Battery‐like Supercapacitors from Vertically Aligned Carbon Nanofiber Coated Diamond - Design and Demonstrator

At a Glance

Metadata	Details
Publication Date	2018-01-17
Journal	Advanced Energy Materials
Authors	Siyu Yu, Nianjun Yang, Michael Vogel, Soumen Mandal, Oliver A. Williams
Institutions	Institute of Metals Research, Cardiff University
Citations	87
Analysis	Full AI Review Included

Technical Documentation and Analysis: High-Performance Battery-like Supercapacitors using CNFs/BDD Hybrid Films

This document analyzes the research detailing the fabrication and performance of vertically aligned Carbon Nanofibers (CNFs) grown on Boron-Doped Diamond (BDD) films for high-performance supercapacitor applications. This hybrid material demonstrates high energy and power densities, competitive with commercial batteries, while utilizing 6CCVD’s core capability: advanced MPCVD diamond substrates.

Executive Summary

The reported research establishes Boron-Doped Diamond (BDD) as a superior, binder-free substrate for high-performance electrochemical capacitors (ECs) by leveraging a hybrid architecture with vertically aligned Carbon Nanofibers (CNFs).

Novel Architecture: Binder-free CNFs were grown directly on highly conductive BDD via Thermal Chemical Vapor Deposition (TCVD), eliminating the stability and conductivity issues associated with traditional organic binders.
Battery-like Performance: The resulting ECs (both Electric Double Layer Capacitors - EDLCs, and Pseudocapacitors - PCs) exhibited characteristics bridging the gap between traditional supercapacitors (high power) and batteries (high energy).
Exceptional Energy Density: The CNFs/BDD Pseudocapacitor achieved a maximal energy density (E) of 44.1 Wh kg^-1, significantly surpassing many reported CNF-based devices and competing with market-available batteries.
High Power Density: The EDLC device achieved a robust maximal power density (P) of 27.3 kW kg^-1, demonstrating excellent fast charge/discharge capabilities suitable for industrial power systems.
Extreme Stability: Both EDLC and PC devices maintained constant capacitance even after 10,000 cycles, confirming the long-term chemical and physical stability derived from the robust C-C covalent bonding between the CNFs and the BDD substrate.
Tunable Properties: Capacitance magnitude was shown to be proportional to CNF length (surface area), offering a straightforward path for performance optimization simply by adjusting the TCVD growth time.

Technical Specifications

Key performance metrics and material parameters extracted from the fabrication and testing of the CNFs/BDD hybrid films:

Parameter	Value	Unit	Context
Max PC Energy Density (E)	44.1	Wh kg^-1	Redox Electrolyte (1.0 M Na₂SO₄ + 0.05 M Fe(CN)₆^3-/4-)
Max EDLC Energy Density (E)	22.9	Wh kg^-1	Inert Electrolyte (1.0 M H₂SO₄)
Max EDLC Power Density (P)	27.3	kW kg^-1	Inert Electrolyte (1.0 M H₂SO₄)
Max PC Capacitance	232.0	mF cm^-2	Three-electrode system, 2 mA cm^-2 current density
Optimized Two-Electrode PC Capacitance	48.1	mF cm^-2	Symmetrical device, 10 mV s^-1 scan rate
Optimized Two-Electrode EDLC Capacitance	30.4	mF cm^-2	Symmetrical device, 10 mV s^-1 scan rate
Capacitance Retention	Unchanged	%	After 10,000 charge/discharge cycles
Electrode Area (Geometric)	0.05	cm²	Two-electrode measurement setup
CNF Thickness (Optimized)	3.6	µm	60 min CNF growth time (t_Cu,s=60 s)
Electrolyte Conductivity (H₂SO₄)	1000	mS cm^-1	High conductivity electrolyte
Electrolyte Conductivity (Na₂SO₄)	80	mS cm^-1	Lower conductivity comparison

Key Methodologies

The CNFs/BDD hybrid electrodes were synthesized using a multi-step CVD and PVD process leveraging diamond substrates.

BDD Substrate Preparation (MWCVD):
- Boron-Doped Diamond (BDD) films were grown onto silicon wafers using the Microwave Plasma Assisted Chemical Vapor Deposition (MWCVD) technique.
Catalyst Deposition (PVD):
- High-purity Copper (Cu, 99.999%) thin films were sputtered onto the BDD surface using RF magnetron Physical Vapor Deposition (PVD).
- Pre-Sputtering: 10 minutes (closed shutter) to clean the target.
- Atmosphere: Argon atmosphere.
- Ar Flow Rate: 50 sccm.
- Pressure (Sputtering): 3.5 - 4.5 x 10^-3 mbar.
- Sputtering Time (t_Cu,s): Varied (15s to 120s); 60s provided optimal CNF structure.
CNF Growth (TCVD):
- Copper-coated BDD films were placed into a quartz tube for Thermal Chemical Vapor Deposition (TCVD).
- Initial Heating: Heated to 250 °C at a rate of 5 °C min^-1.
- Reaction Gas: Acetylene (C₂H₂) was used as the carbon source.
- Growth Pressure: 500 mbar (C₂H₂ reaction gas).
- Catalyst Role: Sputtered Cu nano-films acted as the catalyst for vertical alignment.
Carbonization/Annealing:
- Post-growth, the reactor was rapidly evacuated to below 5 x 10^-2 mbar.
- Carbonization Temperature: 800 °C.
- Carbonization Time: 60 min.
Wet-Chemical Treatment (Wettability Enhancement):
- To improve surface wettability (changing contact angle from 110.4° to 24°), the CNFs/BDD films were immersed for 30 minutes in a mixture of H₂SO₄ and HNO₃ (v/v = 3:1), followed by cleaning and N₂ drying.

6CCVD Solutions & Capabilities: Enabling Next-Generation Diamond Electrodes

This research validates the use of Boron-Doped Diamond (BDD) as the foundational material for creating highly stable, high-performance hybrid energy storage devices. 6CCVD is uniquely positioned to supply and enhance the diamond materials required to replicate or scale this breakthrough for industrial or academic implementation.

Applicable Materials: Boron-Doped Diamond (BDD) Substrates

The high conductivity and chemical inertness of the BDD substrate are crucial for the long-term cycling stability and performance demonstrated in this paper.

6CCVD Material	Specifications & Relevance
Heavy Boron Doped PCD	Ideal conductive substrate for CNF growth. We supply highly conductive polycrystalline diamond (PCD) wafers up to 125mm in diameter, suitable for large-area production scale-up.
Thick BDD Plates	The paper utilized BDD films (µm range). 6CCVD provides custom BDD substrates up to 500µm in thickness, offering enhanced structural integrity and thermal management for demanding industrial devices.
Polishing Control	We offer ultra-smooth polishing down to Ra < 5nm for inch-size PCD, enabling precise control over the initial BDD surface morphology, which the research showed directly influences CNF alignment and density.

Customization Potential: Optimizing Hybrid Structures

The CNF growth requires a highly controlled catalyst layer (copper) to achieve vertical alignment and optimal surface area. 6CCVD’s in-house metalization capabilities directly support this requirement.

Custom Metalization Services: While the paper used PVD sputtering for Cu, 6CCVD provides advanced metalization services, including high-purity deposition of Cu, Ti, W, Au, Pt, and Pd. We can deposit the precise catalyst thickness needed (e.g., matching the optimal 60s sputtering time equivalent) on your BDD wafers, ready for subsequent CNF growth.
Custom Dimensions and Etching: The electrodes used were 0.05 cm². 6CCVD offers high-precision laser cutting and shaping services. We can supply BDD substrates pre-cut or etched to complex geometries and custom dimensions (up to 125mm) required for specific device fabrication processes.
Thickness Scalability: The research indicated performance scales with CNF length (thickness). 6CCVD provides BDD substrates with thickness uniformity suitable for advanced material growth techniques like TCVD and MWCVD, ensuring reproducible results even at high aspect ratios (e.g., CNFs reaching 8.0 µm or longer).

Engineering Support

6CCVD’s in-house PhD materials science team specializes in electrochemical diamond applications, including sensing, water treatment, and advanced energy storage.

Consultation for Energy Storage: Our team can assist researchers and engineers in selecting the optimal BDD properties (boron doping level, crystallographic orientation, surface finish/roughness) required to maximize charge transfer efficiency and cycling stability for hybrid supercapacitor or battery development projects.
Scale-Up Expertise: We offer guidance on transitioning laboratory-scale CNFs/BDD production to larger, industrial-grade platforms using our extensive inventory of large-area PCD and SCD wafers.

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

View Original Abstract

Abstract To fabricate battery‐like supercapacitors with high power and energy densities, big capacitances, as well as long‐term capacitance retention, vertically aligned carbon nanofibers (CNFs) grown on boron doped diamond (BDD) films are employed as the capacitor electrodes. They possess large surface areas, high conductivity, high stability, and importantly are free of binder. The large surface areas result from their porous structures. The containment of graphene layers and copper metal catalysts inside CNFs leads to their high conductivity. Both electrical double layer capacitors (EDLCs) in inert solutions and pseudocapacitors (PCs) using Fe(CN) 6 3−/4− redox‐active electrolytes are constructed with three‐ and two‐electrode systems. The assembled two‐electrode symmetrical supercapacitor devices exhibit capacitances of 30 and 48 mF cm −2 at 10 mV s −1 for EDLC and PC devices, respectively. They remain constant even after 10 000 charging/discharging cycles. The power densities are 27.3 and 25.3 kW kg −1 for EDLC and PC devices, together with their energy densities of 22.9 and 44.1 W h kg −1 , respectively. The performance of these devices is superior to most of the reported supercapacitors and batteries. Vertically aligned CNF/BDD hybrid films are thus useful to construct high‐performance battery‐like and industry‐orientated supercapacitors for future power devices.

Tech Support

Original Source

DOI: https://doi.org/10.1002/aenm.201702947