Compared with other energy storage devices, supercapacitors have superior qualities, including a long cycling life, fast charge/discharge processes, and a high safety rating. The practical use of supercapacitor devices is hindered by their low energy density. Here, we briefly review the factors that influence the energy density of supercapacitors. Furthermore, possible pathways for enhancing the energy density via improving capacitance and working voltage are discussed. In particular, we offer our perspective on the most exciting developments regarding high-energy-density supercapacitors, with an emphasis on future trends. We conclude by discussing the various types of supercapacitors and highlight crucial tasks for achieving a high energy density.
The preparation, characterization, and photochromic properties of a mononuclear gold(I) complex (1oo) with two identical DTE-acetylides and a dinuclear gold(I) complex (2ooo) with both DTE-acetylide and DTE-diphosphine are described. Both gold(I) complexes exhibit multistep and multiple photocyclization/cycloreversion reactions. Particularly, four-state and four-color photochromic switch is successfully achieved for the dinuclear gold(I) complex upon irradiation with appropriate wavelengths of light. In contrast, fully ring-closed form is unattained through multiple photocyclization for the two corresponding model organic compounds coupling with the same DTE units as gold(I) complexes but without gold(I)-participation. It is demonstrated that coordination of gold(I) ion to DTE-acetylides exerts indeed a crucial role in achieving stepwise and selective photocyclization and cycloreversion reactions for both gold(I) complexes, in which the coordinated gold(I) atom acts as an effective "barrier" to prohibit intramolecular energy transfer between multi-DTE moieties.
A series of platinum(II) complexes with 1,3-bis(2-pyridylimino)isoindoline (BPI) derivatives were prepared by substitution of the coordinated Cl in the precursor complex Pt(BPI)Cl with a N-heterocyclic ligand such as pyridine, phthalazine or phenanthridine. These complexes display orange to red luminescence in fluid dichloromethane solutions and in the solid states at room temperature. The photophysical properties were tuned by introducing electron-withdrawing -NO(2) or electron-donating -NH(2) to the BPI ligand. The DFT computational studies suggest that the emission in the N-heterocyclic ligand substituted platinum(II) complexes originates mainly from the (3)[π→π*(BPI)] (3)IL triplet excited state, mixed with some (3)[dπ(Pt)→π*(BPI)] (3)MLCT character. Compared with the precursor Pt(BPI)Cl, both the low-energy absorption and the emission in the N-heterocyclic ligand substituted platinum(II) complexes exhibits a distinct blue-shift due to an obviously enhanced contribution from the (3)IL state and a reduced (3)MLCT character.
Reactions of 1,3-bis(2-pyridylimino)isoindoline (HL1), 1,3-bis(2-pyridylimino)benz(f)isoindoline (HL2), or 5,6-dihydro-2,3-diphenyl-5-(pyridin-2-ylimino)pyrrolo[3,4-b]pyrazin-7-ylidene)pyridin-2-amine (HL3) with Pt(tht)(2)Cl(2) (tht = tetrahydrothiophene) afforded the corresponding Pt(L)Cl complexes. A series of neutral platinum(II) alkynyl complexes Pt(L)(C[triple bond]CR) were prepared by reactions of the precursors Pt(L)Cl with alkynyl ligands through CuI-catalyzed platinum acetylide sigma coordination. Crystal structural determination of Pt(L3)Cl (3), Pt(L1)(C[triple bond]CPh) (4), and Pt(L1)(C[triple bond]CC(6)H(4)Bu(t)-4) (6) by X-ray crystallography revealed that the neutral platinum(Pi) complexes with monoanionic tridentate L ligands display more perfect square-planar geometry than that in platinum(II) complexes with neutral tridentate 2,2':6',2''-terpyridyl ligands. Both the Pt(L)Cl and Pt(L)(C[triple bond]CC(6)H(4)R-4) complexes exhibit low-energy absorption at 400-550 nm, arising primarily from pi --> pi*(L) intraligand (IL) and 5d(Pt) --> pi*(L) metal-to-ligand charge-transfer (MLCT) transitions as suggested from density functional theory calculations. They display bright-orange to red room-temperature luminescence in fluid dichloromethane solutions with microsecond to submicrosecond ranges of emissive lifetimes and 0.03-3.79% quantum yields, originating mainly from (3)IL and (3)MLCT excited states. Compared with the emissive state in Pt(L)Cl complexes, substitution of the coordinated Cl with C[triple bond]CC(6)H(4)R-4 in Pt(L)(C[triple bond]CC(6)H(4)R-4) complexes induces an obviously enhanced contribution from the (3)[pi(C[triple bond]CC(6)H(4)R-4) --> pi*(L)] ligand-to-ligand charge-transfer (LLCT) triplet state. The photophysical properties can be finely tuned by modifying both the L and alkynyl ligands. The calculated absorption and emission spectra in dichloromethane coincide well with those measured in a fluid dichloromethane solution at ambient temperature.
Platinum(II) complex [Pt(Me(3)SiC[triple bond]CbpyC[triple bond]CSiMe(3))(C[triple bond]CPh)(2)] (1) with 5,5-bis(trimethylsilylethynyl)-2,2'-bipyridine (Me(3)SiC[triple bond]CbpyC[triple bond]CSiMe(3)) and phenylacetylene (PhC[triple bond]CH) exhibits unusual luminescence vapochromism to volatile organic compounds (VOCs) including CH(2)Cl(2), CHCl(3), and CH(3)I, which is useful for detection of volatile halohydrocarbon with one carbon atom and molecular weight less of than 150. Crystal structural determination of 1, 1 x CHCl(3), 1 x 1/2(CH(2)ClCH(2)Cl), and 1 x 1/2(toluene) revealed a staggered packing for 1 x CHCl(3) whereas there was an antiparallel packing for other three structures in stacking of planar platinum(II) moieties. The Pt...Pt distance is 3.302(1) A in 1 x CHCl(3), whereas it is >4.0 A in the other three structures. Complex 1 displays bright orange luminescence in dichloromethane solution, arising from pi(phenylacetylide)-->pi*(Me(3)SiC[triple bond]CbpyC[triple bond]CSiMe(3)) (3)LLCT and d(Pt)-->pi*(Me(3)SiC[triple bond]CbpyC[triple bond]CSiMe(3)) (3)MLCT triplet states which are supported by DFT calculation. The solid-state emission occurs at approximately 762 nm for 1 x VOC (VOC = CH(2)Cl(2), CHCl(3), and CH(3)I), whereas it was at approximately 562 (603sh) or 603 (562sh) nm for 1 and other 1 x VOC, corresponding to a vapochromic response shift of approximately 160-200 nm. The dramatic vapochromism and vapoluminescence of 1 to the vapor of CH(2)Cl(2), CHCl(3), or CH(3)I are induced by a reversible conversion of the emissive state from (3)MLCT/(3)LLCT character to (3)MMLCT/(3)LLCT state.
Semiconductor p‐n junctions have been explored and applied in photoelectrochemical (PEC) water splitting, but serious carrier recombination and sluggish oxygen evolution reaction (OER) dynamics have demanded further progress in p‐n junction photoelectrode design. Here, via a controllable NH3 treatment, we construct sandwiched p‐n homojunctions in three‐unit‐cells n‐type SnS2 (n‐SnS2) nanosheet arrays using nitrogen (N) as acceptor dopants. The optimal N‐doped n‐SnS2 (pnp‐SnS2) with such unique structure achieves a record photocurrent density of 3.28 mA cm−2, which is 21 times as high as that of n‐SnS2 and the highest value among all the SnS2 photoanodes reported so far. Moreover, the stoichiometric O2 and H2 evolution from water was achieved with Faradaic efficiencies close to 100 %. The superior performance could be attributed to the facilitated electron–hole separation/transfer, accelerated surface OER kinetics, prolonged carrier lifetime, and improved structural stability.
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