Diamond Supercapacitors - Towards Durable, Safe, and Biocompatible Aqueous-Based Energy Storage

At a Glance

Metadata	Details
Publication Date	2022-05-20
Journal	Frontiers in Chemistry
Authors	Andre Chambers, Steven Prawer, Arman Ahnood, Hualin Zhan
Institutions	RMIT University, Australian National University
Citations	13
Analysis	Full AI Review Included

Technical Documentation & Analysis: Diamond Supercapacitors for Bioelectronics

Executive Summary

This perspective highlights the critical role of MPCVD diamond, specifically Boron-Doped Diamond (BDD) and Nanocrystalline Diamond (NCD), in developing next-generation aqueous-based supercapacitors for durable and safe energy storage, particularly in biomedical and implantable devices.

Core Value Proposition: Diamond electrodes offer an exceptionally wide electrochemical potential window (up to 3.5 V in aqueous electrolytes, potentially 5 V with Fluorine termination), significantly exceeding conventional carbon materials (typically 1.2 V).
Biomedical Suitability: Diamond is durable, biocompatible, resistant to biofouling, and stable in corrosive physiological solutions, making it ideal for chronic implantable bioelectronic devices (biosensors, stimulators).
Material Requirements: Optimal performance relies on highly conductive BDD or NCD/UNCD materials, engineered with high specific surface area (SSA) through porous morphology (nanostructuring, etching, or overgrowth).
Optimization Focus: Device performance (Energy/Power density) is directly proportional to the square of the potential window. Optimization requires precise control over surface chemistry (Oxygen termination for capacitance, Fluorine termination for maximum voltage window) and pore geometry.
6CCVD Advantage: 6CCVD provides the necessary high-quality, custom-dimensioned BDD and PCD/NCD wafers and plates, serving as the foundational material for the complex nanostructuring and surface modification techniques required by this research.
Application Niche: While current diamond EDLCs may not match the highest energy density of transition metal oxide PCs, their superior stability, durability, and wide voltage window make them the best choice for long-term, low-energy, high-power applications like chronic biosensing.

Technical Specifications

The following hard data points were extracted from the analysis of diamond-based supercapacitors, highlighting key performance metrics and material properties.

Parameter	Value	Unit	Context
Electrochemical Potential Window (Aqueous)	Up to 3.5	V	Boron-Doped Diamond (BDD)
Electrochemical Potential Window (Optimized)	Up to 5	V	Fluorine-terminated diamond in aqueous solution
Gravimetric Capacitance (BDD EDLC)	13.7	F g^-1	BDD network, 3 µm film
Areal Capacitance (N-UNCD EDLC)	28.5	mF cm^-2	N-UNCD, 32 µm film, surface etched
Gravimetric Capacitance (PAni:BDD:CF PC)	527	F g^-1	Pseudocapacitor composite (PAni/BDD/Carbon Fiber)
Target Power (Biomedical Standby)	1 - 100	µW	Typical requirement for implantable sensors
Target Power (Biomedical Active)	Up to 30	mW	Typical requirement for implantable stimulators
SCD/PCD Thickness Range (6CCVD Capability)	0.1 µm - 500	µm	Standard SCD/PCD films
Substrate Thickness Range (6CCVD Capability)	Up to 10	mm	SCD/PCD Substrates

Key Methodologies

The research emphasizes that achieving high performance in diamond supercapacitors requires precise control over surface morphology and chemistry, typically achieved through post-growth processing of conductive diamond films.

Porous Surface Creation (Morphology Optimization):
- Top-Down Etching: Utilizing hard masks (metal/metal oxide) followed by plasma etching, RIE, or metal particle etching to create rough or nanoporous surfaces.
- Bottom-Up Overgrowth: Growing diamond over nanostructured templates (e.g., silicon nanowires, porous silicon, anodized aluminum oxide (AAO)) to control pore size and shape.
- Annealing: Applying high temperature in vacuum, air, or steam to etch graphitic grain boundaries, resulting in roughened diamond or onion-like carbon structures.
- Selective Etching: Growing diamond within a sacrificial matrix (e.g., silicon carbide) and subsequently removing the non-diamond material via wet chemical etch.
Surface Chemistry Tailoring (Potential Window & Wettability):
- Oxygen Termination (O-termination): Achieved via oxygen plasma or chemical treatment. Results in lower band bending, reduced Faradaic reaction rate, greater capacitive behavior, and enhanced hydrophilicity/biocompatibility.
- Hydrogen Termination (H-termination): Causes upward band bending, increasing Faradaic reaction rate and surface hole conductivity, but exhibits lower biocompatibility (hydrophobic).
- Fluorine Termination (F-termination): Demonstrated to achieve the widest potential window (up to 5 V) in aqueous solutions, crucial for maximizing energy and power density (E and P are proportional to V²).
Composite Fabrication (Pseudocapacitors - PCs):
- Combining conductive diamond (as a rigid, stable scaffold) with conventional PC materials (e.g., conducting polymers like PAni, PPy, PEDOT, or transition metal oxides like MnO₂, RuO₂). This enhances the cyclic and environmental stability of the PC material.

6CCVD Solutions & Capabilities

6CCVD is uniquely positioned to supply the foundational MPCVD diamond materials and specialized processing required to replicate and advance the research presented in this paper, particularly for high-reliability biomedical applications.

Applicable Materials

To achieve the high conductivity and stability required for both EDLCs and PC scaffolds, 6CCVD recommends the following materials:

Boron-Doped Diamond (BDD): Essential for high conductivity and wide electrochemical window in aqueous electrolytes (up to 3.5 V). 6CCVD supplies BDD films and substrates tailored for electrochemical applications.
Polycrystalline Diamond (PCD) / Nanocrystalline Diamond (NCD): Ideal for creating the high specific surface area (SSA) and porous morphologies required for high capacitance. 6CCVD can supply PCD wafers up to 125 mm in diameter, suitable for large-scale nanostructuring via etching or templated growth.
Single Crystal Diamond (SCD) Substrates: Available up to 10 mm thickness for applications requiring ultra-high purity, low defect density, or use as robust, thick electrode supports.

Customization Potential

The research emphasizes the need for precise control over film thickness, surface morphology, and integration with current collectors. 6CCVD offers comprehensive customization services:

Custom Dimensions and Thickness: We provide SCD and PCD films in the critical thickness range (0.1 µm to 500 µm) cited in the research, necessary for optimizing mass loading and ion transport dynamics. Custom plates/wafers are available up to 125 mm (PCD).
Advanced Polishing: For applications requiring high-quality interfaces or integration with other materials (e.g., composite fabrication), 6CCVD offers ultra-smooth polishing down to Ra < 1 nm (SCD) and Ra < 5 nm (PCD), ensuring optimal adhesion and minimal parasitic resistance.
Integrated Metalization: The fabrication of supercapacitors requires robust current collectors. 6CCVD offers in-house metalization services, including deposition of Ti, Pt, Au, Pd, W, and Cu, allowing researchers to receive pre-metalized diamond electrodes ready for subsequent nanostructuring or device assembly.
Laser Cutting and Shaping: We provide custom laser cutting and shaping services to meet the specific small-scale and planar device geometries required for micro-supercapacitors and implantable bioelectronics.

Engineering Support

The optimization of diamond supercapacitors is highly dependent on controlling the electrode-electrolyte interface, a complex challenge involving surface chemistry and computational modeling.

Surface Chemistry Consultation: 6CCVD’s in-house PhD team specializes in MPCVD growth and post-processing techniques. We offer expert consultation on achieving specific surface terminations (e.g., optimizing for O-termination for capacitance or F-termination for maximum potential window) necessary for chronic biosensing and stimulation projects.
Material Selection for Composites: We assist engineers in selecting the optimal diamond scaffold (BDD vs. PCD) and thickness for integration with pseudocapacitive materials (polymers, metal oxides), ensuring the final composite benefits from diamond’s superior stability and durability.

For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly.

View Original Abstract

Durable and safe energy storage is required for the next generation of miniature bioelectronic devices, in which aqueous electrolytes are preferred due to the advantages in safety, low cost, and high conductivity. While rechargeable aqueous batteries are among the primary choices with relatively low power requirements, their lifetime is generally limited to a few thousand charging/discharging cycles as the electrode material can degrade due to electrochemical reactions. Electrical double layer capacitors (EDLCs) possess increased cycling stability and power density, although with as-yet lower energy density, due to quick electrical adsorption and desorption of ions without involving chemical reactions. However, in aqueous solution, chemical reactions which cause electrode degradation and produce hazardous species can occur when the voltage is increased beyond its operation window to improve the energy density. Diamond is a durable and biocompatible electrode material for supercapacitors, while at the same time provides a larger voltage window in biological environments. For applications requiring higher energy density, diamond-based pseudocapacitors (PCs) have also been developed, which combine EDLCs with fast electrochemical reactions. Here we inspect the properties of diamond-related materials and discuss their advantages and disadvantages when used as EDLC and PC materials. We argue that further optimization of the diamond surface chemistry and morphology, guided by computational modelling of the interface, can lead to supercapacitors with enhanced performance. We envisage that such diamond-based supercapacitors could be used in a wide range of applications and in particular those requiring high performance in biomedical applications.

Tech Support

Original Source

DOI: https://doi.org/10.3389/fchem.2022.924127

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