Carbon Quantum Dots Derived from Spent Local <i>Liberica</i> Coffee Grounds for Paper Thermoelectric Devices

Wearable thermoelectric devices have great potential for renewable energy applications. However, challenges remain in achieving sustainability, durability, and excellent performance. This study aimed to develop an eco-friendly and flexible thermoelectric generator (fTEG) for wearable applications. Utilizing a paper-based substrate and incorporating carbon quantum dots (CQDs) synthesized from spent coffee grounds, we proposed an alternative solution to reduce waste and maximize the utilization of biomass. The synthesized CQDs exhibited blue luminescence with a maximum quantum yield (QY) of 85.95% and a band gap energy of approximately 3.0 eV. The high QY was correlated with enhanced electron mobility, leading to an optimal Seebeck coefficient of −2.38 mV/K for a 1.5 wt % n-type fTEG device. This value was further enhanced to 5.44 mV/K when the 1.5 wt % n-type was paired with a p-type film. The output power reached a high value of 10.5 nWatt/cm2 at a temperature difference of 20 K for a surface area of 1.0 cm2. The prototype fTEG device demonstrated an open-circuit voltage of 30 mV when tested on a human arm. These promising results indicated excellent potential for future flexible and wearable TEG devices based on sustainable materials.