Development of catalysts to split water continues to be a focus for researchers because of the potential for efficiently producing hydrogen, which remains an ideal substance for use in generation of electricity through a renewable pathway. Hydrogen is needed for use in polymer electrolyte membrane fuel cells (PEMFC), which continue to be an important alternative power source for automobiles. 

Water splitting has actively been researched as a new way to generate hydrogen using electricity generated from renewable sources such as solar and wind. In this electrochemical process, oxygen formation from water, also known as the oxygen evolution reaction (OER), is the slow reaction step. Finding the means to improve the efficiency and stability of the catalyst is important because it will help to accelerate the production of hydrogen. 

Hong Yang, Alkire Chair professor of chemical and biomolecular engineering at the University of Illinois in Urbana-Champaign, Ill., says, “Operating the water splitting reaction under acidic conditions is preferred because of the high activity and good stability of the proton exchange membrane used in the electrolyzer. Hydrogen evolution, which occurs when two protons and two electrons combine, is very efficient under these conditions because there is almost no barrier. The challenge is improving the rate of the OER under these conditions where the pH of the system is approximately one.”

Favored catalysts that have been utilized are membranes prepared from pyrochlore-type oxides based particularly on ruthenium and iridium. Yang says, “Catalysts with these two metal oxides exhibit good stability and high activity under acid conditions. While iridium oxides are the more stable of the two metals, this element is one of the least abundant found on Earth and is very expensive.”

This means that it would be desirable to synthesize acid stable oxide catalysts based on ruthenium and other metals besides iridium. One factor that can help in preparing an iridium-free candidate is to produce porous structures. Yang says, “The main benefit of a porous catalyst is the high surface area. In the water splitting reaction, this facilitates small molecule diffusion as well as access for electrons on the catalyst surface.”

Preparation of porous pyrochlore-type oxides has been difficult due to the severe reaction conditions required. Yang says, “Use of high temperatures is often required to generate the right structure with high quality. Unfortunately the catalyst particles also sintered to form dense solids under high-temperature conditions.”

A new approach is necessary to prepare a stable, porous iridium-free pyrochlore-type oxide catalyst. Such an approach has now been reported.

Yttrium ruthenium oxide
Yang and his colleagues synthesized a porous, pyrochlore-type oxide that is based on oxides of yttrium and ruthenium. The key was the use of perchloric acid as the porogen, which was the agent that helped to create the porous structure in the catalyst. 

The yttrium ruthenium oxide catalyst was produced using a two-step process that started with crosslinking yttrium and ruthenium cations homogeneously using citric acid as the chelating agent. Perchloric acid (ca. 70% solution) was added as the porogen, and the reaction mixture heated to 80 C until water evaporated. After placing the mixture in a quartz boat and transferring it to a tube furnace, the porous framework formed as perchloric acid decomposed at 120 C and further heating took place to 600 C. The final product formed after the temperature was increased to 960 C. 

The researchers also experimented with other acids in the synthesis such as acetic, sulfuric and nitric but did not isolate the same kind of porous catalytic structures. Yang says, “We used perchloric acid because it has been evaluated as an electrolyte in fuel cells and electrolyzers. Using other acids, we did produce the catalysts but with denser structures and fewer pores. The porous structure of the yttrium ruthenium oxide remained even after changing the concentration of the citric acid and perchloric acid. We are not sure why perchloric acid is so much better than the other acids, though its decomposition is fast in the same temperature range as formation of the porous structure.”

Figure 3 shows an electron microscope image of the porous structure of the catalyst. 


Figure 3. This electron microscope image shows a new porous catalyst that displays potential for use in generating oxygen during the splitting of water. (Figure courtesy of the University of Illinois at Urbana-Champaign.)

The performance of yttrium ruthenium oxide catalyst was evaluated using a rotating disk electrode method where a carbon disk is used as the working electrode. Yang says, “This method involves rotating the electrode at high speed to minimize any diffusion related issues. The focus is in evaluating the performance of the catalyst.”

Energy-dispersive X-ray spectroscopy and X-ray diffraction studies showed that the yttrium ruthenium oxide catalyst is a mixed B-site cubic structure. The researchers found that this catalyst exhibited a turnover frequency (a measure of catalytic activity) that is two orders of magnitude higher than the activity of the OER catalyst, ruthenium oxide. 

Yang says, “We found that the yttrium ruthenium oxide exhibits more reactivity and better stability than many of the other OER catalysts we tested including ruthenium oxide, in an acidic environment.”

The researchers will focus in the future on improving the stability and performance of this catalyst. Yang says, “Stability is a continuing issue and we need to further improve the stability of the porous yttrium ruthenium oxide so it can operate under even harsher conditions. We also need to test out the catalyst performance in an electrolyzer, the device that is needed to produce both hydrogen and oxygen gases from water electrochemically.”

Additional information can be found in a recent article (1) or by contacting Yang at hy66@illinois.edu. 

REFERENCE
1. Kim, J., Shih, P., Qin, Y., Al-Bardan, Z., Sun, C. and Yang, H. (2018), “A Porous Pyrochlore Y₂[Ru_1.6Y_0.4]O_7- Electrocatalyst for Enhanced Performance towards the Oxygen Evolution Reaction in Acidic Media,” Angewandte Chemie, International Edition, 57 (42), pp. 13877-13881.