Tianran Zhang

Electrocatalysis of polysulfide conversion by sulfur-deficient MoS2 nanoflakes for lithium–sulfur batteries

Lithium–sulfur batteries are promising next-generation energy storage devices due to their high energy density and low material cost. Efficient conversion of lithium polysulfides to lithium sulfide (during discharge) and to sulfur (during recharge) is a performance-determining factor for lithium–sulfur batteries. Here we show that MoS2−x/reduced graphene oxide (MoS2−x/rGO) can be used to catalyze the polysulfide reactions to improve the battery performance. It was confirmed, through microstructural characterization of the materials, that sulfur deficiencies on the surface participated in the polysulfide reactions and significantly enhanced the polysulfide conversion kinetics. The fast conversion of soluble polysulfides decreased their accumulation in the sulfur cathode and their loss from the cathode by diffusion. Hence in the presence of a small amount of MoS2−x/rGO (4 wt% of the cathode mass), high rate (8C) performance of the sulfur cathode was improved from a capacity of 161.1 mA h g−1 to 826.5 mA h g−1. In addition, MoS2−x/rGO also enhanced the cycle stability of the sulfur cathode from a capacity fade rate of 0.373% per cycle (over 150 cycles) to 0.083% per cycle (over 600 cycles) at a typical 0.5C rate. These results provide direct experimental evidence for the catalytic role of MoS2−x/rGO in promoting the polysulfide conversion kinetics in the sulfur cathode.

Balancing the chemisorption and charge transport properties of the interlayer in lithium–sulfur batteries

This study introduces an improved design of the interlayer between the cathode and separator of rechargeable lithium–sulfur batteries to mitigate the polysulfide crossover problem of the latter. The design involves integrating carbon nanotubes with titanium dioxide by a facile room-temperature hydrolytic method to form a titanium dioxide coated carbon nanotube composite (CNT@TiO2) with customizable TiO2 content. The CNT@TiO2 composite was then coated on a separator to form an interlayer much thinner than other standalone interlayers. The TiO2 coating on the CNT surface provides the facility for lithium polysulfides (LiPS) interception by chemisorption, and the underlying CNT core renders the intercepted LiPS electrochemically viable in charging and discharging. A good balance between the chemisorption properties of TiO2 and the charge transport properties of the CNTs is required to deliver a good interlayer performance because of the complementarity of these functions. Consequently, a battery with an optimized CNT@TiO2 interlayer composition could deliver a high initial capacity of 1351 mA h g−1 and a discharge capacity of 803 mA h g−1 after 200 cycles at 0.1C, for less than half of the thickness of a typical standalone interlayer (12 μm).

Improving the Electrochemical Oxygen Reduction Activity of Manganese Oxide Nanosheets with Sulfurization-Induced Nanopores

Low‐cost and high‐activity electrocatalysts for the oxygen reduction reaction (ORR) are necessary for the development of fuel cells and metal‐air batteries. Manganese oxide would be a good candidate because of its low cost, abundant supply, and environmental benignity if not for its relatively low activity compared with noble metals. To improve the ORR activity of manganese oxide, we developed a sulfurization process to create pores in 2 D manganese oxide nanosheets. The nanoporous MnO2 nanosheets (np‐MnO2‐ns) prepared as such contain 7 nm pores in the nanosheets and their half‐wave potential (0.73 V) is 40 mV more positive than that of pore‐free MnO2 nanosheets (0.69 V). The higher ORR activity of np‐MnO2‐ns may be attributed to the combination of a large surface area and the presence of high‐ORR‐activity Mn3+/4+ sites. The np‐MnO2‐ns also showed an enhanced oxygen evolution reaction activity and delivered a good performance in rechargeable Zn‐air batteries.

Monoclinic oxygen-deficient tungsten oxide nanowires for dynamic and independent control of near-infrared and visible light transmittance

The transmittance of near-infrared (NIR) and visible (VIS) light spectral regions can be dynamically and independently controlled using a single-component material – monoclinic oxygen-deficient tungsten oxide nanowires, without the need for compositing with other electrochromic materials. A localized surface plasmon resonance and phase-transition assisted mechanism and bandgap transition electrochromism are individually responsible for the modulation of the NIR and VIS light transmissions.

Metal-doped TiO2 colloidal nanocrystals with broadly tunable plasmon resonance absorption

We report here the discovery of metal-doped colloidal TiO2 nanocrystals (NCs) with broadly tunable plasmon resonance absorption; and their synthesis by a facile and scalable one-pot method. A strong localized surface plasmon resonance (LSPR) absorption peak occurs in the as-synthesized Mo, W, and Nb-doped TiO2 NCs in the visible, near-infrared (NIR) and mid-infrared regions respectively. Density functional theory calculations indicate a dopant perturbation of the TiO2 electronic structure and the resultant increase in the electron density at the Fermi level as the likely cause for the strong LSPR absorption. The W-doped TiO2 NCs are the most versatile since their LSPR absorption in the NIR region can be varied from 980 to 1700 nm by tailoring the dopant concentration and the NC morphology. The method of synthesis can also be scaled up to gram-level production in batch reactors. Tunable LSPR properties and the ease and scalability of synthesis are the strong features of these metal-doped TiO2 NCs for plasmonic applications.

Unconventional noble metal-free catalysts for oxygen evolution in aqueous systems

The oxygen evolution reaction (OER) is a known impediment in the development of electrochemical energy conversion and storage devices such as water-splitting electrolyzers and rechargeable metal–air batteries. The intrinsically slow OER kinetics can only be mitigated by effective catalysis. The search for low-cost alternatives to the conventional noble metal-based catalysts is a research priority and has thus far been focused mostly on metal oxides. It is the purpose of this review to outline the opportunities and available options besides the noble metals and metal oxides. These unconventional catalysts include transition metal phosphates, borates, chalcogenides, phosphides, nitrides and borides as well as metal-free carbon-based materials. They are all based on earth-abundant elements with some of them showing higher catalytic performance than the common metal oxides in aqueous solution. The review begins with the introduction of the evaluation criteria for OER catalysts. The development and breakthroughs in the unconventional catalysts are then succinctly summarized with discussion of some current scientific issues. We then present our perspectives on these issues and suggest some areas of further work. We hope this review can raise the interest in less common options to broaden the search for practical OER catalysts to beyond the metal oxides.

Al3+ intercalation/de-intercalation-enabled dual-band electrochromic smart windows with a high optical modulation, quick response and long cycle life

Dual-band electrochromic smart windows with independent control of the transmittance of near-infrared (NIR) and visible (VIS) light can contribute significantly to the reduction of building energy consumption. Cost and inadequate electrochromic performance are the current technical challenges. We present here a dual-band electrochromic smart window design based on the intercalation/de-intercalation of Al3+ cations to replace the common use of monovalent cations in electrochromic applications. The Al3+ intercalation/de-intercalation-enabled electrochromic smart window delivers not only efficient and independent control of NIR and VIS light transmittance, but also impressive electrochromic performance – a high optical modulation of the full solar spectrum (93.2%, 91.7%, 88.5%, and 86.8% at 633, 800, 1200, and 1600 nm, respectively), high coloration efficiencies (254 and 121 cm2 C−1 at 1200 and 633 nm, respectively), fast switching times (8/5 s and 16/13 s at 1200 and 633 nm, respectively, for coloration/bleaching), and high bistability and cyclability (a 5.5% capacity loss after 2000 cycles). The good electrochromic performance can be attributed to the effective diffusion of Al3+ in the electrochromic material (as good as that of Li+); and a shallow intercalation/de-intercalation depth enabled by the ability of Al3+ to support three-electron redox reactions. The performance of Al3+ intercalation/de-intercalation-enabled dual-band electrochromism was also verified in laboratory prototype devices to confirm its suitability for dual-band smart windows.

Controlled Crumpling of Two-Dimensional Titanium Carbide (MXene) for Highly Stretchable, Bendable, Efficient Supercapacitors

Two-dimensional MXene materials have demonstrated attractive electrical and electrochemical properties in energy storage applications. Adding stretchability to MXene remains challenging due to its high mechanical stiffness and weak intersheet interaction, so the assembling techniques for mechanically stable MXene architectures require further development. We report a simple fabrication by harnessing the interfacial instability to generate higher dimensional MXene nanocoatings capable of programmed crumpling/unfolding. A sequential patterning approach enabled the design of sequence-dependent MXene textures across multiple length scales, which were utilized for controllable wetting surfaces and high-areal-capacitance electrodes. We next transferred the crumpled MXene nanocoating onto an elastomer to fabricate an MXene/elastomer electrode with high stretchability. The accordion-like MXene can be reversibly folded/unfolded and still preserve efficient specific capacitances. We further fabricated asymmetric MXene supercapacitors, and the devices demonstrated efficient electrochemical performance and large deformability (180° bendability, 100% stretchability). Our texturing techniques can be applied to large MXene families for designing stretchable architectures in wearable electronics.