Complexing Additives to Reduce the Immiscible Phase Formed in the Hybrid ZnBr₂Flow Battery

Abstract

The zinc-bromine redox flow battery (RFB) is one of a very few commercially viable RFB energy storage systems capable of integration with intermittent renewable energy sources to deliver improved energy management. However, due to the volatility of the electrogenerated bromine and potential for its crossover from positive to negative electrolytes, this system requires the use of quaternary ammonium complexes (N-methyl-N-ethylpyrrolidinium, (MEP)) to capture this bromine. This produces an immiscible phase with the Br 2 which requires a complex network of pipes, pumps and automated controls to ensure access to the electroactive material during discharge. In this work, the use of novel quaternary ammonium complexes to capture the electrogenerated bromine but to keep it in the aqueous phase is examined. Three compounds, 1-(carboxymethyl) pyridine-1-ium, 1-(2-carboxymethyl)-1-methylmorpholin-1-ium and 1-(2-carboxymethyl)-1-methylpyrrolidin-1-ium, were found to successfully reduce the volume of the immiscible phase formed on complexing with the polybromide (Br x − ) whilst displaying similar enthalpy of vaporization values as that of MEP . Electrochemical analysis also revealed that these compounds did not impact on the electrode kinetics of the Br − /Br x − reaction indicating that the resulting surface film formed with these compounds behaved as a chemically modified electrode, in contrast to the surface film formed with MEP. Renewable energy features in many countries' energy agendas. Current political focus seeks to reduce carbon emissions, as agreed by the Paris Agreement, by using energy more efficiently and to have power generation from zero carbon technologies.1 Installed capacity of renewable energy sources have increased over the decade to 2014 by 175 GW for solar and by 322 GW for wind.2 This increase in capacity has impacted on the global share of energy from renewable sources (excluding hydropower) from 2.2% (2004) to 9.7% (2013) with many countries promising to accelerate their installation of these technologies in the coming years.3 However, energy supplied from renewable sources is often intermittent and can fluctuate depending on weather and location. 4,5 This creates a problem in that energy can be generated in excess or in deficit in relation to energy demand. To solve this issue, energy storage is required to balance these peaks and troughs in order to stabilize energy flow into the electrical grid. This would lead to a better management of renewable sources. 6 Redox flow batteries (RFBs) are one means of achieving large scale energy storage which can provide a more efficient link between energy production and energy demand. This type of battery system has the advantage of having a lower cost, a low level of self-discharge and is considered to have a much safer operation compared to other battery systems such as the sodium sulfur and lithium ion batteries. 7,8 Additionally, as with all batteries, it has the advantage of being more flexible and mobile in relation to pumped hydro and compressed air t...

Catalyst-Inspired Charge Carriers for High Energy Density Redox Flow Batteries

“…Due to the low reduction potential values associated with the +3 → +2 step, the OCV MeCN and MeCN values of these complexes are very similar. In contrast, MeCN+IL [74], which offers one of the highest cell voltages due to the release of two electrons per atom of zinc, are appreciably lower than that of complex II in the EW of MeCN (OCV MeCN = 3.68 V). OCV values of aqueous RFBs are principally lower than those of nonaqueous systems because the expanded EW of the organic solvent provides an opportunity to access redox events outside the EW of water.…”

Advanced Materials for Zinc‐Based Flow Battery: Development and Challenge

“…However, the use of the bromide ion as the complexing agent leads to a low utilization of the active material and further reducing the energy density of a battery. Thus, to liberate the bromide ion that is employed to capture the bromine and to further increase the utilization of the active material, a series of bromine organic salts ( Figure a), such as N ‐ethyl‐ N ‐methyl pyrrolidinium bromide (MEP), 1‐ethylpyridinium bromide ([C 2 Py]Br) and 1‐(carboxymethyl) pyridine‐1‐ium (QBr1), have been designed and proposed as novel bromine complexing agents (BCAs) . The utilization of these complexing agents can suppress the bromine crossover from positive to negative electrolyte because of the large size of the organic salts, thus, further increasing the CE of the battery.…”

“…This action further leads to a small part of the bromine participating in the electrochemical reaction, which in turn results in zinc accumulation at the negative side and further decreases the cycle life of the battery. The oil phase also requires a complicated network of pipes, pumps and automated controls to ensure access to the active material during discharge, thereby increasing the system cost. To address the above issues, novel complexing agents containing carboxyl or hydroxyl group have been designed and proposed.…”

A Single‐Flow Battery with Multiphase Flow

“…Zinc‐bromine batteries, in particular, are often designed to operate without a membrane by leveraging bromine complexing agents (BCAs), such as quaternary ammonium bromide salts (QBr) [24–28] . These sequester the majority of the bromine in a polybromide phase with reduced activity, [6,29] where the polybromide phase is typically restricted to the bottom of the catholyte tank [30,31] . Due to the resulting low aqueous phase concentration of bromine, usually between 10 and 200 m m (at T=25 °C and 0 % state of charge), [32,33] crossover of bromine leading to zinc corrosion is significantly reduced.…”