The impact of imidazolium with steric hindrance on the dissociation of phosphoric acid and the performance of high-temperature proton exchange membranes†

Xi Sun^ab, Huiting Yu^ab, Jiayu Guan^ab, Bin Zhang^ab, Jifu Zheng*^a, Shenghai Li^ab and Suobo Zhang*^ab
aKey Laboratory of Polymer Ecomaterials, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China. E-mail: jfzheng@ciac.ac.cn; sbzhang@ciac.ac.cn
^bUniversity of Science and Technology of China, Hefei 230026, China

First published on 19th August 2024

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Introduction

Fuel cells are clean and efficient energy conversion devices that have become one of the ports in the large-scale application of hydrogen energy.^1–3 High-temperature proton exchange membrane fuel cells (HT-PEMFCs) operate in the temperature range of 120–250 °C.⁴ Compared with other low and medium temperature polymer electrolyte membrane fuel cells, they have the advantages of faster kinetics of electrode reactions, enhanced tolerance to carbon monoxide (CO), and simpler water and heat management, and have attracted increasing research attention in recent years.^4–6 Due to the high operating temperature of HT-PEMFCs, high boiling point acids are chosen as proton transport media, including phosphoric acid (PA), phosphotungstic acid, and so on.⁷ Meanwhile, the harsh working environments also place strict demands on materials. High-temperature proton exchange membranes (HT-PEMs) as the core component of membrane electrode assemblies (MEAs) not only need conducting protons and insulating reactants, but also need good thermal stability and operating stability.^8,9

Polybenzimidazole (PBI) is an ideal polymer material for HT-PEMs due to its excellent thermal stability, good chemical stability and outstanding mechanical behavior.^10–13 It adsorbs PA through acid–base interaction and hydrogen bonds, and conducts protons mainly through the Grotthuss mechanism.¹³ The proton conductivity of PBI/PA is approximately 0.050 S cm⁻¹ at 150 °C, which meets the requirement for HT-PEMFC assembly, but the relatively low conductivity also severely limits the performance of fuel cells.^14,15 In addition, PA leaching in PBI remains a major problem affecting the stability of HT-PEMFCs.^16–19 The preparation of HT-PEMs with high conductivity and good stability to realize efficient and durable HT-PEMFCs remains a great challenge.

Over the past few decades, research on HT-PEMs has focused on enhancing proton conductivity and preventing PA leaching.^19–34 The main methods employed by researchers to improve the conduction efficiency of HT-PEMs are increasing the acid doping level (ADL) and constructing fast conduction channels. Lee et al.²⁰ used a cross-linking strategy to increase the density of basic sites in PBI-type polymers, thereby improving proton conductivity. Li et al.²⁷ found that the ultra-microporous structure based on Tröger's base-derived polymers could increase the ADL and accelerate conduction in HT-PEMs. Zhu et al.³² proposed that the doping of porous aromatic frameworks with an alkaline nitrogen structure (PAF-6) provided additional proton hopping sites, which could facilitate the construction of fast conduction channels. To prevent PA leaching in HT-PEMs, the methods of introducing additional interactions and constructing an internal structure have been proposed. Kim et al.²⁴ demonstrated that the stable ionic pair complexes in quaternary ammonium–biphosphate ion-pair-coordinated polyphenylene HT-PEMs enabled the PA to tolerate water condensation or absorption. Pak and Lee et al.²⁶ found that an increase in the degree of cross-linking would result in a denser membrane structure, thereby reducing the phenomenon of water penetration. These methods have been experimentally validated to be effective in optimizing the performance of HT-PEMs. Nevertheless, they usually require a complex construction process, and there is a lack of research on promoting the dissociation of PA.

Herein, we report a strategy to accelerate proton conduction and improve the operating stability of HT-PEMs by blending polyimidazolium dihydrogen phosphate (P-Im [H₂PO₄]) with strong steric hindrance in the poly[2,2′-(p-oxydiphenylene)-5,5′-benzimidazole] (OPBI) membrane (Fig. 1). By analyzing the computational and experimental results, it is found that, for one thing, the steric effect contributes to the formation of a stable ion-association complex between imidazolium and dihydrogen phosphate (H₂PO₄⁻), which reduces the dissociation energy of PA molecules. For another, it helps to strengthen the acid–base interaction and hydrogen bonds between polyimidazolium (P-Im) and PA, thus increasing the interaction energy between them. Benefiting from these advantages, the dissociation of PA is promoted and the leaching of PA is inhibited, and the performance of blend HT-PEMs can be enhanced comprehensively and effectively. Additional cation–π interactions also help to improve mechanical properties and inhibit PA leaching. This work mainly reveals the effect of steric hindrance of HT-PEMs materials on the dissociation behavior of PA, and provides a new approach to design efficient and durable HT-PEMs.

Results and discussion

To evaluate the effect of the chemical structure of P-Im [H₂PO₄] on proton conduction of HT-PEMs, six types of imidazolium dihydrogen phosphate (Im [H₂PO₄]) with different substituents and steric hindrance were selected and the dissociation energies (E_ds) of PA exposed to different Im [H₂PO₄] environments were computed using the density functional theory (DFT) method.³⁵ The specific results are shown in Fig. 2. Proton carriers with lower E_d values can more easily release protons under the same conditions. For PA molecules, the energy required to dissociate a proton (H⁺) and a dihydrogen phosphate ion (H₂PO₄⁻) was 14.13 eV (Fig. 2a). When PA was exposed to Im [H₂PO₄], a stable ion-association complex between imidazolium (Im) and H₂PO₄⁻ was formed by the ionic bond interaction. Acid–base and ionic interactions acted synergistically to further promote PA dissociation. As presented in Fig. 2b, the E_d of PA exposed to Im-1 was reduced to 13.34 eV. In addition, the steric effect could prevent the attack of other particles and enhance the stability of the ion-association complex. As a consequence, PA exposed to different Im [H₂PO₄] exhibited different E_ds. Among them, for Im-4 with strong steric hindrance, the E_d of PA was 13.10 eV, which was much lower than that of Im-1. For Im-6, due to conjugation with the benzene ring, ionic interactions were weakened, resulting in an increase in E_d. The corresponding E_d of PA was 13.19 eV. These results suggested that additional ionic interaction and steric effect contributed to the formation of a stable ion–association complex which could promote the dissociation of PA molecules (Fig. 2c). Besides that, H₂PO₄⁻ in Im [H₂PO₄] could also provide additional proton conduction sites to accelerate proton conduction in HT-PEMs.

The interaction energies (E_ints) between PA molecules and different Im [H₂PO₄] were also computed to predict the leaching behavior of PA in HT-PEMs (Fig. 3a).^24,33 As computed, E_int between PA molecules and Im-1 was −9.43 kJ mol⁻¹. In contrast, for Im-6 with a stronger steric effect, the E_int was increased to −28.50 kJ mol⁻¹. We believe that the increase in E_int is mainly due to the steric effect, which makes the interactions including acid–base interaction and hydrogen bonds between Im [H₂PO₄] and PA molecules more stable. Due to the stronger interaction, PA molecules were tightly bound around the center of Im [H₂PO₄]. This could effectively inhibit the leaching of PA in HT-PEMs.

Based on the computational results, we attempted to introduce Im [H₂PO₄] with strong steric hindrance to improve the performance of HT-PEMs. Two types of P-Im [H₂PO₄] were selected for blending in OPBI to further confirm the above conclusion through experiments (Fig. 3b). The definitions of P-PhIm and P-MeIm are P-Im [H₂PO₄] with methyl and phenyl substituents at the C4/C5 positions of imidazole, respectively. The membranes were prepared via solution casting, with a thickness of 40 ± 5 μm. The blend membranes were named OPBI&P-MeIm-x% and OPBI&P-PhIm-x%, respectively, where x% was the mass fraction of P-Im [H₂PO₄]. Experimental details for the synthesis of P-Im can be found in ref. 36 and 37 and the counterions were changed to H₂PO₄⁻. The relevant properties of the blend HT-PEMs are listed as follows.

X-ray diffraction (XRD) was used to explore the interstitial space of membranes. The amorphous structure of the membranes was verified by a broad peak in XRD spectra presented in Fig. 4b. The peak appearing at approximately 5.50° corresponded to the formation of ion clusters through the aggregation of ion groups. And the peak at around 19.00° was attributed to the distance of polymer chains, which shifted to a higher 2θ value for the blend membranes compared with OPBI. This is due to the introduction of extra cation–π interaction between imidazolium and phenyl, tightening the chain packing (Fig. 4a). Calculated by Bragg's law (2dsinθ = nλ), the distance of polymer chains decreased from 4.7 Å (2θ = 18.79°) in OPBI to 4.6 Å (2θ = 19.45°) in OPBI&P-MeIm-10% and 4.5 Å (2θ = 19.70°) in OPBI&P-PhIm-10%. In addition, for the blend membranes, the peak shifted to a lower 2θ value with the increase of the content of P-PhIm [H₂PO₄]. And the corresponding distance of polymer chains increased from 4.5 Å (2θ = 19.70°) in OPBI&P-PhIm-10% to 4.8 Å (2θ = 18.49°) in OPBI&P-PhIm-30%. It was the steric hindrance of P-Im [H₂PO₄] that was mainly responsible for the increase in polymer chain spacing. Although the wider chain spacing might weaken the interactions between the chains, it could accommodate more PA molecules. As shown in Fig. 4c, the presence of cation–π interaction was also indirectly demonstrated using Ultraviolet-visible (UV-vis) spectroscopy. Based on cation–π interaction, the absorption peak of the benzimidazole structure appearing at 325 to 360 nm was blue-shifted and became broader.³⁸ This indicated that the chemical environment of the benzimidazole structure had changed.

Mechanical behavior is one of the critical properties for HT-PEMs assembled in hydrogen fuel cells.³⁹ Both OPBI and blend membranes were robust and flexible and they demonstrated excellent mechanical behavior. All of their stresses were above 90 MPa and strains were above 9.5% before doping with PA (Fig. 5c). Among them, both OPBI&P-MeIm-10% and OPBI&P-PhIm-10% membranes demonstrated higher stresses compared with OPBI. This was due to the additional cation–π interaction and smaller chain spacing, which resulted in an increase in intermolecular interaction and an improvement in the tensile strength of the membrane materials. However, for the blend membranes, the mechanical strength decreased with the increase of the P-PhIm [H₂PO₄] content. This was due to the introduction of P-Im [H₂PO₄], which has a strong steric hindrance and rigid structure, reducing the toughness of membranes. Furthermore, as shown in Fig. 5d, the OPBI&P-PhIm-10%/PA membrane had a higher stress and strain at break than OPBI/PA, with 8.6 MPa stress and 51.7% strain. This was owing to the reduced swelling ratio caused by the lower PA uptake (Table 1), resulting in a shorter chain spacing and a greater intermolecular interaction. In addition, all PA doped blend membranes showed higher strains compared with the OPBI/PA membrane. This was due to the interaction of PA molecules with P-Im [H₂PO₄], so that PA played the role of a cross-linker, thus improving the plasticity and toughness of the membranes. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were used to determine the thermal stability of HT-PEMs. As shown in Fig. 5e, thermal decomposition curves of all membranes were recorded within the temperature range of room temperature (RT) to 800 °C. The two stages of weight loss occurring at approximately 70 °C and 200 °C were attributed to the evaporation of water and residual solvents, respectively. The weight loss at approximately 350 °C was due to the decomposition of the carbon–hydrogen skeleton. Based on the DSC profiles presented in Fig. 5f, no characteristic endothermic or exothermic phenomenon was detected in the temperature range of −45 to 200 °C for all membranes. These results confirmed that all of the membrane materials have good thermal stability within the working temperature range of HT-PEMFCs.

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PA molecules were adsorbed and doped in the OPBI membrane mainly due to acid–base interaction and hydrogen bonds. PA uptake and swelling ratio are key factors in determining the proton conductivity and mechanical behavior of HT-PEMs.⁴⁰ As presented in Table 1, compared with OPBI, all blend membranes showed lower PA uptake and lower swelling ratio. And both of these two parameters decreased with the increase of the content of P-PhIm [H₂PO₄]. This was because of the fewer number of imidazole functional groups and smaller chain spacing in the blend membranes. The PA uptake and swelling ratio of the OPBI/PA membrane were 225.16% and 216.23%, respectively. For the OPBI&P-PhIm-10%/PA membrane, they were only 170.88% and 185.18%, respectively. In addition, the ADL of the blend membranes increased with the increase of P-PhIm [H₂PO₄] content, which was due to that the wider polymer chain spacing could accommodate more PA molecules. This result corresponded to the XRD profiles (Fig. 4b).

The proton conductivities of the blend membranes, as well as OPBI for comparison, were plotted versus temperature as shown in Fig. 6a. As was commonly observed, the conductivity increased as a function of temperature. The conductivities of both OPBI&P-PhIm-20%/PA and OPBI&P-PhIm-30%/PA membranes were lower than that of OPBI/PA. This was because their PA uptake was much lower (Table 1). In contrast, the conductivity of OPBI&P-PhIm-10%/PA was 0.117 S cm⁻¹ at 160 °C, which was significantly higher than that of OPBI/PA (0.097 S cm⁻¹) by a factor of 1.20 under the same conditions. According to Table 1, the PA uptake and ADL of OPBI&P-PhIm-10% were all lower than those of OPBI, which were only 75.9% and 79.7% of OPBI, respectively. For the OPBI&P-MeIm-10%/PA membrane, it also had a higher conductivity than OPBI/PA. In other words, blending small amounts of P-Im [H₂PO₄] could significantly increase the conductivity of the OPBI/PA membrane. As presented in Fig. 6b, the Arrhenius plots were drawn and the activation energy (E_a) of proton conduction was calculated. Among them, the OPBI&P-PhIm-10%/PA membrane had the lowest E_a (28.89 kJ mol⁻¹). This peculiar increase in conductivity could be attributed to the different chemical environment to which PA was exposed. By introducing P-Im [H₂PO₄], it was much easier for protons to dissociate from PA molecules due to the additional ionic interaction and steric effect. This effectively reduced the proton conduction energy barrier. The experimental phenomenon was found to agree with the computational results (Fig. S1†).

PA leaching in the humid environment of hydrogen fuel cells was one of the key reasons for the degradation of cell performance. We chose to record the weight change of PA doped membranes versus time at 80 °C and 40% relative humidity (RH) to calculate the PA retention (Fig. 6c). Compared with OPBI, all blend membranes exhibited higher PA retention. Moreover, the higher the content of P-PhIm [H₂PO₄], the higher the PA retention of the membrane. After 100 hours of test, the PA retention of the OPBI&P-PhIm-30% membrane was 82.45%, which was 1.23 times higher than that of OPBI (66.84%) under the same conditions. Moreover, even with a P-PhIm [H₂PO₄] content of 10% in the blend membrane, the PA retention was as high as 80.71%. This experimental result is consistent with the computational E_int between PA and polymer structure units (Fig. S2†). For P-PhIm [H₂PO₄], the steric effect increased the E_int with PA and could effectively inhibit the leaching of PA. The additional cation–π interaction between imidazolium and phenyl also played an important role. The changes in weight of the membranes immersed in Fenton's reagent (3% H₂O₂, 2 ppm Fe²⁺) at 80 °C were recorded to assess the oxidative stability. As shown in Fig. 6d, compared with OPBI, the blend membranes exhibited less weight change during the test. And as the content of P-PhIm [H₂PO₄] increased, the weight changes decreased. After 5 hours of test, the remaining weight of OPBI&P-PhIm-30% was 92.92%. As for OPBI, it was only 85.89%. It was because that the ether bond in OPBI was easily attacked and broken by free radicals, leading to the degradation of the membrane material. The higher remaining weight of the blend membranes indicated that the P-Im [H₂PO₄] structure had better oxidative stability than OPBI. Meanwhile, the cation–π interaction could also help to prevent the attack of free radicals.

To further evaluate the performance of the blend membranes in practical applications, OPBI&P-PhIm-10%/PA and OPBI/PA membranes were assembled in hydrogen fuel cells, respectively, and the relevant performance was tested. Except for HT-PEM materials, both of the MEAs had the same electrode catalyst (Pt/C), ionomeric binder (PIM-P)⁴¹ and fabrication process. As expected, the cell equipped with the OPBI&P-PhIm-10%/PA membrane exhibited good power density and operating stability. The polarization and power density curves are shown in Fig. 7a and d. The open-circuit voltage (OCV) of the cell equipped with the OPBI&P-PhIm-10%/PA PEM was 1.00 V at both 160 °C and 200 °C, illustrating the excellent gas barrier properties that could prevent hydrogen crossover. In contrast, it was only 0.95 V for the OPBI/PA PEM at 160 °C. The better gas barrier properties of the OPBI&P-PhIm-10%/PA membrane were also confirmed using the linear sweep voltammetry (LSV) method. As shown in Fig. 7b, the OPBI&P-PhIm-10%/PA PEM exhibited relatively lower hydrogen crossover current densities than OPBI/PA. For one thing, the lower hydrogen permeability of the OPBI&P-PhIm-10%/PA PEM was due to the smaller polymer chain spacing resulting from intermolecular multiple interactions. For another, it was because of the lower swelling ratio. Moreover, the higher proton conductivity of the OPBI&P-PhIm-10%/PA PEM resulted in a peak power density of 568 mW cm⁻² at 160 °C in a H₂/O₂ fuel cell, which was 12.9% higher than that of the OPBI/PA PEM (503 mW cm⁻²) under the same conditions (Fig. 7a). Particularly, for the polarization curves of the cells with OPBI&P-PhIm-10%/PA and OPBI/PA PEMs, they demonstrated different downward trends in the ohmic polarization region which corresponded to the different ohmic losses. The Nyquist plots obtained using in situ electrochemical impedance spectroscopy (EIS) presented the ohmic resistance (R_ohm) of the two cells clearly (Fig. 7c). For OPBI&P-PhIm-10%/PA, the R_ohm was 1.38 ohm cm², which was 9.2% lower than that for OPBI/PA (1.52 ohm cm²). This result corresponded to the different proton conductivities of the two PEMs. Besides, for the HT-PEMFC with the OPBI&P-PhIm-10%/PA PEM, the conductivity of the PEM and the activity of the catalyst layer increased with temperature, and the performance of the fuel cell improved, with a peak power density of 746 mW cm⁻² at 200 °C without backpressure (Fig. 7d). In situ durability of the H₂/O₂ fuel cell based on the OPBI&P-PhIm-10%/PA PEM was tested at a constant current density of 200 mA cm⁻² at 160 °C (Fig. 7e). Due to the excellent PA retention capacity, mechanical behavior and thermal stability of the blend membrane, the corresponding single cell showed good long-term stability. No significant voltage decay was detected within the first 40 hours. Within 100 hours of test, the voltage loss was 9.8% and the decay rate was 0.647 mV h⁻¹. For OPBI/PA, they were 12.1% and 0.758 mV h⁻¹, respectively. The diminished voltage decay of the cell equipped with OPBI&P-PhIm-10%/PA confirmed that it was effective in inhibiting PA leaching during in situ operation. Moreover, the OCV of the single cell did not decay significantly and remained around 1 V (Fig. S4†) during the long-term stability test. This also indirectly proved the excellent mechanical properties and thermal stability of the OPBI&P-PhIm-10%/PA PEM, which could prevent hydrogen crossover even after 100 hours of operation at 160 °C and several cold starts. It could be concluded that the voltage decay of the single cell mainly came from the leaching of PA in the HT-PEM.

Conclusions

In summary, an efficient strategy of blending polyimidazolium with strong steric hindrance in HT-PEMs was proposed to accelerate proton conduction and improve operating stability, and the feasibility of this strategy was confirmed by both computational and experimental results. For one thing, the steric hindrance and ionic bonds of P-Im [H₂PO₄] contributed to strengthening intermolecular interactions, promoting PA dissociation and inhibiting PA leaching. For another, it helped to improve mechanical strength. Benefiting from these advantages, the performance of blend HT-PEMs could be enhanced comprehensively and effectively. The proton conductivity of the OPBI&P-PhIm-10% membrane was 0.117 S cm⁻¹ at 160 °C, which was significantly higher than that of OPBI (0.097 S cm⁻¹) by a factor of 1.20 under the same conditions, with a 24.1% lower PA uptake. After 100 h of test, the PA retention for the OPBI&P-PhIm-10%/PA membrane was as high as 80.71%. When it was assembled in the H₂/O₂ fuel cell, the peak power density was 568 mW cm⁻² at 160 °C, and reached 746 mW cm⁻² at 200 °C without backpressure. The additional cation–π interactions between imidazolium and phenyl also helped to improve mechanical strength and inhibit PA leaching. This work provides a new approach for the preparation of efficient and durable HT-PEMs by means of introducing additional ionic interactions and regulating the steric effect of functional groups, which has the potential to be generalized to other fields.

Data availability

The data supporting this article have been included as part of the ESI.†

Author contributions

Jifu Zheng and Suobo Zhang designed and supervised the overall project. Xi Sun conducted the experiments with the help of Jifu Zheng and Suobo Zhang. Jifu Zheng and Suobo Zhang revised the manuscript together. All of the authors discussed the results and commented on the manuscript.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We gratefully acknowledge the financial support from the National Natural Science Foundation of China (no. 22075276, 52273219, and U22B6012).

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Footnote

† Electronic supplementary information (ESI) available: Experimental methods and characterization details. See DOI: https://doi.org/10.1039/d4ta03948c

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