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 highly porous 2D nanomaterial, holey graphene oxide (hGO), is synthesized directly from holey graphene powder and employed to create an aqueous 3D printable ink without the use of additives or binders. Stable dispersions of hydrophilic hGO sheets in water (≈100 mg mL-1 ) can be readily achieved. The shear-thinning behavior of the aqueous hGO ink enables extrusion-based printing of fine filaments into complex 3D architectures, such as stacked mesh structures, on arbitrary substrates. The freestanding 3D printed hGO meshes exhibit trimodal porosity: nanoscale (4-25 nm through-holes on hGO sheets), microscale (tens of micrometer-sized pores introduced by lyophilization), and macroscale (<500 µm square pores of the mesh design), which are advantageous for high-performance energy storage devices that rely on interfacial reactions to promote full active-site utilization. To elucidate the benefit of (nano)porosity and structurally conscious designs, the additive-free architectures are demonstrated as the first 3D printed lithium-oxygen (Li-O2 ) cathodes and characterized alongside 3D printed GO-based materials without nanoporosity as well as nanoporous 2D vacuum filtrated films. The results indicate the synergistic effect between 2D nanomaterials, hierarchical porosity, and overall structural design, as well as the promise of a freeform generation of high-energy-density battery systems.

Extrusion‐Based 3D Printing of Hierarchically Porous Advanced Battery Electrodes

Nanotechnology has opened up new frontiers in materials science and engineering in the past several decades. Considerable efforts on nanostructured electrode materials have been made in recent years to fulfill the future requirements of electrochemical energy storage. Compared to bulk materials, most of these nanostructured electrode materials improve the thermodynamic and kinetic properties of electrochemical reactions for achieving high energy and power densities. Here we briefly review the state-of-the-art research activities in the area of nanostructured positive electrode materials for post-lithium ion batteries, including Li–S batteries, Li–Se batteries, aqueous rechargeable lithium batteries, Li–O2 batteries, Na-ion batteries, Mg-ion batteries and Al-ion batteries. These future rechargeable battery systems may offer increased energy densities, reduced cost, and more environmental benignity. A particular focus is directed to the design principles of these nanostructured positive electrode materials and how nanostructuring influences electrochemical performance. Moreover, the recent achievements in nanostructured positive electrode materials for some of the latest emerging rechargeable batteries are also summarized, such as Zn-ion batteries, F- and Cl-ion batteries, Na–, K– and Al–S batteries, Na– and K–O2 batteries, Li–CO2 batteries, novel Zn–air batteries, and hybrid redox flow batteries. To facilitate further research and development, some future research trends and directions are finally discussed.

Nanostructured positive electrode materials for post-lithium ion batteries

Ruthenium (Ru)‐based electrocatalysts as platinum (Pt) alternatives in catalyzing hydrogen evolution reaction (HER) are promising. However, achieving efficient reaction processes on Ru catalysts is still a challenge, especially in alkaline media. Here, the well‐dispersed Ru nanoparticles with adjacent Ru single atoms on carbon substrate (Ru1,n‐NC) is demonstrated to be a superb electrocatalyst for alkaline HER. The obtained Ru1,n‐NC exhibits ultralow overpotential (14.8 mV) and high turnover frequency (1.25 H2 s‐1 at −0.025 V vs reversible hydrogen electrode), much better than the commercial 40 wt.% Pt/C. The analyses reveal that Ru nanoparticles and single sites can promote each other to deliver electrons to the carbon substrate. Eventually, the electronic regulations bring accelerated water dissociation and reduced energy barriers of hydroxide/hydrogen desorption on adjacent Ru sites, then an optimized reaction kinetics for Ru1,n‐NC is obtained to achieve superb hydrogen generation in alkaline media. This work provides a new insight into the catalyst design in simultaneous optimizations of the elementary steps to obtain ideal HER performance in alkaline media.

Synergic Reaction Kinetics over Adjacent Ruthenium Sites for Superb Hydrogen Generation in Alkaline Media

The aprotic Li-O2 battery has attracted a great deal of interest because, theoretically, it can store far more energy than today’s batteries. Toward unlocking the energy capabilities of this neotype energy storage system, noble metal-catalyzed high surface area carbon materials have been widely used as the O2 cathodes, and some of them exhibit excellent electrochemical performances in terms of round-trip efficiency and cycle life. However, whether these outstanding electrochemical performances are backed by the reversible formation/decomposition of Li2O2, i.e., the desired Li-O2 electrochemistry, remains unclear due to a lack of quantitative assays for the Li-O2 cells. Here, noble metal (Ru and Pd)-catalyzed carbon nanotube (CNT) fabrics, prepared by magnetron sputtering, have been used as the O2 cathode in aprotic Li-O2 batteries. The catalyzed Li-O2 cells exhibited considerably high round-trip efficiency and prolonged cycle life, which could match or even surpass some of the best literature results. How...

Reversibility of Noble Metal-Catalyzed Aprotic Li-O2 Batteries

Developing a high-performance Li–O2 battery demands an air electrode with a high-efficiency bifunctional catalyst. Here we designed a new type of bifunctional cathode catalyst by mounting ruthenium nanoparticles on reduced graphene oxide co-doped with nitrogen, iron, and cobalt. The catalyst exhibited significantly higher ORR and OER activities than a commercial Pt/C catalyst in both aqueous and non-aqueous electrolytes. With this novel catalyst as the cathode, the battery exhibited an ultra-high reversible capacity of 23 905 mA h g−1 at a current density of 200 mA g−1. Furthermore, the battery also exhibited an excellent cycling stability—after 300 cycles of limited capacity, the discharge plateau potential decreased only slightly, and the energy efficiency was still above 60%. The battery also demonstrated good rate performance; with discharge current densities of up to 1000 and 2000 mA g−1, the capacities still reached 14 560 and 6420 mA h g−1, respectively. We suggest that the excellent performance of our catalyst can be ascribed to the excellent ORR performance of the multielement co-doped graphene and the excellent OER performance of the mounted Ru nanoparticles. In addition, the nanosheet structure with high surface area of the multielement co-doped graphene may result in the formation of uniform Li2O2 nanocrystals, which make the formation (discharge) and decomposition (charge) processes much more reversible.

Ruthenium nanoparticles mounted on multielement co-doped graphene: an ultra-high-efficiency cathode catalyst for Li–O2 batteries

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Ruthenium nanoparticles mounted on multielement co-doped graphene: an ultra-high-efficiency cathode catalyst for Li–O2 batteries