The effects of anode additives towards suppressing dendrite growth and hydrogen evolution reaction

Abstract

Chapter 1 focuses on the developing a cost effective Zn-air battery and the effects of different anode additives towards suppressing dendrite growth and hydrogen evolution reaction in Zn-air secondary batteries. We revealed interesting findings during an extensive study on Zn-air batteries using 3 wt. % BiO as anode additive. In our continual effort to develop a cost effective rechargeable Zn-air batteries, herewith we demonstrated Zn anodes comprising of (i) 30 wt. % Zn : 3 wt. % bismuth oxide : 10 wt. % potassium sulfide (ZBK) (ii) 30 wt. % Zn : 3 wt. % bismuth oxide : 5 wt. % lead (II) oxide (ZBP) and (iii) 30 wt. % Zn : 3 wt. % bismuth oxide : 10 wt. % potassium sulfide : 5 wt. % lead (II) oxide additives (ZBKP) in 6 M KOH aqueous solutions and 1.88 wt. % polyacrylic acid as the gelling agent. KOH gel constituted the remaining mass of the anode wt. %. Results were confirmed via cycle voltammetry (CV), Tafel, electrochemical impedance spectroscopy (EIS) etc. measurements. Among the various Zn anodes analyzed, ZBKP showed a superior cathodic peak of -1.805 and 1.950 V vs. Hg/HgCl at 5th and 40th cycles during electrochemical cycle voltammetry. Tafel fitting on linear polarization test shows that ZBK exhibits the highest corrosion behavior follow by ZBP while ZBKP has the lowest corrosion behavior with an estimated corrosion inhibition efficiency of 36.06 %. Furthermore, ZBKP display the lowest dendrite growth, least corrosion rate and superior capacity even at 60th cycles compared to ZBK and ZBP. In view of our finding, ZBKP has a higher positive electrode potential compare to ZBK and ZBP electrodes. Thus, ZBKP is the most suitable for aqueous battery due to its low minimal side effect. Field emission-scanning electron microscopy/energy dispersive X-ray spectroscopy (FE-SEM/EDS) images confirm that the various elemental additives were evenly deposited on the Zn anode surface. Ex situ spectroscopy and electrochemical performance studies also verified that the dendrite-free nature of improved Zn anode and the modified interfaces between electrolyte and Zn plays vital roles towards advancing the energy storage performance. In chapter 2, we report the performance and degradation behavior of carbonyl Fe – MoS2 composite as anode material in Fe-air batteries using half-cell. We identified hydrogen evolution reaction (HER) and passivation as the two technical limitations of Fe-air batteries. HER account for low charging efficiency while passivation in Fe-air batteries account for its inability to fully discharge at a high rate due to the formation of iron hydroxide. As a result of this, many studies has been dedicated to inhibit these problems. Just like in recent literatures, the enhancement of Fe-air anode for commercialization is not trivial. With high purity carbonyl Fe-MoS2 composites electrode and the influence of 2 mM Na2S additives in 6M KOH electrolyte solution and 1.88 wt. % polyacrylic acid as the gelling agent, we have demonstrated a high performance carbonyl Fe anodes comprising of 3 wt. % MoS2 (F3M), 5 wt. % MoS2 (F5M), and 10 wt. % MoS2 (F10M) additives in Fe anodes. The result of the various electrodes characterized via field emission scanning electron microscopy (FE-SEM) and X-ray diffraction (XRD) reveals a distinct surface morphology that correspond to fundamental crystallographic growth patterns. Additionally, Energy dispersive spectroscopy (EDS) and SEM mapping affirms that both the Fe and additive (weight (wt. %) and atomic percent (at. %)) were well dispersed in the electrode. According to Tafel test, F3M, F5M, and F10M exhibit corrosion inhibition efficiency of 51.2 %, 21.1 % and 5.6 % respectively. Thus, the corrosion rate decreased in the order of bare Fe > F10M > F5M > F3M. While drastic capacity retention drops occur in the bare Fe electrode around 300th cycle, we were able to regenerate the battery back to full capacity via our choice of additives (F3M). During regeneration at 800th cycles, the capacity retention of F3M, F5M, and F10M electrodes were 97%, 93%, and 76%, respectively. Therefore, in conjunction with the unique structure and synergistic effect that characterizes two excellent anode materials, F3M is best choice for a high-performance Fe-air battery due to its excellent performance, low side effects (corrosion and passivation), low float current, superior capacity retention and efficiency during cycling compare to F5M and F10M.