Reinforced poly(fluorenyl-co-terphenyl piperidinium) anion exchange membranes for fuel cells

Poly(Aryl-co-Aryl Piperidinium) Copolymers for Anion Exchange Membrane Fuel Cells and Water Electrolyzers

Next-generation cost-effective anion exchange membrane (AEM) fuel cells (AEMFCs) and AEM water electrolyzers (AEMWEs) have emerged as promising alternatives to costly proton exchange membrane (PEM) fuel cells and water electrolyzers due to the possibility of utilizing platinum-group-metal (PGM)-free catalysts and phasing out unsustainable perfluorosulfonic acid polymers. Anion exchange polyelectrolytes (AEPs), which can be utilized as AEMs or ionomers, are pivotal materials in AEM devices. Despite extensive exploration in the past decade, the application of AEPs has been significantly impeded by their poor ionic conductivity, insufficient alkaline stability, and unfavorable mechanical properties. Therefore, developing highly conductive and robust AEPs is critical to the success of AEMFCs and AEMWEs. (i) Our group has developed a series of highly conductive and durable poly(aryl-co-aryl piperidinium) (c-PAP) AEPs to address the aforementioned issues. c-PAP AEMs and ionomers enable outstanding OH- conductivity (>160 mS cm-1 at 80 °C), alkaline stability (1 M NaOH at 80 °C > 2000 h), dimensional stability, and mechanical properties (tensile strength > 80 MPa), giving them all the properties required for applications in AEM devices. (ii) Based on c-PAP AEMs and ionomers, we have developed high-performance AEMFCs and AEMWEs, as well as provided insights into the ionomer research and the design of membrane electrode assemblies. Typically, c-PAP AEMFCs reached the topmost peak power densities (PPDs) of 2.7 W cm-2 at 80 °C in H2-O2 along with 1000 h cell durability. Moreover, cathode-dried AEMWEs achieved a record-breaking current density of 17 A cm-2 in 1 M KOH, and the cell can be run stably at a 1.5 A cm-2 current density for over 2000 h. The remarkable performances achieved by this new class of c-PAP AEPs identify them as the most promising candidates for practical applications in AEMFCs and AEMWEs. In this account, we will elaborate on our strategies and methodologies associated with c-PAP AEPs and AEM devices, covering the screening and identification of highly durable cation head groups and molecular-engineering approaches to design c-PAP AEMs and ionomers. Moreover, we underscore our strategy in terms of developing highly efficient and durable AEMFCs and AEMWEs. We also elucidate different approaches for further enhancing the ion conductivity and mechanical stability of c-PAP AEMs, including the design of backbones and side chains, cross-linking, and reinforcement. We firmly believe that our series of studies has made substantial contributions to the fields of AEM, ionomers, AEMFCs, and AEMWEs, which have advanced AEM technology to be on par with PEM technology, opening a new avenue for commercialization of AEMFCs and AEMWEs.

Interfacially Assembled Anion Exchange Membranes for Water Electrolysis

High-performance and durable anion exchange membranes (AEMs) are critical for realizing economical green hydrogen production through alkaline water electrolysis (AWE) or AEM water electrosysis (AEMWE). However, existing AEMs require sophisticated fabrication protocols and exhibit unsatisfactory electrochemical performance and long-term durability. Here we report an AEM fabricated via a one-pot, in situ interfacial Menshutkin reaction, which assembles a highly cross-linked polymer containing high-density quaternary ammoniums and nanovoids inside a reinforcing porous support. This structure endows the membrane with high anion-conducting ability, water uptake (but low swelling), and mechanical and thermochemical robustness. Consequently, the assembled membrane achieves excellent AWE (0.97 A cm-2 at 1.8 V) and AEMWE (5.23 A cm-2 at 1.8 V) performance at 5 wt % KOH and 80 °C, significantly exceeding that of commercial and previously developed membranes, and excellent long-term durability. Our approach provides an effective method for fabricating AEMs for various energy and environmental applications.

Separators and Membranes for Advanced Alkaline Water Electrolysis

Traditionally, alkaline water electrolysis (AWE) uses diaphragms to separate anode and cathode and is operated with 5-7 M KOH feed solutions. The ban of asbestos diaphragms led to the development of polymeric diaphragms, which are now the state of the art material. A promising alternative is the ion solvating membrane. Recent developments show that high conductivities can also be obtained in 1 M KOH. A third technology is based on anion exchange membranes (AEM); because these systems use 0-1 M KOH feed solutions to balance the trade-off between conductivity and the AEM's lifetime in alkaline environment, it makes sense to treat them separately as AEM WE. However, the lifetime of AEM increased strongly over the last 10 years, and some electrode-related issues like oxidation of the ionomer binder at the anode can be mitigated by using KOH feed solutions. Therefore, AWE and AEM WE may get more similar in the future, and this review focuses on the developments in polymeric diaphragms, ion solvating membranes, and AEM.

Triptycene Branched Poly(aryl-co-aryl piperidinium) Electrolytes for Alkaline Anion Exchange Membrane Fuel Cells and Water Electrolyzers

Alkaline polymer electrolytes (APEs) are essential materials for alkaline energy conversion devices such as anion exchange membrane fuel cells (AEMFCs) and water electrolyzers (AEMWEs). Here, we report a series of branched poly(aryl-co-aryl piperidinium) with different branching agents (triptycene: highly-rigid, three-dimensional structure; triphenylbenzene: planar, two-dimensional structure) for high-performance APEs. Among them, triptycene branched APEs showed excellent hydroxide conductivity (193.5 mS cm-1 @80 °C), alkaline stability, mechanical properties, and dimensional stability due to the formation of branched network structures, and increased free volume. AEMFCs based on triptycene-branched APEs reached promising peak power densities of 2.503 and 1.705 W cm-2 at 75/100 % and 30/30 % (anode/cathode) relative humidity, respectively. In addition, the fuel cells can run stably at a current density of 0.6 A cm-2 for 500 h with a low voltage decay rate of 46 μV h-1 . Importantly, the related AEMWE achieved unprecedented current densities of 16 A cm-2 and 14.17 A cm-2 (@2 V, 80 °C, 1 M NaOH) using precious and non-precious metal catalysts, respectively. Moreover, the AEMWE can be stably operated under 1.5 A cm-2 at 60 °C for 2000 h. The excellent results suggest that the triptycene-branched APEs are promising candidates for future AEMFC and AEMWE applications.

Recent Development of Anion Exchange Membrane Fuel Cells and Performance Optimization Strategies: A Review

Anion exchange membrane fuel cells (AEMFCs) are the most promising low-temperature fuel cells and have received extensive attention. Compared to PEMFCs, the cost per unit of power can be significantly reduced for AEMFCs because, in theory, they allow the usage of non-precious metal catalysts and low-cost cell components. Owing to the development of advanced materials and performance improvement strategies, AEMFCs have achieved new records in both initial performance and durability. However, the high performance currently achieved is contingent on certain conditions, e. g., high Pt loading, large gas flowrates, and operation in pure O2 , which are far from practical applications. Therefore, the transition to commercially relevant performance and durability is the next goal of AEMFCs. This paper reviews the performance data of H2 -fueled AEMFCs since 2010 and summarizes possible performance optimization schemes, which can provide useful insights for developing next-generation AEMFCs.

Fluorinated poly(p-triphenyl piperidine) anion exchange membranes with robust dimensional stability for fuel cells

Anion exchange membrane fuel cells (AEMFCs), which are more economical than proton exchange membrane fuel cells (PEMFCs), stand out in the context of the rapid development of renewable energy. Superacid-catalyzed ether-free aromatic polymers have recently received a lot of attention due to their exceptional performance, but their development has been hampered by the trade-off between the dimensional stability and ionic conductivity of anion exchange membranes (AEMs). Here, we introduced fluoroketones containing different numbers of fluorinated groups (x = 0, 3 and 6) in the main chain of p-terphenyl piperidine because of the favorable hydrophobic properties of fluorinated groups. The results show that fluorinated AEMs can enhance OH- conductivity by building more aggregated hydrophilic channels while ensuring dimensional stability. The PTF6-QAPTP AEM with more fluorinated groups has the most excellent performance at 80 °C with an OH- conductivity of 142.7 mS cm-1 and a swelling ratio (SR) of only 4.55 %. Additionally, it exhibits good alkali durability, with the OH- conductivity and quaternary ammonium (QA) cation retaining at 93.45% and 92.6%, respectively, after immersion in a 2 M NaOH solution at 80 °C for 1200 h. In addition, the power density of the PTF6-QAPTP based single cell reaches 849 mW cm-2 when the current density is 1600 mA cm-2. The PTF6-QAPTP based cell has a voltage retention of 88% after 80 h of stability testing at a constant current density of 300 mA cm-2 at 80 °C.

Poly(Ethylene Piperidinium)s for Anion Exchange Membranes

The lack of anion exchange membranes (AEMs) that possess both high hydroxide conductivity and stable mechanical and chemical properties poses a major challenge to the development of high-performance fuel cells. Improving one side of the balance between conductivity and stability usually means sacrificing the other. Herein, we used facile, high-yield chemical reactions to design and synthesize a piperidinium polymer with a polyethylene backbone for AEM fuel cell applications. To improve the performance, we introduced ionic crosslinking into high-cationic-ratio AEMs to suppress high water uptake and swelling while further improving the hydroxide conductivity. Remarkably, PEP80-20PS achieved a hydroxide conductivity of 354.3 mS cm-1 at 80 °C while remaining mechanically stable. Compared with the base polymer PEP80, the water uptake of PEP80-20PS decreased by 69 % from 813 % to 350 %, and the swelling decreased substantially by 85 % from 350.0 % to 50.2 % at 80 °C. PEP80-20PS also showed excellent alkaline stability, 84.7 % remained after 35 days of treatment with an aqueous KOH solution. The chemical design in this study represents a significant advancement toward the development of simultaneously highly stable and conductive AEMs for fuel cell applications.