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Rechargeable lithium−oxygen batteries (LOBs) have attracted increasing attention due to their high energy density but highly rely on the development of efficient oxygen catalysts for reversible Li2O2 deposition/decomposition. Herein, highly porous carbon nanocubes with a specific surface area up to 1600 m2 g−1 are synthesized and utilized to tightly anchor Ru nanoparticles for using as the oxygen-cathode catalyst in LOBs, achieving a low charge/discharge potential gap of only 0.75 V, a high total discharge capacity of 17,632 mA h g−1, and a superb cycling performance of 550 cycles at 1000 mA g−1. Comprehensive ex situ and operando characterizations unravel that the outstanding LOB performance is ascribed to the highly porous catalyst structure embedding rich active sites that synergistically function in reducing overpotentials, suppressing parasitic reactions, accommodating reaction products, and promoting mass and charge transportation.

MATERIALS FOR HUMANITY (MH 21)

Ru-embedded Highly Porous Carbon Nanocubes Derived from Metal Organic Frameworks for Catalyzing Reversible Li2O2 Formation

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technical paper

Co-doped molybdenum disulfide nanosheet (denoted Co-MoS2) was designed as a cathode catalyst in a lithium-sulfur battery, and was prepared by a hydrothermal method. It was demonstrated that cobalt doping induces the transformation of MoS2 from the 2H phase to the metallic 1T phase. The consequents increase the proportion of 1T phase MoS2 (1T-MoS2) with high catalytic activity while improving the stability of the catalytic site. The introduction of Co atoms also provides abundant electrons to the basal plane of the MoS2 matrix, thus improving the conductivity of semiconductor MoS2. Consequently, a sulfur cathode with Co-MoS2 contributes to a high rate performance (943 mAh g−1 at a high rate of 2 C relative to 1220 mAh g−1 at 0.3 C) as well as long cyclic stability (an ultralow capacity fading rate of 0.029% per cycle over 1000 cycles at the high current rate of 0.2 C).

Co-doped Molybdenum Disulfide Facilitating Polysulfides Transformation in Advanced Lithium-sulfur Batteries

Hybrid capacitive deionization (HCDI) is emerging as an energy-efficient alternative for brackish water desalination with low expenditure cost and favourable ion removal effectiveness by adopting battery-type electrodes in the traditional capacitive deionization (CDI) system. Herein, an unscalable electrospinning method is introduced to contrive cathode material for HCDI system. Co3O4 and nitrogen-doped carbon nanotube decoration are formed successfully and uniformly distributed with the hollow structure of free-standing carbon nanofibers (Co3O4@CNF@CNT). The conductive “bridges” provided by the CNF matrix significantly shorten the diffusion length of Na+ and promote the electrical conductivity of the Co3O4 nanoparticles. Moreover, benefiting from the incorporation of nitrogen-doped CNTs as the “medium”, the contact between electrolyte and electrode material is improved and the electrical conductivity is further enhanced. The as-prepared Co3O4@CNF@CNT cathode shows excellent pseudocapacitive performance of 395 F g-1 at a scan rate of 1 mV s-1 and superior rate performance of 279 F g-1 at 100 mV s-1. The HCDI system delivers an outstanding salt adsorption capacity (SAC) of 58.6 mg g-1 and a highest salt adsorption rate (SAR) of 12.27 mg g-1 min-1 with a potential difference of 1.4 V, indicating the great potential of applying Co3O4@CNF@CNT in the practical HCDI system.

Hierarchical tubular Co3O4/CNT structures for Efficient Hybrid Capacitive Deionization

Li-O2 battery is one of the most desired energy storages form due to its extraordinary high energy density and the readily available oxygen source from atmosphere. However, significant challenges still line up ahead from both the anode and cathode sides before the technology can be finally deployed to benefit human life. Herein, a two-birds-one-stone tactic is implemented to fabricate high-performance and flexible Li-O2 batteries utilizing only one structural backbone composed of graphene oxide (GO) aerogel embedding hollow NiCo2O4 (NCO) microspheres for both the anode and cathode. While the composite Li anode fabricated through lithium infusion exhibits an ultrahigh capacity of 3398.4 mAh g-1 and significantly inhibited dendrite growth and volume expansion, the freestanding oxygen cathode demonstrates great capability in promoting energy efficiency by catalyzing reversible Li2O2 formation. When coupled together, the resultant Li-O2 batteries manifest a superb long-term cycling stability with highly reversible discharge/charge for more than 400 cycles, apart from the remarkable mechanical deformability demonstrated by flexible pouch cells surpassing 400 hours of steady operation. By employing a full spectrum of ex-situ and operando spectroscopic and microscopic tools, mechanistics into the remarkable Li-O2 performance is unraveled.

High‐Performance Li–O2 Batteries Based on All‐Graphene Backbone

Silver bismuth iodide has been explored extensively for its potential as absorbers in perovskite inspired solar cells, due to its less toxic environment friendly nature. However, the conventional spin coating methods employed for synthesis of thin films make them dubious when it comes to commercial applications. These methods can be tedious and can facilitate fabrication of one to a few devices in a single run, making it unfitting for commercial applications, though they may be useful for lab scale device fabrication intended for research purposes. Here we propose a modified evaporation method which can be employed for large scale fabrication of silver bismuth iodide thin films. The method involves sequential evaporation of metallic layers of silver and bismuth followed by exposure to sublimated iodine vapours to obtain highly uniform pin hole free silver bismuth iodide thin films. Following this method, with multiple optimization steps, we were able to synthesise Ag3BiI7 thin films with a band gap of around 1.86eV. The photo voltaic devices fabricated, using the hence developed films as absorbers yielded an efficiency of 0.68% for devices with active area of around 36mm2. Thus, we believe that this work would help develop commercially feasible absorbers for perovskite inspired solar cells and provide a new dimension for the associated research for large area devices.

Silver Bismuth Iodide for Photovoltaic Applications Using Modified Evaporation Method

Surface passivation has been proven as an effective strategy to improve power conversion efficiency and stability of perovskite solar cells. However, the rationale of choosing appropriate passivator is still not clear yet, especially the binding interaction of different molecules with perovskite beneath. Here, we elucidated the binding interaction of two judiciously chosen molecules, fluorinated phosphonic acid and its corresponding phosphonate salt, and monitor the extent of interaction between these molecules and perovskite interface. Higher photoluminescence in perovskite film treated with phosphonate salt was observed which possesses stronger binding interaction with perovskite. The effect on photovoltaic performance and its stability were also examined. The corresponded device shows enhancement in conversion efficiency from 18.27% to 19.44%. Furthermore, water contact angle of treated perovskite film exceeding 110.9° was demonstrated which allows super hydrophobic nature, enabling excellent long term stability of devices by retaining over 90% of their initial efficiency after 90 days.

Toward Efficient and Stable Perovskite Photovoltaics with Fluorinated Phosphonate Salt Surface Passivation

Understanding the fundamental properties of buried interfaces in perovskite photovoltaics is of paramount importance to the enhancement of device efficiency and stability. Nevertheless, accessing buried interfaces poses a sizeable challenge because of their non-exposed feature. In this presentation, the buried mysteries in full device stacks will be unveiled by a series of dedicated techniques including the lift-off strategy, the in-situ mapping spectroscopy, and the cross-sectional high-resolution microscopy. By establishing the microstructure–property relations, the basic losses at the contact interfaces are systematically presented, which are induced by both the sub-microscale extended imperfections and lead-halide inhomogeneities. Furthermore, a brand new mechanism for the most popular ammonium-halide post-treatment is explored and called molecule-assisted microstructural reconstruction. The losses can be considerably mitigated by the use of the passivation-molecule-assisted microstructural reconstruction, which unlocks the full potential for improving device performance.

Buried interfaces in halide perovskite photovoltaics

The CsPbI2Br perovskite solar cells based on carbon electrode (CE) are considered to be low-cost and stable devices. Nevertheless, the insufficient contact and energy level mismatch in the interface of CsPbI2Br/CE hinder the hole extraction and limit the enhancement of cell efficiency. Herein, a carbon black (CB) interlayer was introduced between the CsPbI2Br and CE. [1] The hole extraction was facilitated due to the larger contact area and suitable energy band alignment in the CsPbI2Br/CB interface. Moreover, the carbon based CsPbI2Br solar cells maintained over 90% of original efficiency after constant 85 oC heating for 2000 hours, showing excellent thermal stability of carbon based CsPbI2Br solar cells. Further investigation indicated the diffusion of CB nanoparticles from CE or CB layer to CsPbI2Br film after a certain period of time. We disclosed the formation of a CB-CsPbI2Br bulk heterojunction structure due to the carbon diffusion, which resulted in an efficiency enhancement. As a result, a high efficiency of 13.13% was achieved for carbon based CsPbI2Br solar cells. This work indicates that the diffusion of CB nanoparticles in CB-contained perovskite solar cells is universal and inevitable, although this kind of diffusion results in the enhancement of the cell efficiency.

Carbon black interlayer facilitates hole extraction and improves the efficiency of carbon based CsPbI2Br solar cells

Surface imperfections created during fabrication of halide perovskite (HP) films could induce formation of various defect sites that affect device performance and stability. In this work, all‐organic surface modifiers consisting of alkylammonium cations and alkanoate anions are introduced on top of the HP layer to passivate interfacial vacancies and improve moisture tolerance. Passivation using alkylammonium alkanoate does not induce formation of low‐dimensional perovskites species. Instead, the organic species only passivate the perovskite's surface and grain boundaries, which results in enhanced hydrophobic character of the HP films. In terms of photovoltaic application, passivation with alkylammonium alkanoate allows significant reduction in recombination losses and enhancement of open‐circuit voltage. Alongside unchanged short‐circuit current density, power conversion efficiencies of more than 18.5 % can be obtained from the treated sample. Furthermore, the unencapsulated device retains 85 % of its initial PCE upon treatment, whereas the standard 3D perovskite device loses 50 % of its original PCE when both are subjected to ambient environment over 1500 h.

Effects of All-organic Interlayers Surface Modifiers on Perovskite Solar Cells Efficiency and Stability

Halide perovskite-based solar cell (PSC) has emerged as the fastest photovoltaic material to have reached the power-conversion efficiency (PCE) of larger than 25%. Despite unprecedented growth of research interest to study the fundamental aspects as well as to optimize interfacial layers in solar cells to achieve high-performing PSCs, moisture stability still remains as roadblock to get attention PSCs deserve for commercialization and mass production. To obtain first degree of protection, there have been reports on using additives into perovskite precursors and applying protective layers directly on perovskite active materials. For instances, hydrophobic molecules such as functionalized aliphatic compounds, fullerenes, and ionic liquids have been reported to increase the moisture-resistance of PSCs when used as additives in perovskite precursor solution. Separately, hydrophilic polymers such as poly(ethylene glycol) and poly(acrylic acid) were found to maintain device stability despite their water-absorbing nature. However, the beneficial effect of amphiphilic organic molecules has been scarcely discussed. In our study, novel amphiphilic corannulene-pentakis-(triethyleneglycol-monomethyl ether)-sulfone (Cor-TEG) was utilized to explore the mutual effect of both hydrophobic corannulene and hydrophilic side chains (ethylene glycol oligomers) in increasing the moisture resistance of PSCs. It is found that the Cor-TEG additive modulated the perovskite crystal growth to form large and columnar grains with reduced defects and enhanced photocarrier extraction. Notably, the Cor-TEG modified perovskite devices showed improved stability under humid Singapore ambient air, retaining 87% of initial PCE for two weeks of exposure.

Amphiphilic additive for moisture-resistant perovskite solar cells

Lead-free halide perovskite semiconductors are necessary due to the atmospheric instability and lead toxicity associated with the 3D lead halide perovskites. However, a stable lead-free perovskite with an ideal bandgap (1.2-1.4 eV) for photovoltaics is still missing. In this work, we synthesized organic-inorganic gold halide double perovskites ((CH3NH3)2Au2X6, X = Br, I) through a solution-processed route that offers an ideal direct bandgap for photovoltaic applications. Density functional theory calculations confirm the direct nature of bandgap nature with reasonable absorption coefficients in the visible range and excellent charge transport properties. In addition, the Au-halide perovskites show high chemical stability and photoresponse. These combined properties demonstrate that Au-based halide perovskites can be a promising group of compounds for optoelectronic applications.

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