Electrochemical Properties and Structure of Membranes from Perfluorinated Copolymers Modified with Nanodiamonds

At a Glance

Metadata	Details
Publication Date	2023-10-25
Journal	Membranes
Authors	В. Т. Лебедев, Yu. V. Kulvelis, A. V. Shvidchenko, O. N. Primachenko, Alexei S. Odinokov
Institutions	Institute of Macromolecular Compounds, Joint Institute for Nuclear Research
Citations	1
Analysis	Full AI Review Included

Electrochemical Properties and Structure of Nanodiamond-Modified PFSA Membranes

Executive Summary

This study successfully engineered Aquivion^®-type perfluorosulfonic acid (PFSA) membranes modified with detonation nanodiamonds (DNDs) to enhance proton conductivity, focusing on the role of DND surface functionalization.

Optimal Modifier Identified: DND Z+ (hydrogen/hydroxyl saturated, positively charged) proved the most effective modifier, achieving a stable conductivity increase of 20-30% at 50 °C (0.5 wt.% loading) compared to the pristine copolymer.
Mechanism of Enhancement (DND Z+): The positive charge of DND Z+ particles facilitated electrostatic attraction to the negatively charged SO₃H groups of the copolymer, leading to the formation of stable, hybrid conductive channels and increased membrane hydration.
Structural Stability: Small-Angle Neutron Scattering (SANS) confirmed that DND Z+ integration maintained the inherent ionomer structure, with a stable channel packing period (L_c ~ 3 nm), while increasing the total channel surface area.
Negative Charge Effect (DND Z-): DND Z- (carboxyl functionalized, negatively charged) showed mutual repulsion with SO₃H groups, causing particle segregation, minimal conductivity gain (~10%), and disruption of the conductive network at higher concentrations.
Hydrophobic Disruption (DND-F): Hydrophobic, fluorinated DND-F particles severely degraded performance, causing a 4-fold decrease in conductivity at just 1 wt.% loading by interacting with the non-polar fluorocarbon backbone and fragmenting the ion channel network.
Conclusion: The type of DND functionalization is the key factor controlling the formation of the diamond-polymer interface, which directly dictates the membrane’s ability to adsorb water and transport protons.

Technical Specifications

Parameter	Value	Unit	Context
DND Particle Size (d_p)	4-5	nm	Average size of detonation nanodiamonds
DND Concentration Range	0.25-5.0	wt.%	Range tested in composite membranes
Pristine Conductivity (κ₀)	0.131 ± 0.002	S/cm	At 20 °C, maximum equilibrium moistening
Max Conductivity Gain (DND Z+)	Up to 30	%	0.5 wt.% DND Z+, measured at 50 °C
DND Z+ Temperature Effect (ΔTκN)	~20-30	%	Normalized conductivity difference (50 °C vs. 20 °C)
DND-F Conductivity Change	4-fold decrease	N/A	At 1 wt.% DND-F, 20 °C and 50 °C
Ionomer Peak Position (q*)	~2	nm^-1	SANS data, characteristic of channel packing
Channel Packing Period (L_c)	~2.6-3	nm	Derived from SANS q* (L_c = 2π/q*)
Polymer Ion Channel Diameter (δ_c)	~1	nm	Estimated for the pure copolymer
DND Z+ Water Shell Thickness (d)	2.8	nm	Calculated for 1 wt.% DND Z+ composite
DND Z+ Aggregation Fractal Dimension (D_f)	2.2-2.4	N/A	Indicates branched chain structures
DND Z- Aggregation Fractal Dimension (D_f)	1.5-1.7	N/A	Indicates linear, less branched chains
Copolymer Equivalent Weight (EW)	890	g-eq/mol	Short side chain Aquivion^®-type PFSA
Pristine Water Content (W₀)	37.0	wt.%	Maximum equilibrium water content

Key Methodologies

The composite membranes were prepared using a casting and thermal stabilization process, followed by structural and electrochemical characterization.

DND Synthesis and Functionalization:
- Detonation nanodiamonds (DNDs) were produced (UDD-STP) and purified chemically (HF/HCl etching).
- DND Z+ (Hydrophilic): Purified powder annealed in H₂ flow at 600 °C for 3 h to graft H and OH groups, followed by ultrasonic dispersion and centrifugation.
- DND Z- (Carboxyl): Purified powder annealed in air at 430 °C for 6 h to graft COOH groups.
- DND-F (Hydrophobic): Modified via interaction with molecular fluorine at 450 °C, resulting in 97% hydrogen substitution with fluorine.
Copolymer Preparation:
- Short side chain PFSA Aquivion^®-type copolymer (EW = 890 g-eq/mol) was synthesized via aqueous emulsion copolymerization.
- Hydrolysis converted the precursor copolymer powder from -SO₂F form to -SO₃Li form (5% LiOH solution at 90 °C).
- A 2 wt.% dispersion of the copolymer in dimethylformamide (DMF) was prepared using ultrasonic treatment.
Composite Membrane Fabrication:
- DND dispersions (0.25-5.0 wt.%) were mixed with the copolymer dispersion in DMF using mechanical stirring and 10 min ultrasonic treatment.
- Membranes were cast onto glass substrates and dried in an air chamber at 70-72 °C for 5 h.
- Structural Stabilization: Membranes were annealed at 150 °C.
- Acid Conversion: Membranes were converted to the final proton-conducting -SO₃H form using 15 wt.% HNO₃ solution, followed by washing in distilled water.
Electrochemical and Structural Analysis:
- Proton Conductivity: Measured using Impedance Spectroscopy (Z-3000X, 4-electrode circuit) at 20 °C and 50 °C after saturation (boiling in water for 1 h).
- Water Content (W): Determined by comparing wet mass (M_ws) and dry mass (M_DR) after vacuum drying at 80 °C.
- Structure: Small-Angle Neutron Scattering (SANS) performed on dry films (YuMO spectrometer, JINR) to analyze scattering cross sections (σ(q)) and determine ionomer peak characteristics (A_int, q_m, Γ) and fractal dimensions (D_f).

Commercial Applications

The successful modification of PFSA membranes using functionalized nanodiamonds offers significant advantages for high-performance electrochemical systems.

Proton Exchange Membrane Fuel Cells (PEMFCs):
- Enhanced Performance: DND Z+ modification provides a stable 20-30% increase in proton conductivity at elevated temperatures (50 °C), crucial for improving PEMFC efficiency.
- Improved Water Management: The hybrid diamond-polymer interface enhances the membrane’s ability to retain water, addressing the critical issue of dehydration in PEMFCs operating at high temperatures and low humidity.
Advanced Energy Storage:
- Redox Flow Batteries (RFBs): The modified membranes offer high ion conductivity and improved mechanical stability, making them ideal solid electrolytes for large-scale energy storage applications.
High-Durability Electrochemical Devices:
- Chemical and Thermal Stability: Utilizing chemically inert and thermally stable nanodiamonds strengthens the membrane matrix, increasing its resistance to oxidative stress and degradation common in long-term operation.
- Reduced Fuel Crossover: The stabilized and regularized network of ion channels, particularly with DND Z+, helps minimize hydrogen crossover, enhancing the selectivity and lifespan of fuel cells.
Specialized Filtration/Separation:
- Radiation-Tolerant Materials: DNDs impart radiation resistance, suggesting potential applications in filtration or separation membranes used in nuclear environments or high-energy physics facilities.

View Original Abstract

In this study, we aimed to design and research proton-conducting membranes based on Aquivion®-type material that had been modified with detonation nanodiamonds (particle size 4-5 nm, 0.25-5.0 wt. %). These nanodiamonds carried different functional groups (H, OH, COOH, F) that provided the hydrophilicity of the diamond surface with positive or negative potential, or that strengthened the hydrophobicity of the diamonds. These variations in diamond properties allowed us to find ways to improve the composite structure so as to achieve better ion conductivity. For this purpose, we prepared three series of membrane films by first casting solutions of perfluorinated Aquivion®-type copolymers with short side chains mixed with diamonds dispersed on solid substrates. Then, we removed the solvent and the membranes were structurally stabilized during thermal treatment and transformed into their final form with -SO3H ionic groups. We found that the diamonds with a hydrogen-saturated surface, with a positive charge in aqueous media, contributed to the increase in proton conductivity of membranes to a greater rate. Meanwhile, a more developed conducting diamond-copolymer interface was formed due to electrostatic attraction to the sulfonic acid groups of the copolymer than in the case of diamonds grafted with negatively charged carboxyls, similar to sulfonic groups of the copolymer. The modification of membranes with fluorinated diamonds led to a 5-fold decrease in the conductivity of the composite, even when only a fraction of diamonds of 1 wt. % were used, which was explained by the disruption in the connectivity of ion channels during the interaction of such diamonds mainly with fluorocarbon chains of the copolymer. We discussed the specifics of the mechanism of conductivity in composites with various diamonds in connection with structural data obtained in neutron scattering experiments on dry membranes, as well as ideas about the formation of cylindrical micelles with central ion channels and shells composed of hydrophobic copolymer chains. Finally, the characteristics of the network of ion channels in the composites were found depending on the type and amount of introduced diamonds, and correlations between the structure and conductivity of the membranes were established.

Tech Support

Original Source

DOI: https://doi.org/10.3390/membranes13110850

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