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 mm thick LDPE-based radiation-grafted anion-exchange membrane (RG-AEM) with high performance characteristics when fully hydrated. This RG-AEM had a OH anion conductivity of 200 mS cm 1 (80 C in 100% relative humidity (RH) environments), which led to a H2/ O2 anion-exchange membrane fuel cell (AEMFC) performance of 2.0 W cm 2 (80 C, RH 1⁄4 92% environments, a PtRu/C anode, and a Pt/C cathode) and a H2/air (CO2-free) AEMFC peak power density of 850 mW cm 2 with a (non-platinum-group) Ag/C cathode electrocatalyst. When hydrated in a RH 1⁄4 100% N2 (CO2-free) atmosphere, the OH form of this RG-AEM shows <7% degradation after 500 h at 80 C, with the extent of degradation being highly similar to that when measured using three different techniques (decrease in conductivity, decrease in ammonium content as measured using Raman spectroscopy, and decrease in ion-exchange capacity).


Background and context
Anion-exchange membranes (AEMs) are polymeric membranes that typically contain covalently bound cations (e.g.][18][19] There is now a wide range of AEMs that have been developed. 20he AEMs developed by the groups of Jannasch, 21,22 Hickner, 23,24 Holdcro, 25,26 Mamlouk, 27 Xu, [28][29][30][31] and Zhuang 32,33 are particularly noteworthy with respect to being alkali stable and/or high performance as well as having potential for scale-up synthesis/ fabrication or recyclability.7][38] We have recently demonstrated that the fabrication of poly(ethylene-co-tetrauoroethylene)-(ETFE)-based RG-AEMs led to high performance AEMFCs, even when using Ag/C cathodes. 39We have also shown that using lowdensity polyethylene (LDPE), rather than ETFE, leads to RG-AEMs that are more mechanically robust, such that they can be routinely applied to AEMFCs that are operated at 80 C. 12 This article extends on these prior ndings and reports the development of a sub-30 mm thick LDPE-based RG-AEM with high AEMFC performances at 80 C. In the past, the synthesis of ultrathin radiation-graed AEMs proved to be difficult due to the resulting mechanical instability.With this new AEM, we successfully achieved both aspects.An important challenge for the development of high performance AEMFCs is the area specic resistance, which can be improved by reducing the thickness of the AEM.This was the original motivation for this study.However, as we show below, an additional, important nding was that the move to an ultrathin AEM resulted in benecial reductions in AEMFC mass-transfer losses.We hypothesise that this is due to ultra-facile H 2 O transport from the anode to the cathode, through the AEM, despite this H 2 O transport being in the opposite direction to OH À conduction.

Chemicals and materials for RG-AEM synthesis
Low density polyethylene (LDPE, product code ET311115) lms of 15 mm thickness were purchased from Goodfellow (UK) and were used without any pre-treatment steps (e.g.removal of additives or change of crystallinity).The vinylbenzyl chloride (VBC) monomer was supplied by Sigma-Aldrich (product code 338729, mixture of 3-and 4-isomers) and used without removal of inhibitors (initial concentrations on purchase: 50-100 ppm tert-4-butylcatechol and 700-1100 ppm nitromethane).The 1octyl-2-pyrrolidone dispersant and aqueous trimethylamine (TMA, 45 wt%) were also purchased from Sigma-Aldrich, and toluene and propan-2-ol were of reagent grade (Fisher Scientic, UK).All chemicals were used as received, and the ultrapure water (UPW) was of 18.2 MU cm resistivity.

Synthesis of the optimised LDPE15-AEM
The below describes the synthesis of the RG-AEM fabricated from the 15 mm LDPE lms using optimised graing parameters (denoted from now on as LDPE15-AEM).The optimisation study (variables ¼ graing temperature and duration) is discussed below in the Results and discussion section (Table 1).The RG-AEM fabrication process is summarised in Scheme 1.
The LDPE lms were irradiated in air to an absorbed dose of 100 kGy using a 4.5 MeV Dynamatron continuous electronbeam unit (Synergy Health, South Marston, UK): the dose rate was fast with the LDPE lms being irradiated to an absorbed dose of 10 kGy per pass under the e-beam.As this is the peroxidation-type pre-irradiation graing method, this process leads to the "activation" of the LDPE lms by the introduction of peroxy-groups covalently bound to the LDPE polymer chains. 40ence, the LDPE lms can act as a solid-state initiator for subsequent free-radical graing of vinyl monomers.Aer irradiation, the lms were transported in dry ice back to the laboratory at the University of Surrey, where they were subsequently stored in a freezer at À40 C until use: the lm in this study was used within a 12 week storage time.
For the graing step, the e-beam LDPE lm (15 Â 15 cm) was removed from the freezer and immersed in a glass vessel containing an aqueous mixture of VBC (5% vol) and the 1-octyl-2-pyrrolidone dispersant (1% vol).This mixture was then purged with N 2 at room temperature for 2 h before being sealed to prevent any egress of O 2 .The graing process was conducted by heating the vessel at 40 C for 6 h.Aer graing, the LDPE-gpoly(VBC) intermediate membrane was washed with copious amounts of toluene (to remove any un-graed poly(VBC) homopolymer) before being dried at room temperature for 16 h.The degree of graing (DoG) was calculated as follows: where m g ¼ mass of the graed LDPE-g-poly(VBC) intermediate membrane and m i ¼ mass of the irradiated LDPE lm.
For the amination step, the LDPE-g-poly(VBC) intermediate membrane was submerged in an aqueous TMA solution at room temperature for 24 h.The resulting crude RG-AEM was then thoroughly washed with UPW and then heated in UPW for 1 h at 60 C.This procedure was adopted to remove any excess TMA and remaining un-graed (now aminated) homopolymer.The nal production of the Cl À anion form of the LDPE15-AEM was assured by a nal ion-exchange process, which involved immersion of the RG-AEM in aqueous NaCl (1.0 mol dm À3 ) for 15 h with one change of NaCl solution during this period.Aer a nal, thorough UPW washing step (to remove any excess Na + and Cl À ions, such that the only Cl À anions present are those that are counter balancing the +ve charges on the graed polymer chains), the nal LDPE15-AEM was stored in UPW until required (and was not allowed to dry out at any point before the below measurements and experiments were conducted).

Characterisation of the LDPE15-AEM in the Cl À anion form
The LDPE15-AEM (in the Cl À anion form) was characterised for ion-exchange capacity (IEC, mmol Cl À anions per g of dry AEM) gravimetric water uptake (WU, %), through-plane swelling (TPS, %), and Cl À conductivity (s/mS cm À1 , immersed in UPW, 4-Table 1 Results from optimisation of the VBC grafting step.The precursor LDPE films (15 mm thick) were irradiated in air with an electron-beam (4.5 MeV) to an absorbed dose of 100 kGy.The grafting mixture consisted of VBC (5% v/v), ultrapure water (94% v/v) and the 1octyl-2-pyrrolidone dispersant (1% v/v).Errors in the ion-exchange capacities (IECs) of the final AEMs (produced on amination of the grafted membranes with trimethylamine) are sample standard deviations from measurements on n ¼ 3 samples of each AEM probe) by Raman spectro-microscopy, and tensile stress-strain testing using extensively reported routine methods: we did not deviate from the methods published in ref. 12.
Determining the OH À conductivity of the LDPE15-AEM The conductivity (s/mS cm À1 ) of the LDPE15-AEM in the OH À anion form was determined at various temperatures up to 80 C in relative humidity RH ¼ 100% N 2 atmospheres.We used a method that was a minor adaptation of the method recently reported by Ziv and Dekel. 41LDPE15-AEM samples were submerged in aqueous KOH (1.0 mol dm À3 ) solution at room temperature for 1 h, before being thoroughly washed with UPW to remove excess KOH species.As this process was not conducted in a CO 2 -free environment, the anions in the samples would be an indeterminate mixture of OH À , HCO 3 À , and CO 3 2À anion forms.The samples of the LDPE15-AEM (in the mixed anion form) were then transferred into a BekkTech BT-112 conductivity cell containing 2 outer Pt-mesh electrodes (for current ow) and 2 inner Pt-wire voltage sensing electrodes.The BT-112 cell was then tted into a Fuel Cell Technologies fuel cell xture (supplied by Scribner Associates, USA) between two graphite ow elds (serpentine ow-pattern, 5 cm 2 ).This entire assembly was then connected to a Scribner 850C fuel cell test station and supplied with 500 cm 3 min À1 humidied (RH ¼ 100%) high purity N 2 gas.The OH À forms of the LDPE15-AEM samples were in situ generated by passing a 0.1 mA d.c.current between the outer Ptmesh electrodes (initially at 25 C for 24 h) as per the method by Ziv and Dekel. 41These were time consuming experiments as each sample was held at each temperature (25 / 40 / 60 / 80 C) for 24 h, with the d.c.current owing, before the resistances were determined via electrochemical a.c.impedance spectroscopy (EIS) using a Solartron 1260/1287 electrochemical set-up controlled with ZPlot soware (Scribner Associates, USA).EIS spectra were collected by applying an a.c.voltage (frequency range 0.3 Hz to 100 kHz, 10 mV amplitude) between the 2 inner Pt-wire electrodes and recording the a.c.current response between the outer Pt-mesh electrodes.Ionic resistances (Rs) were extracted from the low frequency x-axis intercepts in the collected EIS spectra.The OH À conductivity for each sample of the LDPE15-AEM (n ¼ 3 samples were tested) was calculated using: where l is the distance between the Pt-wire sensing electrodes (0.425 cm) and w and t are the width and thickness of the LDPE15-AEM sample being tested.
500 h alkali stability test (80 C, RH ¼ 100% N 2 atmosphere) Aer the nal 80 C data point was recorded during the OH À conductivity testing of the nal LDPE15-AEM sample (dened as 0 h in this study), the sample was retained inside the conductivity test assembly at 80 C (RH ¼ 100% N 2 500 cm 3 min À1 gas ow) for 500 h with the 0.1 mA d.c.current owing and with periodic EIS measurements to record the changes in R (and hence s) with time.The change in s between the start and end of the 500 h test was quantitatively compared to the change in IEC and the change in Raman spectral data (discussed in detail later).

AEMFC membrane electrode assembly (MEA) preparation
The catalysed gas diffusion electrode (GDE) method was used for fabricating the AEMFC electrodes.Prior to formulation of the electrocatalyst ink, a previously synthesised ETFE-based RG anion-exchange ionomer (AEI) powder, containing benzyltrimethylammonium functional groups with an IEC ¼ 1.26 AE 0.06 mmol g À1 , was ground with a pestle and mortar for 10 min. 42This was the AEI powder used in previous studies All AEI-containing electrodes and RG-AEMs were immersed in aqueous KOH solution (1 mol dm À3 ) for 1 h and then thoroughly washed with water (to remove all excess KOH ions) before assembly between two graphite bipolar ow eld plates using 5 N m torque (Fuel Cell Technologies fuel cell xture supplied by Scribner Associates (USA), serpentine ow-pattern, 5 cm 2 ).Each MEA consisted of an anode, a cathode and a RG-AEM.No prior hot-pressing of the MEA was used: the lamination of the electrodes to the RG-AEM occurs in situ.

AEMFC testing procedures
An 850E fuel cell test station (Scribner Associates, USA) was used for testing.The fuel cell temperature was controlled at 80 C. The H 2 and O 2 gas feeds were supplied to the anode and cathode, respectively, with ow rates of 1 SLPM and with no back-pressurisation (it was estimated that there was 0.2 bar pressure drop across each ow eld): both gas feeds contained ca. 10 ppm CO 2 by the time they had been piped to the fuel cell testers.Testing was also conducted with a 1 SLPM puried air (<1 ppm CO 2 ) gas supply to the cathode.The dew-points for the anode and cathode gas supplies were 78 C and 78 C, respectively (calculated RH ¼ 92%).The MEAs were activated by discharging the cell at a constant voltage of 0.5 V during cell heating, until a cell temperature of 80 C was obtained, followed by retention of this cell voltage until a steady current density was observed.Initial AEMFC performance data were collected with galvanostatic discharge steps where data (at each current density) were only recorded once the potentials had stabilised.The internal ohmic resistances were estimated using the internal current interrupt method of the fuel cell tester.

RG-AEM characterisation
As discussed above, we recently developed peroxidated radiation-graed anion-exchange membranes based on LDPE that combined high conductivity with mechanical robustness at higher temperatures. 12However, the dry thickness of this prior developed RG-AEM (made from 25 mm thick LDPE precursor lms, designated from now on as LDPE25-AEM) was 45 mm, which risks less than optimal H 2 O transport from the anode (where H 2 O is generated) to the cathode (where H 2 O is electrochemically consumed).This was the principal rationale for the development of a thinner LDPE15-AEM (described in detail below).
An outline of the synthesis of the LDPE15-AEM can be found in Scheme 1 (made from 15 mm LDPE that was electron-beam irradiated to an absorbed dose of 100 kGy).Table 1 shows the results of a study focused on the optimisation of the key parameters of the VBC (monomer) graing step.The optimised temperature for fabricating the LDPE15-AEM was found to be 40 C, which is lower than the 55 C needed to fabricate the LDPE25-AEM (see Table S1 †), while LDPE15-AEM synthesis required a shorter graing time (6 h vs. the 16 h graing time that was required to synthesise the thicker, prior-developed LDPE25-AEM).The shorter graing time and temperature required stem from the shorter monomer diffusion distances involved when using a thinner LDPE precursor lm.
The key properties of the nal optimised LDPE15-AEM are summarised in Table 2 alongside data for the LDPE25-AEM (extracted from ref. 12) that was made using the 25 mm LDPE precursor lm (also using electron-beam irradiation to an absorbed dose of 100 kGy).Both LDPE-based RG-AEMs had IECs above the target 2.5 mmol g À1 , which led to both having high anion conductivities (in both Cl À and OH À forms).For the LDPE15-AEM, both the lower IEC and higher WU (diluting the concentration of Cl À anions in the fully hydrated RG-AEM) lead to slightly reduced Cl À conductivities (Table 2 and Fig. S1 in the ESI †).As a standard, we always report the Cl À conductivity of RG-AEMs in water (fully hydrated conditions) as this is the most reliable and rapid measurement that can be conducted (that suffers from no CO 2 interferences), which facilitates interlaboratory comparisons.The Cl À conductivities (fully hydrated) and OH À conductivities (RH ¼ 100% atmosphere) of the LDPE15-AEM are compared in Fig. 1.
Increases in the conductivities of ion-exchange membranes, such as AEMs, generally correlate with increasing WUs until this causes such excessive swelling that the concentration of ionic charge carriers in the membranes is diluted enough to adversely affect (reduce) conductivity.However, an advantage of this class of LDPE-AEMs is that they retain relatively low dimensional swelling even at high water uptakes.Hence, even with a WU of 149%, our LDPE15-AEM achieves 208 mS cm À1 OH À conductivity at 80 C. The ability to retain high conductivity with high water contents is vital for assisting the maximising of alkali stability of AEMs (hydrated AEMs degrade less than dehydrated AEMs).
The distribution of functionality through the core (thickness/cross-section direction) of the LDPE15-AEM was investigated using Raman microscopy (Fig. 2a).The maps present the ratio of the following peak areas (ca. 1 mm spatial resolution): the integration of the 753 cm À1 peak (related to the trimethylammonium group) normalised to the integration of the 1130 cm À1 peak (related to the LDPE backbone).These data give an indication of the homogeneity of both graing and amination through the core of the RG-AEM.The distribution of  ammonium groups was observed to be relatively uniform for both the LDPE15-AEM and LDPE25-AEM (Fig. 2).The lower proportion of less graed zones in the LDPE15-AEM (cf.LDPE25-AEM) explains the higher water contents (and hydration numbers) of the LDPE15-AEM even with the lower IEC.Despite the higher water content of the LDPE15-AEM, its dimensional swelling (TPS) remains below 30%: an ideal AEM can maintain high water contents (for high water transport ability) without excessive dimensional expansion.

Ex situ alkali stability data
The literature now commonly reports the development of AEMs that have "excellent" stability at temperatures of 60 C or below, 44 but we feel that such tests are not very helpful.Investigating the stabilities of AEMs at 80 C (especially under controlled RH conditions rather than when submerged in water) is deemed much more meaningful.Ultimately, long-term testing needs to be conducted in situ (inside a fuel cell with high current densities). 17However, alongside the AEM itself, membrane electrode assemblies (MEAs) generally contain ionomers (ETFE-based radiation-graed powders in this study), electrocatalysts (see later), and carbon-based catalyst supports and gas diffusion media, which can all undergo different types and rates of degradation.As this is an AEM development study, we decided to record alkali stability test data by monitoring OH À conductivities (in a 4-probe cell located inside a fuel cell test xture) over 500 h at 80 C when the LDPE15-AEM was exposed to a CO 2 -free atmosphere supplied with N 2 gas ows at RH ¼ 100% (gas ows and humidity controlled by a Scribner fuel cell test station).The OH À form of the RG-AEM was generated (and maintained as the OH À form) inside the conductivity cell following a method that was adapted from the one developed by Dekel et al. (see experimental descriptions above). 41s can be seen from Fig. 3, the s(OH À ) of the LDPE15-AEM decreased from 202 mS cm À1 to 189 mS cm À1 aer 500 h at 80 C (in a RH ¼ 100% atmosphere): this represents a retention of conductivity of 93.8% (Table 3).A linear regression of the 500 h data yields a degradation rate of 28 AE 12 mS cm À1 h À1 (this error being the condence intervals at the 95% condence level).The regression over 3500 h, if constant degradation is assumed, is presented visually in Fig. S2 in the ESI † as this gives a good initial estimate of retention of conductivity aer several 1000 h, but clearly this needs to be veried in a future study that compares long term ex situ and in situ degradation.Taking the standard errors for the regression (both the slope and y-axis intercept) the worst-case s(OH À ) value aer 3500 h of degradation is 84 mS cm À1 and in the best-case scenario it is Fig. 2 Raman microscopic mapping through the core of a sample of the (a) LDPE15-AEM and (b) LDPE15-AEM (both in the dehydrated Cl À form).The through core direction is on the x-axis.The colour bars represent the ratios of the integrated area of the peak at 753 cm À1 (related to the ammonium groups) normalised to the area of the peak at 1130 cm À1 (related to the LDPE base material).Laser l ¼ 532 nm (8 mW).  a The integrated area of the peak at 753 cm À1 (related to the benzyltrimethylammonium groups that are lost in all common degradation mechanisms) normalised to the area of the peak at 1130 cm À1 (derived from the LDPE based material).The before and aer Raman spectra are presented in Fig. 4.
130 mS cm À1 (assumptions made: no catastrophic mechanical failure, pin-hole formation or disproportionate change in the H 2 O content).
As we have regularly preached ourselves,1 relying solely on conductivity vs. time data is not 100% reliable (as some degradation products may contribute towards conductivity), so we also measured the change in IEC and Raman data before and aer this ex situ conductivity test with the following health warning: due to the small sample size and time consuming nature of the experiment (with limited access to test equipment), we could only record before and aer data on a single sample of the LDPE15-AEM.As can be seen in Table 3, there was a 94.1% retention of IEC aer the 500 h alkali stability test (the RG-AEM also visually maintained its toughness and exibility).This retention of conductivity also precisely matches the quantitative change in the Raman spectra (Fig. 4) recorded before and aer the 500 h alkali stability test: there was a 93.8% retention of the area of the peak at 753 cm À1 (related to the trimethylammonium groups that are lost in all common degradation mechanisms) when normalised to the area of the peak at 1130 cm À1 (derived from the LDPE based material).For additional context, there was a 94.7% retention in the area of the 753 cm À1 peak when normalised to the area of the 1610 cm À1 benzene-ring-derived peak, while there was a 98.1% retention in the area of the 1610 cm À1 peak when normalised to the area of the 1130 cm À1 peak: this suggests that the degradation of the LDPE15-AEM is primarily based on nucleophilic processes at the quaternary ammonium group (leading to loss of -N + Me 3 from the benzyl graed chains) with a only minor contribution from the loss of complete benzyltrimethylammonium moieties (as we have discussed in detail previously). 36Post-degradation, a new peak is clearly seen at 1700 cm À1 , which is related to either a ketone C]O or C]C stretching; 45 formation of such functional groups is related to the minor cleavage of whole benzyltrimethylammonium groups, which we know occurs in benzyltrimethylammoniumcontaining RG-AEMs. 34,36The morphological changes of the surface of the LDPE15-AEM (before and aer the stability test in Fig. 3) are presented in Fig. S6.† The surface of the pre-degraded sample was smooth in appearance, whilst the post-degradation sample was rougher in appearance but with no evidence of critical mechanical failure (cracks nor holes).
H 2 /O 2 AEMFC performance data at 80 C The H 2 /O 2 AEMFC performances at 80 C for the LDPE15-AEM and LDPE25-AEM 12 are presented in Fig. 5 (MEA fabrication conditions were the same for both including the use of PtRu/C anodes and Pt/C cathodes).Moving to a thinner RG-AEM led to lower internal ohmic resistances (r ¼ 35 mU cm2 at peak power density for the LDPE15-AEM cf.49 mU cm 2 for the LDPE25-AEM) and the ability to generate higher current densities before the onset of mass-transport limitations: the latter is hypothesised to be due to enhanced, more rapid H 2 O transport from the anode to the cathode. 14,46The result of this was an impressive 50% increase in peak power density (from 1.35 W cm À2 to 2.02 W cm À2 ), and this fully highlights the advantages of operating AEMFCs with the thinnest AEM possible (as long as this does not compromise mechanical robustness and integrity).
An initial evaluation of MEAs containing select non-Pt oxygen reduction reaction (ORR) electrocatalysts was also conducted (SEM/EDX images and maps of the cathodes tested are presented in Fig. S3-S5 in the ESI †).The loadings of each catalyst in the cathode was optimised to maximise peak power density in the H 2 /O 2 AEMFCs, and the optimal loadings are presented in Table 4.Both non-Pt cathodes could achieve peak power densities in H 2 /O 2 AEMFCs >1 W cm À2 under our standardised test conditions (Fig. 6), despite activation losses that were larger than with Pt/C.This is especially noteworthy considering the FeCoPc/C catalyst had a metal loading <0.01 mg cm À2 (a simple extrapolation suggests 140 MW of beginning-of-life AEMFC peak power density could be achieved with a batch of catalyst containing 1 kg of the Fe + Co metal content).The ohmic area resistances were 35, 34, and 41 mU cm 2 at peak power density for the Pt/C, Ag/C and FeCoPc/C catalysts, respectively.However, the results of 20 h in situ evaluation of the relative durabilities of each cathode catalyst (Fig. 7) showed that the stabilities of the non-Pt-catalyst-containing MEAs were poorer over short timespans than that of the Fig. 4 The Raman spectra (l ¼ 532 nm) of the LDPE15-AEM before (bottomblack) and after (topred) the 500 h alkali stability test in Fig. 3.The data in (Fig. 8) shows the polarisation curves obtained from H 2 AEMFC tests with CO 2 -free air supplies to the cathode (conducted directly aer testing of the respective MEA in H 2 /O 2 mode, ca.2-4 h aer initial assembly of each MEA in the fuel cell xtures).These data show that at low current densities, the Pt/C cathode exhibited higher performances due to better electrokinetics.However, the Pt/C cathode exhibited the on-set of mass transport losses at relatively low current densities (also previously observed and discussed with the LDPE25-AEM), 12 and this resulted in the Ag/C catalyst producing a superior H 2 / air (CO 2 -free) AEMFC peak power density.At this stage, this appears to be due to better diffusion of O 2 in the Ag/C cathode when supplied with CO 2 -free air, especially when the O 2 concentration in the gas feed is more depleted as it passes around the ow elds.With the H 2 /O 2 fuel cell data (Fig. 6) we also observed that the onset of mass transport appears to be more severe with Pt/C compared to Ag/C.Again, this superiority of Ag/C (cf.Pt/C) when using CO 2 -free air at the cathode has been previously observed in AEMFC testing of a sub-30 mm thick ETFE-based RG-AEM. 39

Conclusions
This article describes the fabrication of a new sub-30 mm thick LDPE-based radiation-graed anion-exchange membrane (RG-AEM).This thin RG-AEM exhibited a OH À anion conductivity of 200 mS cm À1 at 80 C in a 100% relative humidity atmosphere, which decayed by <7% over a period of 500 h.Fast H 2 O transport through an AEM is crucial for achieving high-performance anion-exchange membrane fuel cells (AEMFCs).We clearly demonstrate that the use of a thinner AEM, synthesised via an optimised graing process, leads to enhanced in situ H 2 O transport characteristics without sacricing mechanical strength.This led to a H 2 /O 2 AEMFC with a peak power density of 2 W cm À2 at 80 C (Pt/C cathode and PtRu/C anode) and 850 mW cm À2 in a H 2 /air (CO 2 -free) AEMFC containing a (non-Pt-group) Ag/C cathode.This thinner RG-AEM has performance characteristics that are now high enough to facilitate detailed investigations into the short-to medium-term AEMFC performances when operating with a variety of parameters    (temperatures up to 80 C, different electrocatalysts, and gas contaminants).

Fig. 1
Fig. 1 The 4-probe (in-plane) conductivities of the LDPE15-AEM under different conditions.Error bars are from measurements on n ¼ 3 samples of each anion form of the AEM (some error bars are smaller than the symbols).

Fig. 3 A
Fig. 3 A 500 h ex situ stability test with the LDPE15-AEM in the OH À form at 80 C in a N 2 (CO 2 -free, RH ¼ 100%) gas flow.A d.c.current of 0.1 mA was used to maintain the OH À form of the AEM.The extended linear regression (to 3500 h) is presented in Fig. S2 in the ESI.†

Fig. 6
Fig. 6 H 2 /O 2 AEMFC performance data at 80 C for the LDPE15-AEM with the different cathode electrocatalysts.All other test conditions were as described in Fig. 5 (including RH ¼ 92% gas supplies).

Fig. 7 A
Fig.7A short-term relative H 2 /O 2 AEMFC durability comparison of the following cathodes: Pt/C (solid), Ag/C (dashed) and FeCoPc/C (dotted).These 20 h tests (at 80 C and 0.4 V cell discharge) were conducted immediately after the polarisation curves presented in Fig.8below were recorded (without removal of the MEAs from the AEMFC).All other test conditions were as described in Fig.5(including RH ¼ 92% gas supplies).

Fig. 8
Fig.8H 2 /air (CO 2 -free) AEMFC performance data at 80 C for the LDPE15-AEM with the different cathode electrocatalysts.These H 2 /air (CO 2 -free) tests were conducted immediately after the polarisation curves presented in Fig.6were recorded (without removal of the MEAs from the AEMFC).All other test conditions were as described in Fig.5(including RH ¼ 92% gas supplies).
Outline of the synthesis of the LDPE15-AEM in the OH À form. a 12,37,39and was synthesised via the radiation-graing of VBC onto an ETFE powder (Fluon Z8820X, supplied by AGC Europe) with subsequent amination using TMA.For the anode GDEs, PtRu/C (Alfa Aesar, Johnson Matthey HiSpec 12100, 50 wt% Pt and 25 wt% Ru) and AEI powder (20 wt% of the total solid mass) were mixed together with 1 cm 3 water and 9 cm 3 propan-2-ol.This cathode catalyst ink was homogenised with ultrasound for 30 min, sprayed onto a Toray TGP-H-60 carbon paper gas diffusion substrate (Alfa Aesar, PTFE-treated), and then dried in air.For the cathode GDEs, either Pt/C (Alfa Aesar, Johnson Matthey HiSpec 4000, 40wt% Pt), Ag/C (BASF Fuel Cell, 40 wt% Ag on Vulcan XC-72), or FeCoPc/C (the catalyst denoted as FePc-CoPc/ C (600) in ref.43) was used alongside the AEI powder (20 wt% of the total solid mass).The geometric surface area of all GDEs was 5.0 cm 2 , while the Pt loading for all Pt-based anodes and cathodes was 0.40 AE 0.02 mg Pt cm À2 (geometric).The optimised metal loadings for the non-Pt cathodes are discussed in the Results and discussion section.

Table 2
The key properties of the LDPE15-AEM (this study) and LDPE25-AEM (extracted from ref.12).All data, apart from the OH À conductivity data, are for the AEMs in the Cl À anion form.All errors are sample standard deviations from measurements on n ¼ 3 samples of each AEM a The number of water molecules per Cl À anion in the fully hydrated AEM, calculated as: l H 2 O ¼ WU/(100 Â 18.02 Â IEC).b s ¼ 4-probe, inplane ion conductivities at 80 C in the indicated anion form and under test conditions (relative humidities, RH, where relevant, are indicated in the []).c Measured on the test set-up at the Colorado School of Mines, USA (details in ref. 12).

Table 3
Characterisation data for a sample of the LDPE15-AEM recorded before and after the 500 h 80 C OH À conductivity test presented in Fig.3

Table 4
The optimal catalyst loadings for the cathodes containing the three ORR electrocatalysts tested along with select H 2 /O 2 AEMFC data (80 C)