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 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 FeO, FeO, and HfO 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, and required only 470 mV overpotential to reach a 50 mA cm 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.