Researchers from the University of Illinois at Urbana-Champaign have mixed metal compounds with perchloric acid to create a stable, efficient electrocatalytic material for the oxygen evolution reaction in acidic media.

The resulting porous Y₂[Ru_1.6Y_0.4]O_7−δ electrocatalyst exhibits a turnover frequency (TOF) of 560 s⁻¹ (at 1.5 V versus RHE) for the oxygen evolution reaction—two orders of magnitude higher than that of the RuO₂ reference catalyst (5.41 s⁻¹). A paper on their work is published in the journal Angewandte Chemie.

Electrolyzers use electricity to break water molecules into oxygen and hydrogen. The most efficient of these devices use corrosive acids and electrode materials made of the metal compounds iridium oxide or ruthenium oxide. Iridium oxide is the more stable of the two, but iridium is one of the least abundant elements on Earth, so researchers are in search of an alternative material.

Much of the previous work was performed with electrolyzers made from just two elements—one metal and oxygen. In a recent study, we found if a compound has two metal elements—yttrium and ruthenium—-and oxygen, the rate of water-splitting reaction increased.

—Hong Yang, a co-author and professor of chemical and biomolecular engineering at Illinois

Yao Qin, a co-author and former member of Yang’s group, first experimented with the procedure for making this new material by using different acids and heating temperatures to increase the rate of the water-splitting reaction.

The researchers found that when they used perchloric acid as a catalyst and let the mixture react under heat, the physical nature of the yttrium ruthenate product changed.

The material became more porous and also had a new crystalline structure, different from all the solid catalysts we made before.

—Jaemin Kim, the lead author and a postdoctoral researcher

The superior activity results from both high surface area of the porous structures and the optimized energy band structure due to the oxygen lattice defects, which arise from the mixed oxidation state of Ru^4+/5+ due to the partial substitution by Y³⁺ on the B-site. This work shows a new synthetic strategy for making high-temperature stable porous metal oxides and a valuable approach to the design of electrocatalysts.

—Kim et al.

Yang and his team looked at the structure of their new material with an electron microscope and found that it is four times more porous than the original yttrium ruthenate they developed in a previous study, and three times that of the iridium and ruthenium oxides used commercially.

It was surprising to find that the acid we chose as a catalyst for this reaction turned out to improve the structure of the material used for the electrodes. This realization was fortuitous and quite valuable for us.

—Jaemin Kim

The next steps for the group are to fabricate a laboratory-scale device for further testing and to continue to improve the porous electrode stability in acidic environments, Yang said.

Resources

• Jaemin Kim, Pei‐Chieh Shih, Yao Qin, Zaid Al‐Bardan, Cheng‐Jun Sun, Hong Yang (2018) “A Porous Pyrochlore Y₂[Ru_1.6Y_0.4]O_7–δ Electrocatalyst for Enhanced Performance towards the Oxygen Evolution Reaction in Acidic Media” Angewandte Chemie doi: 10.1002/anie.201808825