Fig. 3: Characterization of the hydrated K⁺-containing birnessite phase formed during exposure with KOH.

a Operando XRD performed during the electrochemical oxidation in 0.1 M KOH solution of α-Li₂IrO₃ from OCV (0.8 V) to 1.55 V vs. RHE at a scan rate of 0.1 mV s⁻¹, and then hold for 10 h. b Rietveld analysis for the hydrated K⁺-containing birnessite phase, and c the corresponding structural model with K⁺ cations and H₂O molecules distributed between the honeycomb Li_1/3Ir_2/3O₂ layer. d XRD diffractograms collected for pristine α-Li₁IrO₃ and after cycling in different alkaline solutions (LiOH, NaOH, and KOH). e ¹H 2D exchange spectroscopy (using a 10 ms mixing time) of the birnessite between interlayer H₂O (0.9 ppm peak) and structural hydroxide groups (37.8 ppm peak). f Electron diffraction patterns (right) and HAADF-STEM image (left) of the birnessite phase. The O3 structure with d₀₀₁ ≈ 6.2 Å interleaves with lamellas of the O1 structure with d₀₀₁ ≈ 4.5 Å (marked with arrowheads).