CENmat | Green Technology | Cutting-Edge Nanomaterials | Waldenbuch

Energy storage for 0.1 $ per kWh with only salt water

Reborn out of the box ideas I

Sometimes decades pass by in the scientific world before somebody stumbles over a dusty but yet truly novel idea. Putting it into work, investigating it with the current state of the art techniques and relive an idea which could potentially be one solution to our many problems of today is something which has always fascinated us at CENmat. That’s why we love reading old science papers at CENmat in order to feed our creative side hoping to stumble over such a dusty but novel idea ourselves one day. Hamelers et al. certainly was the lucky who came across such an idea. He built a battery for energy storage by using just table salt, Glauber’s salt, a couple of ion exchange membranes (read more here). His work was published in the international journal of energy research in 2017. Funny enough or maybe even a little bit sad is that the whole work does not cite the origin of the idea, the dusty idea which after all didn’t have the opportunity to shine. But CENmat is there to give it the shine it deserves – We know we deserve the red hero cape so much! OK, the original work was published by Holmstrom et al. in 1982 who describes his idea as follows: “A battery based on bipolar membranes could be built up by unit cells consisting of one anion exchange membrane, one bipolar membrane and one cation-exchange membrane. The unit cell does not need any electrode since water splitting is a membrane action. Thus, several unit cells can be stacked upon each other to form a battery (Fig. 1). This procedure allows a compact design giving any desired voltage at the ends of the stack, where electrodes are placed to extract power from the battery. The energy-storage system should consist of tanks for salt solution, acid and hydroxide, battery unit, and pumps to direct the flows of solutions between tanks and battery. When charging of the battery starts, all membrane spacings are filled with salt solution. The current forced through the battery will cause the bipolar membranes (CA in Fig. 1) to send protons left and hydroxyl ions to the right. If the salt used is NaCl, sodium ions will flow left through the cation-exchanging membranes (C in Fig. 1) and chlorine ions right through the anion exchanging membranes (A). Thus, three solutions will be produced: a more concentrated NaCl-solution, HCI-solution, and NaOH-solution. The NaCl solution may be recycled inside the battery, while acid and hydroxide are pumped to separate storage tanks (Fig. 2). In the tanks, the acid and hydroxide may be stored for an indefinite time. When the charging current is stopped, diffusion will transform all solutions still inside the battery into salt solution. When power is needed, the pumps are started in the reverse direction. All reactions will change direction and power will be delivered to an outside load. The start-up time will be a matter of tens of seconds.1

Ultimately he designed a Battery with 7 units with a cross section of 7 cm and the membrane separation of 3mm. After charging the Battery for 2 hours with a current ranging from 39 to 1 mA. After this time he discharged the battery across a resistor recording the initial voltage to be 1.8 V delivering a current of 1mA. In 2017 Hamelers et al. who has investigated the same construction could achieve power densities up to 3.7 W m−2 per membrane, energy densities up to 2.9 Wh kg−1, and round‐trip efficiencies up to 13.5%. Both scientists however, were aware that these values were and are still far away for practical use. The major energy losses were caused by the high area resistance of the membranes and the high co-ion transport that was estimated to be 39-65%.   Nevertheless a battery solely working with salt water is to our opinion a generous way to store surplus energy. We hope that many polymer chemistst are reading this finding it interesting enough to start working on better cation, anion and bipolar membranes for the greener, safer, and independent energy storage technology of the future.