Shape-Dependent Electrocatalytic Activity of Iridium Oxide Decorated Erbium Pyrosilicate toward the Hydrogen Evolution Reaction over the Entire pH Range KarfaParamita MajhiKartick C. MadhuriRashmi 2018 The synthesis of iridium oxide decorated erbium pyrosilicate (Er<sub>2</sub>Si<sub>2</sub>O<sub>7</sub>:IrO<sub>2</sub>) 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 Er<sub>2</sub>Si<sub>2</sub>O<sub>7</sub>:IrO<sub>2</sub> were obtained and their catalytic activities were tested toward the hydrogen evolution reaction (HER). Among the different morphologies of Er<sub>2</sub>Si<sub>2</sub>O<sub>7</sub>:IrO<sub>2</sub>, cube-shaped nanoparticles (Er<sub>2</sub>Si<sub>2</sub>O<sub>7</sub>:IrO<sub>2–5</sub>) with sharp edges provided promising HER activity over a wide pH range from 0 to 14 in acidic, neutral, and basic media. Er<sub>2</sub>Si<sub>2</sub>O<sub>7</sub>:IrO<sub>2–5</sub> exhibited an onset potential of −0.076 V with a very high current density of 252 mA cm<sup>–2</sup> (at −0.54 V). The overpotential and Tafel slope for the HER using Er<sub>2</sub>Si<sub>2</sub>O<sub>7</sub>:IrO<sub>2–5</sub> 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 H<sub>2</sub>SO<sub>4</sub>, 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 IrO<sub>2</sub> 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.