technology, which can literally convert ambient heat into high value-added electricity. [2] Compared to its analogous heatto-electricity technologies, TE takes the advantage of light weight, quietness, zero pollution emission, and an infinite lifetime (Figure 1b), making it ideal for selfpowering applications that are equipped on human body or installed in rural areas. [3] The performance of TE devices is mainly governed by the dimensionless figure-of-merit of their constituent TE materials, defined as ZT = S 2 σT/κ, where T refers to absolute temperature, S, σ, and κ are the Seebeck coefficient, electrical conductivity, and thermal conductivity, respectively. [4] Practically, "Z" reflects the premise of TE materials being used for power generation, namely as a thermoelectric generator, while "T" regulates the working temperature under which heat transfer is valid. [5] As can be seen in Figure 1c, TE materials with high heat transfer property and a Z of >10 −3 K −1 (corresponds to ZT > 0.3 at room temperature) are suitable for making temperature sensors; in comparison, TE materials with a much higher Z of 10 −2 K −1 (corresponds to ZT > 3 at room temperature) and low heat transfer property are preferable for making wearable devices.On the attainment of satisfactory Z value, a large S, which is proportional to the magnitude of thermovoltage per temperature difference (∆T), is highly desirable. However, the S of conventional electronic thermoelectric (e-TE) materials is limited to an order of 10 1 -10 2 µV K −1 due to relatively low electronic enthalpy of inorganic materials, i.e., the S of electron gas is merely ≈87.5 µV K −1 at room temperature. [6] Considering a small ∆T of 20 °C as that between human skin and ambient environment, more than 100 inorganic TE legs are required to work comparably with a 1.5 V solid battery via calculation, which indicates that e-TE materials are unsuitable for low-grade (<130 °C) applications. As most of waste heat is emitted from low-grade sources such as industrial plants and vehicle exhaust, traditional e-TE devices are in niche market. As another form of charge carrier, ions can induce a much larger S in an order of a few mV K −1 due to higher ionic enthalpy than electronic enthalpy. [7] As a result, an ionic thermoelectric (i-TE) device needs only ≈8 legs to generate a 1.5 V voltage at a ∆T of 20 °C, indicating that i-TE device is feasible for microminiaturization and can be incorporated with wearable devices and electronic skins. Other advantages of i-TEs over e-TEs include i) good flexibility stemming from the normally organic matrix, As an extended member of the thermoelectric family, ionic thermoelectrics (i-TEs) exhibit exceptional Seebeck coefficients and applicable power factors, and as a result have triggered intensive interest as a promising energy conversion technique to harvest and exploit low-grade waste heat (<130 °C). The last decade has witnessed great progress in i-TE materials and devices; however, there are ongoing disputes about the inherent fundamentals ...

Flexible thermoelectric materials and devices show great potential to solve the energy crisis but still face great challenges of high cost, complex fabrication, and tedious postprocessing. Searching for abnormal thermoelectric materials with rapid and scale-up production can significantly accelerate their applications. Here, we develop superlarge 25 × 20 cm2 commercial graphite-produced composite films in batches, achieved by a standard 10 min industrial process. The high cost effectiveness (S 2σ/cost) of 7250 μW g m–1 K–2 $–1 is absolutely ahead of that of the existing thermoelectric materials. The optimized composite film shows a high power factor of 94 μW m–1 K–2 at 150 °C, representing the optimal value of normal carbon materials so far. Furthermore, we design two types of flexible thermoelectric devices fabricated based on such a novel composite, which achieve an output open-circuit voltage of 3.70 mV using the human wrist as the heat source and 1.33 mV soaking in river water as the cold source. Our study provides distinguished inspiration to enrich flexible and cost-effective thermoelectric materials with industrial production.

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Owing to intrinsically high electrical conductivity and low thermoelectric conductivity, poly(3,4‐ethylenedioxithiophene):poly(styrenesulfonate) (PEDOT:PSS) shows promising thermoelectric properties. However, its relatively low power factor limits the practical applications of PEDOT:PSS. Here, unique dual post‐treatments by sodium sulfite (Na2SO3) and formamide (CH3NO) to boost the thermoelectric performance of flexible PEDOT:PSS films with an optimized power factor of 74.09 µW m–1 K–2 are used. Comprehensive characterizations confirm that CH3NO reduces the excessive insulating PSS and thereby increases the electrical conductivity, while Na2SO3 lowers the reduction of the doping level of PEDOT, leading to an increased Seebeck coefficient. Furthermore, the rationally post‐treated PEDOT:PSS films are assembled into a flexible thermoelectric device that exhibits an open‐circuit voltage of 2.8 mV using the heat from the human arm and an output power density of 2.56 µW cm–2 by a temperature difference of 25 K, indicating great potential for practical applications on sustainably charging low‐grade wearable electronics.

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