Molecular metal oxide-base materials for electrooxidation of Glycerol (GOR)

Abstract

The growing need for sustainable systems has intensified interest in green hydrogen as a fuel. Although water electrolysis is a promising route, its efficiency is limited due to the oxygen evolution reaction at the anode. To address this issue, the development of an efficient and earth-abundant electrocatalyst is essential. In parallel, replacing the OER with the oxidation of small organic molecules, in this case glycerol, an abundant byproduct, offers an attractive strategy to reduce the energy consumption needed while generating high value chemicals. This study focuses on the synthesis, characterization and electrocatalytic evaluation of three heterometal-substitute polyoxometalates (POMs): two Lindqvist dimers and one Keggin-type POM incorporating cobalt or iron. The aim of this project is to understand how both the POM structural framework and the identity of the incorporated metal centre influence catalytic activity in the oxygen evolution reaction (OER) and glycerol oxidation reaction (GOR). These studies represent new contribution for both POM families, as the electrocatalytic behaviour of Lindqvist structures in OER and GOR has not been reported, and the Keggin structure has not yet been explored for GOR. Only a limited number of studies exist on POMs as active catalysts for glycerol oxidation coupled with the hydrogen evolution reaction (HER). The parent tungstate Lindqvist cluster, (TBA)2[W6O19], was synthesised through both acidic and basic routes and subsequently transformed into cobalt- and iron- substituted dimers. In parallel, lacunary Keggin precursors were also synthesized, characterized and used to obtain the cobalt-substituted Keggin structure. All compounds were characterized by infrared spectrometry, cyclic voltammetry, electrospray ionization mass spectrometry and NMR spectrometry to confirm the successful synthesis and metal incorporation. The electrochemical studies reveal a clear structure-activity relationship: the Co-Keggin complex shows the highest catalytic activity for both OER and GOR, followed by the Co-Lindqvist dimer, while the Fe-Lindqvist analogue show the lowest performance. Product analysis of the GOR by 1H NMR confirms the oxidation of glycerol through the formation of dihydroxyacetone and formic acid. Despite the confirmed electrocatalytic activity, all three POMs exhibit relatively low current densities, showing they are not strong candidates for practical OER or GOR applications when working on their own, where higher activity and robustness are required. Overall, this study provides a systematic comparison of structure, stability, and catalytic behaviour in Co- and Fe-substituted POMs, highlighting the superior performance of the Keggin framework and the key role of cobalt in oxidative electrocatalysis, while also underscoring the limitations that prevent these systems from being competitive electrocatalysts.