Electrochemical Studies On Porosity and Additives In Hot-Pressed Iron-Based Anodes

Abstract

The reliability of battery performance is crucial for the implementation of long-term large-scale energy storage. The promising Iron-Air battery is known for its suitable properties relying on its advantageous iron-based anode, characterized by its high volumetric energy density, robustness, material abundance, low cost and scalability. The anode does however present drawbacks, such as self-discharge, hydrogen evolution reaction (HER) and initial- and gradual surface passivation. In previous studies the effect of porosity and specific additives have been presented as important factors in tackling these drawbacks. This multidisciplinary research aimed to study porosity and additive effects on the electrochemical behaviour and performance of iron-based mechanically-stable anodes, manufactured at elevated temperature and pressure. Specifically, the effect of 10wt% pore former K2CO3 on the initial capacity and on the passivation behaviour was studied by galvanostatic charge-discharge, with prior investigation on pore formation. In addition, the effect of high-performance additives (Bi2S3, Bi2O3, ZnS and FeS) was investigated on HER kinetics and the passivation behaviour, by cyclic voltammetry (CV) and galvanostatic cycling, respectively. Surface characterization was performed by optical microscopy, scanning
electron microscopy (SEM), and profilometry. Post-test analysis was carried out by optical microscopy and X-ray Photoelectron Spectroscopy (XPS). An intensification of surface cracks was observed for the K2CO3-rich sample before testing and its initial discharge capacity is 27 mAh/g less than a sample without pore former, but passivated later than the pristine sample in the full cell. Amongst the additives, the highest cycle of 100% capacity retention was marked at cycle 41 for FeS and for Bi2S3 at cycle 29 in the full cell, both showing 100% retention in the half cell up to 36 cycles. FeS might benefit from its readily available soluble reservoir of S2− ions. Bi2O3 showed the lowest capacity retention which might be explained by low conductivity, low solubility and/or lack of beneficial role of S2− ions. However, the additive Bi2O3 showed great reversibility of discharge products in the CV, confirmed by the lower O1s peaks and the lower respective Fe2O3 and FeOOH peak in XPS spectra. The pristine sample showed in the CV over the cycles increased current density and slope near the HER potential, with low reversibility. Apart from Bi2O3 and ZnS, the pristine sample showed lower capacity retention than FeS and Bi2S3, confirming the effective working of these additives. This systematic study portrays a good starting point for further studying porosity and additive effects on the electrochemical behaviour in hot-pressed anodes for the upcoming Iron-Air battery. Improvements can be assigned to cell design, electrolyte control and further detailed porosity characterization. Another type of current collector
might withstand higher current densities and anode thickness reduction can lead to higher discharge capacities over its lifespan.