Tiny defects deliver big gains: Controlling oxygen vacancies boosts thermoelectric efficiency by 91%

A research team has dramatically enhanced the efficiency of converting heat into electricity. The key lies in controlling tiny defects known as oxygen vacancies.

Their findings were published as a front cover article in the journal Advanced Science. The team was led by Professor Hyungyu Jin and Dr. Min Young Kim from the Department of Mechanical Engineering at POSTECH, in collaboration with Professors Donghwa Lee and Si-Young Choi from the Department of Materials Science and Engineering, and Professor Joseph P. Heremans from the Ohio State University.

Each day, enormous amounts of heat are lost around us: hot steam from factory chimneys, heat from car engines, and even the warmth generated by smartphones and computers. This waste heat is typically left unused, but if it could be converted back into electricity, it would offer a powerful solution to both energy inefficiency and environmental challenges.

The technology gaining attention among scientists in this context is thermoelectric technology, which converts heat into electricity through temperature gradients. In particular, transverse thermoelectric technology generates a voltage drop that is perpendicular to the direction of heat flow. This method holds great promise as an eco-friendly energy technology due to its simple structure and high efficiency. However, it has faced a persistent limitation: its performance largely depends on the intrinsic properties of the material, limiting the scope of viable industrial applications.

The POSTECH-led team focused their attention on oxygen vacancies, the minute gaps left when oxygen atoms are absent from a materials crystal lattice. Although they may appear to be mere defects, the researchers discovered that precisely controlling their number can substantially improve performance.

In their experiments, they prepared three types of samples with different amounts of oxygen vacancies. The results were remarkable: the sample with the most oxygen vacancies showed an extraordinary 91% enhancement in thermoelectric performance.

This phenomenon can be explained by two mechanisms. First, when a material has more oxygen vacancies, the difference in entropy inside it becomes larger, which in turn drives electric charges to flow more strongly along the temperature gradient, making electricity generation more efficient.

Additionally, the vacancies slightly distort the material’s crystal structure, causing some of the normally straight-flowing charge flow to shift sideways. This is similar to how a car wheel, when subjected to lateral torque, changes its direction of travel, ultimately leading to a substantial increase in transverse thermoelectric efficiency.

The greatest significance of this research is that performance was dramatically improved not by developing complex and expensive new materials, but simply by controlling defects within existing ones.

Professor Jin explained, “This strategy can be broadly applied to various thermoelectric materials, making energy harvesting technologies more efficient and practical.”