Shuang Xiao

A Strongly Coupled Graphene and FeNi Double Hydroxide Hybrid as an Excellent Electrocatalyst for the Oxygen Evolution Reaction

Cost-effective electrocatalysts for the oxygen evolution reaction (OER) are critical to energy conversion and storage processes. A novel strategy is used to synthesize a non-noble-metal-based electrocatalyst of the OER by finely combining layered FeNi double hydroxide that is catalytically active and electric conducting graphene sheets, taking advantage of the electrostatic attraction between the two positively charged nanosheets. The synergy between the catalytic activity of the double hydroxide and the enhanced electron transport arising from the graphene resulted in superior electrocatalytic properties of the FeNi-GO hybrids for the OER with overpotentials as low as 0.21 V, which was further reduced to 0.195 V after the reduction treatment. Moreover, the turnover frequency at the overpotential of 0.3 V has reached 1 s(-1), which is much higher than those previously reported for non-noble-metal-based electrocatalysts.

Metallic Iron-Nickel Sulfide Ultrathin Nanosheets As a Highly Active Electrocatalyst for Hydrogen Evolution Reaction in Acidic Media

We report on the synthesis of iron-nickel sulfide (INS) ultrathin nanosheets by topotactic conversion from a hydroxide precursor. The INS nanosheets exhibit excellent activity and stability in strong acidic solutions as a hydrogen evolution reaction (HER) catalyst, lending an attractive alternative to the Pt catalyst. The metallic α-INS nanosheets show an even lower overpotential of 105 mV at 10 mA/cm(2) and a smaller Tafel slope of 40 mV/dec. With the help of DFT calculations, the high specific surface area, facile ion transport and charge transfer, abundant electrochemical active sites, suitable H(+) adsorption, and H2 formation kinetics and energetics are proposed to contribute to the high activity of the INS ultrathin nanosheets toward HER.

Cobalt-Embedded Nitrogen Doped Carbon Nanotubes: A Bifunctional Catalyst for Oxygen Electrode Reactions in a Wide pH Range

Electrocatalysts for the oxygen reduction and evolution reactions (ORR/OER) are often functionally separated, meaning that they are only proficient at one of the tasks. Here we report a high-performance bifunctional catalyst for both ORR and OER in both alkaline and neutral media, which is made of cobalt-embedded nitrogen doped carbon nanotubes. In OER, it shows an overpotential of 200 mV in 0.1 M KOH and 300 mV in neutral media, while the current density reaches 50 mA cm(-2) in alkaline media and 10 mA cm(-2) in neutral media at overpotential of 300 mV. In ORR, it is on par with Pt/C in both alkaline and neutral media in terms of overpotential, but its stability is superior. Further study demonstrated that the high performance can be attributed to the coordination of N to Co and the concomitant structural defects arising from the transformation of cobalt-phthalocyanine precursor.

Carbon quantum dots as a visible light sensitizer to significantly increase the solar water splitting performance of bismuth vanadate photoanodes

Here, we demonstrate that carbon quantum dots (CQDs), as a low cost, chemically stable, and environmentally friendly photosensitizer, can dramatically broaden the light absorption range to the entire visible range. Consequently, the NiOOH/FeOOH/CQD/BiVO4 (NFCB) photoanode has achieved a remarkable photocurrent density of 5.99 mA cm−2 at 1.23 V vs. RHE under AM 1.5G in KH2PO4 aqueous solution without a hole scavenger (pH = 7) and a record high applied bias photon-to-current efficiency of 2.29% at 0.6 V vs. RHE for BiVO4-based photoanodes. This novel NFCB photoanode could operate stably for 10 h with a Faraday efficiency of ∼95%, demonstrating the great potential of using CQDs for solar water splitting.

A three-dimensional hexagonal fluorine-doped tin oxide nanocone array: a superior light harvesting electrode for high performance photoelectrochemical water splitting

Photonic nanostructures hold great promise in promoting light harvesting. Here we report the first design and construction of a three-dimensional (3D) hexagonal nanocone array of fluorine-doped tin oxide (FTO) on glass as an excellent electrode for photoelectrochemical (PEC) water splitting. The PEC current density with suitably deposited Ti-doped hematite at 1.23 V vs. the reversible hydrogen electrode (RHE) was increased by 86% to 2.24 ± 0.02 mA cm−2 compared to that with the planar counterpart, mainly ascribable to the special light harvesting effect and the electrode surface area provided by 3D FTO. Upon the embedment of a gold layer to concentrate the incident light onto the hematite layer and the deposition of the Co–Pi catalyst with a modified procedure, the photocurrent experienced a large cathodic shift of onset potential by 360 mV and soared to a high value of 3.39 ± 0.01 mA cm−2 (at 1.23 V), yielding a power conversion efficiency of 0.70% at a potential as low as 0.88 V vs. RHE.

Profiling the organic cation-dependent degradation of organolead halide perovskite solar cells

Operational stability is one of the main obstacles that may hold back the commercialization of perovskite solar cells (PVSCs). In this paper, we provide a detailed account of the ion migration accelerated PVSC degradation by comparatively studying perovskite materials with two different organic cations (methylammonium (MA+) and formamidinium (FA+)). Using time of flight secondary ion mass spectrometry (TOF-SIMS), we have uncovered the ion migration accelerated degradation of PVSCs at the device level. Not only did mobile iodide (I−) ions from the perovskite layer diffuse out, but Ag atoms/ions from the metal electrode also diffused into the perovskite layer, which resulted in severe device degradation. Besides, we identified I− species in the hole transport material (HTM) layer for even freshly prepared PVSC devices, which was responsible for the degradation of devices kept under inert conditions. This also testifies the existence of ion migration on the device level of PVSCs. Compared with MAPbI3, the ion migration process can slow down in FAPbI3 devices which accounts for a better stability of FAPbI3 devices. This work underscores the impact of organic cation substitution on PVSC degradation and provides solid evidence for mobile ion migration in perovskite materials and the consequent degradation in specific device settings such as the n–i–p type perovskite solar cells.

A PCBM Electron Transport Layer Containing Small Amounts of Dual Polymer Additives that Enables Enhanced Perovskite Solar Cell Performance

A polymer/PCBM hybrid electron transport layer is reported that enables high‐performance perovskite solar cells with a high power conversion efficiency of 16.2% and with negligible hysteresis. Unlike previous approaches of reducing hysteresis by thermal annealing or fullerene passivation, the success of our approach can be mainly attributed to the doping of the PCBM layer using an insulating polymer (polystyrene) and an amine‐containing polymeric semiconductor named PFNOX.

An amorphous precursor route to the conformable oriented crystallization of CH3NH3PbBr3 in mesoporous scaffolds: toward efficient and thermally stable carbon-based perovskite solar cells

CH3NH3PbBr3 (MAPbBr3)-perovskite solar cells (Br-PSCs) have attracted much attention due to their green-long wavelength transparency and high open-circuit voltage originating from their large bandgap (2.2 eV). However, the efficiency of carbon-based Br-PSCs without organic hole transport materials (HTM) is still low due to the inappropriate quality of MAPbBr3 deposited in a relatively thick porous scaffold. Herein, an amorphous precursor route based on a two-step sequential method is exploited to conformably and seamlessly grow MAPbBr3 in the TiO2 porous scaffold. In the first step, the amorphous Pb–Br precursor containing a large amount of DMF molecules was prepared by lowering the post-treatment temperature to 25 °C, affording full pore filling and smooth surface capping. The conversion to MAPbBr3 in the second step was accelerated by the molecular exchange between DMF and MABr in IPA solution. Moreover, by solvent engineering through the addition of non-polar cyclohexane into the MABr IPA solution, the molecular exchange process was tuned in such a way to separate the nucleation and growth of MAPbBr3 crystals, leading to the preferential [001] orientation with an even surface finishing and the subsequent light absorption enhancement and trap state reduction. Using MAPbBr3 films in carbon-based PSCs has boosted their efficiency to 8.09% (Voc = 1.35 V), a record value for HTM-free Br-PSCs, also comparable to that of the best HTM-based Br-PSCs. Significantly, non-encapsulated devices showed no efficiency decay after storage in dry air (25–30 °C and 10–20% humidity) for 90 days. What is more, the efficiency was retained up to about 90% after storage for 15 days under high heat stress (air, 80 °C and 50–85% humidity).

Co(II)1–xCo(0)x/3Mn(III)2x/3S Nanoparticles Supported on B/N-Codoped Mesoporous Nanocarbon as a Bifunctional Electrocatalyst of Oxygen Reduction/Evolution for High-Performance Zinc-Air Batteries

Rechargeable Zn-air battery is an ideal type of energy storage device due to its high energy and power density, high safety, and economic viability. Its large-scale application rests upon the availability of active, durable, low-cost electrocatalysts for the oxygen reduction reaction (ORR) in the discharge process and oxygen evolution reaction (OER) in the charge process. Herein we developed a novel ORR/OER bifunctional electrocatalyst for rechargeable Zn-air batteries based on the codoping and hybridization strategies. The B/N-codoped mesoporous nanocarbon supported Co(II)1-xCo(0)x/3Mn(III)2x/3S nanoparticles exhibit a superior OER performance compared to that of IrO2 catalyst and comparable Zn-air battery performance to that of the Pt-based battery. The rechargeable Zn-air battery shows high discharge peak power density (over 250 mW cm(-2)) and current density (180 mA cm(-2) at 1 V), specific capacity (∼550 mAh g(-1)), small charge-discharge voltage gap of ∼0.72 V at 20 mA cm(-2) and even higher stability than the Pt-based battery. The advanced performance of the bifunctional catalysts highlights the beneficial role of the simultaneous formation of Mn(III) and Co(0) as well as the dispersed hybridization with the codoped nanocarbon support.

Pinning Down the Anomalous Light Soaking Effect toward High-Performance and Fast-Response Perovskite Solar Cells: The Ion-Migration-Induced Charge Accumulation

The light soaking effect (LSE) is widely known in perovskite solar cells (PVSCs), but its origin is still elusive. In this study, we show that in common with hysteresis, the LSE is owed to the ion migration in PVSCs. Driven by the photovoltage, the mobile ions in the perovskite materials (MA+/I-) migrate to the selective contacts, forming a boosted P-i-N junction resulting in enhanced charge separation. Besides, the mobile ions (MA+) can soften and suture the PCBM/perovskite interface and thus reduce the trap density, in keeping with a higher open-circuit voltage. Finally, almost LSE-free PVSCs can be prepared by using 0.1 wt % MAI-doped PCBM as the electron transport material, whereas overdoping (1 wt % MAI doping) makes the LSE even more pronounced due to excess mobile ions that need time to migrate to reach a new quasi-static state.