Improving membranes for the acid-base flow battery: Membrane assessment, design, and performance optimization.
Nadia Boulif has successfully defended her PhD thesis at the Department of Chemical Engineering and Chemistry on May 27th.
The deployment of renewable energy sources such as solar and wind energy is limited by their intermittent nature. Therefore, implementing large scale energy storage solutions is essential to enable their continuous use. The acid-base flow battery (ABFB) is a technology that can store electrical energy in a mixture of table salt and water. By applying excess renewable electricity to a membrane stack, the salty water is converted to acid and base, which are then stored in external storage tanks. The energy density for a concentration of 58 g/L acid and base is roughly equivalent to a hydropower system with a height difference of 4 kilometres. When the energy is needed, the acid and base are recombined, releasing the stored energy, and returning the system to its original state: table salt and water.
However, this technology still requires further development to achieve long-term stability and energy efficiency. This can be achieved by developing better ion exchange and bipolar membranes, which are the core of the ABFB. An ion exchange membrane is a very thin plastic sheet with fixed charges (either positive or negative) that enables the selective transport of either negative or positive ions from the table salt. A bipolar membrane consists of both a positive and a negative sheet together, which can be used to split water molecules (H2O) into H+ and OH-, or recombine H+ and OH- into H2O (water). The goal of this PhD thesis is to identify the critical membrane properties and explore how they can be improved.
The first research project shows that due to the limitations of the currently available membranes, there is a trade-off between energy density, energy efficiency and long-term stability of the ABFB. Therefore, there is a need to develop ion exchange membranes with higher selectivity and lower resistance, which was the focus of the subsequent research projects.
In the second project, ion exchange membranes are made by incorporating clay particles to an ion exchange polymer (a polymer that has fixed negative charges). The added clay particles have a flat surface that acts as a highway for the transport of sodium ions while maintaining a similar selectivity.
Next, the clay particles are incorporated between the positively and negatively charged layers of the bipolar membrane. Their presence facilitates the dissociation of water molecules into H+ and OH-, which leads to lower energy consumption, and therefore more energy efficient energy storage.
Finally, salt contamination of the acid and base causes energy loss as salt ions block the transport of H+ and OH- within the bipolar membrane. To address this, a thin polymeric (plastic) layer is coated on a commercial bipolar membrane to prevent salt ions from penetrating into the bipolar membrane, making the battery discharge more efficiently.
These findings contribute to the enhancement of energy storage capabilities in the ABFB, making them a promising solution for large-scale renewable energy integration to the electrical grid.