John Varcoe

Giron Rodriguez et al. [ACS Sustainable Chem. Eng., 2023, 11, 1508] previously showed that radiation-grafted anion-exchange membranes containing N-benzyl-N-methylpiperidinium headgroups (MPIP-RG-AEM) are promising for use in CO2 electrolysis (cf. commercial and other RG-AEM types). For a more sustainable synthesis, MPIP-RG-AEMs have now been fabricated using a reduced 1.1 times excess of amine reagent (historically made using >5 times excess). A resulting RG-AEM promisingly had a bulk amination level that was comparable to those made with the traditional large excess. Unexpectedly, however, it had a significantly reduced water content, with two further batches showing that this observation was repeatable (and reproducible via measurements collected on a single batch using different techniques in different labs). The ionic conductivities of the RG-AEM made with a controlled 1.1 excess of amine were also lower, with higher activation energies. Terahertz time-domain spectroscopy measurements showed that the lower water uptake RG-AEMs, made with the 1.1 amine excess, contained smaller amounts of bulk water relative to bound water (a repeatable observation with different counter-anions). This lack of bulk water, yielding reduced water diffusion coefficients, led to a change in the water management when such RG-AEMs were tested in CO2 electrolysis cells, with significantly affected in situ performances. Small angle scattering data (X-ray and neutron) indicated that MPIP-RG-AEM fabrication with the 1.1 excess of amine reduced the size of the amorphous lamella domains on hydration, and this change is suspected to be the cause of the lower water uptakes and swelling. The finding that chemically similar AEMs can have significantly different hydration properties is potentially important to all ion-exchange membrane users and developers (beyond the CO2 electrolysis scope of this study).

The performance of zero-gap CO 2 electrolysis (CO 2 E) is significantly influenced by the membrane's chemical structure and physical properties due to its effects on the local reaction environment and water/ion transport. Radiation-grafted anion-exchange membranes (RG-AEM) have demonstrated high ionic conductivity and durability, making them a promising alternative for CO 2 E. These membranes were fabricated using two different thicknesses of ethylene-tetrafluoro-ethylene polymer substrates (25 and 50 μm) and three different headgroup chemistries: benzyl-trimethylammonium, benzyl-N-methyl-pyrrolidinium, and benzyl-N-methylpiperidinium (MPIP). Our membrane characterization and testing in zero-gap cells over Ag electrocatalysts under commercially relevant conditions showed correlations between the water uptake, ionic conductivity, hydration, and cationic-head groups with the CO 2 E efficiency. The thinner 25 μm-based AEM with the MPIP-headgroup (ion-exchange capacities of 2.1 ± 0.1 mmol g −1) provided balanced in situ test characteristics with lower cell potentials, high CO selectivity, reduced liquid product crossover, and enhanced water management while maintaining stable operation compared to the commercial AEMs. The CO 2 electrolyzer with an MPIP-AEM operated for over 200 h at 150 mA cm −2 with CO selectivities up to 80% and low cell potentials (around 3.1 V) while also demonstrating high conductivities and chemical stability during performance at elevated temperatures (above 60 °C).

Semipermeable polymeric anion exchange membranes are essential for separation, filtration and energy conversion technologies including reverse electrodialysis systems that produce energy from salinity gradients, fuel cells to generate electrical power from the electrochemical reaction between hydrogen and oxygen, and water electrolyser systems that provide H2 fuel. Anion exchange membrane fuel cells and anion exchange membrane water electrolysers rely on the membrane to transport OH− ions between the cathode and anode in a process that involves cooperative interactions with H2O molecules and polymer dynamics. Understanding and controlling the interactions between the relaxation and diffusional processes pose a main scientific and critical membrane design challenge. Here quasi-elastic neutron scattering is applied over a wide range of timescales (100–103 ps) to disentangle the water, polymer relaxation and OH− diffusional dynamics in commercially available anion exchange membranes (Fumatech FAD-55) designed for selective anion transport across different technology platforms, using the concept of serial decoupling of relaxation and diffusional processes to analyse the data. Preliminary data are also reported for a laboratory-prepared anion exchange membrane especially designed for fuel cell applications.

Radiation-grafted anion-exchange membranes (RG-AEM) are being developed to evaluate a range of chemistries that have relevance to a variety of electrochemical applications including reverse electrodialysis (RED) salinity gradient power. RG-AEMs are typically fabricated using an electron-beam activated (peroxidated) polymer substrate film. These activated films are first grafted with a monomer, such as vinylbenzyl chloride (VBC) and then reacted with a variety of tertiary amines to yield the desired RG-AEMs. The amination process forms covalently bound quaternary ammonium (QA) head-groups that allow the RG-AEMs to conduct anions such as Cl−. RG-AEMs are of interest as they exhibit high conductivities (100 mS cm−1 at elevated temperatures when containing Cl− anions). However, the current generation of RG-AEMs have two main Achilles' heels: (1) they exhibit low permselectivities; and (2) they exhibit a high degree of swelling in water. Introducing covalent crosslinking into ion-exchange membranes is a well-known strategy to overcome these issues but it often comes with a price – a significantly lowered conductivity (raised in situ resistance). Therefore, the level of crosslinking must be carefully optimised. RG-AEMs can be primarily crosslinked using two methods: (1) introduction of a divinyl monomer into the monomer mixture used during grafting; or (2) introduction of a diamine agent into the amination process. This study looks into both methods where either divinylbenzene (DVB) is added into the grafting mixture or N,N,N′,N′-tetramethylhexane-1,6-diamine (TMHDA) is added into the amination mixture. We show that on the balance of two application-relevant properties (resistances in aqueous NaCl (0.5 mol dm−3) solution and permselectivity), the diamine crosslinking method is the most effective for RG-AEMs being used in RED cells.

To reduce the cost of fuel cell stacks and systems, it is important to create commercial catalysts that are free of platinum group metals (PGMs). To do this, such catalysts must have very high activity, but also have the correct microstructure to facilitate the transport of reactants and products. Here, we show a high-performing commercial oxygen reduction catalyst that was specifically developed for operation in alkaline media and is demonstrated in the cathode of operating anion-exchange membrane fuel cells (AEMFCs). With H2/O2 reacting gases, AEMFCs made with Fe–N–C cathodes achieved a peak power density exceeding 2 W cm−2 (>1 W cm−2 with H2/air) and operated with very good voltage durability for more than 150 h. These AEMFCs also realized an iR-corrected current density at 0.9 V of 100 mA cm−2. Finally, in a second configuration, Fe–N–C cathodes paired with low-loading PtRu/C anodes (0.125 mg PtRu per cm2, 0.08 mg Pt per cm2) demonstrated a specific power of 10.4 W per mg PGM (16.25 W per mg Pt).

In the past few years, developments in anion exchange membranes (AEMs) have led to a significant increase in hydroxide conductivities, ultimately yielding striking improvements in the performance of anion exchange membrane fuel cells (AEMFCs) at low operating temperatures, usually at 40–80 °C. Aside from these remarkable achievements, the literature is void of any work on AEMFCs operated at temperatures above 100 °C, despite the consensus from various models remarking that working at higher cell temperatures may lead to many significant advantages. In this work, we present the first high-temperature AEMFC (HT-AEMFC) tested at 110 °C. The HT-AEMFC exhibits high performance, with a peak power density of 2.1 W cm−2 and a current density of as high as 574 mA cm−2 measured at 0.8 V. This initial work represents a significant landmark for the research and development of the fuel cell technology, opening a wide door for a new field of research we call hereafter, HT-AEMFCs.

There is a need to understand the water dynamics of alkaline membrane fuel cells undervarious operating conditions to create electrodes that enable high performance and stable,long-term operation. Here we show, via operando neutron imaging and operando micro X-raycomputed tomography, visualizations of the spatial and temporal distribution of liquid waterin operating cells. We provide direct evidence for liquid water accumulation at the anode,which causes severe ionomer swelling and performance loss, as well as cell dryoutfrom undesirably low water content in the cathode. We observe that the operating conditionsleading to the highest power density during polarization are not generally the conditions thatallow for long-term stable operation. This observation leads to new catalyst layer designs andgas diffusion layers. This study reports alkaline membrane fuel cells that can be operatedcontinuously for over 1000 h at 600 mA cm−2with voltage decay rate of only 32-μVh−1–the best-reported durability to date.

It has been long-recognized that carbonation of anion exchange membrane fuel cells (AEMFCs) would be an important practical barrier for their implementation in applications that use ambient air containing atmospheric CO₂. Most literature discussion around AEMFC carbonation has hypothesized: (1) that the effect of carbonation is limited to an increase in the Ohmic resistance because carbonate has lower mobility than hydroxide; and/or (2) that the so-called “self-purging” mechanism could effectively decarbonate the cell and eliminate CO₂-related voltage losses during operation at a reasonable operating current density (>1 A cm−2). However, this study definitively shows that neither of these assertions are correct. This work, the first experimental examination of its kind, studies the dynamics of cell carbonation and its effect on AEMFC performance over a wide range of operating currents (0.2–2.0 A cm¯²), operating temperatures (60–80 °C) and CO₂ concentrations in the reactant gases (5–3200 ppm). The resulting data provide for new fundamental relationships to be developed and for the root causes of increased polarization in the presence of CO₂ to be quantitatively probed and deconvoluted into Ohmic, Nernstian and charge transfer components, with the Nernstian and charge transfer components controlling the cell behavior under conditions of practical interest.

Herein we detail the development of a new high-density polyethylene-(HDPE)-based radiation-grafted anion-exchange membrane (RG-AEM) that achieves a surprisingly high peak power density and a low in situ degradation rate (with configurations tailored to each). We also show that this new AEM can be successfully paired with an exemplar non-Pt-group cathode.

Broader context: A primary motivation for the development of anion-exchange membrane (AEM) fuel cells (AEMFCs) is the broader range of sustainable, non-precious-metal catalysts that are feasible; if costs are lowered enough, AEMFCs would be deployable in a range of stationary power sectors (e.g. back-up and off-grid). However, as the performance of AEMFCs typically drop when Pt-based electrodes are replaced with non-Pt types, it is essential that the highest performing polyelectrolytes are developed, both membranes and ionomers (the latter incorporated to impart ionic conductivity in the electrodes). The findings with the high conductivity AEM reported herein will also be of interest to developers of AEMs for metal–air and redox-flow batteries, electrolysers (both H2O → H2 and CO2 → high-value chemicals and fuels), and salinity gradient power.

Efficient and durable nonprecious metal electrocatalysts for the oxygen reduction (ORR) are highly desirable for several electrochemical devices, including anion exchange membrane fuel cells (AEMFCs). Here, a 2D planar electrocatalyst with CoOx embedded in nitrogen-doped graphitic carbon (N-C-CoOx) was created through the direct pyrolysis of a metal organic complex with a NaCl template. N-C-CoOx showed high ORR activity, with excellent half-wave (0.84 V vs. RHE) and onset (1.01 V vs. RHE) potentials. This high intrinsic activity was translated to operating AEMFCs (containing radiation-grafted polymer electrolyte materials), with the N-C-CoOx catalyst able to achieve extremely high power density (1.05 W cm-2) and mass transport limiting current (3 A cm-2) for a precious metal free electrode. The N-C-CoOx cathode also showed excellent stability over 100 hours of operation at 600 mA/cm2 under H2/air (CO2-free) reacting gas feeds. The N-C-CoOx cathode catalyst was also paired with a very low loading PtRu/C anode catalyst, to create AEMFCs with a total PGM loading of only 0.10 mgPt-Ru cm-2 capable of achieving 7.4W mg-1PGM as well as supporting a current of 0.7 A cm-2 at 0.6 V with H2/air (CO2 free) – creating a cell that was able to meet the 2019 DOE target of initial performance of 0.6 V at 0.6 A cm-2 under H2/air with a PGM loading < 0.125 mg cm-2 with AEMFCs for the first time.