In our attempt to improve Zn-air batteries performance we decided to use a pH 5 electrolyte which allows for better Zn anode stability and cyclability. However, catalyst that can react in this pH conditions are also needed. In this project we studied, metal oxide nanoclusters, known as polyoxometalates (POM) first time as bifunctional catalyst to promote both oxygen evolution and reduction reactions. These materials have outstanding electrochemical properties including an excellent reversibility, which makes them ideal for rechargeable energy storage devices. The electrocatalytic activity of various POMs in near neutral electrolytes was investigated using several electrochemical techniques. Tuning the chemical composition and their structure, Co-containing POMs with good activity were prepared. Initial lab-scale Zn-air batteries were assembled and the performance of POMs as bifunctional catalyst in real operating conditions was assessed. Results are encouraging and, while there is plenty of room for improvements, suggest that combination of tailored-made POMs and near-neutral electrolytes could really boost the performance of Zn-air batteries.
Electrolytes for Zn-ion supercapacitors were formulated following two different approaches, but with the same goal: reduce water-Zn interaction to increase coulombic efficiency and prevent self-discharge. The first strategy considered in the addition of organic additives, in relatively low amounts, to a simple aqueous electrolyte containing zinc sulfate. Two compounds were chosen with different interaction mechanism leading to an improvement in Zn stability, namely, a complexing agent and a leveler. Both additives improve the cycling stability of Zn anodes, although through a synergy between them, the best outcomes were observed with both present in the solution. The results of this work indicate that combining additives that affect the electrochemical process at distinct stages is a simple and efficient way to boost Zn devices performance. At the same time the study demonstrates the valuable information already published by the Zn-plating community which could serve the battery field to develop novel electrolytes. A second approach consisted in the use of highly concentrated electrolytes, known as water-in-salt, combined with acetonitrile, a non-toxic organic solvent. This hybrid electrolyte not only decreases water reactivity protecting the anode, it also delivers a wider potential stability range, which translates to higher energy densities for supercapacitors. As a results, the electrolyte proposed brought improvements in capacity and cycle life of Zn-ion supercapacitors.
Zn-ion batteries rely on intercalation chemistries on the cathode, in a similar way as Li-ion does. Several materials ´++have been proposed that allow for reversible Zn2+ intercalation within their structure. Among this, Prussian blue and its analogues can provide higher operating voltages and acceptable energy densities, being composed of highly abundant and cheap elements. However, this present low conductivity and poor cyclability. To overcome. These issues, Prussian blue (PB) crystals were grown directly on carbon materials to obtain composite materials with high electronic conductivity, which retains the electrochemical properties of the active phase. The potential of PB grown in-situ on Ketjen Black (KB) conductive carbon (PB@KB) as cathode materials for Zn2+ intercalation was evaluated. Two synthesis methods were used to obtain PB@KB: a standard thermal synthesis using mechanic agitation and an ultrasound-assisted methodology. The effect of reaction parameters (temperature, KB initial content and time) was evaluated for the second method. The main objective of this work was to correlate the materials properties, i.e. PB crystallinity and size, KB content, among others, with the electrochemical response towards Zn2+ insertion to optimize Zn-ion devices. The performance as cathode for Zn-ion batteries was assessed by estimation of the capacity, energy density and both coulombic and energy efficiency. The results show that the ultrasonic irradiation can provide a similar material, in terms of PB content and reaction yield, after just 6 hours (compared to the 20 hs needed for mechanical agitation). In addition to this, the average PB size is significantly decreased. Results also indicate that temperature is crucial to achieve good reaction yield and that KB provides nucleation sites based on its impact on the PB size. The synthesized materials can intercalate Zn2+ efficiently, although a fast capacity fading was observed. Further research revealed that this is related to a process in which Na+ (from the synthesis step) is replaced by Zn2+ irreversibly. The use of electrolytes containing sodium could solve this problem delivering good specific capacities and excellent cyclability.
