Periodic Reporting for period 1 - POMZAB (High energy density and long cycle life near-neutral Zn-air rechargeable batteries using polyoxometalates nanoclusters as homogenous catalysts)
Reporting period: 2022-08-01 to 2024-07-31
Even though Li-ion batteries have dominated the market so far, they lack the desired properties for large scale electric grids or electric vehicles. Besides, there are some concerns regarding the safety and raw-materials availability in the future. This is top-up by the geopolitical distribution of these materials, highly concentrated in regions where mining practices are questionable. In this context, Zn-based energy storage devices, including batteries (Zn-air and Zn-ion) and supercapacitors, emerge as the rather promising candidates to be the post-lithium technologies for their lower cost, higher storage capacity per mass and volume, and inherent safety. However, their cycle life is still below the required standards. Therefore, the goal of this proposal is to develop efficient electrically rechargeable Zn-based devices with high energy density and cycling stability. To achieve this, near neutral electrolytes were used instead of traditional highly alkaline solution employed in non-rechargeable devices (Zn-air or Zn-Mn). The latter leads to significant improvements in Zn anode cyclability increasing battery life. However, to fully exploit the potential of this technology, materials specifically designed to operate in this pH conditions are necessary and still missing and just changing the electrolyte pH is usually not enough. This project sought to address these matters working simultaneously on the formulation of novel electrolytes and the design of cathode materials with better performance than those currently available. The motivation of this project is to take Zn-rechargeable technologies to the next level, contributing to their pathway through commercialization. This in turn, will provide a sustainable alternative to Li-ion batteries, helping Europe in their transition towards a green and circular energy economy.
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.
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.
The results obtained during this project provide the foundations for future research, from which it will be possible to achieve widespread usage of Zn-based decives in the future. POMs were proved to be efficient and versatile materials for Zn-air cathode development as well as other technologies, such as water splitting or fuel cells, whose working principle is linked to either oxygen evolution or reduction reactions. Likewise, the electrolytes prepared and tested can be used as starting points for optimization of supercapacitors having metal anodes. The knowledge can also be extended to other Zn energy storage chemistries, in order to maximize the life and the energy output of these devices. Finally, our preliminary work on Prussian blue cathodes for Zn-ion suggest that a reduced crystal size together with integration of the active and conductive materials in a hybrid one can bring a step forward the performance of this sustainable, and thus attractive, material. We have also been able to identify possible degradation mechanism, which should be verify in the future used for proper material design and optimization.