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...

The incessant growth in energy demand has resulted in the deployment of renewable energy generators to reduce the impact of fossil fuel dependence. However, these generators often suffer from intermittency and require energy storage when there is over-generation and the subsequent release of this stored energy at high demand. One such energy storage technology that could provide a solution to improving energy management, as well as offering spinning reserve and grid stability, is the redox flow battery (RFB). One such system is the 200 kW/400 kWh vanadium RFB installed in the energy station at Martigny, Switzerland. This RFB utilises the excess energy from renewable generation to support the energy security of the local community, charge electric vehicle batteries, or to provide the power required to an alkaline electrolyser to produce hydrogen as a fuel for use in fuel cell vehicles. In this article, this vanadium RFB is fully characterised in terms of the system and electrochemical energy efficiency, with the focus being placed on areas of internal energy consumption from the regulatory systems and energy losses from self-discharge/side reactions.

Zinc-bromine redox flow batteries (RFB) are energy storage devices capable of integrating with renewable generators and so improve energy management from these intermittent sources. However, this system is susceptible to self-discharge via the electrogenerated bromine transferring through the separator and reacting with the plated zinc. Here we examine the use of novel quaternary ammonium complexes to capture the electrogenerated bromine but keep it in the aqueous phase (as opposed to the immiscible phase formed with N-methyl-N-ethylpyrrolidinium, MEP). Electrochemical analysis indicates that these complexes do not impact on the electrode kinetics and exhibit similar physical properties as to the MEP. However, the latter still shows an enhanced ability to complex the bromine to higher polybromide states (i.e. Br 5 −).

The zinc-bromine hybrid redox flow battery (RFB) is one of the few battery systems that have seen implementation for the medium to large scale energy storage.  One of the issues identified with this flow battery is that of bromine crossover from the positive electrode to the zinc electrode, leading to loss of current efficiency.  One of the ways in which this problem can be overcome is through the use of organic-based complexing agents which tie up the bromine as a thin surface layer and an immiscible liquid phase which is pumped from the electrode surface into an external reservoir.  In this paper, we examine in detail the kinetics and chemistry of formation of these complexed bromine ion pairs and establish the concentrations of the various complexing agents required to achieve full complexation of the electrogenerated bromine species.  The initial phases of the study used small chain (2,3 and 4-carbon) symmetrical tetraalkylammonium bromide salts to investigate the bromide oxidation and the stages of film formation on a variety of carbon-composite electrodes commonly employed in RFB systems.  These data were then compared to the classical responses obtained on Pt and glassy carbon electrodes.  What is clearly evident from the study is that the electrochemical reversibly of the Br-/Br2 reaction is attenuated on the carbon-composite surfaces in the presence of the complexing agents and leads to a second oxidation process at more positive potentials (Figure 1).   The nature of this second oxidation and of the surface species formed is investigated using in-situ Raman spectroscopy and its electrochemical characteristics examined using electrochemical impedance spectroscopy and potentiodynamic polarisation techniques.  Substantial changes in the electrochemical responses are recorded when film formation occurs and this event also led to a large increase in the measured impedance, requiring the use of an additional low frequency time constant element in the Randles equivalent circuit employed to fit the impedance spectrum acquired.  The magnitudes and trends observed in the impedance parameters at the different concentration and complexing species employed will be discussed in the presentation.  Since in an operational Zn-Br2 RFB system during charge, the oxidation reaction will be occurring on composite carbon surface covered with the Brx - complex, how this film impacts on the kinetics of the charge and discharge reaction will impact on the voltage efficiency.  The understanding of the behaviour of these simpler complexing agents will thus enable the electrochemistry of more complex species, e.g. 1-methyl-1-ethylpyrrolidinium bromide, to be better understood and so lead to improvements in the efficiency of the process occurring at the positive electrode of the zinc-bromine hybrid RFB. Figure 1

No abstract