First row transition metal compounds for the oxygen evolution reaction

Abstract

The aim of this work was to find alternative catalyst systems for the oxygen evolution reaction (OER) in alkaline solutions. The state-of-the-art catalyst systems consist mainly of expensive and rare noble metal catalysts, like platinum, iridium, or ruthenium compounds. First-row transition metal compounds were identified to be promising alternatives for noble metal catalyst systems. For this purpose, in the first part of this work, manganese-based oxides (MnOx) were investigated. Firstly, the manganese-based oxides were synthesized as thin films by a plasma-enhanced chemical vapor deposition process onto titanium substrates. By varying the deposition process parameters, different MnOx phases were produced and subsequently characterized. Afterwards, the determined process parameters were used to successfully deposit the MnOx catalysts onto several other substrate materials. It was shown that the catalytic activity of the manganese oxides for the OER is influenced by the oxidation state of the manganese and, in addition, by the contact between the catalyst and the used substrate material. In particular, a strong reduction of the catalytic activity was found for the deposition onto titanium substrates, which was attributed to the formation of a native oxide layer on the titanium surface, inhibiting a low resistive charge carrier transport. In the second part of this work, powdered nano-structured cobalt boride compounds were analyzed as a starting material to investigate their applicability for the alkaline OER. In a second step, a ternary compound was formed by the incorporation of iron (Co1-xFex)2B and nickel (Co1-xNix)2B (with 0 ≤ x ≤ 0.5) into the dicobalt boride system. The transition metal borides were synthesized by a low-temperature solution synthesis with a subsequent calcination step. For the electrochemical investigation, the binary and ternary compounds were embedded into NafionTM containing ink and drop coated on polished glassy carbon substrates. It was shown that the transition metal borides oxidize under OER reaction conditions forming metal oxide/hydroxide or rather an oxyhydroxide species. The activity of the di-cobalt borides (Co2B) was improved by an incorporation of up to 50wt% of iron or up to 20wt% of nickel. Especially the incorporation of up to 30wt% of iron showed a positive impact on the onset potential of the OER. Finally, the stability of the best performing catalysts was confirmed over an extended time period.