Di Yan Wang

Highly Active and Stable Hybrid Catalyst of Cobalt-Doped FeS2 Nanosheets–Carbon Nanotubes for Hydrogen Evolution Reaction

Hydrogen evolution reaction (HER) from water through electrocatalysis using cost-effective materials to replace precious Pt catalysts holds great promise for clean energy technologies. In this work we developed a highly active and stable catalyst containing Co doped earth abundant iron pyrite FeS(2) nanosheets hybridized with carbon nanotubes (Fe(1-x)CoxS(2)/CNT hybrid catalysts) for HER in acidic solutions. The pyrite phase of Fe(1-x)CoxS(2)/CNT was characterized by powder X-ray diffraction and absorption spectroscopy. Electrochemical measurements showed a low overpotential of ∼0.12 V at 20 mA/cm(2), small Tafel slope of ∼46 mV/decade, and long-term durability over 40 h of HER operation using bulk quantities of Fe(0.9)Co(0.1)S(2)/CNT hybrid catalysts at high loadings (∼7 mg/cm(2)). Density functional theory calculation revealed that the origin of high catalytic activity stemmed from a large reduction of the kinetic energy barrier of H atom adsorption on FeS(2) surface upon Co doping in the iron pyrite structure. It is also found that the high HER catalytic activity of Fe(0.9)Co(0.1)S(2) hinges on the hybridization with CNTs to impart strong heteroatomic interactions between CNT and Fe(0.9)Co(0.1)S(2). This work produces the most active HER catalyst based on iron pyrite, suggesting a scalable, low cost, and highly efficient catalyst for hydrogen generation.

FeS2 Nanocrystal Ink as a Catalytic Electrode for Dye‐Sensitized Solar Cells

In the last decade, dye-sensitized solar cells (DSSCs) have attracted great interest for the fabrication of low-cost largearea photovoltaic devices as an alternative to conventional inorganic counterparts. The counter electrode (CE) is a critical component in DSSCs, where electrons are injected into the electrolyte to catalyze iodine reductions (I3 to I ). The most commonly used CE is based on indium-doped tin oxide (ITO)-coated glass loaded with platinum by sputtering. Platinum has a high catalytic activity for triiodide reduction and presents sufficient corrosion resistance. However, Pt is expensive because of its scarcity, and thus, the development of so-called Pt-free CEs for DSSCs using cheaper and abundant materials becomes technologically desirable. Recently, carbon-based materials, such as graphite, graphene, carbon nanotubes, and conducting polymers, have been used to replace Pt as electrocatalysts for triiodide reduction in DSSCs, although these devices still suffer from poor thermal stability and weak corrosion resistance. Extensive research has been performed on using inorganic compounds such as transitional metal carbides, nitrides, oxides, and sulfides as a new class of alternative catalytic materials for Pt in DSSC systems. Therefore, pursuing low-cost and stable CE materials as alternatives to expensive Pt is crucial to make DSSC systems more competitive for future commercial applications. Pyrite iron disulfide (FeS2, so-called fool s gold) is an interesting next-generation photovoltaic material candidate that is abundant in nature and is nontoxic. It is ranked as having the highest material availability among 23 existing semiconducing photovoltaic systems that could potentially lead to lower costs compared to conventional silicon solar cells. Colloidal pyrite nanocrystals (NCs) were recently synthesized and characterized, providing great potential for developing low-cost fabrications of FeS2-based photovoltaic devices using solution processes. We first demonstrated pyrite NC-based photodiode devices with a spectral response extended to near infrared (NIR) wavelengths because of its large optical absorption coefficient (> 10 cm ) and narrow band gap of 0.95 eV, which provided a crucial step toward success in producing colloidal pyrite NCs thin films as photovoltaic absorption layers. This study demonstrates an important photovoltaic application using FeS2 nanocrystal pyrite ink to fabricate a cost-effective CE in DSSCs, which has the unique advantages of earth abundance and of being solution-processable. The DSSC device with the CE using the FeS2 NC ink exhibits a promising power conversion efficiency of 7.31% comparable to that of the cell using the precious metal of Pt deposited by sputtering (7.52 %), as well as remarkable electrochemical stability of greater than 500 consecutive cycle scans. Solution-processable and semi-transparent FeS2 NC-based CEs also enable the fabrication of flexible and bifacial DSSCs. The results indicate that FeS2 NC ink is an extremely promising candidate for replacing Pt to substantially reduce the cost of DSSCs in future commercial applications and have also shed light on employing the low-cost FeS2 NC catalyst in other electrochemical cells. The FeS2 NCs were prepared using wet solution-phase chemical syntheses with a number of modifications according to our previous reports. 25] Figure 1a shows a highresolution transmission electron microscopy (HR-TEM) image of an FeS2 NC with a diameter of 15 3 nm. The clear lattice fringes of the FeS2 NCs with a lattice spacing of 0.31 nm matched the (111) plane of pyrite. The fast Fourier transform (FFT) patterns shown in Figure 1b exhibited various index facets, including {210}, {211}, and {311} on the NC, showing typical signatures of a pyrite-phased crystal structure.Figure 1 c shows a photograph of the FeS2 NCs ink. For fabricating the FeS2 NC CE, FeS2 NC ink of concentration 30 mg mL 1 was spin-coated onto an ITO glass substrate at 4000 rpm for 20 s, as shown in Figure 1d. Because as[*] Y.-C. Wang, Dr. D.-Y. Wang, H.-A. Chen, Prof. C.-W. Chen Department of Materials Science and Engineering National Taiwan University No. 1, Sec. 4, Roosevelt Rd., Taipei, 10617 (Taiwan) E-mail: chunwei@ntu.edu.tw

A mini review on nickel-based electrocatalysts for alkaline hydrogen evolution reaction

High gravimetric energy density, earth-abundance, and environmental friendliness of hydrogen sources have inspired the utilization of hydrogen fuel as a clean alternative to fossil fuels. Hydrogen evolution reaction (HER), a half reaction of water splitting, is crucial to the low-cost production of pure H2 fuels but necessitates the use of electrocatalysts to expedite reaction kinetics. Owing to the availability of low-cost oxygen evolution reaction (OER) catalysts for the counter electrode in alkaline media and the lack of low-cost OER catalysts in acidic media, researchers have focused on developing HER catalysts in alkaline media with high activity and stability. Nickel is well-known as an HER catalyst and continuous efforts have been undertaken to improve Ni-based catalysts as alkaline electrolyzers. In this review, we summarize earlier studies of HER activity and mechanism on Ni surfaces, along with recent progress in the optimization of the Ni-based catalysts using various modern techniques. Recently developed Ni-based HER catalysts are categorized according to their chemical nature, and the advantages as well as limitations of each category are discussed. Among all Ni-based catalysts, Ni-based alloys and Ni-based hetero-structure exhibit the most promising electrocatalytic activity and stability owing to the fine-tuning of their surface adsorption properties via a synergistic nearby element or domain. Finally, selected applications of the developed Ni-based HER catalysts are highlighted, such as water splitting, the chloralkali process, and microbial electrolysis cell.

Solution-Processable Pyrite FeS2 Nanocrystals for the Fabrication of Heterojunction Photodiodes with Visible to NIR Photodetection

A heterojunction photodiode with NIR photoresponse using solution processable pyrite FeS(2) nanocrystal ink is demonstrated which has the advantages of earth-abundance and non-toxicity. The device consists of a FeS(2) nanocrystal (NC) thin film sandwiched with semiconducting metal oxides with a structure of ITO/ZnO/FeS(2) NC/MoO(3) /Au, which exhibits an excellent photoresponse with a spectral response extended to NIR wavelengths of up to 1150 nm and a high photocurrent/dark current ratio of up to 8000 at -1 V under AM1.5 illumination (100 mW cm(-2) ).

Extended red light harvesting in a poly(3-hexylthiophene)/iron disulfide nanocrystal hybrid solar cell

A polymer solar cell based on poly(3-hexylthiophene) (P3HT)/iron disulfide (FeS2) nanocrystal (NC) hybrid is presented. The FeS2 NCs of 10 nm in diameter were homogeneously blended with P3HT to form an active layer of a solar cell. An extended red light harvesting up to 900 nm resulting from the NCs in the device has been demonstrated, compared to a typical absorption edge of 650 nm of a pristine P3HT. The environmentally friendly and low-cost FeS2 NCs can be used as a promising candidate for an acceptor in the polymer solar cell device application with an enhanced photovoltaic response in the extended red light region.

Advanced rechargeable aluminium ion battery with a high-quality natural graphite cathode

Recently, interest in aluminium ion batteries with aluminium anodes, graphite cathodes and ionic liquid electrolytes has increased; however, much remains to be done to increase the cathode capacity and to understand details of the anion–graphite intercalation mechanism. Here, an aluminium ion battery cell made using pristine natural graphite flakes achieves a specific capacity of ∼110 mAh g−1 with Coulombic efficiency ∼98%, at a current density of 99 mA g−1 (0.9 C) with clear discharge voltage plateaus (2.25–2.0 V and 1.9–1.5 V). The cell has a capacity of 60 mAh g−1 at 6 C, over 6,000 cycles with Coulombic efficiency ∼ 99%. Raman spectroscopy shows two different intercalation processes involving chloroaluminate anions at the two discharging plateaus, while C–Cl bonding on the surface, or edges of natural graphite, is found using X-ray absorption spectroscopy. Finally, theoretical calculations are employed to investigate the intercalation behaviour of choloraluminate anions in the graphite electrode.

Efficient Light Harvesting by Photon Downconversion and Light Trapping in Hybrid ZnS Nanoparticles/Si Nanotips Solar Cells

A hybrid colloidal ZnS nanoparticles/Si nanotips p-n active layer has been demonstrated to have promising potential for efficient solar spectrum utilization in crystalline silicon-based solar cells. The hybrid solar cell shows an enhancement of 20% in the short-circuit current and approximately 10% in power conversion efficiency compared to its counterpart without integrating ZnS nanoparticles. The enhancement has been investigated by external quantum efficiency, photoluminescence excitation spectrum, photoluminescence, and reflectance to distinct the role of ZnS quantum dots for light harvesting. It is concluded that ZnS nanoparticles not only act as frequency downconversion centers in the ultraviolet region but also serve as antireflection coating for light trapping in the measured spectral regime. Our approach is ready to be extended to many other material systems for the creation of highly efficient photovoltaic devices.

Blending Cr2O3 into a NiO–Ni Electrocatalyst for Sustained Water Splitting

The rising H2 economy demands active and durable electrocatalysts based on low-cost, earth-abundant materials for water electrolysis/photolysis. Here we report nanoscale Ni metal cores over-coated by a Cr2 O3 -blended NiO layer synthesized on metallic foam substrates. The Ni@NiO/Cr2 O3 triphase material exhibits superior activity and stability similar to Pt for the hydrogen-evolution reaction in basic solutions. The chemically stable Cr2 O3 is crucial for preventing oxidation of the Ni core, maintaining abundant NiO/Ni interfaces as catalytically active sites in the heterostructure and thus imparting high stability to the hydrogen-evolution catalyst. The highly active and stable electrocatalyst enables an alkaline electrolyzer operating at 20 mA cm(-2) at a voltage lower than 1.5 V, lasting longer than 3 weeks without decay. The non-precious metal catalysts afford a high efficiency of about 15 % for light-driven water splitting using GaAs solar cells.

Self-Crack-Filled Graphene Films by Metallic Nanoparticles for High-Performance Graphene Heterojunction Solar Cells

Graphene, with cracks filled with gold nanoparticles, is grown by chemical vapor deposition on a Cu substrate. The crack-filled graphene not only exhibits superior electrical properties but also forms a better junction with other semiconductors. A high-quality crack-filled graphene/Si Schottky junction solar cell is achieved, demonstrating the highest fill factor (0.79) and best efficiency (12.3%).

Tunable properties of PtxFe1−x electrocatalysts and their catalytic activity towards the oxygen reduction reaction

We present the controlled synthesis of bimetallic Pt(x)Fe(1-x) nanoparticles with tunable physical properties and a study of their catalytic activity towards the oxygen reduction reaction (ORR). Composition-induced variations in alloying extent and Pt d-band vacancies in Pt-Fe/C catalysts are systematically investigated. Density functional theoretical calculations are performed in order to realize the electronic effect caused by alloying Pt with Fe. The DFT computational observations revealed that iron donates electrons to platinum, when the Fe 3d and Pt 5d orbitals undergo hybridization. The Pt(x)Fe(1-x) catalysts with various Pt-to-Fe atomic ratios are characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), cyclic voltammetry (CV), and X-ray absorption spectroscopy (XAS). TEM images indicate that the dispersion of the metal nanoparticles is uniform and the XAS technique provides significant insight on Pt d-band vacancies and the alloying extent of Pt and Fe in Pt(x)Fe(1-x) nanoparticles. Rotating-disk voltammetry of Pt(x)Fe(1-x) nanoparticle catalysts with various Pt : Fe atomic compositions (3 : 1, 1 : 1, and 1 : 3) revealed that the Pt(1)Fe(1)/C nanocatalyst showed a greater enhancement in ORR activity than platinum. The enhanced catalytic activity toward ORR is attributed to the higher alloying extent of platinum and iron as well as the promising electronic structure offered by the lower unfilled Pt d states in Pt(x)Fe(1-x) nanoparticles when compared to pure Pt.