Converting body heat into electricity is a promising strategy for supplying power to wearable electronics. To avoid the limitations of traditional solid-state thermoelectric materials, such as frangibility and complex fabrication processes, we fabricated two types of thermogalvanic gel electrolytes with positive and negative thermo-electrochemical Seebeck coefficients, respectively, which correspond to the n-type and p-type elements of a conventional thermoelectric generator. Such gel electrolytes exhibit not only moderate thermoelectric performance but also good mechanical properties. Based on these electrolytes, a flexible and wearable thermocell was designed with an output voltage approaching 1 V by utilizing body heat. This work may offer a new train of thought for the development of self-powered wearable systems by harvesting low-grade body heat.

Converting body heat into electricity is a promising strategy for supplying power to wearable electronics. To avoid the limitations of traditional solid-state thermoelectric materials, such as frangibility and complex fabrication processes, we fabricated two types of thermogalvanic gel electrolytes with positive and negative thermo-electrochemical Seebeck coefficients, respectively, which correspond to the n-type and p-type elements of a conventional thermoelectric generator. Such gel electrolytes exhibit not only moderate thermoelectric performance but also good mechanical properties. Based on these electrolytes, a flexible and wearable thermocell was designed with an output voltage approaching 1 V by utilizing body heat. This work may offer a new train of thought for the development of self-powered wearable systems by harvesting lowgrade body heat.Given the recent developments in the area of wearable electronics and e-skins, [1][2][3][4][5][6] the emerging need for selfpowered energy supply has heightened the interest in energy harvesting from the environment or human beings. Among recognized energy-harvesting technologies, such as solar cells [7,8] and triboelectric and electret generators, [9,10] thermal energy is a potential power source that is widely available in the environment and in industrial processes. [11,12] However, human bodies are also a permanent heat source, with a surface temperature of about 32 8C and possibly tens of degrees temperature difference between the human body and its environment. [13,14] Hence, it is of practical meaning to convert body heat energy, a type of low-grade heat, into electricity for directly powering wearable electronics. [15][16][17] The most convenient strategy to utilize low-grade heat is thermal-electric conversion. Traditional thermoelectric generators utilizing the Seebeck effect are mainly based on solidstate semiconductors or conducting polymers, [18,19] with output voltages limited by the relatively low Seebeck coefficient (several hundreds mV K À1 ). Meanwhile, the frangibility and expensiveness of thermoelectric materials as well as their complicated fabrication processes are other obstacles restricting their application in wearable electronics. [20] Alternatively, a large thermovoltage can be derived from thermogalvanic effects, resulting from temperature-dependent entropy changes during electron transfer between redox couples and electrodes. [21][22][23][24] Previous reports mainly focused on the exploration of electrode materials, such as carbon nanotubes (CNTs) and graphene, [25][26][27][28] to achieve high thermal-electric conversion efficiencies. However, because of the aqueous electrolytes used in thermocells, large-scale integration and packaging of the units would be more difficult in applications, especially for wearable devices. [29] Inspired by the successful application of gel electrolytes in solid-state electrochemical energy storage systems and stretchable ionic conductors, [30][31][32][33] we surmised that solid-state or quasi-solidstate gel...

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