Studies on the reduction of birnessite thin layers: Influence of medium

This paper describes studies on the reduction of birnessite thin layers electrodeposited onto a cheap transparent semiconductor substrate, tin dioxide (SnO2), in neutral sulphate solutions. A coupled approach based on electrochemical measurements (cyclic voltammetry and chronoamperometry) and X-ray diffraction (XRD) characterisation allowed us to give information about reduction mechanism of birnessite in presence or absence of Mn(II) in solution. In absence of Mn(II), birnessite is reduced only at low potential (E = −0.6 V) into β-MnOOH (feitkneichtite), if Qreduction is lower than Qsynthesis, and in amorphous Mn(II) compound, if Qreduction is equal to Qsynthesis. In presence of Mn(II) in solution, hausmannite (Mn3O4) was detected, and even at high potentials (E = 0.15 V). These results signify that a complex between Mn(III), coming from the reduction of Mn(IV) species, and Mn(II) present in solution, can be formed near the surface and leads to the formation of Mn3O4, a very resistive compound. These results are important for studies devoted to environmental applications, chemical sensors and also for energy storage.

► Determination of compounds during reduction of birnessite layer.
► Importance of Mn(II) ions in solution on the nature of resulting compounds.
► Proposition of reduction mechanisms of birnessite thin films.
► Interests of XRD characterisation coupled to electrochemistry.