2. Halogen Hybrid Flow Batteries Perspective Concepts Analysis
2.1. Hybrid Principle Basis
The hybrid device may lose performance in some way, but this change can push a device from completely non-functional to viable in terms of functionality. So, in the case of lithium batteries, the lithium-metal electrode was sacrificed and replaced by lithium-intercalated graphite, which led to a 30% decrease in theoretical energy density, nevertheless making it possible to create a battery with a long-life cycle
[15][88].
In general, a natural consequence of this approach is the free combination of design and chemistry for the cathode and anode half-cells of the electrochemical system in a way to balance the system as much as possible in terms of performance. When doing this optimization, there is no need to stay within the same design or operation principle for the power source: the ability to combine different technologies is the key to obtaining new energy storage devices that simultaneously combine the advantages of capacitors, electrochemical or fuel cells, flow batteries and other technologies.
At present, this approach to the development of new energy sources is used everywhere. Thus, an increase in the energy capacity of a conventional double-layer capacitor with the reversible redox electrochemical processes on the electrode near-surface layer leads to a new type of electrochemical capacitor–supercapacitor
[16][17][18][19][20][21][89,90,91,92,93,94]. The combination of various chemistries of galvanic cells has led to the appearance of lithium-sulfur batteries with outstanding performance in terms of specific energy capacity—a value of more than 500 W h has been experimentally achieved
[22][23][24][25][26][27][28][95,96,97,98,99,100,101]. Even higher energy intensity (comparable to the energy intensity of gasoline fuel) is demonstrated by a hybrid of a fuel cell and a lithium-ion battery, a lithium-air battery, where electric current is generated due to oxidation of lithium at the anode and oxygen reduction from the air at the cathode
[29][30][31][32][33][34][35][36][37][102,103,104,105,106,107,108,109,110].
Despite a number of difficulties associated with the use of metal anodes, the choice of catalysts and electrolytes, metal-air electrochemical sources are the prominent result of combining the idea of electrolyte circulating through the system and the design of a galvanic cell, for example, for transport applications
[38][39][40][41][42][43][44][45][46][47][111,112,113,114,115,116,117,118,119,120].
The principle of combining various electrochemical approaches can also play a decisive role in solving the problems of traditional fuel cells, where you can sacrifice air oxygen by replacing it with another reagent (whereas the hydrogen reaction is stored at an anode) because such a replacement immediately eliminates the crucial problems of conventional fuel cells: the high cost of the system due to utilization of platinum and other catalysts, and dissolution of cathode catalyst
[48][49][50][121,122,123].
However, an adequate choice of new oxidizing agent for a hybrid flow battery is an extremely difficult task, since several requirements must be met simultaneously: electrolyte must have a high energy density and its reduction reaction must be kinetically fast to ensure the high energy efficiency of the device. Additionally, all reagents and products of the cathodic reaction must be nontoxic, stable and non-flammable, preferably liquids or gases, since, e.g., solids, pastes and suspensions are often commercially less preferable in flow battery design.
2.2. Halogen Hybrid Flow Batteries
Considering the above-mentioned requisites one can recall the hydrogen-halogen hybrid flow batteries, primarily due to rapid, reversible kinetics, which leads to excellent system performance and the use of inexpensive reagents
[10][51][52][53][54][11,75,124,125,126]. In such a system a halogen oxidizer X
2 (for example, Br
2, Cl
2, or I
2) is used in the form of an aqueous solution and hydrogen (H
2) is used as a reducing agent. The potential difference between the halogen and hydrogen electrodes
V is equal to the EMF (from 0.54 V for I
2 to 1.4 V for Cl
2). It should be noted that F
2, which is not considered here, has the largest potential difference relative to the standard hydrogen electrode (3.05 V), however, due to technological difficulties in the separation of gaseous fluorine from a mixture of gases, its use is unreasonable due to energy considerations and the problem of materials’ compatibility with this reagent.
In this battery the following reactions undergo on electrodes (from left to right—charge mode, from right to left—discharge):
The protons are transferred from the anode through the membrane into the catholyte (solution in contact with the cathode), whose composition changes inside the discharge unit from X
2 to HX, and the HX solution enters the corresponding reservoir.
The most promising halogens that can be used as oxidizing agents in such a system are Cl
2 and Br
2 because the standard potentials of the corresponding redox couples for these substances are the highest (1.36 V and 1.09 V, respectively, for Cl
2/Cl
− and for Br
2/Br
−). Recently, the interest of researchers has mainly shifted towards the hydrogen-bromine system due to its higher specific power with voltage efficiency of more than 90%, as well as a higher oxidant solubility in an aqueous solution.
A serious disadvantage of hydrogen-halogen systems based on chlorine and bromine is the high toxicity and corrosiveness of the halogen oxidizer, whose concentrated solution must be stored in a tank. Another limitation for these systems is the risk of the catalytic layer degradation on the negative electrode due to halogen crossover through the separator, which should not only provide high selectivity but also have low internal resistance to minimize voltage losses.
The transition from traditional vanadium-based redox flow batteries to hydrogen-bromine hybrid chemistry has led to a significant increase in the power density of flow systems. To date, specific power values of 1.4–1.5 W cm
2 have been experimentally achieved for the H
2/Br
2 system
[55][56][127,128], while for vanadium redox flow batteries, the highest peak power density for laboratory-scale devices reaches 1.3 W cm
−2 [57][58][59][60][129,130,131,132]. The researchers develop new concepts and models: Dr. Wlodarczyk recently explored the bromine cathode thermodynamics for hydrogen–bromine chemistry in concentrated solutions
[61][133]. Dr. Ronen proposed the perspective concept of the hydrogen-bromine flow battery, which does not need separation of the cathode and anode half-cells via proton exchange membrane. He also completed a thorough analysis of the catholyte complexation reaction in homogeneous conditions vs. the efficiency of such a flow battery system
[62][63][134,135]. Recently Dr. Hou reported a chlorine redox flow battery that reversibly performs electrolysis of NaCl aqueous solution until Cl
2 is generated and transformed into the carbonaceous tetrachloride (CCl
4)
[64][136]. Dr. Fisher and some other scientific groups thoroughly investigated the complexation of bromine with different chemical agents (BCA) in aqueous electrolytes of halogen-containing flow systems. The problem of bromine vapor pressure reduction and halogen storage in the insoluble matter or solid material form is of interest in these works
[65][66][67][137,138,139]. As a result, they showed that bromine can be stored in the fused salt of very high concentrations up to 13.6 M, rushing theoretical energy capacity up to 730 A h L
−1 [65][66][67][137,138,139]. Recently this group also measured current densities in the diffusion-limited regime and corresponding current distribution mappings vs. the flow rate values for the H
2-Br
2 RFB
[68][140].
Despite the recent developments in the H-Br
2 battery, the solubility of bromine in water under normal conditions is about 0.21 mol l
−1 [56][128]. Hydrogen-bromine systems usually use a mixture of bromine dissolved in water and hydrobromic acid (up to 8 mol l
−1)
[56][128], which makes it possible to achieve an energy capacity of about 170 W h kg
−1 [69][141], much higher than most common vanadium flow batteries energy density of 20 to 50 W h kg
−1 [70][71][72][73][19,34,76,142], nevertheless still much lower than, for example, for lithium-ion batteries
[74][143]. Thus, it can be concluded that, despite promising performance in terms of power density, both vanadium-based compounds and halogens show insufficient solubility in water, which leads to limitations on the value of specific energy capacity.
2.3. Halate Hybrid Flow Batteries
The solution to the problem of the insufficient energy capacity of hydrogen-halogen flow batteries was proposed in 2015 by Yu.V. Tolmachev in
[14][15]—to abandon the use of dissolved halogens in favor of a new family of aqueous multielectron oxidants—solutions of halogen oxoacids.
Lithium bromate LiBrO
3, which should be converted at the cathode to lithium bromide LiBr, serves as a promising object of study in
[14][15]. Due to the very high solubility of both substances in water (according to
[14][15] even at room temperature, the concentration of a saturated solution of lithium bromate is about 9 mol l
−1, and bromide—more than 10 mol l
−1) and the transfer of 6 electrons for bromate-bromide transition results in very high charge densities, more than 1000 A h kg
−1, which are more than an order of magnitude higher than similar indicators for other redox flow batteries systems proposed to date.
Other advantages of this oxidizing agent, mentioned in
[14][15], and demonstrated experimentally in
[75][144], are its long-term chemical stability (at moderately low pH), low toxicity of the reagent and product, the utilization of commodity chemicals, low cost per unit of generated electrical energy (due to both a relatively inexpensive reagent and the absence of expensive catalysts for the cathodic reaction, in particular, noble metals), low self-discharge rate and the absence of fire and explosion hazards.
Additionally, in addition to lithium bromate, the use of commercially available sodium bromates with water solubility up to 2 mol l
−1, seems very promising
[14][15].
The reason why the bromate anion (as well as other oxohalogenate ions) did not attract attention for applications in electrochemical energy earlier is its low electrochemical activity: on all the studied electrodes, including noble metals, its direct electroreduction reaction at high-rate proceeds at high overvoltages. For the first time, it has been experimentally confirmed that despite the direct electroreduction of bromate anion via heterogeneous reactions cannot be performed without the huge cathode overvoltage, nevertheless, it can be carried out via redox mediator autocatalysis (EC″ mechanism) through a combination of the following bromine reduction reactions:
which takes place reversibly even on electrodes without expensive catalysts, with the process of comproportionation in the electrolyte bulk
which proceeds at sufficiently high acidity of the solution.
The results of theoretical and experimental studies
[76][77][78][79][145,146,147,148] confirmed a number of extremely interesting and unexpected effects: the presence of an anomalous dependence of current density on the intensity of the convective mixing of the solution (which is uncharacteristic of any other known electrochemical mechanisms) combined with an anomalously high cathodic current density of order ~ A cm
−2 in bromate solutions of the molar concentration range.
Successful confirmation of the developed analytical model predictions with experimental rotating disk electrode data for several solutions of bromate anions in sulfuric and phosphoric acids in the molar range of concentrations is presented in the work
[80][149].
Thus, the quantitative agreement of analytical predictions and experimental data allows
reus
earchers to make the following fundamental conclusions on the kinetics of chemical processe:
-
The high reversibility of the electrochemical step (5) of the EC″ mechanism for the bromine/bromide redox couple used in the system, even on carbon electrodes. That is a prerequisite for high current density, sufficient voltage for under load conditions and the specific power of the system;
-
Irreversibility of the chemical step (5) of the EC″ mechanism due to sufficiently high solution acidity (acid volume concentration approximately in the molar range);
-
The first order of the comproportionation reaction (5) with respect to the anions of bromate and bromide, the second order—with respect to the activity of protons:
-
where aH = 10−pH proton activity (in the form of hydroxonium ions), fBrO3 and fBr are activity coefficients of the bromate and the bromide anions, correspondingly.
-
The six-electron nature of the electrochemical process (the Br atom changes its oxidation state from +5, which corresponds to BrO3−, to −1, which corresponds to Br−) during the cycle is a prerequisite for ensuring a high specific energy capacity of the system.
Prof. Cho et al. have performed a numerical study of this redox-mediated bromate-based electrochemical energy system focusing on the transient cell behavior analysis and mass transport analysis of the main reagent, which resulted in strengthening the conclusions made above about the prospects of such approaches
[81][150]. The numerical model provided by Dr. Chinannai and Dr. Ju has also demonstrated the beneficial features of the H
2/Br
2/BrO
3− system, demonstrating the discharge characteristics of an H
2/Br
2 cell with BrO
3− measured under different C-rates
[82][151]. The bromate system was found to have the innovative features to boost cell performance due to its characteristic autocatalytic reaction to keep regenerating enough reactant for the electrochemical reaction, which may lead to a “no-limiting current condition (i.e., no mass transfer loss)” in well-designed cell and operating conditions.
Thorough research by Dr. Modestov’s group showed for 50 cm
2 membrane-electrode assembly of hydrogen-bromate flow battery 0.74 W cm
−2 power density was reached at the cell voltage 0.7 V while catholyte utilization rate was 0.93 in a single pass through membrane-electrode assembly, see
Figure 1 and
Figure 2 [75][83][144,152]. As a result, based on the high-grade assessments one can conclude that such a system can be of interest to break through the challenging issues in conventional batteries
Figure 1. Evolution of hydrogen bromate hybrid flow batteries: Single flow cell (
a–c): current density I ~ 1.0 A cm
−2, power density P ~ 0.7 W cm
−2, stack (
d): I ~ 0.6 A cm
−2, power density P ~ 0.3 W cm
−2 [75][83][84][85][144,152,153,154].
Figure 2. Dependence of current density and specific power vs. voltage for a laboratory membrane electrode assembly prototype (see Figure 13a) with an active surface area of 50 cm2 at different acidic concentrations of the bulk solution: (a) 1 mol L−1 H2SO4, (b) 0.25 mol L−1 H2SO4, (c) 0.1 mol L−1 H2SO4.