Showing papers by "Licheng Sun published in 2018"

Dendritic core-shell nickel-iron-copper metal/metal oxide electrode for efficient electrocatalytic water oxidation.

Abstract: Electrochemical water splitting requires efficient water oxidation catalysts to accelerate the sluggish kinetics of water oxidation reaction. Here, we report a promisingly dendritic core-shell nick ...

Direct Observation of Structural Evolution of Metal Chalcogenide in Electrocatalytic Water Oxidation

Abstract: As one of the most remarkable oxygen evolution reaction (OER) electrocatalysts, metal chalcogenides have been intensively reported during the past few decades because of their high OER activities. It has been reported that electron-chemical conversion of metal chalcogenides into oxides/hydroxides would take place after the OER. However, the transition mechanism of such unstable structures, as well as the real active sites and catalytic activity during the OER for these electrocatalysts, has not been understood yet; therefore a direct observation for the electrocatalytic water oxidation process, especially at nano or even angstrom scale, is urgently needed. In this research, by employing advanced Cs-corrected transmission electron microscopy (TEM), a step by step oxidational evolution of amorphous electrocatalyst CoS x into crystallized CoOOH in the OER has been in situ captured: irreversible conversion of CoS x to crystallized CoOOH is initiated on the surface of the electrocatalysts with a morphology change via Co(OH)2 intermediate during the OER measurement, where CoOOH is confirmed as the real active species. Besides, this transition process has also been confirmed by multiple applications of X-ray photoelectron spectroscopy (XPS), in situ Fourier-transform infrared spectroscopy (FTIR), and other ex situ technologies. Moreover, on the basis of this discovery, a high-efficiency electrocatalyst of a nitrogen-doped graphene foam (NGF) coated by CoS x has been explored through a thorough structure transformation of CoOOH. We believe this in situ and in-depth observation of structural evolution in the OER measurement can provide insights into the fundamental understanding of the mechanism for the OER catalysts, thus enabling the more rational design of low-cost and high-efficient electrocatalysts for water splitting.

Vertically Aligned Oxygenated-CoS2–MoS2 Heteronanosheet Architecture from Polyoxometalate for Efficient and Stable Overall Water Splitting

Abstract: To achieve efficient conversion of renewable energy sources through water splitting, low-cost, earth-abundant, and robust electrocatalysts for the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) are required. Herein, vertically aligned oxygenated-CoS2–MoS2 (O-CoMoS) heteronanosheets grown on flexible carbon fiber cloth as bifunctional electrocatalysts have been produced by use of the Anderson-type (NH4)4[CoIIMo6O24H6]·6H2O polyoxometalate as bimetal precursor. In comparison to different O-FeMoS, O-NiMoS, and MoS2 nanosheet arrays, the O-CoMoS heteronanosheet array exhibited low overpotentials of 97 and 272 mV to reach a current density of 10 mA cm–2 in alkaline solution for the HER and OER, respectively. Assembled as an electrolyzer for overall water splitting, O-CoMoS heteronanosheets as both the anode and cathode deliver a current density of 10 mA cm–2 at a quite low cell voltage of 1.6 V. This O-CoMoS architecture is highly advantageous for a disordered structure, exposure of acti...

Promoting Active Sites in Core–Shell Nanowire Array as Mott–Schottky Electrocatalysts for Efficient and Stable Overall Water Splitting

Abstract: Developing earth-abundant, active, and robust electrocatalysts for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) remains a vital challenge for efficient conversion of sustai ...

Metal-organic frameworks (ZIF-67) as efficient cocatalysts for photocatalytic reduction of CO2 : The role of the morphology effect

Abstract: Metal–organic frameworks (MOFs), ZIF-67, with different morphologies were synthesized via a solvent-induced method at room temperature. The photocatalytic performances towards the reduction of CO2 were evaluated by using ZIF-67 materials as cocatalysts cooperating with a ruthenium-based complex as the photosensitizer. It has been demonstrated that the two-dimensional ZIF-67 with a leaf-like morphology exhibited the best photocatalytic activity and stability due to the highest CO2 adsorption capability and efficient electron transfer from the excited [Ru(bpy)3]2+ to ZIF-67.

Inorganic Hole‐Transporting Materials for Perovskite Solar Cells

Abstract: In the last few years, inorganic–organic metal halide perovskite solar cells (PSCs) have attracted a great deal of attention as a promising next-generation solar-cell technology because of their high efficiencies and low production cost. Hole-transporting materials (HTMs) play an essential role in effective charge extraction and thus in achieving high overall efficiency. Therefore, searching for an efficient, stable, and low-cost HTM in PSCs has been one of the hottest research topics in this field. Inorganic p-type semiconductors that possess several appealing characteristics, such as suitable energy levels, high hole mobility, and high chemical stability, as well as low production cost, etc., are promising HTM candidate materials in PSCs. Here, specific attention is paid to the recent progress in inorganic HTMs being explored for PSCs. A variety of methods developed for the fabrication of these inorganic HTMs are summarized in detail, together with their corresponding performance in PSCs. Finally, an outlook on further enhancements of highly efficient PSCs based on inorganic HTMs is presented.

Assembly of highly efficient photocatalytic CO2 conversion systems with ultrathin two-dimensional metal–organic framework nanosheets

Abstract: An ultrathin two-dimensional Zn porphyrin-based metal-organic framework (Zn-MOF nanosheets) is developed and used for the first time in photoreduction of CO2 to CO. Consequently, two novelty noble- ...

The Importance of Pendant Groups on Triphenylamine-Based Hole Transport Materials for Obtaining Perovskite Solar Cells with over 20% Efficiency

Abstract: Tremendous progress has recently been achieved in the field of perovskite solar cells (PSCs) as evidenced by impressive power conversion efficiencies (PCEs); but the high PCEs of >20% in PSCs ha ...

Atomically Thin Mesoporous In2O3-x/In2S3 Lateral Heterostructures Enabling Robust Broadband-Light Photo-Electrochemical Water Splitting

Abstract: Atomically thin 2D heterostructures have opened new realms in electronic and optoelectronic devices. Herein, 2D lateral heterostructures of mesoporous In2O3-x/In2S3 atomic layers are synthesized th ...

D-A-D-Typed Hole Transport Materials for Efficient Perovskite Solar Cells : Tuning Photovoltaic Properties via the Acceptor Group

Abstract: Two D–A–D-structured hole-transport materials (YN1 and YN2) have been synthesized and used in perovskite solar cells. The two HTMs have low-lying HOMO levels and impressive mobility. Perovskite-based solar cells (PSCs) fabricated with YN2 showed a power conversion efficiency (PCE) value of 19.27% in ambient air, which is significantly higher than that of Spiro-OMeTAD (17.80%). PSCs based on YN1 showed an inferior PCE of 16.03%. We found that the incorporation of the stronger electron-withdrawing group in the HTM YN2 improves the PCE of PSCs. Furthermore, the YN2-based PSCs exhibit good long-term stability retaining 91.3% of its initial efficiency, whereas PSCs based on Spiro-OMeTAD retained only 42.2% after 1000 h lifetime (dark conditions). These promising results can provide a new strategy for the design of D–A–D HTMs for PSC applications in future.

Integration of FeOOH and Zeolitic Imidazolate Framework‐Derived Nanoporous Carbon as an Efficient Electrocatalyst for Water Oxidation

Abstract: As a cost-effective catalyst for the oxygen evolution reaction (OER), the potential use of FeOOH is hindered by its intrinsic poor electron conductivity. Here, the significant enhancement of OER ac ...

Electrical Behavior and Electron Transfer Modulation of Nickel-Copper Nanoalloys Confined in Nickel-Copper Nitrides Nanowires Array Encapsulated in Nitrogen-Doped Carbon Framework as Robust Bifunctional Electrocatalyst for Overall Water Splitting

Abstract: Probing robust electrocatalysts for overall water splitting is vital in energy conversion. However, the catalytic efficiency of reported catalysts is still limited by few active sites, low conducti ...

3D Core–Shell NiFeCr Catalyst on a Cu Nanoarray for Water Oxidation: Synergy between Structural and Electronic Modulation

Abstract: Low-cost transition metal-based electrocatalysts for water oxidation and understanding their structure–activity relationship are greatly desired for clean and sustainable chemical fuel production. Herein, a core–shell (CS) NiFeCr metal/metal hydroxide catalyst was fabricated on a 3D Cu nanoarray by a simple electrodeposition–activation method. A synergistic promotion effect between electronic structure modulation and nanostructure regulation was presented on a CS-NiFeCr oxygen evolution reaction (OER) catalyst: the 3D nanoarchitecture facilitates the mass transport process, the in situ formed interface metal/metal hydroxide heterojunction accelerates the electron transfer, and the electronic structure modulation by Cr incorporation improves the reaction kinetics. Benefiting from the synergy between structural and electronic modulation, the catalyst shows excellent activity toward water oxidation under alkaline conditions: overpotential of 200 mV at 10 mA/cm2 current density and Tafel slope of 28 mV/dec. T...

Chemical Dopant Engineering in Hole Transport Layers for Efficient Perovskite Solar Cells: Insight into the Interfacial Recombination

Abstract: Chemical doping of organic semiconductors has been recognized as an effective way to enhance the electrical conductivity In perovskite solar cells (PSCs), various types of dopants have been developed for organic hole transport materials (HTMs); however, the knowledge of the basic requirements for being efficient dopants as well as the comprehensive roles of the dopants in PSCs has not been clearly revealed Here, three copper-based complexes with controlled redox activities are applied as dopants in PSCs, and it is found that the oxidative reactivity of dopants presents substantial impacts on conductivity, charge dynamics, and solar cell performance A significant improvement of open-circuit voltage (Voc) by more than 100 mV and an increase of power conversion efficiency from 132 to 193% have been achieved by tuning the doping level of the HTM The observed large variation of Voc for three dopants reveals their different recombination kinetics at the perovskite/HTM interfaces and suggests a model of an

Why nature chose the Mn4CaO5 cluster as water-splitting catalyst in photosystem II: a new hypothesis for the mechanism of O–O bond formation

Abstract: Resolving the questions, namely, the selection of Mn by nature to build the oxygen-evolving complex (OEC) and the presence of a cubic Mn3CaO4 structure in OEC coupled with an additional dangling Mn (Mn4) via μ-O atom are not only important to uncover the secret of water oxidation in nature, but also essential to achieve a blueprint for developing advanced water-oxidation catalysts for artificial photosynthesis. Based on the important experimental results reported so far in the literature and on our own findings, we propose a new hypothesis for the water oxidation mechanism in OEC. In this new hypothesis, we propose for the first time, a complete catalytic cycle involving a charge-rearrangement-induced MnVII-dioxo species on the dangling Mn4 during the S3 → S4 transition. Moreover, the O-O bond is formed within this MnVII-dioxo site, which is totally different from that discussed in other existing proposals.

One plus one greater than two: high-performance inverted planar perovskite solar cells based on a composite CuI/CuSCN hole-transporting layer

Abstract: The low-cost and stable inorganic p-type semiconductor copper(I) iodide (CuI) is a promising hole-transporting layer (HTL) material for inverted planar perovskite solar cells (PSCs). However, the power conversion efficiencies (PCEs) of inverted planar PSCs based on CuI HTLs reported so far are not satisfactory and far behind those of their organic counterparts. Herein, we demonstrate a simple but effective approach to improve the performance of inverted planar PSCs based on the CuI HTL through the incorporation of copper thiocyanate (CuSCN) into the CuI HTL. As compared to pristine CuI, the introduction of CuSCN significantly improves the quality of the film, resulting in a smooth and uniform film while maintaining relatively high electrical conductivity. As a consequence, the champion device based on the composite CuI/CuSCN HTL affords an impressive PCE of 18.76% under full sun illumination (100 mW cm−2, AM 1.5G), which is substantially higher than the corresponding values of the respective devices containing pristine CuI (14.53%) and CuSCN (16.66%). This value is one of the highest efficiencies reported thus far for CuI- and CuSCN-based HTLs in PSCs. This work demonstrates the great potential of low-temperature solution-processed CuI/CuSCN composites as hole-selective layers for low-cost and efficient PSCs as well as other optoelectronic devices.

Simultaneous oxidation of alcohols and hydrogen evolution in a hybrid system under visible light irradiation

Abstract: Water oxidation as a multi-electron transfer and endothermal reaction has been considered to be the bottleneck of solar-driven water splitting into oxygen and hydrogen. Herein, an alternative approach for solar energy conversion is developed by coupling H2 generation with the selective oxidation of alcohol. In a closed redox system containing molecular ruthenium catalyst (RuCat) and Pt modified g-C3N4 (Pt-g-C3N4) composite, hydrogen production is concomitant with the oxidation of benzyl alcohols to aldehydes with over 99% selectivity in the presence of visible light and pure water. By contrary, the system lacking molecular catalyst only exhibits low to moderate selectivities towards aldehydes. The remarkably improved selectivity is attributed to the formation of highly active Ru(IV) = O intermediate through efficient hole transfer from g-C3N4 to RuCat.

Achieving High Open-Circuit Voltages up to 1.57 V in Hole-Transport-Material-Free MAPbBr3 Solar Cells with Carbon Electrodes

Abstract: An open-circuit voltage (V-oc) of 1.57 V under simulated AM1.5 sunlight in planar MAPbBr(3) solar cells with carbon (graphite) electrodes is obtained. The hole-transport-material-free MAPbBr(3) sol ...

Ultrafast Relaxation Dynamics in Zinc Tetraphenylporphyrin Surface-Mounted Metal Organic Framework

Abstract: Ordered porphyrin-based metal organic frameworks (MOFs) may serve as a model for mimicking the natural photosynthesis with highly ordered chlorophylls, i.e., porphyrin-like chromophores. Study of light harvesting and energy transfer as the primary event of photosynthesis is of great importance leading to improvement of photovoltaics overall performance. Detailed characterization of ultrafast dynamics of zinc tetraphenylporphyrin (ZnTPP) surface mounted metal organic framework (SURMOF) is reported by using various steady-state and time-resolved laser spectroscopic techniques, i.e., time-correlated single photon counting, fluorescence up-conversion and transient absorption pump–probe with 20 fs resolution. Obtained results in these nanoporous materials were compared with corresponding results for ZnTPP in ethanol measured under the same conditions. Dramatic quenching of both upper excited singlet state S2 and first excited state S1 was observed. Subpicosecond and picosecond lifetimes were detected in transi...

Planar FAPbBr3 Solar Cells with Power Conversion Efficiency above 10

Abstract: Bromide-based hybrid perovskites are of particular interest not only due to the fact that they offer a way to go beyond the Shockley–Queisser limit via the tandem cell scheme but single-junction devices of them can also achieve reasonably high efficiency with high stability for solar energy conversion. However, the highest power conversion efficiency achieved up to now for FAPbBr3 single-junction solar cells is only 8.2%, which is far below the efficiency of ∼17% predicted from detailed balance analysis. Here, a two-step method (the intermolecule exchange pathway) was systematically optimized for the fabrication of high-quality FAPbBr3 films. A molecule of urea, structurally similar to formamidinium, is introduced as an additive to tune the intermolecular ion exchange procedure. SnO2 is introduced as an electron-selective contact to the planar structured FAPbBr3 solar cells. As a result, a power conversion efficiency of 10.61% and a Voc of 1.56 V are achieved with planar structured solar cells, both of wh...

Photon Up-Conversion via Epitaxial Surface-Supported Metal–Organic Framework Thin Films with Enhanced Photocurrent

Abstract: We report a new triplet–triplet annihilation photon up-conversion (TTA-UC) system using an epitaxial Zn-perylene surface-supported metal–organic framework (SURMOF) grown on metal oxide surface as “emitter”, and a platinum octaethylporphyrin (PtOEP) as “sensitizer” in [Co(bpy)3]2+/3+ acetonitrile solution. It has been demonstrated that the photocurrent can be significantly enhanced relative to epitaxial Zn-perylene SURMOF due to the TTA-UC mechanism. This initial result holds promising applications toward SURMOF-based solar energy conversion devices.

Orienting Active Crystal Planes of New Class Lacunaris Fe2PO5 Polyhedrons for Robust Water Oxidation in Alkaline and Neutral Media

Abstract: Developing efficient and stable oxygen evolution reaction (OER) electrocatalysts is essential for realizing sustainable energy conversion, such as solar fuels. Although modulating active sites and ...

Perovskite Hydroxide CoSn(OH)6 Nanocubes for Efficient Photoreduction of CO2 to CO

Abstract: Perovskite hydroxide CoSn(OH)6 nanoparticles were synthesized and used for the first time in the photocatalytic reduction of CO2 to CO. Under mild reaction conditions and using [Ru(bpy)3](PF6)2 as the photosensitizer, a high photocatalytic efficiency of 19.3 μmol for CO evolution with a high selectivity of 86.46% was obtained. The photocatalytic activity and CO selectivity were further improved by adding weak Bronsted acids, as proton sources, to the system.

Electronic and Structural Effects of Inner Sphere Coordination of Chloride to a Homoleptic Copper(II) Diimine Complex.

Abstract: The reaction of CuCl2 with 2,9-dimethyl-1,10-phenanthroline (dmp) does not lead to the formation of [Cu(dmp)2](Cl)2 but instead to [Cu(dmp)2Cl]Cl, a 5-coordinated complex, in which one chloride is directly coordinated to the metal center. Attempts at removing the coordinated chloride by changing the counterion by metathesis were unsuccessful and resulted only in the exchange of the noncoordinated chloride, as confirmed from a crystal structure analysis. Complex [Cu(dmp)2Cl]PF6 exhibits a reversible cyclic voltammogram characterized by a significant peak splitting between the reductive and oxidative waves (0.85 and 0.60 V vs NHE, respectively), with a half-wave potential E1/2 = 0.73 V vs NHE. When reduced electrochemically, the complex does not convert into [Cu(dmp)2]+, as one may expect. Instead, [Cu(dmp)2]+ is isolated as a product when the reduction of [Cu(dmp)2Cl]PF6 is performed with l-ascorbic acid, as confirmed by electrochemistry, NMR spectroscopy, and diffractometry. [Cu(dmp)2]2+ complexes can be synthesized starting from Cu(II) salts with weakly and noncoordinating counterions, such as perchlorate. Growth of [Cu(dmp)2](ClO4)2 crystals in acetonitrile results in a 5-coordinated complex, [Cu(dmp)2(CH3CN)](ClO4)2, in which a solvent molecule is coordinated to the metal center. However, solvent coordination is associated with a dynamic decoordination-coordination behavior upon reduction and oxidation. Hence, the cyclic voltammogram of [Cu(dmp)2(CH3CN)]2+ is identical to the one of [Cu(dmp)2]+, if the measurements are performed in acetonitrile. The current results show that halide ions in precursors to Cu(II) metal-organic coordination compound synthesis, and most likely also other multivalent coordination centers, are not readily exchanged when exposed to presumed strongly binding and chelating ligand, and thus special care needs to be taken with respect to product characterization.

Molecular Engineering of Triphenylamine-Based Non-Fullerene Electron-Transport Materials for Efficient Rigid and Flexible Perovskite Solar Cells

Abstract: There has been a growing interest in the design and synthesis of non-fullerene electron transport materials (ETMs) for perovskite solar cells (PSCs), which may overcome the drawbacks of traditional fullerene derivatives. In this work, a novel donor-acceptor (D-A) structured ETM termed TPA-3CN is presented by molecular engineering of triphenylamine (TPA) as the donor group and (3-cyano-4,5,5-trimethyl-2(5 H)-furanylidene) malononitrile as the acceptor group. To further improve the electron mobility and conductivity and achieve excellent photovoltaic performance, a solution processable n-type dopant is introduced during the ETM spin-coating step. After device optimization, PSCs based on the doped TPA-3CN exhibit an impressive power conversion efficiency (PCE) of 19.2% with a negligible hysteresis. Benefitting from the low temperature and good solution processability of ETM TPA-3CN, it was further applied in flexible inverted PSCs and an impressive PCE of 13.2% was achieved, which is among the highest values reported for inverted flexible fullerene-free PSCs.

Efficient and Stable Dye-Sensitized Solar Cells Based on a Tetradentate Copper(II/I) Redox Mediator

Abstract: The identification of an efficient and stable redox mediator is of paramount importance for commercialization of dye-sensitized solar cells (DSCs). Herein, we report a new class of copper complexes containing diamine-dipyridine tetradentate ligands (L1 = N, N'-dibenzyl- N, N'-bis(pyridin-2-ylmethyl)ethylenediamine; L2 = N, N'-dibenzyl- N, N'-bis(6-methylpyridin-2-ylmethyl)ethylenediamine) as redox mediators in DSCs. Devices constructed with [Cu(L2)]2+/+ redox couple afford an impressive power conversion efficiency (PCE) of 9.2% measured under simulated one sun irradiation (100 mW cm-2, AM 1.5G), which is among the top efficiencies reported thus far for DSCs with copper complex-based redox mediators. Remarkably, the excellent air, photo, and electrochemical stability of the [Cu(L2)]2+/+ complexes renders an outstanding long-term stability of the whole DSC device, maintaining ∼90% of the initial efficiency over 500 h under continuous full sun irradiation. This work unfolds a new platform for developing highly efficient and stable redox mediators for large-scale application of DSCs.