Zhifeng Ren

Bio: Zhifeng Ren is an academic researcher from Texas Center for Superconductivity. The author has contributed to research in topics: Thermoelectric effect & Thermoelectric materials. The author has an hindex of 122, co-authored 695 publications receiving 71212 citations. Previous affiliations of Zhifeng Ren include Massachusetts Institute of Technology & University of Cincinnati.

Deep defect level engineering: a strategy of optimizing the carrier concentration for high thermoelectric performance

Abstract: Thermoelectric properties are heavily dependent on the carrier concentration, and therefore the optimization of carrier concentration plays a central role in achieving high thermoelectric performance. The optimized carrier concentration is highly temperature-dependent and could even possibly vary within one order of magnitude in the temperature range of several hundreds of Kelvin. Practically, however, the traditional doping strategy will only lead to a constant carrier concentration, and thus the thermoelectric performance is only optimized within a limited temperature range. Here, we demonstrate that a temperature-dependent carrier concentration can be realized by simultaneously introducing shallow and deep defect levels. In this work, iodine (I) and indium (In) are co-doped in PbTe, where iodine acts as the shallow donor level that supplies sufficient electrons and indium builds up the localized half-filled deep defect state in the band gap. The indium deep defect state traps electrons at a lower temperature and the trapped electrons will be thermally activated back to the conduction band when the temperature rises. In this way, the carrier concentration can be engineered as temperature-dependent, which matches the theoretically predicted optimized carrier concentration over the whole temperature range. As a result, a room temperature ZT of ∼0.4 and a peak ZT of ∼1.4 at 773 K were obtained in the n-type In/I co-doped PbTe, leading to a record-high average ZT of ∼1.04 in the temperature range of 300 to 773 K. Importantly, since deep defect levels also exist in other materials, the strategy of deep defect level engineering should be widely applicable to a variety of materials for enhancing the thermoelectric performance across a broad temperature range.

High thermoelectric conversion efficiency of MgAgSb-based material with hot-pressed contacts

Abstract: The efficiency of heat-to-electricity conversion based on the thermoelectric effect depends on the materials' nondimensional figure of merit zT. While recent years saw an increasing number of reports of large peak zT in thermoelectric materials, efficiency data are scarce. High conversion efficiency requires not only a large average zT in the operational temperature range, but also good electrical and thermal contacts to the material. In this work, we experimentally demonstrate a record high thermoelectric conversion efficiency of 8.5% with a single thermoelectric leg based on a recently reported p-type MgAgSb-based compound operating between 20 and 245 °C. The efficiency can exceed 10% by increasing the hot side temperature to 295 °C. The sample is fabricated with silver contact pads using a one-step hot-press technique eliminating a typically required sample metallization process. This significantly simplifies the fabrication of thermoelectric elements with low electrical and thermal contact resistances.

Amorphous NiFe layered double hydroxide nanosheets decorated on 3D nickel phosphide nanoarrays: a hierarchical core–shell electrocatalyst for efficient oxygen evolution

Abstract: The rational design of efficient and earth-abundant electrocatalysts for the oxygen evolution reaction (OER) plays a paramount role in hydrogen production by water electrolysis. Here we report a 3D hierarchical core–shell nanostructured OER electrocatalyst, in which amorphous NiFe layered double hydroxide (LDH) nanosheets are decorated on 3D conductive nickel phosphide nanoarrays. The integrated 3D core–shell electrode simultaneously offers excellent electrical conductivity for fast electron transfer, a large surface area with numerous active edge sites, and a hierarchical nanostructure for rapid release of gas bubbles, thus contributing to outstanding catalytic performance: low overpotentials (197, 243, and 283 mV for current densities of 10, 100, and 300 mA cm−2, respectively), a small Tafel slope (46.6 mV dec−1), and superior stability, which are better than those of almost all reported LDH-based OER catalysts. When this hybrid catalyst is combined with nickel phosphide for overall water splitting, the two-electrode cell achieves current densities of 10 mA cm−2 at 1.52 V and 100 mA cm−2 at 1.68 V in alkaline media, which are even superior to those of benchmark IrO2 and Pt. This work paves an effective approach to design 3D hierarchical hybrid electrocatalysts for energy conversion and storage.

I and i

Abstract: There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality. Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

疟原虫var基因转换速率变化导致抗原变异[英]／Paul H, Robert P, Christodoulou Z, et al//Proc Natl Acad Sci U S A

Abstract: 抗原变异可使得多种致病微生物易于逃避宿主免疫应答。表达在感染红细胞表面的恶性疟原虫红细胞表面蛋白1（PfPMP1）与感染红细胞、内皮细胞、树突状细胞以及胎盘的单个或多个受体作用，在黏附及免疫逃避中起关键的作用。每个单倍体基因组var基因家族编码约60种成员，通过启动转录不同的var基因变异体为抗原变异提供了分子基础。

Carbon Nanotubes--the Route Toward Applications

Abstract: Many potential applications have been proposed for carbon nanotubes, including conductive and high-strength composites; energy storage and energy conversion devices; sensors; field emission displays and radiation sources; hydrogen storage media; and nanometer-sized semiconductor devices, probes, and interconnects. Some of these applications are now realized in products. Others are demonstrated in early to advanced devices, and one, hydrogen storage, is clouded by controversy. Nanotube cost, polydispersity in nanotube type, and limitations in processing and assembly methods are important barriers for some applications of single-walled nanotubes.