Structure-transport relationships of poly(aryl piperidinium) anion-exchange membranes: Eeffect of anions and hydration

Long Period Voltage Oscillations Associated with Reaction Changes between CO2 Reduction and H2 Formation in Zero-Gap-Type CO2 Electrochemical Reactor

Zero-gap-type reactors with gas diffusion electrodes (GDE) that facilitate the CO2 reduction reaction (CO2RR) are attractive due to their high current density and low applied voltage. These reactors, however, suffer from salt precipitation and anolyte flooding of the cathode, leading to a short lifetime. Here, using a zero-gap reactor with a transparent cathode end plate, we report periodic voltage oscillations under constant current operation. Increases in cell voltages occur at the same time as the reactor switches from the hydrogen evolution reaction (HER) to predominant CO2RR; decreases in cell voltage occur with the switch from the CO2RR to HER. Further, real time visual observations show that salt precipitation occurs during the CO2RR, whereas salt dissolution occurs during the HER. Slow flooding triggers the transition from the CO2RR to HER. A number of processes combine to slowly reduce the water content in the microporous layer, which triggers the transition back to the CO2RR.

Aryl ether-free polymer electrolytes for electrochemical and energy devices

Anion exchange polymers (AEPs) play a crucial role in green hydrogen production through anion exchange membrane water electrolysis. The chemical stability of AEPs is paramount for stable system operation in electrolysers and other electrochemical devices. Given the instability of aryl ether-containing AEPs under high pH conditions, recent research has focused on quaternized aryl ether-free variants. The primary goal of this review is to provide a greater depth of knowledge on the synthesis of aryl ether-free AEPs targeted for electrochemical devices. Synthetic pathways that yield polyaromatic AEPs include acid-catalysed polyhydroxyalkylation, metal-promoted coupling reactions, ionene synthesis via nucleophilic substitution, alkylation of polybenzimidazole, and Diels-Alder polymerization. Polyolefinic AEPs are prepared through addition polymerization, ring-opening metathesis, radiation grafting reactions, and anionic polymerization. Discussions cover structure-property-performance relationships of AEPs in fuel cells, redox flow batteries, and water and CO2 electrolysers, along with the current status of scale-up synthesis and commercialization.

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.

Swelling-Induced Quaternized Anthrone-Containing Poly(aryl ether ketone) Membranes with Low Area Resistance and High Ion Selectivity for Vanadium Flow Batteries

A vanadium flow battery (VFB) is one of the most promising electrochemical energy storage technologies. However, membranes for VFBs still suffer from high cost or low conductivity and poor stability. Here, we report new quaternized anthrone-containing poly(aryl ether ketone) (QAnPEK) membranes for VFBs. QAnPEK membranes with moderate ion exchange capacity (1.26 mmol g^-1) were swelling-induced in H₃PO₄ (50 wt %) to form wider ion transport pathways that significantly enhanced membrane conductivity (e.g., 0.49 Ω cm² for the QAnPEK-virgin membrane and 0.12 Ω cm² for the swelling-induced QAnPEK-90 membrane). The bulky rigid anthrone-containing backbone provided high swelling resistance and enabled QAnPEK membranes to have high ion selectivity. As a result, QAnPEK membranes displayed low area resistance, high ion selectivity, and robust mechanical strength. The QAnPEK-90 membrane yielded excellent energy efficiencies (92.4% at 80 mA cm^-2, 85.1% at 200 mA cm^-2, and 80.3% at 280 mA cm^-2). Moreover, QAnPEK membranes exhibited outstanding in situ and ex situ stability, for example, the VFB with the QAnPEK-40 membrane demonstrated highly stable battery performance for 3000 cycles at 160 mA cm^-2. QAnPEK membranes are attractive candidates for VFB application.

Properties of Anion Exchange Membranes with a Focus on Water Electrolysis

Recently, alkaline membrane water electrolysis, in which membranes are in direct contact with water or alkaline solutions, has gained attention. This necessitates new approaches to membrane characterization. We show how the mechanical properties of FAA3, PiperION, Nafion 212 and reinforced FAA3-PK-75 and PiperION PI-15 change when stress−strain curves are measured in temperature-controlled water. Since membranes show dimensional changes when the temperature changes and, therefore, may experience stresses in the application, we investigated seven different membrane types to determine if they follow the expected spring-like behavior or show hysteresis. By using a very simple setup which can be implemented in most laboratories, we measured the “true hydroxide conductivity” of membranes in temperature-controlled water and found that PI-15 and mTPN had higher conductivity at 60 °C than Nafion 212. The same setup was used to monitor the alkaline stability of membranes, and it was found that stability decreased in the order mTPN > PiperION > FAA3. XPS analysis showed that FAA3 was degraded by the attack of hydroxide ions on the benzylic position. Water permeability was analyzed, and mTPN had approximately two times higher permeability than PiperION and 50% higher permeability than FAA3.

Three-Electrode Study of Electrochemical Ionomer Degradation Relevant to Anion-Exchange-Membrane Water Electrolyzers

Among existing water electrolysis (WE) technologies, anion-exchange-membrane water electrolyzers (AEMWEs) show promise for low-cost operation enabled by the basic solid-polymer electrolyte used to conduct hydroxide ions. The basic environment within the electrolyzer, in principle, allows the use of non-platinum-group metal catalysts and less-expensive cell components compared to acidic-membrane systems. Nevertheless, AEMWEs are still underdeveloped, and the degradation and failure modes are not well understood. To improve performance and durability, supporting electrolytes such as KOH and K₂CO₃ are often added to the water feed. The effect of the anion interactions with the ionomer membrane (particularly other than OH^-), however, remains poorly understood. We studied three commercial anion-exchange ionomers (Aemion, Sustainion, and PiperION) during oxygen evolution (OER) at oxidizing potentials in several supporting electrolytes and characterized their chemical stability with surface-sensitive techniques. We analyzed factors including the ionomer conductivity, redox potential, and pH tolerance to determine what governs ionomer stability during OER. Specifically, we discovered that the oxidation of Aemion at the electrode surface is favored in the presence of CO₃^2-/HCO₃^- anions perhaps due to the poor conductivity of that ionomer in the carbonate/bicarbonate form. Sustainion tends to lose its charge-carrying groups as a result of electrochemical degradation favored in basic electrolytes. PiperION seems to be similarly negatively affected by a pH drop and low carbonate/bicarbonate conductivity under the applied oxidizing potential. The insight into the interactions of the supporting electrolyte anions with the ionomer/membrane helps shed light on some of the degradation pathways possible inside of the AEMWE and enables the informed design of materials for water electrolysis.

Electrocatalysis in Alkaline Media and Alkaline Membrane-Based Energy Technologies

Hydrogen energy-based electrochemical energy conversion technologies offer the promise of enabling a transition of the global energy landscape from fossil fuels to renewable energy. Here, we present a comprehensive review of the fundamentals of electrocatalysis in alkaline media and applications in alkaline-based energy technologies, particularly alkaline fuel cells and water electrolyzers. Anion exchange (alkaline) membrane fuel cells (AEMFCs) enable the use of nonprecious electrocatalysts for the sluggish oxygen reduction reaction (ORR), relative to proton exchange membrane fuel cells (PEMFCs), which require Pt-based electrocatalysts. However, the hydrogen oxidation reaction (HOR) kinetics is significantly slower in alkaline media than in acidic media. Understanding these phenomena requires applying theoretical and experimental methods to unravel molecular-level thermodynamics and kinetics of hydrogen and oxygen electrocatalysis and, particularly, the proton-coupled electron transfer (PCET) process that takes place in a proton-deficient alkaline media. Extensive electrochemical and spectroscopic studies, on single-crystal Pt and metal oxides, have contributed to the development of activity descriptors, as well as the identification of the nature of active sites, and the rate-determining steps of the HOR and ORR. Among these, the structure and reactivity of interfacial water serve as key potential and pH-dependent kinetic factors that are helping elucidate the origins of the HOR and ORR activity differences in acids and bases. Additionally, deliberately modulating and controlling catalyst-support interactions have provided valuable insights for enhancing catalyst accessibility and durability during operation. The design and synthesis of highly conductive and durable alkaline membranes/ionomers have enabled AEMFCs to reach initial performance metrics equal to or higher than those of PEMFCs. We emphasize the importance of using membrane electrode assemblies (MEAs) to integrate the often separately pursued/optimized electrocatalyst/support and membranes/ionomer components. Operando/in situ methods, at multiscales, and ab initio simulations provide a mechanistic understanding of electron, ion, and mass transport at catalyst/ionomer/membrane interfaces and the necessary guidance to achieve fuel cell operation in air over thousands of hours. We hope that this Review will serve as a roadmap for advancing the scientific understanding of the fundamental factors governing electrochemical energy conversion in alkaline media with the ultimate goal of achieving ultralow Pt or precious-metal-free high-performance and durable alkaline fuel cells and related technologies.

Tunable Anion Exchange Membrane Conductivity and Permselectivity via Non-Covalent, Hydrogen Bond Cross-Linking

Ion exchange membranes (IEMs) are a key component of electrochemical processes that purify water, generate clean energy, and treat waste. Most conventional polymer IEMs are covalently cross-linked, which results in a challenging tradeoff relationship between two desirable properties─high permselectivity and high conductivity─in which one property cannot be changed without negatively affecting the other. In an attempt to overcome this limitation, in this work we synthesized a series of anion exchange membranes containing non-covalent cross-links formed by a hydrogen bond donor (methacrylic acid) and a hydrogen bond acceptor (dimethylacrylamide). We show that these monomers act synergistically to improve both membrane permselectivity and conductivity relative to a control membrane without non-covalent cross-links. Furthermore, we show that the hydrogen bond donor and acceptor loading can be used to tune permselectivity and conductivity relatively independently of one another, escaping the tradeoff observed in conventional membranes.

OH- Conductive Properties and Water Uptake of Anion Exchange Thin Films

Several investigations have indicated that proton conduction and hydration properties of acidic ionomers differ from those of membranes. However, relations between the OH^- conductivity and water uptake in thin film forms of anion exchange membranes have not been reported yet. For this study, new in situ measurements were established to elucidate the OH^- conductivity and water uptake without allowing any influence of CO₂ from the air. Poly[(9,9-bis(6'-(N,N,N-trimethylammonium)-hexyl)-9H-fluorene)-alt-(1,4-benzene)], denoted as PFB⁺ , was synthesized as a model ionomer. The highest OH^- conductivity of 273 nm-thick PFB⁺ film was 5.3×10^-2  S cm^-1 at 25 °C under 95 % relative humidity (RH), which is comparable to the reported OH^- conductivity of PFB⁺ membrane. Reduced OH^- conductivity was found in the thinner film at 95 % RH. The decreased OH^- conductivity is explainable by the reduced number of water molecules contained in the thinner film. The OH^- conductivity was reduced only slightly under the same water uptake.

Study of Anion Exchange Membrane Properties Incorporating N-spirocyclic Quaternary Ammonium Cations and Aqueous Organic Redox Flow Battery Performance

Flexible cross-linked anion exchange membranes (AEMs) based on poly (p-phenylene oxide) grafted with N-spirocyclic quaternary ammonium cations were synthesized via UV-induced free-radical polymerization by using diallylpiperidinium chloride as an ionic monomer. Five membranes with ion exchange capacity (IEC) varying between 1.5 to 2.8 mmol Cl-·g-1 polymer were obtained and the correlation between IEC, water uptake, state of water in the membrane and ionic conductivity was studied. In the second part of this study, the influence of properties of four of these membranes on cell cycling stability and performance was investigated in an aqueous organic redox flow battery (AORFB) employing dimethyl viologen (MV) and N,N,N-2,2,6,6-heptamethylpiperidinyl oxy-4-ammonium chloride (TMA-TEMPO). The influence of membrane properties on cell cycling stability and performance was studied. At low-current density (20 mA·cm-2), the best capacity retention was obtained with lower IEC membranes for which the water uptake, freezable water and TMA-TEMPO and MV crossover are low. However, at a high current density (80 mA·cm-2), membrane resistance plays an important role and a membrane with moderate IEC, more precisely, moderate ion conductivity and water uptake was found to maintain the best overall cell performance. The results in this work contribute to the basic understanding of the relationship between membrane properties and cell performance, providing insights guiding the development of advanced membranes to improve the efficiency and power capability for AORFB systems.

Operando cathode activation with alkali metal cations for high current density operation of water-fed zero-gap carbon dioxide electrolyzers

Continuous-flow electrolyzers allow CO2 reduction at industrially relevant rates, but long-term operation is still challenging. One reason for this is the formation of precipitates in the porous cathode from the alkaline electrolyte and the CO2 feed. Here we show that while precipitate formation is detrimental for the long-term stability, the presence of alkali metal cations at the cathode improves performance. To overcome this contradiction, we develop an operando activation and regeneration process, where the cathode of a zero-gap electrolyzer cell is periodically infused with alkali cation-containing solutions. This enables deionized water-fed electrolyzers to operate at a CO2 reduction rate matching that of those using alkaline electrolytes (CO partial current density of 420 ± 50 mA cm-2 for over 200 hours). We deconvolute the complex effects of activation and validate the concept with five different electrolytes and three different commercial membranes. Finally, we demonstrate the scalability of this approach on a multi-cell electrolyzer stack, with a 100 cm2 / cell active area.