Role of Pore Architecture on the Confinement of Electrogenerated Iodine in Iodide-Based Energy Storage Systems

Abstract

Iodine-iodide redox reactions are pivotal in diverse energy storage and production technologies. The electrooxidation of iodide ions (I − ) involves the formation of a solid I 2 film on the electrode surface that impedes the further electrooxidation process and therefore has a significant implication on the performance of iodine-iodide redox chemistry-based systems. In this work, we have illustrated the role of pore structure in the accumulation of the I 2 film and its consequent effect on charge storage capabilities. Freestanding electrospun carbon nanofibers (CNFs) with varying pore architectures were synthesized through strategic selection of sacrificial material. The mechanistic understanding of the I 2 -film formation within these porous CNF electrodes is elucidated compared to that of a planar electrode. The cyclic stability tests demonstrated a loss in capacitance, attributed to gradual I 2 -film accumulation on the CNFs over multiple redox cycles. A mathematical model has been proposed to simulate this capacitance loss and understand its dependence on the pore structure in the CNFs. It was deduced that while macropores contribute to an exponential reduction in capacitance, meso-/micropores result in a near-linear reduction in capacitance with cycling. This is attributed to the lower diffusional resistance to the transport of electroactive I − ions offered by the macropores and, thereby, a higher propensity toward I 2 -film formation and accumulation. Furthermore, self-discharge studies revealed that surfaces with a higher fraction of meso-/micropores demonstrated relatively slower self-discharge compared to macropores.