Study of Mn(III) Disproportionation Reaction Using Vanadium and Titanium Additives: Application to Redox Flow Batteries

摘要

Manganese, an earth-abundant transition metal, exists as Mn(III)/Mn(II) redox couple at a standard potential of +1.51V vs. SHE. It has drawn interest for implementation as positive side in V/Mn redox flow battery (RFB) due to higher achievable energy density than all-vanadium RFB. Unfortunately, Mn(III) disproportionation into Mn(II) and MnO_2(s) led to loss of capacity, pressure drop increase and passivation of electrode surface during RFB cycling. We studied the influence of Ti(IV) or V(V) additives on Mn(III) stability in high acidic medium, by formulating 4 different electrolytes at equimolar ratio (Mn, Mn:Ti, Mn:V:Ti, Mn:V). Electrochemical characterizations reveals Mn(III) involvement in a complex nucleation/growth process at the electrode surface, along with its disproportionation. SEM combined to XPS demonstrates different oxide layers morphologies and chemical nature: MnO_2 was formed in the presence of Mn and Mn:Ti, and MnO in the presence of V(V). Spectroelectrochemical study of Mn electrolytes highlights the displacement of the disproportionation reaction equilibrium in the presence of Ti(IV) or V(V) additives. Below 10% of electrolyzed Mn(III) was involved in the disproportionation reaction in the presence of Ti(IV) and V(V) (e.g., 25% without additives). V(V) was observed to enhance the stability of Mn(III) as compared to Ti(IV), which is of high interest for redox flow battery applications. In fact, Mn:V electrolyte could be used instead of Mn:Ti as the positive side of a Mn-V RFB will considerably reduce cross-contamination issues. As observed in the spectroelectrochemical study, the battery might be charged up to 90% SOC without loss of capacity. Moreover, in case of MnO particles formation, the strong acidic conditions will directly dissolve the particles preventing the battery from electrode passivation or/and pressure drop concerns.