Shape-Dependent Electrocatalytic Activity of Iridium Oxide Decorated Erbium Pyrosilicate toward the Hydrogen Evolution Reaction over the Entire pH Range

The synthesis of iridium oxide decorated erbium pyrosilicate (Er₂Si₂O₇:IrO₂) was performed by a simple sol–gel technique. By simple alteration of the reaction parameters, different shapes (i.e., cubes, rods, large spheres, small spheres, and sheets) of Er₂Si₂O₇:IrO₂ were obtained and their catalytic activities were tested toward the hydrogen evolution reaction (HER). Among the different morphologies of Er₂Si₂O₇:IrO₂, cube-shaped nanoparticles (Er₂Si₂O₇:IrO_2–5) with sharp edges provided promising HER activity over a wide pH range from 0 to 14 in acidic, neutral, and basic media. Er₂Si₂O₇:IrO_2–5 exhibited an onset potential of −0.076 V with a very high current density of 252 mA cm^–2 (at −0.54 V). The overpotential and Tafel slope for the HER using Er₂Si₂O₇:IrO_2–5 were found to be 130 mV and 49 mV/dec, 170 mV and 59 mV/dec, and 190 mV and 67 mV/dec in 0.5 M H₂SO₄, 1.0 M KOH, and 2.0 M PBS, respectively. The low cost, highly active electrocatalyst shows robust durability over acidic medium for nearly 250 min. Such a superior catalytic activity can be attributed to the synergistic effect between IrO₂ and erbium pyrosilicate, in which the 4f orbital (not fully occupied) of rare-earth elements may have occupied the 5d orbital and become valence electrons, resulting in the improved HER activity.