Flexible thermocouple using a thermoelectric graphene fiber with a seamless junction

Temperature is an important physical variable that indicates the condition of the human body and artificial systems. Advanced wearable applications require the development of temperature sensors with different form factors. In this study, a fiber-shaped thermoelectric temperature sensor is fabricated using a continuous graphene fiber whose two halves possess different reduction states. A seamless junction is formed by partially reducing a wet-spun graphene oxide fiber with hydroiodic acid (HI) solutions of different concentrations. One-half of the fiber is mildly reduced with 0.97 wt% HI, while the other half is highly reduced with 30.6 wt% HI. The different reduction states of the graphene composite fiber result in different Seebeck coefficients, allowing for the fabrication of a fiber-shaped graphene thermocouple without any laborious assembly. The flexible graphene thermocouple exhibits high sensitivity with a thermopower of 12.5 μV K^–1 in the temperature range of room temperature to ∼70 °C. Furthermore, it exhibits high linearity with a correlation coefficient exceeding 0.995 and fast response with a time constant of 0.24 s. Owing to its mechanical robustness and flexibility, the stand-alone graphene thermocouple can be knitted into a cotton fabric glove, which presents a fast response to environmental changes without any external power source. This work offers a unique fabrication method for producing a high-performance, flexible thermocouple that features a seamless and clear junction without the use of additional materials. This alternative method eliminates the complicated assembly processes typically required for conventional thermocouples.