Anion Exchange Membranes with 1D, 2D and 3D Fillers: A Review

Hydroxide exchange membrane fuel cells (AEMFC) are clean energy conversion devices that are an attractive alternative to the more common proton exchange membrane fuel cells (PEMFCs), because they present, among others, the advantage of not using noble metals like platinum as catalysts for the oxygen reduction reaction. The interest in this technology has increased exponentially over the recent years. Unfortunately, the low durability of anion exchange membranes (AEM) in basic conditions limits their use on a large scale. We present in this review composite AEM with one-dimensional, two-dimensional and three-dimensional fillers, an approach commonly used to enhance the fuel cell performance and stability. The most important filler types, which are discussed in this review, are carbon and titanate nanotubes, graphene and graphene oxide, layered double hydroxides, silica and zirconia nanoparticles. The functionalization of the fillers is the most important key to successful property improvement. The recent progress of mechanical properties, ionic conductivity and FC performances of composite AEM is critically reviewed.


Introduction
The anion exchange membranes (AEM) are the central element of many technologically relevant devices [1][2][3], first and foremost alkaline membrane fuel cells (FC) [4][5][6]. These fuel cells can significantly reduce the amount of noble metal catalysts for the oxygen reduction reaction (ORR) and may represent the future of FC development. However, although AEM may revolutionize future fuel cell technology, currently they lack several properties that are paramount for the viability of this technology, including long-term stability in the alkaline medium that impacts their durability in operating conditions [7][8][9][10]. One favoured way to improve the properties of ionomeric and more generally polymeric membranes is the development of composite materials, especially using inorganic fillers that are supposed to enhance the mechanical properties and the FC performance. The reduction in ionic conductivity consecutive to the addition of an inert second phase can be mitigated by fillers that present an intrinsic ionic conductivity or by space charge effects near the interfaces between ionomer and inorganic nanoparticles. Furthermore, the composite material may in certain cases show higher long-term stability in alkaline conditions by synergistic effects, due, e.g., to a physical cross-linking (such as acid-base interactions) or a partial crystallization of the polymer near the inorganic phase. Although the application of nanoparticle fillers is promising, their practical investigation started in earnest only a decade ago, as shown in the number of publications and citations on the topic "composite anion exchange membranes" (Figure 1). No comprehensive and critical review exists on this topic, although some related topics have been addressed in the last years [11][12][13][14][15]. review exists on this topic, although some related topics have been addressed in the last years [11][12][13][14][15]. This review intends thus to summarize the main findings reported in the literature since 2010, including AEM for use in fuel cells, but also in redox flow batteries and water purification applications. We chose a simple and coherent presentation of this wide field by subdividing the literature according to the dimensionality of the fillers. We start from 1D materials, essentially nanotubular solids, including titanate and carbon nanotubes (NTs). Titanate NT exhibit high chemical stability, high specific surface area and good performance in alkaline environments and their aspect ratio can increase the mechanical properties. Carbon NT are also characterized by extreme strength and flexibility so that an increase in mechanical properties is expected. Organic functionalization is the key to obtaining high ionic conductivity. The following chapter summarizes 2D materials with a large part on graphene and graphene oxide (GO). Graphene and GO are mechanically extremely resistant and at the same time present great flexibility. Composites, especially with functionalized graphene and/or reduced GO, are expected to have improved FC performance and conductivity thanks to a better filler distribution and sometimes with the help of ionic liquids. Layered This review intends thus to summarize the main findings reported in the literature since 2010, including AEM for use in fuel cells, but also in redox flow batteries and water purification applications. We chose a simple and coherent presentation of this wide field by subdividing the literature according to the dimensionality of the fillers. We start from 1D materials, essentially nanotubular solids, including titanate and carbon nanotubes (NTs). Titanate NT exhibit high chemical stability, high specific surface area and good performance in alkaline environments and their aspect ratio can increase the mechanical properties. Carbon NT are also characterized by extreme strength and flexibility so that an increase in mechanical properties is expected. Organic functionalization is the key to obtaining high ionic conductivity. The following chapter summarizes 2D materials with a large part on graphene and graphene oxide (GO). Graphene and GO are mechanically extremely resistant and at the same time present great flexibility. Composites, especially with functionalized graphene and/or reduced GO, are expected to have improved FC performance and conductivity thanks to a better filler distribution and sometimes with the help of ionic liquids. Layered double hydroxides (LDHs) can mitigate the loss of hydroxide ion conductivity, being themselves anion conducting materials. They can also improve the mechanical properties even at 100% RH. The final part discusses 3D fillers, with a large section on SiO 2 and silicates, as well as other metal oxides including alumina, titania and zirconia nanoparticles as well as metal-organic frameworks (MOFs). The organic functionalization of the oxides plays a decisive role in increasing the performance of composite materials. The first type of polymers used as matrix are inert, non-ionic, used as an absorbent for alkaline solutions, including synthetic poly(vinyl alcohol) with the general formula [-CH 2 CH(OH)-]n and chitosan, an aminated polysaccharide, composed of N-acetyl-D-glucosamine and D-glucosamine linked by β (1)(2)(3)(4) bonds. The second type of matrix are ionomers that present an intrinsic ionic conductivity. They include fluorinated and hydrocarbon polymers, especially commercial, inexpensive aromatic polymers such as poly (2,6-dimethyl-1,4-phenylene oxide) (PPO) and polysulfone (PSU). Both are thermoplastics, known for their toughness, resistance at high temperature and especially PPO for alkaline stability.
We cover in this review particularly ionic conductivity, mechanical properties and device performance, especially in fuel cells. Several tables in the manuscript summarize information on the reviewed AEM.

Titanate (TNTs) and Halloysite Nanotubes (HNTs)
Titanate nanotubes (TNTs) were first described in the 1990s. These nanostructured tubular materials present excellent characteristics and performances [16]. Among them, titanium dioxide TiO 2 NT exhibit high chemical stability and catalytic activity, a high specific surface area, strong metal-support interaction, and good performances in alkaline and acidic environments. TiO 2 NT have a one-dimensional (1D) structure and exists as bundled tubes with a length between 1 and 220 µm and a diameter between 30 and 80 nm. The morphology of TNT depends on which synthesis method was used, including electrochemical anodization [17,18], a template-assisted method, sol-gel chemistry [19], etc. given all these characteristics, among other applications, TNT are used as filler in fuel cell membranes [16].
One of the earliest examples of composite AEM with TNT was presented in 2018 by Elumalai and Sangeetha [20]. The same authors incorporated TNT covalently linked with an imidazolium-based ionic liquid (IL) into quaternized polysulfone (QPSU) with triethylamine (TEA) [21]. The presence of the IL increased the IEC; the best conductivity was observed for 5 wt% IL-TNT (21 mS cm −1 , 30 • C), due to the uniform distribution of the filler without any cluster formation. The same composite membrane reached a maximum tensile strength (TS) of 43 MPa and a fuel cell power density of 302 mW cm −2 at 60 • C under 80% RH. Quaternized TNT (QTNT) were also incorporated in quaternized poly(arylene ether ketone) (QPAEK) [22].
The incorporation of 5 wt% of HNTs elevated the TS and Young's modulus (E) to 52 MPa and 1100 MPa, respectively. The membranes containing QHNTs functionalized with carbethoxyl groups showed the best hydroxide conductivity under hydrated conditions: with 7.5 wt% the conductivity reached 17 mS cm −1 at 90 • C, which is still modest.

Carbon Nanotubes (CNTs)
Carbon nanotubes are a relaxation of the spherical structure of fullerene rolled up on itself, obtaining the typical cylindrical structure. They can be divided into two types:  The incorporation of 5 wt% of HNTs elevated the TS and Young's modulus (E) to MPa and 1100 MPa, respectively. The membranes containing QHNTs functionalized w carbethoxyl groups showed the best hydroxide conductivity under hydrated conditio with 7.5 wt% the conductivity reached 17 mS cm −1 at 90 °C, which is still modest.

Carbon Nanotubes (CNTs)
Carbon nanotubes are a relaxation of the spherical structure of fullerene rolled up itself, obtaining the typical cylindrical structure. They can be divided into two types: Very promising in their electrical properties, they are also characterized by extre strength and flexibility, making them suitable for use as reinforcing fibers in composi Several authors incorporated functionalized multi-walled carbon nanotubes (MWCN into various types of AEM.

Poly(vinyl alcohol) (PVA)
Poly(vinyl alcohol) (PVA) is an inert polymer, soluble in warm water, that acqu ion conductivity through the addition of KOH or other alkaline solutions. One of the f examples of composite with PVA and MWCNT was reported by Pan et al. in 2011 a applied in direct methanol alkaline fuel cells (DMAFC). The methanol permeability creased from ~3.6 × 10 −7 cm 2 s −1 for pristine PVA to 3.0 × 10 −7 cm 2 s −1 for composite me branes. The conductivity increased with the presence of MWCNT and was reported tween 52 and 118 mS cm −1 (in 2 M and 6 M KOH at 30-60 °C). Figure 3 presents DMAFC performance and the improvement due to the incorporation of CNTs [24]. Very promising in their electrical properties, they are also characterized by extreme strength and flexibility, making them suitable for use as reinforcing fibers in composites. Several authors incorporated functionalized multi-walled carbon nanotubes (MWCNTs) into various types of AEM.

Poly(vinyl alcohol) (PVA)
Poly(vinyl alcohol) (PVA) is an inert polymer, soluble in warm water, that acquires ion conductivity through the addition of KOH or other alkaline solutions. One of the first examples of composite with PVA and MWCNT was reported by Pan et al. in 2011 and applied in direct methanol alkaline fuel cells (DMAFC). The methanol permeability decreased from~3.6 × 10 −7 cm 2 s −1 for pristine PVA to 3.0 × 10 −7 cm 2 s −1 for composite membranes. The conductivity increased with the presence of MWCNT and was reported between 52 and 118 mS cm −1 (in 2 M and 6 M KOH at 30-60 • C). Figure 3 presents the DMAFC performance and the improvement due to the incorporation of CNTs [24].  PVA-functionalized MWCNTs were also prepared using an ozone-mediated method [25]. The MWCNTs were added to PVA solution and stirred at 80 °C for 3 h to graft the PVA polymer onto the MWCNT. The dry PVA/MWCNT films were then immersed in 1-8 M KOH solutions. PVA containing 0.05% of functionalized MWCNT exhibited the maximum power density of 39 mW cm −2 obtained with 2 M MeOH and 6 M KOH at 60 °C, consistent with the previous work.
Functionalized MWCNTs with pendant Fe3O4 nanoparticles (FeCNT) were incorporated into a PVA film [26]. High resolution HRTEM images showed a diameter of the PVA-functionalized MWCNTs were also prepared using an ozone-mediated method [25]. The MWCNTs were added to PVA solution and stirred at 80 • C for 3 h to graft the PVA polymer onto the MWCNT. The dry PVA/MWCNT films were then immersed in 1-8 M KOH solutions. PVA containing 0.05% of functionalized MWCNT exhibited the maximum Functionalized MWCNTs with pendant Fe 3 O 4 nanoparticles (FeCNT) were incorporated into a PVA film [26]. High resolution HRTEM images showed a diameter of the MWCNT of 20−30 nm with Fe 3 O 4 nanoparticles on the MWCNT surface. For a PVA-0. 15 FeCNT sample an open-circuit potential of 0.87 V and a maximum power density of 88 mW cm −2 were obtained in DMAFC (2 M MeOH in 6 M KOH at 60 • C). To improve the chemical compatibility between CNTs and PVA, the CNTs were functionalized with PVA chains [27]. The maximum power density for 3 M EtOH in 5 M KOH solution was 65 mW cm −2 for the composite and 31 mW cm −2 for pristine PVA film.
Membranes composed of a semi-interpenetrating network structure of cross-linked PVA, poly-diallyldimethylammonium chloride (PDDA) and hydroxylated MWCNTs-OH were tested in AEMFC [28]. The cross-link was obtained by thermal treatment of PVA and subsequent immersion in acid solution with 10 wt% glutaraldehyde (GA). The tensile stress at break of PVA/PDDA membranes with 3 wt% MWCNTs-OH was 40 MPa, the elongation at break (%) and Young's modulus were 12% and 783 MPa, respectively. The maximum OH − conductivity was 30 mS cm −1 at RT with an improved alkaline stability. The (PVA/PDDA/1 wt% MWCNTs-OH) MEAs in an AEMFC with 0.5 mg Pt cm −2 at the anode and cathode side attained a power density of 66 mW cm −2 at 40 • C; using 3.2 mg cm −2 of cobalt phthalocyanine at the cathode side, the power density decreased to 14.0 mW cm −2 .

Chitosan (CS)
Only a few examples of composites containing chitosan and CNTs are present in the literature. CS was used as a matrix for MWCNTs-OH and ionic liquids quaternized with isoquinoline moieties bearing ammonium groups [29]. Quaternized chitosan (QCS) and functionalized CNTs were prepared with the aim to improve the mechanical properties [30]. CNTs were also functionalized with quaternized silica obtained by in situ sol-gel methods [31]. The incorporation of 5 wt% of functionalized CNTs enhanced the conductivity to 43 mS cm −1 at 80 • C, about two times higher than bare QCS. DMAFC tests with composite membranes produced a maximum power density at 60 • C of 81 mW cm −2 , higher than pure QCS (52 mW cm −2 ).

Aromatic Polymers
Except in one case [32], the imidazolium group (Im) was the preferred choice in all composites containing aromatic polymers.
Jin and Bai in 2013 blended methyl-Im quaternized PPO (PPO-MIm) with MWCNTs functionalized with a polymeric ionic liquid containing Im moieties (PIL(BF4)-MWCNTs) [33]. The conductivity increased in the presence of the filler (95% more with respect to PPO-MIm) together with an enhancement in TS up to 13 MPa.
Imidazolium ionic liquids (ImILs) modified carbon nanotubes (IL@CNT) were also inserted into imidazolium-modified poly(ether ether ketone) (Im-PEEK) [34]. Two types of ionic liquids (IL-M and IL-B) with different alkaline stability and chain length were chemically attached to CNTs (Figure 4). The authors stated that the introduction of IL@CNT provided additional ion hopping positions and 1D long range ion-conducting channels.
The composite containing IL-B@CNT showed a higher conductivity (135 mS cm −1 at 70 • C and 100% RH) confirming the beneficial effect of the long chain (pure Im-PEEK 80 mS cm −1 ), while the residual hydroxide conductivity (Im-PEEK/IL-B@CNT-8, after 48 h in 2 M KOH at 50 • C) was 77%. The peak power density in AEMFC was 81 mW cm −2 at 50 • C [34]. PPO-MIm) together with an enhancement in TS up to 13 MPa.
Imidazolium ionic liquids (ImILs) modified carbon nanotubes (IL@CNT) were also inserted into imidazolium-modified poly(ether ether ketone) (Im-PEEK) [34]. Two types of ionic liquids (IL-M and IL-B) with different alkaline stability and chain length were chemically attached to CNTs (Figure 4). The authors stated that the introduction of IL@CNT provided additional ion hopping positions and 1D long range ion-conducting channels. The composite containing IL-B@CNT showed a higher conductivity (135 mS cm −1 at 70 °C and 100% RH) confirming the beneficial effect of the long chain (pure Im-PEEK 80 mS cm −1 ), while the residual hydroxide conductivity (Im-PEEK/IL-B@CNT-8, after 48 h in 2 M KOH at 50 °C) was 77%. The peak power density in AEMFC was 81 mW cm −2 at 50 °C [34].
Im-PEEK was also used as a hosting matrix for poly(vinyl imidazole) functionalized carbon nanotubes (PVI@CNT) [35]. The imidazolium groups of the filler reacted with the Im-PEEK to form a cross-linking structure along the nanotubes. The nano-hybrid membrane with 15 wt% of the filler (Im-PEEK/PVI@CNT-15) showed a conductivity of 121 mS cm −1 at 70 °C and 100% RH. According to the authors, the improvement of the hydroxide conductivity was due to additional anion transport pathways built along the interface between the PVI@CNT and the polymer matrix, with an increase in the connectivity and the ion transport. The maximum power density in a single H2/O2 fuel cell was 129 mW cm −2 at 60 °C under 100% RH.
Via co-electrospinning, an imidazolium functionalized MWCNTs was incorporated into imidazolium functionalized polysulfone (Im-PSU) [36]. In fully hydrated conditions, the tensile stress increased from 5.6 MPa for a cast Im-PSU to 24.4 MPa for electrospun ImPSU with 0.2 wt% of MWCNTs. The maximum value of OH-conductivity was obtained Im-PEEK was also used as a hosting matrix for poly(vinyl imidazole) functionalized carbon nanotubes (PVI@CNT) [35]. The imidazolium groups of the filler reacted with the Im-PEEK to form a cross-linking structure along the nanotubes. The nano-hybrid membrane with 15 wt% of the filler (Im-PEEK/PVI@CNT-15) showed a conductivity of 121 mS cm −1 at 70 • C and 100% RH. According to the authors, the improvement of the hydroxide conductivity was due to additional anion transport pathways built along the interface between the PVI@CNT and the polymer matrix, with an increase in the connectivity and the ion transport. The maximum power density in a single H 2 /O 2 fuel cell was 129 mW cm −2 at 60 • C under 100% RH.
Via co-electrospinning, an imidazolium functionalized MWCNTs was incorporated into imidazolium functionalized polysulfone (Im-PSU) [36]. In fully hydrated conditions, the tensile stress increased from 5.6 MPa for a cast Im-PSU to 24.4 MPa for electrospun ImPSU with 0.2 wt% of MWCNTs. The maximum value of OH-conductivity was obtained for 0.4 wt% of FMWCNT with 100 mS cm −1 at 60 • C, but the residual conductivity after immersion in 1 M NaOH at 60 • C for 168 h was 62%. Table 1 summarizes information on the various composites, including polymer and 1D filler type, remarks and references. Im-PEEK poly(vinyl imidazole)-CNT maximum power density 129 mW cm −2 at 60 • C (AEMFC) [35] Im-PSU Im-MWCNTs TS 24 MPa (0.2 wt%) [36] We have seen in this section different 1D nanofillers such as CNT, halloysite and titanate NT, commonly used for the fabrication of hybrid materials. One-dimensional tubular and well designed nanofillers can provide ion conducting groups, construct ion conducting pathways along the interphase region and improve the anti-swelling property of the nanohybrid membrane. For these reasons, as shown previously, both the mechanical properties and the residual conductivities after the alkaline test increase, and above all the performances in FC show an improvement compared to the bare polymers.

Layered Double Hydroxides (LDHs)
Layered double hydroxides (LDHs) are inorganic lamellar ionic materials belonging to the group of anionic clays and their synthesis has a low cost. The structure of LDH is based on Mg(OH) 2 brucite-type blocks where the replacement of M 2+ with M 3+ cations gives positively charged layers, balanced by mobile anions in the interlayer, which can be reversibly inserted. The lamellae are linked by Van der Waals forces. LDHs with an acceptable anionic conductivity and excellent stability in alkaline media can be used successfully as filler in AEM [37]. Well dispersed LDHs increase the mechanical properties of the matrix and can help to mitigate the loss of conductivity observed in the case of non-conducting fillers.

Poly(vinyl alcohol) (PVA) and Chitosan (CS)
One of the first attempts to use LDH in a composite date back to 2012 when Zhao et al. dispersed LDH in crosslinked PVA [38]. SEM images showed a good homogeneity without aggregates or chunks until 30 wt% of LDH (30LDH). FC with a PVA/20LDH membrane achieved a maximum power density of 82 mW cm −2 at 80 • C.
Exfoliated LDH nanosheets, obtained via the filtration process, were inserted in quaternized PVA [39]. The conductivity of the LDH(NO 3 − ) membrane after exfoliation and restacking was around 200 mS cm −1 at 80 • C, which seems surprisingly high especially for nitrate ions and also higher than the OH − form. The improvement of conductivity was attributed to the exposed surfaces of the nanosheets, the enhanced water uptake, and longrange ordered ionic channels. For the composite with 10 wt% of PVA, (LDH(NO 3 − )/QPVA-10) was reached at the same temperature 172.1 mS cm −1 .
Other examples concern the use of mixed membranes composed of QCS and PVA; they hosted LDH intercalated with glycine betaine [40], an LDH flower-like hierarchical structure wrapped on SiO 2 nanospheres (LDH@SiO 2 ) [41], and carbon nanotubes coated with LDH (LDH@CNTs) [42]. In the last example, LDH nanosheets were anchored on carbon nanotubes by a NH 4 F-assisted in situ coating method ( Figure 5) [42].

Polysulfone (PSU)
An attempt to use an inexpensive commercial polymer was proposed in 2017 by Di Vona et al. [37]. The composites were prepared from QPSU grafted with TMA or 1,4-diazabicyclo[2.2.2]octane (DABCO) and 14 wt% of LDH, synthesized by the urea method with a composition Mg 0.62 Al 0.38 (OH) 2 (Cl) 0.38 ·0.6H 2 O. The ionic conductivity at 60 • C as a function of RH is reported in Figure 6: composite membranes showed lower conductivity values due to the lower water uptake, while polymers with TMA were more conductive because of the higher water content. The mechanical properties in fully humidified conditions were improved, with a three-fold increase in Young's modulus for composites (~600 MPa vs.~200 MPa). The membranes were treated in alkaline conditions at 60 • C without losing their properties. with a composition Mg0.62Al0.38(OH)2(Cl)0.38•0.6H2O. The ionic conductivity at 60 °C as a function of RH is reported in Figure 6: composite membranes showed lower conductivity values due to the lower water uptake, while polymers with TMA were more conductive because of the higher water content. The mechanical properties in fully humidified conditions were improved, with a three-fold increase in Young's modulus for composites (~600 MPa vs. ~200 MPa). The membranes were treated in alkaline conditions at 60 °C without losing their properties. Meanwhile, Zhang prepared LDH by the urea method, which was incorporated from 3 to 10 wt% into TMA-PSU membranes [43]. The 5 wt% LDH nanocomposite membrane showed the highest TS (21 MPa), the lowest elongation at break (11%), and the highest ionic conductivity (24 mS cm −1 at 60 °C). Pizzoferrato et al. proposed a composite membrane based on PSU-DABCO and ZnAl-LDH containing an ionic liquid (1-butyl-3-methylimidazolium hydrogen sulfate, BmimSO4) [44]. This strategy enabled the intercalation of IL into the interlamellar space of LDH with the aim of enhanced ionic conductivity. In fact, at 100% RH and 25 °C, the membranes with 7 wt% of filler reached a conductivity of 25 mS cm −1 in OH − form and maintained 16 mS cm −1 after alkaline treatment in 2 M KOH at 60 °C for 24 h [44].

Poly(phenylene oxide) (PPO)
A porous-sandwich structure was synthesized by electrostatic spraying of ammonium layered double hydroxide (QLDH) on the surface of triple-cation side chains of PPO (TC-PPO) [45,46]. The composite membranes with higher IEC values (from 3.12 to 3.39 meq g −1 ), due to the presence of ammonium groups in QLDH layers, showed a high hydroxide conductivity (122 mS cm −1 at 80 °C). The FC performance at 60 °C achieved a maximum power density of 267 mW cm −2 at a current density of 554 mA cm −2 , values Meanwhile, Zhang prepared LDH by the urea method, which was incorporated from 3 to 10 wt% into TMA-PSU membranes [43]. The 5 wt% LDH nanocomposite membrane showed the highest TS (21 MPa), the lowest elongation at break (11%), and the highest ionic conductivity (24 mS cm −1 at 60 • C). Pizzoferrato et al. proposed a composite membrane based on PSU-DABCO and ZnAl-LDH containing an ionic liquid (1-butyl-3methylimidazolium hydrogen sulfate, BmimSO 4 ) [44]. This strategy enabled the intercalation of IL into the interlamellar space of LDH with the aim of enhanced ionic conductivity. In fact, at 100% RH and 25 • C, the membranes with 7 wt% of filler reached a conductivity of 25 mS cm −1 in OH − form and maintained 16 mS cm −1 after alkaline treatment in 2 M KOH at 60 • C for 24 h [44].

Poly(phenylene oxide) (PPO)
A porous-sandwich structure was synthesized by electrostatic spraying of ammonium layered double hydroxide (QLDH) on the surface of triple-cation side chains of PPO (TC-PPO) [45,46]. The composite membranes with higher IEC values (from 3.12 to 3.39 meq g −1 ), due to the presence of ammonium groups in QLDH layers, showed a high hydroxide conductivity (122 mS cm −1 at 80 • C). The FC performance at 60 • C achieved a maximum power density of 267 mW cm −2 at a current density of 554 mA cm −2 , values higher than those of membranes without QLDH but still far from those recorded for from Nafion 112 [46]. The same authors synthetized LDH functionalized with 3-hydroxy-6azaspiro [5.5] undecane cations (OH-ASU) to increase the ion exchange capacity [45]. The ASU-LDH filler was combined with TC-PPO to fabricate three-decker ASU-LDH/TC-PPO hybrids with IEC up to 3.90 meq g −1 . The OH − conductivity, shown in Figure 7, increased from 79 mS cm −1 for TC-PPO to 112 mS cm −1 for ASU-LDH/TC-PPO-50. After stability tests in 1 M NaOH at 80 • C for 588 h, only an 11.5% drop in OH − conductivity was observed.
higher than those of membranes without QLDH but still far from those recorded for from Nafion 112 [46]. The same authors synthetized LDH functionalized with 3-hydroxy-6azaspiro [5.5] undecane cations (OH-ASU) to increase the ion exchange capacity [45]. The ASU-LDH filler was combined with TC-PPO to fabricate three-decker ASU-LDH/TC-PPO hybrids with IEC up to 3.90 meq g −1 . The OH − conductivity, shown in Figure 7, increased from 79 mS cm −1 for TC-PPO to 112 mS cm −1 for ASU-LDH/TC-PPO-50. After stability tests in 1 M NaOH at 80 °C for 588 h, only an 11.5% drop in OH − conductivity was observed. An electric field was sometimes applied to induce the formation of ion channels along the through-plane direction [47,48]. Aligned LDH functionalized with N-spirocyclic ammonium groups and PPO showed a maximum ion conductivity of 110 mS cm −1 at 80 °C and an enhanced alkali resistance with 83% residual conductivity [48]. The good stability was attributed to the formation of electric field-induced ion channels that reduced the local alkaline concentration near quaternized groups and decreased the possibility of the hydroxide to attack ammonium groups.
Composite membranes with DABCO-PPO and 30 wt% MgAl-LDH were prepared in 2020 by Pasquini et al. [49]. The high amount of the second phase was chosen in order to understand the effect of the inorganic filler on alkaline and hydrolytic degradation. The stability behaviour, studied by different techniques, was comparable to the initial ionomer showing a fast degradation in the first hours.

Poly(vinylidene fluoride) (PVDF)
Only one example was reported in the literature concerning PVDF: a bioinspired geometrical templating of PVDF substrate containing electrospun MgAl-LDH for solid-state AFC [50]. The hybrid substrate was morphologically similar to the coconut flower (Cocosnucifera). Efficient pore-filling by LDH was demonstrated by SEM and the inorganic phase enhanced TS two-fold (14 MPa). The Cl − conductivity at 80 °C was 92 mS cm -1 at 100% RH and 18 mS cm -1 at 50% RH. An electric field was sometimes applied to induce the formation of ion channels along the through-plane direction [47,48]. Aligned LDH functionalized with N-spirocyclic ammonium groups and PPO showed a maximum ion conductivity of 110 mS cm −1 at 80 • C and an enhanced alkali resistance with 83% residual conductivity [48]. The good stability was attributed to the formation of electric field-induced ion channels that reduced the local alkaline concentration near quaternized groups and decreased the possibility of the hydroxide to attack ammonium groups.
Composite membranes with DABCO-PPO and 30 wt% MgAl-LDH were prepared in 2020 by Pasquini et al. [49]. The high amount of the second phase was chosen in order to understand the effect of the inorganic filler on alkaline and hydrolytic degradation. The stability behaviour, studied by different techniques, was comparable to the initial ionomer showing a fast degradation in the first hours.

Poly(vinylidene fluoride) (PVDF)
Only one example was reported in the literature concerning PVDF: a bioinspired geometrical templating of PVDF substrate containing electrospun MgAl-LDH for solidstate AFC [50]. The hybrid substrate was morphologically similar to the coconut flower (Cocosnucifera). Efficient pore-filling by LDH was demonstrated by SEM and the inorganic phase enhanced TS two-fold (14 MPa). The Cl − conductivity at 80 • C was 92 mS cm −1 at 100% RH and 18 mS cm −1 at 50% RH.

MXenes
MXenes are a family of 2D layered materials which include transition metal nitrides, carbides, and carbonitrides produced by selective exfoliation of MAX phases, where "M" refers to early d-block transition metals, "A" to groups 13 and 14 (main sp elements) and "X" to carbon or nitrogen atoms [51]. MXenes present several properties, such as hydrophilicity, fast ion transport and intercalation, large surface area, etc., provoking interest in many scientific applications including FC.
By solution blending method two kinds of 2D nanomaterial Ti 3 C 2 T x (T x stands for the hydrophilic surface terminations-O, -OH, -F, etc.) were incorporated into QPSU [52].
The mechanical strength and the conductivity were slightly increased adding 3 wt% of LiF-Ti 3 C 2 T x and NH 4 HF 2 -Ti 3 C 2 T x . The maximum power density in a single cell at 60 • C with H 2 /O 2 atmospheric pressure and Pt loading of 0.4 mg cm −2 in both sides for QPSU/LiF-Ti 3 C 2 T x and QPSU/NH 4 HF 2 -Ti 3 C 2 T x attained 74 mW cm −2 and 101 mW cm −2 , respectively. In 2018, Wang et al. dispersed imidazolium functionalized MXene in a CS matrix [53]. The pure CS membrane had a TS of 28 MPa, after the addition of QMXene-NH 2 , the TS improved significantly up to 41 MPa. The conductivity of AEM with 7.5 wt% of QMXene-NH 2 at 100% RH increased from 1.5 to 4 mS cm −1 [53].

Graphene Oxide (GO) and Graphene
A very large amount of work and effort was devoted to composite AEM containing graphene oxide (GO) or graphene. The main features for graphene are an extreme mechanical resistance and great flexibility, for GO its hydrophilicity and dispersibility in water, due to epoxy and hydroxyl groups on the basal planes, and carboxyl and carbonyl groups at the edges of layers. In particular, GO was functionalized with many groups creating original and sometimes complex structures.
Magnetite nanoparticles on GO were distributed along the through-plane direction in a QPVA matrix by applying an external magnetic field (MF) during the film drying step [58]. The QPVA/0.1%Fe 3 O 4 @GO nanocomposites presented the highest ionic conductivity with 55 mS cm −1 and a maximum power density of 200 mW cm −2 at 60 • C and 2 M MeOH [59].
TMA-PSU was cross-linked (XL-QPSU) by rGO modified with short-and long-chain tertiary amines (SrGO and LrGO) [69]. Membranes with elastic long-chain (LrGO) displayed higher ion conductivity due to major nanophase separated morphology. The XL-QPSU-2%-LrGO OH − conductivity was 76 mS cm −1 at RT, with respect to a conductivity of 49 mS cm −1 for the pristine AEM. The same authors modified rGO with pyrene-containing tertiary amines (TrGO) and polymers (PrGO) via π-π interactions [70]. The functionalized rGO was used to cross-link TMA-PSU. The XL-QPSU-2%-PrGO reached a hydroxide conductivity at 80 • C of 118 mS cm −1 and after alkaline treatment in 1 M NaOH for 500 h at 60 • C, the residual conductivity was 88% higher than un-crosslinked QPSU.
Composite AEMs were also expected to reduce the vanadium permeability in vanadium redox flow batteries (VRFB), due to the Donnan exclusion effect. A long alkyl chain (C16) grafted to quaternized GO was bonded to DABCO-PSU and the resulting composites used in VRFB [71]. The long alkyl chain increased the hydrophilic-hydrophobic nanophase separation. The selectivity, assessed by a ratio of ion conductivity and vanadium ion permeability, for 5 wt% of the modified GO, was 19 × 10 5 S min cm −3 and the Coulombic efficiency (CE), was 98% at 60 mA cm −2 . The good results were attributed to the high ionic conductivity and the extremely low vanadium cross-over.

Poly(phenylene oxide) (PPO)
Although PPO is widely employed as a polymer in AEMs, only a few examples concern its use in composites with GO.
Brominated PPO and polyethyleneimine (PEI)-modified GO was proposed by Kulshrestha et al. [72]. A blend of GO (QGO) and cellulose (QCel) quaternized with DABCO, and subsequently XL with DABCO-PPO, displayed an OH − conductivity of 114 mS cm −1 at 25 • C [73]. The stability decreased with the increase in QCel amount, due to its oxygenated functionalities susceptible for the nucleophilic attack, and increased with the content of QGO, very stable in alkaline conditions. An imidazolium-functionalized PPO (Im-PPO) and an ionic liquid-functionalized GO were used to prepare composite membranes with a hydroxide conductivity of 79 mS cm −1 at 80 • C [74]. The alkaline test in 2 M NaOH at 80 • C over 480 h showed a decrease in IEC from 1.90 to 1.34 meq g −1 . FC tests at 60 • C for 0.5 wt% of IL-GO composite presented a maximum power density of 136 mW cm −2 with a current density of 300 mA cm −2 under 100% RH.
Recently, commercial membranes based on PPO (Fumion ® FAA) were filled with different amounts of commercial graphene with a surface area of 500 m 2 /g [75]. The SEM morphology showed a uniform graphene inclusion into the polymer matrix; no crack formation and increased roughness were found. The maximum value of OH − conductivity was reached for the sample containing 50 mg of graphene and was 113 mS cm −1 in 0.01 M KOH at 80 • C.

Polybenzimidazole (PBI)
Polybenzimidazole (PBI) is a well-known matrix in proton exchange membranes due to its performance at high temperatures; in AEM, PBI can be used after alkaline doping. To overcome the barrier due to the release of alkaline dopants in operative conditions, various strategies were used with the support of GO or rGO.
Zeng et al. proposed a MEA formed by a porous PBI membrane (sp-PBI) and a composite as binder formed by PBI and rGO on which the electrocatalyst was deposited [77]. The MEA reached a maximum power density of 544 mW cm −2 , higher than conventional MEA (397 mW cm −2 ), as shown in Figure 8.
GO nanosheets were coated onto a PBI surface using a spin coater. The thickness of spin-coated with 2 wt% GO was 1-2 µm [78]. The Young's modulus was improved to 1040 MPa and the tensile strength reached 50 MPa, effects attributed to hydrogen bonds between the edges of GO nanosheets. In DMFC the maximum power density at 80 • C reached 200 mW cm −2 with Pt-based catalysts [78]. Two types of GO nanosheets were prepared by microwaves (MGO) and by the modified Hummer's method (NGO) [79]. MGO was less hydrophilic and more thermally resistant due to it having fewer ether groups and more sp 3 C-C bonds. The PBI composite with 1 wt% of filler showed an increased conductivity by 38% for NGO and 29% for MGO with respect to pure PBI. Voc and Pmax for NGO were 0.7 V and 310 mW cm −2 , respectively.
Recently, DABCO quaternized poly(arylene ether) (QPAE) was filled with ammonium functionalized GO (GO-(APTS-c-PTMA)) using (3-aminopropyl)triethoxysilane (APTS) and (3-bromopropyl)trimethyl ammonium bromide (PTMA) as chemical cross linkers [83]. GO nanosheets were coated onto a PBI surface using a spin coater. The thickness of spin-coated with 2 wt% GO was 1-2 μm [78]. The Young's modulus was improved to 1040 MPa and the tensile strength reached 50 MPa, effects attributed to hydrogen bonds between the edges of GO nanosheets. In DMFC the maximum power density at 80 °C reached 200 mW cm −2 with Pt-based catalysts [78]. Two types of GO nanosheets were prepared by microwaves (MGO) and by the modified Hummer's method (NGO) [79]. MGO was less hydrophilic and more thermally resistant due to it having fewer ether groups and more sp 3 C-C bonds. The PBI composite with 1 wt% of filler showed an increased conductivity by 38% for NGO and 29% for MGO with respect to pure PBI. Voc and Pmax for NGO were 0.7 V and 310 mW cm −2 , respectively.
A GO multilayer paper was also employed as a matrix. The membranes were treated with KOH and used in FC [88]. and poly(4,4′-diphenylether-5,5′-bibenzimidazole) (DPEBI) [87]. The synthesis of GO/IM is reported in Figure 9. The nanohybrid membranes attained an OH − conductivity of 102 mS cm −1 at 100 °C. The activation energy was between 16 and 19 kJ mol −1 corresponding to a Grotthuss-type mechanism; the lower value was observed for 2 and 3 wt% filler content.
A GO multilayer paper was also employed as a matrix. The membranes were treated with KOH and used in FC [88].

Carbon and Boron Nitride (BN)
Recently, some examples were reported in the literature on the use of nitrides, characterized by extreme hardness, to improve the mechanical properties and decrease the crossover of membranes.

Carbon and Boron Nitride (BN)
Recently, some examples were reported in the literature on the use of nitrides, characterized by extreme hardness, to improve the mechanical properties and decrease the crossover of membranes.
Graphitic carbon nitride (g-C 3 N 4 ) nanosheets were used as filler in modified quaternary aminated poly(arylene ether sulfone) (QPAES) [89]. The Young's moduli were in the range of 1150-1670 MPa and increased with respect to bare QPAES (1110 MPa). AEMFC tests at 60 • C, for 0.6 wt% filler, showed the highest power density with 68 mW cm −2 . g-C 3 N 4 nanosheets were also inserted into PAEK functionalized by Menshutkin reaction with TMA (QPAEK) [90]. The Young's modulus and the TS increased as expected; the maximum power density in a single H 2 /O 2 FC was reached for the QPAEK-CN-0.5 membrane with 49 mW cm −2 at 80 • C. PPO quaternized with N-methyl morpholine was used by Rathod et al. in 2020 to prepare composite membranes with a different amount (1-5 wt%) of functionalized boron nitride (BN) [91]. Membranes with 5 wt% exhibited the best performances; the ionic conductivity was acceptable (6.3 mS cm −1 at 30 • C) and the MeOH permeability decreased (5.15 × 10 −8 cm 2 s −1 ) indicating that BN acted as a barrier for methanol limiting the cross-over. Table 3 summarizes information on the various composites, including the type of polymer and other 2D filler, remarks and references.   GO multilayer membranes GO and GO KOH water uptake GO KOH 1099 wt% [88] TMA-PAES g-C 3 N 4 maximum power density 68 mW cm −2 at 60 • C (0.6 wt%) [89] TMA-PAEK g-C 3 N 4 nanosheets conductivity 35 mS cm −1 at 80 • C (0.5 wt%) [90] N-methyl morpholine-PPO f-BN Yield stress 37 MPa (5 wt%) [91] We have seen in this section various 2D nanofillers, such as LDH, MXene, graphene and graphene oxide, etc., commonly used for the fabrication of hybrid materials especially with commercial and low cost polymers such as PPO and PSU. The purpose was to maintain good conductivity, also by functionalizing the inorganic fillers with ionic conducting groups, while trying to enhance the mechanical properties and reduce the permeability of gases in the FC, thanks to the 2D geometry of these materials.

Silica and Silicates
Silicon dioxide and silicates are the most abundant classes of minerals in nature, and their cost is practically negligible. Phyllosilicates, such as montmorillonite and palygorskite, are hydrated silicates of aluminium and/or magnesium with the ability to exchange ions. Notable successes were obtained by using Si and its derivatives as fillers in proton exchange membranes [92,93]; later their use was extended to AEM. In this review, we will limit the description of the progress achieved to silica and derivatives not obtained via the sol-gel process. Sol-gel silica composites were described in a recent review by Sgreccia et al. [94] [95]. The membrane with 2 wt% of SiO 2 showed the best specific conductivity (3 mS cm −1 ). More recently silica-coated PVDF (SiO 2 @PVDF) electrospun nanofibers were quaternized and inserted in QCS [96]. Tests in alkaline DMFC, performed at various MeOH concentrations, showed a maximum power density at 80 • C up to 99 mW cm −2 (2 M MeOH), presenting only 4% of performance loss after 100 h in chronoamperometry test.

Aromatic Polymers
Aromatic polymers were also a preferred choice in the building of silica-containing membranes for fuel cells, diffusion dialysis (DD), and redox flow batteries. Pan et al. in 2015 improved the efficiency of acid recovery by DD with the fabrication of a quaternized PPO/SiO 2 hybrid material (QPPO-SiO 2 ) obtained by electrospinning and posttreatment (solvent fumigation and hot-press) [102]. The main fiber diameters were between 200-300 nm with an average of~250 nm as shown in Figure 10. In a simulated polishing waste solution containing 1 M HCl and 0.225 M FeCl2, the membrane exhibited both a higher acid permeability (UH) with 0.053 m h −1 and selectivity (S, 68.05) in comparison with the direct casting QPPO-SiO2 hybrid membrane. In addition, compared with a commercial DF-120 membrane the hot-pressed electrospun QPPO-SiO2 membrane showed more than seven times higher UH and approximately three times higher S, underling its advantage in DD application [102]. Quaternized polyethersulfone (TMA-PES) composite membranes were fabricated with the incorporation of three functionalized SiO2 nanoparticles containing propylamine (ASi-I), trimethylpropylamine (ASi-II) and melamine-based dendrimer amine groups (ASi-III) [103].
Recently Chen et al. prepared composites with a "hamburger structure" using quaternized PPO with a triple-cation precursor (TA-PPO) as matrix and 1,2-dimethylimidazolium-silica (Im-SiO2) as dopant. Im-SiO2 was placed on the surface of TA-PPO membrane to protect the matrix from the attack of OH − and radicals [104]. The composite membrane showed a low swelling ratio (8.2%), and a high OH − conductivity (105 mS cm −1 at 80 °C).
Quaternized mesoporous silica nanoparticles (QMSNs) were mixed with PSU quaternized with TEA [105] and TMA [106]. The homogeneous dispersion of QMSN in the QPSU matrix, highlighted by the SEM images, improved the mechanical properties: the Young's modulus increased from 1990 for pure QPSU to 2250 MPa for 20 wt% of QMSNs while the composite with 15 wt% of filler showed the highest CO3 2− conductivity, reaching 20 mS cm −1 at 80 °C [106]. Afterwards mesoporous silica (SBA-15) was function- In a simulated polishing waste solution containing 1 M HCl and 0.225 M FeCl 2 , the membrane exhibited both a higher acid permeability (UH) with 0.053 m h −1 and selectivity (S, 68.05) in comparison with the direct casting QPPO-SiO 2 hybrid membrane. In addition, compared with a commercial DF-120 membrane the hot-pressed electrospun QPPO-SiO 2 membrane showed more than seven times higher UH and approximately three times higher S, underling its advantage in DD application [102]. Quaternized polyethersulfone (TMA-PES) composite membranes were fabricated with the incorporation of three functionalized SiO 2 nanoparticles containing propylamine (ASi-I), trimethylpropylamine (ASi-II) and melamine-based dendrimer amine groups (ASi-III) [103].
Recently Chen et al. prepared composites with a "hamburger structure" using quaternized PPO with a triple-cation precursor (TA-PPO) as matrix and 1,2-dimethylimidazoliumsilica (Im-SiO 2 ) as dopant. Im-SiO 2 was placed on the surface of TA-PPO membrane to protect the matrix from the attack of OH − and radicals [104]. The composite membrane showed a low swelling ratio (8.2%), and a high OH − conductivity (105 mS cm −1 at 80 • C).
Quaternized mesoporous silica nanoparticles (QMSNs) were mixed with PSU quaternized with TEA [105] and TMA [106]. The homogeneous dispersion of QMSN in the QPSU matrix, highlighted by the SEM images, improved the mechanical properties: the Young's modulus increased from 1990 for pure QPSU to 2250 MPa for 20 wt% of QMSNs while the composite with 15 wt% of filler showed the highest CO 3 2− conductivity, reaching 20 mS cm −1 at 80 • C [106]. Afterwards mesoporous silica (SBA-15) was functionalized with imidazolium-based ionic liquid and inserted in QPSU. The presence of covalent bonds between IL and SBA-15 was verified by solid-state NMR [107]. The QPSU/3%IL-SBA-15 showed the best fuel cell performance with an OCV of 0.87 V and a maximum power density of 278 mW cm −2 .
In VRFB, the CE at 100 mA cm −2 was 99% for both, QPEK-C and QPEK-C/20 wt% TMSP-TMA + , compared to 95% for Nafion ® 212. The battery capacity was 10% lower over 30 charge/discharge cycles (~60 h) while for Nafion ® 212 in similar conditions a loss of 30% was observed [110]. Table 4 summarizes information on the various composites, including the type of polymer and 3D filler, remarks and references.  In VRFB, the CE at 100 mA cm −2 was 99% for both, QPEK-C and QPEK-C/20 wt% TMSP-TMA + , compared to 95% for Nafion ® 212. The battery capacity was 10% lower over 30 charge/discharge cycles (~60 h) while for Nafion ® 212 in similar conditions a loss of 30% was observed [110]. Table 4 summarizes information on the various composites, including the type of polymer and 3D filler, remarks and references.

Metal Oxides and Derivatives
One of the major uses of metal oxides as filler in AEM is in improving the mechanical properties of soft polymers such as PVA. Various oxides are used, including Al 2 O 3 , a well known hard material characterized by poor thermal and electrical conductivity, TiO 2 , non-expensive and non-toxic with high chemical stability, and ZrO 2 , a polymorphous crystalline oxide with high ionic conductivity when doped with acceptor cations and low electronic conductivity. The chosen matrices were mainly PVA and aromatic polymers.

Aluminum Oxides
PVA quaternized with glycidytrimethyl ammonium chloride and 10 wt% Al 2 O 3 (QPVA/Al 2 O 3 ) displayed in DMAFC a maximum power densities of 36 mW cm −2 with 4 M KOH + 4 M CH 3 OH, as shown in Figure 12 [111]. Although SEM images indicated some chunks and aggregates randomly distributed on the surface, the storage modulus slightly increased with respect to pure PVA (172 vs. 151 MPa at 100 • C).
Composite membranes with smooth and dense morphology were formed by quaternary trimethylammonium PSU with different loadings of Al 2 O 3 powder (1-4 wt%) [112]. The presence of filler increased the swelling degree, the water uptake, and the ionic conductivity.

Aluminum Oxides
PVA quaternized with glycidytrimethyl ammonium chloride and 10 wt% Al2O3 (QPVA/Al2O3) displayed in DMAFC a maximum power densities of 36 mW cm −2 with 4 M KOH + 4 M CH3OH, as shown in Figure 12 [111]. Although SEM images indicated some chunks and aggregates randomly distributed on the surface, the storage modulus slightly increased with respect to pure PVA (172 vs. 151 MPa at 100 °C).

Zirconium Oxides
The morphology of composite membranes of TMA-PSU and zirconia was investigated by TEM, revealing a homogenous distribution when the particles size were around 10 nm [113]. The membrane with 10 wt% of ZrO 2 showed the best performance, reaching a maximum conductivity of 15 mS cm −1 . The maximum power density presented a value of 250 mW cm −2 with an OCV of 0.91 V at 60 • C [113]. Based on Im-PSU, recently, Rambabu et al. prepared composites with different percentages of zirconia (2.5, 5, 7.5 and 10 wt%), through a phase inversion method [114]. The conductivity attained was 80 mS cm −1 at 50 • C, 47% higher than pure Im-PSU. The TS showed values of 47 and 43 MPa before and after the alkaline treatment, while pure Im-PSU presented 31 and 25 MPa, respectively. The FC performance, using Pt/C catalysts, showed with 10 wt% of ZrO2 an OCP of 1.04 V, a maximum power density of 270 mW cm −2 and a current density of 640 mA cm −2 .
Another quaternized aromatic polymer (QPAES) was used to prepare composite membranes with nanozirconia [115,116]. The introduction of the second phase enhanced the OH − conductivity of cross-linked multiblock copoly(arylene ether sulfone) (XL TMA-coPAES), with 7.5 wt% of ZrO 2 the composites showing the best conductivity (55 mS cm −1 at 80 • C) and the best alkaline stability, retaining 94% of conductivity after immersion in 1 M NaOH solution at 60 • C for 340 h. The highest Young's modulus was reached for 10 wt% of ZrO 2 with 492 MPa, around two times higher than virgin polymer. Fluoropolymer-based membranes were obtained from a free-radical desulfurization coupling reaction on Nafion NR-50 followed by grafting and filling with ZrO(ClO 4 ) 2 [117]. The chemical stability was evaluated in a reactor with a 7 M KOH solution of EtOH/H 2 O 15%/85% at 120 • C for 2200 h. The IEC after the stability test was practically unchanged. The high stability was ascribed to the fluorinated polymeric structure and to covalent C-C bonds that link the ZrO(ClO 4 ) 2 nanoclusters and polymer side chains [117].
The effect of the doping with hydrophilic (tri(hydroxymethyl)propane, TMP) and hydrophobic (polymethyl-hydrosiloxane, PMHS) TiO 2 nanoparticles was studied using DABCO-PSU as the matrix [122]. The DMA and DSC studies showed a Tg around 250 • C, due to a partial crystallization of the polymer, with a significant decrease in the contact angle (67 • ) for composites with hydrophilic character. The ionic conductivity was higher for the hydrophobic filler (PMHS-TiO 2 ) related to a more homogeneous dispersion, as observed by AFM images.
Some examples of composites concern the use of ionic liquids and nanoparticles with the aim of simultaneously increasing conductivity and mechanical properties. 1-Methy-3methylimidazolium IL and nano-TiO 2 were mixed to TEA-PPO [123]. The conductivity increased with the amount of IL and TiO 2 and the best mechanical properties were reached for 15 wt% IL and 1 wt% of TiO 2 with Young's modulus of 921 MPa and 6% of elongation at break. The degradation rates of the ion conductivity measured after treatment in 4 M NaOH solution at RT for 280 h, showed for 1 wt% of TiO 2 a reduction of around 30% instead of 80% without the nanofiller, confirming the role of TiO 2 in stabilizing the IL in the membranes [123]. The same authors inserted methyl, ethyl, hydroxyethyl IL and TiO2 into a TEA-PPO matrix [124]. The stability tests, in 4 M NaOH at RT for 288 h, revealed for the ethyl derivative a 92% retention of conductivity while the pristine membrane retained 78%.
PVA membranes, filled with spherical calcium titanate nanoparticles (CaTiO 3 ) with orthorhombic perovskite structure, were proposed for redox flow batteries [126]. The distribution was uniform and homogenous when the nano-CaTiO 3 filler was less than 20 wt%.

Other Inorganics
Homogeneous composite membranes with PTFE and Sn 0.92 Sb 0.08 P 2 O 7 exhibited conductivities from~10 to 100 mS cm −1 between 75 and 200 • C [127]. In the fuel cell tests, the maximum power density reached 94 mW cm −2 at 100 • C.
In Direct Borohydride Fuel Cells (DBFCs), PVA membranes with CoOOH − functionalization showed a better performance than pristine membranes [128]. The maximum power densities obtained in the test cells at 30 • C with and without CoSO 4 were 144 and 72 mW cm − 2 , respectively ( Figure 13). for the ethyl derivative a 92% retention of conductivity while the pristine membrane retained 78%. PVA based-membranes were used in microbial fuel cells (MFCs) [125]. The comparison between TiO2-PVA quaternized with trimethylammonium chloride (QAPVA), commercial Nafion 117, and LeHoAM-III (Hangzhou Lvhe Environmental Technology Co., Ltd., China) showed a maximum power density at 35 °C for TiO2-QAPVA (125 mW cm −2 ), two and three times higher than Nafion 117 and LeHOAM-III, respectively. The TiO2-QAPVA superior performances were attributed to a good oxygen resistance of the membrane.
PVA membranes, filled with spherical calcium titanate nanoparticles (CaTiO3) with orthorhombic perovskite structure, were proposed for redox flow batteries [126]. The distribution was uniform and homogenous when the nano-CaTiO3 filler was less than 20 wt%.

Metal Organic Frameworks (MOFs)
Crystalline metal-organic frameworks (MOFs) are formed by metal ions coordinated with rigid organic ligands to form high porosity structures and are therefore characterized by very large internal surface areas. With tunable functionality and well-defined channels, MOFs have inspired a new class of ion-conductive compounds. In contrast to the extensive studies on proton-conductive MOFs [132] and related membranes, rare reports focus on MOFs in the preparation of AEM.
Vinyl benzyl chloride (VBC) monomers were impregnated into nanopores of zeolitic imidazolate framework (ZIF-8) and then aminated to obtain poly vinyl benzyl trimethylammonium chloride (PVBTAC) as described in Figure 14 [133]. The Brunauer-Emmett-Teller (BET) surface analysis showed, after incorporation of the polymer, a pore volume of ZIF-8 from 0.49 to 0.26 mL g −1 . The absence of a cross-linking agent in the polymerization ensured the formation of linear chains in the structure.
ZIF-8 was also added in a different amount to PVA [134]. To enhance the electrochemical properties at low humidity, choline hydroxide ionic liquids were used in PVA-ZIF-8 system [135]. The conductivities of IL@ZIF-8/IL/PVA composites were tested at 33% of RH and improved from 0.11 mS cm −1 at 25 • C to about 1 mS cm −1 at 60 • C for 20 wt% of IL@ZIF-8.
PVA-ZIF-8 composite membranes were also used for DMAFCs [136]. The alkaline stability test for PVA-40.5% ZIF-8 after immersion in 6 M KOH for 24 h and 168 h showed a decrease in conductivity by 14%, for pure PVA 31%. For the same composition, a maximum power density of 173 mW cm −2 was achieved in DMAFC at 60 • C.
A sandwiched AEM formed by porous bromomethylated-PPO with entrapped cationic MOFs coated with PVA on the two sides was prepared for DMAFCs [137]. PVA coating limited the MeOH crossover and entrapped cationic MOFs worked as OH − conductive channels. The OH − conductivity was 145 mS cm −1 at 80 • C and the MeOH permeability was 3.68 × 10 −7 cm 2 s −1 [137].
The incorporation of chloromethylated MIL-101(Cr) into chloromethylated PEEK, followed by quaternization with imidazolium, gave Im-PEEK/ImMIL-101(Cr) membranes [138]. ImMIL-101(Cr) was uniformly distributed into the Im-PEEK matrix up to 1.0 wt%; above this value a slight aggregation of the filler was observed. The TS of the composite membranes reached the highest value of 35 MPa with the incorporation of 10 wt% of ImMIL-101(Cr), 47.5% higher than pure Im-PEEK, the presence of filler reducing the elongation at break and increased the rigidity of membranes.
on MOFs in the preparation of AEM.
Vinyl benzyl chloride (VBC) monomers were impregnated into nanopores of zeolitic imidazolate framework (ZIF-8) and then aminated to obtain poly vinyl benzyl trimethylammonium chloride (PVBTAC) as described in Figure 14 [133]. The Brunauer-Emmett-Teller (BET) surface analysis showed, after incorporation of the polymer, a pore volume of ZIF-8 from 0.49 to 0.26 mL g −1 . The absence of a cross-linking agent in the polymerization ensured the formation of linear chains in the structure. ZIF-8 was also added in a different amount to PVA [134]. To enhance the electrochemical properties at low humidity, choline hydroxide ionic liquids were used in PVA-ZIF-8 system [135]. The conductivities of IL@ZIF-8/IL/PVA composites were tested at 33% Figure 14. Synthesis of ion-exchange polymer (PVBTAH) inside the porous network of ZIF-8. Reprinted with permission from Ref. [133]. Copyright (2014) American Chemical Society.

Carbon Dots (CDs)
Although the CD particle is classified with zero dimensions, we will deal with it in this section for simplicity.
Yuan et al. in 2019 prepared nanocomposites formed by an Im-PSU matrix and quaternized carbon dots (QCDs) derived from citric acid and ethylenediamine as shown in Figure 15 [139].
The Young's modulus rose to 1600 MPa; the TS 70 MPa and the elongation at break rose to 15.5 %. For Im-PSU-1.0%-QCDs, the stability tests showed 61% of OH − conductivity retention, (original value 109.3 mS cm −1 ), after 500 h in 1 M NaOH at 60 • C.
Composite AEMs based on chloromethylated PSU were developed for the DD process in the recovery of acid from the waste stream in 2020 [140]. The composites were prepared with different wt% (0.1, 0.5, and 1.0) of graphene quantum dots (GQDs) synthesized by chemical oxidation of MWCNTs, followed by quaternization with TMA (called SCE-0.1, SCE-0.5, and SCE-1.0). The conductivity for SCE-0.1 attained was 15 mS cm −1 . TS and elongation at break increased, respectively, by 37% and 28% for a composite with 1.0 wt% GQDs with respect to pristine SCE. Acid recovery with 3 M HCl solutions was 30% for pristine membranes, while for SCE-1.0 it increased to 44%. Table 5 summarizes information on the various composites, including the type of polymer and other 3D filler, remarks and references.

Carbon Dots (CDs)
Although the CD particle is classified with zero dimensions, we will deal with it in this section for simplicity.
Yuan et al. in 2019 prepared nanocomposites formed by an Im-PSU matrix and quaternized carbon dots (QCDs) derived from citric acid and ethylenediamine as shown in Figure 15 [139]. The Young's modulus rose to 1600 MPa; the TS 70 MPa and the elongation at break rose to 15.5 %. For Im-PSU-1.0%-QCDs, the stability tests showed 61% of OH − conductivity retention, (original value 109.3 mS cm −1 ), after 500 h in 1 M NaOH at 60 °C.
Composite AEMs based on chloromethylated PSU were developed for the DD process in the recovery of acid from the waste stream in 2020 [140]. The composites were prepared with different wt% (0.1, 0.5, and 1.0) of graphene quantum dots (GQDs) synthesized by chemical oxidation of MWCNTs, followed by quaternization with TMA (called SCE-0.1, SCE-0.5, and SCE-1.0). The conductivity for SCE-0.1 attained was 15 mS cm −1 . TS and elongation at break increased, respectively, by 37% and 28% for a composite with 1.0   TMA-PSU TMA-GQDs conductivity 15 mS cm −1 at RT (0.1 wt%) [140] We have seen in this section different 3D nanofillers such as silica, ZrO 2 , MOF and CD commonly used for fabrication of hybrid materials especially with commercial, low-cost polymers such as PPO, PSU and very attractive PVA. The greatest efforts have focused on silica and functionalized derivatives and oxides, especially zirconia. The purpose was to increase conductivity, slightly improve IEC, enhance mechanical properties especially Young's modulus and increase the power density in FC tests. Generally, the stability in an alkaline environment improves, but not significantly except for fluoropolymer-based anion-conducting membranes containing ZrO(ClO 4 ) 2 where the stability seems to exceed 2000 h.

Conclusions
This review on composite anion exchange membranes with inorganic fillers allows some general conclusions. We divided the results according to the dimensionality of the fillers. Some interesting synthetic efforts versus the innovative functionalization of nanoparticles can be recognized. Generally, an enhancement of mechanical properties (TS, Young's modulus) is observed in composites. The ionic conductivity is often reduced with some possible mitigation by an intrinsic ion conducting fillers or little improvement with functionalized fillers.
Frequently an enhancement of FC performances is reported, but not much improvement on membrane stability in alkaline conditions. For 1D fillers, the major work was perfromed on carbon nanotubes with various functionalization and some papers on titanate nanotubes. One-dimensional tubular nanofillers can provide ion conducting groups, construct ion conducting pathways along the interphase region and improve the anti-swelling property of the nanohybrid membrane. The carbon nanotubes are also characterized by extreme strength and flexibility; the increase in mechanical properties is evident but the increase in FC performance is modest. The use of imidazole as functionalizing molecule contributes to attainment of a high conductivity; a composite with imidazole PEEK and imidazole CNT reached a hydroxide conductivity of 135 mS cm −1 at 70 • C.
For 2D fillers, by far the largest amount of results was reported on graphene oxides; significant work was also performed on inexpensive layered double hydroxides which are intrinsically anionic conductors. A perfluorinated anion-conducting polymer with ionicliquid-functionalized graphene nanoribbons attained a conductivity of 121 mS cm −1 at 80 • C in water. Sandwiched-porous PBI and reduced graphene oxide attained a maximum power density at 90 • C of 544 mW cm −2 . The membrane GO/cellulose/PPO displayed a conductivity of~114 mS cm −1 at 25 • C and~215 mS cm −1 at 80 • C. PVA and exfoliated LDH(NO 3 − ) membrane exhibited a conductivity of 172.1 mS cm −1 at 80 • C. A poroussandwich structure based on triple-cation side chain PPO and quaternary-ammoniummodified LDH exhibited a maximum power density of 267 mW cm −2 at 60 • C.
For 3D fillers, most works were reported on functionalized silica and zirconia nanoparticles. The ionic conductivity generally did not improve much, remaining at acceptable values around 10 mS cm −1 at 60 • C for most composites with silica. A maximum power density of 298 mW cm −2 at 60 • C was attained for a QPSU with triethylamine functionalized mesoporous silica. The stability in an alkaline environment was improved, but not significantly except for a fluoropolymer-based AEM containing ZrO(ClO 4 ) 2 where the measured stability seems to exceed 2000 h. As for the MOFs, we can see a good conductivity for MIL 101 with 145 mS cm −1 at 80 • C.
Still open challenges that must be overcome for a large-scale application of AEMFCs as alternative to PEMFCs are: (i) the creation of alkaline stable ionomers, (ii) the creation of efficient and stable non-PGM catalysts for HOR/ORR necessary to make a low-cost final device. Other points must be optimized including the components of the MEA and their assembly, support for the catalyst, etc. As far as composites are concerned, the improvement from a mechanical point of view is evident, but they still suffer from low alkaline stability at high temperatures. Other points of discussion are certainly: (i) clarifying the interaction at the interface between the composite membrane and the catalyst, (ii) how to improve water management, (iii) how to avoid carbonation and (iv) the use of easily reproducible test conditions for a true comparison of membrane performances. Concerning the composite membranes reported in the review, few cases are highlighted with relatively new materials such as MOFs, CDs and MXenes; we expect in the future an increase in the use of these materials by optimizing their properties as needed and making them more stable, e.g., with the formation of covalent bonds between organic and inorganic phases. Another point to explore is replacing the commercial AEM that suffers from a significant degradation both at the level of the cationic group and of the backbone, with more stable polymers such as polynorbornene, polystyrene, and ionic groups, such as n-dimethyl piperidiniumtype. Even if these materials are more expensive, this disadvantage can be mitigated by a future wide commercialization of AEM capable of operating with high current densities at temperatures of 80-95 • C.