As the world decarbonizes the energy systems, energy storage is an important pillar that cannot be ignored. Renewable energy sources are intermittent and non-flexible. Therefore, energy needs to be stored for shorter and longer periods. Since most renewable energy sources provide electricity, chemical battery storage is the most obvious first choice for storage.

And indeed, for short- and medium-term storage, chemical battery storage is economically feasible. However, the current reigning technology, the lithium-ion battery, is far too expensive for world-wide deployment for grid-scale storage. This comes down to material cost, which is connected to resource abundancy. Lithium, nickel, cobalt and other metals are too expensive. One should also not forget that their concentration in only a few places on Earth, makes for a weak supply chain.

New battery chemistries need to be found, that are solely based on abundant and/or renewable materials. Sodium, magnesium, iron, zinc and aluminum are metals that could replace lithium in batteries, while expensive transition metal oxides might be replaced by easily synthesized organic molecules. This thesis explores a few examples of materials that could be used in magnesium batteries.

In chapter 2 perylene diimides are introduced. These versatile and robust molecules have shown promise as battery electrode material. This chapter describes a new and mild synthesis route towards them, the mild imidization. Traditionally perylene diimide synthesis requires harsh conditions, high temperature and corrosive solvents. In this new approach, one only needs the biocompatible solvent DMSO and potassium carbonate at 100 oC. Also, a room temperature variant of the procedure has been developed that uses DBU as a base. The resulting products are also easily obtained through a simple precipitation in water, without the need for purification. The reaction kinetics and substrate scope are also investigated.

Perylene diamic acids, a sister compound to perylene diimides, are described in chapter 3. These are the intermediates in the imide synthesis. When the reaction is done with a secondary amine, the imide cannot be formed, and one is left with a water-soluble salt. The high solubility in water makes this class of molecules stand out in the perylene family. When diamic acids are exposed to acid, the reaction is undone and perylene dianhydride is formed. This compound is very insoluble in water. Chapter 3 shows that very controlled acidification can yield highly colored hydrogels. The gels are characterized by different techniques. The hydrogels might be of interest to material scientists, since they undergo uniform shrinkage over time. In their soluble form, perylene diamic acids are of interest as water-soluble compound in aqueous batteries.

The perylene family is known to be ‘redox tunable’. By changing the side groups of the perylene, one can change the redox potential at which it reacts, which determines the potential at which a battery operates. Chapter 4 looks into a new way to gain more control over the substitution of perylene tetraesters, by making a dibromo-dichloro perylene tetraester. In theory, this gives more control over the substitution pattern compared to tetrachloro and tetrabromo derivatives. This chapter investigates the kinetics of substitution reactions.

Chapter 5 presents a new aqueous polymer electrolyte for magnesium batteries, magnesium alginate. Alginate is a biopolymer that is harvested from algae. It is a polysaccharide of two monomers that both contain carboxylate groups. The difference between the monomers is only the spatial orientation of the carboxylate group. When an alginate solution is mixed with a solution that contains a multivalent cation, it will form a hydrogel. The carboxylate group from crosslinks with the cations, making a covalent-link hydrogel. Magnesium ions, two plus charged, are an exception, judged on the macroscale, they do not form a hydrogel. However, it is known that magnesium makes microscopic instable hydrogels that constantly form and break apart. From this, we can say that although magnesium is mobile in the solution, it is still heavily associated with the alginate backbone, and this is an indication that this electrolyte can act as a ‘water-in-salt’ electrolyte. Chapter 5 shows that magnesium alginate can have good ionic conductivity, even at very low water content. After some cycling, a black layer forms on the magnesium electrodes. This layer enhances the stability of magnesium in water.

Alginate hydrogels can also be used as matrices to encapsulate transition metal cations. Chapter 6 brings a new approach to make a ‘simple’ manganese-iron battery. By encapsulating manganese(II) on one electrode and iron(III) in alginate, we were able to cycle this simple ion pair multiple times. This material might have applications in semi-solid flow battery.

We present magnesium alginate as an aqueous polymer electrolyte for use in magnesium batteries. Alginates are polysaccharides extracted from algae, which form hydrogel materials upon interaction with divalent and trivalent cations. They are renewable, non-toxic, biocompatible materials that are widely used in the food and pharmaceutical industries. Mg2+ is weakly bound to an alginate polymer, which results in a hydrogel-like material that contains mobile magnesium ions. We propose that this is the ideal situation for an electrolyte that behaves in a similar way as a ‘water-in-salt’ system. Magnesium alginate was successfully synthesized and characterized by FTIR, XRD, and PDF. Ionic conductivity was measured with EIS measurements; a 2 wt% magnesium electrolyte shows a conductivity of 1.8 mS/cm. During conductivity experiments, we noticed the formation of a black layer on magnesium electrodes, which can improve the ionic conductivity between the electrodes. We carefully characterized this layer with XPS and saw that it mainly consists of alginate derivatives.

Perylene dianhydride (PDA) hydrogels are made from highly accessible perylene diamic acid salts (PDAA salts) by a versatile protonation–hydrolysis mechanism. Very weak gels are formed initially by π-stacking of PDAAs and subsequent hydrolysis yields much stronger PDA hydrogels. Hydrogels are readily made at concentrations down to 0.5 mM, exhibit storage moduli around 600 Pa @ 1 mM and undergo significant syneresis in time.

The upcoming energy transition requires not only renewable energy sources but also novel electricity storage systems such as batteries. Despite Li-ion batteries being the main storage systems, other batteries have been proposed to fulfil the requirements on safety, costs, and resource availability. Moving away from lithium, materials such as sodium, magnesium, zinc, and calcium are being considered. Water-based electrolytes are known for their improved safety, environmentally friendliness, and affordability. The key, however, is how to utilize the negative metal electrode, as using water-based electrolytes with these metals becomes an issue with respect to oxidation and/or dendrite formation. This work studied magnesium, where we aimed to determine if it can be electrochemically deposited in aqueous solutions with alginate-based additives to protect the magnesium. In order to do so, atomic force microscopy was used to research the morphological structure of magnesium deposition at the local scale by using a probe—the tip of a cantilever—as the active electrode, during charging and discharging. The second goal of using the AFM probe technology for magnesium deposition and stripping was an extension of our previous study in which we investigated, for lithium, whether it is possible to measure ion current and perform nonfaradaic impedance measurements at the local scale. The work presented here shows that this is possible in a relatively simple way because, with magnesium, no dendrite formation occurs, which hinders the stripping process.

We present a sustainable, inherently safe battery chemistry that is based on widely available and cheap materials, that is, iron and manganese hosted in alginate bio-material known from the food and medical industry. The resulting battery can be recycled to allow circularity. The electrodes were synthesised by the alginate caging the multi-valent metals to form a hydrogel in an aqueous environment. Characterisation includes FTIR, XPS and Mössbauer spectroscopy. The electrochemical performance of the electrodes was investigated by performing cyclic voltammetry (CV) and (dis)charge experiments. Mn and Fe ions show good co-ordination with the alginic acid with higher oxidation states demonstrating complex bonding behaviour. The non-optimised iron and manganese alginate electrodes already exhibit a cycling efficiency of 98% and 69%, respectively. This work shows that Fe and Mn atomically disperse in a bio-based host material and can act as electrodes in an aqueous battery chemistry. While demonstrated at cell level, it is furthermore explained how these materials can form the basis for a (semi-solid) flow cell.

A novel protocol for the synthesis of perylene diimides (PDIs), by reacting perylene dianhydride (PDA) with aliphatic amines is reported. Full conversions were obtained at temperatures between 20 and 60 °C, using DBU as the base in DMF or DMSO. A "green"synthesis of PDIs, that runs at higher temperatures, was developed using K2CO3 in DMSO. The reaction sequence for the imidization process, via perylene amic acid intermediates (PAAs), has been confirmed experimentally aided by the synthesis and full characterization of stable model amic acid salts and amic esters. Kinetic studies, using absorption spectroscopy, have established that PDI formation proceeds via fast amic acid formation, followed by a slow conversion to imides. Solubility of the intermediate PAA salts is found to be low and rate-limiting. Based on this finding, quantitative PDI synthesis at room temperature was achieved by diluting the reaction mixture with water, the solvent in which PAA salts have better solubility. Thus, the otherwise harsh synthesis of PDIs has been transformed into an extremely convenient functional group tolerant and highly efficient reaction that runs at room temperature.