邁向膠體熱電紀元：開發新型氧化還原性熱賈法尼凝膠

Abstract

熱賈法尼電池 (Thermogalvanic Cells, TGCs) 為一種以氧化還原反應驅動之離子型熱電裝置，具備高賽貝克係數 (Seebeck coefficient)、低熱導率、材料彈性高等特點，尤其適用於低階廢熱 (<100 °C) 之回收利用，對於實現碳中和與綠色能源發展具有高度潛力。然而，傳統 TGCs 多受限於質傳與動力學遲滯和電荷轉移效率低落等問題，導致其整體能量轉換效率與功率密度仍難以突破。本研究提出兩種創新設計策略，針對熱賈法尼凝膠材料進行熱力學與動力學性能提升：(1) 溶劑化結構工程，與 (2) 高分子結構熵調控。 第一部分針對 Fe(CN)64−/3−之熱賈法尼系統，該研究探討添加胺類分子 formamide (FA)對其氧化還原性離子之溶劑化結構、反應熵變與電荷傳輸之影響。FA為結構促進型 (Structure-making) 分子，能與水分子形成氫鍵網絡，並在電極界面降低離子反應能障，從而同時提升熱電勢與短路電流密度 (JSC)。在FA濃度為 1.12 M 時，裝置熱電性能達最大化，α 提升至 1.60 mV K⁻¹，JSC /ΔT 為 0.55 A m−2 K−1，Pmax/ΔT² 達 0.274 mW m−2 K−2，較未添加 FA (0.165 mW m−2 K−2) 提升約 60%。此現象透過 DFT 計算進行佐證，FA–水作用能 (0.52 eV) 明顯高於水–水作用能 (0.35 eV)，說明 FA 有效穩定自由水，促進反應進行。此外，透過電化學阻抗分析 (Electrochemical impedance spectroscopy, EIS) 與循環伏安法 (Cyclic voltammetry, CV) 評估其電荷轉移行為，發現 FA 在低濃度下可降低電荷轉移電阻 (Rct)，提升電子轉移速率 (k0)，最大值達 1.5×10−3 cm s−1。 第二部分則以分子設計角度切入，合成新型 TEMPO 基自由基側鏈取代之聚丙醯胺 (PTAm) 高分子，探討其於 TGC 系統中之應用潛力。TEMPO 為一具有高氧化還原可逆性與快速電荷轉移特性的穩定自由基官能基，鍵結至高分子主鏈上後，可藉由氧化還原過程所伴隨之構型變化產生額外構型熵變(Conformational entropy)。構型熵變可用於預測熱電勢。實驗中，PTAm 經電化學氧化後展現良好對水溶解性與可逆性，展現優異之熱電性能 (α = −0.76 mV K−1; Pmax/ΔT2 = 1.18 mW m−2 K−2)。 綜合以上，本研究從溶劑層級與分子層級提出兩種創新性設計策略，分別針對傳統無機氧化還原對 (Fe(CN)64−/3−) 與2,2,6,6-四甲基哌啶-1-氧化物自由基系統 (TEMPO) 進行性能優化，顯著提升熱電性能並同時探討其熱力學與動力學本質。研究成果不僅深化對熱賈法尼系統中熵變與溶劑化結構間相互關係的理解，也提供未來設計高效、低成本、可彎曲式熱電材料的重要參考架構，為實現低溫廢熱回收與永續能源利用開闢新方向。

Thermogalvanic cells (TGCs) represent a promising class of ionic thermoelectric systems for harvesting low-grade waste heat due to their high thermopower, low thermal conductivity, and continuous power generation capabilities. However, their performance remains limited by factors such as sluggish charge-transfer kinetics and low entropy change during redox reactions. In this work, two distinct strategies are explored to enhance the performance of gel-state TGCs through molecular and solvation engineering. First, solvation structure engineering is demonstrated using formamide (FA) as a structure-making amino-additive in Fe(CN)64−/3−-based thermogalvanic hydrogels. FA selectively interacts with water to stabilize the solvation shell, lowering desolvation barriers at the electrode interface and simultaneously increasing both the redox entropy change and charge-transfer kinetics. These effects lead to a significant enhancement in thermopower (α increases from 1.38 to 1.80 mV K−1) and a 60% improvement in normalized power output (Pmax/ΔT² = 0.274 mW m−2 K−2). Electrochemical impedance spectroscopy and DFT simulations support the dual thermodynamic and kinetic role of FA at low concentrations, while revealing performance degradation at high concentrations due to solvation shell overcrowding and viscosity increase. Second, a novel TEMPO-substituted polyacrylamide (PTAm) was synthesized and investigated as a redox-active polymer for thermogalvanic applications. The incorporation of the nitroxide radical (TEMPO) onto a flexible polymer backbone enables fast redox kinetics and a significant conformational entropy change during oxidation-reduction When deployed in aqueous media, the oxidized-PTAm polymer demonstrated excellent solubility and electrochemical reversibility, offering a robust platform for n-type thermogalvanic energy conversion. Together, these two approaches demonstrate the importance of combining solvation tuning and molecular design to improve performance in ionic thermoelectric systems. This work not only provides a deeper understanding of the fundamental mechanisms governing entropy and kinetics in TGCs, but also introduces scalable, low-cost strategies for designing high-performance thermogalvanic gels. The findings open new avenues for efficient, soft-matter-based thermal energy harvesting devices aimed at next-generation green energy technologies.