Investigating the Impedance of an Iron/Iron Redox Flow Battery at Different State of Charge Conditions – a Distribution of Relaxation Times Analysis

Abstract

New energy storage technologies are needed to offset the erratic power supply of renewable energy sources due to the rising global energy demand and the urgency of climate change action. The Iron/Iron redox flow battery is a viable contender for large-scale energy storage, even if there are still issues with operational parameters and efficiency. Due to the positive electrode's moderate pH and low voltage, which preserves minimum toxicity and aggressivity, iron is a very affordable active material, making it feasible to develop inexpensive energy storage systems and batteries. The Electrochemical impedance spectroscopy technique was utilized to understand the degradation of the iron/iron redox flow battery during charging and discharging operation modes. However, discriminating between processes occurring at similar time scales is difficult since they overlap in the generally used Nyquist or Bode representation. Distribution of relaxation times (DRT) analysis tackles this issue by converting the EIS data with respect to the time constants of the individual processes. Individual processes such as electrochemical reactions inside the iron/iron redox flow cell, transport processes through porous structures, and iron ion transport were investigated.The impedance measurements were carried out to evaluate the ohmic resistance during the charging and discharging phases of the iron/iron redox flow battery with varied states of charge ranging from 0-100% while charging and 100-0% while discharging. In this series, a DC offset load of 100 mA in the charging and discharging directions was used to test the resistance differential associated with the battery's state of charge. In the charging phase of the battery of three DC offset load parameters, the iron ions were transported faster in the process while the state of charge of a battery increased at lower frequencies. The impedance increased when the state of charge decreased at a higher frequency range, involving the iron ions transported through the negative half-cell in an iron/iron redox flow battery. The overall ohmic resistance in the charging phase of the battery decreased up to 6 % until the battery was fully charged, but in discharging phase, the overall ohmic resistance increased to 2 %. This indicated that the DC offset load in the charging and discharging direction were dependent on the battery's state of charge. Figure 1