3.1.2. Improving Membrane/Separator

The membrane/separator is a critical material in RFBs as well, mainly influencing the RFB performance of the battery to a great extent, especially the CE and capacity retention. The membrane/separator divides the negative and positive half-cells to refrain from battery short circuits. Meanwhile, the membrane provides ion transportation pathways to make a conductive circuit to optionally enable H⁺ or specific ions to pass through, avoiding the crossover between the catholyte and anolyte. To achieve a higher battery, CE requires a higher ionic selectivity of the membrane. Hence, the ideal ion-selective membranes for RFBs should satisfy the following requirements: excellent mechanical properties, high cycle stability, good ionic conductivity and selectivity, and low active species crossover and self-discharge rate. For ZIRFBs, the only concerned metallic ions which may permeate through the membranes and lead to capacity fade are Feⁿ⁺ and Zn²⁺. The radius of Feⁿ⁺ is between 63–92 pm, which is much smaller than that of Zn²⁺ (139 pm). Hence, the crossover of Feⁿ⁺ takes place much easier than Zn²⁺. It was reported that the permeability of the Feⁿ⁺ ion through Nafion was 5.5 × 10⁻⁵ cm²/min, which was 18.9~20.7 times higher than that of the vanadium ion (2.9 × 10⁻⁶ cm²/min). For the modification and improvement of membranes for RFB applications, inorganic–organic hybrid membranes and polymer blending composite membranes are widely used to reduce the undesired permeation of metallic ions and improve the ion selectivity of IEMs (ion-exchange membranes) ^[6][39][6,75]. The main hazard of zinc dendrites is to pierce through the membrane/separator and result in the battery short circuit. To avoid zinc dendrites from piercing the membrane/separator, membranes with high mechanical strength can be selected, such as the PBI (polybenzimidazole) membrane ^[34][51]. The PBI membrane with heterocyclic rings may ensure the rapid transportation of OH^{- [34][40][43]}[36,51,91]. Concurrently, the PBI membrane owns strong mechanical stability and can resist zinc dendrites well, thus, ensuring the long-term cycling stability of alkaline ZIRFBs. At the same time, the use of porous ion-conducting membranes instead of traditional IEMs solves the problem of an increased internal resistance of the membrane due to iron ion pollution, and improves the conductivity of ions from the neutral medium through the membrane, which greatly improves the performance and stability of neutral ZIRFBs. 

3.1.3. Adding Additives to the Electrolyte

The electrolyte is the source that affects the generation and growth of zinc dendrites. Therefore, the employment of additives into the catholyte/anolyte is a common method to suppress zinc dendrites by direct intervention in the formation of crystal nuclei. Additives can be mainly divided into three categories: metal ions, organic molecules, and polymers. Metallic ions may influence Zn nucleation, and thus, affect the growing process. Therefore, a compact and homogeneous Zn deposit layer is obtained ^[44][98]. Zhang et al. reported that the aqueous CaCl₂ solution containing NH₄Cl is appropriate to be a supporting electrolyte ^[45][99]. Severe Zn dendrites are detected by SEM in 0.1 M ZnCl₂ solution. Meanwhile, bulk Zn metal is detected with 1 M NH₄Cl as the supporting electrolytes. The cyclic voltammogram (CV) curves show the redox peaks sharpen obviously in the presence of NH₄Cl and independent of the amounts of NH₄Cl, meaning that the nucleation hysteresis decreases significantly. It can be confirmed that the addition of NH₄Cl may promote Z-P/S significantly. However, no prominent Zn dendrite, but only bulk Zn with random holes, is detected in the CaCl₂/H₂O (3.5 m) solution with 0.5 M NH₄Cl. The CV curves are analogous to those in aqueous NH₄Cl solutions, but the redox peak currents enhance with the addition of NH₄Cl. Therefore, for Z-P/S, a preferable supporting electrolyte has been an aqueous CaCl₂ and NH₄Cl solution. The charge–discharge curves under various current densities demonstrate a clear plateau with an average voltage of 1.5 V. The CE and EE reach 94% and 75% at 20 mA cm⁻², respectively.

3.1.4. Flow Field Regulation

The electrolyte flow acts a vital role in Zn dendrites, not only owing to changing the gradient distribution of zinc ions, but also reshaping the orientation of dendritic growth. When electrolyte flow velocity is 50 mL min⁻¹, the species concentration distribution is uniformly obtained by numerical simulation ^[46][87]. It is very clear that two different zinc-depositing morphologies can be observed under the conditions of the quiescent electrolyte and the flowing electrolyte ^[47][101]. A higher flow rate of the electrolyte may enhance the transport velocity of Zn²⁺ which accelerates the diffusion process on the electrode accessory surface and the mass-transfer process in the bulk electrolyte, thus, finally reducing the Zn²⁺ concentration gradients and constraining the formation and growth of dendrites. Furthermore, a relatively high flow rate results in mitigating dendritic growth, as the formed dendrites are washed away directly by the electrolyte [17].

3.2. Fe(III) Hydrolysis Suppression

Both Fe²⁺ and Fe³⁺ have hydrolytic reactions in an aqueous solution. It has been reported that the hydrolysis product of iron ions will combine with the sulfonic group in the membrane to increase membrane resistance ^[48][103]. The hydrolytic reactions of Fe³⁺ are much stronger than those of Fe²⁺ and, consequently, occur at a much lower pH ^[49][104]. However, the hydrolysis of Fe³⁺ is easier to be suppressed in a hydrochloric acid environment ^[45][50][99,105]. In the catholyte of acidic ZIRFBs, polymerization takes place more seriously during the Fe²⁺ oxidation reaction, and ferrihydrite precipitation takes place during the Fe³⁺ hydrolysis process. The polymerization and hydrolysis reactions are rapidly promoted by enhancing H⁺/OH⁻ ions formed due to water electrolysis. To address this issue, seven types of Fe²⁺-complexing ligands are tested and reported, but some issues remain if itswe consider the binding stability and electrochemical performance of the Fe²⁺-ligand complex ^[51][82]. It can be concluded that a novel Fe²⁺-pyridine complexation in the catholyte has been presented for acidic ZIRFBs with a long cycle life and high performance over other Fe²⁺-complexing ligands. In comparison to other complexing ligands, the Fe²⁺-complexation with pyridine presents the highest electrochemical activity and reversibility ^[51][82]. 

3.3. Electrolyte Optimization

3.3.1. Concentration and Additives

By optimizing the composition of the electrolyte, Yuan et al. made the concentration of the Fe(CN)₆³⁻/Fe(CN)₆⁴⁻ redox couples achieve 1 mol L⁻¹, far exceeding the previously reported concentration (0.4 mol L⁻¹) ^[34][51]. The high concentration of active redox couples enables the system with a high-energy density. The battery can realize 500 cycles of charge–discharge cycling under 80 and 160 mA cm⁻², and still maintain an EE over 80% and CE over 99% at 160 mA cm⁻². The results verified the outstanding stability of this system. Most important of all, the functionality of this work is further verified by assembling a kW battery stack at a capital cost of less than USD 90 per kWh.

3.3.2. Zinc–Bromide Complexation

To ensure the long-term operation stability of neutral ZIRFBs, Yang et al. proposed the use of Br^- ions to stabilize Zn²⁺ through complexation interactions in the neutral electrolytes ^[52][56]. The results of cyclic voltammetry indicate that the redox reversibility has been significantly enhanced between Zn²⁺ and Zn. To tackle the issue of the sluggish kinetics of the coordination interaction between Br⁻ and Zn²⁺, ZnBr₂ as the electrolyte additive was directly selected to boost the process of complexation. By employing active K₃Fe(CN)₆ in the catholyte and modified species in the anolyte, the proposed neutral ZIRFB demonstrates excellent efficiencies and cycle stability (without obvious capacity decay) during 2000 cycles (356 h) ^[52][56].

3.3.3. pH

For ZIRFBs, plate electrodes or porous CFs are generally adopted for Z-P/S. During charging, zinc ions or zincate ions are continuously converted to zinc metal, and then, finally, are completely plated on the electrode. Once the deposition is finished on the electrode completely, no further electroplating will be carried out. Further charging will lead to a sharp increase in the charge potential, thus, resulting in the irreversible HER in the negative half-cell. Liu et al. investigated the effect of several inorganic and organic additives on water migration in alkaline ZIRFBs ^[53][83]. Although all these additives are proved to be effective to suppress water migration, the organic additive, such as xylitol, sorbitol, and mannitol with several hydroxyl groups, can increase the alkalinity of the electrolyte, which in turn, accelerates the corrosion rate of zinc metal and further aggravates the HER of the battery.

3.3.4. Mix System

Fe–Cr RFB in the mixed electrolyte was first invented to tackle the cross-contamination issue [13]. Hybrid RFBs, such as Zn–Fe, all-Fe, Sn–Fe, have been widely explored in order to get rid of the bondage of high-cost membrane materials. Zhou et al. reported an Sn–Fe RFB, employed SnCl₂ and FeCl₂ as both an anolyte and catholyte, and delivered 78.5% of EE and 0.96‰ per cycle of the capacity decay rate at 200 mA cm^{−2 [54]}[108]. At present, a long lifetime and a high-power density of Zn–Fe RFBs are achieved through additional operation and structural design.

Abbreviations

RFBs	redox flow batteries
ZIRFBs	zinc–iron redox flow batteries
ZBRFB	zinc-based RFB
VRFB	vanadium RFB
R.T.	room temperature
OCV	open-circuit voltage
OCP	open-circuit potential
CEM	cationic exchange membrane
IEM	ion exchange membrane
n-IEMs	non-ionic membranes
PES	poly (ether sulfone)
PEG	polyethene glycol
SPEEK-K	sulfonated polyether ether ketone
PBI	polybenzimidazole
BMImCl	1-butyl-3-methylimidazolium chloride
CF	carbon felts
Z-P/S	zinc plating/stripping
CV	cyclic voltammogram
EE	energy efficiency
CE	current efficiency
VE	voltage efficiency
HER	hydrogen evolution reaction
SOC	state of charge
MC	microporous carbon
THEED	N, N, N′ N′-Tetra(2- hydroxyethyl) ethylenediamine