Microwave-Hydrothermal Tuning of Spinel-Type Co 3 O 4 Water Oxidation Catalysts.

Karla Lienau,C A Triana,Greta R Patzke,Lukas Reith,Sebastian Siol

doi:10.3389/fchem.2020.00473

Abstract

Water oxidation is the bottleneck reaction for overall water splitting as a direct and promising strategy toward clean fuels. However, the development of robust and affordable heterogeneous water oxidation catalysts remains challenging, especially with respect to the wide parameter space of synthesis and resulting material properties. Oxide catalysts performance in particular has been shown to depend on both synthetic routes and applied catalytic test methods. We here focus on spinel-type Co3O4 as a representative case for an in-depth study of the influence of rather subtle synthetic parameter variations on the catalytic performance. To this end, a series of Co3O4 samples was prepared via time-saving and tunable microwave-hydrothermal synthesis, while systematically varying a single parameter at a time. The resulting spinel-type catalysts were characterized with respect to key materials properties, including crystallinity, oxidation state and surface area using a wide range of analytical methods, such as PXRD, Raman/IR, XAS and XPS spectroscopy. Their water oxidation activity in electrocatalytic and chemical oxidation setups was then compared and correlated with the obtained catalyst properties. Both water oxidation methods displayed related trends concerning favorable synthetic parameters, namely higher activity for lower synthesis temperatures, lower precursor concentrations, addition of hydrogen peroxide and shorter ramping and reaction times, respectively. In addition to the surface area, structural features such as disorder were found to be influential for the water oxidation activity. The results prove that synthetic parameter screening is essential for optimal catalytic performance, given the complexity of the underlying performance-properties relationships.

Highlights

Water splitting is a promising renewable energy approach due to the superior storage options of fuels compared to electricity generated by solar cells
We found that the applied synthetic method exerted a major influence on the catalytic water oxidation
In order to screen their influence on the water oxidation catalysts (WOCs) activity of cobalt oxide nanoparticles, the selected standard microwave hydrothermal method was varied with respect to the following parameters: temperature, precursor concentration, amount of hydrogen peroxide added to the synthesis mixture, the ramping and holding times of the synthesis and the stirring speed

Summary

Introduction

Water splitting is a promising renewable energy approach due to the superior storage options of fuels compared to electricity generated by solar cells. After decades of research into water splitting, the optimal catalytic systems are yet to be found, in terms of efficiency and considering stability and applicability (Chu et al, 2017). To this end, the water oxidation half reaction with its four electron transfer steps remains the bottleneck of the process. In order to derive clear design guidelines for heterogeneous WOCs, the investigation of their reaction mechanisms is indispensable This remains a challenging task and requires sophisticated in situ analyses. In a recent representative study on Co3O4 WOCs by Zhang et al (2014a), the HO–Co2(μ-O/OH)2-OH edge site cobalt motif, i.e., two adjacent Co ions coupled via an oxygen bridge, was shown to be essential for efficient catalysis

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