Abstract
High-performance proton exchange membranes are of great importance for fuel cells. Here, we have synthesized polycarboxylate plasticizer modified MIL-101-Cr-NH, (PCP-MCN), a kind of hybrid metal-organic framework, which exhibits a superior proton conductivity. PCP-MCN nanoparticles are used as additives to fabricate PCP-MCN/Nafion composite membranes. Microstructures and characteristics of PCP-MCN and these membranes have been extensively investigated. Significant enhancement in proton conduction for PCP-MCN around 55 °C is interestingly found due to the thermal motion of the PCP molecular chains. Robust mechanical properties and higher thermal decomposition temperature of the composite membranes are directly ascribed to strong intermolecular interactions between PCP-MCN and Nafion side chains, i.e., the formation of substantial acid-base pairs (-SO3 - - - "H-NH-), which further improves compatibility between additive and Nafion matrix. At the same humidity and temperature condition, the water uptake of composite membranes significantly increases due to the incorporation of porous additives with abundant functional groups and thus less crystallinity degree in comparison to pristine Nafion. Proton conductivity (с) over wide ranges of humidities (30 - 100% RH at 25 °C) and temperatures (30 - 98 °С at 100% RH) for prepared membranes is measured. The o in PCPMCN/Nafion composite membranes is remarkably enhanced, i.e. 0.245 S/cm for PCP-MCN-3wt.%/Nafion is twice that of Nafion membrane at 98 °C and 100% RH, because of the establishment of well-interconnected proton transport ionic water channels and perhaps faster protonation- deprotonation processes. The composite membranes possess weak humidity-dependence of proton transport and higher water uptake due to excellent water retention ability of PCP-MCN. In particular, when 3 wt.% PCP-MCN was added to Nafion, the power density of a single-cell fabricated with this composite membrane reaches impressively 0.480, 1.098 W/cm? under 40% RH, 100% RH at 60 °C, respectively, guaranteeing it to be a promising proton exchange membrane.
Keywords: Nafion composite membrane; Surface-modified MIL-101-Cr-NH,; Proton conductivity; Single-cell performance
(ProQuest: ... denotes formulae omitted.)
1. Introduction
Proton exchange membrane fuel cell (PEMFC) is a chemical device that converts chemical energy possessed by fuel directly into electrical energy and is known as the fourth generation of power source [1,2]. In the foreseeable future, PEMFC is an ideal choice to meet the decarbonization and cleanliness of society. Proton exchange membrane (PEM), as one pivotal component, plays an important role in separating hydrogen and oxygen, transporting protons from anode to cathode, and insulating + > electrons [3,4]. Dupont's a Nafion, a as a
commercial PEM, is highly attractive due to its decent proton conductivity, chemical stability, and low gas permeability [5- 7]. However, the practical application of Nafion in low-humidity or high-temperature environments has encountered challenges because its proton conductivity is heavily dependent on water content and the integrity of the proton transport channels in the membrane. The power of PEMFC dramatically drops in reduced humidity (RH)/high temperature (T > 100 °C) conditions, which is attributed to the collapse of the hydrogen bonding networks, shrinking and isolation of water ionic clusters because of water loss inside the Nafion membrane at such conditions [4,8]. Besides, when PEMFC is working, if the environmental T and RH repeatedly change, PEM will cyclically shrink and swell. This will reduce the mechanical properties and durability of the PEMFC [9]. Thus, it is necessary to maintain a well-interconnected hydrogen bonding network in membranes under low humidity conditions and to enhance the mechanical stability and durability of them.
Hence, researchers [10-12] are striving to improve the conductivity of PEMs under tough conditions (particularly under operating low RH conditions), reducing the dependence of the membrane in the external environment. People also need to maintain the high stability and durability of the membrane, adapting to long-term operation [13]. It is mainstream practice to introduce some hydrophilic and hygroscopic inorganic matters (sulfonated graphene [14], silica [6,10], titanium dioxide [15], ceria [13,16]) and porous materials (metal-organic frameworks, MOFs [17-19], and covalentorganic frameworks, COFs [20,21]) into Nafion matrix, which can improve the integrity of the hydrogen bonding networks within the membrane at a low RH or high T [22], resulting in increased water adsorption and proton transport sites in the hydrophobic region. For instance, Hasmukh A. et al. [18] reported that Nafion-SZM membranes possessed higher proton conduction boosted by 23% in comparison to Nafion membrane because Brgnsted acidic sites and micropores in the SZM frameworks could retain ample amounts of water molecules at a low RH. Hou et al. [23] synthesized Zn-MOF-NHz/ Nafion-5 membrane, whose proton conductivity is 5.47 times of that of pristine Nafion membrane at 80 °C and 58% RH. This result is attributed to the creation of additional hydrogen bonds (H,O-H·-NH;), which provides extra paths for proton transport. Mohanraj's [13] and Li's [24] groups mentioned that basic groups (such as-NH>, -NH-, and NH3) are more suitable for conducting protons at reduced RH, because of their self-ionization property. Proton donor groups (HSO4H) of the Nafion chain and proton acceptor groups (-NH>) on the MOFs can form acid-base pairs [25], which can accelerate proton dissociation and transport [24,26]. Wu's group [27] used different functional MOFs (UIO-66-SO;H and UIO-66-NH,) to synergistically enhance the proton conductivity of the corresponding Nafion composite membrane to 0.256 S/cm at 90 °C and 95% RH, which is 1.17 times higher than that of pristine Nafion. Moderate loading with functionalized MOFs may facilitate proton conductivity for Nafion composite membrane, under low RH or high T environments. Herein, we prepared MIL-101-Cr-NH, (a kind of MOF, abbreviated as MCN) through hydrothermal process of metal sources ch with organic linkers (2-aminoterephthalic acid), which has the following advantages: i. many micro/ mesopores and high specific surface area (~ 1500 m"/g); ii. plentiful Lewis acids (metal unsaturated sites) generating -OH and protons through hydrolysis [28]; ii. likely formation of acid-base pairs by -SO;H and-NH,. Nevertheless, when rigid MOF particles are directly introduced to the Nafion matrix, the dispersion and incompatibility problems, resulting in a reduction of the mechanical property and proton conductivity for composite membranes, may be occurred according to the rule of "like dissolves like" [18,29,30] and the previous reports [31,32]. In order to maintain a good consistency between the additive and the Nafion matrix, some researchers tried to introduce organic fillers (such as carbon nanotubes [33], ionic liquid [34], and organic polymer [35,36]) into Nafion matrices, which enhance the mechanical integrity of Nafion by chemical cross-linking and robust guest-host interactions. Guo et al. [37] fabricated Nafion/APNB membrane by introducing macromolecules, exhibiting better compatibility and tensile strength. Na et al. [35] synthesized PA-PEEK-PBI membrane by using polybenzimidazole as a macromolecular cross-linker, displaying excellent dimensional stability and tensile strength in comparison to a pristine Nafion membrane. Liu et al. [38] used PTFE as a reinforced material to prepare a thinner (10 pm) Nafion/PD@PTFE membrane. Comb-like polycarboxylate plasticizers (PCPs) have some advantages of molecular designability, high thermal stability, and high water adsorption [39,40]. On one hand, PCP, polymer organics, has good compatibility with Nafion matrix and it can be introduced into Nafion matrix as cross-linking sites to enhance the mechanical properties of the composite membrane. On the other hand, PCP (Fig. S1) contains hydrophilic groups such as carboxyl, sulfonate groups, and ether bonds, which provide proton hopping sites. It is likely beneficial for the refined construction of the hydrogen bonding networks of the Nafion membrane, helping facile and rapid transport of proton. Besides, PCP is an anionic surfactant that can tightly anchor cations by electrostatic attraction [41]. By employing the property of PCP, we have successfully combined PCP and porous MCN with plentiful amino groups to prepare PCPMCN composite. Accordingly, the introduction of PCPmodified MCN into Nafion may be an effective solution to improve not only the water retention ability and proton conductivity of the membrane but also the overall membrane compatibility and mechanical integrity [30].
In this work, PCP-modified MIL-101-Cr-NH, (PCP-MCN) nanocomposite was synthesized via strong electrostatic interaction between PCP and Ст? in a hydrothermal process. Different amounts of PCP-MCN were incorporated into Nafion matrix to prepare composite membranes. The microstructures, water uptake and proton conductivity of PCP-MCN influenced by PCP are investigated. It has been found that the conductivity of PCP-MCN steeply increases around 55 °C. Further, influence of the additive of PCP-MCN on the mechanical and thermal properties, and proton conductivities over wide ranges of humidities (30-100% RH at a fixed 25 °C) and temperatures (30-98 °C at a fixed 100% RH) for PCPMCN/Nafion composite membranes are extensively studied. The composite membranes show increased break strength, longer elongation and enhanced proton conductivity in comparison to pristine Nafion. Membrane electrodes (MEAs) fabricated with the modified composite membranes exhibit outstanding power output performance in a wide range of RH at a relatively low temperature of 60 °C.
2. Experimental section
2.1. Synthesis of MCN and PCP-MCN
1.6 в Cr(NO3);-9H,0 (4 mmol, AR), 0.72 в 2-aminoterephthalic acid (4 mmol, >98.0%), and 0.32 g NaOH (8 mmol, AR) were dissolved in 30 mL deionized water (DI) and then stirred for 30 min. Afterward, these mixtures were transferred to the Teflon-lined autoclave at 160 °C for 12 h. MCN was generated after the autoclave had been naturally cooled to room temperature. The product was thoroughly washed three times with N, N-dimethylformamide (DMF), ethanol, and DI, sequentially. The resultant MCN was dried in a vacuum to remove impurities at 150 °C [42]. Meanwhile, PCP-MCN was synthesized by the following process. Firstly, 0.3 g PCP (CQJ-JSSO2, purchased by Shanghai Chenqi Chemical Technology Co.) and 1.6 g Cr(NO3);-9H,0 were mixed and dissolved in 15 mL DI and magnetically stirred for 10 min. During this process, carboxyl groups on PCP main chains may anchor the positively charged EP, Secondly, 0.32 g sodium hydroxide and 0.72 g 2-aminoterephthalic acid were mixed and dissolved in 15 mL DI and stirred for 10 min. Next, the above two mixtures were blended and stirred for 20 min. Finally, the mixtures were transferred to a Teflon reactor. The remaining preparation process is identical to that of MCN.
2.2. Preparation of PCP-MCN/Nafion composite membranes
In brief, the process for preparing composite membranes is similar to the previous method [14,33]. Firstly, Nafion solution (DuPont, DE-520) was dried into resin at 60 °C, and then a certain amount of Nafion resin was fully dissolved in DMF. Secondly, a pre-weighed amount of PCP-MCN was added to the Nafion/DMF solution and magnetically stirred at 60 °C for 24 h. The resultant mixture was transferred into a flat quartz dish and evaporated at 45 °C for 8 h. Then, the resulting membranes were further dried at 140 °C for 2 h in a vacuum oven. The fabricated Nafion composite membranes were named as PCP-MCN-X/Naf, where X = 3, 5 wt.% represents the weight percent of PCP-MCN, respectively. For comparison, the MCN-doped Nafion composite membrane was also prepared by a similar way, and it is abbreviated as MCN-3/ Naf, where 3 wt.% represents the weight percent of MCN. Pristine Nafion membrane was prepared by an identical procedure without any fillers. Finally, all Nafion-based membranes were activated through a standard procedure [6,14]. The thickness of different membranes was about 35 um.
2.3. Characterization
Fourier transform infrared spectra (Nicolet 170 SXIR, FTIR) for the additives and membranes were recorded in the range of 500-4000 cm ' at room temperature. Surface morphologies of the MCN and PCP-MCN were investigated by field emission transmission electron microscope (JEM-F200, TEM). Surface and cross-section of the membranes were observed after being sputtered with Pt by scanning electron microscope (Hitachi, S-4300SE, SEM). X-ray diffractometer (Rigaku, XRD) was employed to study crystal structure of the additives and crystallinity degree of these membranes. Nanophase separation structure about surface morphology for hydrated membranes was detected through an atomic force microscope (Burker mode 8, AFM). To evaluate the thermal stability of various samples, thermogravimetric data of synthesized samples were collected by a thermogravimetry instrument (Netzsch STA449C, TG) at 10 °C/min under N,. Differential scanning calorimetry (DSC) was used to detect endothermic peaks of materials at 2 °C/min under №. A static specific surface area analyzer (JW-BK122W, BET) was used to determine the specific surface area and the pore size distribution of PCP, MCN, and PCP-MCN at 77 K. Positron annihilation lifetime spectroscopy (PALS) was implemented for the characterization of pores for the MCN, and PCP-MCN. Before the test, all samples were pressed into two plates (pressure 6 MPa). During testing, a kapton-encapsulated positron source (··Na) was sandwiched between the two pellets at 23 °C and 50% RH. The LT program [43] was used to resolve the obtained positron annihilation lifetime spectra. More details about PALS measurement can be found in our previous papers [44,45].
2.4. Measurements
2.4.1. Water uptake, area swell rate, and break strength
Water uptake (WU) and area swell rate (SW) are measured by the following process. Firstly, a quantity of sample is placed in a drying oven at 60 °C for 12 h to dry it. After that, they were placed at 0% RH for a long time in order to obtain the mass (M4) and area (A4) of the dry sample. Secondly, the samples were placed at specific temperature and different humidity levels, and the mass (M;) and the swelling (A;) of the samples were measured in such conditions. The WU and SW of the samples were calculated based on the following equations,
... (1)
... (2)
Break strength test is carried out to assess the mechanical integrity of the membrane. Prior to this experiment, the membranes were cut into long strips, and their thickness (7) and width (W) were obtained by micrometers. The break strength of the membrane was calculated by the following equation,
... (3)
where F is the maximum force when the membrane breaks, and P (MPa) is the break strength of the membrane.
2.4.2. Proton conductivity and H2/02 single fuel cell
In-plane proton conductivity (с) of the membrane was measured using a two-point probe system through electrochemical impedance spectra (EIS) measurement by an electrochemical workstation (CS35011, Corrtest, China) under different controlled-T&RH levels monitored via accurate hygrometer (CENTER 310) and thermometer (AS877). Prior to the measurement, all samples were kept at different RH levels for at least 10 h for saturation of water uptake. When measured at different Ts, they were kept for 0.5 h before testing. EIS of the MCN, PCP, PCP-MCN pellets (the thickness is about 1.5 mm), and various membranes were obtained in different RH (20-100% RH) and T (25-100 °C) environments. These resistance measurements were repeated several times until no further resistance variation at each selected condition. The proton conductivity values are calculated by the following equation,
... (4)
where с (S/cm) is the proton conductivity, and L (cm), 5 (em?), and R (©) are the length along samples between the two electrodes, the cross-sectional areas and resistances of the pellets/membranes, respectively [23]. Next, apparent activation energies (E,, kJ/mol) of proton transport for the membranes and pellets were calculated by linear fitting the experimental data of с vs. temperature according to the Arrhenius equation as follows,
... (5)
where с is the proton conductivity at a given T, and со, R (J/ mol K) and T (K) are the pre-exponential factor, universal gas constant and temperature, respectively. Furthermore, the pristine Nafion membrane and the modified Nafion membranes were fabricated to MEAs to evaluate their output performance at different RH and T conditions. The prepared MEAs have a platinum loading (Johnson Matthey) of 0.5 mg/ ст? on both sides of the cathode and the anode. They were packed in a assembled single-cell [46], and 80 SCCM of humidified H, is fed to the anode and 150 SCCM of humidified O, is fed to the cathode [14,47]. Polarization curves of MEAs are measured by a high-precision electronic load instrument (PLZ70UA, KIKUSUI). More details about proton conductivity and H,/O, single-cell measurements can be found in our previous papers [14,46].
3. Results and discussion
3.1. Structures and morphologies of MCN and PCP-MCN
Morphologies of synthetic MCN and PCP-MCN are shown in Fig. la-d. The SEM (Fig. la) and TEM (Fig. 1b) images of MCN show the formation of aggregated nanoparticles and well-defined MCN particles of ~50 nm size. The SEM (Fig. lc) of PCP-MCN also exhibits stacked nanoparticles, while its TEM (Fig. 1d) demonstrates the vague edges of PCPMCN particles compared to that of MCN particles, revealing a slight change in the morphology due to the introduction of PCP. Meanwhile, clear lattice fringes (Fig. S2) of PCP-MCN hint a good crystallinity. Their crystal structures are investigated using XRD (Fig. le). The XRD patterns of synthesized MCN is basically consistent with the simulated ones (Fig. le) of standard MIL-101-Cr as well some previous works [42,48], suggesting it has high crystallinity and some cage-like pore structures. The XRD of PCP-modified MCN exhibits the same diffraction peaks as that of MCN, and the XRD diffraction peaks of PCP aren't found, implying that the introduction of PCP did not hinder the generation of MCN crystal structure. The FTIR spectrum of PCP in Fig. If shows some of the functional groups contained in PCP: the absorption peaks at 1560, 1457, and 1410 ст"! corresponding to the carboxyl group on the main chain, the peak at 2887 em"! describing the hydrophobic alkyl group on the graft chain, the peak at 1105 cm" representing the hydrophilic ether bond on the side chain, and the peak at 1150 cm ' attributing to the sulfonate [39,49-51]. Fig. 1f shows the FTIR spectra of MCN and PCPMCN. It can be observed that FTIR spectra of MCN and PCPMCN show a wide vibration peak of the H-O bonds for H,0 molecules and the N-H for hydrophilic amino groups (between 3200 and 3700 cm), indicative of plentiful water molecules absorption in them due to capillary condensation. It's rational that plenty of water can be adsorbed in MOFs with hydrophilic functional groups on the frameworks even in low humidity. In the PCP-MCN's FTIR spectrum, the absorption peaks at 1258, 1341 ст"! correspond to С-М stretching vibration of aromatic amines, the peak at 1658 em"! is attributed to C-N bending vibrations of aromatic amines, and the peaks at 3353, 3436 cm ' are described to symmetric and asymmetric stretches of the amine moieties, respectively [42,48,52]. Meanwhile, PCP-MCN's spectrum is an integration of that of PCP and MCN, further indicative of the successful synthesis of composite materials.
Next, pore structures of MCN and PCP-MCN are investigated by BET and PALS. From the nitrogen adsorption and desorption curves (Fig. 1g), it can be seen that MCN contains a large number of micropores and a small number of mesopores, and it has a large specific surface area (BET specific surface area: 1582 m"/g). PCP-MCN has similar adsorption and desorption curves compared to that of MCN, but its specific surface area is smaller (BET specific surface area: 790 m°/g), which is because PCP has no pore structure and does not contribute to specific surface area value (Fig. S3). The result of the pore size distribution (Fig. 1h) demonstrates the presence of -0.7 nm micropores (chromium octahedra with oxygen atoms form trimers, and then trimers with terephthalic acid esters form ortho-tetrahedral micropores), -2 nm mesopores (the ortho-tetrahedral structure and terephthalate are linked by organic ligands to form mesopores) and ~3.5 nm stacked pores in both MCN and PCP-MCN [53], representing that the modified-MCN's pore structure remains unchanged (consistent with the XRD results as mentioned above). The pore characteristics of the samples are studied using PALS further. The raw positron annihilation lifetime spectra and the resulting o-Ps lifetimes of 73, 74 and their corresponding intensities (7; and 7,) decomposed by LT program [54] for MCN, PCP and PCP-MCN are shown in Fig. li and listed in Table S1, respectively. According to the extended Tao-Eldrup model [55], the average pore sizes corresponding to 73 and 74 for MCN are 0.9 nm and 1.98 nm, and the o-Ps intensities are in the order 7; < I,, indicating that the interconnectivity between micropores and mesopores is well because o-Ps easily diffuses from small pores to large pores [56]. After being modified MCN by PCP, 7; is dramatically declined to 2.21 ns on careful observation, which is similar to the lifetime (2.29 ns) of the positron in PCP, indicating that the presence of the PCP phase in PCP-MCN. 7, of PCP-MCN is almost unchanged, indicating that the mesopores of MCN are remained. Considering o-Ps lifetime in the small micropores of PCP-MCN is identical to that of PCP, from the above results it's reasonable to state that the frameworks of MCN are intact, and the grain surfaces are surrounded by PCP. In addition, the lifetime intensities (73, 14) of the positrons for PCP-MCN are reduced, indicative of the presence of electron scavengers. Considering the ability of PCP tightly anchoring the cations (Cr··) in solution as mentioned above, as schematically shown in Fig. S4, some Cr·· cations are widely dispersed in the PCP moiety of PCP-MCN presenting as the electron scavengers, which inhibit the formation of positronium. The incorporation of these porous MOF materials into Nafion might effectively improve the hydrophilic and retention water capacity of the composite membrane, which is conducive to the construction and refinement of hydrogen bonding networks and the achievement of high proton conductivity.
3.2. Water uptake, thermal stability and proton conductivity of MCN and PCP-MCN
Water uptake and proton conductivity of MCN and PCPMCN were measured at different RH&T conditions. As depicted in Fig. 2a, the WU of three samples is increased with the increasing RH at room T. The WU of MCN dramatically rises from 6.43 wt.% to 76.06 wt.% in the medium RH region of 35-75% RH, which is explained by the high specific surface area, the abundant nanopores and suspended amino groups in MOFs, and eventually reaches 100.4 wt.% at saturated RH, exhibiting an impressive water uptake. The water absorption of PCP increases from 2.06 wt.% to 30.1 wt.% in the 35-85% RH ranges, which is ascribed to the hydrophilic functional groups (carboxyl, sulfonic, and ether bonds), and eventually reaches 99.9 wt.% at a saturation RH, which is due to the PCP being in a fully swollen state. Meanwhile, the WU of MCN from 8.06 wt.% to 73.14 wt.% in the ranges of 35-100% RH, which also demonstrates that PCP-MCN possesses extremely high water adsorption capacity. Thermogravimetric (TG) tests were performed to determine the thermal stability of samples. TG results (Fig. 2b) manifest that the decomposition temperature of PCP, MCN, and PCP-MCN is about 350 °C, 200 °C, and 210 °C, respectively. The mass loss below the decomposition temperature (below 200 °C) of the samples is attributed to the adsorbed free water in MOFs pores and bonded water combined with coordinatively unsaturated metal sites (UMS) in MOFs [20,28]. From Fig. 2b, it can also be concluded that the water absorption capacity is in the following order: MCN > PCP-MCN > PCP, which is consistent with the results of the WU measurements above.
As far as the proton transport in MCN and PCP-MCN, water molecules play an important role in either Grotthuss [57] or Vehicle [58] mechanisms. Given their superior water uptake, various hydrophilic groups, and plentiful UMS, the successive hydrogen bonding networks may be well-established in ordered porous structures of MOFs. EIS of MCN and PCP-MCN were implemented to obtain their proton conductivity (0) under different environmental conditions. Fig. 2c and d show the variation of the proton conductivity for each sample at different RH and different T conditions, respectively. Obviously, the o of MCN and PCP-MCN progressively increases with the increasing RH and T. MCN's с increases from 1.36 x 107° to 2.26 x 107? mS/cm in the 25-90% RH ranges. This dramatic growth may be attributed to the large amount of H,O adsorbed inside the nanopores of MOFs and the formation of many continuous hydrogen-bonding networks. The suspended amino group possessing the lone pair electrons of the nitrogen atom readily accepts a proton to form -NH3, and then some feasible hydrogen bonds, likely including -NH,-H·-O-H, NH,-H·-N-H, and HO-H-O-H, are established in MCN. The protons can jump depending on these networks, or hydrated hydrogen ions formed from protons and water molecules diffuse along regular and interconnected frameworks of MCN. When the sample is placed under a reduced RH, the number of H,O is too small to form a coherent hydrogen-bonding network, resulting in the severe disruption of the transfer channels. Meanwhile, the с of PCP was tested, exhibiting a proton conductivity of 8.34 x 10? mS/cm at 90% RH (Fig. S5) owing to its intrinsic hydrophilicity and specific functional groups, such as carboxyl, ether bond and sulfonic group. The o of PCP-MCN reaches 7.36 x 107% mS/cm at 25% RH, which is 50 times higher than that of MCN under identical conditions, and is enhanced to 3.8 x 107? mS/cm upon the increment of humidity to 90% RH, which is increased by ~70% compared to that of MCN. Since PCP possesses relatively higher intrinsic proton conductivity at a certain humidity as mentioned above, the higher proton conductivity of PCPMCN is likely ascribed to the fact that abundant proton transport channels are formed because of high water uptake in the PCP moiety among the MCN particles, resulting in facile diffusion of proton with respect to the absence of the PCP. Furthermore, Fig. 2d displays the temperature-dependent proton conductivity of MCN and PCP-MCN at 100% RH, showing a gradually increasing trend. For MCN, the с is 5.2 x 107% S/em at 30 °C, which increases with the increasing temperature, reaching 1.24 x 107? S/em at 95 °C. The results can be ascribed to the more intense thermal motion or rotation of functional groups transferring the proton and the greater diffusion coefficient of hydrated ions at relatively higher temperatures. The conductivity of PCP-MCN enhances from 6.6 x 107· S/em at 30 °C to 2.61 x 107 at 95 °C, which is competitive with various recently reported proton conducting materials (Table 1) based on MOFs, such as PCMOF2'/, [59] and Im-Fe-MOF [60]. This higher conductivity is achieved as a result of its high water content, the numerous proton carriers, and the synergistic effect of PCP and MCN working together to build proton transfer channels.
Next, we further insight into the mechanism of proton transfer by fitting the experimental values with equation (5) to obtain their apparent proton activation energies (E,). MCN's E, is 0.49 eV (Fig. 2e), ascribing to the Vehicle mechanism, while PCP-MCN's E, is 0.32 eV at 30-52 °C, and 0.24 eV at 55-95 °C, respectively, ascribing to the hopping mechanism according to the Grotthuss theories. Interestingly, a significant reduction in resistance (seeing Nyquist plots of PCP-MCN in Fig. S6) and an increment in conductance are found at 55 °C. To understand the reason for this, DSC and variable-temperature XRD were performed. DSC (Fig. 2f) presents an obvious endothermic peak for PCP near 50 °C, and an endothermic peak for PCP-MCN near 55 °C. Meantime, the variabletemperature XRD results (Fig. 2g) of PCP demonstrate the transformation of the side chain crystalline region of PCP into amorphous regions due to the thermal motions of its molecular chain, as schematically shown in Fig. 2h. The slightly higher melting temperature for PCP-MCN may be a result of the intermolecular interaction between PCP and MCN. Therefore, the significant enhancement in conductivity for PCP-MCN at ~55 °C is attributed to the higher proton conductivity in PCP with much stronger molecular motions in a rubbery state, as displayed in Fig. 2i. This figure graphically illustrates that the sudden increase in conductivity is due to the higher water uptake and more strong molecular motion upon heating in the PCP moiety in the PCP-MCN composite. The reduced activation energy for PCP-MCN may be explained for the generation of well-connected proton transport paths due to higher proton conductivity and water adsorption of PCP among MCN particles. In short, the higher o and lower E, for PCP-MCN under different RH&T conditions confirm the establishment of 3D consecutive and facile proton hopping transfer networks, which are well-interconnected networks of hydrogen bonds formed by free and lattice water, MCN, and PCP. Hence, it's expected that the proton conductivity of a Nafion composite membrane can be significantly improved by introducing PCP-MCN with high water absorption, lots of amino and UMS, and decent conductivity into the Nafion matrix.
3.3. Mechanical property, thermal stability, water uptake and area swell rate of PCP-MCN/Naf composite membranes
Nafion, MCN-3/Naf, and PCP-MCN-3, 5/Naf composite membranes were successfully prepared and extensively characterized by various technologies. Dense surfaces and compact cross-sections of pure Nafion and the composite membranes are observed, as shown in Fig. 3a-f, and further cross-section images of higher magnification (2 pm scale) for the three membranes are also shown in Fig. S7, suggesting a nice compatibility between PCP-MCN nanoparticle and Nafion matrix. Compared to the neat surface (Fig. 3a) of the pure membrane, with the addition of more PCP-MCN, agglomeration of sample particles is found to disperse well on the surface (Fig. 3c) of the composite membrane, indicating that the additive is likely to be well distributed in the Nafion matrix. The agglomeration of additives has been found by many researchers [14,20,23,37,66], which may be detrimental to the mechanical and electrical properties of PEMs. From the FTIR plots (Fig. S8) of the membranes, it can be seen that with the introduction of PCP-MCN, these faint characteristic peaks of PCP-MCN rationally appear at 1394 em"! and 1626 cm". In Fig. S8b, the absorption peak representing the C-O-C bond on the fluorocarbon backbone shifts from 967 ст"! to a relatively higher value of 969.5 cm™', and its peak intensity reduces with more additives, indicative of the presence of electrostatic interactions of the fillers with the fluorocarbon backbone [67]. The peak derived from the symmetric stretching vibration of -SO3 shifts from 1054 cm"! to 1057 em", which may hint at some acid-base pairs formation (-SO3 ... TH-NH-) because of intermolecular interaction between PCP-MCN and Nafion side chains [22,37,68,69]. The strong interaction of the additives with the Nafion matrix is further confirmed by measuring the mechanical property and thermal stability of the membranes. As displayed in Fig. 3g, the break strength of Nafion, MCN-3/Naf, and PCP-MCN-3, 5/Naf composite membranes are 15.5, 20.1, 19.0 and 17.2 MPa, respectively, and their elongations at break are 182.8, 339.2, 502.4 and 378.0%, respectively. Comparing with the tensile strength of 15.5 MPa of Nafion, the higher strength (20.1 MPa) for MCN-3/Naf again provides convincing evidence for strong electrostatic interactions between the suspended -NH, on the MCN and the -SO;H on the Nafion side chains. MCN nanoparticles act as physical cross-linkers in Nafion matrix, reinforcing the break strength of the composite membrane. Meanwhile, the break strength and elongation at break for PCP-MCN-3/Naf are significantly increased, which is due to synergistic effect of the robust guest-host interactions and plasticizing of PCP, respectively. Nevertheless, the break strength and elongation for PCP-MCN-5/Naf are reduced to some extent in comparison to PCP-MCN-3/Naf, which may be ascribed to the agglomeration of additives in Nafion matrix. The increased break strength and longer elongation at break for the composite membranes demonstrate excellent compatibility and the strong intermolecular interaction between the additive and Nafion matrix. Subsequently, thermogravimetry (TG) was implemented for these membranes. As indicated by the arrows in Fig. 3h, the desulfonation temperature reasonably rises from ~284 °C of Nafion to ~301 °C of PCP-MCN/Naf because of the stronger molecular electrostatic interaction between host and guest.
WU and SW are important macroscopic properties of PEMs, because water is the medium for proton conduction in PEMs, and the swelling of PEMs is closely related to their lifetime and practical use [20,70]. Fig. 3(i and j) presents the water uptake of the membranes at different RH and T levels, respectively. With the gradually increasing RH at room temperature, the water content within the pristine Nafion membrane increases from 11.1 wt.% at 25% RH to 18.0 wt.% at 100% RH, which is due to the enlarged free volume because of the swelling of polymer chains and the formation of numerous ionic water clusters around the hydrophilic -SO;H of Nafion side chains at high humidity. The composite membrane loaded with 3 wt.% PCP-MCN displays superior WU from 31.5 wt.% at 25% RH to 60.5 wt.% at 100% RH due to the high porosity, plentiful hydrophilic groups, and plasticizing of the filler. With the gradually increasing temperature at a fixed 100% RH, the water absorption within membranes further is enhanced. The water uptake within Nafion membrane increases from 18.3 wt.% at 30 °C to 39.7 wt.% at 70 °C, while the water uptake within PCP-MCN-3/Naf membrane rises from 61.0 wt.% at 30 °C to 73.0 wt.% at 70 °C. Accompanied by the WU, the SW (Fig. 3k) of Nafion gradually increases from 20.6% at 30 °C to 26.8% at 70 °C, while the SW of PCPMCN-3/Naf progressively elevates from 38.5% at 30 °C to 46.2% at 70 °C. These results may be attributed to the larger free volumes and more amorphous regions in Nafion matrix being formed due to the stronger motion of the Nafion chains at higher temperatures. Further, the relative crystallinity degrees of the three samples are investigated by XRD measurement by fitting the amorphous peak around 16.1° and the crystalline peak at 17.5° [71]. Nanophase separation and surface morphology for hydrated Nafion and PCP-MCN-3/Naf membranes are investigated by AFM measurement [72,73]. As shown in Fig. 59, after impregnation with PCP-MCN into Nafion matrix, the crystallinities of the composite membranes (7.30% and 13.23% for PCP-MCN-3,5/Naf, respectively) are severely reduced compared to that of Nafion membrane (20.25%), indicating that the additive is essential for the rearrangement and cross-linking of Nafion chains, again confirming the strong electrostatic interaction between them. Meanwhile, the surface morphology and corresponding nanophase separation for Nafion membrane and PCP-MCN-3/ Naf membrane are shown in Fig. S10, respectively. More pronounced nanophase separation of the composite membrane (Fig. S10b) is observed compared to that of pure Nafion (Fig. S10a) due to the introduction of PCP-MCN, indicative of the growth of ionic water clusters nanophases and the interconnection of them. Therefore, the higher WU and SW of the composite membrane are associated with its lower crystallinity degree and more hydrophilic water cluster phases, which is conducive to the diffusion and adsorption of water molecules in the composite membrane matrix. As a result, the PCPMCN/Naf membrane integrates the advantages of MCN and PCP, demonstrating excellent hydrophilicity, mechanical stability and lower crystallinity with respect to Nafion membrane. Moreover, many acid-base pairs within the membrane are able to be formed through the riched -NH, on MCN with -SO; on the Nafion side chains, which could accelerate proton dissociation and transport, especially in reduced RH environments. Hence, it is reasonable to believe that the PCP-MCN/Naf membranes must possess high proton conduction and improved single-cell performance.
3.4. Proton conductivities and performances for H>/0> single-cell of PCP-MCN/Naf composite membranes
Proton conductivities of Nafion, PCP-MCN-3,5/Naf under different conditions (various RH levels at a fixed room temperature and different Ts at a fixed 100% RH) are displayed in Fig. 4. It's rational that the o for the three membranes gradually improves with the increasing humidity and temperature. Fig. S11 presents original AC impedance spectra at different RH and different Ts for PCP-MCN-3/Naf membrane, which displays a semicircle in the high-frequency and an inclined tail in the low-frequency, showing a typical proton transfer behavior. With the increase of temperature and humidity, an obvious decrease in semicircle size indicates that the proton conductivity significantly increases. In detail, as shown in Fig. 4a, the o of Nafion increases from about 10 $/ст at 30% RH to 1.85 x 1072 S/cm at a saturated RH. This is because the concentration of water molecules in Nafion increases during the process of increasing RH, which makes the internal ionic water clusters larger and more coherent, and further the proton transfer channels are thus constructed. With the introduction of PCP-MCN into Nafion matrix, the с of PCP-MCN-3/Naf astonishingly increases from 3.1 x 1073 S/ ст at 30% RH to 7.1 x 107? S/em at 100% RH, while the o of the PCP-MCN-5/Naf significantly rises from 1.53 x 107? at 30% RH to 4.21 x 10 S/cm at 100% RH. Notably, the o of the composite membranes increases by two orders of magnitude at 30% RH compared to that of pristine Nafion membrane, which is explained by the fact that water channels linked by water ionic clusters can be formed at low humidity due to the high water uptake of the Nafion composite membranes (Fig. 3i) [44]. As the ambient humidity gradually increases to 100% RH, the WU of the Nafion composite membranes increases, and then the water channels interconnect inside them, achieving a significantly elevated proton conductivity. Next, the slope of conductivity (Fig. 4b) as a function of humidity decreases from 0.029 for Nafion to 0.021 for PCP-MCN-3/Naf, which is mainly attributed to the formation of interconnected ionic-water clusters within the composite membrane at low humidities due to its water content above the percolation threshold of hydrated ion channel networks [6]. Additionally, acid-base pairs between the -SO;H of Nafion and the -NH> on the additive can promote proton transfer via the protonation-deprotonation process [74]. These results suggest that the composite membranes show a relatively weak humidity-dependence of proton conduction because of their high water uptake.
Additionally, the temperature-dependent proton conduction is also shown in Fig. 4d. Reasonably, the o for different samples gradually increases with raising the temperature, which can be attributed to the elevated diffusion coefficient of water molecules and more water absorption in expanded free volumes in Nafion matrix at a higher temperature. Particularly, when the PCP-MCN loading reaches 3 wt.%, at 98 °C the с rises to a maximum of 0.245 S/cm, being 2 times that of Nafion membrane (0.120 S/cm). As the filler continues to increase to 5 wt.%, с reaches 0.174 S/cm, which is 1.45 times the o of Nafion at 98 °C. As predicted, the conductivity of the membranes after loading PCP-MCN shows multiple obvious increments at different Ts. The apparent activation energy (Е, Fig. 4e) of proton transport is declined from 14.13 kJ/mol for Nafion to 8.18 kJ/mol for PCP-MCN-3/Naf. This significantly reduced E, can be well explained for the well-interconnected proton channels owing to the reduced crystallinity degree in comparison to Nafion membrane, improved water uptake of the composite membranes, and probably a low-energy barrier manner for proton migration via acid-base pairs [75]. These collective contributions allow the formation of long-range ordered and low-energy hopping channels for protons transfer within the membrane, directly boosting its macroscopic conductivity, as schematically shown in Fig. 4c, which depicts the possible transport processes of protons through the Nafion matrix and the additive in the composite membrane. Nevertheless, it is noteworthy that the composite membrane loaded with more PCP-MCN particles (5 wt.%) exhibits slightly lower conductivity and higher E, value than those of PCPMCN-3/Naf. Likewise, the WU of the composite membrane of PCP-MCN-5/Naf is lower than that of PCP-MCN-3/Naf under identical conditions. This observation, based on SEM and mechanical property results, is probably due to the fact that the more nanoscale dopants there are, the more difficult it is to disperse them due to the surface energy. The PCP-MCN agglomerates can make the connectivity of the proton pathway worse, since the с of the PCP-MCN pellet actually is an order of magnitude lower than that of raw Nafion. It is common for this filler agglomeration to slightly lower the с of the composite membrane [18,20,66]. Moreover, considering the prominent stability, time-dependent proton conduction of Nafion and PCP-MCN-3/Naf at 80 °C and 100% RH is explored. Fig. S12 illustrates that the conductivity of Nafion membrane is almost unchanged under 80 °C and 100 % RH in 33 h. While the conductivity of PCP-MCN-3/Naf is diminished from 0.187 S/cm to 0.138 S/cm at ~20 h, beyond which it almost remains unchanged. Although the composite membrane has a conductivity degradation, it still has a significant improvement by 62% in comparison with that of pristine Nafion membrane under identical conditions.
Further, the single fuel cell of membrane electrode assemblies (MEAs) fabricated with Nafion, PCP-MCN-3,5/Naf composite membranes were implemented under 40% RH (Figs. 4f) and 100% RH (Fig. 4g) at 60 °C. Whether in a low RH or a high RH environment, the open circuit voltage of the respective samples is above 0.9 V, indicative of a high gas barrier capacity of these membranes. Maximum power density (W max) at a reduced RH for each sample is significantly lower than Wax at a saturated RH, e.g., W max for Nafion at 40% RH is 0.176 W/cm? compared to its 0.567 W/cm" at 100% RH. It is recognized that the external humidity condition might be an extremely critical factor in the output performance of Nafion membrane. Desorption of H,O molecules from the ionic water clusters of Nafion at low RH conditions leads to the pinch-off of the proton transport water channels formed by numerous H,O molecules and -SOz, and thus the ion mobility is interrupted [33]. Surprisingly, compared with Nafion (0.176 W/ em"), the composite membranes with PCP-MCN exhibit higher Wimax, Which can be attributed to their high proton conductivity especially in the reduced humidity conditions. Compared to the Мах of 100% RH, the Wax of Nafion, PCP-MCN-3/Naf, and PCP-MCN-5/Naf at 40% RH is attenuated to 31%, 43.6%, and 60.2%, respectively. These results further confirm that the prepared composite membranes possess a weakly humidity-dependent conductivity compared to pure Nafion membrane. At the saturated RH and 60 °C, the impressive single-cell power output of PCP-MCN-3, 5/Naf reaches 1.098 and 0.737 W/cm? and is improved by nearly 94% and 30%, respectively, compared with that of Nafion (0.567 W/em°). This superior performance is derived from the higher proton conductivity and the lower E, for the composite membranes because of their water-retention ability. In addition, a Wmax of 0.863 W/em" (Fig. S13) of MCN-3/Naf is much lower than that of PCP-MCN-3/Naf (1.098 W/cm?), which confirmed the contribution of PCP to cell performance. Table S2 shows that the single-cell performance, breaking strength and elongation at break of MCN-PCP-3/Naf membrane are comparable to those of other remarkable Nafion proton-conducting materials reported in the literature. The data demonstrate the outstanding proton conductivity over wide humidity ranges and enhanced mechanical property for the present MCN-PCP-3/Naf membrane.
Nyquist plots of Nafion and PCP-MCN-3/Naf composite membrane, obtained by electrochemical impedance measurement at the output current density of 0.6 A/cm? under 60 °C and 100% RH, are shown in Fig. 4h. The Z axis intercept of the arc at high-frequency represents area-specific resistance (ASo) of single-cell [14], correlating to the proton transport resistance in the membrane. The ASa (0.23 O cm?) of a singlecell fabricated with PCP-MCN-3/Naf membrane is lower than that of the one fabricated with Nafion membrane (0.28 Q cm"). It is further observed that the diameter of the semicircle for the composite membrane is dramatically reduced in comparison to that of Nafion, which could be due to the decrement of charge transfer resistance. The enhanced output power and reductive internal cell resistance of the composite membrane are tightly ascribed to its high proton conductivity and water uptake, and establishment of well-interconnected proton transport networks because of the incorporation of PCP-MCN to Nafion matrix.
4. Conclusions
PCP-modified MIL-101-Cr-NH, is synthesized by hydrothermal reaction, and PCP-MCN particles of different amounts are introduced into the Nafion matrix to prepare composite membranes. XRD, BET and PALS demonstrated that the regular and porous crystal structure of MCN is retained in PCP-MCN, which exhibits high water uptake capacity and high proton conductivity of 2.61 x 107? S/em at 95 °C and 100% RH. The significant enhancement in proton-conduction for PCP-MCN at 55 °C is attributed to the activation of molecular motions of PCP chains, accompanied by enlargement of the free volumes in PCP. The dense surface and compact cross-section for PCP-MCN/Naf composite membranes suggest good compatibility between the additives and Nafion matrix, which is further confirmed by their enhanced mechanical properties and higher thermal stabilities. Simultaneously, because of the strong intermolecular interaction with PCP-MCN and Nafion side chains, the crystallinity degree is found to reduce in the composite membranes. Further, their water uptake and area swell ratio for the composite membranes are found to significantly increase due to the reduced crystallinity degree and the introduction of PCP-MCN. The с of pristine Nafion increases from about 10 S/cm at 30% RH to 1.85 x 107? S/cm at a saturated RH. Reasonably, with the introduction of PCP-MCN into Nafion matrix, the o of PCPMCN-3/Naf astonishingly increases from 3.1 x 1073 $/ст at 30% RH to 7.1 x 107? S/em at 100% RH. Notably, the o of the composite membrane is increased by two orders of magnitude at 30% RH, and it's improved by 283% at 100% RH in comparison to those of pristine Nafion. The resultant o of PCP-MCN-3/Naf composite membrane ultimately reaches a 0.245 $/ст at 98 °C, 100% RH, being 2 times of that of the Nafion membrane (0.120 S/cm), and a reduced E, of 8.18 kJ/ mol, which is due to its high water adsorption, retention abilities and the formation of well-interconnected ionic water channels within the matrix of Nafion composite membrane. Nevertheless, when the loading amount of PCP-MCN is excessive (up to 5 wt.%), PCP-MCN grains may aggregate in the Nafion composite membrane, which is not conducive to its overall performance. Furthermore, the Мах of MEA fabricated with PCP-MCN-3/Naf is 0.480 and 1.098 W/cm" at 40% RH and 100% RH (60 °C), which is improved by 173% and 94% in comparison to that of Nafion (0.176 and 0.567 W/ cm? under the same conditions), respectively. This study demonstrates that surface-modified MOFs/Nafion composite membrane is an attractive solid electrolyte in PEMFC.
CRediT authorship contribution statement
Xu Li: Writing-original draft, Methodology, Investigation, Visualization, Formal analysis. Dongwei Zhang: Conceptualization, Investigation, Formal analysis. Si Chen: Investigation, Visualization. Yingzhao Geng: Investigation, Formal analysis. Yong Liu: Conceptualization, Formal analysis. Libing Qian: Investigation, Visualization. Xi Chen: Formal analysis. Jingjing Li: Visualization, Formal analysis. Pengfei Fang: Conceptualization, Resources. Chunging He: Conceptualization, Resources, Writing - review & editing, Supervision, Project administration, Funding acquisition.
Declaration of competing interest
There are no conflicts to declare.
Acknowledgements
This work was supported by the National Natural Science Foundation of China (NSFC) (Nos. 12075172, 12375288, 12205089, and 12105048), National Key R&D Program of China (Grant No. 2019YFA0210003), Guangdong Basic and Applied Basic Research Foundation (No.2020A1515110817). We thank the Core Facility of Wuhan University for supporting the characterization and analysis of the materials. Dr. Y. Ma was appreciated for the AFM measurement.
Received 21 September 2023; revised 22 October 2023; accepted 30 October 2023 Available online 31 October 2023
* Corres OITGSPONCMIE-AUEIOE, di thor.
E-mail address: [email protected] (?. He).
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Appendix A. Supplementary data
Supplementary data to this article can be found online at https://doi.org/10.1016/j.gee.2023.10.007.
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Abstract
High-performance proton exchange membranes are of great importance for fuel cells. Here, we have synthesized polycarboxylate plasticizer modified MIL-101-Cr-NH, (PCP-MCN), a kind of hybrid metal-organic framework, which exhibits a superior proton conductivity. PCP-MCN nanoparticles are used as additives to fabricate PCP-MCN/Nafion composite membranes. Microstructures and characteristics of PCP-MCN and these membranes have been extensively investigated. Significant enhancement in proton conduction for PCP-MCN around 55 °C is interestingly found due to the thermal motion of the PCP molecular chains. Robust mechanical properties and higher thermal decomposition temperature of the composite membranes are directly ascribed to strong intermolecular interactions between PCP-MCN and Nafion side chains, i.e., the formation of substantial acid-base pairs (-SO3 - - - "H-NH-), which further improves compatibility between additive and Nafion matrix. At the same humidity and temperature condition, the water uptake of composite membranes significantly increases due to the incorporation of porous additives with abundant functional groups and thus less crystallinity degree in comparison to pristine Nafion. Proton conductivity (с) over wide ranges of humidities (30 - 100% RH at 25 °C) and temperatures (30 - 98 °С at 100% RH) for prepared membranes is measured. The o in PCPMCN/Nafion composite membranes is remarkably enhanced, i.e. 0.245 S/cm for PCP-MCN-3wt.%/Nafion is twice that of Nafion membrane at 98 °C and 100% RH, because of the establishment of well-interconnected proton transport ionic water channels and perhaps faster protonation- deprotonation processes. The composite membranes possess weak humidity-dependence of proton transport and higher water uptake due to excellent water retention ability of PCP-MCN. In particular, when 3 wt.% PCP-MCN was added to Nafion, the power density of a single-cell fabricated with this composite membrane reaches impressively 0.480, 1.098 W/cm? under 40% RH, 100% RH at 60 °C, respectively, guaranteeing it to be a promising proton exchange membrane.
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Details
1 Key Laboratory of Nuclear Solid-State Physics Hubei Province, School of Physics and Technology, Wuhan University, Wuhan, 430072, China