction of 2D monolayer interface featuring a specific facet simultaneously optimizes both its kinetic behavior and wetting behavior, thereby facilitating efficient charge and mass transfer from the IrO2 surface for OER.
We further calculated the specific current densities at 1.50 V vs. RHE to study their OER performances. To present a comprehensive performance evaluation of different catalysts, the OER performances were evaluated by normalizing surface areas obtained using different approaches, such as GCE (glassy carbon electrode) geometric areas and electrochemically active surface areas (ECSA)40. IrO2(101) monolayer exhibits both the highest geometrical activity of 47.75 mA cmgeo−2 and ECSA-based activity of 0.027 mA cmECSA−2 at 1.50 V vs. RHE, nearly 10 times higher than those of IrO2 NP, C-IrO2, and C-Ir/C (Supplementary Fig. 17a, b). The methods to determine the ECSA-based surface areas of different catalysts and clean GCE are summarized in Methods section (Supplementary Figs. 18 and 19). In addition, the mass activities at 1.50 V vs. RHE were calculated to compare their intrinsic activities. By normalizing the Ir loading, rationally designed IrO2(101) monolayer performs 238.75 mA mgIr−1 at 1.5 V vs. RHE, almost 10 times higher than that of other rutile IrO2 (Fig. 5c). To gain a comprehensive view of the intrinsic performance, the OER performance of IrO2(101) monolayer was also evaluated using 0.5 M H2SO4 as the electrolyte. IrO2(101) monolayer exhibits the lowest overpotential of 245 mV at the current density of 10 mA cmgeo−2, compared to 304, 344, and 305 mV for IrO2 NP, C-IrO2, and C-Ir/C, respectively (Supplementary Fig. 20a). The Tafel slope of IrO2(101) monolayer in 0.5 M H2SO4 was measured to be only 47.47 mV dec− 1 (Supplementary Fig. 20b). In addition, both mass activities and EIS results illustrate that IrO2(101) monolayer shows the highest intrinsic activity and lowest electron transfer resistance (Supplementary Fig. 20c, d). Above OER measurements conducted in 0.5 M H2SO4 electrolyte further demonstrates that the IrO2(101) facet provide an ideal catalytic platform for OER process by favoring mass transport, mechanical behavior, and even electron transfer.
In the nucleation and growth process, both NH3·H2O amount and temperature play vital roles in guiding the (101)-oriented growth of IrO2 nuclei at a monolayer thickness. A perfect IrO2(101) monolayer is predicted to possess highest electrocatalytic performance for OER compared to other traditional nanostructures, due to the ~ 100% exposure of the (101) facet on the monolayer. LSV curves reveal that IrO2(101) monolayer exhibits the lowest overpotential of 227 mV to attain a current density of 10 mA cm− 2geo, which is 80 mV, 36 mV and 22 mV lower, respectively, than its counterparts synthesized with NH3·H2O amounts of 0 g, 3.2 g and 12.8 g (Supplementary Fig. 21). Insufficient ammonization is unfavorable for the preferred orientation growth of the highly active (101) facet, whereas excessive ammonization may disrupt the orderliness and integrity of the (101) crystal facet. In terms of annealing temperature, the overpotential of IrO2(101)-350 and IrO2(101)-500 were measured to be 257 mV and 357 mV at 10 mA cm− 2geo, respectively, significantly higher than that of IrO2(101) monolayer (227 mV) (Supplementary Fig. 22). Thus, 450 oC is identified as the optimal temperature to favor the (101)-oriented growth of 2D monolayer structure.
In addition to activity, balancing the operating stability in acidic media poses another significant challenge when considering the structural stability at industrial-level current densities under practical OER condition41, 42. In a three-electrode system, both IrO2 NPs and C-IrO2 suffer from dramatically activity loss during the 100 h chronopotentiometry (CP) tests while the working potential of IrO2(101) monolayer at 50 mA cm− 2geo maintains a consistent value after 910 h (Fig. 5d). The overpotential decay rate was calculated to be only 15 µV h− 1, demonstrating the superior stability of IrO2(101) monolayer. The inset in Fig. 5d further exhibits that the linear polarization curves of IrO2(101) monolayer before and after the CP test perfectly overlap. It also operates stably at the high current of 200 mA cm− 2geo during a 167 h stability test, confirming its great potential for commercial application (Supplementary Fig. 23). We also found that the 2D monolayer structure seems to be more insensitive to the working potential than previously reported nanostructured IrO2. For instance, a strong redox peak of IrⅢ/Ⅳ appears at 0.86 V vs. RHE once an anodic potential is applied, indicating that Ir3+ has been oxidized to Ir4+ or an even higher state (Supplementary Fig. 24). Even after 10000 CV cycles, no obvious shift in the redox peak has been detected43. The polarization curves and overpotentials of IrO2(101) monolayer before and after the 2,000th /10,000th CV cycles tests also prove that its activity loss is negligible despite being a 2D monolayer structure (Supplementary Fig. 25). To provide a comprehensive evaluation of the stability property of IrO2(101) monolayer, the inductively coupled plasma-optical emission spectrometer (ICP-OES) was further operated to calculate the dissolution amount of the Ir element after 910 h durability test at 50 mA cm− 2geo. Form the ICP-OES data in Supplementary Table 7, the S-number for each IrO2 catalyst was calculated as the ratio of the molar amount of evolved O2 to the molar amount of dissolved Ir44. The S-number of IrO2(101) monolayer after the 910 h OER was calculated to be 4.64×106, over 11 and 40 times higher than the S-numbers of IrO2 NP (4.18×105) and C-IrO2 (1.14×105), respectively (Supplementary Table 8). In addition, IrO2(101) monolayer exhibits superior stability compared to other recently reported Ir-based catalysts, including pristine Ir or Ir oxides (1 ~ 9.2×105)45, Ir-based alloys (5 ~ 2.49×105)41, and IrO2@TiO2 nanoparticles (1.0×104)46. Given the high current density and the superlong test time, the performance stability of 2D IrO2(101) monolayer in acidic media is further highlighted, pointing to its significant potential for commercialization even at monolayer thickness. As shown in Supplementary Figs. 26 and 27, the monolayer structure and uniform (101) facet orientation of the IrO2(101) monolayer remain intact after ~ 1000 h stability test at 50 mA cm− 2geo. TEM analysis reveals an initial sheet-like morphology with no reconstructed nanoparticles or nanoclusters on the surface. XRD characterization further confirms the presence of only a prominent (101) diffraction peak at 34.93°, without any signals from other rutile facets, demonstrating high stability in the crystallographic orientation during long-term operation. To gain insights into the electronic structure changes after the stability test, XPS analysis was performed. As shown in Supplementary Fig. 28, XPS spectrum of the IrO2(101) monolayer exhibits a slight blue shift of 0.15 eV compared to the initial state, suggesting the partial oxidation of surface Ir atoms under acidic OER conditions. Above characterizations after OER stability test confirm that the single-(101)-facet structure realizes a win-win strategy in terms of activity and stability, positioning IrO2(101) monolayer as a promising candidate for PEM applications.