Ionomer Effect in Dry Cathode AEM Water Electrolysis
Ionomer Effect in Dry Cathode Alkaline Anion Exchange Membrane Water Electrolysis
In dry cathode alkaline anion exchange membrane (AEM) water electrolysis (AEMWE) technology, the AEM is the core component determining system performance. An ideal AEM must simultaneously meet multiple performance indicators: high hydroxyl (OH⁻) ionic conductivity to ensure ion transport efficiency, excellent mechanical strength and thermal stability to adapt to electrolysis conditions, moderate water absorption (WU) to balance ion conduction and structural stability, and good chemical stability to resist alkaline corrosion. The core structural feature of the AEM is the cationic groups grafted onto the polymer backbone. These groups not only endow the membrane with selective permeability to anions but are also the core carriers of ion conductivity.
Ionomers, as a key component of the AEM, have anion transport capacity that largely depends on the membrane’s water retention capacity (i.e., WU)—OH⁻ migration requires hydrated ions, and the maintenance of moisture within the membrane directly affects the integrity of the ion transport channels. While the Dornan repulsion effect can significantly inhibit the permeation of electrolytes such as potassium hydroxide (KOH), trace amounts of residual permeation may still occur in ion-solubilized membrane systems or high-concentration KOH (≥1 M) environments. This permeation can impair the ion selectivity of the membrane and may accelerate the performance degradation of the electrode catalyst layer; therefore, it must be carefully avoided in practical applications.
The ion exchange capacity (IEC) of the membrane is a core physical parameter for controlling water retention capacity (WU), making IEC crucial in dry cathode configurations, especially in high-current-density long-term operation scenarios, where water management efficiency directly determines the stability and energy consumption level of the electrolysis system. By definition, IEC represents the number of exchangeable ions per unit weight of AEM, commonly expressed in meq・g⁻¹ or mmol・g⁻¹. Currently, mainstream IEC measurement methods include titration, spectroscopic methods (such as UV-Vis spectroscopy), and ion-selective methods. These methods can achieve precise quantification by directly or indirectly measuring the exchange capacity of H⁺ or OH⁻.
The core characteristic of dry cathode configurations is the reliance on water generated at the anode to diffuse to the cathode to maintain the reaction environment. Therefore, optimizing water management becomes a crucial issue in this technology, making it particularly important to clarify the relationship between IEC, WU, and the electrolytic cell voltage. The molecular structure of an AEM consists of a polymer backbone and functional groups. Traditional designs often use quaternary ammonium groups as cationic functional groups, linked to polymer backbones such as polystyrene, polysulfone, polyethersulfone, or polyphenylene ether via benzylmethylene groups. In recent years, imidazolium cationic groups and metal-based cationic groups have also been increasingly used, with different functional groups exhibiting distinct performance differences: quaternary ammonium groups offer the advantage of high OH⁻ conductivity, while imidazolium groups demonstrate superior chemical stability. Currently, commercially available membranes are still predominantly based on quaternary ammonium groups, with commercial products using imidazolium cationic groups being relatively few.
To adapt to the water transport characteristics of dry cathodes, the development of ionomers and AEMs with gradient IEC and WU has become a research hotspot. Previous studies have designed four piperidinium-based ionomers and selected three of these ionomers from the same series to prepare aqueous electrolytic membranes (AEMs). Experimental data show that a certain ionomer-based AEM exhibits a water retention capacity (WU) as high as 73% at an IEC of 2.80 mmol·g⁻¹; while under the same testing conditions, the WU of an enhanced commercial AEM is only 36%, highlighting the advantage of laboratory-customized ionomers in water retention performance.
It is noteworthy that in the full range of IEC tests, the WU values of commercial and laboratory membranes did not show a clear linear correlation. This irregularity may be related to the coupling of multiple factors such as the membrane’s microporous structure, the uniformity of functional group distribution, and the polymer chain aggregation state. Regarding electrolysis performance testing, under constant current density conditions of 60°C and 1 A·cm⁻², the electrolytic cell voltage of the laboratory membrane is generally slightly lower than that of the commercial membrane, but it also does not show a regular trend matching the IEC or WU, further illustrating the complexity of multi-parameter interactions in the dry cathode system.
Besides the membrane’s inherent performance, the amount of ionomer and binder content on the electrode side also significantly impact system performance. Tests with cathode ionomer contents ranging from 10% to 40% showed that 25% was the optimal ratio, enabling AEMWE to achieve a low electrolysis voltage of 2 V at 60°C and a current density of 3.5 A·cm⁻². In studies of anode PTFE binder contents (5-20 wt%), samples with low binder content exhibited higher current densities initially at a constant voltage of 1.8 V, but showed poor long-term stability; while samples with a high binder content of 20 wt% maintained a current density of 1.07 A·cm⁻² at 1.8 V after 1300 cycles, significantly higher than the low-content samples. This performance reversal stems from the fact that higher binder content reduces catalyst loss and optimizes the electrode pore structure to improve mass transfer efficiency and reaction kinetics.
To further elucidate the water management mechanism of the dry cathode, researchers used neutron imaging to observe the water distribution characteristics of two types of IEC samples (1.8–2.2 meq·g⁻¹) and high (2.3–2.6 meq·g⁻¹) at high current densities, confirming a significant water distribution imbalance within the membrane electrode assembly (MEA). Experiments revealed that the cathode configuration supplied with 0.1 M KOH exhibited significantly lower electrolysis voltage at current densities > 0.6 A·cm⁻² compared to the dry cathode configuration due to its superior membrane wettability. A 6-hour degradation test at 1 A·cm⁻² further demonstrated that the initial 2-hour degradation rate of the dry cathode configuration was significantly higher, with the medium IEC sample reaching 75 mV·h⁻¹ and the high IEC sample reaching 45 mV·h⁻¹, while the KOH-supplied configuration consistently exhibited the best stability.
The chemical stability of the AEM in an alkaline environment is another key factor determining the system’s lifetime. The nucleophilicity of OH⁻ can lead to polymer backbone breakage or functional group destruction, thereby reducing IEC and mechanical properties. In conventional systems, the main degradation pathways of functional groups include nucleophilic substitution (SN2), Hoffmann elimination, and Yride intermediate formation. However, in dry cathode configurations, the concentration of free OH⁻ is significantly reduced, which can inhibit the aforementioned nucleophilic attack-dependent degradation reactions. Nevertheless, locally generated reactive oxygen species during electrolysis can still trigger free radical oxidative degradation. Several reviews have systematically elucidated the degradation mechanisms of AEMs with different structures in conventional systems, but specific research on dry cathode configurations remains lacking, posing a challenge to our understanding of membrane/ionomer degradation patterns.
Future research needs to make breakthroughs in two aspects: First, by combining techniques such as Fourier transform infrared spectroscopy (FTIR), nuclear magnetic resonance (NMR), and electrochemical impedance spectroscopy (EIS), an in-situ analysis method for AEM degradation under dry cathode conditions should be established to clarify the degradation pathway and microscopic mechanism; Second, IEC control experiments should be systematically carried out to construct a quantitative correlation model of IEC-WU-electrolysis voltage, providing theoretical support for the structural design of AEMs specifically designed for dry cathodes.
In the membrane electrode fabrication stage of dry cathode AEMWE, ultrasonic spraying technology, with its unique atomization deposition mechanism, provides a feasible path for the precise coating of ionomers and catalyst layers. The ultrasonic sprayer atomizes the ionomer solution or catalyst slurry into micron-sized droplets through high-frequency vibration, which are then uniformly deposited on the membrane surface or gas diffusion layer under the assistance of a low-pressure carrier gas, achieving precise control of thickness from nanometer to micrometer levels. Compared with traditional blade coating and brush coating, ultrasonic coating produces droplets with more uniform particle size and more controllable deposition density, effectively reducing ionomer agglomeration and catalyst layer cracks, thereby optimizing the ion transport channels and three-phase interface structure within the electrode. Addressing the stringent water management requirements of dry cathode systems, ultrasonic spraying can construct a functional layer with gradient IEC and WU distributions by adjusting parameters such as feed rate, ultrasonic power, scanning path, and substrate temperature. This allows the anode side to maintain adequate water retention to sustain OH⁻ migration, while the cathode side controls the ionomer loading to avoid excessive water absorption and mass transfer blockage. Furthermore, the non-contact nature of ultrasonic coating reduces mechanical damage to ultrathin AEMs, making it suitable for large-area, repetitive membrane electrode assembly fabrication processes. Future integration of ultrasonic spraying with ionomer molecular design, IEC gradient control, and in-situ characterization techniques holds promise for providing a systematic solution for high-performance, long-life membrane electrode fabrication in dry cathode AEMWEs.
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