High-performance proton exchange membrane employing water-insoluble hybrid formed by chemically bonding phosphotungstic acid with polydopamine

: Heteropolyacids can retain water in a proton exchange membrane to increase proton conductivity at high temperatures and low humidity; however, their high solubility in water leads to leaching, which limits their further application. Herein, we used phosphotungstic acid (HPW) and polydopamine (PDA) particles to prepare a water-insoluble PDA/HPW hybrid (PDW) via hydrothermal reaction. The amino groups of PDA in PDW chemically bonded to HPW and acted as an anchor for HPW. The proton conductivity of the sulfonated poly(ether ether ketone) (SPEEK) composite membrane containing 15wt% PDW (SPEEK/PDW-15) in liquid water was 0.052 S⸱cm –1 at 25 ℃, which was 63% higher than that of the SPEEK control membrane (0.032 S⸱cm –1 ). The SPEEK/PDW-15 composite membrane also showed stable proton conductivity during 80 days of testing while immersed in water.


Introduction
Global climate change has become a major environmental threat and challenge for all kinds of creatures on the earth, including humans [1][2][3].Various strategies have been employed to reduce fossil fuel over-consumption and environmental pollution, which are responsible for global climate change [4][5][6].The utilization of clean energy is one of the most important strategies.Hydrogen energy is a globally recognized ideal solution for clean energy, and the fuel cell is an electrochemical device that directly converts hydrogen energy into electrical energy [7].Proton-exchange-membrane fuel cells (PEMFCs) have become the focus of research globally due to their advantages of high theoretical efficiency, high power density, simple operation, and pollution-free emission [8][9][10][11][12].Theoretically, PEMFCs present a higher energy conversion efficiency when operating at higher temperatures.However, as the core component of PEMFCs to conduct protons, the proton exchange membrane (PEM) tends to dehydrate at high temperatures and low humidity, which severely reduces its proton conductivity.This phenomenon hinders the further development and commercialization of PEMFCs [13].
Heteropolyacids (HPAs), such as phosphotungstic acid (H3PW12O40, HPW), are widely used to enhance the proton conductivity of PEMs because of their excellent thermal stability and strong acidity, as well as good water retention properties at low humidity [14][15][16][17].However, HPAs in composite membranes are highly water-soluble and can be easily dissolved.It has been reported that the mass loss of HPW reached CITATION Lu Z, Yuan X, Jia X, et al.Highperformance proton exchange membrane employing water-insoluble hybrid formed by chemically bonding phosphotungstic acid with polydopamine.Clean Energy Science and Technology.2024; 2(2): 138.https://doi.org/10.18686/cest.v2i2.13 8 93.5 wt% after immersing a 30wt% HPW-doped sulfonated poly(ether ether ketone) (SPEEK) composite membrane in water for 30 days at 80 ℃ [18].Therefore, it is of great importance to immobilize HPAs in composite membranes to achieve high performance.
Dopamine, the main component of adhesion proteins in mussels, can be oxidized and self-polymerized to form polydopamine (PDA) in weakly alkaline aqueous solutions.PDA can be easily and stably deposited on almost all types of substrates [19][20][21][22].In addition, the chemical structure of PDA contains many functional groups, such as catechols and amines [23][24][25].These two characteristics endow PDA the ability to modify a substrate to immobilize HPW in PEMs [17,[26][27][28][29].For example, He et al. [30] used PDA-coated halloysite nanotubes (DHNTs) as binding sites to immobilize HPW.The proton conductivity of the nanocomposite membrane loaded with 15wt% DHNTs and 42.9wt% HPW (0.117 S⸱cm -1 ) increased by 114% compared with that of the SPEEK control membrane.Owing to the acid-base interactions between amino groups in PDA and HPW, only a slight decrease in proton conductivity was found under the long-term water immersion test.Wei et al. [28] incorporated PDA-coated polyimide (PI) into a SPEEK membrane to anchor HPW via acid-base pairs, and the membrane proton conductivity reached a maximum of 0.212 S⸱cm -1 at 60 ℃ and 100% relative humidity (RH).After a six-week test, the loss of proton conductivity was reduced by 51% compared with that of the composite membrane without PDA-coated PI.
Nevertheless, it is difficult to completely solve the leaching problem of soluble proton carriers through electrostatic interactions, and so chemical bonding to anchor HPW has been considered a better option.Zhai et al. [31] synthesized nanohybrids using the imine groups of covalent organic frameworks to react with HPW via a hydrothermal method to immobilize HPW, and they found that no HPW dissolved after immersing nanohybrids in deionized water for three months.Zhang et al. [32] encapsulated HPW in MIL-101(Fe) using a hydrothermal method, which was then doped into a sulfonated poly(arylene ether ketone sulfone) (SPAEKS) matrix.The proton conductivity of the composite membrane reached up to 0.072 S⸱cm -1 (1.8 times that of pure SPAEKS) at 80 ℃ and 100% RH and remained almost unchanged at 30 ℃ for 30 days.
Inspired by the two different strategies mentioned above, we believe that HPW is capable of bonding to amino groups under certain conditions to form a stable structure.Therefore, in this work, we utilized a simple one-step hydrothermal method to synthesize water-insoluble PDA/HPW (PDW) hybrid, where HPW and PDA were bonded together by chemical bonding.PDW can act as the proton conductor in the composite membrane, shortening the distance of proton hopping and effectively increasing the proton conductivity of the composite membrane.The water-insoluble nature of PDW also can ensure the stable proton conductivity of the composite membrane over the course of the 80-day test.In addition, the excellent water retention ability of HPW can endow the composite membrane with good proton conductivity even at low RH.Hence, the structure, morphology, and physicochemical properties of the composite membrane with PDW hybrid were investigated.the range of 600-2000 cm -1 .The powders were observed using a transmission electron microscope (TEM) (JEM-2100Plus, JEOL) at an accelerating voltage of 200 kV.Scanning electron microscopy (SEM) (Quattro S, Thermo Scientific) was used to observe the morphology of the samples at an accelerating voltage of 15 kV, and energy dispersive X-ray spectroscopy (EDS) was used to probe the distribution of the elements in the PDW.X-ray photoelectron spectroscopy (XPS) (K-Alpha, Thermo Scientific) was performed on the powders using an Al-Kα source at a working pressure of about 3 × 10 -7 mba.Thermogravimetric analysis (TGA) (TGA 550, TA) was carried out under a nitrogen atmosphere from 30 ℃ to 800 ℃ with a heating rate of 20 ℃ min -1 .
The proton exchange membranes were immersed in DI water at room temperature to ensure complete hydration.
The water uptake and swelling ratio of the membranes were determined according to Equation (1) and Equation (2), separately: where Mw is the weight of the wet membrane after the rapid removal of surface moisture, and Md is the weight of the dried membrane, while Sw and Sd are the areas of the wet and dried membranes, respectively.The ion exchange capacity (IEC) of the membranes was determined via the acidbase titration method.Typically, a dried membrane sample was immersed in 20 mL NaCl solution (2.00 mol⸱L -1 ) for 24 h to release H + ions and then titrated with a standard NaOH solution (0.01 mol⸱L -1 ) using phenolphthalein as an indicator.The formula for IEC is as follows: where VNaOH is the volume of the NaOH solution used for titration.The proton conductivity (σ) of the membranes was tested via an electrochemical workstation (ZENNIUM PRO, Zahner) using the AC impedance technique.A membrane sample was first placed in a four-electrode conductivity clamp (BT110, BekkTech LLC) and then put in liquid DI water or a humidity chamber (YSGDS-50, YISHUO, Shanghai) to reach the desired humidity.Proton conductivity is determined using the following equation: where L is the distance between the two inner electrodes in the conductivity clamp (0.50 cm), S is the cross-sectional area of the membrane sample, and R is membrane resistance.

Structure and morphology of PDW
To investigate the crystal structure of the hybrid particles, the XRD analysis of HPW, PDA, and PDW was carried out.As shown in Figure 1(a), there was a broad diffraction peak at 2θ = 24° in the XRD pattern of PDA, which was consistent with those in previous works, attributed to the fact that PDA samples are typically amorphous polymers [33,34].For HPW, a typical Keggin structure can be observed [35].After the hydrothermal reaction of HPW with PDA, the XRD curves changed significantly.This phenomenon was due to the strong interaction between HPW and PDA during the hydrothermal process, resulting in a change in the crystal structure of PDW compared with that of HPW.Notably, the physical mixing of PDA and HPW at a mass ratio of 1:5, noted as PDA/HPW-mixed in Figure 1(a), was analyzed and compared.The XRD pattern of PDA/HPW-mixed was the same as that of HPW.The physical cladding of PDA did not change the crystal structure of HPW.This phenomenon further confirmed that the PDW was not a mechanical mixture of HPW and PDA.
The structures of HPW, PDA, and PDW were further investigated using FTIR analysis.As shown in Figure 1(b), four characteristic peaks of HPW can be observed.The peak at 1076 cm -1 corresponded to the stretching vibrations of P-Oa bonds in the PO4 unit.Moreover, the peaks at 956 cm -1 , 880 cm -1 , and 786 cm -1 were associated with W=Od, W-Ob-W, and W-Oc-W bonds of Keggin units, respectively [36,37].For PDA, the peaks at 1591 cm -1 , 1508 cm -1 , and 1280 cm -1 belonged to C=C stretching vibrations, N-H shear vibrations, and C-N shear vibrations, respectively [38,39].As for PDW, the characteristic peaks of PDA at 1200-1600 cm -1 still existed and the characteristic peaks of the Keggin structure from HPW can also be seen clearly.However, the peaks of the Keggin structure in PDW showed a slight offset compared with those of HPW.This suggested that there was an interaction between HPW and PDA during the hydrothermal process, which was also confirmed by the XRD characterization.energy due to the interaction between HPW and PDA.The significant enhancement of the peak at 402.1 eV for PDW can be attributed to the formation of -NH 3+ [40].In Figure 2(c), the O1s peak corresponding to C-O in PDA shifted from 532.1 eV to 531.3 eV in PDW.This implied that C-O-W covalent bonds may have been formed after the hydrothermal reaction [41].In Figure 2(d), the W4f characteristic peaks of HPW at 37.9 eV and 35.8 eV shifted to 38.5 eV and 36.4 eV in PDW, respectively.This suggested a decrease in the electron density of HPW in PDW and the presence of an electron transfer between HPW and PDA [31].Consequently, the creation of the chemical bonding between PDA and HPW in PDW was further confirmed by the XPS results.The thermal stability of PDA, HPW, and PDW was investigated using thermogravimetric analysis (TGA).As shown in Figure 3, there were three main stages of the thermal decomposition of PDA.The weight loss below 120 ℃ was mainly due to the evaporation of water in PDA.The second stage of weight loss was related to the decomposition of the catechol fraction.The weight loss above 350 ℃ was mainly attributed to the degradation of the PDA backbone, which is in agreement with that in a previous work [42].For HPW, the two weight loss stages below 200 ℃ were mainly due to the evaporation of physically adsorbed water and water of crystallization.Apart from that, there was almost no further weight loss for HPW at 200 ℃-800 ℃.As for PDW, it had better thermal stability below 450 ℃ compared with those of PDA and HPW.By having approximately 86% of the mass of PDW remaining at 800 ℃, the weight ratio of HPW to PDA in PDW can be calculated to be ~5:1.

Structures and properties of composite membranes
The distribution of PDW in the SPEEK matrix and the microstructures of the membranes were observed via SEM. Figure 5 shows the cross-sectional images of SPEEK and SPEEK/PDW-x membranes.The SPEEK control membrane exhibited a dense morphology.PDW was well dispersed in the composite membranes when the content of PDW was less than 15 wt%.In addition, the fracture surfaces of the SPEEK/PDW composite membranes were all very homogeneous with no voids or defects, as observed in Figure 5(b-d), which suggested that the PDW fillers had good compatibility with the SPEEK matrix.This phenomenon may be mainly due to the strong electrostatic interactions between the hydrophilic groups in PDW and SPEEK.Nevertheless, the content of PDW reached 20 wt%, defects, such as the aggregation of PDW hybrid and holes, can be observed.As shown in Figure 6(a), the proton conductivity of the SPEEK/PDW composite membrane increased significantly with the increase in filler loading.The SPEEK/PDW-15 membrane exhibited proton conductivity of 0.052 S⸱cm -1 , which was 63% higher than that of the SPEEK control membrane (0.032 S⸱cm -1 ).For comparison, the proton conductivity of the SPEEK/PDA composite membrane filled with 3wt% PDA was tested, which showed a slight decrease to 0.029 S⸱cm -1 (blue dashed line in Figure 6(a)) compared with that of the SPEEK control membrane.This indicated that PDA itself was unfavorable for proton transport in composite membranes.Therefore, the significant enhancement of proton conductivity in the SPEEK/PDW composite membranes should be due to the super-strong acidity of HPW.However, when the content of PDW increased to 20 wt%, the proton conductivity of the composite membrane decreased, mainly due to filler aggregation and defects.The comparison of the proton conductivity and promotion ratio (relative to the SPEEK control membrane) between the SPEEK/PDW-15 membrane and other membranes developed in previous works is shown in Table 1.IL@MIL-125-NH2/SPEEK-5 25 0.016 60 [45] SPEEK/ATP-IL-5% 25 0.050 39 [46] SPEEK/MoS2@CNTs-1 20 0.042 55 [47] SPS-3 30 0.038 36 [48] 1.3%NU6@PPNF-SPEEK 60 0.132 25 [49] Figure 6(b) demonstrates the water uptake and area swelling of the SPEEK control and SPEEK/PDW composite membranes in liquid water as a function of filler content.The water uptake of the composite membrane showed an increasing and then decreasing trend with the increase in PDW loading.The increase in water uptake was attributed to the stronger water absorption ability of PDW with hydrophilic groups, such as phosphotungstate and -NH2.The water uptake of the composite membrane decreased when the filler content exceeded 15 wt%.This may be related to the aggregation of PDW, as shown in the SEM images, which resulted in interfacial defects between the SPEEK matrix and the PDW fillers.In contrast, there was very little change in the area swelling of the composite membranes compared with that of the SPEEK control membrane.This phenomenon is mainly due to the strong acid-base pair interaction between PDW and SPEEK controlling the degree of swelling of the membranes upon water uptake.The relatively low swelling of the membranes can improve the stability and durability of membrane electrode assemblies, which is favorable for their application in PEMFCs.
As shown in Figure 6(c), the IEC of the SPEEK/PDW composite membrane decreases almost linearly with the increase in PDW loading.This may be due to the reaction between HPW and PDA, which led to a decrease in the IEC of PDW from HPW (1.04 mmol⸱g -1 ).Therefore, the IEC of PDW was lower than that of the SPEEK control membrane (1.66 mmol⸱g -1 ).Notably, a linear extrapolation of the IEC curve to a filler content of 100 wt% (red dashed line in Figure 6(c)) yielded an IEC of 0.9 mmol⸱g -1 , which was consistent with our reasoning.
Figure 6(d) shows the proton conductivity stability of the SPEEK/PDW-15 composite membrane immersed in liquid water at room temperature (25 ℃).The proton conductivity stability of the SPEEK composite membrane with 15wt% unmodified HPW added (SPEEK/HPW-15) was also tested for comparison.The proton conductivity of the SPEEK/HPW-15 membrane decreased significantly within a short period due to the fact that the HPW in the SPEEK/HPW-15 membrane dissolved in water, resulting in the loss of HPW.In contrast, the proton conductivity of the SPEEK/PDW-15 membrane remained essentially unchanged over nearly three months, indicating that most of the PDW particles remain in the composite membrane.This phenomenon can be attributed to the amino groups on PDA bonding with HPW to form a stabilizing structure, which served to immobilize HPW.
To further investigate the proton transport mechanism in SPEEK/PDW composite membranes, the proton conductivity of the SPEEK control and SPEEK/PDW-15 composite membranes in liquid water at different temperatures was measured, and the corresponding Arrhenius plots of proton conductivity are shown in Figure 7(a).Moreover, the activation energy (Ea) of proton transport in the membranes was calculated based on the Arrhenius plot [50].The Ea values of the SPEEK control and SPEEK/PDW-15 membranes were 18.3 kJ⸱mol -1 and 15.8 kJ⸱mol - 1 , respectively, dropping in the range of 14-40 kJ⸱mol -1 [51].This result indicated that the proton transport in the membranes was mainly dominated by the Grotthuss mechanism, where protons were transported by hydrogen bonding through jumps between proton conductors [51][52][53].The Ea of the SPEEK/PDW-15 membrane was lower than that of the SPEEK control membrane.This suggested that doping PDW in the composite membranes reduced the potential barrier for proton transport and effectively shortened the proton hopping distance (Figure 7(b)).
The water uptake and proton conductivity of the SPEEK control and SPEEK/PDW-15 composite membranes at low RH are shown in Figure 8(a) and Figure 8(b), respectively.The water uptake of the SPEEK/PDW-15 composite membrane was always higher than that of the SPEEK control membrane at the same RH (Figure 8(a)).This indicated that the water absorption capacity of PDW was stronger than that of SPEEK at low RH, which was mainly due to the excellent water retention capacity of the HPW component in PDW.The proton exchange membrane was more favorable for proton transport at low RH when it had a stronger water retention capacity and more water.According to Figure 8(b), the proton conductivity of the SPEEK/PDW-15 composite membrane decreased much slower than that of the SPEEK control membrane as RH decreased, especially when the RH was lower than 60%.The proton conductivity of the SPEEK/PDW-15 membrane was about an order of magnitude higher than that of SPEEK when the RH was below 45%.In addition to the higher water uptake of the composite membrane, the Keggin structure and the stronger acidity of HPW in PDW also played positive roles in proton transport.Figure 9 illustrates the stress-strain curves of the SPEEK control and SPEEK/PDW composite membranes.The tensile strength and elongation at the break of the SPEEK control membrane were 49.9 MPa and 101%, respectively.For the SPEEK/PDW composite membranes, both their tensile strength and elongation at the break were higher than those of the SPEEK control membrane when the loading of PDW was below 15 wt%.The tensile strength of the SPEEK/PDW-5 membrane was the highest (59.6 MPa), which was considerably higher than that of the commercial Nafion 212 membrane (16.1 MPa) [54].The elongation at the break of the SPEEK/PDW-15 membrane was the largest at 112%.The enhancement of the mechanical strength and toughness of the composite membranes can be attributed to the interactions generated between PDW and the SPEEK matrix, including acid-base interactions and hydrogen bonding [55,56].When the filler content reached 20 wt%, the tensile strength of the composite membrane decreased due to the severe aggregation of the PDW hybrid.

Conclusion
This work described the preparation of a water-insoluble PDW hybrid by the hydrothermal treatment of HPW and PDA.The XRD, FTIR, and XPS characterization results showed that HPW and PDA in PDW chemically bonded upon hydrothermal treatment, which resulted in the water-insolubility of PDW.The SEM analyses demonstrated that the SPEEK/PDW composite membranes exhibited excellent compatibility due to the strong interaction between sulfonic acid groups in the SPEEK matrix and PDW.This interaction also enabled the SPEEK/PDW composite membranes to maintain essentially unchanged area swelling in the presence of increased water uptake.The SPEEK/PDW-15 membrane achieved the highest proton conductivity of 0.052 S⸱cm -1 , which was 63% higher than that of the SPEEK control membrane.The proton conductivity of the SPEEK/PDW-15 membrane was about an order of magnitude higher than that of SPEEK when RH was lower than 45%.Moreover, the proton conductivity of the SPEEK/PDW-15 composite membrane remained almost unchanged during the 80-day water immersion test.Thus, the present work provides a facile and promising method to prepare water-insoluble solid proton conductors for high-performance composite proton exchange membranes, which could be potentially applied to the fabrication of PEMFCs operating at high temperatures and low RH.

Figure 4 .
Figure 4. (a) SEM image of PDA.(b) SEM image of PDW.(c) EDS maps of PDW.(d) TEM image of PDA.(e) TEM image of PDW in low resolution.(f) TEM image of PDW in high resolution.

Figure 6 .
Figure 6.(a) Proton conductivity, (b) water uptake and area swelling, and (c) IEC of SPEEK control and SPEEK/PDW composite membranes at 25 ℃.(d) Proton conductivity of SPEEK/PDW-15 composite membrane at various times during water immersion test at 25 ℃.

Figure 7 .
Figure 7. (a) Arrhenius plots of proton conductivity as function of temperature for SPEEK control and SPEEK/PDW-15 composite membranes.(b) Proposed mechanism for proton conduction in SPEEK/PDW composite membrane.
Author contributions: Data curation, ZL, XJ and XY; Conceptualization, JL and SH; methodology, JL and XJ; investigation, ZL, XJ and XY; writing-original draft -5 SPEEK/PDW-10 SPEEK/PDW-15 SPEEK/PDW-20 preparation, ZL; writing-review and editing, SH; visualization, ZL and SH; supervision, JL and SH; project administration, JL and SH; funding acquisition, JL and SH.All authors have read and agreed to the published version of the manuscript.

Table 1 .
Comparison between SPEEK/PDW-15 and similar membranes from other studies.