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, Russian Scientific Center “Applied Chemistry”
Citations	1
Analysis	Full AI Review Included

Technical Documentation & Analysis: Nanodiamond-Modified Proton Exchange Membranes

Executive Summary

This analysis focuses on the successful modification of Aquivion®-type Perfluorosulfonic Acid (PFSA) membranes using detonation nanodiamonds (DNDs) to enhance proton conductivity for advanced electrochemical applications, such as fuel cells.

Core Achievement: Modification with positively charged, hydrogen-saturated nanodiamonds (DND Z+) increased proton conductivity by up to 30% at 50 °C compared to the pristine copolymer matrix.
Mechanism Validation: Small-Angle Neutron Scattering (SANS) confirmed that DND Z+ particles integrate effectively with the negatively charged sulfonic acid groups (-SO₃H) of the copolymer via electrostatic attraction, stabilizing the ion channel network.
Material Specificity: The positive effect is highly dependent on diamond surface functionalization; negatively charged (DND Z-) or hydrophobic (DND-F) diamonds either caused minimal gain or resulted in a severe 4-fold decrease in conductivity by disrupting the ion channel structure.
Structural Insight: DND Z+ modification led to the formation of hybrid conductive channels, increasing the total volume fraction of channels and enhancing water adsorption capacity.
Application Relevance: These findings demonstrate a viable pathway for engineering high-performance, structurally stable Proton Exchange Membranes (PEMs) suitable for operation at elevated temperatures (up to 50 °C tested), crucial for next-generation fuel cell technology.
6CCVD Value Proposition: 6CCVD provides the high-purity, customizable Single Crystal Diamond (SCD) and Polycrystalline Diamond (PCD) substrates necessary for developing, testing, and scaling up advanced diamond-based electrochemical materials, including Boron-Doped Diamond (BDD) electrodes and thermal management solutions for fuel cell stacks.

Technical Specifications

The following hard data points were extracted from the research concerning the composite membrane performance and material properties:

Parameter	Value	Unit	Context
Maximum Conductivity Increase (DND Z+)	30	%	At 0.5 wt. % filler, 50 °C
Pristine Membrane Conductivity (κ₀)	0.131 ± 0.002	S/cm	At 20 °C
DND Particle Size (dp)	4-5	nm	Detonation Nanodiamonds (DND)
DND Concentration Range (C)	0.25-5.0	wt. %	Used for modification
Membrane Thickness	~50	µm	Final film thickness
Copolymer Equivalent Weight (EW)	890	g-eq/mol	Aquivion®-type PFSA
Ionomer Peak Position (q*)	~2	nm^-1	Corresponds to channel packing period
Channel Packing Period (L_c)	~3	nm	In pristine Aquivion®-type copolymer
DND Z+ Surface Potential	Positive	mV	In aqueous media (~30-70 mV)
DND Z- Surface Potential	Negative	mV	In aqueous media (~30-70 mV)
DND Z+ Water Shell Thickness (d)	2.8	nm	Estimated around 4.5 nm particle

Key Methodologies

The experiment relied on precise material synthesis and advanced structural characterization techniques to correlate nanodiamond surface chemistry with membrane performance.

DND Synthesis and Purification:
- Detonation synthesis (UDD-STP) was used to produce nanodiamonds (4-5 nm).
- Chemical purification involved etching in HF and HCl acids, followed by annealing and ultrasonic dispersion for deagglomeration.
Surface Functionalization:
- DND Z+ (Positive): Hydrogenation via annealing in H₂ flow at 600 °C for 3 h to graft H and OH groups.
- DND Z- (Negative): Annealing in air at 430 °C for 6 h to graft COOH groups.
- DND-F (Hydrophobic): Modification with molecular fluorine at 450 °C, substituting 97% of hydrogen atoms with fluorine.
Copolymer Synthesis and Membrane Casting:
- Short side chain PFSA Aquivion®-type copolymer (EW 890 g-eq/mol) was synthesized via aqueous emulsion copolymerization.
- Composite membranes were prepared by mixing the copolymer dispersion (in -SO₃Li form) with DND dispersions in dimethylformamide (DMF).
- Films were cast on glass substrates, solvent removed (70-72 °C), followed by thermal stabilization (annealing at 150 °C), and final conversion to the active -SO₃H form using 15 wt. % HNO₃ solution.
Electrochemical and Structural Characterization:
- Proton Conductivity: Measured via impedance spectroscopy (10-150,000 Hz) using a 4-electrode circuit on water-saturated samples (boiled at 100 °C for 1 h) at temperatures ranging from 20 °C to 50 °C.
- Structure: Small-Angle Neutron Scattering (SANS) was performed on dry films to analyze the ionomer peak, channel packing, and fractal dimension of diamond aggregates.
- Ancillary Methods: FTIR, Dynamic Light Scattering (DLS), TEM, SEM, and AFM were used to confirm particle morphology and surface chemistry.

6CCVD Solutions & Capabilities

6CCVD is uniquely positioned to support the next phase of research and commercialization for advanced diamond-based electrochemical materials, offering high-purity MPCVD diamond substrates and customization services essential for scaling and integration.

Applicable Materials for Electrochemical Systems

While this research utilized detonation nanodiamonds (DNDs) as a filler, the core requirement is high-purity, functionalizable diamond material. 6CCVD’s expertise in MPCVD diamond provides superior platforms for related electrochemical and thermal management applications:

Boron-Doped Diamond (BDD): Highly recommended for advanced electrochemical applications. BDD is a robust, conductive material ideal for use as electrodes or current collectors in fuel cells and redox flow batteries, offering exceptional chemical inertness and stability, surpassing the performance of traditional carbon materials.
Optical Grade Single Crystal Diamond (SCD): Available in thicknesses from 0.1 µm to 500 µm. SCD offers the highest purity and thermal conductivity (up to 2200 W/mK), making it ideal for thermal management layers in high-power fuel cell stacks where heat dissipation is critical for maintaining optimal operating temperatures (like the 50 °C tested here).
Polycrystalline Diamond (PCD): Available in large formats (up to 125 mm plates) and thicknesses up to 500 µm. PCD provides a cost-effective, large-area substrate for scaling up membrane research or serving as a robust, chemically resistant support layer for composite PEMs.

Customization Potential for PEM Integration

The successful integration of advanced PEMs into commercial devices requires precise dimensional control and robust interfaces, areas where 6CCVD excels:

Capability	Relevance to PEM/Fuel Cell Research	6CCVD Specification
Custom Dimensions	Scaling up membrane testing and device integration.	Plates/wafers up to 125 mm (PCD).
Custom Thickness	Optimizing membrane support or BDD electrode performance.	SCD/PCD from 0.1 µm to 500 µm. Substrates up to 10 mm.
Advanced Polishing	Ensuring smooth interfaces for thin-film deposition and membrane casting.	Ra < 1 nm (SCD), Ra < 5 nm (Inch-size PCD).
Custom Metalization	Creating robust electrical contacts and current collectors for fuel cell stacks.	Internal capability for Au, Pt, Pd, Ti, W, Cu.
Functionalization Support	Assisting researchers in developing custom surface chemistries (like the DND Z+ modification) on MPCVD diamond surfaces for targeted integration.	In-house expertise in surface modification and doping (BDD).

Engineering Support

6CCVD’s in-house team of PhD material scientists can assist researchers and engineers in selecting the optimal diamond material (SCD, PCD, or BDD) and customization parameters required to replicate or extend this research, particularly for projects focused on:

High-Stability Electrochemical Systems: Utilizing BDD for superior electrode performance in acidic environments typical of PFSA membranes.
Thermal Management: Integrating high-thermal-conductivity SCD/PCD layers to manage heat generated in high-current density fuel cells.
Scaling and Manufacturing: Providing large-area PCD substrates for pilot-scale production of advanced composite membranes.

Call to Action: For custom specifications or material consultation regarding diamond substrates, BDD electrodes, or thermal management solutions for similar Proton Exchange Membrane and Fuel Cell projects, visit 6ccvd.com or contact our engineering team directly. We offer global shipping (DDU default, DDP available) to ensure timely delivery of your specialized diamond materials.

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

References

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