Fumasep FAA-3: Moisture-Swing AEM for DAC

• Fumasep FAA-3 is a poly(phenylene oxide)-derived alkaline anion-exchange membrane with moderate functionalization and humidity-dependent molecular ordering for CO2 capture.
• It exhibits multiscale features including short-range ionic channels, nanostructured water clusters, and mesoscale voids that facilitate rapid ion and gas transport.
• Its nonlinear mechanical behavior under varied temperature and humidity requires constitutive modeling to guide the design of durable, efficient direct air capture systems.

Fumasep FAA-3 is a commercially available alkaline anion-exchange membrane (AEM) polymer that has been investigated for low-energy, moisture-driven direct air capture (DAC). In the moisture swing (MS) process, the membrane absorbs CO$_2$ as it dries and releases it when water is added, making the coupling between ionic chemistry, humidity response, structure, and mechanics central to performance and lifetime. Recent work characterizes FAA-3 as a hierarchically organized charged polymer with humidity-dependent molecular ordering, clustering, porosity, swelling, and water-channel formation, while a separate mechanical study identifies FAA-3 as a membrane whose swelling, stiffness, strength, plastic deformation, and stress relaxation are sufficiently consequential to warrant constitutive modeling under varying temperature and humidity conditions (Yogaganeshan et al., 15 Aug 2025, Sarbaz et al., 3 Aug 2025).

1. Chemical identity and functional mechanism

Fumasep FAA-3 is built on a poly(phenylene oxide) (PPO) backbone—repeat unit –[Ph–O–Ph]–—that is benzyl-chloromethylated and subsequently quaternized. Each chloromethyl pendant is converted to a quaternary ammonium (–CH$_2$–N$^+$R$_3$ Cl$^-$), then exchanged to –N$^+$R$_3\cdot$OH$^-$. In this formulation, the quaternary ammonium sites and hydroxide counterions define the membrane’s ion-exchange function and its relevance to moisture-swing CO$_2$ capture (Yogaganeshan et al., 15 Aug 2025).

The MS chemistry reported for these materials is:

$$\text{Dry state: } \mathrm{OH^- + CO_2 \rightleftharpoons HCO_3^-}$$

$_2$0

with the net relation

$_2$1

Commercial FAA-3 films are reported as 30 $_2$2m thick. The structural study does not report an exact ion-exchange capacity (IEC), but it notes that typical Fumasep FAA-3 grades have IEC $_2$3–$_2$4 mmol g$_2$5, and that by mass balance of PPO repeating units and IEC, roughly one quaternary site per 10–12 phenylene oxide rings is inferred, corresponding to $_2$6–10 mol% functionalization. This suggests a moderately functionalized PPO-derived ionomer rather than an extremely high-charge-density membrane (Yogaganeshan et al., 15 Aug 2025).

2. Structural probes and measurement framework

FAA-3 has been characterized by a multimodal workflow spanning reciprocal-space and real-space methods. X-ray diffraction (XRD) used Cu K$_2$7 radiation with $_2$8 \AA, samples mounted both parallel and perpendicular to the film plane, and the standard relations

$_2$9

and

$^+$0

The XRD detector was a Dectris Eiger, with 180$^+$1 rotation, 0.1$^+$2 step, and 10 s per frame. Small- and wide-angle X-ray scattering employed a Xenocs Xeuss 3.0 with Cu K$^+$3 beam, $^+$4 keV and $^+$5 \AA. Sample-to-detector distances were 900 mm for SAXS, giving $^+$6–$^+$7 \AA$^+$8, 370 mm for MAXS, giving $^+$9–$_3$0 \AA$_3$1, and 50 mm for WAXS, giving $_3$2–$_3$3 \AA$_3$4. A humidity chamber enabled measurements at 25, 50, 75, and 95% RH after 60 min equilibration, followed by 360$_3$5 azimuthal averaging and background subtraction (Yogaganeshan et al., 15 Aug 2025).

Surface and internal morphology were further examined by AFM, FIB-SEM, and TEM. AFM in air used tapping and contact modes on Bruker MM8/Icon with tip radius 20–60 nm and $_3$6 N/m; AFM in liquid used a fluid cell with ScanAsyst-Liquid+ probes of tip radius $_3$7 nm. FIB-SEM used a TESCAN AMBERX plasma FIB-SEM with Xe$_3$8 ions at 30 kV and Pt sputter-coating of 20 nm for conductivity; the milling strategy produced “bridge” lamellae with successive beam current reductions to $_3$9 nm thickness, while cryo-FIB used plunge-frozen samples after 100% hydration. TEM used a JEOL F200 at 200 kV and FIB-prepared lamellae. This measurement stack implies deliberate resolution matching from angstrom-scale chain packing to nanometer- and submicron-scale clustering (Yogaganeshan et al., 15 Aug 2025).

3. Multiscale structure of FAA-3

In dry FAA-3 at approximately 30% RH, WAXS shows a clear peak at $^-$0 \AA$^-$1 in the parallel direction and $^-$2 \AA$^-$3 in the perpendicular direction, corresponding to $^-$4 \AA\ and $^-$5 \AA, respectively. The interpretation given is partial ordering of PPO repeat units and spacing between quaternized side-chains. At larger scales, FAA-3 shows a SAXS “shoulder” around $^-$6 \AA$^-$7, corresponding to a domain size $^-$8 \AA. These data identify both short-range molecular packing and larger-scale structural organization within the same film (Yogaganeshan et al., 15 Aug 2025).

Real-space imaging corroborates that hierarchy. AFM of the dry FAA-3 film gives rms roughness of approximately 4 nm over a 1 $^-$9m$^+$0 scan, with phase-contrast domain patches of approximately 20–30 nm. In hydrated contact-mode AFM, expanded features of approximately 8 nm high are observed, confirming surface swelling. TEM of FAA-3 lamellae parallel to the film plane shows granular grains of approximately 5–10 nm diameter, while perpendicular lamellae show a network of approximately 10–20 nm voids. Room-temperature FIB-SEM lamellae under $^+$1 nm thickness verified granular and clustered morphologies. A common simplification is to treat FAA-3 as a structurally uniform ionomer film; these combined observations instead indicate a heterogeneous charged polymer with ordering, grains, voids, clustering, and humidity-sensitive nanoscale features (Yogaganeshan et al., 15 Aug 2025).

4. Humidity response, swelling, and channel formation

Humidity alters both reciprocal-space signatures and bulk dimensions. The WAXS-derived spacing in FAA-3 evolves as follows:

RH (%)	Peak $^+$2 (\AA$^+$3)	$^+$4 (\AA)
25	1.61	3.90
50	1.63	3.85
75	1.65	3.81
95	1.68	3.74

As RH increases, $^+$5 increases and $^+$6 decreases by about 4%, which is interpreted as water intercalation between chains bringing quaternary sites slightly closer. In the perpendicular direction, an emergent hump appears at $^+$7 \AA$^+$8 at 95% RH, corresponding to a super-domain size $^+$9 \AA\ with a $_3\cdot$0 \AA\ tail, interpreted as nano-water channels in the 20–60 \AA\ range (Yogaganeshan et al., 15 Aug 2025).

Bulk swelling is also directly reported. The film thickness increases from 30 $_3\cdot$1m in the dry state to approximately 34 $_3\cdot$2m at 95% RH, giving a swelling ratio

$_3\cdot$3

or 13%. The thickness returns to 30 $_3\cdot$4m upon drying, indicating minimal hysteresis. The study further cites a qualitative BET-like approximation for water uptake,

$_3\cdot$5

and states that the onset of the super-domain hump at RH $_3\cdot$6% suggests a percolation threshold for connected water channels. AFM and TEM are reported to confirm surface networks that facilitate OH$_3\cdot$7 conduction. This suggests that humidity does not merely swell the membrane isotropically; it reorganizes transport-relevant domains and channels across multiple length scales (Yogaganeshan et al., 15 Aug 2025).

5. Mechanical behavior and constitutive modeling

Mechanical behavior is identified as an essential but incompletely reported aspect of FAA-3 in the 2025 mechanical study. The abstract states that one anion exchange membrane, Fumasep FAA-3, was tested under mechanical loading and various temperature and humidity conditions to measure swelling, stiffness, strength, plastic deformation, and stress relaxation. It further states that experimental results were used to identify a mechanical model for FAA-3 that can be used to predict the material’s nonlinear viscous behavior under various loads and environments (Sarbaz et al., 3 Aug 2025).

The associated detailed text, however, does not contain the experimental methods, mechanical-model equations, fitted parameters, swelling values, stress–strain curves, or relaxation and creep data. As a result, the literature record presently supports a high-level conclusion—that FAA-3 exhibits nonlinear viscous behavior requiring environment-dependent modeling—without disclosing the constitutive form or calibrated parameter set in the text at hand. This is significant because the same humidity changes that generate transport-enabling channel formation and reversible thickness change are also the conditions under which stiffness, strength, plasticity, and relaxation must be managed to prolong sorbent lifetime (Sarbaz et al., 3 Aug 2025).

6. Functional implications for direct air capture and proposed design targets

The structural study links FAA-3’s multiscale morphology to moisture-swing DAC function. It states that 3.7–4.2 \AA\ molecular channels control HCO$_3\cdot$8 / CO$_3\cdot$9 binding distances, that 20–60 \AA\ water channels enable rapid water and ion diffusion with $^-$0, and that macroporous clusters in the 100–300 nm range support bulk gas ingress and egress. It also notes that humidity-induced structural changes emphasize the role of moisture in CO$^-$1 capture and release. A plausible implication is that FAA-3 performance depends on simultaneous access to short-range ionic coordination, mesoscale hydrated pathways, and larger gas-accessible domains rather than on any single structural descriptor (Yogaganeshan et al., 15 Aug 2025).

The same source presents explicit design recommendations. It proposes targeting super-domain spacing $^-$2–40 \AA\ to balance conductivity and mechanical integrity; controlling swelling to $^-$3 and limiting $^-$4% at 95% RH; increasing IEC to approximately 1.5 mmol g$^-$5 while monitoring brittleness; aiming for $^-$6 S cm$^-$7 at 50% RH for rapid cycle times below 5 min; and promoting microphase separation by incorporating a hydrophobic PPO block alternating with a cationic block such as poly(vinylbenzyltrimethylammonium). It also recommends limiting extreme pore sizes above 100 nm, which collapse on cycling, and targeting nanopores below 50 nm stabilized by a rigid backbone. These are proposed engineering targets rather than measured FAA-3 requirements, but they summarize the study’s view of how hierarchical structure, humidity response, and durability should be co-optimized in FAA-3-derived DAC materials (Yogaganeshan et al., 15 Aug 2025).