Perchlorate Fusion-Hydrothermal Synthesis of Nano-Crystalline IrO2: Leveraging Stability and Oxygen Evolution Activity

Abstract

Iridium oxides are the state-of-the-art oxygen evolution reaction (OER) electrocatalysts in proton-exchange-membrane water electrolyzers (PEMWEs), but their high cost and scarcity necessitate improved utilization. Crystalline rutile-type iridium dioxide (IrO₂) offers superior stability under acidic OER conditions compared to amorphous iridium oxide (IrO_x). However, the higher synthesis temperatures required for crystalline phase formation result in lower OER activity due to the loss in active surface area. Herein, a novel perchlorate fusion-hydrothermal (PFHT) synthesis method to produce nano-crystalline rutile-type IrO₂ with enhanced OER performance is presented. This low-temperature approach involves calcination at a mild temperature (300 °C) in the presence of a strong oxidizing agent, sodium perchlorate (NaClO₄), followed by hydrothermal treatment at 180 °C, yielding small (≈2 nm) rutile-type IrO₂ nanoparticles with high mass-specific OER activity, achieving 95 A g_Ir ^-1 at 1.525 V_RHE in ex situ glass-cell testing. Most importantly, the catalyst displays superior stability under harsh accelerated stress test conditions compared to commercial iridium oxides. The exceptional activity of the catalyst is confirmed with in situ PEMWE single-cell evaluations. This demonstrates that the PFHT synthesis method leverages the superior intrinsic properties of nano-crystalline IrO₂, effectively overcoming the typical trade-offs between OER activity and catalyst stability.

Keywords: catalyst stability; electrocatalyst synthesis; hydrothermal synthesis; nano‐crystalline iridium dioxide; oxygen evolution reaction; perchlorate fusion; water electrolysis.

Figure 1

Schematic of the perchlorate fusion–hydrothermal (PFHT) synthesis.

Figure 2

a) X‐ray diffractogram of the IrO₂‐PFHT catalyst, corrected for the scattering background of the sample holder (red). The purple and green reference lines correspond to rutile‐type IrO₂ (PDF 00‐015‐0870) and cubic Ir metal (PDF 00‐046‐1044), respectively. A fit with the Lorentzian‐broadened IrO₂ reference peaks is shown, in good agreement with the pattern of the IrO₂‐PFHT catalyst. b) Normalized particle size distribution and c,d) representative HRSTEM images of the IrO₂‐PFHT catalyst. e) HRSTEM image showing the lattice fringes correlating to the (101) lattice plane of rutile IrO₂. f) XPS peak fitting of the Ir 4f spectrum and the associated contributions of formal iridium valencies for the IrO₂‐PFHT catalyst.

Figure 3

Cyclic voltammogram (CV) of the IrO₂‐PFHT catalyst, recorded at a sweep rate of 50 mV sec⁻¹ mV/s between 0.050–1.200 V versus RHE in 0.5 м H₂SO₄ solution, in comparison to the commercial benchmark iridium oxides. Comm. 1‐IrO_x is a highly active Ir‐based catalyst, whilst Comm. 2‐IrO₂ catalyst is a crystalline iridium oxide selected for its inherent stability. The OER activity and stability of the prepared catalyst were investigated ex situ in a three‐electrode glass‐cell setup in 0.5 м H₂SO₄ electrolyte.

Figure 4

a) Tafel plots of the OER currents for the IrO₂‐PFHT catalyst, commercial IrO_x, and IrO₂ benchmark catalysts (labeled Comm.1 and Comm. 2, respectively) before (solid lines) and after (dotted lines) the AST stability evaluation. b) Ir‐mass‐specific current densities at 1.525 V_RHE before and after the AST. c) Current‐potential curves of the IrO₂‐PFHT catalyst and the commercial IrO_x and IrO₂ benchmarks before and after the AST. The curves shown are averaged forward and backward cyclic voltammetry sweeps measured from 1.400–1.600 V_RHE at a scan rate of 10 mV dec⁻¹. d) AST chronoamperometry at 1.600 V_RHE for 4 h including online iR‐correction accounting for 85% of the Ohmic drop. The RDE disc was rotated at 2000 rpm during the AST (catalyst loading on the electrode: 100 µg_cat cm_geo ⁻²; electrolyte: 0.5 м H₂SO₄).

Figure 5

In situ, single‐cell PEM water electrolysis performance of CCMs consisting of IrO₂‐PFHT (blue) and Comm. 2‐IrO₂ catalyst (green) as the anodic catalysts. a) Ambient pressure polarization curves obtained at 60 °C b) Tafel plots constructed from the kinetic region (0.01–0.08 A cm⁻²) of the polarization curves.