Exploring the combustion synthesis for yttrium ruthenate pyrochlores as OER electrocatalysts

Abstract

Yttrium ruthenate pyrochlores have become popular as electrocatalysts for the oxygen evolution reaction in water electrolysis. However, the traditional synthesis routes used to prepare these Y₂Ru_2−xY_xO₇ electrocatalysts require calcination at temperatures higher than 1000 °C for extended periods of time, resulting in highly sintered particles. We propose an alternative synthesis route, a glycine combustion method, that reduces the calcination time to only two hours to obtain porous pyrochlores. These pyrochlores contain a small RuO₂ impurity phase that is eliminated when the combusted product is calcined for nine hours instead of two. When the combustion synthesis is combined with a molten salt synthesis (MSS), there is no impurity phase, but large pyrochlore crystallites in a porous matrix are produced. The electrochemically active surface area (ECSA) is an order of magnitude lower than the other pyrochlores prepared, negatively affecting the electrocatalytic activity. In addition to altering the fuel and calcination conditions used in the synthesis, the oxidiser : fuel ratio (ϕ) has also been altered to manipulate the intensity of the combustion reaction. A ϕ = 1, an explosive reaction takes place. The intensity is reduced to a slow burn when adjusting ϕ to 0.3. The synthesised pyrochlores have been analysed with X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS) and Raman spectroscopy. They have also been tested as OER electrocatalysts in 0.5 M H₂SO₄. Combustion-synthesised Y₂Ru_2−xY_xO₇ calcined for nine hours has an active-area normalised current of 1.52 mA cm⁻² at 1.6 V, which is almost two times higher than that of the same pyrochlore prepared by citric acid, and three times higher than that of RuO₂ and IrO₂.