Simple thermoelectric generators based on cellulose paper and thin layers of non-toxic metals promise a cheap and eco-friendly means of heat recovery [Mulla et al., Materials Today Communications 29 (2021) 102738, https://doi.org/10.1016/j.mtcomm.2021.102738].
Around two-thirds of energy generated is lost as waste heat, making recovery systems highly desirable to improve efficiency and reduce use of fossil fuels. By converting a temperature gradient into useful electrical power, thermoelectric materials serve as the basis for such waste heat recovery systems. Electrical power is generated from a temperature difference in a thermoelectric device via the Seebeck effect. For thermoelectric systems to be useful practically, a high temperature gradient must be maintained across the device. Performance can be boosted in various ways such as using thin films, which afford greater specific power densities than bulk material. Thin film devices also hold the potential to be flexible – and even wearable.
Until now, however, most thermoelectric devices have relied on expensive and toxic materials, as well as energy-intensive manufacturing processes. But now Charles W. Dunnill and colleagues at Swansea University have designed and fabricated thermoelectric devices from cellulose paper and low-toxicity, cost-effective metallic conductors.
Copper iodide (CuI) and bismuth (Bi) thin films are deposited onto cellulose paper substrates using sputter coating at room temperature as the p- and n-type layers, respectively. As well as being highly conductive, CuI has extremely low thermal conductivity and a high Seebeck coefficient. Similarly, while Bi behaves in a semi-metallic manner, it possesses lower thermal conductivity and a higher Seebeck coefficient than other metals. Following deposition of the metallic layers, the cellulose paper is cut up into strips and arranged in alternating p- and n-type stripes on a polyimide tape and connected with copper foil and carbon conductive paint. The thermoelectric devices can sustain a high temperature gradient and generate promising power outputs at low temperatures.
A key feature of the cellulose-paper-based thermoelectric devices is their flexibility, which makes them conformable to a range of curved or uneven surfaces. As an example, the researchers wrapped their thermoelectric device around a cylindrical beaker filled with hot water and generated power outputs of 21-38 mV at temperatures of 40-60°C. Moreover, the devices can be repeatedly flexed without serious loss of performance, while the output remains stable.
The researchers believe that device performance could be improved further by better connections between the layers or directly connecting the Bi and CuI films before encapsulation with thermally stable material such as Kapton tape. Ultimately, the abundance and biodegradability of cellulose paper make these devices promising for cheap, disposable, eco-friendly thermoelectrics.
Thermoelectric device comprising ten pairs of alternating p- and n-type, Bi- and CuI-coated, cellulose paper strips on polyimide tape connected with conductive carbon paste and copper tape.