Positronium chemistry of a Fe^2+/3+ solution under electrochemical control

The positronium chemistry of a Fe^2+/3+ solution is studied under full electrochemical control. For this novel approach to positronium
electrochemistry, a suitable cell setup is used, which allows simultaneously both electrochemical measurements and positron annihilation
spectroscopy. For the Fe^2+/3+ redox couple, positronium serves as an ideally suited atomic probe owing to the rather different positronium
chemistry of Fe²⁺ (spin conversion) and Fe³⁺ (total positronium inhibition and oxidation). This enabled the precise in situ monitoring of
oxidation and reduction by means of positron lifetime upon slow cycling voltammetry or galvanostatic charging. The variation of the mean
positron lifetime with the Fe^2+/3+ concentration ratio could be quantitatively described by a reaction rate model for positronium formation
and annihilation. An asymmetric behavior of the variation of the mean positron lifetime with applied potential, as compared to the simultaneously
recorded symmetric current–potential curve, could be explained by the stronger influence of Fe³⁺ on the characteristics of positronium
formation and annihilation. The highly reversible galvanostatic charging behavior monitored by positron lifetime underlines the attractive
application potentials of positronium electrochemistry for in situ studies of iron-based redox-flow battery electrolytes.