Measuring Proton Conductivity in MOF-Based Mixed Matrix Membranes by Electrochemical Impedance Spectroscopy

Summary

This protocol describes a reproducible methodology for the fabrication of sustainable metal-organic framework/polymer mixed-matrix membranes and accurate measurement of their in-plane proton conductivity under variable temperature and humidity using electrochemical impedance spectroscopy, which can be useful for proton exchange membrane fuel cells (PEMFCs).

Abstract

This protocol describes a reproducible method to measure the in-plane proton conductivity of metal-organic framework (MOF)/polymer mixed-matrix membranes (MMMs) through electrochemical impedance spectroscopy (EIS) under controlled temperature and humidity conditions. The MMMs were fabricated by solution casting method using the non-fluorinated, water-processable polymer, poly (vinyl alcohol) (PVA) and the proton-conducting MOF MIP-207-SO₃H. The synthesis of MIP-207-SO₃H is detailed, followed by membrane fabrication, assembly of in-plane conductivity measurement cells, and acquisition of membrane resistance through EIS. This enables accurate acquisition and determination of membrane resistance, determination of proton conductivity, and activation energy. The representative results demonstrate that MIP-207-SO₃H-containing membranes exhibit enhanced proton conductivity under the test conditions compared to the membrane without MOF incorporation.

Introduction

The world is shifting from fossil fuel-based generation, finite and environmentally harmful, toward low-carbon and renewable energy sources^1,2. Within this transition, the move from a hydrocarbon-based economy toward a hydrogen economy has been proposed to meet sustainable energy demands across the transportation, domestic, and commercial sectors, with fuel cells providing efficient route to convert hydrogen into electricity^1,2,3. Among the various fuel-cell technologies, proton exchange membrane fuel cells (PEMFCs) are particularly attractive due to their high-power density and rapid start–stop capability make them especially attractive for both distributed power and mobility applications, with only water as a byproduct^1,2,3,4,5. In this work, we present a reproducible method to fabricate sustainable MOF/polymer mixed-matrix membranes and to accurately measure their in-plane proton conductivity under controlled temperature and humidity.

The heart of a PEMFC is the polymer electrolyte membrane (PEM), which ensures the proton transport from the anode to the cathode⁶. Proton transport occurs either via a vehicular mechanism, where hydrated proton species diffuse with water or water clusters, or a hopping (Grotthuss) mechanism, where protons hop along a hydrogen-bond network^6,7. Nafion, a perfluoro sulfonic acid (PFSA) ionomer featuring a perfluorinated backbone and pendant perfluoro ether side chains terminated with sulfonic acid (–SO₃H) groups, remains the benchmark commercial PEM⁸. Despite its excellent proton conductivity (10^-2-10^-1 S·cm^-1) at 70–80 °C and relative humidity (RH) of 95–100%, Nafion shows a pronounced conductivity drop when temperature and humidity deviate from these conditions, limiting performance under low-RH and/or high operating temperature^8,9. In addition, other limitations like gas crossover, high fabrication cost, and polyfluoroalkyl substances (PFAS) related environmental / toxicity concerns during membrane fabrication and use have motivated the development of alternative, more sustainable PEMs that deliver robust proton transport alongside improved environmental compatibility and reduced cost^7,9.

Among the alternative materials for this application, Metal-Organic Frameworks (MOFs) appear as promising candidates. These are porous crystalline solids, composed of metal nodes (ions/clusters/chains…) and organic linkers (carboxylates, azolates, phosphonates…), offering an exceptional chemical and structural versatility that allows to target specific properties. Proton conductivity in MOFs has been explored over the past two decades^10,11,12, and different approaches can be used to enhance proton conductivity, including linker or inorganic cluster functionalization, the incorporation/impregnation of acidic dopants, and defect control. These strategies have led to the development of MOFs with enhanced proton conductivity, in some cases comparable or exceeding Nafion^{13,14,15,16,17}. Ponomareva et al.¹⁷ impregnated the mesoporous MIL-101(Cr) (MIL refers to Matériaux de l’Institut Lavoisier) with nonvolatile strong acids (aqueous H₂SO₄ or H₃PO₄), then removed excess liquid and dried to form H₂SO₄@MIL-101 and H₃PO₄@MIL-101, effectively confining concentrated acid inside the cages as additional proton sources. The confined acid/water phase provided a Grotthuss-like proton transport stabilized by capillary confinement, enabling high temperature conduction under very low humidity (up to 1 × 10^-2 S·cm^-1 at 150 °C, 1.1% RH). Taylor et al.¹⁵ tuned UiO-66 (UiO refers to the University of Oslo) into a better proton conductor through defect control. They controlled missing-linker/non-bridging-ligand defects during synthesis by adjusting metal/linker ratios and a monocarboxylic-acid modulator, which generates Lewis-acid Zirconium (Zr) sites and increases pore volume and hydrophilicity. These defect sites raised the proton carrier concentration and, crucially, the enlarged pore network boosts proton transport within the hydrated H-bond network, giving a nearly 3-order-of-magnitude conductivity enhancement up to 6.93 × 10^-3 S·cm^-1 at 65 °C and 95% RH.

Recently, our group has successfully developed several proton-conducting MOFs in green conditions (using water and non-toxic solvents or reactants), displaying high bulk conductivity, such as MIP-202(Zr), MIP-177-SO₄H, and MIP-207-SO₃H(Ti) (MIP refers to Materials of the Institute of Porous Materials from Paris)^14,18,19. MIP-202(Zr) is a microporous amino acid (aspartic acid) based Zr-MOF, integrating NH₃⁺/Cl⁻ pairs, which give to this MOF a zwitterionic character. The Brønsted-acidic –NH₃⁺ groups, combined with µ₃-OH sites of the Zr-oxocluster and the water network in the pores, enable a high proton conductivity of 1.1 × 10⁻² S·cm^-1 at 90°C and 95% RH. Another particularly interesting MOF is MIP-207-SO₃H, due to its 2D layered ultra-microporous structure, where the linker 1,3,5-benzenetricarboxylic acid (BTC) linkers are partially replaced by 5-SO₃H-isophthalic acid (SO₃H-IPA), introducing pore-facing -SO₃H Brønsted-acid sites while retaining the same framework topology as the unsubstituted MIP-207(Ti). The -SO₃H groups, together with the remaining -CO₂H groups, act as proton donors and, with adsorbed water, form a continuous H-bond network enabling Grotthuss-type transport, leading to a proton conductivity of 2.6 x 10^-2 S·cm^-1 at 90 °C and 95% RH for a degree of partial substitution of 28 % ¹⁴.

Although many MOFs show excellent intrinsic proton conduction, they are typically obtained as brittle microcrystalline powders, so a bulk MOF PEM would behave like a packed particulate layer rather than a dense, continuous film like Nafion. Under a membrane electrode assembly (MEA) clamping pressure and repeated hydration/dehydration, the particle–particle boundaries can open into micro voids or cracks, creating nonselective pathways that increase reactant gas crossover rather than forcing transport through the intended proton-conducting network²⁰. For this reason, MOFs nanoparticles are usually shaped or embedded in a polymer matrix²¹. Although numerous examples with Nafion or other usual polymers as SPEEK exist, biopolymers such as Chitosan, Alginate, Nanocellulose or Lignin are currently being heavily investigated as attractive PEM matrices as they are naturally abundant, water processable, non‑fluorinated, and can be processed into free‑standing films^22,23,24. Similarly, synthetic biocompatible polymers such as Poly (vinyl alcohol) (PVA) are widely being investigated as a sustainable membrane matrix or blend component that provides mechanical integrity, good hydration, while MOFs supply proton-donor/transport sites and help suppress gas crossover by creating more tortuous diffusion pathways²⁵. Besides their possible contribution to the proton percolation within the composite, these polymers also offer the possibility to modulate the mechanical properties of the composite, as well as mitigate/control the crossover properties of the composite membranes.

Considering the relevance of proton conductivity as a defining performance parameter for PEM, in this work, we present a reproducible step-by-step workflow for membrane fabrication and to quantify the in-plane proton conductivity of the polymer, PVA, and MIP-207-SO₃H composite membranes using electrochemical impedance spectroscopy (EIS) in a temperature and humidity-controlled setup. Unlike many through‑plane conductivity cells, where the membrane must be compressed between electrodes, the measured resistance can be strongly influenced by electrode-membrane contact resistance and, when mesh electrodes are used, the mesh imprints locally change the effective contact area and membrane thickness, which can affect conductivity measurements²⁶. The two‑electrode in‑plane setup minimizes interfacial contributions and improves measurement reproducibility²⁶. This method provides improved reproducibility, reduced electrode–membrane interfacial effects, and more reliable comparison between membranes with differing compositions. Our protocol briefly details the synthesis of MIP-207-SO₃H, the preparation of PVA/MIP-207-SO₃H membranes, followed by a presentation of the relevant steps involved in proton conductivity measurements, including mounting in the measurement cell, acquisition of impedance spectra for defined RH/temperature set points, and finally proton conductivity calculation. This protocol is intended to enable researchers designing proton‑conducting membranes to carry out in-plane proton conductivity measurements for different membrane formulations under well‑defined humidity and temperature conditions.

Protocol

Ethics statement

This study did not involve human subjects, animals or regulated biological materials. All experiments used standard chemical synthesis and materials characterization techniques.

1. Synthesis of MIP-207-SO3H

Note: The synthesis of MIP-207-SO₃H follows the protocols previously reported by Wang et al.²⁷ and Nandi et al¹⁴. A schematic illustration of the synthesis process is shown in Figure 1.

1. Place a magnetic stirring bar in a 50 mL round-bottom flask and add 1,3,5-benzenetricarboxylic acid (588 mg, 2.40 mmol) and 5-LiSO₃-isophthalic acid (5-LiSO₃-IPA, 303 mg, 1.21 mmol) to the flask.
   CAUTION: In a fume hood, add 10 mL acetic anhydride to the flask and stir at 25 °C for 5 min until homogeneous. Then add 10 mL glacial acetic acid, followed by dropwise addition of Titanium (IV) Isopropoxide (Ti(iPrO)₄, 800 µL, 2.70 mmol) under stirring. Acetic anhydride/acid and Ti alkoxide are corrosive, irritant, moisture-sensitive, and flammable; keep away from ignition sources.
2. Fit the flask with a reflux condenser, heat the reaction mixture to reflux, and ensure reflux is maintained for 24 h under continuous stirring.
3. After completion of the reaction, allow the mixture to cool to room temperature, then isolate the resulting solid product by centrifugation. Centrifuge the mixture at an appropriate speed depending on particle size, in general around ~27,000 x g.
4. Wash the collected solid three times with boiling acetone to remove all remaining unreacted linker, then dry on a bench under air as it is required for subsequent characterization or membrane fabrication.
5. Collect a PXRD pattern of the dried MOF powder. Validate the MOF structure and purity by overlaying the collected PXRD with the reported/simulated MIP-207-SO₃H pattern. Fourier Transform Infrared spectroscopy (FTIR) and Thermogravimetry Analysis (TGA) can also be performed to verify the quality of the MOF (free linker, defects, etc).

Figure 1: Schematic illustration of MIP-207-SO₃H synthesis procedure. Please click here to view a larger version of this figure.

2. Preparation of mixed-matrix membranes

1. Disperse PVA powder (10% w/w) in deionized water and heat continuously (90 °C) until a clear, homogeneous solution is obtained.
2. To prepare the mixed-matrix membrane, disperse the required amount of MIP‑207‑SO₃H in deionized water.
3. Sonicate the solution using an ultrasonication probe (10% amplitude for 5 min) to obtain a uniform MOF suspension. Adjust the sonication duration depending on the nature of the MOF.
   Note: Further confirmation of the quality of the dispersion can be investigated using particle-size analysis via dynamic light scattering (DLS) or through morphological observations using scanning electron microscopy (SEM).
4. Add the dissolved PVA solution dropwise to the pre-dispersed MIP‑207‑SO₃H suspension over 1 h under continuous stirring and mix until the dispersion appears visually uniform (no visible aggregates or phase separation). Adjust the amount of PVA solution to obtain a final concentration of 7% w/w.
5. Cast the PVA–MIP‑207‑SO₃H solutions onto clean glass plates using a film applicator to form wet films of defined thickness (e.g., 1 mm). The thickness of the membrane can be selected depending on the concentration and viscosity of the polymer solution, typically ranging from about 10 µM to over 200 µM in the dry film.
6. Dry the cast films under ambient conditions under a hood until solvent evaporation is complete, then peel the membranes from the glass plates to obtain flexible, dense, and transparent PVA and PVA/MIP‑207‑SO₃H membranes. A schematic illustration of the fabrication of a Mixed Matrix Membrane (MMM) is presented in Figure 2.
7. Prepare a PVA-only control membrane without MIP‑207‑SO₃H using the same fabrication procedure to determine the contribution of the pristine polymer matrix to the proton conductivity.

Figure 2: Schematic illustration of the fabrication of PVA and PVA/MIP‑207‑SO₃H membranes fabrication via the solution casting method. Please click here to view a larger version of this figure.

3. Proton conductivity measurement

1. Cut the fabricated membrane samples according to the in-plane measurement cell geometry (e.g., 2 cm x 2 cm).
2. Measure the sample dimensions and thickness at several positions using a micrometer, then calculate and record the average thickness (t) (see Figure 3, step 2).
   Note: It is recommended to take at least three precise thickness measurements near the center of the membrane.
3. Assemble the conductivity cell by placing the membrane between two electrodes with a known geometric contact area, A, then sandwich the assembly between two Teflon cells and gently tighten with screws and hex nuts to ensure good interfacial contact. A stepwise process of assembling the membrane conductivity cell is presented in Figure 3.
4. Place the assembled Teflon cells inside a climatic humidity chamber, which is directly connected to a potentiostat and set to the desired temperature and RH conditions.
5. Directly program the temperature and RH set points (often ranging from 30 °C to 90 °C depending on the system and increasing RH from 60% to 95%) in a programmable humidity chamber. It will automatically change over time without manual intervention.
6. Connect the in-plane conductivity cell to the potentiostat equipped with a frequency response analyzer and place the setup at the selected temperature/RH condition.
   Caution: Allow the membrane to equilibrate for 1 h before measurement.
7. Specify a frequency range using the EIS software, from 1 MHz to 1 Hz with at least two measurements per frequency, set the AC perturbation amplitude to 20 mV, then start the measurement and record the impedance spectrum for subsequent analysis.
8. Save the impedance spectrum for subsequent analysis after the measurement is complete. A schematic illustration of the in-plane proton conductivity measurement setup is provided in Figure 4 to facilitate visualization of the experimental configuration.
9. After turn the humidity chamber off and allow it to cool for 1 h until the temperature is below 30 °C. Take the measurement cells out of the humidity chamber, disassemble, and confirm the membrane stays mechanically intact with no visible wear and tear in the conditions used for the proton conductivity measurement.
10. Dry the membrane and store it in a dedicated plastic bag for further analysis (e.g., PXRD, mechanical test, etc.) to verify structural and mechanical properties after the conductivity measurements.
11. To analyze the EIS results, the membrane resistance is determined from the Nyquist plots (−Im(Z) versus Re(Z)).
    The resistance can be read as the intersection between the semicircle at high frequencies and the real axis. The resistance can also be read from the Bode plot (log(|Z|) versus the frequency). It corresponds to the ordinate at the slope rupture.
12. Calculate the proton conductivity, σ(S/cm), from the measured membrane resistance using Eq-1. Use L (in cm) as the distance between the electrodes (defined by the set-up), R (Ω) as the bulk membrane resistance obtained from impedance spectroscopy, and A (cm²) as the membrane cross-sectional area carrying in-plane current (A is defined as the product between the membrane width, w, and its thickness T).
    NOTE: Proton conductivity, σ (S·cm^-1), versus temperature plots can then be generated using any available data analysis tool.
    
13. Construct an Arrhenius plot by plotting In(σ) versus 1/T (or 1000/T, with T in Kelvin) to extract the activation energy, E_a, from the linear fit (Eq-2) and decide whether proton transport occurs via vehicular (E_a > 0.5 eV) or hopping (Ea < 0.4 eV) mechanism.
14. Use σ (S.cm^-1) as the measured proton conductivity, A as the pre-exponential factor, E_a as the activation energy (eV), k_B as the Boltzmann constant = 8.62 × 10^-5 eV K^-1, and T as the temperature.
    

Figure 3: Two‑electrode cell for in‑plane proton conductivity measurement cells, showing the internal configuration (step 1), membrane placement (step 2), and assembled configuration (step 3). The scale shown indicates the physical dimensions of the membrane piece used in the measurement cells. Please click here to view a larger version of this figure.

Figure 4: Schematic of the in-plane proton conductivity measurement setup under controlled temperature and humidity conditions, showing the membrane assembly, electrode configuration, and connection to the electrochemical workstation. Please click here to view a larger version of this figure.

Results

Figure 5 A and B represent the structural illustration and PXRD diffractograms (CuK_α) for the synthesized MIP-207-(SO₃H-IPA)_x(BTC)_1−x (x=0.28) and MIP-207 simulated pattern, calculated from the crystallographic information file (CIF) reported in the original study by Wang et al²⁷. The close match in the positions of these characteristic Bragg reflections, notably the dominant low‑angle peaks at 2θ ≈ 5.0° and 2θ ≈ 12-13° confirm that the sulfonated derivative remains isostructural with MIP‑207. In addition, these signature MIP-207-SO₃H peaks can be observed in the fabricated composite membrane, which indicates successful incorporation of the proton-conducting MOF in the matrix of PVA. MOF structure should not be affected during membrane fabrication and defective membranes (with intrinsic voids or strong MOF particles agglomeration) can also display high quality PXRD patterns. Hence, further analysis of the membranes through scanning electron microscopy (SEM) is required. Moreover, prior to EIS measures the integrity of the membrane after exposure to high relative humidity should also be verified.

Figure 5: (A) Crystal structure of MIP-207(Ti): (left top) Ti–oxo cluster inorganic unit; (left bottom) Benzene tricarboxylic acid (BTC) or 5-substituted isophthalic acid linker (5-substituted IPA), the pink atom represents the –COOH and –SO₃H functional groups, Ti is shown in blue, C in gray, acetate groups in green, and O in red); (right) extended 3D framework with periodic pore channels, (B) Chemical structure of PVA, and (C) Experimental PXRD pattern (CuK_α) of MIP-207-SO₃H, simulated MIP-207 pattern, PVA, and PVA/MIP-207-SO₃H membranes. Please click here to view a larger version of this figure.

Nyquist plot and proton conductivity of pristine PVA (without MOF) and PVA/MIP-207-SO₃H membranes evaluated by EIS over 30-60 °C at 95% RH are presented in Figure 6 A and B. The Nyquist plots for the PVA/MIP‑207‑SO₃H membrane show a reduction of the semi-circle with increasing temperature (refer to Figure 6 A), indicating decreased bulk resistance. Figure 6 B reveals a monotonic increase in proton conductivity with temperature for both membranes. This trend indicates thermally activated proton transport behavior consistent with the expected membrane performance. A successful result is characterized by a complete semicircle in the Nyquist plot with a clear high-frequency intercept representing bulk membrane resistance. Deviations such as irregularly shaped arcs or highly asymmetric plots may suggest experimental issues including poor membrane-electrode contact, insufficient membrane hydration, improper membrane-electrode assembly, or faulty potentiostat connections. The composite membrane, PVA/MIP‑207‑SO₃H, has a proton conductivity of 7.4 x 10^-4 S·cm^-1 compared to pristine PVA with a conductivity of 3.1 x 10^-4 S·cm^-1, which is 140% higher at 60 °C and RH 95%. Long-term durability under continuous operation was not evaluated in this study and remains an important aspect for future investigations.

Figure 6: (A) Nyquist plots (Imaginary impedance (-Z") versus Real impedance (Z') of the PVA/MIP-207-SO3H membrane measured at 95 % RH (30-60 °C), and (B) Proton conductivity (S/cm) as a function of temperature for bare PVA and PVA/MIP-207-SO₃H membranes. Please click here to view a larger version of this figure.

Arrhenius plots of In(σ) versus 1000/T (95 % RH, 30-60 °C) show linear fits for both pristine PVA and PVA/MIP-207-SO₃H membranes (refer to Figure 7). The Arrhenius plot here is expressed as ln(σ) versus 1000/T, which is mathematically equivalent to plotting ln(σ) versus 1/T but provides a more convenient x-axis scale for data visualization and extraction of the activation energy. The composite exhibits an activation energy of E_a = 0.65 eV, while bare PVA has an E_a = 0.46 eV. Overall, these values indicate proton transport occurs via a predominantly vehicular proton transport mechanism for the composite membrane, while both vehicular and hopping mechanisms occur in the pristine PVA membrane. These results confirm that the protocol enables reliable extraction of activation energy and identification of proton transport mechanisms. Ineffective experimental outcomes are characterized by non-linear Arrhenius plots with significant scatter and poor coefficient of determination (R²) values, suggesting that alternative models such as the Vogel-Tammann-Fulcher (VTF) equation may be more appropriate for describing temperature-dependent conductivity behaviors. Additionally, abnormally high activation energies (E_a > 1.0 eV) or conductivity values below 10^-6 S/cm indicate poor MOF dispersion, insufficient hydration, or discontinuous proton-conducting pathways.

Figure 7: Arrhenius plots: In(σ) versus 1000/T for bare PVA and PVA/MIP-207-SO₃H membranes measured at 95 % RH (30-60 °C); symbols are experimental data and dashed lines are linear fits used to extract the activation energy. Please click here to view a larger version of this figure.

Discussion

When preparing MOF/polymer composites, one of the most important protocol steps is to ensure crystallinity and the phase purity of the MOF before membrane fabrication, since the structural integrity of the MOF strongly impacts water uptake, and ultimately, proton conductivity. In addition, the particle size and polydispersity of the MOF also significantly influence both the membrane’s final properties and the colloidal stability of the suspension. Moreover, the interfacial compatibility between the MOF and the chosen polymer plays a critical role in determining the overall membrane quality. Thus, the comparison of the experimental PXRD pattern with reported/simulated MOF pattern (herein MIP-207-SO₃H) is highly recommended, as well as complementary characterization techniques, which are not described here in this process paper (for instance, Infrared Spectroscopy (IR), Thermogravimetric Analysis (TGA), Nitrogen (N₂) porosimetry, and SEM. If the MOF’s characteristic Bragg reflections are missing or if extra peaks appear after synthesis, the synthesis should be repeated before moving forward to blending with the polymer mixture. The same confirmation must be made again after composite membrane fabrication. A second key step is achieving a stable, aggregate-free MOF suspension before mixing with the polymer, since MOF aggregation can create non-uniform pathways that can compromise the MOFs/membrane’s proton-conducting ability²¹. Nanoparticles with controlled size (< 200 nm) improve membrane uniformity. Finally, ensuring colloidal stability of the suspension before combining the MOF and polymer solutions is essential, as it directly influences the quality of the MMM²⁸. This protocol can be adapted by selecting alternative polymers or MOFs with compatible chemistry, or modifying preparation conditions such as solvents, type of crosslinking agents, or other membrane fabrication conditions like thickness, temperature, and polymer/MOF loading.to suit different membrane systems.

The casting parameters are also central to the success of the method, since the membrane’s uniformity directly affects the conductivity calculation through the thickness. For membrane fabrication via solution casting, the uniformity of the final dry thickness is essentially governed by the film applicator gap; therefore, explicitly controlling and, when needed, adjusting the casting gap is essential to obtain comparable dry thickness across different membranes. In addition to the applicator gap, membrane uniformity can be affected by the casting speed but also by polymer concentration, viscosity, and mixture homogeneity^7,28. During mixture or membrane fabrication, if visible aggregates are present, priority should be given to improving polymer–MOF homogeneity (including controlling polymer viscosity and MOF particle size/distribution/colloidal stability) and to optimizing the drying conditions, since aggregation and poor polymer-MOF adhesion can introduce defects, locally alter thickness, and disrupt continuous proton-conduction pathways in the MMM²⁸. If poor conductivity or inconsistent impedance spectra are observed, ensure the membrane fabrication quality before repeating the measurements. Specifically, verify the MOF uniformly dispersed within the membrane matrix, membrane fabrication conditions (i.e., polymer mixture ratio, mixing technique, and crosslinking), or membrane integrity under humid conditions. Beyond these important parameters, other critical aspects for conductivity measurements such as a stable membrane-electrode contact, stable temperature, and RH in the humidity chamber, proper connection between assembled measurement cells and the potentiostat, adequate equilibration at each of the selected temperature/RH, and consistent extraction of resistance values from the Nyquist plots²⁹.

In general, the conductivity values are strongly dependent on the temperature and relative humidity; nonetheless, in MOF/polymer MMMs, the MOF loading can be adjusted to maximize proton conductivity. Increasing MOF loading can promote a more continuous polymer and MOF-assisted transport pathways, which lead to significant enhancement in proton conductivity^14,18,19. However, it should be noted that higher loadings also increase the likelihood of MOF aggregation and interfacial defects due to incompatibility between the MOF/polymer²⁸. The incorporation of MOF nanoparticles increases the number of water adsorption sites and hydration channels when dispersed in a polymer matrix, facilitating proton transport¹⁴. In addition to enhancements of water adsorption, MOFs can also act as radical scavengers (one of the critical aspects for Nafion durability), limit gas crossover, while reinforcing the tensile strength, thermal, and mechanical durability of the membranes^7,30,31.

Compared with through‑plane conductivity measurements, where the membrane is commonly compressed between electrodes to minimize interfacial contact resistance, the in‑plane setup avoids sustained through‑thickness loading during the measurement²⁶. When temperature and humidity change, it becomes advantageous for biopolymer PEMs because hydration‑induced swelling and plasticization/softening make the effective contact area drift under compression³². As a result, in‑plane configuration can improve reproducibility when comparing membranes with different MOF loadings (which can differ in water uptake and mechanical properties, etc.) and dispersion quality²⁶. Compared to conventional through-plane measurements, this approach offers improved reproducibility and reduced interfacial contact resistance, making it particularly suitable for comparative evaluation of membranes with varying compositions.

One limitation of this method is that, although it reduces the electrode–membrane interfacial effects compared with through-plane conductivity measurement setup, it does not directly reflect the through-thickness proton transport and should therefore be regarded as complementary rather than equivalent to through-plane methods. In summary, this method provides a reproducible and accessible approach for evaluating proton conductivity in sustainable MMMs, enabling systematic comparison of materials under controlled temperature and humidity. This approach can be applied to the development and optimization of PEMFCs.

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Equipment
Centrifuge	Thermo Fisher Scientific	Sorvall LYNX 6000	Solid isolation after synthesis.
Climatic humidity chamber	Memmert	HCP50	Adjust humidity and temperature for proton conductivity measurement
Film applicator	Elcometer Limited	4340 film applicators	Control wet film thickness during casting.
Glass plates	Various	N/A	Casting substrate.
Magnetic stirrer hotplate + stir bars	Various	N/A	Stirring and heating during synthesis and polymer prep.
Micrometer	Mitutoyo	Quantumike	Membrane thickness measurement (average from multiple points).
Other laboratory utensils	Various	N/A	Spatula, fume hood, pipettes/syringes for chemical addition, etc.
Potentiostat	Biologic	VSP-3e	For EIS measurement
PTFE (Teflon) cells, screws, hex nuts	Equilabrium	BT-112	Mechanical tightening to ensure good electrode contact.
PXRD instrument	Bruker	D8 Advance	For phase/crystallinity check of MOF powder.
Reflux condenser	Various	N/A	Maintain reaction temperature during MOF synthesis
Round-bottom flask	Various	N/A	MOF synthesis reaction vessel and polymer dissolution
Ultrasonic Probe	Branson	Ultrasonic Sonifier SFX550	Disperse MOF in water before mixing with PVA.
Ultrasonic Probe	Branson	Ultrasonic Sonifier SFX550	Disperse MOF in water before mixing with PVA.
Film applicator	Elcometer Limited	4340 film applicators	Control wet film thickness during casting
Glass plates	Various	N/A	Casting substrate
Micrometer	Mitutoyo	Quantumike	Membrane thickness measurement (average from multiple points)
Other laboratory utensils	Various	N/A	Spatula, fume hood, pipettes/syringes for chemical addition, etc.
Chemicals
1,3,5-Benzenetricarboxylic acid	Thermo Scientific	A15947.18	MOF organic linker
5-sulfoisophthalic acid lithium salt	Thermo Scientific	A18028.36 (95%)	Sulfonated linker source used to introduce acidic sites.
Acetic anhydride	Thermo Scientific	149490010 (99+%,)	Used in synthesis mixture
Titanium (IV) isopropoxide	Sigma Aldrich	205273 ((97%)	Titanium precursor used for Ti-MOF formation
Acetone	Carlo Erba	002100DD14150	Used for boiling washes/activation
Poly (vinyl alcohol) (PVA)	Alfa Aesar	ALF-041243-14	Polymer matrix for membranes