Energy plays a critical role in the society of today, from the lights in your room to the medical equipment in the city hospital, human life depends greatly on energy production. Between the energy crisis of the 70s and the increasing carbon emissions in the air, renewable and alternative energy has seen increasing demand in research. While solar and wind plants are effective, they face challenges in their inability to hold excess power to be evenly distributed when required during maximum and non-maximum peak hours. To address this issue, energy-storage devices, such as secondary batteries and hydrogen fuel cells have undergone research, testing, and eventual commercial use. Among these energy-storage devices is the Redox Flow Battery (RFB), a secondary battery with a simple but productive design. While commercialisation and design on a large-scale has been hit and miss for most systems, the Redox Flow Battery still provides an effective energy storage tool for now and the future.
To begin with, the Redox Flow Battery follows a non-complex design; most RFBs contain a positive and negative half-cell that is separated by an ion-exchange membrane. Each half-cell contains its own electrode to allow energy to flow through the system in the form of an electrolyte solution, which stores the energy, that is pumped to and from separate electrolyte storage tanks for
Figure 1 – Redox Flow Battery Design
each half-cell. True to its name, oxidation reactions occur during charging and reduction reactions occur during discharging in the positive half-cell, while the reverse occurs in the negative half-cell. A major advantage to the RFB compared to conventional batteries is the use of fully soluble redox couples and electrodes, removing the undesirable changes to the electrode structure. The RFB's storage capacity is exclusively determined by the solution concentration and the size of the solution tanks, while its power is determined by the number of cells; this enables a RFB's performance parameters to be modified individually and accordingly. Additionally RFB's have some other benefits such as their simple electrode reactions or not requiring high temperatures for operation. Despite its apparent advantages, the RFB design has a couple concerns with it; a RFB can either use inexpensive or very expensive electrode and redox couple materials that can amount to a very large cost ratio. Furthermore, the sheer bulk of the electrolyte tanks limit a RFB to usage in stationary stand-alone applications such as load-leveling devices at power generating facilities.
The primary features sought in a RFB are the speeds of its energy transfer capabilities and the efficiency of which it can do it at. In the past few decades, several designs have been researched and developed but few have made commercialisation. Out of these next four designs, two have failed in commercial applications, the iron-chromium system and the bromine/polysulfide system (RGN-ESS), but the...