Nanomaterials to Enhance the Performance of Thermoelectric Materials and Catalysts for Water Splitting

Solutions to the growing energy demand and environment concerns can be increasing the current energy conversion efficiency or seeking alternative energy sources. Thermoelectric materials can directly harvest and convert waste heat from industrial, residential, commercial and transportation to electricity. However, the low conversion efficiency of thermoelectric material severely hinders its mass commercial application in past few decades, because thermoelectric parameters including electrical conductivity, Seebeck coefficient, and electronic thermal conductivity are intimately interdependent on each other by carrier concentration. Nanosized materials exhibit advantages to decouple the thermoelectric parameters. In my work, thermoelectric performances of n-type Mg3Sb1.5Bi0.5-based compounds are enhanced through grain alignment and carrier concentration optimization. Due to its typical layered crystal structure, partial texturing in the (001) plane is achieved by hot forging. Hall mobility is significantly improved to 105 cm2 V-1 s-1 in the (00l) plane, resulting in higher electrical conductivity, and power factor of 18 μW cm-1 K-2 at room temperature. Additionally, all of the Mg vacancies in Mg3Sb1.5Bi0.5-based compounds are almost eliminated by simple Y doping, and n-type conduction was successfully achieved without adding extra Mg in the initial composition. The carrier concentration is optimized through the combination of Y and Mg, leading to a record peak ZT of ~1.8 at 773 K in Mg3.02Y0.02Sb1.5Bi0.5.

Electrochemical water splitting is capable of converting electricity to chemical energy storaged as hydrogen, which is considered to be one of the most promising energy alternatives. Oxygen evolution reaction (OER) as one-half reaction of water splitting suffers from multiple steps of proton-coupled electron transfer and displays a sluggish kinetic process, it is challenging to develop efficient OER catalysts in order to match well with hydrogen evolution reaction for overall water splitting. We develop a new route to synthesize highly efficient and robust bulk catalyst of Ni1-xFex layered double hydroxide (Ni1-xFex-LDH) for OER by ball milling and sintering. The nano-sheet catalysts achieve 100, 500, and 1000 mA cm-2 at overpotentials of 244, 278, and 300 mV, respectively, as well as a low Tafel slope of 58 mV dec-1 in 1 M KOH.