High-performance and eco-friendly flexible thermoelectric generators based on Cu₃Se₂ and waste pencil lead composites

The novel properties of waste pencils lead for flexible thermoelectric applications, along with the need to enhance the thermoelectric power density of flexible thermoelectric generators for low-temperature applications, have motivated this work. Here, screen printable novel inks of the composites consisting of widely available and low toxic copper selenide with HB, 2B, and 4B pencil lead in a ratio of 99:1 were formulated using a bio-degradable binder. Among three pencil leads, 2B lead composite ink-based flexible thermoelectric generators exhibited superior thermoelectric performance. On optimizing the concentration of 2B pencil lead (0.5–2.0 wt%) in copper selenide, it was observed that 1 wt% of 2B pencil lead was found to exhibit 1970 % higher power output than pure copper selenide. Under external load, the optimum structural, compositional, and thermal properties of 2B pencil lead resulted in power output, power factor, and power density of 135.34 nW, 56.39 nW/m²K², and 77.25 mW/m², respectively, at 100 °C temperature gradient, which is superior to many reported novel thermoelectric generators. This work demonstrates an eco-friendly, sustainable, low-cost approach for fabricating a flexible thermoelectric generator with excellent performance.