Moderate heat treatment of CoFe Prussian blue analogues for enhanced oxygen evolution reaction performance

Prussian blue analogues (PBAs) with inherent ordered structures and abundant metal ion sites are widely explored as precursors for various electrochemical applications, including oxygen evolution reaction (OER). Using a range of characterization techniques including Fourier-transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and energy dispersive spectroscopy (EDS), this work discloses the process of replacement of K ⁺ by NH ₄ ⁺ in the interstitial spaces of the CoFe PBA by a hot aqueous urea solution, which influences the transformation of PBAs under further heat treatment and the OER performance of the derivatives. After heat treatment at 400 °C under Ar flow, high-resolution transmission electron microscopy (HRTEM) images reveal that CoFe alloy nanoparticles grew on the crystalline cubes of CoFe PBA with K ⁺, while CoFe PBA cubes with NH ₄ ⁺ become amorphous. Besides, the derivative of CoFe PBA with NH ₄ ⁺ (Ar-U-CoFe PBA) performs better than the derivative of CoFe PBA with K ⁺ (Ar-CoFe PBA) in OER, registering a lower overpotential of 305 mV at 10 mA cm ⁻², a smaller Tafel slope of 36.1 mV dec ⁻¹, and better stability over a testing course of 20 h in 1.0 M KOH. A single-cell alkaline electrolyzer, using Ar-U-CoFe PBA and Pt/C for the anodic and cathodic catalyst, respectively, requires an initial cell voltage of 1.66 V to achieve 100 mA cm ⁻² at 80 °C, with negligible degradation after 100 h.