Elucidating electro-oxidation kinetics of Fe(II) in the anode of air–cathode fuel cells from an Fe(II) speciation perspective

Abstract

The electro-oxidation of ferrous ion (Fe(II)) can be found in many air–cathode fuel cell systems, such as acid mine drainage fuel cell (AMD-FC) and sediment microbial fuel cell (SMFC). To deeply understand these iron-related systems it is essential to elucidate the kinetics and mechanisms involved in the electro-oxidation of Fe(II). Therefore, in this work we examine the Fe(II) oxidation process in an electrochemical system analogue to the anode of air–cathode fuel cells. A kinetic model is developed that accurately describes the Fe(II) oxidation rate in carbonate solution as a function of pH and applied potential. The speciation of Fe(II) is integrated into the kinetic model and contribution of individual Fe(II) species to the overall Fe(II) oxidation rate is quantitatively evaluated. The results show that the electro-oxidation of Fe(II) follows a pseudo first-order behavior under the experimental conditions. The pseudo first-order rate constant generally increases with the increase of applied potential, and presents a first-order linear dependence upon solution pH. The Fe(II)-carbonate complexes play a significant role in the electro-oxidation of Fe(II). In acidic solution, the Fe(II) oxidation kinetics is primarily controlled by the oxidation of FeCO30 and FeOH+ species. As the pH increases, the oxidation of Fe(CO3)22- and Fe(OH)20 species becomes more important to determine the overall Fe(II) oxidation rate. Above pH 8.0, the Fe(CO3)22- is the dominant species responsible for Fe(II) oxidation rate. It is anticipated the kinetic model developed here will provide valuable information for a better understanding and manipulation of the iron-related fuel cell systems for various applications.