Studies on Kinetics and Mechanism for Rechargeable-Iron Electrodes Using in Situ x-Ray Diffraction Technique in Conjunction with Electrochemistry

Iron is the fourth most abundant element on the earth’s crust. It is cost effective, has large theoretical specific capacity and is non-toxic. Iron-based accumulators are both mechanically and electrically rugged. Besides, iron electrodes are also environmentally benign unlike other battery electrode materials, such as cadmium and lead.¹ Open-circuit potential of a charged alkaline-iron electrode is always negative to hydrogen evolution reaction. Consequently, iron is thermodynamically unstable and suffers corrosion through concomitant evolution of hydrogen. Hydrogen evolution also occurs while charging alkaline iron electrodes bringing about a decrease in their charge acceptance. Several sulfide additives, like FeS, PbS and Bi₂S₃, have been used to increase hydrogen over-potential to mitigate this problem. Recently, we reported a carbon-grafted alkaline iron electrode for iron-based accumulators with specific capacity values in excess of 400 mAh g^-1 with faradaic efficiency of about 80%.² In this study, we report in situ x-ray diffraction investigations on charge and discharge products of carbon-grafted iron electrodes with and without Bi₂S₃ additive to throw light on a feasible charge and discharge mechanism in these electrodes. When iron electrodes with Bi₂S₃ additives are subjected to formation cycle Fe₃O₄ is reduced to metallic α-Fe. Interestingly, carbon-grafted iron electrodes with Bi₂S₃ exhibit complete conversion of Fe₃O₄ to α-Fe at the end of formation cycles and, consequently, it is surmised that carbon grafting into the iron active material promotes utilization of active material.

 References:

1. S.R. Narayanan, G.K. Surya Prakash, A. Manohar, Bo. Yang, S. Malkhandi and Andrew Kindler, Solid State Ionics, 216, 105 (2012).
2. A. Sundar Rajan, S. Sampath and A.K. Shukla, Energy Environ. Sci., 7, 1110 (2014).