FeHf binary hydroxide/oxide nanostructures as catalysts for oxygen evolution

We present pulsed electrodeposition (PED) of FeHf binary hydroxide/oxide (FeHf-BH) nanocomposites from aqueous electrolyte baths containing $NO_{3}$$^{–}$ ions. The deposition was carried out on a graphite foil at room temperature. This study, for the first time, demonstrated a controlled variation of Fe (5.9–49.9 avg. at. %) and Hf (2.4–58.7 avg. at. %) in the deposited materials. We showed the high scalability of FeHf-BH deposition by tuning the PED parameters. The morphology, composition, chemical structure, and oxidation states of metals in the materials were investigated by using scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS). The deposited materials consist of agglomerated nanoparticles sized 50–150 nm. Thermal annealing studies revealed improved crystallinity, with the appearance of thermodynamically stable oxide phases of $Fe_{3}O_{4}$, $Fe_{2}O_{3}$, and $HfO_{2}$ in the composites. The oxygen evolution activity of the materials was analyzed in an alkaline medium based on the Hf content. The optimized material containing 11.9 avg. at. % Hf demonstrated an OER onset potential of 1.63 V vs RHE, Tafel slope of 47 mV $dec^{–1}$, and required only 470 mV overpotential to reach a 50 mA $cm^{–2}$ OER current. These PED strategies of designing FeHf-BH materials may open an avenue for designing other catalytically active and stable multimetallic hydroxides/oxides composites.