Carbon fibre paper coated by a layered manganese oxide: a nano-structured electrocatalyst for water-oxidation with high activity over a very wide pH range

Abstract

Manganese oxide coated carbon fibre paper electrodes (MnO_x/CFP), prepared via an easily scalable redox-deposition route, have been reported as promising materials for electrochemical water oxidation catalysis (WOC). In the study presented here, the catalyst mass-dependence of the activity of such nanostructured WOC electrodes and their activity and stability in phosphate buffer electrolytes over nearly the entire pH range (pH 1–14) was explored. For all pH values, current densities of >1 mA cm⁻² at overpotentials η of 350–500 mV were observed, which are very good values for electrodes containing only earth-abundant elements (Mn, O and C in this case). The very decent performance observed for the mildly acidic pH regime (pH 2–6) was especially noteworthy. For the dependence of η on the pH, an ideal Nernstian behaviour was detected for the pH window pH 1–10. Above pH 10, a pronounced deviation from the expected trend was found, as most electro-kinetic parameters indicated even higher activities for the strongly alkaline regime. Concerning anode stabilities, current-densities of 2 mA cm⁻² could be maintained at all studied pHs over at least 24 h of continuous operation. Pre- and post-operando spectroscopic analyses (e.g. vibrational and X-ray absorption spectroscopy, scanning electron microscopy) revealed only minor changes of the catalyst structure, composition, morphology or the average Mn oxidation state induced by the electrocatalytic operation, which confirms the good stabilities found in the electrochemical measurements. Thus, we report on an earth-abundant, easily-prepared type of WOC electrode, which exhibits promising activities and stabilities for applications in alkaline, neutral and even acidic electrolytes.