Structural and Photo(Electro)-Catalytic Properties of ZnO/RuO2 Composites depending on ZnO to RuO2 Mass Ratio

Abstract

Owing to its high energy density and environmentally friendly properties, hydrogen is recognized as promising fuels in the future energy system. Among the various techniques for H2 production, electrochemical water splitting, concerning both the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), is established as an efficient one. However, the kinetics of electrocatalytic water splitting reaction is significantly slowed down by the large energy barriers of the two-electron transfer (HER) and four-electron-proton coupled reaction (OER) pathways. The sluggish kinetics of water splitting processes can be overcome by employing electrocatalysts. It has been shown that Pt-based electrocatalysts exhibit the best HER activity, while IrO2 and RuO2-based electrocatalysts demonstrate the best OER activity [1,2]. Though, scarcity of noble metals and high price hindered their commercial applications in water splitting. Hence, it is an imperative to replace noble metal-based catalysts or reduce their amount through combination with abundant materials. The aim of this study was to examine an influence of ZnO to RuO2 mass ratio in the ZnO/RuO2 composites on their structural properties in order to prepare commercially acceptable materials with an enhanced photo(electro)-catalytic activity for water splitting. Composites of ZnO and RuO2 with mass ratios of 1:1, 2:1 and 10:1, named 1ZnO/1RuO2, 2ZnO/1RuO2, 10ZnO/1RuO2, respectively, were synthesized using the microwave processing. The structural investigations of the composites were completed using TEM/HRTEM and ADF/STEM analysis, including EDXS elemental mapping (FEI Talos F200X microscope operating at 200 kV, Thermo Fisher Scientific, Waltham, MA, United States). The electrocatalytic (EC) activity of the prepared composites towards OER and HER was examined through linear sweep voltammetry (LSV) in both acidic (0.1 M H2SO4, pH ∼ 1) and alkaline (0.1 M NaOH, pH ∼ 13) electrolytes. An Ivium VertexOne potentiostat/galvanostat and a conventional three-electrode quartz cell setup, including a glassy carbon as the working electrode, a platinum foil as the counter electrode, and a saturated calomel electrode as the reference electrode, were used for the EC measurements. The working electrode, with a surface area of 0.3 cm2, was coated with a catalyst ink prepared by previously published procedure [3]. The results of TEM analysis, Figure 1 (a-c), show that 2ZnO/1RuO2 composite is consisted of the smallest particles which are uniform in size and shape, while the both decreases of ZnO content (in 1ZnO/1RuO2 composite) and its increases (in 10ZnO/1RuO2) provoke appearance of larger, irregularly shaped particles. Besides, EDXS elemental mapping reveals that 2ZnO/1RuO2 composite possesses particles with the most homogeneous distribution of all constituting elements, Figure 1 (d-f). Figure 2 shows LSV curves illustrating the HER/OER catalytic performance of ZnO/RuO2 composites in a (a) 0.1 M H2SO4, and (b) 0.1 M NaOH electrolyte solutions. Solid lines represent LSV results obtained under dark conditions while dashed lines represent LSV results obtained after 60 minutes of simulated solar-light illumination. As can be seen from Figure 2, the 2ZnO/1RuO2 composite demonstrated superior HER/OER photo(electro)-catalytic activity in both electrolytes. Comprehensive structural analysis completed with Raman, PL and UV-Vis DR spectra indicates that superior HER/OER photo(electro)-catalytic activity of the 2ZnO/1RuO2 composite can be attributed to an optimal amount of oxygen vacancies in its crystal structure and homogeneous distribution of all constituting elements in the particles.