Radiation-grafted Anion-exchange Membranes Research Articles

A high conductivity ultrathin anion-exchange membrane with 500+ h alkali stability for use in alkaline membrane fuel cells that can achieve 2 W cm−2 at 80 °C

This article describes the development of a sub-30 μm thick LDPE-based radiation-grafted anion-exchange membrane (RG-AEM) with high performance characteristics when fully hydrated.

Radiation-induced grafting of a butyl-spacer styrenic monomer onto ETFE: the synthesis of the most alkali stable radiation-grafted anion-exchange membrane to date

Adding a butyl spacer between the benzene and N-methylpyrrolidinium group led to double the ex situ alkaline stability compared to use of a methylene spacer.

Degradation of radiation grafted anion exchange membranes tethered with different amine functional groups via removal of vinylbenzyl trimethylammonium hydroxide

abstract Low-density polyethylene (LDPE)-based anion exchange membranes (AEMs) with 65% degree of grafting of vinylbenzyl chloride (VBC) were tethered with different amine functionalities namely, trimethyl amine (TMA), 1,4-diazabicyclo[2.2.2]octane (DABCO), 1-azabicyclo[2.2.2]octane (ABCO) and N-methylpiperidine (NMP), and were subjected to degradation test by immersing the OH − exchanged AEMs in deionised water at 60 °C, a condition analogous to fuel cell and electrolyser environment. All the quaternised membranes, regardless of the tethered amine functional group, exhibited similar degradation loss of ca. 5% IEC per month. Benzylic peroxide was detected in the degradation solution in all the tested AEMs. The observed degradation of the OH − exchanged AEMs was mainly attributed to peroxide and hydroxide radical attacks on the ternary (benzylic) carbon resulting in the release of vinylbenzyl trimethylammonium hydroxide (VBTMA) as a whole which was also detected. The degradation test performed on TMA-functionalised membrane under nitrogen, oxygen and 3 wt% H 2 O 2 showed similar degradation products namely benzylic peroxide and VBTMA suggesting that the degradation mechanisms under these three conditions are similar. The over three-fold increase in the degradation rate under oxygen saturated solution compared to nitrogen is due to the higher peroxide concentration produced from oxygen reduction to superoxide via ylide.

Performance of polyethylene based radiation grafted anion exchange membrane with polystyrene-b-poly (ethylene/butylene)-b-polystyrene based ionomer using NiCo2O4 catalyst for water electrolysis

A soluble anion exchange ionomer with high OH− ion conductivity comparable to that of H+ conductivity of Nafion is synthesised by chloromethylation of polystyrene-b-poly (ethylene/butylene)-b-polystyrene (SEBS) and used with NiCo2O4 electro-catalyst for water electrolysis. The ionomer has an ion exchange capacity of 1.9 mmol g−1 and ionic conductivity of 0.14 S cm−2 at 50 °C. The cell voltage at 20 °C at 100 mA cm−2 is 1.77 and 1.72 V in, 0.1 and 1.0 M NaOH, respectively, for an optimum loading of 10 mg cm−2 NiCo2O4. At 10 mg cm−2 NiCo2O4 electrolyser cell performance is at least equal to or superior to that of IrO2 at 2 mg cm−2 with excellent stability over 1 h. When the catalyst is sprayed on the GDL instead of CCM, the performance is further improved to 1.65 V at 100 mA cm−2 at 60 °C & 0.1 M KOH. The limited AEM electrolyser performance when operating with deionised water in comparison to PEM and alkaline electrolyser arises from the sluggish OER in the AEM environment equivalent to pH of 11.5 and the two orders of magnitude lower HER activity with respect to acid medium combined with the high Tafel slope of 120 mV dec−1.

High Performance Radiation-Grafted Anion Conducting Polymer Electrolytes for Energy Applications

This presentation will describe the latest research at the University of Surrey and the development of radiation-grafted anion-exchange membranes (RG-AEM) and ionomer powders (AEI) for use in devices such as alkali membrane fuel cells. The presentation will provide an overview of the following recent achievements: Synthesis of RG-AEMs with heterocyclic chemistries that are more stable to alkali that the those containing the benzyltrimethylammonium benchmark;Synthesis protocols that require reduced radiation doses, reduced amounts of monomer and the use of water rather than organic solvents;Synthesis of thinner RG-AEMs;The ability to produce RG-AEMs based on low density polyethylene (LDPE) rather than partially fluorinated ETFE;Synthesis of an interesting class of solid-state AEI powders;AEMs with high hydroxide conductivities and high performances in H2/O2 fuel cells. For example, the graph show the beginning-of-life 60°C H2/O2 fuel cell performances achieved with Pt-based catalysts and RG-AEMs made from 25 μm ETFE (red) and 13 μm ETFE (green) with no gas back-pressurization applied.

Degradation of radiation grafted hydroxide anion exchange membrane immersed in neutral pH: removal of vinylbenzyl trimethylammonium hydroxide due to oxidation

Degradation tests in deionised water of radiation grafted anion exchange membranes for fuel cell/electrolyser operation showed loss of vinylbenzyl trimethylammonium groups.

A Raman spectro-microscopic investigation of ETFE-based radiation-grafted anion-exchange membranes

Raman spectro-microscopy on a radiation-grafted anion-exchange membrane detected alkali degradation throughout its cross-section that quantitatively matched loss of ion-exchange capacity.

An optimised synthesis of high performance radiation-grafted anion-exchange membranes

The improved synthesis of radiation-grafted anion-exchange membranes (AEM) using water as a diluent and with reduced electron-beam absorbed doses and monomer amounts.

The first anion-exchange membrane fuel cell to exceed 1 W cm-2 at 70 °C with a non-Pt-group (O2) cathode.

Anion-exchange membrane fuel cells face two challenges: performance and durability. Addressing the first, we demonstrate high performance with both O2 and CO2-free air supplies, even when using a Ag/C cathode. This was enabled by the development of a radiation-grafted anion-exchange membrane that was less than 30 μm thick when hydrated.

(Invited) Improvements in the Anion Exchange Membrane Transport of Carbonate and Bicarbonate for Low-Temperature CO2 Capture and Energy Conversion

In recent years, anion exchange membrane (AEM) devices have become a topic of significantly increased interest. Unlike liquid electrolyte alkaline fuel cells, which precipitate potassium carbonate salts when exposed to carbon dioxide, the lack of mobile cations in anion exchange membranes prevents the rapid cell death that results from salting. Though carbonate and bicarbonate transport in anion exchange membranes has typically been poor, recent improvements have facilitated better mobility and opened the door for many different types of carbonate based devices.1 Electrochemical carbon dioxide separators2, 3, room temperature carbonate fuel cells4, and methane activation reactors5 are all promising applications of carbonate based anion exchange membranes, but with typical current densities reported in the single mA/cm2range, improvements in carbonate transport are necessary to improve performance. In a typical carbonate-based device, oxygen is reduced with carbon dioxide at the cathode, generally through an indirect pathway first to hydroxide, then (bi)carbonate. This pathway results in a mixed anion composition throughout the AEM where three very different anions can carry the charge across the membrane. Bicarbonate and carbonate (especially bicarbonate) have lower conductivities than hydroxide, but this cannot alone explain the 2 to 3 order of magnitude performance loss from a carbonate-free cell (typically 100s of mA/cm2 to 1000s of mA/cm2). In a carbon dioxide pump, the oxygen evolution reaction results in evolved carbon dioxide, while in a carbonate fuel cell hydrogen oxidation results in carbon dioxide without evolving oxygen. The cell potential and resulting half reactions influence the mixed transport and the amount of carbonate, bicarbonate, and hydroxide anions transported. In both carbon dioxide pumps and carbonate fuel cells a relationship is observed between the evolution of carbon dioxide at the anode and the flow of electrons in the cell. This relationship between current and evolved CO2in both pump mode and fuel cell mode is indicative of the anion ratio and the exchange mechanisms occurring at the anode the cathode. In this study, a peak current power density of 400 mA/cm2 was achieved using a carbonate-exchange membrane fuel cell (Figure 1a) and 100 mA/cm2 was achieved in CO2 pumping mode (Figure 1b). The difference in the current is a reflection of the difference in the dominating anion in the membrane as well as the difference in anode reactivity of hydrogen oxidation vs. carbonate electrolysis. This performance was achieved with radiation grafted anion exchange membranes and ionomers.6In this work, it was found that performance was very sensitive to AEM humidification; there is a delicate balance between maintaining a high water content in the membrane to facilitate anion transport and catalyst and GDL flooding. The cell anion balance was examined in these high performing cells. The order of magnitude increase in the carbonate/bicarbonate transport observed in this work significantly increases the viability of membrane based carbon dioxide pumps, fuel cells, and fuel conversion devices. However, our results also point to advances that are needed in both catalysts and membranes to increase the faradaic efficiency in high current density carbonate devices.

High performance aliphatic-heterocyclic benzyl-quaternary ammonium radiation-grafted anion-exchange membranes

High performance benzylpyrrolidinium anion-exchange membrane.

Radiation-Grafted Anion-Exchange Polymer Electrolytes for Clean Energy Applications

Anion-exchange polymer electrolyte materials are being developed and studied for an increasing diversity of clean energy technologies, such as alkaline polymer electrolyte fuel cells, redox flow batteries, biological fuel cells, and reverse electrodialysis cells. These technologies include the application of the anion-exchange polymer electrolytes in conditions that span the pH range from highly acidic, through more neutral environments, and finally to highly alkaline. This presentation will highlight the versatility of the radiation-grafting methodology in that it allows the production of polymer electrolytes containing numerous different morphologies and chemistries, including both cation-exchange and anion-exchange types. Once initial (small scale) experiments have been conducted and synthesis protocols have been developed, this methodology has the advantage that reasonably large (for lab-scale) quantities of materials can be synthesised, e.g. 10 – 100s of grams and several m2 sized batches, with excellent repeatabilities; this also allows for a large number of tests and experiments to be conducted on the same batch of material. This synthesis platform therefore facilitates research into such materials including research that compares ion-exchange materials where a single variable has varied: e.g. anion-exchange polymer electrolyte materials with the same ion-exchange capacities and polymer backbones but where the chemistries of the cationic head-groups is varied, anion-exchange polymer electrolyte materials with the same ion-exchange capacities and cationic head-groups but where the polymer backbones are varied, or anion-exchange polymer electrolyte materials with the same polymer backbone and head-group chemistries but where the ion-exchange capacity is varied.The materials to be discussed include radiation-grafted anion-exchange membranes (with similar ion-exchange capacities) that possess an array of cationic head-group chemistries, including the benchmark benzyltrimethylammonium, benzyltriethylammonium, benzylimidazoliums, benzylpyridiniums, benzylpiperidiniums, and benzylmorpholinium. Some of these anion-exchange polymer electrolytes have reasonable stabilities at alkaline pHs (e.g. for use in alkaline polymer electrolytes fuel cells), whilst others can only be applied to systems where the pHs, that the materials are exposed to, are more neutral (e.g. materials for use in reverse electrodialysis cells). The ionic conductivities of these materials appear to be high enough for use in the above mentioned technologies. As a small aside, it will also be demonstrated that cation-exchange membranes with different chemistries can also be synthesised and this is of interest to applications such as the more well-known proton-exchange membrane fuel cells as well as reverese electrodialysis. Figure 1

The alkali stability of radiation-grafted anion-exchange membranes containing pendent 1-benzyl-2,3-dimethylimidazolium head-groups

Radiation-grafted anion-exchange membranes (AEM) containing pendent benzyltrimethylammonium, 1-benzyl-3-methylimidazolium and 1-benzyl-2,3- dimethylimidazolium functional head-groups were synthesised with ion-exchange capacities in the range 1.7-1.9 meq g. The ionic conductivities of the AEMs were also comparable (24.5 ± 1.8 mS cm at 50°C). The alkali stability (in aqueous potassium hydroxide (1 mol dm ) at 60°C) of the 1-benzyl-2,3-dimethylimidazolium head-groups was superior to the 1-benzyl-3-methylimidazolium but inferior to the benzyltrimethylammonium benchmark head-groups. Radiation-grafted AEMs containing pendent 1-benzyl-2,3-dimethylimidazolium head-groups are not suitable for application in electrochemical devices containing highly alkaline environments. © 2013 The Royal Society of Chemistry.

Poly(ethylene-co-tetrafluoroethylene)-Derived Radiation-Grafted Anion-Exchange Membrane with Properties Specifically Tailored for Application in Metal-Cation-Free Alkaline Polymer Electrolyte Fuel Cells

A new class of stable poly(ethylene-co-tetrafluoroethylene)-based alkaline anion-exchange membrane (AAEM) with enhanced tensile strength has been synthesized in response to the poor mechanical properties of previously reported poly(tetrafluoroethylene-co-hexafluoropropylene) radiation-grafted AAEMs; this type of AAEM exhibits significant through-plane conductivities (up to 0.034 ± 0.004 S cm-1 at 50 °C in water: conductivities that match requirements for application in fuel cells). The methanol permeabilities of this new AAEM class were found to be substantially reduced relative to Nafion-115 proton-exchange membranes; this offers the prospect that thin, low-resistance membranes may be used in direct methanol alkaline fuel cells with reduced methanol crossover. The fuel cell power performances obtained in a H2/O2 single fuel cell at 50 °C with this AAEM is now within 1 order of magnitude of state-of-the-art Nafion-based fuel cells. It is evident that the alkaline ionomers are not the primary performance limiters of alkaline membrane fuel cells; performances are currently limited by the electrode architectures that have been optimized for use in PEM fuel cells but not alkaline fuel cells. The need for electrodes and catalyst structures that have been specifically tailored for use in AAEM-containing fuel cells is highlighted.