Nanocatalyst-Assisted Fine Tailoring of Pore Structure in Holey-Graphene for Enhanced Performance in Energy Storage

Scalable Dry-Pressed Electrodes Based on Holey Graphene

ConspectusHoley graphene (hG) is a structural derivative of graphene with arrays of through-thickness holes of a few to tens of nanometers in diameter, randomly distributed across the nanosheet surfaces. In most bulk preparation methods, the holes on hG sheets are preferentially generated from the pre-existing defects on graphene. Therefore, contrary to intuitive belief, hG is not necessarily more defective than the intact graphene. Instead, it retains essential parent properties, including high electrical conductivity, high surface area, mechanical robustness, and chemical inertness. Furthermore, the added holey structural motif imparts unique properties that are not present in unmodified graphene, making hG advantageous in numerous applications such as sensing, membranes, reinforcements, and electrochemical energy storage. In particular, the presence of holes enhances the mass transport through the nanosheet plane and thus significantly reduces tortuosity. This difference is a key advantage for using hG in energy storage applications where the transport of ions through the thickness becomes more hindered as the electrode thickness increases to meet practical energy density requirements.An unexpected discovery is that the holes of the hG sheets enable the dry hG powder to be directly compressed into robust monoliths. hG not only can be pressed into monoliths by itself but also can host other electrochemically active materials as a compressible matrix. This important yet unique property, which is not available for other carbon materials including intact graphene, significantly broadens the application horizon in energy storage applications. With the dry compressibility, electrodes with ultrahigh mass loading and thus ultrahigh areal capacity may be conveniently fabricated without toxic solvents or parasitic binders, which are required in conventional slurry-based approaches for electrode fabrication. The dry-press electrode preparation process can be completed within minutes regardless of mass loading. In comparison, high-mass-loading electrodes for advanced battery chemistries using conventional fabrication methods often need stringent and time-consuming process control. hG can also be combined with electrochemically active battery materials while maintaining dry compressibility. This has allowed the unprecedented, convenient manipulation of a wide variety of thick electrode compositions and architectures, which provides not only outstanding performance but also new physical insights for various battery chemistries.In this Account, we first present some basic observations on the dry compressibility of hG as well as the mechanistic investigations from atomistic modeling rationalizing this unique property. We then showcase the applications of neat and composite dry-pressed hG electrodes for various energy storage platforms including supercapacitors, lithium (Li) ion batteries, Li-O₂ batteries, and Li-S/Se batteries. The preparation and performance of thick electrodes with practical mass loadings and unique electrode architecture manipulation, both enabled by the dry compressibility of hG, are highlighted and discussed.

Custom-made holey graphene via scanning probe block co-polymer lithography

Oxidative chemical etching of metal nanoparticles (NPs) to produce holey graphene (hG) suffers from the presence of aggregated NPs on the graphene surface triggering heterogeneous etching rates and thereby producing irregular sized holes. To encounter such a challenge, we investigated the use of scanning probe block co-polymer lithography (SPBCL) to fabricate precisely positioned silver nanoparticles (AgNPs) on graphene surfaces with exquisite control over the NP size to prevent their aggregation and consequently produce uniformly distributed holes after oxidative chemical etching. SPBCL experiments were carried out via printing an ink suspension consisting of poly(ethylene oxide-b-2-vinylpyridine) and silver nitrate on a graphene surface in a selected pattern under controlled environmental and instrumental parameters followed by thermal annealing in a gaseous environment to fabricate AgNPs. Scanning electron microscopy revealed the uniform size distribution of AgNPs on the graphene surface with minimal to no aggregation. Four main sizes of AgNPs were obtained (37 ± 3, 45 ± 3, 54 ± 2, and 64 ± 3 nm) via controlling the printing force, z-piezo extension, and dwell time. Energy dispersive X-ray spectroscopy analysis validated the existence of elemental Ag on the graphene surface. Subsequent chemical etching of AgNPs using nitric acid (HNO₃) with the aid of sonication and mechanical agitation produced holes of uniform size distribution generating hG. The obtained I _D/I _G ratios ≤ 0.96 measured by Raman spectroscopy were lower than those commonly reported for GO (I _D/I _G > 1), indicating the removal of more defective C atoms during the etching process to produce hG while preserving the remaining C atoms in ordered or crystalline structures. Indeed, SPBCL could be utilized to fabricate uniformly distributed AgNPs of controlled sizes on graphene surfaces to ultimately produce hG of uniform hole size distribution.

Sulfonated carbons from agro-industrial residues: simple and efficient catalysts for the Biginelli reaction

An eco-friendly catalyst prepared from rice husk was used to synthesize dihydropyrimidinones (DHPMs) and achieved a yield of 92%.

Synthesis of holey graphene for advanced nanotechnological applications

Holey or porous graphene, a structural derivative of graphene, has attracted immense attention due to its unique properties and potential applications in different branches of science and technology. In this review, the synthesis methods of holey or porous graphene/graphene oxide are systematically summarized and their potential applications in different areas are discussed. The process-structure-applications are explained, which helps relate the synthesis approaches to their corresponding key applications. The review paper is anticipated to benefit the readers in understanding the different synthesis methods of holey graphene, their key parameters to control the pore size distribution, advantages and limitations, and their potential applications in various fields.

Separation of CH4, H2S, N2 and CO2 gases using four types of nanoporous graphene cluster model: a quantum chemical investigation

Nanoporous graphene is being regarded as a promising candidate for reliable gas separation and purification applications. In the present research, the permeation barrier, selectivity and all thermodynamic functions for passing of four different molecules including CH₄, H₂S, N₂ and CO₂ gases on four types of porous graphene which is doped by two, three and six nitrogen atoms using quantum mechanical modelling, based on the density functional theory, B97D, and cc-pVTZ basis set have been evaluated. We find that the permeation barrier of all studied gases especially carbon dioxide decreased by considering the functionalized porous graphene by two, three and six nitrogens-doped, respectively. The results of our study propose using a porous graphene sheet as highly efficient and highly selective membranes for gas separations.

Solving Gravimetric-Volumetric Capacitive Paradox of 2D Materials through Dual-Functional Chemical Bonding-Induced Self-Constructing Graphene-MXene Monoliths

High electrical conductivity and all-open microstructure characteristics intrinsically endow both graphene and MXenes with superior electrochemical energy storage capability. However, the above two-dimensional (2D) thicker electrodes (>20 μm) severely dilute their unique rapid electronic-ionic transferring characteristic, posing a paradox of high gravimetric and high volumetric capacitive properties due to massively excessive macropores or an unduly restacked issue. Herein, we elaborately construct novel monolithic NH₂-graphene and Ti₃C₂T_x MXene (NG@MX) composites through dual-functional induced self-assembly with the help of both covalent and hydrogen bonding interactions. Notably, much thicker monolithic NG@MX electrodes (>90 μm) fabricated by a conventional roll-coating method without any further compaction treatment can simultaneously deliver two times gravimetric (gra.) and volumetric (vol.) performance than those of pure graphene (in vol.) or MXene (in gra.) materials. Moreover, monolithic NG@MX-based supercapacitors can remarkably present two times energy density as that of graphene and four times as MXene, respectively. Such greatly enhanced electrochemical properties are closely related to the appropriate equilibrium of the volumetric density and the open structure, which can effectively guarantee the rapid transfer of both electrons and ions in the thick monolithic NG@MX electrodes. Undoubtedly, dual-functional chemical bonding-induced self-constructing NG@MX monoliths efficiently solve the long-existing gra. and vol. capacitive paradox of the thicker 2D materials used in supercapacitors, which will guide the design of high-performance capacitive materials and promote their practical application in electrochemical energy storage.