1. Introduction to Alkali Metal–Air Batteries
The gradual depletion of fossil fuels and environmental concerns associated with their use have been challenging the energy sector. Thus, vast development and deployment of sustainable renewable energy sources, such as solar and wind are required [1
]. Wind and solar are known to be the world’s fastest-growing energy sources [2
]. Despite their economic feasibility and environmental friendliness, their intermittent nature and geographical limitations are the major challenges for their full employment as next-generation energy sources. To counteract their fluctuating energy outputs and thus improve the stability of the electrical grid, an efficient and stable electrical energy storage system is needed [3
Over the years, various electrical energy storages have been identified and used. Currently, lithium (Li)-ion and lead–acid batteries are the leading energy storage technologies. Lead–acid batteries are well-established electrochemical energy storages for both automotive and industrial applications [6
]. However, they have low energy density (30–50 Wh/kg), low cycling life (500–1000 cycles), and are dependent on toxic lead [7
]. Similarly, Li-ion batteries play an important role in our daily lives, as they are the most commonly used battery in electric vehicles and electronics today. Unfortunately, their high cost per kWh and recent concerns over their safety have restricted their application, thus requiring the development of new storage technologies for the next generation [3
]. The thermal runaway of the cell, which leads to whole battery pack failure, is believed to be caused by mechanical, electrical, or thermal abuses [11
]. Moreover, Li-ion batteries’ energy density is only about 100–200 Wh/Kg, which cannot provide the extended use needed for electric vehicles and large stationary applications [12
]. Therefore, safe and high-energy-density energy storage systems are extremely desired. Figure 1
shows the chemistries and principal components of lead–acid and Li-ion batteries.
Among several potential candidates, metal–air batteries are a promising and competitive high-energy alternative to Li-ion batteries [13
]. Metal–air batteries are high energy density electrochemical cells that use air at the cathode (+ve) and metal as the anode (−ve) with an aqueous electrolyte [14
]. Roughly speaking, a metal–air battery consists of four components: metal anode, electrolyte, membrane separator, and an air cathode. The metal anode can be an alkali metal (e.g., Li, Na, and K), alkaline earth metal (e.g., Mg), or first-row transition metal (e.g., Fe and Zn) [15
]. The electrolyte can be aqueous (Zn–air, Fe–air, Al–air, and Mg–air) or non-aqueous (Li–air, Na–air, and K–air). Based on the electrolyte used, metal–air batteries can be classified as acidic, neutral, or alkaline [16
]. As shown in Figure 2
, during discharge, oxygen transforms to hydroxide ions at the cathode and metal transforms to metallic ions at the anode. The hydroxide ion functions as the main charge carrier. Figure 2
shows the main processes involved in alkali metal (Zn, Fe, Al)–air batteries, as they are the main concern of this review paper.
Alkaline metal–air batteries (Zn–air, Al–air and Fe–air) are known to be inexpensive and non-toxic [17
]. These batteries, due to their lower reactivity, easier handling, and safety, can be chosen over Li-ion batteries. Similarly, there is an intense interest in rechargeable Zn–air batteries, since Zn is the most active metal (less passive) that can be plated from an aqueous electrolyte [18
]. Fe–air batteries are also very interesting as there is no dendrite formation in the negative electrode, unlike Zn–air batteries [19
]. Equally important is the fact that these batteries use the most abundant elements in the Earth’s crust, which makes them very promising as cheap energy storage devices; indeed, the elements involved, O2
, Al, Fe, and Zn are the 1st, 3rd, 4th, and 24th most abundant elements in the Earth’s crust, respectively.
As shown in Table 1
, due to their open configuration, the oxygen can be received from the atmosphere instead of prior incorporation, thus giving the cells high theoretical energy densities [20
]. Therefore, they can be applicable across a wide range of energy transfer stations and energy storage device applications, including automotive and larger passenger vehicles and large stations for stationary application. Table 1
summarizes the reactions, nature, and characteristics of the three alkali metal–air batteries.
Research works on alkali metal–air batteries began earlier than those on Li-ion batteries. For instance, the first Zn–air battery was designed in 1878 (Figure 3
) and commercialized in 1932 [33
]. It is now one of the most common commercially available metal–air batteries worldwide [16
]. On the other hand, Fe–air and Al–air batteries were developed in the 1960s [34
]. The use of Al metal as anode material was proposed for the first time by Zaromb in 1962. However, surprisingly, despite their early beginning and becoming very promising, none are applicable for large-scale industrial deployment at the moment [15
]. Among the alkali metal–air batteries, electrically rechargeable Zn–air batteries seem more likelyto become commercially available any time soon. EOS Energy Storage [36
], NantEnergy [37
], and ZincNyx [38
] are companies that recently began offering Zn–air batteries as grid energy storage systems. Moreover, many projects, such as EU-funded projects (including PowAir [39
], ZAS [40
] and FlowCamp [41
]), are underway with the objective of achieving a low-cost, next-generation, rechargeable Zn–air flow battery. Figure 3
shows the major developments during the history of Fe–air, Al–air, and rechargeable Zn–air batteries.
In agreement with the increase in the research devoted to these batteries in the last few years, the number of published papers has shown a steady increase, especially in the last ten years, as shown in Figure 4
. It seems that in recent years, there has been significant research effort and progress with regard to the development of alkali metal–air batteries. In particular, the number of papers published on Zn–air and Fe–air batteries has shown a steady increase in the past 10 years (2011–2019). There were only a few papers published in the years before that (2000–2010, not included in the graph) in all of these batteries. Among the three, Zn–air and Fe–air batteries seem to have higher research activities compared to Al–air batteries, in terms of the number of published papers.
However, despite their early beginning and being active research topics, their development and commercialization have been hampered by several remaining challenges associated with their components, such as the metal anode (corrosion, forming passivation layers, dendritic formation, electrode deformation, and energy loss due to self-discharging), air cathode (lack of efficient catalysts for both oxygen reduction and evolution reactions, affecting electrolyte stability, and gas diffusion blockage by side reaction products), and electrolyte (side reaction with the anode, reaction with CO2
from the air, and low conductivity) [15
] (Table 1
). These problems have been well studied and reviewed in the literature [22
]. As a result of the low shelf-life and cell irreversibility of the batteries, especially for Fe–air and Al–air batteries, they still remain in the early stages of development [20
Another challenge is the lack of a suitable membrane for the batteries. The main roles of membrane are OH−
ion transportation, avoiding mixing and short-circuit, blocking unwanted species crossover (zincate (Zn(OH)42−
) ions in Zn–air battery), and dendrite growth suppression [45
]. The membrane would act as a conductor for the OH−
ions as well as a separator for the electrolyte solution. Different membrane separator types, such as organic polymer porous membranes, inorganic membranes [46
], composite membranes [47
], cation-exchange membranes [48
], and anion-exchange membranes (AEMs) [49
] have been used in Zn–air battery applications.
Commercial Zn–air batteries commonly use laminated separators (typically Celgard®
5550), which is a porous polyolefin separator. Zn(OH)42−
ion crossover due to their open structure has been reported to be the main drawback associated with the use of such porous membrane separators. This gives rise to an increase in cell polarization and loss of capacity with cycling [45
]. This clearly indicates the substantial need to control the Zn(OH)42−
ion crossover [50
]. To minimize this problem, techniques such as filling pores with inorganic particles (composite membrane) [47
] and coating with anion-selective polymers have been employed [50
]. The former has been reported to significantly increase the membranes’ resistance (applying 0.3 g Mn(OH)2
on one or two 10 cm2
3401 resulted in resistance three orders of magnitude larger), while the latter has not yet been well explored.
To completely solve the problem, the use of AEMs that are selective to the passage of OH−
ions has been widely recommended [25
]. Various advantages can be mentioned from using AEM in alkali metal–air batteries, especially in Zn–air batteries: (i) minimizing or avoiding zincate ion permeation toward the air electrode, (ii) reducing the tendency to form and shape change of dendrites, and (iii) preventing leaching of catalysts from the air electrode to the Zn electrode. Moreover, the ion exchange membrane may also help to prevent the breakdown of the air electrode and to assist in the maintenance of a stable three-phase boundary at the air electrode [51
The use of AEM as a separator in these systems is analogous to the famous use of Nafion®
(and other cation-exchange membranes) in vanadium redox flow batteries, in which VO2+
serve as positive and negative redox couples, respectively, separated by an ion-exchange membrane [54
]. The well-designed, defined nanochannel morphologies of Nafion®
enable it to have high proton conductivity, whereas the selectivity of the membrane is controlled by the size of the nanochannel. Furthermore, to reduce the possible permeation of VO2+
and increase the ion selectivity of Nafion®
, various modification methods, including composite membranes, have been proposed [58
On the other hand, even though the promise of AEMs for Zn–air batteries was identified a long time ago, it remains a surprisingly underinvestigated topic to date, despite the wide recommendations and huge potential. As a result, it remains almost unclear whether AEMs can be used practically in the long term in alkali metal–air batteries [60
]. On top of that, there are only a few companies providing robust AEMs with potential applications in an alkaline environment, and there is not much information available in the literature with regards to most of these membranes’ practical applications in alkali metal–air batteries.
In this review paper, we address the recent advances and remaining challenges (and strategies to solve them) of AEMs in alkali metal–air batteries, mainly Zn–air batteries, which have received the most attention. However, it should be noted that similar performance and challenges can be expected by applying AEMs in other alkali metal–air batteries. The review is divided into three parts. First, recent advances in AEMs for zinc–air batteries are discussed. Next, the remaining current challenges of AEMs and strategies to solve these problems are provided. In the end, we present a summary and outlooks on the topic.
2. Advances in AEMs for Alkali Metal–Air Batteries
A literature survey was done and analyzed to understand the weaknesses and strengths of the AEMs reported in the literature and commercial AEMs, such as A201®
(Tokuyama Corporation, Japan) and FAA-3®
(FumaTech, Germany) when used in alkali metal–air batteries. FAA®
-3 is a slightly cross-linked, non-reinforced AEM consisting of a polyaromatic backbone with a quaternary ammonium group. FAA®
-3-50 is 45–55 µm thick and has an ion-exchange capacity (IEC) of 2 meq/g in chloride form [61
(IEC = 1.7 mmol/g and 28 µm thick) employees quaternary ammonium groups and hydrocarbon main chain [64
] AEMs used in Zn–air batteries are mainly discussed, as these batteries are better explored compared to the others. Among the commercial AEMs, A201®
membrane has been most tested in rechargeable Zn–air batteries. Moreover, in addition to commercial AEMs, preparation of AEMs for Zn–air batteries has been reported in the literature. The cycling stabilities of the batteries have been found to be dependent on ionic conductivity, zincate diffusion (selectivity), water uptake capacity, and anisotropic swelling ratio of the membranes. Anisotropic swelling of membranes is defined as the ratio of through-plane to in-plane swelling of the membrane. Moreover, prospects on the possible use of other commercial AEMs have also been investigated.
Recently, Abbasi et al. [66
] prepared poly (p-phenylene oxide) (PPO)-based AEMs using three different cations—trimethylamine (TMA), 1-methylpyrolidine (MPY), and 1-methylimidazole (MIM)—and tested them in a Zn–air battery. PPO-TMA and PPO-MPY exhibited low zincate diffusion coefficients (1.13 × 10−8
, and 0.28 × 10−8
/min, respectively) and high discharge capacity (about ~800 mAh/gZn
using PPO-TMA). The PPO-TMA membrane was reported to have low conductivity (0.17 mS/cm) despite its high water uptake (89 wt.%). On the other hand, the membranes showed good alkaline stability in a solution typically used in Zn–air batteries (7 M KOH solution at 30 °C) for at least 150 h. Moreover, PPO-TMA showed good electrochemical stability in a range of −1.5 to +1.5 V (stability window of 3 V). It is a well-established fact that PPO-TMA membranes undergo an SN
2 hydroxide attack [67
]. In this degradation process, the C–N bond electrons move towards the nitrogen while the OH−
forms a new bond with the α carbon, producing trimethylamine and benzyl alcohol. Therefore, the relative alkaline stability of the current membranes could be due to the low temperature and the reasonable duration of the test.
In another study, a polysulfonium-cation-based AEM was fabricated and used in a Zn–air battery [49
]. Compared to Celgard®
5550, the prepared membrane demonstrated better ionic selectivity. As a result, the capacity was 6-fold higher than that of reference membrane during discharge. However, the species crossing over was mistakenly considered to be Zn2+
, rather than Zn(OH)42−
. Moreover, the cyclability of the battery was not studied.
A porous alkaline-exchange membrane based on quaternary ammonium (QA)-functionalized nanocellulose (2-QAFC, cellulose nanofibres modified with 200 mol. % concentration of dimethyloctadecyl [3-(trimethoxysilyl) propy l] ammonium chloride) exhibiting high hydroxide ion conductivity (21.2 mS/cm) and water swelling (95.6%) was developed [68
]. Figure 5
presents the procedure followed for the preparation of the 2-QAFC membrane (Figure 5
a), galvanostatic discharge of solid-state Zn–air batteries using the 2-QAFC, A201®
and KOH-PC membranes (1 M KOH-doped pristine cellulose membranes) (Figure 5
b), and galvanostatic charge and discharge cycling (Figure 5
c). Both the prepared 2-QAFC (30 µm thickness) and commercial A201®
membranes (28 µm thickness) were tested in a flexible, solid-state rechargeable Zn–air battery. Initially, the A201®
-based battery had a higher discharge voltage; however, it was quickly surpassed by the 2-QAFC-based battery, showing a voltage plateau about 180 mV higher and a higher discharge capacity.
-based battery exhibited a rapid voltage and capacity loss, which could have been due to the progressive loss of water and ionic conductivity in the membrane during the constant current applied. It seems that water consumption during oxygen reduction in air electrode leads to electrolyte drying problems and a shortened battery life. Since water plays an important role in the ion transport, its loss can directly reduce ionic transport limitation inside the air electrode (decrease of the OH−
mobility, degrading the catalyst/electrolyte interface) and inside the membrane, resulting in a large ohmic polarization of the battery. Nevertheless, it must be noted that by wetting the membrane with distilled, de-ionized water, it is possible to regenerate the performance of the battery. As a consequence, the A201®
-based battery deteriorated after a few cycles (Figure 5
c), showing large discharge and charge polarizations.
On the other hand, the 2-QAFC-based battery exhibited superior cycling stability in both the charge and discharge (Figure 5
c). This superior cycling stability was assumed to be due to the battery’s holding a higher amount of water (95.6%, OH-
form) and having a smaller anisotropic swelling ratio (1.1) of 2-QAFC than A201®
membrane (44.3% water uptake and 4.4 anisotropic swelling ratio). In other words, the 2-QAFC membrane could tolerate the periodic stress and dehydration during the discharge and charge processes.
Similarly, a QA-functionalized, crosslinked nanocellulose/graphene oxide (QAFCGO) membrane was prepared and assembled in a flexible rechargeable Zn–air battery [69
]. Batteries employing the QAFCGO and A201®
membranes exhibited similar high open-circuit voltages (≈1.4 V). The QAFCGO-based battery showed a better performance compared to the A201®
-based battery, with smaller over potentials for both discharge and charge processes. At high current densities (above 20 mA/cm2
), the QAFCGO-based battery showed a remarkable advantage over the A201®
-based battery (Figure 6
a). As shown in Figure 6
b, the QAFCGO-based battery exhibited much higher cycling stability performance than that of A201®
-based battery. Furthermore, the former battery had higher peak power density (44.1 mW/cm2
) than the latter (33.2 mW/cm2
) (Figure 6
Similarly to the finding of Fu et al. [68
], the A201®
-based battery showed a clear performance decline (with large charge and discharge polarizations) after relatively few cycles (Figure 6
b). On the other hand, the QAFCGO-based battery was reported to continue without any sign of performance loss after 30 cycles. As clearly noted previously, the superior cyclability and performance stability of the QAFCGO-based battery compared to the A201®
-based battery was attributed to the QAFCGO membrane’s higher water uptake (5 times higher than that of the A201®
membrane) and smaller anisotropic swelling degree (half of that of the A201®
Furthermore, AEMs composed of both cross-linked chitosan (CS) and poly (diallyldimethylammonium chloride) (PDDA) and A201®
membranes were tested in all-solid-state Zn–air batteries [60
] (Figure 7
a). The prepared CS-PDDA membrane exhibited high OH−
conductivity (24 mS/cm), strong alkaline stability (216 h at 8 M KOH), and a low degree of anisotropic swelling (1.7), all of which are very important membrane properties required for long-term and superior electrochemical performance in all-solid-state Zn–air battery. The CS-PDDA-based battery exhibited a high open-circuit voltage (1.3 V) and superior peak power density to the A201®
-based cell (48.9 vs. 41.4 mW/cm2
) under the same measurement conditions (Figure 7
b). Additionally, the CS-PDDA-based battery initially had a higher discharge voltage (1.14 vs. 1 V), and exhibited lower discharge and charge polarization and longer cycle times (even if only a few cycles were shown) than the battery with the commercial A201®
membrane (Figure 7
As mentioned in previous studies [69
], the superior performance of the prepared membrane over A201®
was due to its smaller anisotropic swelling and higher water uptake (4 times higher water uptake than the A201®
membrane). It should be noted that the anisotropic swelling ratio of commercial A201®
membrane was different in all the studies, indicating the lack of a standardized testing protocol.
Moreover, in addition to polysulfonium and QAs, imidazolium cations have been used to prepare AEMs for Zn–air batteries. Zarrin et al. [70
] prepared a graphene oxide membrane functionalized with 1-hexyl-3-methylimidazolium chloride molecules (HMIM/GO) with potential for wearable electronics, including flexible Zn–air batteries. The prepared 5-HMIM/GO (5 refers to weight ratio of HMIM to GO, 27 μm) and A201®
membranes were tested in flexible Zn–air batteries. The 5-HMIM/GO membrane was reported to have a hydroxide conductivity of 44 mS/cm at room temperature and 30% relative humidity. Both membranes exhibited stable charge/discharge performances for 60 cycles. The 5-HMIM/GO-membrane-based flexible Zn–air battery exhibited a charge–discharge voltage polarization at low relative humidity and room temperature that was comparable to that of A201®
-based battery in a humidified environment. This was attributed to the high rate of ion transfer of the former membrane in the studied conditions.
All in all, all the presented studies indicate the need for development of an alkaline AEM with high hydroxide conductivity (at room temperature) and smaller anisotropic swelling degree. It can be concluded that the A201® membrane may not be practical for long-term rechargeable Zn–air batteries; in the studied systems, it was reported that its performance began to deteriorate after only a few hours. Therefore, in addition to preparing new, high-performing AEMs, testing other commercially available AEMs should be done to understand and determine their potential applications in such batteries.
Alkaline AEMs from Fumatech BWT GmbH (typically, fumapem®
FAA and fumasep®
FAP) are suggested by the company to be suitable separators for Zn–air batteries [71
]. However, there have not been many studies in the literature to date about their practical use. Anion-exchange polymer (AEP) resin (FAA®
-3-SOLUT-10 in NMP, Fumatech BWT GmbH) was used to prepare a separator and used as the separator to prepare transparent, bendable secondary Zn–air batteries [72
]. The membrane was prepared using a solution (10% of AEP solution) casting method. The produced battery exhibited a maximum power density of 9.77 mW/cm2
. The cells were reported to be stable for at least 100 cycles. In another study, a fumatech®
-FAA membrane doped with KOH was used to prepare the membrane electrode assembly for a Zn–air battery [73
]. The battery exhibited a peak power density of 170 and 164 mW/cm2
based on Fe-LC-900 (FeCl3
–leather, pyrolysis temperatures of 900 °C) and Pt/C-catalyst-based air electrodes, respectively. However, in both studies, not much information was providing regarding the effects, weakness, and strength of the membranes used.
According to the technical datasheet provided by the company, fumapem®
FAA-3-50 membrane, in its OH−
form, has 40 wt. % H2
O uptake and a dimensional swelling (in H2
O) of 17% at 25 °C [74
]. The membrane’s in-plane swelling ratio, and thus anisotropic swelling ratio, has not been reported to date. All in all, considering its relatively low water uptake, low performance can be expected. This is due to potential periodic stress and dehydration of the membrane, similarly to A201®
membrane. However, testing in a real system is the only way to observe and understand its real strengths and weaknesses.
4. Summary and Outlooks
In summary, this paper briefly reviewed the recent advances of AEMs for alkali metal–air batteries. Metal–air flow batteries have the potential to become the energy storage of choice in the future because of their low cost, environmental friendliness, and huge energy storage capacity. As a result, in recent years, they have been given intense research and development. To achieve their goal, an excellent cathode, anode, and electrolyte are required in addition to suitable, excellent, and low-cost membrane.
One of the many challenges preventing these batteries (typically, rechargeable Zn–air batteries) from reaching their full potential as the next generation of electrochemical energy storage is the lack of suitable membrane separators. Despite their critical importance, the preparation and development of suitable membranes have been given only a little attention. Various membranes, including porous membranes and modified porous membrane separators, have been tested as separators. Usually, porous membrane separators taken from Li-based batteries are used. Despite their superior mechanical strength and broad electrochemical stability window, they are not able to block the crossover of zincate ions due to their too large pores, thus leading to increased cell polarization and decreased long-term durability of the batteries. To solve this problem, composite and cation-exchange membranes have tested; however, their OH− conductivities have been reported to be low.
The use of AEMs could address this problem by selectively limiting the crossover of unwanted ions. Despite these high recommendations, there have been few papers published studying the performance of alkaline AEMs in alkali metal–air batteries. In this review paper, correlations between the properties of the AEMs (such as conductivity, water uptake, and swelling ratio) and battery performances (such as capacity and cyclic performance stability) were established. This review mainly discussed the use of AEMs in Zn–air batteries. However, similar results and challenges should be expected in other batteries as well.
Among the commercial membranes, the A201® membrane has been relatively tested to a certain extent. Since the membranes were not initially designed and tuned for this specific application, the performance and durability of A201®-based batteries are limited. The A201® membrane’s high anisotropic swelling ratio and relatively low uptake have been critically found to be the main reasons. On the other hand, AEMs specifically designed for these applications have been shown to be promising. Based on the collected results of membrane properties and battery performances, AEMs with small anisotropic swelling and high water uptake are required for superior performance of batteries. An AEM with structural stability via decrease of swelling in both the through-plane and in-plane directions is needed for long-term battery cycling operation. Testing other commercially available AEMs in alkali metal–air batteries is also needed.
To fully implement these membranes, there are some challenges that need to be overcome first. The main problem associated with the use of AEMs is their limited alkaline stability. Different strategies have been proposed and employed to improve and solve this problem. The major degradation mechanisms, especially for the commonly used cations and polymer backbones, are discussed in this review. Moreover, a perspective on how to avoid these degradations is outlined. For example, quaternized, aryl-ether-free polyaromatics are promising AEM materials because of their outstanding alkaline stability. In general, polymers that are free of electron-withdrawing groups in their backbone such as poly(phenylene) and carbon–carbon bonded polymers have shown promising results and are expected to provide the required level of stability. On the other hand, the stability of cations at harsh and high temperatures is a hot research topic at the moment. AEMs incorporating relatively stable cations and unique aromatic polymers free of electron-withdrawing groups in the backbone have been reported with promising results. Additionally, the morphology of AEMs has been reported to play a vital role in determining the rate of chemical degradation. Therefore, more efforts are needed to develop more stable membranes by employing these strategies in combination.
The degradation of membranes by OH− is known to speed up with increasing temperature. However, unlike most AEM fuel cells, the operating temperature of alkaline metal–air batteries is room temperature. This low-temperature operation plays an important role in increasing the lifespan of the membrane. AEMs normally regarded to be less stable (tests performed at 80 °C) have shown good alkaline and electrochemical stability in Zn–air batteries when tested at room temperature.
Another possible problem associated with the use of AEMs in alkali metal–air batteries application is their low ion conductivity at low temperatures. The low conductivity of these membranes is due to the large size of hydroxide ions (compared to protons) and to the carbonation process. There has been a lot of improvement in this regard as well. AEMs that are highly conductive have been reported in the literature. However, with an increase in water uptake and hydroxide conductivity, care must be taken not to affect the integrity and selectivity of the membranes. Therefore, an adequate conductive membrane with unaffected mechanical/structural integrity and selectivity is required.
Another vital property of AEMs, which should be given huge attention, is the size of their ionic nanochannels, and their conductivity and orientation, in order to decrease the membrane tortuosity and increase the through-plane membrane conductivity. The size of the nanochannels determines the selectivity of the membrane. Therefore, attention should be given to membrane synthesis in order to form channels with an optimal size, which would allow the passage of hydroxide ions and block other species such as zincate ions.
Moreover, the lack of standardizing testing protocols is another challenge. For instance, the water uptake and anisotropic swelling ratio even of a commercial A201® membrane has been found to be different in different studies, indicating the need for a standardized the testing protocol.