In SituEvolution of Ru4Al13Crystals into a Highly Active Catalyst for the Hydrogen Evolution Reaction

Materials that catalyze the hydrogen evolution reaction (HER) are important for producing H2as a zero-emission fuel. Ru-based materials are becoming increasingly studied as HER catalysts because they often perform comparably to Pt, the benchmark for this reaction. Despite the availability of a growing number of high surface area Ru-based HER catalysts, these materials can be challenging to synthesize. In contrast, bulk Ru-based materials can be readily synthesized by using established solid-state chemistry techniques. While bulk compounds are undesirable as practical catalysts because of their low surface areas, they allow candidate catalysts that are not yet accessible as high surface area materials to be evaluated and studied. Using this approach for Ru-based materials, we show here that the surfaces of millimeter-scale single crystals of the intermetallic compound Ru4Al13become rough, pitted, and enriched in Ru, consistent with computed Pourbaix diagrams, upon exposure to acid due to selective Al leaching. The resulting material, having a Ru-rich Ru–Al surface and a Ru4Al13core, catalyzes the HER in 0.5 M H2SO4with overpotentials of 18 and 39 mV at current densities of −10 and −100 mA/cm2, respectively, when normalized to geometric surface areas. These values are comparable to higher surface area nanoparticle catalysts, including Pt. This in situacid-mediated evolution of a bulk crystal into a surface-roughened derivative demonstrates a pathway for engineering catalytic materials that can be readily made as bulk crystals but not yet as higher surface area nanostructures.