Time-Dependent Impedance Spectroscopy to Unravel Non-cyclability and Current Fading in Hydroelectric Cells

Abstract

In addressing the energy crisis, a strategic emphasis on green energy generation is essential, with water dissociation-based cells named hydroelectric cells (HECs) playing a pivotal role in sustainable production. The multifunctionality of HECs, such as the capability of generating both green and clean energy alongside hydrogen, underscores their significance. In this work, the composite material of exfoliated graphite and lithium−nickel ferrite is utilized in these cells, and a maximum short circuit current density of 14.65 mA/cm 2 with an off-load power density of 2.17 mW/cm 2 is achieved, which is suitable for low power applications. However, in the longer run, we observed the current fading behavior. To unravel the potential electrochemical processes involved in this current fading over the operational lifespan of HECs, we utilized time-dependent electrochemical impedance spectroscopy. By observing electrochemical processes at different aging times over which the current is decaying, it was found that water dissociation resistance and charge transfer resistance increase with time. The former is attributed to the reduction in oxygen vacant sites, and the latter is due to the electrode fouling caused by the formation of the Zn(OH) 2 layer at the Zn electrode. In the face of escalating R ct , the persistent stability of Warburg impedance throughout the cell's lifespan indicates a consistent diffusion process and the availability of unutilized ions in the system. Exploring and mitigating these intricacies promises to utilize available ions, thereby enhancing both these cells' overall efficiency and longevity. Thus, we recommend measuring time-dependent impedance spectra over HEC's lifespan to understand better the different degradation mechanisms responsible for rapid current fading.