Since the first report of using micromechanical cleavage method to produce graphene sheets in 2004, graphene/graphene-based nanocomposites have attracted wide attention both for fundamental aspects as well as applications in advanced energy storage and conversion systems. In comparison to other materials, graphene-based nanostructured materials have unique 2D structure, high electronic mobility, exceptional electronic and thermal conductivities, excellent optical transmittance, good mechanical strength, and ultrahigh surface area. Therefore, they are considered as attractive materials for hydrogen (H2) storage and high-performance electrochemical energy storage devices, such as supercapacitors, rechargeable lithium (Li)-ion batteries, Li–sulfur batteries, Li–air batteries, sodium (Na)-ion batteries, Na–air batteries, zinc (Zn)–air batteries, and vanadium redox flow batteries (VRFB), etc., as they can improve the efficiency, capacity, gravimetric energy/power densities, and cycle life of these energy storage devices. In this article, recent progress reported on the synthesis and fabrication of graphene nanocomposite materials for applications in these aforementioned various energy storage systems is reviewed. Importantly, the prospects and future challenges in both scalable manufacturing and more energy storage-related applications are discussed.

Graphene‐Based Nanocomposites for Energy Storage

A novel nanocomposite cathode consisting of sulfur and hollow-mesoporous titania (HMT) embedded within carbon nanotubes (CNT), which is designated as S-HMT@CNT, has been obtained by encapsulating elemental sulfur into the pores of hollow-mesoporous, spherical TiO2 particles that are connected via CNT. A carbon-paper interlayer, referred to as dual functional porous carbon wall (DF-PCW), has been obtained by filling the voids in TiO2 spheres with carbon and then etching the TiO2 template with a chemical process. The DF-PCW interlayer provides a medium for scavenging the lithium polysulfides and suppressing them from diffusing to the anode side when it is inserted between the sulfur cathode and the separator. Lithium–sulfur cells fabricated with the thus prepared S-HMT@CNT cathode and the DF-PCW interlayer exhibit superior performance due to the containment of sulfur in TiO2 and improved lithium–ion and electron transports. The Li–S cells display high capacity with excellent capacity retention at rates as high as 1C, 2C, and 5C rates.

High‐Energy, High‐Rate, Lithium–Sulfur Batteries: Synergetic Effect of Hollow TiO2‐Webbed Carbon Nanotubes and a Dual Functional Carbon‐Paper Interlayer

The most important challenge in the practical development of lithium–sulfur (Li–S) batteries is finding suitable cathode materials. Due to the complexity of this system, various factors have been investigated during the last years, but still, the roadmap for designing the best cathode candidates is not vivid. This review attempts to create a better picture of the cathode process to pave the path for developing more practical cathode materials. The most promising candidates as the host cathode material are porous carbon nanomaterials, which are highly conductive and lightweight while having the capability for fabricating freestanding electrodes. In this case, there is no need for a binder and current collector, and thus, a significantly higher energy density can be expected. Despite the good performance of these carbon-based sulfur cathodes, the presence of some additives anchoring the sulfur molecules to the electrode backbone seems necessary for practical performance. Metal oxides and sulfides are among the best options as additives, as they can act as mediators in the electrochemical redox system of Li/S. Despite their similarities, these additives might mediate in the battery system but via entirely different mechanisms. In addition to carbon nanomaterials, other porous materials, such as metal–organic frameworks, can also provide a cage-like architecture for the construction of the sulfur cathode.

Cathode materials for lithium–sulfur batteries: a practical perspective

Photocatalytic hydrogen production from overall water splitting is a clean and renewable technology that can convert solar energy into chemical energy, for which developing an efficient and stable photocatalyst has been the central scientific topic. Herein, CoO/g-C3N4 heterojunction photocatalysts were fabricated through a facile solvothermal method for overall water splitting. Simultaneous evolution of H2 and O2 from pure water with the stoichiometric ratio of about 2:1 achieved with all the CoO/g-C3N4 heterojunctions as catalysts under visible light irradiation. Among of them, 30 wt.% CoO/g-C3N4 with H2 evolution rate of 2.51 μmol/h and O2 evolution rate of 1.39 μmol/h also exhibited remarkably higher photocatalytic performance and stability (over 15 cycles) than single CoO or g-C3N4. This enhanced photocatalytic activity of CoO/g-C3N4 heterojunction can be ascribed to the synergistic effect of junction and interface formed between CoO and g-C3N4. In addition, the sufficient long lifetime stability of CoO/g-C3N4 comes from the complementary advantages effect between CoO and g-C3N4, as is proved, CoO can protect g-C3N4 from H2O2 poisoning, and simultaneously the photo-induced heat from CoO during the photocatalytic process responsible for the rapid deactivation can be timely conducted to g-C3N4.

CoO and g-C3N4 complement each other for highly efficient overall water splitting under visible light

Lithium–sulfur (Li–S) batteries have attracted considerable attention due to their high theoretical energy density and low cost. However, their practical applications are greatly limited by rapid capacity degradation because of the unfavorable reaction between soluble intermediate polysulfides and the lithium anode. Recently, various strategies have been devoted to addressing the above polysulfide shuttle issue and to improving the performance of Li–S batteries. Herein, a comprehensive review related to sulfur-based cathode designs, separator functional-modifications, lithium anode improvements, and novel electrolyte systems is presented for extending the cycle life of Li–S batteries. In addition, the prospects of promising approaches regarding the development of practical Li–S batteries are presented.

Effective strategies for long-cycle life lithium–sulfur batteries

A new layered porous nanostructure with ZrO2 nanoparticles attached on the reduced graphene oxide (ZrO2/RGO) was synthesized by a facile solvothermal process. The resulting ZrO2/RGO composite with well-designed mesoporous structure and excellent conductivity not only served as scaffold to house sulfur but also as polysulfide reservoir for lithium–sulfur batteries. This nanostructured S@ZrO2/RGO electrode exhibits enhanced cycling stability, high specific capacity, and superior coulombic efficiency.

A layered porous ZrO2/RGO composite as sulfur host for lithium─sulfur batteries

Porous cobalt–nitrogen-doped hollow graphene spheres were prepared by a template synthesis method. As a catalyst for the oxygen reduction reaction, they exhibit an excellent electrocatalytic activity, superior methanol tolerance and strong durability, not only in alkaline solution, but also in acidic solution. The unprecedented electrocatalytic performance of the catalyst is attributed to the well-defined morphology, high specific surface area (321 m2 g−1), large pore volume (1.8 cm3 g−1) and homogeneous distribution of cobalt–nitrogen active sites.

Porous cobalt–nitrogen-doped hollow graphene spheres as a superior electrocatalyst for enhanced oxygen reduction in both alkaline and acidic solutions

3D hierarchical porous graphene laminates (GLs) with high surface area and porosity were synthesized through self-assembly of functionalized graphene oxide embedded with SiO2 in situ. Thus, as a cathode material for Li–S batteries, the obtained GLs loaded with 73 wt% sulfur, deliver a high discharge capacity of 800 mA h g−1 after 100 cycles at a current density of 0.2C.

Embedding SiO2 into graphene oxide in situ to generate 3D hierarchical porous graphene laminates for high performance lithium–sulfur batteries

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A layered porous ZrO2/RGO composite as sulfur host for lithium─sulfur batteries

Porous cobalt–nitrogen-doped hollow graphene spheres as a superior electrocatalyst for enhanced oxygen reduction in both alkaline and acidic solutions

Embedding SiO2 into graphene oxide in situ to generate 3D hierarchical porous graphene laminates for high performance lithium–sulfur batteries