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Page 2 of 17 Schertzer et al. J. Mater. Inf. 2025, 5, 5 https://dx.doi.org/10.20517/jmi.2024.69

INTRODUCTION
Fuel cells have emerged as a promising solution to the global energy crisis. A large body of prior research
[1,2]
has focused on advances in proton exchange membrane (PEM) fuel cells ; however, several factors hinder
their widespread adoption. Fluorinated polymers and platinum catalysts used in PEMs to prevent corrosion
are costly and environmentally harmful [1,3,4] , and effective water and carbon monoxide management is also
crucial to avoid rapid degradation of performance [1,5,6] .

Over the past two decades, anion exchange membrane (AEM) fuel cells have emerged as promising
alternatives to PEM fuel cells. AEMs utilize hydroxide ions for ion transport and thus operate in an alkaline
environment, which offers several advantages over the acidic environments present in PEMs, as illustrated
in Figure 1A. Notably, because AEMs operate in alkaline environments, expensive fluorinated polymers and
other acid-tolerant materials are no longer required, potentially leading to significant reductions in material
costs compared to traditional PEMs. Figure 1B shows the chemical structures of Nafion and Sustanion, the
leading PEM and AEM materials, respectively. Although the prospect of non-fluorinated membranes is
exciting, AEMs face major challenges, including low ionic conductivity, limited chemical/alkaline stability,
and mechanical integrity issues. These limitations stem primarily from the conduction of anions - which
have inherently lower diffusion constants compared to protons - and excessive water adsorption and
[1,3]
swelling .

The United States Department of Energy has set ambitious goals for AEM performance over the next five
years, namely: hydroxide conductivity of at least 100 mS/cm, swelling ratio (SR) of no more than 50%,
durability of 25,000 h with no more than 10% loss in current density, and a cost no greater than $40/kW.
These goals were identified with the hope of designing polymers that can outperform PEMs in energy
generation capability and long-term stability without the need for halogenated backbones or functional
groups, with fluorine being the most commonly used halogen to impose ionic conductivity and chemical
[4,7]
stability .
One solution to increase conductivity is to enhance the ion exchange capacity (IEC), defined as the total
number of active sites responsible for ion exchange in the polymer electrolyte membrane (reported in units
of meq/g). However, this often leads to mechanical instability due to excessive water uptake (WU)-induced
swelling, and chemical instability due to the elimination of charged groups under alkaline
environments [2,3,8] . AEMs are also prone to rapid chemical and mechanical degradation . A common
[6,9]
approach for increasing stability is the addition of fluorinated groups, but there is a strong push to move
away from fluorinated polymers amid environmental health concerns [3,5-7] . The primary challenge is thus to
develop an AEM that combines high ionic conductivity with strong mechanical and chemical stability
across a range of temperatures and humidities without the use of fluorinated chemistries, replicating the
benefits of PEMs while significantly reducing the cost and environmental impact of using fluorinated
polymers. Achieving this balance has proven extremely difficult due to conflicting property requirements.
We wish to maximize hydroxide conductivity while limiting the WU and SR, but as indicated in Figure 2,
there exists a positive correlation between these properties of interest, making this a non-trivial task.
Additionally, because of its high alkaline stability, fluorine is present in ~25% of the top-performing
candidates (OH conductivity ≥ 100 mS/cm) that were found in the literature. This highlights the difficulty
-
of designing state-of-the-art polymers for AEMs that contain no fluorine.

AEM performance is intricately linked to several factors, including WU, SR, temperature, relative humidity
(RH), and IEC. RH, expressed as a percentage, measures the level of environmental humidity. WU and SR,
both known to correlate with anion conductivity and mechanical stability, are frequently reported alongside