具極高導電、高熱電性能的可拉伸雙網絡水凝膠

Abstract

隨著工業的發展，環保及永續發展已經成為人類社會的一個大議題。在我們的生活中，大量的低溫熱能常常因為無法轉化成人類可利用的能源而成為一種浪費。近年來，由於人們發現thermoelectric generators（TEGs）具有將熱能轉換為電能的特點而被廣泛的研究。然而，傳統的TEGs所產生的thermopower (又稱Seebeck coefficient) 只有幾百mV K-1，難以使電子裝置正常操作。為了解決這個問題，通過索雷效應(soret effect)的熱充電電容(thermally chargeable supercapacitors，TCCs）顯示的thermopower可達到幾十mV K-1，可解決傳統TEGs所遇到的問題。此外，可穿戴電子設備常可用於醫療保健、溫度感測器，為了讓電子產品不會因人體運動所產生損壞，具有可拉伸性質的穿戴式熱電元件便成為了研究的重點。 在這項研究中，為了實現高性能的熱電轉換和可拉伸的特性。我們使用了具有高機械強度的雙交聯網絡水凝膠作為熱電的系統。並將均勻混和的聚乙烯醇（PVA）、海藻酸鈉（SA）和聚乙二醇（PEG）的溶液透過凍融法(freeze-thaw method)形成水凝膠，隨後浸泡在不同濃度的四氟硼酸鹽(NaBF4)水溶液中製備PVA/SA/PEG/NaBF4 TCCs。並發現在1.5M NaBF4溶液中，PVA/SA/PEG/NaBF4水凝膠具有最高的機械性能，其拉伸性可達114 %，最大拉伸強度為69 kPa。此外，在熱電性能的表現上，在相對溼度60%下，其Seebeck coefficient還可達到50.9 mV K-1，離子導電率更可達到31.4 mS cm-1，Power factor 為 8.1 mW m-1 K-2。因此證明了透過SA的Manning’s “counterion condensation”和PVA聚合物鏈於低溫生成結晶所產生的協同作用下能夠使Na+和BF4-在熱泳效應下產生遷移率上的不同進而大大提升了Seebeck coefficient，使得PVA/SA/PEG/NaBF4 TCCs在熱電可穿戴式元件大大增加了其應用的潛力。

Nowadays, with the development of industry, the environment problem has been a big issue for human society. Abundant and ubiquitous thermal energy also known as low-grade heat is wasted in daily life. Therefore, thermoelectric generators (TEGs) exhibit the characteristic of conversion the heat to electricity, which has been widely investigated recently. However, thermopower for traditional TEGs is only few hundred microvolt per Kelvin, and is hard for power electronic devices. To solve this problem, thermally chargeable capacitor (TCCs) power by Soret effect display thermopower even as large as tens of mV K-1, which overcomes the problem TEGs have met. Besides, wearable electronics are applied in technology like health care, temperature sensors, etc. Therefore, the high thermoelectric performance with high Stretchability must be request. In this research, in order to attain the promising thermoelectric conversion as well as stretchable property. The ionic double-network hydrogel is employed as thermoelectric system, which has highly mechanical porous matrix swelling with liquid. The hydrogel made of elastic polyvinyl alcohol (PVA), sodium alginate (SA) and polyethylene glycol (PEG) are fabricated by the freeze-thaw method followed by soaking into different loading of tetrafluroborate (NaBF4) aqueous solution. The PVA/SA/PEG/ NaBF4 hydrogel at 1.5 M NaBF4 solution has high mechanical property, in which the stretchability is up to 114 % and large tensile strength of 69kPa. Most of all, it also displays a gigantic Seebeck coefficient of 50.9 mV K-1, ionic conductivity of 31.4 mS cm-1 at humidity of 60%, and power factor of 8.1 mW m-1 K-2. The result demonstrates that under the synergistic effect of Manning’s “counterion condensation” from SA and crystallinity characteristic of PVA polymer chains. The chaotropic NaBF4 ions can be selectively tuned, generating the ultrahigh thermopower, which made a PVA/SA/PEG/NaBF4 a potential thermally chargeable capacitor (TCCs) and pave a way for thermoelectric wearable devices to harvest the low-grade heat.