Review: carbon onions for electrochemical energy storage

Abstract

Carbon onions are a relatively new member of the carbon nanomaterials family. They consist of multiple concentric fullerene-like carbon shells which are highly defective and disordered. Due to their small size of typically below 10 nm, the large external surface area, and high conductivity they are used for supercapacitor applications. As electrode materials, carbon onions provide fast charge/discharge rates resulting in high specific power but present comparatively low specific energy. They improve the performance of activated carbon electrodes as conductive additives and show suitable properties as substrates for redox-active materials. This review provides a critical discussion of the electrochemical properties of different types of carbon onions as electrode materials. It also compares the general advantages and disadvantages of different carbon onion synthesis methods. The physical and chemical properties of carbon onions, in particular nanodiamond-derived carbon onions, are described with emphasis on those parameters especially important for electrochemical energy storage systems, including the structure, conductivity, and porosity. Although the primary focus of current research is on electrode materials for supercapacitors, the use of carbon onions as conductive additives and for redox-active species is also discussed.

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Marco Zeiger

Marco Zeiger studied Microfabrication and Nanostructures in the Department of Physics and Mechatronics at Saarland University, Germany. Between 2011 and 2013 he worked in the Metallic Microstructures Group of the INM – Leibniz Institute for New Material on the antibacterial properties of copper and its application in touch surfaces. In 2013 he joined the INM Energy Materials Group working on the synthesis and characterization of nanodiamonds and carbon onions with the focus on electrochemical energy storage. Receiving his M.Sc., he began his PhD studies in 2014 on the development of carbon/metal oxide hybrid materials for redox-enabled energy storage devices.

Nicolas Jäckel

Nicolas Jäckel studied Microfabrication and Nanostructures in the Department of Physics and Mechatronics at Saarland University, Germany. In 2012 he completed his Bachelors thesis on gas sensors at Drägerwerk AG & Co. KGaA, Lübeck. In 2013, he joined the INM Energy Materials Group working on atomic layer deposition and advanced supercapacitors. In 2015 he received his M.Sc. and started his PhD studies. In collaboration with the Group of Prof. Doron Aurbach, Bar-Ilan University, Israel, he is working on the in situ electrochemical methodology for tracking of charging and related mechanical processes in batteries and supercapacitors.

Vadym N. Mochalin

Vadym N. Mochalin received his PhD in Physical Chemistry from L. M. Litvinenko Institute of Physical Organic and Coal Chemistry, National Academy of Sciences of Ukraine, and M.S. in Biochemistry from Donetsk National University, Ukraine. In 2005–2015 he worked with Prof. Yury Gogotsi at Drexel University, leading the research on nanodiamonds. He is now Associate Professor in Chemistry at Missouri University of Science and Technology. His current research interests include synthesis, characterization, purification, chemical modification, computational modeling, and applications of nanodiamonds, MXene, nanoonions, nanocarbons, and other materials for composites, energy storage, biomedical applications, and extreme environments.

Volker Presser

Volker Presser obtained his PhD in Applied Mineralogy in 2006 from the Eberhard Karls University in Tübingen, Germany. As a Humboldt Research Fellow and Research Assistant Professor, he worked between 2010 and 2012 at the A.J. Drexel Nanotechnology Institute in the team of Yury Gogotsi at Drexel University, Philadelphia, USA. As Chair for Energy Materials, he is Full Professor at the Department of Materials Science and Engineering at Saarland University and Group Leader at the INM – Leibniz Institute for New Materials in Saarbrücken, Germany. His research activities include nanocarbon and hybrid nanomaterials for electrochemical energy storage, harvesting, and water desalination.

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1. Introduction

1.1 Electrochemical energy storage with nanocarbons

Carbon materials have always been at the focal point of energy applications, ranging from the ongoing use of coal as the primary energy source to the utilization as the standard anode material for lithium ion batteries. The emergence of new carbon nanoforms, foremost carbon nanotubes and graphene, has propelled research related to electrochemical energy storage and conversion, leading to what has been coined the “carbon new age”.¹ Among all carbon nanoforms and allotropes, carbon onions are one of the most interesting. Carbon onions, also called onion-like carbon or carbon nano-onions, are nanoscopic carbon particles, with a nearly spherical shape made of multiple enclosed fullerene-like carbon shells. The unique combination of high electrical conductivity (comparable to carbon black), large external surface area, and nanoscopic size (commonly below 10 nm), alongside the possibility for large-scale synthesis and chemical modification,² has made this material very attractive for applications, ranging from lubrication and catalysis³ to electrochemical energy storage (EES),⁴ biomedical imaging,² and water treatment.⁵ Carbon onions have been explored intensively for EES applications as electrode materials for ultrafast charge/discharge devices⁶ or as a potent conductive additive to enhance the power handling ability of activated carbon.⁷ With the increasing number of carbon onion studies for advanced electrochemical applications,^8,9 this review summarizes the state-of-the-art and provides a comprehensive overview of relevant topics of the structure as well as electrochemical and relevant properties of carbon onions.

Efficient and adaptable energy storage has emerged as a key enabling technology for the large scale utilization of renewable energy from sustainable (“green”) sources, such as solar or wind power.^10,11 Carbon nanomaterials and hybrids have been at the focal point of many electrochemical energy storage technologies,^12,13 as exemplified by supercapacitors¹⁴ or batteries.^15,16 The electrochemical energy storage mechanisms range from ion electrosorption and interfacial redox-reactions of the electrode material or bulk faradaic reactions of the electrolyte to ion intercalation into the anode or cathode.^12,17,18 The most prominent group of supercapacitors, the electrical double-layer capacitors (EDLCs), capitalizes on fast ion electrosorption to enable very high power handling.¹⁴ Energy is stored at the electrode/electrolyte interface where electric charges are accumulated on the electrode surfaces and ions of opposite charge are arranged on the electrolyte side of both electrodes. A high electrosorption rate and the absence of (electro)chemical reactions (faradaic reactions) enable very high specific power of EDLCs at the cost of a rather low specific energy compared to batteries.¹⁹ An approximate storage capacity for pure ion electrosorption is ∼0.1 F m⁻² = 0.03 mA h m⁻² at 1 V, when normalized to the surface area of a nanoporous carbon electrode.²⁰

Considering the limitations of ion electrosorption in stored energy, it is intriguing to utilize faradaic reactions to enhance the energy storage capacity.²¹ This can be achieved by adding redox-active materials, such as electroactive polymers, surface functional groups, transition metal oxides, or by using a redox-active electrolyte, such as the iodine/iodide redox couple.¹⁷ Depending on the electrochemical response, the resulting behavior may be classified as capacitor-like (pseudocapacitor) or battery-like (sometimes referred to as the supercabattery).^18,22 Operation of such hybrid cells, in contrast to EDLCs, involves the charge transfer between the electrode and electrolyte where, for example, ruthenium changes its oxidation state from Ru(III) to Ru(VI).^23–26 Carbon onions, featuring a fully available outer surface, are a highly attractive material for hybrid designs because of the facile access to the surface and the ease of functionalization with, for example, metal oxides or surface groups, to enable enhanced energy storage capacity.²⁷ For a more substantial review of the energy storage technologies, such as supercapacitors, batteries, and hybrid devices, the reader is referred to recent comprehensive articles on these topics.^14,18,28

1.2 What is a carbon onion?

Carbon onions can be defined by virtue of their structure as spherical or polyhedral carbon nanoparticles, often smaller than 10 nm, consisting of several fullerene-like carbon shells enclosed in a “Russian doll” manner which are defective and disordered to a certain degree.²⁹ This distinct multi-layer architecture is also the name-giving feature for carbon onions. First discovered by Iijima in 1980 and described by Ugarte in 1992,^30,31 carbon onions are a rather novel addition to the carbon materials family. Depending on their synthesis method and the synthesis parameters, the resulting carbon onions differ in size, chemical composition, phase, and morphology. The most commonly used synthesis method for carbon onions is graphitization of nanodiamonds at high temperatures (>1700 °C) in an inert gas or vacuum. The resulting carbon onions are typically smaller than 10 nm and present a high degree of carbon ordering. The amount of non-carbon material is very small (typically in the range of 1–2 mass%) and surface areas can reach up to 600 m² g⁻¹. Key structural characteristics of a carbon onion are summarized in Fig. 1, with more details about the synthesis methods given in the next section.³²

Fig. 1 Typical features of carbon onions.

1.3 Nomenclature

So far, there is no consistent nomenclature for carbon onions in the literature. Consistent nomenclature is a notorious issue throughout the entire field of nanocarbons, especially when it comes to rather new carbon structures.³³ Most reports, often interchangeably, use either “carbon onions”, “carbon nano-onions”, or “onion-like carbon”. All three are not in agreement with the nomenclature for sp²-hybridized carbon nanoforms by Suarez-Martinez, Grobert, and Ewels.²⁹ They suggest consistently calling carbons with an “onion” structure “multi-wall fullerene”. This unifying nomenclature employs purely geometrical classification, independent of the state of carbon ordering (defects), particle size and shape, or synthesis method. When considering very large structures referred to as carbon onions with size more than 100 nm and a high defect density, the conceptual description as “fullerene”, normally applied to molecules, may be arguable. On the other hand, onion-like carbon, which is an umbrella term for any onion-type structure is just as vague as other terms, like diamond-like carbon. Also, compared to carbon nanotubes, the differentiation between single-, double-, few-, and multi-wall architectures is not practical for carbon onions: almost all carbon onions qualify as multi-wall structures. Subtler differentiation in the name based on the shape ranging from spherical to polyhedral is also neither practical nor easily quantifiable. Based on considerations similar to those used in the nomenclature of carbon nanotubes, we suggest application of the term “carbon onions” only to those carbon particles that have at least 4 shells (like multi-wall carbon nanotubes), a size smaller than 100 nm, spherical or polyhedral shape, and a partially defective structure (amorphous domains, or islands of sp³-hybridized carbon). The distinct structural characteristic defining carbon onions should be the clearly visible multi-shell character. This nomenclature also includes hollow carbon onions (hollow core), and carbon onions with a core (e.g., metal clusters or residual nanodiamonds remaining inside the carbon onion due to incomplete transformation).

As with any other material, the issues related to nomenclature for carbon onions are much more difficult to address than in the nomenclature of molecules. The key reason for this, as the authors believe, is the undefined variable composition of a material, whereas the composition of molecules, by definition, is always fixed. Taking into consideration chemical modifications of materials, any attempts to use nomenclature in the sense it is applied to molecules, become meaningless. This necessitates alternative approaches to distinguish materials. A more practical differentiation is based on the type of precursor and/or the synthesis method. For example, clearly stating “nanodiamond-derived carbon onions” versus “arc-discharge carbon onions” unambiguously refers to a specific group of carbons.

Finally, the community may consider to adopt a size-dependent differentiation between carbon onions (in general) and carbon nano-onions, with the latter referring to carbon onions smaller than 10 nm. This suggestion is motivated by the differentiation between nanodiamonds and single-digit nanodiamonds, with the latter covering the range below 10 nm.³⁴ Yet, it is beyond the scope of this review to establish a clear nomenclature recommendation. Thus, in the absence of definite nomenclature, thorough and complete description of the experimental procedure and the obtained carbon onion structures becomes even more important to enable a scientifically useful basis for comparison.

2. Synthesis of carbon onions

2.1 Overview of synthesis methods

Different methods can be used for the synthesis of carbon onions, such as decomposition of carbon-containing precursors including combustion and detonation methods,^35–45 mechanical milling,^46,47 carbon ion implantation,^48,49 underwater arc discharge between graphite electrodes,^50–52 annealing of acetylene black in the presence of an iron catalyst,⁵³ or heating of a carbon filament in liquid alcohol.⁵⁴ Alternatively, carbon onions can also be derived via phase transformation of nanodiamonds by annealing in vacuum,^55–61 argon,^32,62–67 nitrogen,⁶⁸ hydrogen,⁶⁹ or helium,^27,70–75 plasma spraying of nanodiamonds,⁷⁶ laser irradiation of nanodiamonds in liquid alcohol,⁷⁷ electron beam irradiation of carbon materials,^30,78,79 or direct plasma treatment of coal.⁸⁰ The type of precursor and the synthesis conditions have a strong impact on the structure, as seen from transmission electron micrographs of different types of carbon onions given in Fig. 2, but all carbon onions share the multi-shell fullerene-like architecture.

Fig. 2 Transmission electron micrographs of carbon onions synthesized using different methods. (A) Hou et al., counterflow diffusion flame method (with permission from Elsevier);³⁹ (B) Choucair and Stride, flash pyrolysis of naphthalene (with permission from Elsevier);⁴⁰ (C) Gao et al., laser-assisted combustion process using C₂H₄ and O₂ (with permission from IOP Publishing);^41,81 (D) He et al., catalytic decomposition of C₂H₄ on Ni/Al (with permission from Elsevier);³⁷ (E) this work, annealing of nanodiamonds; (F) Zhao et al., carbonization of phenolic resin in the presence of ferric nitrate (with permission from Elsevier);⁴² (G) Bystrzejewski et al., thermolysis of a NaN₃–C₆Cl₆ system under argon or air atmosphere (with permission from Elsevier);⁴³ (H) Lian et al., annealing of acetylene black in the presence of ferric nitrate (with permission from Elsevier);⁵³ (I) Ugarte, electron-beam irradiation of amorphous carbon (with permission from Macmillan Publishers);³⁰ (J) Du et al., radio frequency plasma treatment of coal (with permission from Elsevier);⁸⁰ (K) Xiao et al., laser irradiation of nanodiamonds in liquid alcohol (with permission from American Chemical Society);⁷⁷ (L) Azhagan et al., burning ghee (with permission from The Royal Society of Chemistry);⁸⁶ (M) Sano et al., arc discharge between graphite rods in water (with permission from Macmillan Publishers);⁵² (N) Cabioc'h et al., carbon ion implantation in copper and silver (with permission from Elsevier);⁴⁸ (O) Chen et al., ball milling of graphite (with permission from Elsevier);⁴⁷ (P) Serin et al., chemical vapor deposition using BCl₃, C₂H₂, and H₂ (with permission from Elsevier).³⁶

In 1992, Daniel Ugarte observed the formation of carbon onions during electron beam irradiation of amorphous carbon particles in a transmission electron microscope (TEM).³⁰ Due to irradiation-stimulated graphitization, amorphous carbon can be transformed to nanometer-sized carbon onions with clearly visible nucleation centers (Fig. 2I). While scientifically intriguing, this method is limited in terms of the yield of synthesized carbon onions.³⁰

The production of carbon onions via decomposition of carbon-containing precursors like CH₄ in the presence of Ni/Al composite catalysts (Fig. 2D) can be accomplished at rather moderate temperatures of 600 °C in hydrogen.³⁷ The resulting carbon onions showed a hollow structure with several carbon shells and a diameter larger than 30 nm. The catalytic decomposition enabled the formation of carbon onions with a metallic core. Hou et al. used a counterflow diffusion flame method (Fig. 2A) to obtain carbon onions on a nickel catalyst.³⁹ By mixing ethylene, methane, nitrogen, and oxygen, it was possible to control the yield as well as the size of carbon onions.³⁹ These synthesis methods, together with boron-doped carbon onions from chemical vapor deposition (Fig. 2P),³⁶ the carbonization of phenolic resin with ferric nitrate (Fig. 2F),⁴² and carbon ion implantation in copper and silver (Fig. 2N),⁴⁸ present facile ways for the production of carbon onions, but may be disadvantageous due to a high amount of hetero-atoms in the product.

In a study by Choucair et al. (Fig. 2B), high purity carbon onions with more than 90 mass% carbon and no metal impurities were produced by a catalyst-free flash pyrolysis of naphthalene.⁴⁰ The particles of ca. 50 nm diameter were composed of highly defective carbon shells, as shown by Raman spectroscopy and TEM. Gao et al. (Fig. 2C) used laser-assisted combustion of C₂H₄ in air.⁸¹ The resulting high purity carbon onions with a diameter of ca. 5 nm exhibited a high degree of sintering and the KOH activated materials showed a promising supercapacitor performance with a capacitance of up to 126 F g⁻¹ in 2 M KNO₃ and a relatively high surface area of up to 804 m² g⁻¹ (see Section 4.1 for further discussion).⁸¹

Underwater arc-discharge-derived carbon onions (Fig. 2M)⁵² and nanodiamond-derived carbon onions (Fig. 2E) have been evaluated as electrodes for supercapacitors.^4,82–85 The advantages of these two synthesis methods are the high yield, the high purity, and the good controllability and reproducibility. Yet, not all synthesis routes reported in the context of carbon onion synthesis actually yielded carbon onions as defined above. For example, Fan et al. synthesized carbon particles by heating carbon in liquid alcohol; these particles had a nanoscopic size of ∼50 nm, but without a multi-shell structure.⁵⁴ Structurally, such carbon nanoparticles are virtually indistinguishable from carbon black. Bystrzejewski et al. used the thermolysis of a NaN₃–C₆Cl₆ mixture to produce different types of carbon onions (Fig. 2G).⁴³ In some of their experiments, the multi-shell structure was clearly produced, but large amounts of carbon by-products were generated, too.⁴³ The same was observed by using ball milling of graphite published by Chen et al. (Fig. 2O).⁴⁷

2.2 Nanodiamond-derived carbon onions

Thermal annealing of detonation nanodiamond powders has emerged as the most practical method for producing carbon onions.^8,61 Key benefits are the high purity, the small particle size between 5 nm and 10 nm, large-scale material production with high material homogeneity and a relatively low price of the precursor (currently ∼2 € per gram). Further cost reductions are expected as worldwide synthesis capacities for carbon onions are increasing. Furthermore, the facile synthesis of nanodiamond-derived carbon onions is attractive because the material can be obtained by simple thermal treatment in an inert atmosphere (i.e., dry powder processing).⁸⁷

By now, various nanodiamond precursors, heating rates, annealing temperatures, and durations have been investigated. Considering the use of different synthesis atmospheres like vacuum,^55–61 argon,^32,62–67 nitrogen,⁶⁸ hydrogen,⁶⁹ or helium,^{27,70,73–75} it is difficult to adequately compare the structure and properties of the resulting carbon onions. Typically, detonation nanodiamonds with a mean particle size of ca. 5 nm are used as precursors for carbon onion synthesis. Chemically, nanodiamonds consist of 80–90 mass% of carbon, a few mass% of oxygen, hydrogen, nitrogen, and minor amounts of other impurities.^32,88,89 Oxygen and hydrogen are associated with surface functional groups, whereas nitrogen can also be located in the nanodiamond core.⁸⁸ The detonation synthesis of nanodiamonds yields particles engulfed by a thin layer of amorphous and disordered carbon.^88,90–92 It also leads to particle agglomeration and sintering with agglomerates of several hundreds of nanometers.⁹³

The thermal transformation of nanodiamonds to carbon onions is a multistep process. In general it starts with desorption of water and detachment of oxygen-containing surface functional groups from sp³-hybridized carbon when heating up to around 200 °C.^94–96 Further increasing the temperature will effectively remove functional groups like carboxyl, anhydride, and lactone groups, leading to the emergence of CO and CO₂ gases.^32,94,96 Detachment of functional groups causes the formation of dangling bonds on carbon atoms which can reconstruct and combine to form π-bonds, indicating the onset of graphitization when approaching 800–900 °C. The reconstructive phase transformation forms sp²-hybridized carbon shells on the outside of the nanodiamonds, followed by continuous graphitization to the inside of the particle when increasing the temperature.^{32,59,95,97–100} Structural defects of the nanodiamond surface, stemming from the synthesis or from the detachment of surface functional groups, increase the reactivity of surface carbon atoms which greatly facilitates the phase transformation process.¹⁰⁰ At temperatures between 1100 °C and 1300 °C, the initially highly disordered carbon shells become increasingly more graphitic with a lower defect density yielding a fully transformed highly ordered carbon onion at 1800–2000 °C.^55,101 During the phase transformation, if the nanodiamond particles are in direct contact, the dangling bonds of two adjacent particles may combine, forming a common π-bond; thus local particle sintering may occur, facilitated by carbon etching and redistribution at higher temperatures. Examples for the different stages of the nanodiamond to carbon onion transformation are given in Fig. 3.

Fig. 3 Transformation from nanodiamonds to carbon onions by annealing shown using transmission electron micrographs, optical images, schematic illustrations of intermediate steps, and the assignment of physical effects depending on the annealing temperature (slight variations occur depending on the annealing atmosphere, especially when comparing vacuum with an inert gas atmosphere).

3. Physical and chemical properties of nanodiamond-derived carbon onions

3.1 Properties of nanodiamond-derived carbon onions: overview

A summary of selected physical and chemical properties of nanodiamond-derived carbon onions is given in Fig. 4 in dependence on synthesis temperature. The next sections will address in more detail the structural aspects of the nanodiamond to carbon onion transition and specifically aspects of electrical conductivity and porosity. For electrochemical energy storage, carbon onions synthesized at temperatures of at least 1700 °C present the most attractive set of properties. Due to the high temperature, the nanodiamond transforms completely to carbon onion resulting in a low content of surface functionalities (mostly oxygen-containing functionalities), a low density leading to a relatively high surface area, and a high carbon ordering which is responsible for the high conductivity.

Fig. 4 Properties of nanodiamond-derived carbon onions dependent on the synthesis temperature. The values are approximations shown to demonstrate the trend in selected physical and chemical properties for carbon onions annealed at different temperatures. For more precise values, we recommend the following literature: mass loss and oxygen content,^32,87 ND content,^56,102 skeletal density,^32,56,87 specific surface area (see Fig. 6, Section 3.3), sp² carbon ordering,^{32,65,83,101,103} and electrode conductivity (see Fig. 7, Section 3.4).

Kuznetsov et al. investigated the graphitization mechanism and formulated a model for the gradual phase transition.⁶¹ The exfoliation to graphite sheets preferentially occurs on the (111) diamond facets and involves shrinkage along the graphitic network.^61,104 The decrease in density from nanodiamonds (∼3.3 g cm⁻³) to graphitic carbon (1.9–2.2 g cm⁻³) induces an increase in particle volume.^32,56,61 Therefore, the number of diamond surface atoms was not sufficient to form a closed shell on the outside of the particle. The missing carbon atoms come from the edges or inner diamond layers leading to the complete closure of carbon onion shells. The transition from nanodiamond to carbon onion might be characterized by intermediate steps, for example spiral-like carbon shells or semi-spherical shells, which are yet to be reliably confirmed in experiments.^61,105,106

In a theoretical study by Ganesh et al., Reax force field was used to model the transformation from nanodiamonds to carbon onions.⁹⁷ In agreement with experimental studies, a step-wise transformation takes place, successively from the outside to the inside when increasing the annealing temperature. A full conversion, for example for a 2 nm nanodiamond, was only reached at 1800 °C. For temperatures below 627 °C, no conversion occurred, in agreement with experimental studies on larger nanodiamond particles.^32,84 It is expected that larger nanodiamonds would show a lower degree of transformation for the same temperature. The simulations yielded highly defective shells and an interlayer spacing of 0.34 nm, slightly smaller than 0.35 nm in graphite. This interlayer spacing makes the intercalation of small ions, for example, lithium, into carbon onions possible. Moreover, some of the simulations reported in this work yielded very interesting structures in which the carbon onion resembled a snail shell with one spiral channel going from the surface all the way down to the center of a carbon onion. This structure is in striking contrast to the closed multishell structure thought to be typical for carbon onions before and may change our perception of ionic and other species intercalation into the carbon onions.⁹⁷ Large spiraling channels, which would also allow the uptake of larger ions, are sometimes observed in experimental studies (Fig. 2E).^97,107,108

3.2 Nanodiamond to carbon onion transformation

The physical and chemical properties of nanodiamond-derived carbon onions strongly depend on the precursor, the synthesis conditions, and the post-synthesis treatment. Both annealing of nanodiamonds in vacuum and inert gas lead to the transformation to carbon onions.^55,87,109 X-ray diffraction (XRD) shows for vacuum and inert gas annealing the emergence of the (002)-graphite peak for temperatures higher than 900 °C, whose intensity increases during annealing, while the intensity of the (111) diamond peak decreases (Fig. 5A). The phase transformation progresses at a faster rate at higher temperatures and is nearly completed for temperatures higher than 1700 °C, as is indicated by the small (111)-diamond peak (Fig. 5A).^55,87,109

Fig. 5 (A) X-ray diffractograms of nanodiamond and carbon onions synthesized at different temperatures in an argon atmosphere.³² (B) Raman spectra of nanodiamond-derived carbon onions synthesized in argon measured with a 532 nm laser (recorded using 0.2 mW, 30 accumulations, and 10 s acquisition time; for pristine nanodiamonds and nanodiamonds annealed at 750–900 °C the spectra were only recorded using 1 accumulation to avoid laser-induced heating and were baseline-corrected).³² The inset shows the Raman spectrum of nanodiamonds measured with a 325 nm UV-laser.¹¹⁰

During the transformation, not only the carbon onion/diamond ratio increases, but also the degree of sp² carbon ordering. A high degree of sp² carbon ordering is desirable to enhance the electrical conductivity and can be accomplished by using high synthesis temperatures (above 1500 °C). With higher annealing temperature, carbon onions become more graphitic, leading to an increase of the density states of conductive electrons and furthermore to a higher intrinsic conductivity.¹¹¹ For spherical carbon onions, consisting of small poorly connected domains of graphitic sp²-hybridized carbon, dangling bonds due to structural defects were detected using electron spin resonance (ESR). The π-electrons localized in the small graphitic domains might not provide sufficient conductivity. Gan and Banhart showed that spherical carbon onions are highly stable up to temperatures of more than 1200 °C.¹¹² For comparison, in polyhedral carbon onions, formed at temperatures higher than 1800–1900 °C,^57,83 dangling bonds were reduced and delocalization of π-electrons increased the intrinsic conductivity.⁵⁷

Raman spectroscopy is a powerful tool to characterize the structure of carbon onions.^{101,103,113–115} Carbon onion Raman spectra exhibit a G-mode coming from the vibration of carbon atoms in sp²-hybridized carbon networks and a disordered D-mode related to the breathing of hexagonal carbon rings with defects (Fig. 5B). The G-mode, usually at 1582 cm⁻¹ for planar graphite,¹¹⁶ is commonly downshifted for carbon onions.^113,114 At lower synthesis temperature, such as 750 °C, the Raman spectrum of the material resembles amorphous carbon covering the surface of the particles, similar to the nanodiamond precursor (Fig. 5B).¹¹⁷ Due to the limited ability to study nanodiamonds with visible light excitation, UV-Raman is often preferred. The UV-Raman spectrum of nanodiamonds is additionally presented in Fig. 5B. For a synthesis temperature of 800 °C, the graphitic G-mode and the disordered D-mode emerge from the nanodiamond starting material, presenting the onset of the transformation to carbon onions (Fig. 5B). The G-mode shows a relatively broad shape due to the large bond length variation in the material. Together with a broad background coming from amorphous carbon at ∼1520 cm⁻¹ and the emergence of the D-mode at ∼1340 cm⁻¹, it can be concluded that the sp³-hybridized carbon starts to transform into sp²-hybridized carbon. Higher annealing temperature results in sharper D- and G-modes and lowers the secondary amorphous carbon signal.^87,101

In general, Raman spectra of carbon onions can often be reasonably well explained assuming at least two different carbon phases present, namely the disordered carbon onion structure rather than a specific spectrum of spherical shells¹¹³ and amorphous carbon.⁸⁷ Typically, additional low-intensity Raman modes are observed for wavenumbers below 1000 cm⁻¹. Roy et al. discussed these peaks using the 2D phonon density of states (PDOS) and the dispersion curves of graphite.¹¹⁴

In addition to XRD and Raman spectroscopy, TEM is a facile tool to image and analyze the nanodiamond to carbon onion transformation (see also Fig. 2).^{55,58,61,77,83,101,103,109,118–121} The conversion can be directly performed using electron irradiation of nanodiamond powder in a TEM.¹¹⁸ The shells form from the outside inward. According to electron energy loss spectroscopy (EELS), sp²-hybridized carbon forms in this process, but some sp³-hybridized carbon may remain. Various types of defects in the carbon onion structure were proposed such as holey shells, spiral-like structures, y-junctions, and protuberance, but the experimental investigation of the structure and defects of the shells is difficult.¹¹⁸

A key difference between vacuum and inert gas atmosphere annealing is related to the role of surface functional groups. A high oxygen content in nanodiamonds leads to an increased amount of decomposing functional groups, which form gaseous carbon oxides in the reaction zone.^32,122 If the nanodiamond annealing process is carried out in vacuum, the partial pressure of these carbon oxides remains near zero. However, if the process is carried out in an inert gas atmosphere, the partial pressure may stay high enough to cause local carbon etching and effective redistribution of carbon. The extreme case is the formation of micrometer-sized graphitic particles instead of nanometer-sized carbon onions leading to a total loss of the carbon onion structure.³² Even in less extreme cases, carbon onions from annealing nanodiamonds in an inert gas atmosphere show other characteristic features related to carbon redistribution. As seen from XRD and Raman (Fig. 5A and B), the material after annealing in an inert gas atmosphere consists of two phases: nanometer-sized carbon onions and few-layer graphene nanoribbons (Fig. 6A and B). In contrast, a second carbon phase (i.e., nanoribbons) is absent in carbon onions from vacuum annealing (Fig. 6C and D).⁸⁷

Fig. 6 Transmission electron micrographs of nanodiamond-derived carbon onions synthesized at 1700 °C in argon (A and B) and vacuum (C and D). Reproduced with permission from Elsevier from ref. 87.

3.3 Porosity characteristics of nanodiamond-derived carbon onions

Unlike many other carbon materials for electrochemical energy storage (e.g., activated carbons), a relatively large surface area of nanodiamond-derived carbon onions is commonly believed to be entirely associated with their external surface. The typical values of specific surface area are 300–600 m² g⁻¹ (Fig. 7A) with more than 1 cm³ g⁻¹ interparticle pore volume.⁸⁷ For comparison, activated carbons present usually more than 1500 m² g⁻¹ specific surface area, with a pore volume also around 1 cm³ g⁻¹.⁷ The fully accessible external surface area of carbon onions greatly facilitates ion transport during charge and discharge in electrochemical energy storage devices. This is in contrast to nanopores within micrometer-sized particles (e.g., of activated carbon), as will be shown in Section 4.1. Typically, the pore size distribution (i.e., interparticle pores) of carbon onion powders is relatively broad: from <1 nm up to 40 nm.³² The surface area of nanodiamond-derived carbon onions strongly depends on the synthesis parameters with maximum values just below 650 m² g⁻¹ (Fig. 7A). For carbon onions synthesized by certain methods, such as arc discharge of graphite, even higher values can be reached (up to 984 m² g⁻¹).

Fig. 7 (A) Surface areas of different types of carbon onions. The colored bars give the range in surface area with minimal and maximum values. See the legend for synthesis atmosphere and post-synthesis modification. Bushueva et al.,⁸⁴ Butenko et al.,⁵⁶ McDonough et al.,⁸³ Portet et al.,⁴ Zeiger et al. (circle and square symbolize 2 different nanodiamond precursors),^32,87 Sano et al.,¹²³ Gao et al.,⁸¹ Mykhailiv et al.,⁷⁰ Gu et al.,¹²⁴ Plonska-Brzezinska et al.⁷⁵ (B) Relative change of surface area during annealing of nanodiamond in an inert gas or vacuum.

To understand the dependency of the surface area on the synthesis conditions, we have to consider that the nanodiamond to carbon onion phase transformation not only brings along a structural change, but also significantly decreases the density.^32,56,97 The density of nanodiamonds (3.3 g cm⁻³) is much higher than that of carbon onions (e.g., ∼1.9 g cm⁻³ for carbon onions produced at 1700 °C).³² As a result of the large decrease in density, a volume expansion occurs during the formation of sp²-hybridized carbon shells from the outside to the inside of the particles.^32,56 This expansion has been predicted by modeling and verified experimentally by the increase in surface area (nitrogen gas sorption analysis) and the continuous decrease in density (helium gas pycnometry).³² The higher lattice spacing of graphite compared to diamond leads to a decrease in density, a volume expansion, and to an increase in specific surface area (Fig. 7B). By normalizing the surface areas of different nanodiamond-derived carbon onions to the surface area of the precursor nanodiamonds, one can determine a constant increase in surface area up to around 70% when annealing at 1500 °C. For temperatures higher than 1500 °C, the surface area does not increase and may even decrease, whereas the density keeps decreasing.³² Locally occurring particle–particle sintering, as well as carbon etching and redistribution to larger structures, is an important factor for the decrease in surface area. At the end of this process, even consolidated structures in the form of micrometer-sized graphitic particles may occur when using an inert gas atmosphere for the synthesis.^32,122

To increase the surface area of carbon onions beyond the values of ∼600 m² g⁻¹, chemical or physical activation can be used, such as oxidation in air or acid treatment. So far, the highest value for activated nanodiamond-derived carbon onions was ∼650 m² g⁻¹ using oxidation in air (corresponding to an increase of the surface area by 100% compared to the surface area of the nanodiamond precursor).³² The increase in the surface area is accomplished by the removal of interparticle pore-blocking amorphous carbon and a partial decrease in the carbon onion diameter.³² Higher values of the specific surface area above 800 m² g⁻¹ were achieved by KOH activation of carbon onions produced by laser-assisted combustion of methane.⁸¹ Compared to oxidation in air, KOH etching does not remove carbon shells and amorphous carbon; rather this procedure etches new pores into the carbon onions.⁸¹ The largest surface area reported so far is 984 m² g⁻¹ by using an arc-discharge method with graphite as the precursor and this high value was explained by the emergence of intraparticle pores due to surface roughening.¹²³ In the latter study, TEM images showed particles with a diameter of 4–36 nm which is up to 4-times larger than for nanodiamond-derived carbon onions. The rather low density of 1.64 g cm⁻³ may also indicate the presence of other carbon nanoforms in the sample (which is common for this type of synthesis method). Thus, it is difficult to tell to what extent this large specific surface area of the products can be assigned just to carbon onions.

Until now, the discussion was limited to surface area and pore volume, but ion accessibility to a certain pore can only be understood when we consider the actual pore size distribution (PSD). The PSD can be derived from gas sorption isotherms using, for example, CO₂ at 0 °C (for pores between ca. 0.3 and 1.0 nm) and N₂ at −196 °C (for pores between ca. 0.6 nm and 30 nm). A slit-shaped pore model is typically assumed as the best-fit model for carbon onions among the available density functional theory (DFT) models.⁶⁵ Assuming a series of pores, the PSD of carbon onions can be defined as pores between particles in direct contact (<1 nm), pores between several particles (1–10 nm), and pores between agglomerates (larger than 2 nm).³² The large number of mesopores with the size 2–50 nm facilitate ion transport which makes carbon onions a very attractive high-power material for supercapacitor electrodes.

3.4 Electrical conductivity of nanodiamond-derived carbon onion electrodes

The electrical conductivity of carbon onion electrodes is influenced by the intrinsic electrical conductivity of the material, the degree of particle sintering, the particle arrangement and compaction, as well as the electrode preparation (e.g., use of binder vs. binder-free electrodes). The intrinsic conductivity of carbon onions can be tuned by adjusting the annealing temperatures. High temperatures (>1700 °C) result in a higher degree of carbon ordering. The resulting carbon onions are more graphitic and more electrically conductive (see also Section 3.2 and Fig. 4).¹¹¹ The particle sintering is typically caused by agglomeration of nanodiamonds, and partially provoked by high annealing temperatures.^32,125 In a recent study it was shown that annealing in argon instead of vacuum produces few-layer graphene flakes between the carbon onions, which enhance the electrode conductivity.⁸⁷

The electrical conductivity of nanodiamond-derived carbon onion electrodes may reach ca. 4 S cm⁻¹ measured with a four-point probe on pellets or polymer-bound electrodes.⁴ However, the spread of reported values is very large, ranging from below 0.5 S cm⁻¹ to above 4 S cm⁻¹ (Fig. 8A). The maximum conductivity of ∼4 S cm⁻¹ (2.5–3 S cm⁻¹ for polymer-bound electrodes) is comparable to that of carbon black (ca. 1–2 S cm⁻¹) and an order of magnitude higher than activated carbon (<0.5 S cm⁻¹).⁴ A comprehensive and fair comparison of experimental data is complicated by the use of different synthesis temperatures, atmospheres, and different electrode preparation methods in the literature. For example, polytetrafluoroethylene (PTFE)-bound film electrodes and compressed pellets show significant differences in conductivity. However, we can assume an increased electrical conductivity for carbon onions synthesized at higher temperatures as a result of enhanced carbon ordering, as confirmed by the literature. The large spread of literature values shown in Fig. 8A reflects the large influence of the measurement procedure.

Fig. 8 Conductivities measured using a 4-point probe. (A) Literature values. McDonough et al.: powder compressed with 8 MPa;⁸³ Portet et al.: Teflon cylinder filled with powder, copper electrodes of 22 mm in diameter as probes, electrodes with 15 mass% PTFE (mass density 15 mg cm⁻²) were painted on a current collector covered with conductive paint;⁴ this study: ∼200 mg powder compressed with 0.25 MPa, electrode with 10 mass% PTFE and a thickness of 200 μm; Jäckel et al.: electrodes with 5 mass% PTFE for activated carbon and 10 mass% PTFE for carbon black and a thickness of 200 μm (ref. 7). (B) Carbon onion pellet conductivities: powder of ∼200 mg compressed with 0.25 MPa. (C and D) Electrode densities, and electrode conductivities of carbon onions synthesized at 1700 °C in argon for different electrode thicknesses (10 mass% PTFE, 60–500 μm thickness).

To make a comparison within a cohesive data set, we show in Fig. 8B data for the electrical conductivity of compressed pellets. Nanodiamonds were annealed at different temperatures yielding carbon onions above ca. 1000 °C. Using a four-point probe, we measured an increase from the negligible electrical conductivity of nanodiamonds around the onset of carbon onion formation (i.e., at around 800–1000 °C). By varying the annealing time (exemplified for 1100 °C and 1300 °C), we see a continuous increase of the electrical conductivity. This corresponds to a continuous transformation of nanodiamond powder to carbon onions. A residual diamond core and less perfect graphitic shells result in lower electrical conductivity. Finally, constant values are seen for synthesis temperatures exceeding 1500 °C where it is assumed that nearly full transformation to carbon onions has occurred within less than 1 h of annealing.

We show in Fig. 8C and D the influence of the electrode thickness with 10 mass% PTFE as the binder and packing density on the electrical conductivity measured by a four-point probe. For electrode thicknesses between 200 and 500 μm, the sheet conductivity (not normalized to the electrode thickness) varies only within ±10%, mirroring a moderate increase in electrode density by ca. 25%. Thus, for the electrode thickness values in the range 200–500 μm the sheet conductivity is not significantly influenced by electrode density and thickness. However, for thinner electrodes, the sheet conductivity drops, for example by more than 80% for 60 μm thickness (compared to 500 μm). Fig. 8D presents the conductivity values normalized to the thickness as common in the literature. By decreasing the electrode thickness the normalized conductivity continuously increases. Thin electrodes are commonly obtained by repeated rolling, which leads to a higher degree of particle compaction (i.e., larger contact area) and a possible increase in electrical conductivity. In Fig. 8D, this is exemplified for a 400 μm thick electrode which was rolled two times. In the second rolling step, additional electrode material is used and rolled to a more compact electrode. A higher density is achieved and a higher conductivity can be reached. The nanoscopic size of carbon onions makes it quite difficult to prepare PTFE-bound electrodes with a thickness smaller than 200 μm. Consequently, very thin PTFE-bound electrodes suffer from cracking and similar issues may arise when using other polymer binders and non-freestanding, sprayed electrodes instead of (hot-)rolling. Therefore, in addition to normalizing the electrical conductivity to the electrode thickness (as commonly done in the literature), careful assessment of the electrode density is required when comparing experimental data.

4. Electrochemical properties of carbon onion electrodes

4.1 Carbon onions for electrical-double layer capacitors

The electrochemical performance of carbon onions reflects their physical and chemical properties, as well as the accessible surface area. Nanodiamond-derived carbon onions commonly show capacitance values of up to 52 F g⁻¹ in aqueous 1 M H₂SO₄ (ref. 84) and ca. 40 F g⁻¹ in 1.5 M TEA-BF₄ in acetonitrile (ACN) (Fig. 9).⁴ Chemical and physical activation can be used to improve the relatively small capacitance (for comparison, activated carbon has >100 F g⁻¹) by increasing the surface area. Using a laser resonant excitation of ethylene molecules, carbon onions with a size between 5 nm and 50 nm were produced. For KOH activated carbon onions mentioned before, with surface areas in excess of 800 m² g⁻¹ an enhanced capacitance of 115 F g⁻¹ was achieved in 2 M KNO₃.⁸¹ Carbon onions from the arc-discharge of graphite electrodes have shown a very low capacitance of ca. 5 F g⁻¹ probably due to the low surface area of particles with a diameter of 25–35 nm.⁸² The overall moderate surface area of carbon onions results in a rather low energy storage capacity compared to nanoporous carbons, such as activated carbon or carbide-derived carbon with typical values of 100–200 F g⁻¹ (and surface areas above 1500 m² g⁻¹).¹⁴ However, the facile ion access to the external surface of carbon onions makes them excel in power handling, as will be discussed in more detail later.

Fig. 9 Capacitance values of different types of carbon onions. Bushueva et al.,⁸⁴ Portet et al.,⁴ Plonska-Brzezinska et al.,⁷⁵ Mykhailiv et al.,⁷⁰ Gao et al.,⁸¹ Borgohain et al.⁸² The synthesis methods and the settings for electrochemical characterization are presented in the figure. The panel on the left encloses capacitance values of nanodiamond-derived carbon onions, the one in the middle of carbon onions from the combustion of C₂H₄, and on the right of arc discharge carbon onions. Carbon onions without modification are shown by squares and with modification by circles. For comparison, the capacitance of PTFE-bound activated carbon is shown using 1 M TEA-BF₄ in ACN.⁷

The combination of high intrinsic electrical conductivity (see Section 3.4) and facile ion accessibility via an external surface area (see Section 3.3) makes carbon onions very attractive for high power applications that require fast charge and discharge rates (e.g., pulsed operation in hybrid electric cars or power stabilization for grid scale applications).⁶ The high intrinsic electrical conductivity of nanodiamond-derived carbon onions is achieved by using high annealing temperatures as discussed in Section 3.4.^4,83,87 The higher degree of carbon ordering not only increases the conductivity and power handling but also leads to higher differential capacitances (i.e., potential-dependent capacitance). In a recent study the influence of carbon ordering on the capacitance of carbon supercapacitors was systematically studied using carbon onions as the model material.⁶⁵ Carbon onions with similar porosities, but differing in carbon ordering, were synthesized at 1300 °C, 1500 °C, and 1750 °C in vacuum. Using PTFE-bound electrodes and different organic electrolytes and ionic liquids, the influence of carbon ordering on the electrochemical properties was investigated. Cyclic voltammograms showed a characteristic butterfly shape for all carbon onions, but it was much more pronounced for carbon onions synthesized at high temperatures with a higher degree of carbon ordering (Fig. 10A). The butterfly shape represents an increasing differential capacitance for higher potentials, because the electron density of states changes as a function of applied potential.^65,126 This so-called electrochemical doping¹²⁷ strongly depends on the degree of carbon ordering¹²⁸ and is enhanced for more ordered carbon onions with higher electrical conductivity. The change in electrical resistance measured with an in situ resistance cell is shown in Fig. 10B. At high electrode potentials, the normalized resistance of the electrodes reduces constantly by increasing the annealing temperature.⁶⁵

Fig. 10 (A) Cyclic voltammogram of PTFE-bound electrodes, made of nanodiamond-derived carbon onions synthesized at 1300 °C and 1750 °C in 1 M TEA-BF₄ in ACN at 10 mV s⁻¹.⁶⁵ (B) Resistance of the carbon onion electrodes after charging to a specific potential using an in situ resistivity measurement cell for different electrolytes.⁶⁵ (C) Rate handling behavior of the carbon onion powders compared with activated carbon, carbon black, and the nanodiamond precursor (UD50-detonation soot) measured in 1 M H₂SO₄.⁸³ (D) Rate handling behavior of film electrodes (200 μm, unless stated otherwise) of carbon onions synthesized from nanodiamonds at 1300 °C and 1700 °C in flowing argon or vacuum. All data from ref. 87 were measured in 1 M TEA-BF₄ in ACN using a full cell set-up with a cell voltage of 2.7 V.

The nanoscale size of carbon onions necessitates adding more binder during electrode manufacturing, typically 10 mass% PTFE for PTFE-bound electrodes.⁷ For activated carbons, usually 5 mass% PTFE is sufficient to prepare free-standing, mechanically stable electrodes.¹⁴ Polymer binders add electrochemically dead mass and may partially block pores leading to lower capacitance.⁷ Microcavity electrodes present a facile tool to qualitatively compare different electrode materials without the influence of binders.⁴ In a study by McDonough et al., carbon onions synthesized at different temperatures were electrochemically tested using microcavity electrodes in 1 M H₂SO₄.⁸³ At scan rates up to 15 V s⁻¹, the relative capacitance for carbon onions synthesized at 1800 °C decreased by less than 30% compared to the low-rate value, while carbon onions synthesized at 1300 °C showed a reduction by 45% at 15 V s⁻¹ (Fig. 10C). Compared to carbon onions, other carbon materials like activated carbon suffer from a lower degree of carbon ordering and lower ion mobility resulting in a decrease in capacitance of more than 70% under comparable testing conditions. The enhanced power handling ability of carbon onions synthesized at higher temperatures (e.g., 1700 °C vs. 1300 °C) was also demonstrated for full cells with 200 μm thick electrodes in galvanostatic measurements.⁸⁷ Carbon onions in this study were derived from nanodiamonds via vacuum or argon annealing and higher synthesis temperatures led to higher capacitance retention when increasing the specific current.⁸⁷ By lowering the electrode thickness to 100 μm an even higher power performance and almost no loss in capacitance at 20 A g⁻¹ compared to the low-rate value were reported (Fig. 10D). This again shows the high influence of the electrode design on the resulting electrochemical performance in addition to the intrinsic material properties.

As mentioned in Section 3.2, argon annealing leads to etching and redistribution of carbon onion outer shells and amorphous carbon.^32,87 Oxygen containing gaseous species, formed by the decomposition of functional groups, accumulate in the atmosphere around the sample etching the carbon and redistributing it to form few-layer graphene flakes between the particles (Fig. 6).^65,87,122 This effect was only observed by using ultrafast heating and cooling;⁸⁷ lower heating rates resulted in larger graphitic particles.³² Due to the interconnection of the particles by few-layer graphene, a more open network with higher electrical conductivity was produced with an enhanced capacitance retention of 85% at 20 A g⁻¹ (Fig. 10D) for PTFE-bound electrodes (10 mass% PTFE, 200 μm thick).⁸⁷

Taking advantage of the high rate handling capability, carbon onions have been demonstrated as electrode materials for ultrahigh-power micro-supercapacitors.⁶ Using an electrophoretic deposition technique, nanodiamond-derived carbon onions produced at 1800 °C were directly deposited onto patterned gold current collectors from an ethanol–water suspension. The electrodes showed ultrafast rate handling behavior in 1 M TEA-BF₄ in propylene carbonate (PC) up to 200 V s⁻¹ scan rate.⁶ This superior rate handling is partially due to the ultrathin electrode thickness of just a few micrometers; as demonstrated in ref. 6, the rate handling of conventional activated carbon is also severely enhanced for micro-supercapacitors as compared to conventional supercapacitors with much thicker electrodes (150–250 μm). The high rate handling ability of carbon onions was also shown for carbon onion/carbon fiber composite electrodes with a thickness below 50 μm.¹²⁹ Using 1 M TEA-BF₄ in ACN, a decrease of 37% of the initial low-rate capacitance was found at 100 A g⁻¹ using a full-cell setup (two-electrode geometry) for the composite electrode; for comparison, PTFE-bound electrodes of carbon onions without the highly conductive network of carbon fibers showed a 75% decrease in capacitance already at 50 A g⁻¹.¹²⁹

Electrochemical performance stability is another important parameter for energy storage devices; however, so far, only a few studies provided data in this regard for carbon onions. For example, using micro-supercapacitors with high temperature carbon onions (1800 °C, high vacuum), a capacitance retention of ∼100% was maintained after 10 000 cycles at 10 V s⁻¹ in an organic electrolyte.⁶ Yet, micro-supercapacitor data are not directly transferable to estimate the performance stability of full cells. Two-electrode (full cell) data were provided by another study for the comparison of nanodiamond-derived carbon synthesized at 1300 °C and 1700 °C in argon or high vacuum.⁸⁷ While no significant difference in the performance stability was caused by using different synthesis atmospheres (vacuum vs. argon), a significant difference was identified when comparing the different synthesis temperatures (1300 °C vs. 1700 °C). Using voltage floating at 2.7 V in TEA-BF₄ in ACN, high temperature carbon onions still showed nearly 100% capacitance retention after 100 h, while the capacitance of low temperature carbon onions decreased to ∼90% after 20 h. These data correlate with the measured charge efficiency, which remains virtually unchanged at around 100% for high temperature carbon onions. The charge efficiency for low temperature carbon onions first decreases to around 92% after 20 h, but then recovers over time to ∼98% after 100 h. This effect can be explained by the irreversible reactions of surface functional groups associated with the higher oxygen content after the low temperature synthesis (∼3 mass%) compared to the high temperature synthesis (∼1 mass%).⁸⁷

In summary, the electrochemical properties of carbon onions depend on the synthesis conditions, as it is typical for carbon materials. Due to the lack of comprehensive electrochemical characterization data for non-nanodiamond-derived carbon onions, the following statements mainly concern carbon onions produced by thermal annealing of nanodiamonds.

- Carbon onions synthesized at higher temperatures (>1500 °C) show a higher degree of carbon ordering, higher differential capacitance for higher potentials, higher conductivity, higher electrochemical stability, and better rate handling capability.

- Typically, low temperature carbon onions (<1500 °C) present higher surface area and higher capacitance due to negligible sintering and carbon redistribution.

- By using rapid heating and cooling rates, sintering can be minimized and high conductivity high surface area carbon onions can be obtained for high annealing temperatures (1700 °C). Improved electrical conductivity, rate handling, and electrochemical stability can be obtained when using an inert gas like argon instead of vacuum for the annealing of carbon onions.

- Activation (e.g., by using KOH) can lead to a roughening of outer shells, development of internal porosity, larger surface area, and accordingly higher capacitance.

- The thickness and packing density of the electrodes strongly influences the rate handling behavior, not only for carbon onions, but also for other materials and should be taken into account when comparing data.

4.2 Carbon onions as conductive additives for electrical double-layer capacitors

The use of carbon onions as the primary electrode material for electrical double-layer capacitors is limited due to the low energy storage capacity resulting from a low specific surface area (<600 m² g⁻¹) when compared to commercial activated carbon.⁷ However, their electrical conductivity superior to activated carbon makes them a promising conductive additive, effective in small amounts (typically 5–10 mass%). The very small carbon onions (below 10 nm) as well as their soft agglomerates (typically 100–200 nm) fit into very tight interparticle spaces. This results in a better (more homogeneous) distribution between the micrometer-sized activated carbon particles.⁷ As such, carbon onions (1) fill effectively the gaps between larger carbon particles; (2) increase the overall particle–particle contact area between larger nanoporous carbon particles; and (3) decrease the particle–particle resistance since carbon onions are much more conductive than, for example, activated carbon.^7,130,131 Compared to conventional carbon black and graphite nanoparticles, carbon onions consistently lead to a better gravimetric and volumetric performance.⁷

So far, the use of carbon onions as a conductive aid to enhance power handling of carbon supercapacitor electrodes has been documented when admixing to activated carbon,^7,131 mesoporous carbon,¹³⁰ electrochemically active polymers,¹³² and inserting between graphene oxide sheets.¹³³ Depending on the added amount of carbon onions, the activated carbon electrode sheet resistance drops by 30–80%.⁷ The addition of 10 mass% carbon onions to flexible polyester electrodes leads to a 60% decrease in electrical series resistance.¹³⁰ Also, the performance of polymer electrodes based on polyaniline was enhanced by adding carbon onions, yielding an improved cycle stability and better rate handling ability.¹³²

4.3 Carbon onions as a substrate for redox-enabled hybrid energy storage

Electrical double-layer capacitors show a high specific power due to fast ion electrosorption on high surface area carbons, but suffer from comparatively low specific energy (commonly around 5 W h kg⁻¹ on a device-level).¹³⁴ A particularly attractive approach to enhance the energy storage capacity is the implementation of redox-active materials. Carbon onions are highly suited for this task as they are believed to only present an easily accessible external surface, which can be decorated with redox-active species, like surface functional groups, molecular species (e.g., quinones), metal oxides (e.g., manganese oxide), or conductive polymers (e.g., polyaniline, PANI). This is in contrast to highly porous substrates, such as activated carbon, where adding redox-active materials brings along the issue of pore blocking in addition to the poor electrical conductivity of the substrate (especially activated carbon, see Section 3.4). Several types of redox-active species and/or pseudocapacitive materials were used in combination with carbon onions, including manganese oxide,^86,135–137 ruthenium oxide,²⁶ nickel hydroxide and oxide,²⁷ PANI,^74,132 poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS),⁷³ polypyrrole,¹³⁸ and quinones.^129,139 Among these materials, manganese oxide in combination with carbon onions shows the highest energy storage values with 575 F g⁻¹ at 0.5 A g⁻¹ in 0.5 M H₂SO₄.⁸⁶ A collection of SEM and TEM images of such hybrid materials is given in Fig. 11 and an overview of the capacitance values for redox-enabled carbon onions is presented in Fig. 12.

Fig. 11 Transmission and scanning electron micrographs of carbon onions loaded with different types of redox active species. (A–D) Carbon onions decorated with manganese oxide: Azhagan et al. (with permission from Royal Society of Chemistry),⁸⁶ Makgopa et al. (with permission from Royal Society of Chemistry),¹³⁵ Borgohain et al. (with permission from Royal Society of Chemistry),¹³⁶ and Wang et al. (with permission from Royal Society of Chemistry);¹³⁷ (E) loading with ruthenium oxide: Borgohain et al. (with permission from the American Chemical Society);²⁶ (F) coating with PANI: Kovalenko et al. (with permission from John Wiley & Sons Inc.);¹³² (G) coating with polypyrrole (PPy): Mykhailiv et al. (with permission from John Wiley & Sons Inc.);¹³⁸ (H) loading with nickel hydroxide: Plonska-Brzezinska et al. (with permission from Royal Society of Chemistry)²⁷

Fig. 12 Comparison of capacitance values of carbon onions decorated with surface functional groups, metal oxides, or conductive polymers. Experimental details are given in the figure. Anjos et al.,¹³⁹ Zeiger et al.,¹²⁹ Azhagan et al.,⁸⁶ Makgopa et al.,¹³⁵ Borgohain et al. (MnO₂),¹³⁶ Wang et al.,¹³⁷ Borgohain et al. (RuO₂),²⁶ Plonska-Brzezinska et al. (PEDOT:PSS),⁷³ Mykhailiv et al.,¹³⁸ Plonska-Brzezinska et al. (Ni(OH)₂, the capacitance value in the publication was 1225 F g⁻¹, which is normalized to the carbon content. We adjusted the value by normalization to the electrode mass to compare with other literature values),²⁷ Plonska-Brzezinska et al. (PANI),⁷⁴ Kovalenko et al.,¹³² Gu et al.,²⁸ Liu et al.¹⁴⁰

The vast majority of electrochemical applications of carbon onions relates to their use for supercapacitor electrodes as an active material, conductive additive, or substrate for redox-hybrid systems. In contrast, only a few studies have investigated their use as anode materials for battery systems, such as lithium ion batteries. In a study by Han et al., carbon onions were chemically synthesized at 600 °C by a reaction between copper dichloride hydrate (CuCl₂·2H₂O) and calcium carbide (CaC₂). The material was tested as an anode material for lithium-ion batteries. In a mixture of 1 M LiPF₆ in ethylene carbonate and dimethyl carbonate (EC/DMC, 1 : 1 by volume), carbon onions showed a capacity of 391 mA h g⁻¹ at a rate of C/10 after 60 cycles which is slightly larger than the theoretical value of graphite with 372 mA h g⁻¹.¹⁴¹ This may be explained by a shorter pathway for the ions and the carbon onion structure which exhibits an enhanced number of active sites for Li ion storage.¹⁴¹ Wang et al. hydrothermally coated carbon onions with manganese oxide using KMnO₄ at 150 °C for use as lithium-ion battery anodes with a two-electrode configuration, lithium foil as the counter electrode, and 1 M LiPF₆ in EC/DMC (1 : 1 by volume) as the electrolyte.¹⁴² The capacity of the manganese oxide composite was increased at 50 mA g⁻¹ to ∼630 mA h g⁻¹ compared to ∼260 mA h g⁻¹ without carbon onions. The increased rate capability and an improved cycling stability were attributed to the structure of carbon onions leading to fast ion and electron transport.¹⁴² In another publication by Wang et al. even 853 mA h g⁻¹ was reached with a coulombic efficiency of 98% after pre-lithiation using MoS₂/onion-like carbon composites measured at a specific current of 50 mA g⁻¹ in 1 M LiPF₆ in EC/DMC (1 : 1 by volume).¹⁴³

4.3.1 Carbon onions decorated with manganese oxide. In a study by Azhagan et al., carbon onions were synthesized by flame burning clarified butter (ghee) and the collected carbon black was annealed at 800 °C in an inert gas to produce graphitic multi-shell carbon onions in the size range of 40–45 nm (Fig. 11A).⁸⁶ A microwave assisted hydrothermal reaction with KMnO₄ as the precursor was used to coat the carbon onions with manganese oxide. The as-synthesized manganese oxide nanoparticles had a diameter of ∼10 nm with the lattice constant of λ-MnO₂ (0.239 nm) measured using TEM. Carbon onion electrodes without metal oxides showed a surface area of 486 m² g⁻¹ and a capacitance of 171 F g⁻¹ at 0.5 A g⁻¹ in 0.5 M H₂SO₄,⁸⁶ which is 5–10 times higher than that for neat nanodiamond-derived carbon onions from McDonough et al.⁸³ With regard to the presence of a redox peak between −1 V and +1 V, carbon onion surface functional groups may contribute to the enhanced capacitance even in the absence of additional decoration with redox-active species (such as quinones or metal oxides). After coating with manganese oxide, the specific capacitance increased to 575 F g⁻¹ at 0.5 A g⁻¹, measured in a two-electrode configuration, with a capacitance retention of 18% at 10 A g⁻¹. The relatively low capacitance retention for higher specific current might not only be the result of the low conductivity of the metal oxide coating, but also come from the carbon onion surface functionalities, undergoing irreversible electrochemical transformations at high currents. As shown in ref. 87, high purity carbon onions with a high degree of carbon ordering exhibit superior rate handling with more than 80% capacitance retention even at 20 A g⁻¹. Therefore, the low capacitance retention of the carbon onions before and after their combination with manganese oxide might be the result of large amounts of surface functionalities or the dissolution of manganese oxide in sulfuric acid. Still, the cycle stability after 2000 cycles was more than 90%.⁸⁶

In a study by Borgohain et al., nanodiamond-derived carbon onions were oxidized in sulfuric and nitric acids, and hybrid electrodes of oxidized carbon onions, polydiallyldimethylammonium chloride (PDDA), and delaminated manganese oxide were synthesized by a sequential chemical deposition technique (Fig. 11C).¹³⁶ Using a two-electrode configuration and 1 M Na₂SO₄, the highest capacitance of 219 F g⁻¹ was reached for the loading with 55 mass% manganese oxide. For comparison, less than 50 F g⁻¹ was measured for neat carbon onions.¹³⁶ The hybrid electrodes presented a capacitance retention of roughly 80% after 1000 cycles, which is smaller than for the material reported by Azhagan et al. with more than 95% after 2000 cycles.^86,136 Borgohain et al. used chemical oxidation of carbon onions before they were combined with manganese oxide.¹³⁶ Due to the oxidation induced formation of surface functional groups, the stability might be negatively influenced as shown for the as-synthesized carbon onions with a reduction of the capacitance by ca. 15% after 1000 cycles. Yet, the decrease was not monotonic: for the first 600 cycles, the capacitance anomalously increased due to faradaic reactions of the surface functional groups and only after their depletion, the capacitance decreased again.¹³⁶ A similar effect was found in a study with carbon onions showing reduced stability in an organic electrolyte due to higher contents of oxygen-containing functional groups.⁸⁷ The manganese oxide/carbon onion hybrids showed a decrease by ca. 15% at 20 mV s⁻¹ of the initial low-rate capacitance.¹³⁶

Wang et al. used phenolic resin-derived carbon onions hydrothermally coated with manganese oxide in KMnO₄ in aqueous solution.¹³⁷ In contrast to Borgohain et al., the capacitance did not increase with the manganese oxide loading but rather showed a maximum value of 190 F g⁻¹ (galvanostatic mode in 1 M Na₂SO₄) for 80 mass% manganese oxide with a flower-like morphology (Fig. 11D). Up to 2 A g⁻¹ in galvanostatic cycling, the electrodes demonstrated good rate handling behavior with more than 93% capacitance retention and almost 100% performance stability over 1000 charge/discharge cycles.¹³⁷ For comparison, materials reported by Borgohain et al. (219 F g⁻¹)¹³⁶ and Azhagan et al. (575 F g⁻¹)⁸⁶ had a much lower rate handling ability. These data are in line with a recent study by Makgopa et al. using nanodiamond-derived carbon onions and a hydrothermal treatment with KMnO₄ producing birnessite-type manganese oxide with a loading of 47 mass% (Fig. 11B).¹³⁵ While the as-synthesized carbon onions only showed a capacitance around 30 F g⁻¹, the hybrid electrodes yielded more than 400 F g⁻¹ in 1 M Na₂SO₄ at 0.1 A g⁻¹ in a three-electrode setup (Fig. 13A). At higher rates up to 5 A g⁻¹, the material still presents ca. 250 F g⁻¹ (i.e., 37% decrease).

Fig. 13 (A) Cyclic voltammogram of carbon onions and carbon onion/manganese oxide nanohybrids in 1 M Na₂SO₄.¹³⁵ (B) Rate handling plot of different types of carbon onions and carbon onion/manganese oxide composites. Makgopa et al.: nanodiamond-derived carbon onions coated in a hydrothermal reaction with manganese oxide; 1 M Na₂SO₄ electrolyte.¹³⁵ Azhagan et al.: burning ghee-derived carbon onions coated using a microwave assisted hydrothermal reaction; 0.5 M H₂SO₄ electrolyte.⁸⁶ Borgohain et al.: nanodiamond-derived carbon onions were combined with polydiallyldimethylammonium chloride (PDDA), and delaminated-manganese oxide by a sequential chemical deposition technique; 1 M Na₂SO₄ electrolyte.¹³⁶ Wang et al.: phenolic resin-derived carbon onions were hydrothermally coated with manganese oxide; 1 M Na₂SO₄ electrolyte.¹³⁷ Activated carbon electrodes were prepared using 5% PTFE as a binder and measured in 1 M Na₂SO₄ using a full cell set up.

The rate handling data of several studies of carbon onion/manganese oxide hybrids are shown in Fig. 13B. The plot also shows values for conventional activated carbon and all data are in absolute values so that a direct comparison is possible. Still, a direct quantitative comparison of the rate handling data is difficult due to the different preparation methods and testing setups. For example, Makgopa et al.¹³⁵ and Wang et al.¹³⁷ used polyvinylidenefluoride PVDF (5 mass% Makgopa et al., 10 mass% Wang et al.) as a binder, with 10 mass% (Wang et al.) or 15 mass% (Makgopa et al.) carbon black as a conductive additive. The improved charge transport in the presence of a conductive additive (carbon black) complicates a direct comparison of the rate handling data. Moreover, the use of highly graphitic carbon onions with high intrinsic conductivity in the studies by Makgopa et al. and Wang et al. makes a comparison difficult with other carbon onions, like ghee-derived carbon onions.⁸⁶ Also, by the use of carbon onions with a lower electrical conductivity and the absence of an additional conductive additive, the rate handling of the materials reported by Azhagan et al. is moderate.⁸⁶ While the power handling is limited, the capacitance is very high (575 F g⁻¹), probably due to additional surface functionalities on the carbon onions and the usage of sulfuric acid (oxidizer) as the electrolyte leading to higher values compared to chemically inert electrolytes.¹⁴⁴

4.3.2 Carbon onions decorated with ruthenium oxide. Ruthenium oxide was also explored as a redox-active coating for carbon onions. Borgohain et al. used 5–7 nm nanodiamond-derived carbon onions synthesized at 1650 °C functionalized with polar carboxylic groups.²⁶ The carbon material was coated with non-agglomerated ruthenium oxide in a hydrothermal reaction using RuCl₃ as the precursor (Fig. 11E). Upon loading of ruthenium oxide, the surface area decreased from 369 m² g⁻¹ for functionalized carbon onions to 126 m² g⁻¹ for carbon onions with 68 mass% ruthenium oxide. Accordingly, the capacitance increased up to 334 F g⁻¹ for the highest loading of ruthenium oxide measured in 1 M H₂SO₄ by cyclic voltammetry using a full cell setup and a scan rate of 20 mV s⁻¹. The material exhibited excellent power handling when increasing the scan rate from 50 mV s⁻¹ to 8 V s⁻¹ with nearly ideal capacitance retention.²⁶ The combination of high electrical conductivity and enhanced surface area (carbon onions) with redox active species (high faradaic charge storage capacity) benefitted improved accessibility of the redox material and allows taking full advantage of the high ion mobility provided by carbon onions. De-agglomerated RuO₂ nanoparticles in the size range of 2–3 nm have a very high surface-to-volume ratio leading to a large ratio of accessible surface to mass per particle. Facile accessibility of the metal oxide nanoparticles to the electrolyte, combined with the good electrical connection to the conductive carbon substrate, enabled a high rate handling behavior. This is paralleled by high performance stability (almost 100% capacity retention after 3000 charge/discharge cycles).²⁶ In contrast, acid-oxidized carbon onions in the same study without metal oxide decoration underwent irreversible redox reactions due to the surface functional groups and only show ∼85% capacitance retention after 3000 cycles.²⁶

4.3.3 Carbon onions decorated with nickel oxide. Plonska-Brzezinska et al. have explored nickel oxide (hydroxide)/carbon onion hybrids.²⁷ In one of their studies, nanodiamond-derived carbon onions synthesized at 1650 °C were further oxidized in air to remove amorphous carbon. The resulting carbon onions were loaded with nickel hydroxide nanoparticles (mass ratio ca. 4 : 1) in the presence of different modifiers, polyvinylpyrrolidone (PVP), (4-dimethylamino)pyridine (4-DMAP), and pyridinium p-toluenesulfonate (PPST). Through further annealing, the composite was transformed to carbon onion and nickel oxide (mass ratio ca. 4 : 1). Maximum capacitance values measured in a three-electrode configuration in 1 M KOH at 5 mV s⁻¹ scan rate were 306 F g⁻¹ for Ni(OH)₂ and 73 F g⁻¹ for NiO hybrid electrodes with carbon onions. These values were normalized to the total electrode mass to enable a direct comparison with the literature; in the original paper, the reported values of 1225 F g⁻¹ and 291 F g⁻¹ were normalized only to the carbon content which accounts for ∼25 mass% of the total electrode mass. The adjusted values align with values reported for other redox-active hybrid systems, such as a high mass loading of ruthenium oxide (334 F g⁻¹)²⁶ and manganese oxide composites (190 F g⁻¹ (ref. 137) to 575 F g⁻¹ (ref. 86)). Rate handling was determined by cyclic voltammetry for the hybrid electrodes of carbon onions and Ni(OH)₂ or NiO carbon onion composites from 5 mV s⁻¹ to 30 mV s⁻¹ with a drop in capacitance of ∼40% when using Ni(OH)₂ and ∼6% for NiO. The latter presented promising power handling but suffers from a low 73 F g⁻¹ initial capacitance. In contrast, Ni(OH)₂ was characterized by a much higher initial capacitance (300% higher) but has relatively poor power handling.

4.3.4 Carbon onions decorated with quinones and functional groups. Redox-active surface species like quinones can significantly enhance the energy storage capacity (preferably in acidic electrolytes).¹⁴⁵ Such systems often do not qualify as pseudocapacitors because their charge-versus-voltage is not linear (i.e., non-capacitive), so the use of Farad as a unit has been questioned recently to characterize the capacity of such redox-active energy storage materials.^17,22 However, in order to be able to compare the literature data, we still report in this review values in F g⁻¹.

In a study by Anjos et al., nanodiamond-derived carbon onions were decorated with different types of quinones and tested in 1 M H₂SO₄ in a three-electrode setup.¹³⁹ According to the quasielastic neutron scattering experiments by Chathoth et al., quinones arrange parallel to the carbon onion surface due to π interactions between the aromatic rings of the quinones and the carbon planes.^146,147 By scanning from 0.1 V to 0.8 V versus standard hydrogen, a redox peak was seen between 0.4 and 0.5 V due to a proton coupled electron transfer reaction.¹⁴⁸ The highest capacitance of 264 F g⁻¹ was reached in cyclic voltammetry at 5 mV s⁻¹ when using 9,10-phenanthrenequinones.¹³⁹ The rate handling behavior of these electrodes is rather moderate with only 75 F g⁻¹ (i.e., −72%) at 1 V s⁻¹ but, in absolute numbers, still better than for activated carbon (ca. 20 F g⁻¹ at that rate).^129,139 In addition, the performance stability was demonstrated to be high, with the capacitance retention more than 90% after 10 000 charge/discharge cycles.¹³⁹ Using nanodiamond-derived carbon onions embedded in a continuous network of conductive carbon nanofibers and 9,10-phenanthrenequinones, the reported capacitance reached values of 288 F g⁻¹ at low scan rates and an enhanced power handling could be utilized (135 F g⁻¹ at 1 V s⁻¹ = −53%).¹²⁹

The cyclic voltammograms of the carbon fiber/carbon onion composite electrodes decorated with different amounts of quinones are shown in Fig. 14A. The redox peak at ca. −0.2 V vs. Pt increased in area when quinone loading is increased, reaching 288 F g⁻¹ capacitance.¹²⁹ For a better comparability, Fig. 14B provides an overview of the rate handling ability for different quinone-decorated carbon onion systems (data from cyclic voltammetry). As can be seen, the capacitance retention for carbon onion/carbon fiber composites nears 100% at high scan rates (2 V s⁻¹) and even though the absolute value is rather moderate (34 F g⁻¹) it is still higher than what can be achieved at this scan rate for conventional activated carbon (3 F g⁻¹) using the same set-up and electrolyte system. Loading with quinones increased the capacitance at low scan rates to a maximum value of 288 F g⁻¹ at 1 mV s⁻¹,¹²⁹ which is more than 2 times higher than 115 F g⁻¹ for activated carbon at the same rate. For a higher scan rate of 1 V s⁻¹, the capacitance values were still 135 F g⁻¹ (ref. 129) and 75 F g⁻¹.¹³⁹ Testing over 10 000 cycles demonstrated sufficient stability for quinone-decorated carbon onions and quinone-decorated carbon onion/carbon fiber composites with ca. 95% (ref. 139) and ca. 90% (ref. 129) capacitance retention, respectively (Fig. 14C).

Fig. 14 (A) Cyclic voltammograms of carbon onion/carbon fiber composites loaded with different amounts of 9,10-phenanthrenequinones at 10 mV s⁻¹ scan rate.¹²⁹ (B) Rate handling data of different types of carbon onions, activated carbon, and carbon onions loaded with quinones or other functional groups. (C) Stability test using 10 000 cycles (CV) with activated carbon, carbon onion/carbon fiber composites, carbon onions and carbon onion/carbon fiber composites loaded with 9,10-phenanthrenequinones. (D) Cyclic voltammograms of carbon onions during electrochemical oxidation. All measurements were performed in 1 M H₂SO₄.¹⁴⁰ Zeiger et al.: nanodiamond-derived carbon onion/carbon fiber composites (1700 °C synthesis temperature) loaded with 9,10-phenanthrenequinones, three-electrode set-up, free-standing mats.⁸⁷ Anjos et al.: nanodiamond-derived carbon onions (1800 °C synthesis temperature), three-electrode set-up, dropcast on glassy carbon.¹³⁹ Gu et al.: nanodiamond-derived carbon onions (1800 °C synthesis temperature) oxidized with H₂SO₄/HNO₃, two-electrode set-up, 1 M LiPF₆ in PC, PTFE-bound electrodes (10 mass%).¹²⁴ Liu et al.: nanodiamond-derived carbon onions (1650 °C synthesis temperature with further oxidation in air at 400 °C) electrochemically oxidized in 1 M H₂SO₄, three-electrode set-up, dropcast on glassy carbon.¹⁴⁰

Apart from loading carbon onions with redox-active or pseudocapacitive materials, an in situ electrochemical oxidation of carbon onions in an acidic electrolyte can lead to enhanced capacitive behavior. Liu et al. used nanodiamond-derived carbon onions and oxidized them in 1 M H₂SO₄ at a potential from 1.0 to 2.0 V vs. Ag/AgCl.¹⁴⁰ The cyclic voltammograms are shown in Fig. 14D. After the first oxidation cycle, the current increases with a maximum at ∼0.3 V and further develops until a capacitance of 99 F g⁻¹ (120 F g⁻¹ for galvanostatic cycling at 0.5 A g⁻¹) was reached.¹⁴⁰ Due to the insignificant change in specific surface area after electrochemical oxidation, the redox peak is seemingly related to redox-active oxygen functionalities (most probably the quinone–hydroquinone redox couple), which is generated at exfoliated graphene segments of the carbon onions.¹⁴⁰ This assumption is supported by the higher oxygen contents for electrochemically oxidized carbon onions measured with X-ray photoelectron emission spectroscopy (XPS).¹⁴⁰ However, with the higher content of functional groups, the capacitance at higher scan rates decreased to 75 F g⁻¹, which is comparable to what was found for quinone-decorated carbon onions (Fig. 14B).¹³⁹ The electrochemical stability was tested for 1000 cycles with nearly 100% capacitance retention in this range. After treating nanodiamond-derived carbon onions in a H₂SO₄/HNO₃ 3 : 1 mixture to prepare surface oxidized carbon onions the surface area increased from 384 m² g⁻¹ to 578 m² g⁻¹. Carbon onion shells were etched, interparticle amorphous carbon was removed, and the oxygen content increased from 0.5 at% to 18.3 at%.¹²⁴ Electrochemical characterization was performed in a two-electrode coin cell using 1 M lithium hexafluorophosphate (LiPF₆) in a 1 : 1 : 1 mixture of dimethyl carbonate (DMC), diethyl carbonate (DEC) and ethylene carbonate (EC), or 1 M LiPF₆ in PC.¹²⁴ The capacitance values for oxidized carbon onions measured in 1 M LiPF₆ in PC are shown in Fig. 14B. The capacitance more than doubled after oxidation compared to untreated carbon onions for all electrolytes. Even when the values were normalized to the enhanced surface area after oxidation, oxidative treatment still increased the normalized capacitance due to redox contributions.¹²⁴ However, compared to carbon onions with other surface functionalizations (Fig. 14B) or activated carbon, these values are rather small. The reason might be the lower amount of redox-active sites or the necessity for acidic/protic electrolytes.

4.3.5 Carbon onions decorated with electroactive polymers. Electroactive polymers present another attractive group of materials to impart carbon onion electrodes with additional redox-activity. Polyaniline (PANI) is one of the most popular electrochemically active conductive polymers with a high specific capacitance of maximum 430 F g⁻¹ in combination with carbon onions.¹³² Conductive polymers, including PANI, commonly suffer from slow charge and discharge compared to carbon materials and poor long-term stability due to volume changes leading to mechanical damage and disrupting conductivity.¹³² Therefore, a conductive aid is needed, to enhance the conductivity of the composite. In a study by Kovalenko et al., nanodiamond-derived carbon onions were coated with PANI using a wet chemical approach (Fig. 12F).¹³² Pure PANI electrodes demonstrated a capacitance of 470 F g⁻¹ in 1 M H₂SO₄ using a symmetric two-electrode configuration (Fig. 15A). The long-term stability after 10 000 cycles was rather low with less than 150 F g⁻¹ (i.e., −68%).¹³² Electrodes composed of PANI and carbon onions (14 mass% carbon onions) present a more ideal capacitive behavior (Fig. 15B) with slightly smaller capacitance (ca. 430 F g⁻¹), but with a much better performance stability (ca. −3% capacitance after 10 000 cycles).¹³² By increasing the carbon onion amount from 3 to 14 mass%, the capacitance of the composite increased but then decreased when using 28 mass%; thus, there seems to be an optimum load with regard to the electrochemically active surface area.¹³² The filling of PANI with nanoparticles lead to a toughening of the polymer and a less distinct volume change during cycling. Interestingly, purified non-conductive nanodiamonds (3 mass%) also significantly enhance the electrode performance. This may sound surprising as purified nanodiamonds have a very low electrical conductivity (≪1 S cm⁻¹) and points to nanodiamond-mediated mechanical toughening and surface roughening of the polymer as the dominant mechanisms responsible for the enhanced electrochemical performance of the PANI-nanodiamond composite (at least at low scan rates). A larger surface area in contact with the electrolyte is a direct result of the nanoscopic size of carbon onions, nanodiamonds, and nanodiamond soot particles and the change in the surface morphology of the PANI (increased roughness) that can be seen with electron microscopy (Fig. 11F). The best enhancement was found when using nanodiamond-containing raw detonation soot. Apparently, this material, which is a mixture of carbon onions, nanodiamonds, amorphous carbon, and other carbon nanoforms, provides an optimal combination of mechanical strength (nanodiamonds), conductivity (carbon onions, graphitic carbon), and surface roughness (all together).¹³²

Fig. 15 Cyclic voltammograms of nanodiamond-derived carbon onions (1800 °C synthesis temperature) coated with PANI. Electrodes are PTFE-bound (3 mass%) and measured in a two-electrode configuration using 1 M H₂SO₄ recorded at a scan rate of 1 mV s⁻¹. (A) Pure PANI electrodes. (B) Carbon onion/PANI composite with 14 mass% carbon onions. Data adapted from ref. 132.

Plonska-Brzezinska et al. used an in situ polymerization technique by first functionalizing nanodiamond-derived carbon onions with p-phenylenediamine or 4-aminobenzoic acid and subsequent growth of PANI on the surface groups using the synthesis method from ref. 149 and 150. The functionalized carbon onions were soluble in polar solvents and an increased hydrophilicity and wettability was achieved in aqueous electrolytes.⁷⁴ The capacitance measured in 0.1 M H₂SO₄ at 5 mV s⁻¹ was 353 F g⁻¹ for pure PANI, 207 F g⁻¹ for the carbon onion/PANI hybrid with 4-amino-benzoic acid (4-ABAc) and carbon onions, and 155 F g⁻¹ for the hybrid with poly-p-PDA and carbon onions. These values were slightly smaller than those reported in ref. 132, maybe due to the lower molarity of the electrolyte.

Another well-known inexpensive electroactive polymer is polypyrrole (Ppy) with comparable high conductivity and high stability in aqueous electrolytes. In the study by Mykhailiv et al., carbon onion/polypyrrole hybrids were prepared by chemical polymerization of pyrrole.¹³⁸ Carbon onions and sodium dodecyl sulfate (SDS) functionalized carbon onions were coated by Ppy using FeCl₃ and pyrrole (Fig. 12G). Negatively charged surface functional groups of carbon onions interact with the pyrrole monomers leading to nucleation centers for the polymerization of pyrrole.¹³⁸ The largest capacitance was 805 F g⁻¹ in 0.1 M TBA-ClO₄ in ACN at 5 mV s⁻¹ scan rate with cyclic voltammetry and smaller values were obtained at higher rates (384 F g⁻¹ at 100 mV s⁻¹, i.e., −52%). The capacitance value of 384 F g⁻¹ is derived from integration of the cyclic voltammogram and therefore more representative than the values derived from the current at a specific potential in the CV (in this case: 805 F g⁻¹ at 0.1 V versus Ag/AgCl). The strong decrease in capacitance with increasing scan rate can be attributed to the inaccessibility of parts of the electrode at higher rates or degradation phenomena. Degradation of the electrode is supported by the low stability after 500 cycles at 100 mV s⁻¹ of ∼20% and ∼35% depending on the potential window of −1 V to +0.3 V and −2 V to +0.6 V, respectively.¹³⁸

Like for PANI and PPy, carbon onions can also be used as conductive substrates for poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS). Plonska-Brzezinska et al. used nanodiamond-derived carbon onions as well as their acid-oxidized form for coating with PEDOT:PSS layers.⁷³ By increasing the amount of PEDOT:PSS in the composite a maximum capacitance of 95 F g⁻¹ was reached (1 : 1 mass ratio) for the composite with neat carbon onions and 47 F g⁻¹ for the composite with oxidized carbon onions.⁷³

5. Conclusions

This work provides an overview of electrochemical applications of carbon onions, and especially of nanodiamond-derived carbon onions. Several synthesis methods exist, but a standard method, with a high yield, reproducibility, and full control of structure, is the annealing of detonation nanodiamond powders. According to the synthesis conditions several properties, such as surface area and conductivity, can be tuned. Carbon onions as supercapacitor electrode materials present a superior rate handling behavior but suffer from a comparatively low specific energy. To overcome this issue, composites with metal oxides, conducting polymers, other redox species, as well as creation of surface functional groups can be used to induce the redox activity of carbon onions.

Carbon onions present a nearly ideal substrate for coating with redox-active substances due to their external and fully accessible surface area, combined with a high degree of sp²-hybridized carbon ordering. In addition, carbon onions can be used as a potent conductive additive which enhances the power handling ability of supercapacitor carbon electrodes more than conventional carbon black or graphite nanoparticles. Further research aimed at enhancing the surface area of carbon onions and improving the rate handling for advanced redox-active hybrid electrodes will help to further promote the use of this unique carbon nanomaterial for electrochemical applications.

Acknowledgements

The authors thank Prof. Eduard Arzt (INM) for his continuing support. We acknowledge funding from the German Federal Ministry for Research and Education (BMBF) in support of the nanoEES^3D project (award number 03EK3013) as part of the strategic funding initiative energy storage framework. This work was supported by the CREATe-Network Project, Horizon 2020 of the European Commission (RISE Project No. 644013). The authors thank Prof. Gleb Yushin (Georgia Tech) for supporting us with experimental data and useful discussions.

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