The all iron flow battery was proposed by Hruska in 1981, and its positive and negative active substances are iron containing compounds with different valence states, solving the problem of electrolyte interconnection. All iron flow batteries can be divided into acidic all iron flow batteries and alkaline all iron flow batteries according to different electrolytes. They utilize two sets of redox pairs, Fe (II)/Fe (0) and Fe (III)/Fe (II). During the discharge process, the deposited Fe (0) on the anode undergoes a transition to Fe (II), while the cathode undergoes a transition from Fe (III) to Fe (II). The charging process is a reverse reaction of the reaction. During the charging process, the reduction potential of divalent iron is -0.44 V (vs. SHE). Such a low reduction potential is bound to cause serious hydrogen evolution reactions in all iron flow batteries under acidic or alkaline conditions, and the generated iron can serve as a catalyst for hydrogen evolution reactions. A strong hydrogen evolution reaction can alter the morphology of the active substance, thereby reducing the current efficiency during battery cycling and having a negative impact on all iron flow batteries.
Therefore, considerable research has been conducted to minimize hydrogen evolution reactions in acidic all iron flow batteries and improve battery efficiency. For example, Jayathilake and Manohar et al. constructed a buffer system on the electrode surface by adding ascorbic acid to the electrolyte of an all iron flow battery, which avoids the precipitation of iron in the form of hydroxides under the condition of electrolyte pH 2. However, the study also found that the chelation between ascorbic acid and iron ions can reduce the electrode reaction rate [2]. In addition, research has also focused on improving electrodes and conductive media. For example, Petek et al. found through the use of multi walled carbon nanotubes as conductive media that as the state of charge of the battery increases, it can promote the deposition of iron on multi walled carbon nanotubes, gradually improving the conductivity of the slurry and thus improving the voltage efficiency of the battery [3]. In terms of commercialization, currently, some companies have also produced commercially applicable all iron flow batteries through efficient electrode structure design and system control. The iron-based redox flow battery developed by ESS company in the United States uses FeCl2 and KCl as electrolytes. According to our previous patent analysis of the company, the pH value of the electrolyte in this system needs to be precisely controlled around 4 through another set of acid-base compensation facilities to avoid dissolving iron metal or precipitating Fe (OH) 2. In addition, in order to avoid the formation of metal dendrites under high current density, this system usually operates below 40mA/cm ² Operates at a current density of.
Alkaline all iron flow batteries often use iron oxides or organic chelates of iron as negative electrode active substances. Due to their own activity and stability issues, the discharge specific capacity is usually controlled below the theoretical specific capacity corresponding to the active substance. And in alkaline environments, the hydroxide passivation layer produced by the dissociated iron ions can lead to poor rate performance of the battery, while also accompanied by hydrogen evolution reactions similar to acidic conditions, thereby reducing battery efficiency. At present, research is being conducted to enhance the electrochemical activity of the negative electrode by combining various active substances of iron with high specific surface area carbon materials. For example, Wang et al. generated crystalline iron oxide FeOx on graphene by liquid phase reaction and gas phase annealing, mixed FeOx/graphene mixture, carbon black and binder, and then coated it on foam nickel to make an electrode, and obtained 377 mAh g-1 specific capacity at a scanning rate of 5 mV s-1 [4]. Hang et al. studied the mixing of different types of carbon materials, iron powder, and PTFE suspension to make electrodes, and found that the electrode capacity using carbon nanotubes was the highest. This is because carbon nanotubes have a larger specific surface area, and during the generation of Fe (OH) 2, the Fe (OH) 2 layer formed is thinner and the reaction activity is stronger, thereby improving the electrode capacity [5].
Gong et al. developed the first fully soluble all iron flow battery using [Fe (TEOA) OH] -/[Fe - (TEOA) (OH)] 2- (Fe TEOA) and Fe (CN) 63-/Fe (CN) 64- (Fe CN) as redox pairs, resulting in a significant improvement in energy efficiency compared to traditional all iron [1]. This technology route has also been commercialized by multiple institutions because it avoids issues such as iron metal dendrites.