Alkaline Membranes toward Electrochemical Energy Devices: Recent Development and Future Perspectives

Anion-exchange membranes (AEMs) that can selectively transport OH-, namely, alkaline membranes, are becoming increasingly crucial in a variety of electrochemical energy devices. Understanding the membrane design approaches can help to break through the constraints of undesired performance and lab-scale production. In this Outlook, the research progress of alkaline membranes in terms of backbone structures, synthesis methods, and related applications is organized and discussed. The evaluation of synthesis methods and description of membrane stability enhancement strategies provide valuable insights for structural design. Finally, to accelerate the deployment of relevant technologies in alkaline media, the future priority of alkaline membranes that needs to be addressed is presented from the perspective of science and engineering.

Are Radicals Formed During Anion-Exchange Membrane Fuel Cell Operation?

In this paper we present a study on stable radicals and short-lived species generated in anion-exchange membrane (AEM) fuel cells (AEMFCs) during operation. The in situ measurements are performed with a micro-AEMFC inserted into a resonator of an electron paramagnetic resonance (EPR) spectrometer, which enables separate monitoring of radicals formed on the anode and cathode sides. The creation of radicals is monitored by the EPR spin trapping technique. For the first time, we clearly show the formation and presence of stable radicals in AEMs during and after long-term AEMFC operation. The main detected adducts during the operation of the micro-AEMFC are DMPO-OOH and DMPO-OH on the cathode side, and DMPO-H on the anode side. These results indicate that oxidative degradation involving radical reactions has to be taken into account when stability of AEMFCs is investigated.

Direct modification of polyketone resin for anion exchange membrane of alkaline fuel cells

A kind of side-chain type anion exchange membranes (AEMs) with high ionic conductivity and good comprehensive stability was prepared via direct modification of commercial engineering plastic polyketone with diamines through Paal-Knorr reaction and quaternization reaction. It was found that the amount of diamine can effectively tune the microphase morphology and properties of the prepared quaternized functionalized-polyketone anion exchange membranes (QAFPK-AEMs). The tensile strength was increased from 18.6 MPa to 38.6 MPa, and the ion exchange capacity (IEC) was increased from 1.11 mmol/g to 2.71 mmol/g depending on the amount of added diamine. The QAFPK-1-6-AEM with the IEC of 1.43 mmol/g showed the highest hydroxide conductivity of 65 mS/cm at 25 °C and 96.8 mS/cm at 80 °C. The high ionic conductivity was achieved through the establishment of effective ionic channels, and it maintained 70% of the initial ionic conductivity after the 192 h treatment in 2 mol/L KOH (aq) at 80 °C. Moreover, a peak power density of 129 mW/cm² was achieved when the assembled single cell with QAFPK-1-6-AEM was operated at 50 °C. Thus, the prepared QAFPK-AEMs showed great potential applications for the anion exchange membrane fuel cells (AEMFCs).

Poly(2,6-dimethyl-1,4-phenylene oxide)s with Various Head Groups: Effect of Head Groups on the Properties of Anion Exchange Membranes

Poly(2,6-dimethyl-1,4-phenylene oxide)s (PPOs)-based anion exchange membranes (AEMs) with four of the most widely investigated head groups were prepared. Through a combination of experimental and simulation approaches, the effects of the different types of head groups on the properties of the AEMs, including hydroxide conductivity, water content, physicochemical stability, and fuel cell device performance were fully explored. Unlike other studies, in which the conductivity was mostly investigated in liquid water, the conductivity of the PPO-based AEMs in 95% relative humidity (RH) conditions as well as in liquid water was investigated. The conductivity trend in 95% RH condition was different from that in liquid water but corresponded well with the actual cell performance trend observed, suggesting that the AEM fuel cell performance under in situ cell conditions (95% RH, 60 °C, H₂/O₂) is more closely related to the conductivity measured ex situ under 95% RH conditions (60 °C) than in liquid water. On the basis of the conductivity data and molecular simulation results, it was concluded that the predominant hydroxide ion-conducting mechanism in liquid water differs from that in the operating fuel cell environment, where the ionomers become hydrated only as a result of water vapor transported into the cells.

Enhanced Conductivity of Anion-Exchange Membrane by Incorporation of Quaternized Cellulose Nanocrystal

High ion conductivity of anion-exchange membrane is essential for the operation of alkaline anion-exchange membrane fuel cell. In this work, we demonstrated an effective strategy to enhance the conductivity of anion-exchange membrane (AEM), by incorporation of quaternized cellulose nanocrystal (QCNC) for the first time. Morphology observation demonstrated a uniform distribution of QCNC within QPPO matrix, as well as a clear QCNC network, which led to significant enhancement in hydroxide conductivities of composite membranes, for example, 2 wt % QCNC/QPPO membrane possessed a conductivity of 160% (60 mS cm^-1, @80 °C) of that of QPPO. Furthermore, H₂/O₂ cell performance of membrane electrode assembly based on 2 wt % QCNC/QPPO AEM showed an excellent peak power density of 392 mV cm^-2 at 60 °C without back pressure, whereas that of neat QPPO AEM was only 270 mV cm^-2.

Synthesis of gemini basic ionic liquids and their application in anion exchange membranes

A gemini-type basic morpholine ionic liquid ([Nbmd][OH]) was synthesized via a two-step method with morpholine, bromododecane and 1,4-dibromobutane as raw materials, and its structure was characterized by 1H NMR and FT-IR spectroscopy. Meanwhile, a series of anion exchange membranes ([Nbmd][OH] x -QCS) were prepared with quaternized chitosan (QCS) as the polymer matrix and [Nbmd][OH] as the dopant owing to its strong alkalinity and good solubility. The structures of the [Nbmd][OH] x -QCS composite membranes were characterized in detail by FT-IR spectroscopy, the OH- conductivity by AC impedance spectroscopy, and the morphological features by scanning electron microscopy (SEM), thermal gravity analysis (TGA), etc. The results show that the [Nbmd][OH] x -QCS composite membranes have uniform surfaces and cross-section morphology. Increasing the content of [Nbmd][OH] not only enhances the thermal stability but also increases the OH- conductivity; the thermal decomposition temperature of the [Nbmd][OH]40-QCS membrane is nearly 20 °C higher than that of the pristine QCS membrane, and the maximum OH- conductivity is approximately 1.37 × 10-2 S cm-2 at 70 °C. The methanol permeability of the [Nbmd][OH]40-QCS membrane in 1 M methanol at room temperature is 2.21 × 10-6 cm-2 s-1, which is lower than that of Nafion®115, indicating a promising potential use in alkaline direct methanol fuel cells. Moreover, the [Nbmd][OH]40-QCS membrane exhibits the best alkaline stability of all the membranes prepared in this work, retaining approximately 81% of its initial conductivity after immersion in 3 M KOH solution for 120 h at 70 °C.

Poly(terphenylene) Anion Exchange Membranes: The Effect of Backbone Structure on Morphology and Membrane Property

A new design concept for ion-conducting polymers in anion exchange membranes (AEMs) fuel cells is proposed based on structural studies and conformational analysis of polymers and their effect on the properties of AEMs. Thermally, chemically, and mechanically stable terphenyl-based polymers with pendant quaternary ammonium alkyl groups were synthesized to investigate the effect of varying the arrangement of the polymer backbone and cation-tethered alkyl chains. The results demonstrate that the microstructure and morphology of these polymeric membranes significantly influence ion conductivity and fuel cell performance. The results of this study provide new insights that will guide the molecular design of polymer electrolyte materials to improve fuel cell performance.

1,2,3-Triazolium-Based Poly(2,6-Dimethyl Phenylene Oxide) Copolymers as Anion Exchange Membranes

Anion exchange membranes (AEMs) based on 1,2,3-triazolium (TAM) were prepared from commercial poly(2,6-dimethyl phenylene oxide) (PPO) via "click chemistry" and subsequent N-alkylation. Flexible and tough membranes with various ion exchange capacities (IECs) were obtained by casting the polymers from NMP solutions. Although the resulting TAM-functionalized PPOs (PPO-TAM) membranes exhibited incomplete ion exchange in 1 M NaOH or NaHCO3 for 24 h even at elevated temperature, the highest hydroxide conductivities of the membranes were above 20 mS/cm at room temperature, which is comparable to many reported AEMs. Alkaline stability tests indicate that the PPO-TAM membranes showed a better alkaline stability than that of membranes containing imidazolium groups in 1 M NaOH at 80 °C, but still require further improvements in long-term stability for alkaline fuel cell application. An investigation of alkaline stability of model compounds demonstrated the instability of TAM cations under alkaline conditions could contribute to the deprotonation of benzylic methylene, C4 and C5 position on the triazolium ring. These results suggests that the alkaline stability of 1,2,3-triazolium cation could be improved by the introduction of substituents at the C4, C5 positions and benzylic methylene, and also provide insight and directions for organic cation designs for AEM application by the facile synthetic strategy of "click chemistry".

Polymerized ionic liquid diblock copolymers: impact of water/ion clustering on ion conductivity

Herein, we examine the synergistic impact of both ion clustering and block copolymer morphology on ion conductivity in two polymerized ionic liquid (PIL) diblock copolymers with similar chemistries but different side alkyl spacer chain lengths (ethyl versus undecyl). When saturated in liquid water, water/ion clusters were observed only in the PIL block copolymer with longer alkyl side chains (undecyl) as evidenced by both small-angle neutron scattering and intermediate-angle X-ray scattering, i.e., water/ion clusters form within the PIL microdomain under these conditions. The resulting bromide ion conductivity in the undecyl sample was higher than the ethyl sample (14.0 mS cm(-1)versus 6.1 mS cm(-1) at 50 °C in liquid water) even though both samples had the same block copolymer morphology (lamellar) and the undecyl sample had a lower ion exchange capacity (0.9 meq g(-1)versus 1.4 meq g(-1)). No water/ion clusters were observed in either sample under high humidity or dry conditions. The resulting ion conductivity in the undecyl sample with lamellar morphology was significantly higher in the liquid water saturated state compared to the high humidity state (14.0 mS cm(-1)versus 4.2 mS cm(-1)), whereas there was no difference in ion conductivity in the ethyl sample when comparing these two states. These results show that small chemical changes to ion-containing block copolymers can induce water/ion clusters within block copolymer microdomains and this can subsequently have a significant effect on ion transport.

Reactive oxygen species accelerate degradation of anion exchange membranes based on polyphenylene oxide in alkaline environments

The degradation of quaternary ammonium-cation-sites in PPO-based AEMs in alkali was considerably faster in the presence oxygen.

Alkyl bisimidazolium-mediated crosslinked comb-shaped polymers as highly conductive and stable anion exchange membranes

Crosslinked poly(arylene ether sulfone)s (PES) with pendant alkyl bisimidazolium units, which act as both crosslinkage sites and hydroxide conductors, were developed as anion exchange membranes (AEMs).

A facile construction of quaternized polymer brush-grafted graphene modified polysulfone based composite anion exchange membranes with enhanced performance

Novel quaternized polymer brush-functionalized graphenes (QPbGs) were synthesized and a series of composite anion exchange membranes for alkaline fuel cells were fabricated by incorporating different amounts of QPbGs into quaternized polysulfone.

Click Chemistry Finds Its Way in Constructing an Ionic Highway in Anion-Exchange Membrane

To find the way to construct an ionic highway in anion-exchange membranes (AEMs), a series of side-chain-type alkaline polymer electrolytes (APEs) based on poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) polymer backbones were synthesized via Cu(I)-catalyzed click chemistry. The resulting triazole groups and quaternary ammonium (QA) groups facilitate the formation of a continuous hydrogen bond network, which will lead to high hydroxide conductivity according to Grotthuss-type mechanism. Microphase separation induced by long alkyl side chains contributes at the same time to further improving the hydroxide conductivity of the resultant AEMs. Hydroxide conductivity as high as 52.8 mS/cm is obtained for membrane TA-14C-1.21 (IEC = 1.21 mmol/g) with the longest pendant chain at 30 °C, and the conductivity can be increased to 140 mS/cm when the temperature was increased to 80 °C. Moreover, the corresponding water uptake is only 8.6 wt % at 30 °C. In the meantime, the membrane properties can be tuned by precisely regulating the hydrophilic/hydrophobic ratio in the cationic head groups. Compared with AEMs containing triazole and quaternized trimethylammonium head groups, enhanced dimensional stability and mechanical properties are obtained by tuning side-chain chemistry. However, the alkaline stability of the membrane is not as stable as anticipated, probably because of the existence of the triazole ring. Further study will be focused on increasing the alkali stability of the membrane. We envisage that the side-chain-type APEs meditated by click chemistry bearing long hydrophobic side chains pendant to the cationic head groups hold promise as a novel AEMs material.