Nitrogen doped graphene with diamond-like bonds achieves unprecedented energy density at high power in a symmetric sustainable supercapacitor.

Supercapacitors have attracted great interest because of their fast, reversible operation and sustainability. However, their energy densities remain lower than those of batteries. In the last decade, supercapacitors with an energy content of ∼110 W h L-1 at a power of ∼1 kW L-1 were developed by leveraging the open framework structure of graphene-related architectures. Here, we report that the reaction of fluorographene with azide anions enables the preparation of a material combining graphene-type sp2 layers with tetrahedral carbon-carbon bonds and nitrogen (pyridinic and pyrrolic) superdoping (16%). Theoretical investigations showed that the C-C bonds develop between carbon-centered radicals, which emerge in the vicinity of the nitrogen dopants. This material, with diamond-like bonds and an ultra-high mass density of 2.8 g cm-3, is an excellent host for the ions, delivering unprecedented energy densities of 200 W h L-1 at a power of 2.6 kW L-1 and 143 W h L-1 at 52 kW L-1. These findings open a route to materials whose properties may enable a transformative improvement in the performance of supercapacitor components.

On-Surface Synthesis of One-Dimensional Coordination Polymers with Tailored Magnetic Anisotropy.

One-dimensional (1D) metalloporphyrin polymers can exhibit magnetism, depending on the central metal ion and the surrounding ligand field. The possibility of tailoring the magnetic signal in such nanostructures is highly desirable for potential spintronic devices. We present low-temperature (4.2 K) scanning tunneling microscopy and spectroscopy (LT-STM/STS) in combination with high-resolution atomic force microscopy (AFM) and a density functional theory (DFT) study of a two-step synthetic protocol to grow a robust Fe-porphyrin-based 1D polymer on-surface and to tune its magnetic properties. A thermally assisted Ullmann-like coupling reaction of Fe(III)diphenyl-bromine-porphyrin (2BrFeDPP-Cl) on Au(111) in ultra-high vacuum results in long (up to 50 nm) 1D metal-organic wires with regularly distributed magnetic and (electronically) independent porphyrins units, as confirmed by STM images. Thermally controlled C-H bond activation leads to conformational changes in the porphyrin units, which results in molecular planarization steered by 2D surface confinement, as confirmed by high-resolution AFM images. Spin-flip STS images in combination with DFT self-consistent spin-orbit coupling calculations of porphyrin units with different structural conformations reveal that the magnetic anisotropy of the triplet ground state of the central Fe ion units drops down substantially upon intramolecular rearrangements. These results point out to new opportunities for realizing and studying well-defined 1D organic magnets on surfaces and demonstrate the feasibility of tailoring their magnetic properties.

Tailoring Electronic and Magnetic Properties of Graphene by Phosphorus Doping.

The electronic and magnetic properties of graphene can be modulated by doping it with other elements, especially those with a different number of valence electrons. In this article, we first provide a three-dimensional reconstruction of the atomic structure of a phosphorus substitution in graphene using aberration-corrected scanning transmission electron microscopy. Turning then to theoretical calculations based on the density functional theory (DFT), we show that doping phosphorus in various bonding configurations can induce magnetism in graphene. Our simulations reveal that the electronic and magnetic properties of P-doped (Gr-P) and/or phosphono-functionalized graphene (Gr-PO3H2) can be controlled by both the phosphorus concentration and configurations, ultimately leading to ferromagnetic (FM) and/or antiferromagnetic (AFM) features with the transition temperature up to room temperature. We also calculate core-level binding energies of variously bonded P to facilitate X-ray photoelectron spectroscopy-based identification of its chemical form present in P-doped graphene-based structures. These results may enable the design of graphene-based organic magnets with tailored properties for future magnetic or spintronic applications.

Thermally reduced fluorographenes as efficient electrode materials for supercapacitors.

There is an urgent need for a simple and up-scalable method for the preparation of supercapacitor electrode materials due to increasing global energy consumption worldwide. We have discovered that fluorographene exhibits great potential for the development of new kinds of supercapacitors aimed at practical applications. We have shown that time control of isothermal reduction of fluorographite at 450 °C under a hydrogen atmosphere led to the fine-tuning of fluorine content and electronic properties of the resulting fluorographene derivatives. Charge transfer resistances (Rct) of the thermally reduced fluorographenes (TRFGs) were decreased with respect to the pristine fluorographene; however, the Rctvs. time-of-reduction plot showed a v-shaped profile. The specific capacitance vs. time-of-reduction of TRFG followed the v-shaped trend, which could be the result of the decreasing content of sp3 carbons and increasing content of structural defects. An optimized material exhibited values of specific capacitance up to 539 F g-1 recorded at a current density of 0.25 A g-1 and excellent cycling durability with 100% specific capacitance retention after 1500 cycles in a three-electrode configuration and 96.7% of specific capacitance after 30 000 cycles in a two-electrode setup.

Tuning the magnetic properties of graphene derivatives by functional group selection.

The recent discovery of hydroxofluorographene G(OH)F, a graphene derivative showing room temperature antiferromagnetic ordering, suggests that there may be other sp-materials based on sp3-functionalized graphene that exhibit magnetic ordering. Here, we report a detailed theoretical study of hydroxofluorographene analogs, G(X)F, where X = -F, -SH, -NH2, -CH3, -BH3, and -BH2, conducted to deeper understand the relation among their structural, electronic and magnetic properties and to identify an effect of the functional group on magnetic transition temperatures. Although the magnetism of all G(X)F materials stems from the presence of aromatic islands with diradicals coupled via functional groups that enable superexchange interactions, the materials exhibited widely varying magnetic transition temperatures. The trends in the studied materials' transition temperatures are discussed in relation to the widths of their spin-flip gaps and the materials' stability. Our findings indicate that the properties of graphene-based magnets can be tuned by changing their functionalization, which may enable the design of organic magnets with tailored properties.

Zigzag sp2 Carbon Chains Passing through an sp3 Framework: A Driving Force toward Room-Temperature Ferromagnetic Graphene.

Stabilization of ferromagnetic ordering in graphene-based systems up to room temperature remains an important challenge owing to the huge scope for applications in electronics, spintronics, biomedicine, and separation technologies. To date, several strategies have been proposed, including edge engineering, introduction of defects and dopants, and covalent functionalization. However, these techniques are usually hampered by limited temperature sustainability of ferromagnetic ordering. Here, we describe a method for the well-controlled sp3 functionalization of graphene to synthesize zigzag conjugated sp2 carbon chains that can act as communication pathways among radical motifs. Zigzag sp2/sp3 patterns in the basal plane were clearly observed by high-resolution scanning transmission electron microscopy and provided a suitable matrix for stabilization of ferromagnetic ordering up to room temperature due to combined contributions of itinerant π-electrons and superexchange interactions. The results highlight the principal role of sp2/sp3 ratio and superorganization of radical motifs in graphene for generating room-temperature nonmetallic magnets.