Operando X-ray Absorption Spectroscopy of Co3Mn-CO3 LDH: Formation and Structural Evolution under Electrochemical Conditions

The purpose of this article is to elucidate the mechanism of formation of CoMn layered double hydroxide (LDH) and to study the role of both metal cations, cobalt and manganese, in the electrochemical behavior of this lamellar compound. Multivariate curve resolution with alternating least squares (MCR-ALS) analysis was applied to time-resolved synchrotron X-ray absorption spectroscopy (QXAS) data, monitoring the speciation of Co and Mn valence states under both Co3Mn LDH formation and electrochemical processes. QXAS experiments at Co and Mn K-edges were recorded all over three steps of the synthesis process: (i) fast coprecipitation and two successive aging steps (ii) under Ar and (iii) in air. In the first step, corresponding to the fast coprecipitation, a brucite-like Co/Mn(OH)2 mixed phase precipitates rapidly and no oxidation of metal cations occurs during the two first steps ((i) and (ii)). Topotactic oxidation of Mn(II) into Mn(III) by oxygen occurs during the final drying step under an air atmosphere (iii), assisted by the intercalation of carbonate, leading then to the formation of Co(II)3Mn(III)-CO3 LDH. Finally, the redox contribution of both metal cations Co(II) and Mn(III) in the electrochemical process is highlighted, and a mechanism of electron hopping between Co sites is proposed.