Direct ethanol fuel cells using an anion exchange membrane

Direct ethanol fuel cells (DEFCs) with a PtRu anode and a Pt cathode were prepared using an anion exchange membrane (AEM) as an electrolyte instead of a cation exchange membrane (CEM), as in conventional polymer electrolyte fuel cells. The maximum power density of DEFCs significantly increased from 6 mW cm⁻² to 58 mW cm⁻² at room temperature and atmospheric pressure when the electrolyte membrane was changed from CEM to AEM. The anode and cathode polarization curves showed a decrease in the anode potential and an increase in the cathode potential for AEM-type DEFCs compared to CEM-type. This suggests that AEM-type DEFCs have superior catalytic activity toward both ethanol oxidation and oxygen reduction in alkaline medium than in acidic medium. The product species from the exhausted liquid from DEFCs operated at a constant current density were identified by enzymatic analysis. The main product was confirmed to be acetic acid in AEM-type, while both acetaldehyde and acetic acid were detected in 1:1 ratio in CEM-type. The anodic reaction of AEM-type DEFCs can be estimated to be the oxidation of ethanol to acetic acid via a four-electron process under these experimental conditions.     

Synthesis and characterization of anion exchange membranes for alkaline direct methanol fuel cells

Alkaline fuel cells suggest solution for the problems of low methanol oxidation kinetics and methanol crossover, which are limiting the development of direct methanol fuel cells. In this work, a novel anion exchange membrane, quaternized poly(aryl ether oxadiazole), was prepared through polycondensation, grafting and quaternization. The ionic conductivity of as-synthesized anion exchange membrane can reach up to 2.79 × 10⁻² S/cm at 70 °C. The physical and chemical stability of the anion exchange membranes could also meet the requirement for alkaline direct methanol fuel cells.     

Radiation grafted membranes for superior anion exchange polymer membrane fuel cells performance

A study of radiation grafted polymers on the conductivity and performance of alkaline anion exchange membrane fuel cells (AAEMFCs) is reported. The aminated poly (LDPE-g-VBC), poly (HDPE-g-VBC) and poly (ETFE-g-VBC) membranes were produced by the using the radiation grafting technique. Differences in grafting behaviour are observed between the studied materials caused by differences in the base polymer film properties as molar mass, crystallinity, orientation or grafting technique used. In plane conductivities increased with Degree of Grafting DoG. At a DoG of 68% the LDPE-g-VBC membrane achieved an in-plane ionic conductivity between 0.18 and 0.32 S cm⁻¹ in the temperature range 20–80 °C. Measured through plane conductivities were lower than that of the in plane ones for all studied membranes. Membranes with the highest degree of swelling showed the highest through plane conductivity of 0.07–0.11 S cm⁻¹. The membrane specific resistance (per MEA cm²) of most of the produced membranes was in the range of 0.09–0.18 Ω cm². While membrane conductivity and hence IR loss is a crucial factor in fuel cell performance, membrane water permeability is a similarly crucial key for optimised water transport to the cathode. The main source of performance loss of AAEMFCs is believed to be restricted mass transport of water to the cathode reaction sites. The highly humidified anode stream along with large amount of water produced at the anode at high current densities could lead to flooding if water is not removed quickly to the cathode via the membrane (back diffusion) where it is consumed.     

ZrO2 incorporated polysulfone anion exchange membranes for fuel cell applications

Novel imidazolium functionalized polysulfone (ImPS) membranes modified with zirconia (ZrO₂) were synthesized through solution casting technique. Structural, morphological, thermal and mechanical analysis of the composite membranes confirmed adhesion and property enhancement caused by ZrO₂. Water absorption investigations revealed better water absorption of the ImPS/ZrO₂ membranes with intact morphology. Maximum ion exchange capacity and ionic conductivity for the composite membranes were obtained as 2.84 mmol/g and 80.2 mS/cm (50 ^°C) which was 21% and 47% higher as compared to pure ImPS membrane. Alkaline stability of the blend membranes was increased due to strong interaction between ZrO₂ and ImPS molecules. Fuel cell performance using Pt/C catalysts exhibited OCP and power density elevation with incremental amounts of ZrO₂ in the composite membrane composition. ImPS membrane with 10% ZrO₂ recorded a highest OCP and power density of 1.04 V and 270 mW/cm² which was 35% and 39% higher than the pure ImPS. Thus, the anion exchange membranes developed by ImPS/ZrO₂ blending could be suiting well for alkaline fuel cells applications.     

Poly(isatin-piperidinium-terphenyl) anion exchange membranes with improved performance for direct borohydride fuel cells

In this work, an effective design strategy for anion exchange membranes (AEMs) incorporating ether-bond free and piperidinium cationic groups promote chemical stability. A series of poly (isatin-piperidium-terphenyl) based AEMs were synthesized by superacid catalyzed polymerization reaction, followed by quaternization. The effect of functionalization on the performance of poly (isatin-N-dimethyl piperidinium triphenyl) (PIDPT-x) AEMs was investigated. Highly reactive N-propargylisatin was introduced into the backbone to achieve high molecular weight polymers (η_a = 2.06–3.02 dL g⁻¹) leading to robust mechanical properties, as well as modulating 1.78–2.00 mmol g⁻¹ of the ion exchange capacity (IEC) of the AEMs by feeding. Apart from that, the rigid non-ionized isatin-terphenyl segment provides AEMs improved dimensional stability with a swelling ratio of less than 12% at 80 °C. Among them, PIDPT-90 exhibited a higher OH⁻ conductivity of 105.6 mS cm⁻¹ at 80 °C. The alkali-stabilized PIDPT-85 AEM was presented, in which OH⁻ conductivity retention maintained 85.6% in a 2 M NaOH at 80 °C after 1632 h. Afterward, the direct borohydride fuel cells (DBFC) with PIDPT-90 membrane as a separator showed an open-circuit voltage of 1.63 V and a peak power density of 75.5 mWcm⁻² at 20 °C. This work demonstrates the potential of poly (isatin- N-dimethyl piperidinium triphenyl) as AEM for fuel cells.     

Preparation and characterization of imidazolium-based membranes for anion exchange membrane fuel cell applications

New anion exchange membranes (AEMs) with high conductivity, good dimensional and alkaline stability are currently required in order to develop alkaline fuel cells into efficient and clean energy conversion devices. In this study, a series of AEMs based on 1, 2-dimethyl-3-(4-vinylbenzyl) imidazolium chloride ([DMVIm][Cl]) are prepared and investigated. [DMVIm][Cl] is synthesized and used as ion carriers and hydrophilic phase in the membranes. The water uptake, swelling ratio, IEC and conductivity of the AEMs increase with increasing the [DMVIm][Cl]. The imidazolium-based AEMs show excellent thermal stability, sufficient mechanical strength, the membrane which containing 30% mass fraction of [DMVIm][Cl] shows conductivity up to 1.0 × 10^?2 S cm^?1 at room temperature and good long-term alkaline stability in 1 M KOH solution at 80 °C. The results of this study suggest that this type of AEMs have good perspectives for alkaline anion exchange membrane fuel cell applications.     

Preparation and characterization of fluorinated poly(aryl ether oxadiazole)s anion exchange membranes based on imidazolium salts

A series of fluorinated poly(aryl ether oxadiazole)s ionomers based on imidazolium salts (FPAEO-xMIM) were synthesized by quaternization of bromomethylated poly(aryl ether oxadiazole)s (FPAEO-xBrTM) with 1-methyl imidazole as aminating reagent. The anion exchange membranes (AEMs) were prepared by casting method and then immerged in aqueous sodium hydroxide for hydroxide ion exchanging. The structure of the obtained ionomers was characterized by ¹H-NMR and FT-IR measurements. The physical and electrochemical properties of the membranes were also investigated. The hydroxide conductivity of FPAEO-xMIM membranes was higher than 10⁻² S cm⁻¹ at room temperature, while the water uptake and swelling ratio was moderate even at elevated temperature. TGA analysis revealed that the membranes based on imidazolium salts had good thermal stability.     

The effect of electrode parameters on the performance of anion exchange polymer membrane fuel cells

An investigation of several electrode parameters on performance of an alkaline membrane fuel cell is described. The studied parameters were: ionomer content, anode and cathode catalyst layer thickness, electrode aminating agent and membrane thickness.It was found that an optimum ionomer content depended on a balance between the OH⁻ ion/water mobility and the oxygen solubility/diffusivity through it and which varied with temperature. Thick catalyst layers were necessary for the anode as thin anode catalyst layers suffered from flooding. 40%Pt/C provided the best thickness (with loading of 0.4 mg_Pt cm⁻²) for cathodes operating with air.An aminated low density poly(ethylene-co-vinyl benzyl chloride) (LDPE-VBC) membrane was shown to be a good membrane for an alkaline membrane fuel cell, giving conductivities up to 0.13 S cm⁻¹ at 80 °C. A Membrane Electrode Assembly (MEA) utilizing this membrane with fully hydrated thickness of 57 μm produced good peak power density, at a high potential of 500 mV, of 337 mW cm⁻² with air (1 bar gauge) at 60 °C.     

Anion-exchange membranes composed of quaternized-chitosan derivatives for alkaline fuel cells

A series of quaternized-chitosan derivatives (QCDs) with various degrees of quaternization was synthesized using glycidyltrimethylammonium chloride as a main quaternized reagent. These QCDs were then processed into hydroxide—form quaternary ammonium salts with aqueous potassium hydroxide solutions. The resultant hydroxide—form QCD gels were further crosslinked into anion-exchange membranes using ethylene glycol diglycidyl ether. The crosslinking density, crystallinity, swelling index, ion exchange capacity, ionic conductivity and thermal stability of the crosslinked membranes were subsequently investigated. It was found that properties of crosslinked membranes were modulated mainly by the degree of quaternization and crosslinking density of membranes. Some membranes exhibited promising characteristics and had the potential for applications in alkaline polymer electrolyte fuel cells in considering their integrative properties.     

A comprehensive study on the stability and ion transport in cross-linked anion exchange membranes based on polysulfone for solid alkaline fuel cells

Anion exchange membranes were prepared based on polysulfone and functionalized by trimethylamine and N,N,N′,N′-tetramethyl-1,6-hexanediamine using chloromethylation, amination and alkalization steps. N,N,N′,N′-tetramethyl-1,6-hexanediamine with a long alkyl chain has integrated crosslinking and amination processes into a single step and enhances psychochemical properties of the membranes. ¹HNMR, FTIR, TGA and DCS analysis were done besides mechanical strength and alkaline stability studies to characterize membranes comprehensively. The prepared membranes exhibited through plane ionic conductivity of 2–42 mS/cm at 25–80 °C in different relative humidities. The ion exchange capacities, anion transport numbers and hydration numbers were within the range of 1.6–2.1 mmol/g, 0.95–0.98 and 9–16, respectively. Furthermore, the membranes showed acceptable water uptake and swelling ratio at different temperatures. A single H₂/O₂ fuel cell test was carried out at 60 °C which resulted in an open circuit voltage of 1.05 V and maximum power density of 110 mW/cm² at current density of 195 mA/cm². Based on the experiments, it can be inferred that suitable proportions of amination and crosslinking agents, could lead to the membranes with well-balanced performance between ion transport and stability. Regarding the improved electrochemical and physicochemical properties, these anion exchange membranes are proposed as a good candidate for solid alkaline fuel cells.     

Electrochemical and fuel cell evaluation of Co based catalyst for oxygen reduction in anion exchange polymer membrane fuel cells

Co based catalyst were evaluated for oxygen reduction (ORR) in liquid KOH and alkaline anion exchange membrane fuel cells (AAEMFCs). In liquid KOH solution the catalyst exhibited good performance with an onset potential 120 mV more negative than platinum and a Tafel slope of ca. 120 mV dec⁻¹. The hydrogen peroxide generated, increased from 5 to 50% as the electrode potential decreased from 175 to −300 mV vs. SHE.In an AAEMFC environment, one catalyst (GP2) showed promising performance for ORR, i.e. at 50 mA cm⁻² the differences in cell potential between the stable performance for platinum (more positive) and cobalt cathodes with air and oxygen, were only 45 and 67 mV respectively. The second catalyst (GP4) achieved the same stable power density as with platinum, of 200 and 145 mW cm⁻², with air at 1 bar (gauge) pressure and air (atm) cathode feed (60 °C), respectively. However the efficiency was lower (i.e. cell voltage was lower) i.e. 40% in comparison to platinum 47.5%.     

Preparation and characterization of quaternary ammonium functionalized poly(2,6-dimethyl-1,4-phenylene oxide) as anion exchange membrane for alkaline polymer electrolyte fuel cells

Anion exchange membrane from poly(phenylene oxide) containing pendant quaternary ammonium groups is fabricated for application in alkaline polymer electrolyte fuel cells (APEFCs). Chloromethylation of poly(phenylene oxide) (PPO) was performed by aryl substitution and then homogeneously quaternized to form an anion exchange membrane (AEM). The influence of various parameters on the chloromethylation reaction was investigated and optimized. The successful introduction of the above groups in the polymer backbone was confirmed by ¹H NMR and FT-IR spectroscopy. Membrane intrinsic properties such as ion exchange capacity, water uptake and ionic conductivity were evaluated. The membrane electrolyte exhibited an enhanced performance in comparison with the state-of-the-art commercial AHA membrane in APEFCs. A peak power density of 111 mW/cm² at a load current density of 250 mA/cm² was obtained for PPO based membrane in APEFCs at 30 °C.     

Quantifying water transport in anion exchange membrane fuel cells

Sufficient water transport through the membrane is necessary for a well-performing anion exchange membrane fuel cell (AEMFC). In this study, the water flux through a membrane electrode assembly (MEA), using a Tokuyama A201 membrane, is quantified using humidity sensors at the in- and outlet on both sides of the MEA. Experiments performed in humidified inert gas at both sides of the MEA or with liquid water at one side shows that the aggregation state of water has a large impact on the transport properties. The water fluxes are shown to be approximately three times larger for a membrane in contact with liquid water compared to vaporous. Further, the flux during fuel cell operation is investigated and shows that the transport rate of water in the membrane is affected by an applied current. The water vapor content increases on both the anode and cathode side of the AEMFC for all investigated current densities. Through modeling, an apparent water drag coefficient is determined to −0.64, indicating that the current-induced transport of water occurs in the opposite direction to the transport of hydroxide ions. These results implicate that flooding, on one or both electrodes, is a larger concern than dry-out in an AEMFC.     

Well-designed mono- and di-functionalized comb-shaped poly(2,6-dimethylphenylene oxide) based alkaline stable anion exchange membrane for fuel cells

To mitigate the membrane stabilities (dimensional and mechanical) and ionic properties, we report well functionalized hydrophilic and hydrophobic (responsible for stabilities) phase separated quaternized anion exchange membrane (AEM). The N,N,N,N-Tetramethyl-1,3-propanediamine quaternized AEM spliced with alkyl chain (DQCP-36) formed self-amassed morphology with larger ionic clusters. The most suitable optimized AEM (DQCP-36) demonstrated enhanced hydroxide ion conductivity (4.66 × 10^?2 S cm^?1), ion-exchange capacity (1.35 meq./g) and lower activation energy (11.52 kJ/mol). These AEMs showed self-amassed morphology (well balanced hydrophilic and hydrophobic domain) and excellent stabilities (thermal, alkaline and dimensional). Under harsh alkaline medium (2 M NaOH) at 60 °C, DQCP-36 AEM showed about 9% reduction in conductivity after 700 h treatment, and assessment to be a suitable candidate for alkaline fuel cells.     

Anionic polysulfone ionomers and membranes containing fluorenyl groups for anionic fuel cells

Poly(arylene ether sulfone) ionomers containing fluorenyl groups and functionalized with benzyltrimethylammonium groups were synthesized through polycondensation, chloromethylation, and amination reactions. The resulting polymers were characterized by ¹H NMR, FT-IR and TGA. Polymer membranes were solvent cast from DMF on Teflon plates to form clear, flexible anion exchange membranes (AEMs). Carbonate anions had conductivities in the AEMs up to 63.12 mS cm⁻¹ at 70 °C and were used in a carbonate fuel cell. The membranes were stable in 1 M carbonate solution (pH 11). However, conductivity loss was observed during soaking in 1 M hydroxide solution (pH > 14) at 50 °C. A carbonate fuel cell operating at room temperature with H₂ at the anode and O₂ and CO₂ at the cathode had a maximum power density of 4.1 mW cm⁻².     

A review of anion exchange membranes prepared via Friedel-Crafts reaction for fuel cell and water electrolysis

Hydrogen is considered a potential, clean, and renewable energy for the future. Anion exchange membranes (AEMs) are significant components in AEM fuel cells and water electrolysis, crucial devices in the hydrogen industry. Friedel-Crafts (F–C) reaction has been widely used in preparing AEMs due to its versatility, high catalytic efficiency, relatively mild reaction conditions, etc. This review article provides a comprehensive literature survey for AEMs prepared via Friedel-Crafts reaction. Firstly, the fundamentals of the F–C reaction were introduced in detail, including the category, mechanism, catalyst and chloromethylating agent. Different types of AEMs, including polysulfones (PSUs), poly(arylene ether)s (PAEs), poly(ether ketones) (PEKs), and poly(2,6- dimethyl-1,4-phenylene oxide) (PPO), etc. were discussed. The cell performance of fuel cells and water electrolysis was investigated and analyzed. Finally, this review addresses the current challenges facing the development of AEM and proposed research implications for future investigations.     

High performance porous polybenzimidazole membrane for alkaline fuel cells

In this study, a highly ion-conductive and durable porous polymer electrolyte membrane based on ion solvating polybenzimidazole (PBI) was developed for anion exchange membrane fuel cells (AEMFCs). The introduction of porosity can increase the attraction of electrolytic solutions (e.g., potassium hydroxide (KOH)) and ion solvation, which results in the enhancement of PBI's ionic conductivity. The morphology, thermo-physico-chemical properties, ionic conductivity, alkaline stability, and the AEMFC performance of KOH-doped PBI membranes with different porosities were characterized. The ionic conductivity and AEMFC performance of 70 wt.% porous PBI was about 2 times higher than that of the commercially available Fumapem^® FAA. All KOH-doped porous PBI membranes maintained their ionic conductivity after accelerated alkaline stability testing over a period of 14 days, while the commercial FAA degraded just after 3 h. The excellent performance and good durability of KOH-doped porous PBI membrane makes it a promising candidate for AEMFCs.     

Novel poly(carbazole-butanedione) anion exchange membranes constructed by obvious microphase separation for fuel cells

To develop anion exchange membranes with excellent chemical stability and high performance. A series of quaternary ammonium functionalized (hydrophilic) hydrophobic rigid poly (carbazole-butanedione) (HOCB-TMA-x) anion exchange membranes were prepared, where x represents the percentage content of hydrophobic unit octylcarbazole (OCB). Due to the introduction of hydrophobic rigid unit octylcarbazole and hexyl flexible side chain, the hydrophilic-hydrophobic microstructure of AEMs was developed. The AEMs exhibit excellent overall performance, specifically the low swelling ratio HOCB-TMA-30 membrane exhibits the highest OH^? conductivity of 152.9 mS/cm at 80 °C. Furthermore, the ionic conductivity of AEM decreased by only 9.5% after 2250 h of immersion in 1 M NaOH. The maximum peak power density of a single cell with a current density of 4.38 A/cm² at 80 °C was 1.85 W/cm².     

Partially crosslinked comb-shaped PPO-based anion exchange membrane grafted with long alkyl chains: Synthesis,characterization and microbial fuel cell performance

Anion exchange membranes (AEMs) with higher ion exchange capacities (IECw) are limited to applications due to excessive swelling and higher water uptake. Crosslinked macromolecular structures have been a strategy to balance between ionic conductivity and swelling in membranes. However, highly crosslinked AEMs are usually mechanically brittle and poorer in ion transport. Thus we report a series of partially diamine crosslinked (X = 10%, 15%, 20%) comb-shaped AEMs functionalized with dimethylhexadecylammonium groups exhibiting improved flexibility, water uptake and swelling properties over conventional un-crosslinked or fully crosslinked materials. The higher conductivities in these PPO AEM(X) (for example, X = 20%, IECw = 1.96 mmol/g, σ(OH⁻) ~ 67 mS/cm at 80 °C) are attributed to the distinct nanophase separation as observed in SAXS and AFM analyses. Finally, the microbial fuel cell performances of the membranes were compared with commercially available cation and anion exchange membranes.     

Facile construction of poly(arylene ether)s-based anion exchange membranes bearing pendent N-spirocyclic quaternary ammonium for fuel cells

Anion exchange membrane (AEM) fuel cells have received significant attention due to their low fuel permeability and the use of non-platinum catalysts. However, the development of AEMs with robust chemical stability and high conductivity is still a great challenge. Herein, we prepare a new type of partially fluorinated backbone bearing pendent N-spirocyclic quaternary ammonium (QA) cations via a facile Williamson reaction, which displays great potential for fuel cells. The integration of the two substructures (a fluorinated moiety into a polymer backbone and a pendent cation structure) is beneficial for the fabrication of a well-defined micro-phase separation structure, thereby facilitating the construction of a highly-efficient ion transporting pathway. Correspondingly, the resulting AEM (PAENQA-1.0), despite its a relatively low ionic exchange capacity (0.93 meq g⁻¹) demonstrates a conductivity of 63.1 mS cm⁻¹ (80 °C). Meanwhile, the constrained ring conformation of N-spirocyclic QA results in improved stability of the AEMs.