Abstract
AbstractWearable electronics are becoming one of the key technologies in health care applications including health monitoring, data acquisitions, and real‐time diagnosis. The commercialization of next‐generation devices has been stymied by the lack of ultrathin, flexible, and reliable power sources. Wearable thermo‐electrochemical cells (TECs), which can convert body heat to electricity via an electrochemical process, are showing great promise as power sources for such wearable systems. TECs harvest orders of magnitude more voltage per temperature difference (Seebeck coefficient (1–34 mV K−1)) when compared to the more common thermoelectric generators (Seebeck coefficient ≈tens or hundreds of µV K−1). However, there still remain great challenges for TECs progressing towards wearable applications. This review summarizes the recent development of potentially wearable TECs with promise for body‐heat harvesting, with a specific focus on flexible electrode materials, solid‐state electrolytes, device fabrication, and strategies toward applications. It also clarifies the challenges and gives some future direction to enhance future investigations on high‐performance wearable TECs for practical and self‐powered wearable devices.
Highlights
They investigated the electrodes in an n-type electrolyte of poly(vinyl alcohol) (PVA)-FeCl2/3 (1 m FeCl2/3) system and suggested that pure PEDOT/PSS film exhibited superior performance of Pmax/(△T)2 of 0.38 mW m−2 K−2, which surpasses all other composite materials such as PEDOT/PSS-carbon nanotubes (CNTs), PEDOT/PSS-edge functionalized graphene (EFG), and PEDOT/PSS-EFG/CNT film. This was attributed to the negative charges on excess PSS in PEDOT/PSS having a high affinity for multivalent cations and enhanced the cation transport through the polymer electrode. These results broaden the application of the flexible PEDOT/PSS electrodes in thermogalvanic cells (TGCs)
The results show that the Pmax/ΔT2 could be improved from 0.00375 to 0.014 mW m−2 K−2 with 5 wt% polyvinylidene difluoride (PVDF), 0.1 m redox couple and an electrode separation of 1 mm
The device consisting of 30 pairs of p–n cells worn on the arm of a person could charge 470 mF supercapacitors and power a green light-emitting diodes (LED) with a voltage booster during long-term wear (Figure 7c,d)
Summary
The configuration and mechanism of a thermogalvanic cell (TGC) is illustrated in Figure 2, which shows a TGC consists of two identical electrodes with an electrolyte containing redox couples. The redox couple has a negative (or positive) sign indicates that the hot electrode behaves as the anode, which is analogs to a p-doped (or n-doped) thermoelectric and defined as p-type (or n-type) redox couple.[60] To amplify the entropy change of redox couples for a high Se, efforts have focused on; varying the solvation shell of the redox active species by using different solvent systems (e.g., water,[42,61] organic solvents,[59] ionic liquids[53,59] or their mixtures[62]); the incorporation of additives (like chaotropic cations for Fe(CN6)3-/4-[63] and α-cyclodextrins for I−/I3−[64,65,66]); and employing different counter ions, such as Cl−, SO42−, NO3−, ClO4− for Fe2+/3+[57,67], NTf2− and Cl− for CoII/III(bpy)32+/3+.[68]. The exchange current density is dependent on the ion concentration and diffusion coefficient of electrolytes,[60,67] the charge transfer resistance on the interface of between the electrode/electrolyte as well as the electric transport ability of the electrode materials.[51,69,70,71,72,73] electrode materials with the high catalytic surface, high electrical conductivity, and large surface area such as platinum, carbon materials and conducting polymers are highly promising
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