High-temperature proton exchange membranes with tunable acidity of phosphonic acid groups by incorporating zwitterionic aromatic moieties

Introduction

High-temperature proton exchange membranes (HT-PEMs) are essential components of high-temperature proton exchange membrane fuel cells (HT-PEMFCs), facilitating efficient proton transport while serving as barriers to gases and electrons^1,2. The electrochemical performance and durability of HT-PEMs depend on the balance between proton conductivity and mechanical stability³. The polyelectrolytes membrane depends on phosphoric acid (PA) as a proton source, which simultaneously promotes a dynamic hydrogen-bond network for proton transfer⁴. The doping level of the proton conductor significantly influences the electrochemical performance of HT-PEMFCs. However, the plasticizing effect of small PA molecules can compromise the mechanical of membrane. Moreover, PA leaching from HT-PEMs during operation may adversely affect precious metal catalysts, thereby diminishing the durability of HT-PEMFCs⁵. Addressing the challenges associated with reducing proton conductor doping and PA leaching, while maintaining proton concentration and establishing a robust proton conduction network, is essential for achieving high-performance HT-PEMs.

Commercially available polybenzimidazole (PBI) is extensively utilized in HT-PEM applications. Alkaline polyelectrolytes, such as PBI, involve a fraction of PA interacting with basic groups through acid-base interactions, while another fraction exists as free PA (f-PA) molecules, stabilized by hydrogen bonding between polymer chains^6,7. The adsorption of high concentrations of proton conductors adversely affects the mechanical properties of the polymer membrane⁸. Conversely, a reduction in the concentration of proton conductors within the membrane significantly compromises its proton conductivity⁹.

Compared to traditional alkaline polymer, phosphonated polymers, in which phosphonated groups are covalently linked to the polymer backbone, exhibit promising potential for HT-PEMs¹⁰. Under high-temperature and low-humidity conditions, phosphonated groups undergo self-dissociation, creating a robust hydrogen bond network that enhances proton conductivity¹¹. Recently, Meng et al. demonstrated that the synergistic interaction between covalently bonded PA (b-PA) groups and f-PA enabled Poly(p-terphenyl-co-isatin piperidinium) to achieve a proton conductivity of 90 mS cm⁻², with a voltage decay rate of 0.45 mV h⁻¹ over 100 h¹². However, this approach revealed limitations, as reliance on b-PA and f-PA as proton conductors does not adequately mitigate the degradation rate of proton conductors of membrane. To address this, we incorporated functional groups capable of forming ion-pair bonded PA (c) groups into the polymer backbone. For example, Kim et al. demonstrated that ion-pair-coordinated PEMs facilitate proton conduction through enhanced interactions between ionic pairs, while simultaneously mitigating phosphoric acid (PA) leaching, thus resulting in remarkable fuel cell performance¹³.

This study introduces a novel phosphonated zwitterionic aromatic polymer (P/QTIP-x) designed for HT-PEMs. The polymer features a unique architecture that incorporates three types of acidic proton conductors (b-PA groups, i-PA groups, and f-PA) to enhance the functionality of membrane. Unlike traditional acid-base interactions, the ionic stabilization from the i-PA groups significantly reduces proton conductor leaching and enhances proton conductivity stability. Additionally, the b-PA groups act as reliable proton sources and help form hydrogen bonding channels that facilitate proton transport. The f-PA molecules further reinforce the proton transport network within the membrane. The synergistic interactions among these three proton-conducting structures lead to exceptional chemical stability and superior electrochemical performance. The design of structure optimizes the distribution of various acidic proton conductors, effectively minimizing leaching and enhancing the stability of proton conductivity. This dual enhancement not only improves the mechanical properties but also elevates the electrochemical performance of the membrane. Ultimately, this innovative strategy establishes a scientific foundation for the development of high-performance and durable HT-PEMFCs.

Results and discussion

Synthesis and characterizations of polymer

P/QPIT-x has been meticulously developed for HT-PEM, integrating i-PA groups, b-PA groups, and f-PA molecules into a multifunctional framework. This strategic synthesis facilitates the precise modulation of proton conduction properties in HT-PEM, thereby enhancing performance for HT-PEMFC applications. As depicted in Fig. 1, linear PTIP-x copolymers are synthesized through a Friedel-Crafts reaction catalyzed by strong acid^14,15. By systematically varying the ratios of ketone monomers, we successfully synthesized a series of PTIP-x copolymers, where x indicates the proportion of isatin. To incorporate p-PA groups into the polymer structure, QPIT-x was synthesized through the quaternization of PTIP-x with CH₃I. The results of the ¹H-NMR spectrum for QPIT-x are presented in Fig. S1. Notably, a significant shift in the peak corresponding to the -CH₃ group was observed, moving from 2.79 ppm to 3.16 ppm, confirming the successful synthesis of QTIP-x. To incorporate b-PA groups into the QPIT-x polymer framework, phosphorylation reaction was performed using POCl₃ as the reagent. As depicted in Fig. S2, the characteristic peak in the ³¹P-NMR spectrum of P/QPIT-x appears around -1 ppm. The presence of this peak, in contrast to the characteristic peak of PA (0 ppm), confirms the successful integration of b-PA groups into the QPIT-x backbone.

Following the reaction, the solubility of the resulting P/QPIT-x in common organic solvents decreased significantly. The introduction of TFA disrupted the hydrogen bonding interactions in P/QPIT-60, thereby improving its solubility. Conversely, P/QPIT-80 exhibited markedly low solubility, remaining insoluble in solvents. The intrinsic viscosities of P/QPIT-20, P/QPIT-40, and P/QPIT-60 in DMSO, measured using an Ubbelohde viscometer, were determined to be 1.47 dL g⁻¹, 1.98 dL g⁻¹, and 2.96 dL g⁻¹, respectively. As shown in Fig. S3, P/QPIT-x can be processed into transparent, flexible films with an approximate thickness of 20 ± 3 μm for further testing.

PA adoption doping, volume swelling, mechanical and chemical stability

The PA adoption level and volumetric swelling ratio of HT-PEMs are critical factors influencing stability of HT-PEMs. Figure 2a details PA absorption and swelling behavior of P/QPIT-x. Due to the robust ion-pair interactions between quaternary amine groups and PA, P/QPIT-x attains saturation through a rapid ion exchange process involving OH⁻ and H₂PO₄²⁻ within a duration of 2 h (Fig. S4). As b-PA groups increasing within structure, the ability to adsorb protons conductor gradually decreases. The equilibrium PA adoption level for P/QPIT-20, P/QPIT-40, and P/QPIT-60 are 105%, 75%, and 50%, respectively. Additionally, the saturation volumetric swelling ratios for P/QPIT-20, P/QPIT-40, and P/QPIT-60 are 68%, 47%, and 27%, respectively. The P/QPIT-x membranes exhibit lower PA adoption and remarkable dimensional stability, which significantly mitigates the risk of physical damage during membrane electrode assembly and enhances the overall durability of HT-PEMFCs.

**Fig. 2: Dimensional stability and mechanical properties of membrane.**

The mechanical performance of HT-PEMs is vital for the durability of HT-PEMFCs, with the level of PA doping serving as a key determinant. As shown in Fig. S5, PBI and P/QPIT-x membranes show excellent mechanical property. However, the plasticizing effect of PA molecules attenuates the interactions forces of polymer backbone chains, consequently resulting in decreased tensile strength of HT-PEMs. As illustrated in Fig. 2b, the tensile strength of 105%PA@P/QPIT-20, 75%PA@P/QPIT-40 and 50%PA@P/QPIT-60 are 13 MPa, 22 MPa, and 30 MPa, respectively. The tensile strength of the obtained 50%PA@P/QPIT-60 was 3.3 times greater than that of 250%PA@PBI. Consequently, P/QPIT-x membranes with lower PA doping exhibit superior mechanical properties, facilitating the fabrication of thinner electrolyte membrane.

The overall thermal and chemical stabilities of membrane are crucial to the endurance of HT-PEMs. Figure S6 depicts the thermal degradation behavior of P/QPIT-x which show no weight loss above 200 °C. Therefore, P/QPIT-x have excellent thermal stability during HT-PEMFCs operation. The primary degradation mechanism of HT-PEMs stems from radical attacks peroxide radicals (·OH and ·OOH), which results in membrane deterioration and poor mechanical strength arising during operational environment. Given the limitations in replicating the genuine operational conditions of HT-PEMFCs, the oxidative stability of HT-PEMs is experimentally evaluated by assessing accelerated degradation rate in Fenton’s reagent. Modifying the ratio of i-PA to b-PA groups within the P/QPIT-x structure demonstrated that an increase in b-PA groups enhances the structure’s tolerance to peroxide radical attack (Fig. S7). Since the actual membrane electrode assemblies utilize polymer electrolyte membrane with PA adoption, this study tested the oxidative stability of PA@P/QPIT-x membranes (Fig. 2c). For PA doped membranes, oxidative stability is influenced by both volumetric stability and the volume swelling ratio, which is dependent on the PA content and the treatment method employed of the membrane. Compared to 250%PA@PBI membranes with larger volumetric swelling which exhibited rapid breakdown with 2 h, 50%PA@P/QPIT–60 membranes with lower volumetric swelling demonstrated superior oxidative stability over 8 h.

As illustrated in Fig. 2d, utilizing reference computational methods from the literature, pKa values of PA groups with diverse structural variations was performed, which reflect their protonation capacity¹⁶. Notably, the pKa values of phosphonated phenol-formaldehyde (1.369)¹⁷ and phosphonated piperidine groups (1.163)¹² are lower than that of phosphoric acid (1.377), indicating pronounced acidity of groups. In the P/QPIT-x structure, the b-PA group exhibits increased acidity due to the influence of the adjacent carbonyl group. Computational results reveal that the pKa of i-PA group (3.871), associated through ion-pair interactions, is notably weaker acidity. Consequently, the P/QPIT-x structure effectively integrates both strongly acidic b-PA groups and weakly acidic i-PA. Subsequent investigations focused on the influence of the acidity of PA groups on the physicochemical properties of the membrane.

Morphology observation

Transmission electron microscopy (TEM) and atomic force microscopy (AFM) were employed to investigate the distribution of clusters and microphase separation within the P/QPIT-x structure. To analyze the distribution of b-PA groups within the polymer, we performed TEM analysis on P/QPIT-x (I⁻ form) without ion exchange in PA solution. Pb²⁺ was utilized to selectively stain the b-PA groups, facilitating high-resolution visualization of their distribution across the matrix^18,19. As illustrated in Fig. 3a–c, the dark regions in the TEM cross-sectional images correspond to clusters formed by the aggregation of b-PA groups. An increased b-PA content leads to a higher density of these clusters within the P/QPIT-x structure²⁰. In the case of P/QPIT-60, the b-PA groups exhibit uniformly distributed clusters with an average size of 7 nm. These ionic aggregates of b-PA groups significantly impact proton transport within the membrane, facilitating rapid proton conduction while also acting as stable proton reservoirs that promote the formation of a dynamic hydrogen-bonding network²¹.

**Fig. 3: Microphase separation structure of membrane.**

AFM images reveal the nanoscale adhesion-force distribution of the P/QPIT-x structure, elucidating the distinct interactions between the tip probe and the functional groups²². Notably, the bright regions within the acid-rich areas display a continuous and dense pattern, indicative of a progressive enhancement of the continuous channel structure in specific membrane sections (Fig. 3d–f). The average adhesion force values for P/QPIT-20, P/QPIT-40, and P/QPIT-60 were recorded at 8.5 nN, 16.5 nN, 24.5 nN, respectively (Fig. 3g–i). These findings suggest a pronounced two-phase separation within the P/QPIT-x polymer, attributed to the varying acidity of phosphonic acid groups, which significantly influences the proton transport behavior of the P/QPIT-x membranes. Concurrently, the trend in adhesion force of the P/QPIT-x membrane corresponds with the fluctuations observed in its static contact angle (Fig. S8).

Proton conductivity and proton conductor stability

Figure 4a illustrates that variations in the ratios of f-PA, b-PA, and p-PA within the P/QPIT-x structure significantly influence proton transport rates over the temperature range of 80–180 °C and anhydrous conditions. The measured proton conductivities for 105%PA@P/QPIT-20, 75%PA@P/QPIT-40, and 50%PA@P/QPIT-60 were 201 mS cm⁻¹, 139 mS cm⁻¹, and 81 mS cm⁻¹ operated at 180 °C, respectively. Notably, the PA@P/QPIT-x membranes exhibited markedly higher proton conductivity compared to the 250%PA@PBI membranes (43 mS cm⁻¹ at 180 °C). The nanoscale adhesion-force distribution for 105% PA@P/QPIT-20 (Fig. 4b) reveals pronounced phase separation, characterized by heterogeneously distributed bright “island-like” regions, indicative of an expanded polar proton-conducting phase²³. The rich-PA region emerges from the synergistic interactions of three distinct proton conductors, promoting the formation of a dense, continuous three-dimensional hydrogen bonding network within the membrane, which significantly enhances proton transport rates. The stability of high-temperature proton conductivity was assessed by monitoring proton transport resistance over an extended duration. Compared with 250%PA@PBI, 105%PA@P/QPIT-20 exhibited higher stability, retaining 68% of initial conductivity after 95 h of testing at 140 °C (Fig. 4c). The stabilized proton conductivity results for P/QPIT-x indicate an enhanced capacity to anchor proton conductors and the existent for novel proton transfer pathways within the designed structure.

**Fig. 4: Proton conductivity and proton conductor retention of membrane.**

During operation, a significant interaction between water vapor generated by electrochemical reactions and phosphoric acid can lead to the leaching of proton conductors from HT-PEM. This leaching process introduces defects within the proton transport channels, consequently leading to a reduction in proton conductivity²⁴. To accelerate simulate the effects of vapor on f-PA, the membrane was suspended above a 100 °C water bath. As illustrated in Fig. 4d, the PA retention rate within the 250%PA@PBI after 6 h is 66%, while the 50% PA@P/QPIT-60 membrane exhibits a significantly higher PA retention rate of 81%. The strong interactions between b-PA and i-PA groups within the polymer chain significantly reduce their susceptibility to leaching. In contrast, f-PA molecules, which interact weakly with the polymer backbone, are more prone to loss. Consequently, the P/QPIT-x membrane, which incorporates zwitterionic aromatic moieties to modulate the acidity of phosphonic acid groups, exhibits significantly improved retention capacity for proton conductor.

Under conditions of elevated temperature and low humidity, the proton conductor in HT-PEM is prone to dehydration through polycondensation, resulting in the generation of non-proton-conductive acid anhydrides^25,26. This phenomenon markedly reduces proton conductivity and overall cell performance. Therefore, the capacity to inhibit the formation of acid anhydrides is essential for evaluating the electrochemical stability of HT-PEM, a process that can be investigated through thermogravimetric analysis. The dehydration condensation temperature was ascertained from the derivative thermogravimetry curve. As depicted in Fig. 4e, the dehydration temperature of the 105%PA@P/QPIT-20, 75%PA@P/QPIT-40, and 50%PA@P/QPIT-60 membrane are 189 °C, 194 °C, and 198 °C, which higher than that of 250%PA@PBI (183 °C). Notably, as the concentrations of i-PA and f-PA in P/QPIT-x decrease, the dehydration temperature of the PA@P/QPIT-x membrane increases, indicating a diminished tendency for PA to undergo dehydration and polycondensation. Therefore, the innovative design of P/QPIT-x incorporates anhydride inhibitors within its structure, thereby enhancing the electronic stabilization of the phosphonate anion.

Fuel cell performance and durability

This section presents a comparative analysis of the fuel cell performance and durability of HT-PEMFCs. The MEA based on 250% PA@PBI demonstrated inferior output performance relative to the P/QPIT-x (Fig. S9). The open-circuit voltages (OCVs) of the MEAs utilizing 105% PA@P/QPIT-20, 75% PA@P/QPIT-40, and 50% PA@P/QPIT-60 membranes were measured at 1.038 V, 0.813 V, and 0.915 V, respectively (Figs. 5a and S10), demonstrating satisfactory gas barrier capabilities under anhydrous conditions at 140 °C. The peak power densities (PPD) for the MEAs were 728 mW cm⁻², 617 mW cm⁻², and 370 mW cm⁻², with corresponding maximum current densities of 2098 mA cm⁻², 1716 mA cm⁻², and 850 mA cm⁻². The superior proton transport rate and mechanical properties of the P/QPIT-x membrane enabled the fuel cells to maintain high OCV and PPD, even under elevated back pressure and a flow rate of 160 °C (Figs. 5b and S11).

Figure 5c illustrates the stable cycling performance of P/QPIT-x based MEAs under different current loading, with a gradual voltage decay observed over 10 h at 140 °C. Cycling performance testing elucidates the polarization losses within HT-PEMFC systems under different load conditions, thereby facilitating a comprehensive assessment of MEA efficiency and reliability. The voltage decline during operation is primarily attributed to activation, ohmic, and concentration losses. The MEA incorporating P/QPIT-x exhibits a diminished electrochemical reaction rate at low current densities, primarily due to activation polarization identified as the predominant factor contributing to voltage decay. Over time, this effect diminishes, giving way to ohmic polarization. The improved voltage and retention observed in P/QPIT-60 can be attributed to the microphase separation structure formed by b-PA and i-PA groups, which facilitates enhanced reactant diffusion and effectively mitigates activation polarization. At elevated current loads, concentration polarization predominantly accounts for voltage losses in HT-PEMFCs. Notably, P/QPIT-20 exhibits enhanced voltage output and retention at higher current densities. As the concentration of strongly acidic b-PA groups within the P/QPIT-x structure decreases, the concentrations of i-PA and f-PA groups increase. This enhancement, facilitated by the presence of fast-migrating proton conductors, fosters improved ion migration and proton conductivity within the structure, thereby optimizing interfacial reaction kinetics and enhancing overall electrochemical performance. The advancement of MEAs capable of enduring high current loads while maintaining elevated voltage presents ongoing challenges due to the substantial influence of internal resistance on ion transfer within the membrane and other components^27,28.

Moreover, high-temperature durability tests conducted at 140 °C under steady-state current loads of 200 mA cm⁻² indicated that the P/QPIT-x based MEAs demonstrated superior fuel cell performance (Figs. 5d and S12). The voltage variation rates for the 105% PA@P/QPIT-20, 75% PA@P/QPIT-40, and 50% PA@P/QPIT-60 membranes over 140 h were recorded at 0.367 mV h⁻¹, 0.418 mV h⁻¹, and 0.974 mV h⁻¹, respectively, with corresponding voltage decay rates of 8.1%, 8.6%, and 23.1%. The methodologies utilized for accelerated testing (Fig. 4d) and durability assessment of HT-PEM (Fig. 5d) under water vapor conditions exhibit inherent inconsistencies. Figure 4d illustrates the rate of f-PA loss from HT-PEM at 100 °C with water vapor, while Fig. 5d presents the performance degradation trend of HT-PEM during actual operation within the HT-PEMFC clamp. Despite the discrepancies observed in degradation trends between the two testing approaches, both consistently demonstrate that the PQPIT-x membrane exhibits superior electrochemical performance compared to PBI membrane.

Figure S13 illustrates the evolution of PPD during cell operation. After 96 h at 140 °C under anhydrous conditions, the P/QPIT-20 maintained a PPD of 543 mW cm⁻², with minimal degradation of 5.4%. In comparison, the P/QPIT-40 cell achieved a PPD of 373 mW cm⁻², while the P/QPIT-60 cell exhibited a significantly lower value of 177 mW cm⁻². Moreover, the P/QPIT-x membranes, characterized by fine structural regulation, exhibited superior performance compared to recent studies on HT-PEMs (Fig. 5e), which typically enhance proton conductivity by increasing the concentration of proton conductors^{6,29,30,31,32,33,34,35,36,37}. Current methodologies, including polymer backbone modification and the incorporation of additives into the PBI matrix, have been shown to effectively reduce the levels of PA doping in membranes. Nevertheless, conventional alkaline polymer membranes frequently demonstrate inadequate performance when doping low-concentration proton conductors. Consequently, a significant challenge in the development of high-performance HT-PEMFCs lies in the effective optimization of proton conductor distribution, which is essential for balancing the proton transport rate with the mechanical properties of the membrane. The regulation of three distinct acidic groups within the structure presents a novel research approach for optimizing proton conductor distribution within the polymer matrix, enhancing PA retention and offering critical insights for the advancement of durable, high-performance HT-PEMFCs.

Conclusions

In conclusion, this study successfully integrates three distinct acidic groups—i-PA groups, b-PA groups, and f-PA molecules—into P/QPIT-x structure through meticulous design. Clusters formed by b-PA groups facilitate the establishment of stable proton transport pathways, while i-PA groups and f-PA molecules further enhance the development of continuous and dense proton transport channels. Importantly, b-PA and i-PA groups function as fixed proton sources, anhydride inhibitors, and free radical scavengers, effectively mitigating the dependence of proton conductivity on f-PA. Furthermore, by regulating the distribution of acidic groups within the structure, HT-PEMs demonstrate exceptional proton conductivity and mechanical properties, thereby presenting a promising avenue for the development of high-performance and durable membranes for HT-PEM fuel cells.