Durability Limits of Pure-Water AEM Water Electrolysis

Durability Limiting Factors of Anion Exchange Membrane Electrolysis of Water in Pure Water Environment

The most significant drawback of anion exchange membrane electrolysis (AEMWE) systems using pure water as feedstock is their low durability. Multiple experimental data have confirmed this characteristic: at a current density of 200 mA/cm² and room temperature, the system using a spinel ferrite catalyst experienced a voltage increase from 1.6 V to 1.75 V in just 3 hours, despite rotating disk electrode (RDE) testing showing that the catalyst’s oxygen evolution reaction (OER) activity remained stable for 4100 hours; while at 200 mA/cm² and 50°C, the voltage of the iridium oxide-catalyzed system surged from 1.75 V to 2.3 V within 35 hours, and post-failure testing revealed no significant degradation traces on the piperidine-functionalized anion exchange membrane (AEM).

It is noteworthy that under pure water feedstock conditions, without the participation of an additional liquid electrolyte, the system’s corrosivity is significantly reduced, meaning that the alkalinity stability of the MEA (membrane electrode assembly) is not the bottleneck to durability. In-depth research indicates that problems related to the ion exchange membrane caused by high operating voltage and high current density are the core issues limiting system lifespan. This article focuses on two key limiting factors—ion exchange membrane detachment from the catalyst surface and ionomer poisoning—analyzing their accelerated degradation mechanisms and mitigation pathways under harsh operating conditions.

Ion Exchange Membrane Detachment from the Catalyst Surface: A Failure Mechanism of Interfacial Stability

The failure of the bond between the ion exchange membrane and the catalyst surface is the primary cause of AEMWE performance degradation. Ion exchange capacity (IEC, the core indicator for measuring the ion exchange capacity of the ion exchange membrane) and operating temperature are key variables affecting this process. A study used high-IEC quaternized polystyrene ionomers to improve system performance. TMA-70 ionomer (IEC = 3.3 mequiv/g) could achieve a high current density of 2.4 A/cm² at 2.0 V and 85℃. However, catalyst particle loss was detected at both the anode and cathode outlets during continuous operation, proving that the high-IEC ion exchange membrane could not stably anchor the catalyst, resulting in a system lifespan of only 7 hours.

Temperature also significantly affects interfacial adhesion: lowering the operating temperature to 60°C improves the bonding strength of the ionomer membrane and extends system durability to 12 hours; further, using the low-IEC TMA-53 ionomer (IEC=2.6 mequiv/g), at 60°C, although the initial voltage increases by approximately 200 mV, the lifespan surges to 4100 hours, and the degradation rate is significantly reduced—clearly revealing the trade-off between AEMWE performance and durability.

The ease with which high-IEC ionomers detach is closely related to their water absorption characteristics. High IEC means a high density of ionic groups and a high water absorption rate in the hydrated state, leading to drastic changes in ionomer size and directly weakening its adhesion to the catalyst surface. This problem is exacerbated by pure water feeding conditions: compared to traditional electrolyte systems, the catalyst-electrolyte interface area is smaller in a pure water environment, resulting in more uneven gas release at the same current density; simultaneously, the gas permeability of the hydrocarbon-based quaternary ammonium ion-exchange membrane is much lower than that of KOH solution, making it difficult for the generated gas to quickly detach from the interface during high-current operation. The mechanical force generated by bubble accumulation further damages the bond between the ion-exchange membrane and the catalyst. Compared to proton exchange membrane electrolysis (PEMWE) systems, the low permeability and high swelling properties of the hydrocarbon-based membrane in AEMWE make it more susceptible to bubble-induced ion-exchange membrane detachment. Experimental data confirms this trend: the system voltage remained stable for 100 hours at 100 mA/cm², but failed after only 40 hours at 300 mA/cm²; the pure water AEMWE catalyzed by nickel-iron hydroxide also exhibits the same trend.

1.1 Core Strategies for Mitigating Ion Exchange Membrane Delamination

Current mitigation approaches revolve around “improving interfacial bonding stability,” primarily encompassing three technical directions:

– Optimizing operating parameters and ionomer selection: Using low-IEC ionomers and lowering the operating temperature is the most direct method, but it comes at the cost of performance degradation—while low IEC increases bonding strength, it reduces ion conduction efficiency, leading to an increase in initial voltage.

– Developing novel ionomer materials: The goal is to prepare balanced materials with “high IEC and low water absorption.” Key synthetic strategies include introducing multiple cationic groups, constructing polar interaction networks, and cross-linking structures. These materials can suppress swelling while ensuring ion conduction, but face three major challenges: low water absorption may reduce conductivity, thus affecting hydrogen production rates; multiple cationic groups may reduce chemical stability; and the synthesis process is complex and costly.

– Enhanced Interface Structure Design: Using non-aqueous dispersants increases the chain entanglement of ionomers, improving membrane adhesion and mechanical strength. It also optimizes the distribution of the ionomer membrane within the electrode, resulting in more uniform gas release. Reducing the size of catalyst nanoparticles also allows for a more dispersed gas evolution reaction distribution, reducing localized bubble impact.

Ionomer Poisoning: Electrochemical Oxidation Damage to the Catalyst Surface

The electrochemical oxidation of phenyl groups in the ionomer membrane at the OER potential is another core durability bottleneck for pure water AEMWEs. The unique aspect of this process is that although the AEMWE anode is free of carbon due to the high OER potential and is prone to carbon corrosion, ionomers cannot completely remove phenyl groups. These groups contaminate the catalyst surface through electrochemical oxidation, leading to decreased activity.

The mechanism of phenyl poisoning can be divided into three steps: First, the phenyl group in the ionomer is tightly adsorbed onto the catalyst surface through the strong interaction between aromatic π electrons and the electron cloud of the catalyst metal atoms—studies show that the adsorption energy of phenyl fragments in the ionomer backbone on the platinum surface is even higher than that of pure benzene. Second, the adsorbed phenyl group is oxidized to phenolic compounds at high potentials. Unlike carbon corrosion, which ultimately produces CO₂, 1,4-substituted phenyl groups are difficult to completely oxidize and remain mostly in the form of phenols, 2-phenylphenol (pKa=9.6), and 2,2′-biphenol (pKa=7.6). Finally, the generated phenolic protons are deprotonated by hydroxide ions from quaternary ammonium groups, and remain stable in alkaline media, continuously occupying the active sites of the catalyst.

The operating characteristics of AEMWEs make their poisoning risk much higher than that of anion exchange membrane fuel cells (AEMFCs): the anode operating voltage of an AEMWE is 1.4-2.2 V, while that of an AEMFC cathode is only 0.6-1.0 V; the higher potential significantly accelerates phenyl oxidation. Experiments confirmed that phenol compounds in benzyltrimethylammonium hydroxide (BTMAOH) solution undergo significant adsorption oxidation after 100 hours of contact with an iridium oxide catalyst at 2.1 V (vs RHE); even at oxygen reduction potentials above 0.6 V, phenyl oxidation occurs, adversely affecting the lifetime of AEMFC.

The adsorption energy of phenyl groups on the catalyst surface is crucial in determining the poisoning rate. Density functional theory (DFT) calculations show that the adsorption energy of BTMAOH phenyl groups parallel to the iridium oxide surface at 1.6 V is 1.2-2.2 eV, significantly higher than that of La₀.₈₅Sr₀.₁₅CoO₃ perovskite catalysts; RDE testing further verified that the phenyl oxidation rate on the iridium oxide surface is approximately three times that of perovskite. The corresponding differences in system durability are extremely significant: the voltage of the iridium oxide-catalyzed AEMWE increased from 1.7 V to 2.1 V within 5 hours, while the voltage of the perovskite catalytic system remained stable at around 1.8 V within 100 hours.

2.1 Technical Pathways for Suppressing Ionomer Poisoning

The core strategy for mitigating phenyl poisoning is to reduce phenyl adsorption from a dual-dimensional perspective of “catalyst-polymer,” specifically including the following directions:

– Selecting OER catalysts with low phenyl adsorption energy: Transition metals (platinum, palladium, iridium) have high phenyl adsorption energy, while alloy catalysts can reduce adsorption energy by adjusting the central electron structure of the d-band—for example, the adsorption energy of BTMA-group phenyl on the Pt surface is -2.30 eV, which drops to -1.30 eV on the Pt₁Ru₁ alloy surface; perovskite catalysts, due to their surface characteristics, have the lowest phenyl adsorption energy, making them ideal for long-term operation, and they have low pH dependence, making them suitable for pure water conditions.

– Developing low-adsorption-energy polymer electrolytes: Polymer structure directly affects phenyl adsorption capacity. The phenyl adsorption energy of quaternized polyolefins is lower than that of quaternized polyaromatics, and the adsorption energy of non-rotatable phenyl groups (such as fluorene and carbazole) is lower than that of rotatable phenyl groups (such as biphenyl). Three sets of MEA comparative experiments visually illustrate this pattern: Using HTMA-DAPP containing biphenyl/terphenyl units as the AEM and ionomer system resulted in a sharp performance drop within 5 hours; using phenyl-free SES-TMA AEM paired with HTMA-DAPP ionomer resulted in a slow performance degradation within 80 hours; while the combination of SES-TMA AEM and FLN55 ionomer containing fluorene structures (non-rotatable phenyl groups) maintained stable performance within 80 hours.

Summary and Challenges

The two major durability limiting factors for pure water AEMWEs are directly related to the ionomer binder: insufficient interfacial bonding leads to ionomer detachment, and phenyl oxidation triggers catalyst poisoning; both pathways jointly accelerate system degradation. Furthermore, side reactions such as hydrogenation of ionomer fragments and co-adsorption of cations, hydroxides, and water can also lead to deactivation of the hydrogen evolution reaction (HER) catalyst—although the latter is not yet fully studied at the single-cell level, the hydrogenation of benzene compounds on noble metal catalysts has been confirmed, and high concentrations of hydroxides can also reduce the water supply to the catalyst surface, further affecting activity.

The trade-off between performance and durability is the core contradiction: methods to improve durability, such as low IEC and low temperature, sacrifice output performance, while highly active catalysts and high IEC ionomers are prone to poisoning and shedding. Overcoming this trade-off and achieving a synergistic effect of “high activity and long lifespan” is a key technological challenge for the commercialization of pure water AEMWE.

Ultrasonic spraying for the preparation of AEM electrolysis membrane electrodes for pure water systems: Pure water anion exchange membrane electrolysis (AEMWE) does not require high-concentration alkaline solutions and relies on intramembrane ionomers to conduct hydroxide ions. It places stringent requirements on the uniformity of the catalyst layer, pore structure, and interfacial bonding of the membrane electrode. Ultrasonic spraying is the core process for preparing high-performance MEAs. The equipment utilizes 20–120kHz high-frequency vibration to atomize NiFeOx and nickel-based non-precious metal catalyst slurries, forming uniform microdroplets of 10–50μm. A triaxial motion platform precisely coats these droplets onto the anion exchange membrane using a CCM direct film formation process, coupled with segmented drying at a constant temperature of 40–80℃ to avoid membrane swelling and coating cracking issues in pure water solvent systems.

Compared to traditional pneumatic spraying, ultrasonic atomization eliminates the impact of high-pressure airflow, preventing damage to thin AEM membranes. Coating thickness is controllable at 5–15μm with a thickness error of less than 5%, increasing catalyst utilization to over 90% and significantly reducing non-precious metal consumable consumption. The spraying creates multi-level interconnected pores, constructing a sufficient three-phase reaction interface, suitable for low-ion concentration pure water conditions, reducing electrolysis interface impedance, and improving hydroxide ion conduction efficiency.

The formed membrane electrode can operate stably without high-alkali immersion activation. The catalyst layer adheres tightly to the membrane, resisting detachment even after long-term rinsing with pure water, effectively inhibiting gas stripping failure. The entire process is adaptable to pilot-scale R&D and mass production, and the catalyst loading can be flexibly adjusted. It solves the pain points of high polarization and low stability in pure water AEM electrolysis of water, and provides a standardized membrane electrode preparation solution for low-cost, alkali-free green hydrogen production.

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