DOI: 10.35097/vf3j9gx4te0ftpqc
title: Understanding the influence of iridium oxide catalyst state on the performance in oxygen evolution reaction

description: Proton-exchange membrane (PEM) water electrolysis is a critical technology for hydrogen production. The oxygen evolution reaction (OER) kinetics at the anode significantly determines the electrolysis performance, requiring the development of active and stable catalysts for high conversion rates. Despite extensive experimental studies, it is still difficult to fully understand how the catalyst state, i.e., the structure, morphology and oxidation state - which vary by synthesis conditions - affect the OER kinetics and free energies. In this study, we delve into the influence of catalyst calcination on catalyst state and its relationship with OER kinetics by a combination of experimental analysis and microkinetic modeling. Our results show that the increasing degree of crystallinity upon calcination, and thus, the reduced number of active sites is the main reason for the decreasing performance of Ir-oxide nanoparticles. Additionally, water adsorption step becomes thermodynamically more favorable, CUS-mediated PCET and O₂ release are modestly hindered, and the bridge-site redox contribution declines with increasing crystallinity. These subtle, systematic shifts help explain the non-linear structure–activity relationships reported in the literature. This understanding of the interplay between catalyst synthesis conditions and OER performance facilitates the tailored design and optimization of high-performance catalysts for more efficient electrocatalytic water electrolysis.
description: Figure 1a: X-ray diffractograms for as-prepared, and post-calcined (at 400 °C, 500 °C, 600 °C, and 700°C) Ir-oxide nanoparticles (2theta vs. intensity /a.u.).

Figure 1b: X-ray diffractograms for the reference spectrum for the tetragonal rutile IrO2 (JPCD card no. 15-0870), and Ir fcc (COD, 1512514), (2theta vs. intensity /a.u.).

Figure 2a: TEM image for the Ir-oxide nanoparticles calcined at 400 °C.

Figure 2b: TEM image for the Ir-oxide nanoparticles calcined at 500 °C.

Figure 2c: TEM image for the Ir-oxide nanoparticles calcined at 600 °C.

Figure 2d: TEM image for the Ir-oxide nanoparticles calcined at 700 °C.

Figure 3a: Comparison of the high-resolution Ir 4f spectra obtained from the XPS measurements on Ir-oxide nanoparticles (as-prepared, calcined at 400°C and calcined at 700°C), (binding energy / eV vs. intensity /a.u.).

Figure 3b: Comparison of the high-resolution O 1s spectra obtained from the XPS measurements on Ir-oxide nanoparticles (as-prepared, calcined at 400°C and calcined at 700°C), (binding energy / eV vs. intensity /a.u.).

Figure 3c: The hydroxide-to-oxide ratio for the post-calcined Ir-oxide nanoparticles.

Figure 4: Cyclic voltammograms of the post-calcined Ir-oxide nanoparticles (potential, E vs current density, j).

Figure 5a: Measured current densities at E = 1.53 V at 25 mV·s-1, j@1.53V, for all post-calcined Ir-oxide samples (potential, E vs current density, j).

Figure 5b: Change in the calculated total surface area of the particles present in the bulk vs. change in j@1.53V when calcination temperature increased (change / % vs calcination change interval).

Figure 6: Experimental and simulated CV data (at 200 mV·s-1) for the Ir-oxide nanoparticles calcined at 400 °C, 500 °C, 600 °C, and 700 °C (potential, E vs current density, j).

Figure 7a: Experimental and simulated CV curves recorded at the scan rates of 200, 100, 50, and 25 mV·s-1 for the Ir-oxide nanoparticles calcined at 400 °C.

Figure 7b: Experimental and simulated CV curves recorded at the scan rates of 200, 100, 50, and 25 mV·s-1 for the Ir-oxide nanoparticles calcined at 500 °C.

Figure 7c: Experimental and simulated CV curves recorded at the scan rates of 200, 100, 50, and 25 mV·s-1 for the Ir-oxide nanoparticles calcined at 600 °C.

Figure 7d: Experimental and simulated CV curves recorded at the scan rates of 200, 100, 50, and 25 mV·s-1 for the Ir-oxide nanoparticles calcined at 700 °C.

Figure 8a: Current density simulated at E=1.53V, j@1.53V-sim, double layer capacitance, Cdl, density of active CUS sites, ρCUS, versus calcination temperatures; values extracted from the simulations for post-calcined Ir-oxide particles. 

Figure 8b: Current density simulated at E=1.53V, j@1.53V-sim, double layer capacitance, Cdl, versus density of active CUS sites, ρCUS; values extracted from the simulations for the post-calcined Ir-oxide particles.

Figure 9a: Comparison of the free reaction energies of all the elementary steps (Step 1−7) of OER on CUS on the post-calcined Ir-oxide nanoparticles (reaction coordinate vs free reaction energy / eV).

Figure 9b: Comparison of the activation energies of all the elementary steps (Step 1−7) of OER on CUS on the post-calcined Ir-oxide nanoparticles (reaction coordinate vs activation energy / eV).

Figure 9c: Comparison of the interaction energies of all the elementary steps (Step 1−7) of OER on CUS on the post-calcined Ir-oxide nanoparticles (reaction coordinate vs interaction energy / eV).

Figure 9d: Comparison of the free reaction energies for the one-electron redox transition on BRI for the post-calcined Ir-oxide nanoparticles (reaction coordinate vs free reaction energy / eV).

Figure 9e: Comparison of the activation energies for the one-electron redox transition on BRI for the post-calcined Ir-oxide nanoparticles (reaction coordinate vs activation / eV).

Figure 9f: Comparison of the interaction energies for the one-electron redox transition on BRI for the post-calcined Ir-oxide nanoparticles (reaction coordinate vs interaction energy / eV).

Figure 9g: Comparison of the bridge site fraction for the post-calcined Ir-oxide nanoparticles.

Figure 10a_left: Effective reaction rates of the elementary steps of the OER mechanism on CUS for Ir-oxide nanoparticles calcined at 400 °C (potential, E vs reaction rate, r).

Figure 10a_right: Surface coverage of the adsorbed species on CUS for Ir-oxide nanoparticles calcined 400 °C (potential, E vs surface coverage). 

Figure 10b_left: Effective reaction rates of the elementary steps of the OER mechanism on CUS for Ir-oxide nanoparticles calcined at 700 °C (potential, E vs reaction rate, r).

Figure 10b_right: Surface coverage of the adsorbed species on CUS for Ir-oxide nanoparticles calcined 700 °C (potential, E vs surface coverage). 

Figure_S1a: XRD diffractograms of as-prepared Ir oxide nanoparticles (2theta vs. intensity /a.u.).

Figure_S1b: XRD diffractograms of Ir-oxide nanoparticles calcined at 400 °C (2theta vs. intensity /a.u.).

Figure_S1c: XRD diffractograms of Ir-oxide nanoparticles calcined at 500 °C (2theta vs. intensity /a.u.).

Figure_S1d: XRD diffractograms of Ir-oxide nanoparticles calcined at 600 °C (2theta vs. intensity /a.u.).

Figure_S1e: XRD diffractograms of Ir-oxide nanoparticles calcined at 700 °C (2theta vs. intensity /a.u.).

Figure_S1f: XRD diffractograms of tabulated reference for tetragonal rutile IrO2 and fcc Ir metal (2theta vs. intensity /a.u.).

Figure S2a-d: Electron diffraction results of Ir-oxide samples calcined at a) 400 °C, b) 500 °C, c) 600 °C, and d) 700 °C.

Figure S3a: SEM images of Ir-oxide samples calcined 400 °C.

Figure S3b: SEM images of Ir-oxide samples calcined 500 °C.

Figure S3c: SEM images of Ir-oxide samples calcined 600 °C.

Figure S3d: SEM images of Ir-oxide samples calcined 700 °C.

Figure S4: a) High-resolution O 1s spectrum of the Ir oxide nanoparticles calcined at 500 °C and 600 °C; b) High resolution C 1s spectrum of the Ir oxide nanoparticles as-prepared and calcined at 400 °C, 500 °C, 600 °C and 700 °C.

Figure S5: XPS survey scan of the Ir oxide nanoparticles calcined at a) 700 °C, b) 600 °C, c) 500 °C, d) 400 °C and e) as-prepared.

Figure S6a: Experimental CV curves recorded at the scan rates of 200, 100, 50, 25, and 200 mV/s on the iridium oxide nanoparticles calcined 400 °C (potential, E vs current density, j).

Figure S6b: ExperimentalCV curves recorded at the scan rates of 200, 100, 50, 25, and 200 mV/s on the iridium oxide nanoparticles calcined 500 °C (potential, E vs current density, j).

Figure S6c: ExperimentalCV curves recorded at the scan rates of 200, 100, 50, 25, and 200 mV/s on the iridium oxide nanoparticles calcined 600 °C (potential, E vs current density, j).

Figure S6d: ExperimentalCV curves recorded at the scan rates of 200, 100, 50, 25, and 200 mV/s on the iridium oxide nanoparticles calcined 700 °C (potential, E vs current density, j).

Figure S7a: Scan rate–normalized CVs for the Ir-oxide samples calcined at 400 °C.

Figure S7b: Scan rate–normalized CVs for the Ir-oxide samples calcined at 500 °C.

Figure S7c: Scan rate–normalized CVs for the Ir-oxide samples calcined at 600 °C.

Figure S7d: Scan rate–normalized CVs for the Ir-oxide samples calcined at 700 °C.

Figure S8: Comparison of average current density j versus scan rate for all post-calcined Ir-oxide samples.

Figure S9: Surface area–normalized cyclic voltammograms recorded at 200mV/s for the Ir-oxide catalysts calcined at 400 °C, 500 °C, 600 °C, and 700 °C. 

Figure S10a: Comparison of the experimental and simulated CV curves at the scan rate of 200 mV/s, for the Ir-oxide nanoparticles calcined at 400 °C (potential, E vs current density, j).

Figure S10b: Comparison of the experimental and simulated CV curves across all scan rates for the Ir-oxide nanoparticles calcined at 400 °C (potential, E vs current density, j).

Figure S10c: Free reaction energy diagram for all elementary steps of the oxygen evolution reaction (OER) at E=0V, E=1.23V, and E=1.6V for the Ir-oxide nanoparticles calcined at 400 °C (reaction coordinate vs free reaction energy / eV).

Figure S10d: Identified free reaction energy parameters for steps 1−6 at the electrochemical standard conditions in comparison to DFT-calculated values for the Ir-oxide nanoparticles calcined at 400 °C (reaction step vs free reaction energy / eV).

Figure S11a: Comparison of the experimental and simulated CV curves at the scan rate of 200 mV/s, for the Ir-oxide nanoparticles calcined at 500 °C (potential, E vs current density, j).

Figure S11b: Comparison of the experimental and simulated CV curves across all scan rates for the Ir-oxide nanoparticles calcined at 500 °C (potential, E vs current density, j).

Figure S11c: Free reaction energy diagram for all elementary steps of the oxygen evolution reaction (OER) at E=0V, E=1.23V, and E=1.64V for the Ir-oxide nanoparticles calcined at 500 °C (reaction coordinate vs free reaction energy / eV).

Figure S11d: Identified free reaction energy parameters for steps 1−6 at the electrochemical standard conditions in comparison to DFT-calculated values for the Ir-oxide nanoparticles calcined at 500 °C (reaction step vs free reaction energy / eV).

Figure S12a: Comparison of the experimental and simulated CV curves at the scan rate of 200 mV/s, for the Ir-oxide nanoparticles calcined at 600 °C (potential, E vs current density, j).

Figure S12b: Comparison of the experimental and simulated CV curves across all scan rates for the Ir-oxide nanoparticles calcined at 600 °C (potential, E vs current density, j).

Figure S12c: Free reaction energy diagram for all elementary steps of the oxygen evolution reaction (OER) at E=0V, E=1.23V, and E=1.66V for the Ir-oxide nanoparticles calcined at 600 °C (reaction coordinate vs free reaction energy / eV).

Figure S12d: Identified free reaction energy parameters for steps 1−6 at the electrochemical standard conditions in comparison to DFT-calculated values for the Ir-oxide nanoparticles calcined at 600 °C (reaction step vs free reaction energy / eV).

Figure S13a: Comparison of the experimental and simulated CV curves at the scan rate of 200 mV/s, for the Ir-oxide nanoparticles calcined at 700 °C (potential, E vs current density, j).

Figure S13b: Comparison of the experimental and simulated CV curves across all scan rates for the Ir-oxide nanoparticles calcined at 700 °C (potential, E vs current density, j).

Figure S13c: Free reaction energy diagram for all elementary steps of the oxygen evolution reaction (OER) at E=0V, E=1.23V, and E=1.68V for the Ir-oxide nanoparticles calcined at 700 °C (reaction coordinate vs free reaction energy / eV).

Figure S13d: Identified free reaction energy parameters for steps 1−6 at the electrochemical standard conditions in comparison to DFT-calculated values for the Ir-oxide nanoparticles calcined at 700 °C (reaction step vs free reaction energy / eV).

Figure S14a_left: Effective reaction rates of the elementary steps of the OER mechanism for Ir-oxide nanoparticles calcined at 400 °C (potential, E vs reaction rate, r).

Figure S14a_right: Surface coverage of the adsorbed species for Ir-oxide nanoparticles calcined 400 °C (potential, E vs surface coverage).

Figure S14b_left: Effective reaction rates of the elementary steps of the OER mechanism for Ir-oxide nanoparticles calcined at 500 °C (potential, E vs reaction rate, r).

Figure S14b_right: Surface coverage of the adsorbed species for Ir-oxide nanoparticles calcined 500 °C (potential, E vs surface coverage).

Figure S14c_left: Effective reaction rates of the elementary steps of the OER mechanism for Ir-oxide nanoparticles calcined at 600 °C (potential, E vs reaction rate, r).

Figure S14c_right: Surface coverage of the adsorbed species for Ir-oxide nanoparticles calcined 600 °C (potential, E vs surface coverage).

Figure S14d_left: Effective reaction rates of the elementary steps of the OER mechanism for Ir-oxide nanoparticles calcined at 700 °C (potential, E vs reaction rate, r).

Figure S14d_right: Surface coverage of the adsorbed species for Ir-oxide nanoparticles calcined 700 °C (potential, E vs surface coverage).

Figure S15a: Surface coverage of the bridging hydroxyl (BRI-OH), θ_BRI-OH, for Ir-oxide nanoparticles calcined at 400 °C.

Figure S15b: Surface coverage of the bridging hydroxyl (BRI-OH), θ_BRI-OH, for Ir-oxide nanoparticles calcined at 500 °C.

Figure S15c: Surface coverage of the bridging hydroxyl (BRI-OH), θ_BRI-OH, for Ir-oxide nanoparticles calcined at 600 °C.

Figure S15d: Surface coverage of the bridging hydroxyl (BRI-OH), θ_BRI-OH, for Ir-oxide nanoparticles calcined at 700 °C.