Structural hierarchy is ubiquitous in nature and quite important for optimizing the properties of functional materials. Carbon nanomaterials, owing to their unique and tunable physical and chemical properties, have been regarded as promising candidates for various energy storage systems. Constructing hierarchically structured carbon nanomaterials (HSCNs) can boost electrochemical performance of nanocarbons. Therefore, HSCNs have attracted tremendous research attentions in recent years. In this review, we summarized the recent progress in hierarchical structure design of carbon nanomaterials and their potential applications in different energy storage technologies. First we give a brief introduction about carbon nanomaterials and the hierarchical structure merits. Subsequently, recent research works on hierarchical structure design of carbon nanomaterials was summarized and classified according to applications in lithium-ion batteries, sodium-ion batteries, supercapacitors and lithium–sulfur batteries, respectively. In addition, the challenges of HSCNs in different applications were also concluded and reviewed. At last, design principles of HSCNs were summarized and future development trends were prospected.

Organic gels obtained by sol–gel polycondensation reaction followed by subcritical drying in ambient conditions are termed as xerogels which are pyrolyzed to yield carbon xerogels. Resorcinol formaldehyde (RF) derived carbon xerogels have received considerable attention due to their higher carbon yield and ease of tuning their microstructure and therefore physiochemical properties. Recent advances in the synthesis of carbon xerogels have allowed porous as well as non-porous but large external surface area morphologies. Further efforts have been made about increasing the surface area by activation or changing the microstructure by doping with foreign elements. These advances in the area of carbon xerogels synthesis led to their use as high performance anode materials for Li ion batteries recently. This review summarizes these recent studies on electrochemical performance of carbon xerogels to clearly demonstrate their potential as high capacity anode material for Li ion batteries. Notably, given the potential not only for Li ion batteries but also for latest sodium-ion batteries and super-capacitors, this review provides a much needed attention of scientific community to so far unnoticed carbon xerogel materials.

Chemical vapor deposition (CVD) of graphene has attracted high interest in the electronics industry due to its potential scalability for large-scale production. However, producing a homogeneous thin-film graphene with minimal defects remains a challenge. Studies of processing parameters, such as gas precursors, flow rates, pressures, temperatures, and substrate types, focus on improving the chemical aspect of the deposition. Despite the many reports on such parameters, studies on fluid dynamic aspects also need to be considered since they are crucial factors in scaling up the system for homogenous deposition. Once the deposition kinetics is thoroughly understood, the next vital step is fluid dynamics optimization to design a large-scale system that could deliver the gas uniformly and ensure maximum deposition rate with the desired property. In this review, the influence of fluid dynamics in graphene CVD process was highlighted. The basics and importance of CVD fluid dynamics was introduced. It is understood that the fluid dynamics of gases can be controlled in two ways: via reactor modification and gas composition. This paper begins first with discussions on horizontal tubular reactor modifications. This is followed by mechanical properties of the reactant gasses especially in terms of dimensionless Reynolds number which provides information on gas flow regime for graphene CVD process at atmospheric pressure. Data from the previous literature provide the Reynolds number for various gas compositions and its relation to graphene quality. It has been revealed that hydrogen has a major influence on the fluid dynamic conditions within the CVD, hence affecting the quality of the graphene produced. Focusing on atmospheric pressure CVD, suggestions for up-scaling into larger CVD reactors while maintaining similar fluid properties were also provided.

Heteroatom-doped carbon materials have attracted immense interest as advanced supercapacitor electrode materials due to their unique properties. A carbon cloth-supported, nitrogen-doped carbon “spider web” network full of macropores and mesopores is developed via the pyrolysis of polyaniline nanofibers in ammonia atmosphere. The presence of mesopores and macropores can provide ion-buffering reservoirs to shorten the ion diffusion distance to the interior part of the carbon network. Carbonization in ammonia introduced N heteroatoms through gas phase chemical reactions between ammonia and the oxygen functionalities on the carbon surface. The enhanced ion-accessible surface area and improved charge transfer rate can be achieved. The N-doped carbon “spider web” exhibited a high specific capacitance of 266 F/g at a scan rate of 2 mV/s. Even when the scan rate was increased to 500 mV/s, 61% of its capacitance could still be retained, evidencing its excellent rate performance. The demonstrated strategy is anticipated to be generally effective for preparing heteroatom-doped carbon electrodes with other polymers.

Carbon nanofibers are prepared via the electrospinning method accompanied by the phase-separation process using polyacrylonitrile as a carbon precursor. Effects of preoxidation and carbonation temperatures on electrochemical performance are studied and optimized in detail. The morphology and porous structure are characterized by scanning electron microscope, transmission electron microscope, and nitrogen adsorption and desorption measurements, respectively; the electrochemical performances are measured by the CHI660E workstation. The results show that the diameter of carbon nanofibers is about 150–200 nm with a uniform and smooth surface. The optimized preoxidation temperature is 280 °C with a carbonation temperature of 700 °C. The highest capacitance is up to 155 F/g, and the symmetric supercapacitor delivers a maximum energy density of 7.78 W h/kg with a power density of 400 W/kg and a maximum power density of 4000 W/kg with an energy density of 2.0 W h/kg. The symmetric supercapacitor also exhibits good cycle stability 91.0% of initial specific capacitance after 5000 cycles.

Three-dimensional nitrogen-doped graphene frameworks (3DNGFs) hold great promise in the application of supercapacitors for the advantages of superior conductivity, well-connected porous structure, and additional N-containing active sites for charge storage. Nevertheless, the developed techniques for preparing 3DNGFs always suffer from the drawbacks of high cost or complex processes for large-scale application. Herein, a new and cost-effective method has been developed to massively prepare monolithic 3DNGFs by a simple and scalable electrochemical oxidation process. Benefiting from the enhanced electrical conductivity, the increase in the pore volume for fast diffusion increased the electrode–electrolyte contact area and additional active sites resulting from the incorporation of nitrogen species, the 3DNGFs showed a high specific capacitance of 2250.3 mF/cm2 at 4 mA/cm2, with good rate capability. Furthermore, this 3DNGFs electrode also owns an excellent long-term cycling stability that can retain more than 97.5% of its original capacitance after 10,000 cycles.

Porous carbon nanomaterials with significant capacitive performance were successfully prepared through a simple two-step process of thermal-polymerization and carbonization without an additional template. As a result, the as-prepared porous carbon nanomaterials of sample-A and sample-B exhibited an amorphous phase with low graphitization. And sample-A showed a moderate specific surface area of 476.39 m2/g, larger than that of sample-B (280.94 m2/g). The relatively high mass specific capacitance of 205.1 F/g at a scan rate of 5 mV/s and 211 F/g at a current density of 4 A/g was obtained by sample-A, which are higher than those of sample-B (82.6 F/g at 5 mV/s and 78.6 F/g at 4 A/g). Sample-A also showed excellent conductivity and superior cyclic stability with 94.19% capacitance retention after 5000 cycles, which are also higher than those of sample-B. This work proposed a cost-effective, green, and promising strategy for the large-scale preparation of porous carbon nanomaterial electrodes.

In this article, we report the synthesis of highly packed graphene oxide-based electrodes (1.25 g/cm3) with a three-dimensional multiscale porous structure (denoted as MPGP) through the ZnO nanodisk (100–500 nm) template and subsequent H2O2 treatment. Consequently, MPGP with a macropore diameter of 100 nm and a mesopore diameter of 2–3 nm was fabricated as the electrode for supercapacitors (SCs). Significantly, the MPGP achieves a high-volumetric capacitance of 327 F/cm3 (262 F/g) at a current density of 1 A/g and retains 240 F/cm3 (192 F/g) at a current density of 16 A/g in 3 M KOH solution. More importantly, it was also capable of delivering a high-volumetric energy density as well as power density in a SC device. Our work shows that the capability of preparing highly packed graphene-based electrodes with high-volumetric as well as specific capacitance is critical for the application of SCs.

Binary transition metal oxides, such as NiCo2O4 (NCO) electrode have been demonstrated to be promising candidates for high-performance supercapacitors. However, their low electrical conductivity and poor stability made the electrochemical performance of most current NCO electrodes yet below the expectation. Herein, a novel electrode (NCC) consisting of binary transition metal (Ni–Co) nanoparticles embedded N-doped porous carbon matrix on graphite papers (GPs) has been developed with a high specific capacitance of 933.5 F/g at 1 mA/cm2 which is substantially 10 times than that of the NCO electrode (99.3 F/g) and much higher than those of most reported NCO based electrode. Moreover, this NCC electrode has an ultrahigh rate capability of 725.5 F/g at 10 mA/cm2 with excellent electrochemical durability (no capacitance decreases after 10,000 cycles). These results indicate a promising potential application of Ni–Co metal composite embedded carbon matrix for using as an effective electrode material in supercapacitors.

High-voltage (≥4.0 V) operation of supercapacitor devices was demonstrated using carbon nanotubes as active electrode materials combined with room temperature ionic liquids as electrolyte. Pouch cells were assembled with four different ionic liquids, 1-butyl-3-methylimidazolium tetrafluoroborate (BMIM-BF4), diethyl-N-methyl-N-(2-methoxyethyl) ammonium bis(trifluoromethanesulfonyl)imide (DEME-TFSI), diethyl-N-methyl-N-(2-methoxyethyl)ammonium tetrafluoroborate (DEME-BF4), and 1-butyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (Pyr14-TFSI). Cyclic voltammetry showed the maximum operational voltage to be 4.5 V for DEME-TFSI and 4.7 V for DEME-BF4. Compared to electric double layer capacitor (EDLC) cells using propylene carbonate electrolyte at 2.7 V, capacitance increased by 20% using BMIM-BF4 at 4.0 V, DEME-TFSI at 4.5 V, DEME-BF4 at 4.7 V, and Pyr14-TFSI at 4.3 V, with tripling of energy density and comparable power density using Pyr14-TFSI-based EDLCs. Long-term cyclability using BMIM-BF4 ionic liquid electrolyte operating at 4.0 V showed retention of >80% of initial capacitance after 65,000 continuous cycles without doubling of initial cell equivalent series resistance.

A hierarchical porous carbon with bimodal pore size distribution was synthesized using pyrolysis and controlled activation of polymer gel derived from furfuryl alcohol and phloroglucinol using a soft templating approach. Symmetric capacitors made using the synthesized carbon electrodes in neat 1-butyl 3-methylimidazolium tetrafluoroborate showed a specific capacitance of 141 F/g at 1 A/g when cycled between 0 and 3.8 V at room temperature. The capacitor also showed 90% capacitance retention over 5000 cycles. Equivalent circuit modeling of impedance spectra was done to monitor changes and provide microstructural details during cycling and temperature studies. Cyclic voltammetry showed the presence of specific adsorption of the electrolyte ions on the carbon electrode and this was in good agreement with strong dependence of specific capacitance on temperature. Such strong interaction along with the nature of electrical double layer formed in the hierarchical porous carbon results in widening the voltage stability window of the capacitor.

We develop an asymmetric aqueous supercapacitor using iron oxide anode and cobalt oxide cathode. The anode was fabricated using electrospinning of carbon precursor/iron oxide precursor blend followed by pyrolysis and in situ electrochemical conversion (to oxide) to form the binder-free and freestanding composite anode which delivered a capacitance of 460 F/g at 1 A/g and retained 82% capacitance after 5000 cycles. The superior performance is attributed to easy electrolyte accessibility as well as the porous fibrous carbon morphology, facilitating volume expansion of iron oxide. The cobalt oxide cathode was prepared using a simple chemical synthesis technique. The electrodes were chosen based on high over potential to water splitting reactions in 6 M KOH electrolyte resulting in a potential window of 1.6 V. The asymmetric device operated in 1.6 V achieved a capacitance of 94.5 F/g at 0.5 A/g while retaining 75% of its capacitance after 12,000 cycles, delivering energy and power densities of 40.53 W h/kg and 2432 W/kg, respectively.

Lignin-based phenol formaldehyde resin was synthesized using phenolated calcium lignosulfonate, and porous carbon with good wettability was prepared after carbonization and potassium hydroxide (KOH) activation. The results indicated that when the KOH to the carbonized sample mass ratio was 6:1, the prepared carbon had a rich porous structure and higher surface area, with a specific surface area of 1320.13 m2/g. Furthermore, the porous carbon exhibited a maximum specific capacitance of 204.88 F/g at a current density of 0.5 A/g in the potential range −1.0 to 0 V in a 6 M KOH solution and a low equivalent series resistance of 0.64 Ω. The phenolated calcium lignosulfonate-based phenol formaldehyde resin porous carbon demonstrated a favorable electric double-layer performance.

This work designed a facile preparation for an SiO2/C composite as the anode material for lithium ion battery. Both SiO2 and carbon are amorphous. SiO2 and carbon are mixed uniformly. The SiO2/C composite shows high specific capacity, cycle stability, and rate capability in lithium ion battery charge–discharge test. A stable reversible capacity of 1024 mA h/g at the current density of 100 mA/g is reached. The capacity retains 83% after 100 cycles. The uniform mixture of SiO2 and carbon leads to reduced volume change during the lithiation and delithiation of SiO2, together with the amorphous nature of SiO2 explains the high cycling stability. The carbon coating is a key factor for the high capacity and stability due to the increased electrical conductivity and reduced volume change. The resistance of the solid electrolyte interface film and charge transfer resistance of the SiO2/C composite are much smaller than those of pure carbon, which is a direct proof of the improved conductivity of the material by the carbon coating.

A facile synthesis procedure of nitrogen-self-doped porous carbon (NPC) derived from abundant natural biological materials has been presented. The pyrolysis temperature and the weight ratio of Co3O4 to carbon play a key role in determining microscopic structure and electrochemical performances of the final materials. The ordered mesostructures with nanopores in the channel walls provided support for immobilization of well-dispersed Co3O4 nanoparticles. They also served as a highly conductive substrate for effectively alleviating severe particle aggregation during the charge/discharge processes, which prevented capacity fading from deteriorated electric contact between the components. Taking advantage of the interconnected porous structures and high specific surface area (1799 m2/g) of carbon substrate, the Co3O4/NPC composite as anode in lithium-ion battery delivers a stable reversible capacity of 903 mA h/g after 400 cycles. It is expected that by loading other electrode active materials on such carbon material, the manufacture of the promising anode materials with excellent cycle stability is highly possible.

Porous carbon derived from biomass materials with enrich, low cost, clean, and renewable merits, exhibits various physical and chemical properties. So, it is of great significance to rationally utilize biomass materials for producing porous carbon with low cost to reduce overusing fossil fuel and environmental pollution. In this report, porous carbon has been fabricated using fruits shells of the Paulownia tomentosa by a facile method of KOH-activation. The as-obtained porous carbon containing a larger number of micropores and slight mesopores possesses a high specific surface area (1914.4 m2/g) and well hierarchical porosity. As the anode for sodium ion batteries, the porous carbon sample displays superior cycling stability and rate capability, delivering a reversible specific capacity of 179 mA h/g at 50 mA/g after 100 cycles and a discharge specific capacity of 100 mA h/g at 1 A/g.

The catalytic property toward oxygen reduction reaction (ORR) plays a significant role in the power generation of fuel cells (FCs). Here we demonstrate a graphene/activated carbon aerogel (GA/AC) composite to facilitate the ORR process, which is synthesized by a one-step hydrothermal method. The aligned pores and high porosity enable its mass density 20-times lighter than bare AC. Electrochemical studies show that the composite exhibits a remarkably improved electro-catalytic performance. The onset potential shifts positively from 0.68 to 0.83 V, and the number of electrons transferred is increased from 2.85 to 3.52, indicating that a four-electron pathway dominates the ORR process. This composite presents a mesoporous structure containing a large number of multi-scale pores and having a high specific surface area of 758.19 m2/g, which is responsible for its excellent onset potential and charge transfer rate. These aerogel-composites show great potential as ORR catalysts for assembling lightweight FCs and metal-air batteries.

The development of electrocatalysts with high activity and low cost has attracted growing attentions in recent years. Herein, we reported the Mn-doped CoP nanosheet arrays on flexible activated carbon cloth (Mn–CoP/CC) for the effective oxygen evolution reaction (OER) at low overpotential and high current density. Due to the novel 3D nanostructures of the carbon cloth and doping effect of the Mn element, the Mn doped CoP/CC electrode delivered the best overpotential of 317 mV for water splitting with the current density of 10 mA/cm2, a Tafel slope of ∼65.1 mV/dec, and excellent stability over 16 h in 1.0 mol/L KOH, which is superior or comparable to the most of the reported cobalt-based catalysts. Thus outstanding electrocatalytic performance originates from the Mn doping effect, which resulted in increased surface area and fast charge-transfer. It is believed that these findings would help us to develop high effective and stable electrocatalysts for water splitting.

Graphitic carbon nitride (g-C3N4) is considered as a promising heterogeneous catalyst for photocatalytic H2 evolution from water under visible light illustration, and its photocatalytic performance could be controlled through its texture and optical/electronic properties. Herein, we present a facile one-step heating method for the synthesis of B/P/F doped g-C3N4 photocatalysts (BCN, PCN, and FCN). The prepared photocatalysts were characterized by XRD, SEM, UV-vis absorption, FTIR, BET, XPS, PL, and photocurrent measurement. The results show that the B/P/F doping increased the interplanar stacking distance of g-C3N4, enlarged the optical absorption range, and improved the photocatalytic activity of H2 evolution. FCN exhibits the highest photocatalytic activity, followed by BCN, and PCN that has the lowest performance. This work studies the doping effects of the nonmetal elements on the photocatalytic activities, the electronic structures as well as the band gaps of g-C3N4, to provide a feasible modification pathway to design and synthesize highly efficient photocatalysts.