(Invited) Sustainable and Energy-Efficient (nano)Structured Oxidation Electrocatalysts in Acidic Media from Earth Abundant Metals

Abstract

The establishment of a sustainable renewable fuels technology will depend on the electrocatalytic reduction of substrates into fuels, but also on the electrocatalytic oxidation of water or other feedstock to extract the needed electrons and protons. The requirements for the oxidation electrocatalysts are typically more demanding regarding long term stability, since these catalysts must be fast, selective and efficient, but also robust to high oxidation potentials, water, oxygen, electrolytes, acids or bases, etc. Several earth abundant metal oxides are excellent electrocatalysts in neutral and basic conditions, with multiple examples from different labs.1 However, most active transition metal oxides are unstable in acidic media, where they just dissolve even in open circuit conditions. Only noble metal oxides, such as IrO2, are able to exhibit competitive electrooxidation activity in aqueous acidic conditions (pH < 3), where reduction electrochemistry is easier and faster, and proton exchange membrane (PEM) technology may facilitate high current density performance in compact electrolyzer architectures.In this communication we will report our latest results in the search for efficient, low cost oxidation electrocatalysts from earth abundant, non-critical raw materials able to work in aqueous acidic media. We showed how Co-containing polyoxometalates (POMs), are active electrocatalysts for the oxygen evolution reaction (OER) in acidic conditions when stabilized by a partially hydrophobic support.2 Although with some limitations, a hybrid composite of an insoluble salt of the polyoxoanion [Co9(H2O)6(OH)3(PW9O34)3]16– blended with a carbon paste electrode exhibited superior performance and higher current densities than the corresponding IrO2-based electrodes in the low overpotential range. We are extending this same strategy to bulk and nanostructured transition metal oxides to provide fast and robust OER at pH < 1. Furthermore we will show how some of these metal oxides can also catalyze alternative electrochemical oxidation processes of high interest for the chemical industry, such as the selective oxidation of alcohols to aldehydes or carboxylic acids.[1] J. R. Galan-Mascaros, ChemElectroChem, 2015, 2, 37. [2] M. Blasco-Ahicart, J. Soriano-Lopez, J. J. Carbó, J. M. Poblet, J. R. Galan-Mascaros, Nat. Chem. 2018, 10, 24.