Figure 9 (a) Voltage (straight line) and power density (dashed line) versus current density of the Pt, Ni, and carbon electrodes.[81] Reproduced from ref. 81 with permission from Elsevier Ltd, copyright 2021. (b) Current-voltage and power-voltage curves for LTC and TC-LTC using different electrodes at a ΔT of 70 K. The best performance of the device was obtained using a 3D multi-structured Cu electrode.[82] Reproduced from ref. 82 with permission from Elsevier Ltd, copyright 2021. (c) Schematic diagrams of hybrid therermos-electrochemical cells (TECs). (d) Voltage (straight line) and power density (dashed line) versus current density of the Pt, W, and GC electrodes at ΔT of 50 K.[83]Reproduced from ref. 83 with permission from Elsevier Ltd, copyright 2021.
In the case of a W-based electrode, the \(S\) can be improved by two synergistic reactions, as depicted in Figure 9c: the redox reaction of the electrolyte and the oxidation reaction of the W electrode.[83] The resulting hybrid thermal cell using W electrodes achieved a \(S\) of 1.66 mV K–1and a power density of 425 mW m–2. Notably, this power density is 70% higher than that achieved by Pt or C electrodes, as shown in Figure 9d. This study presents a new thermal cell design employing metal-based electrodes, where both the electrodes and electrolytes undergo redox reactions. Generally, most metal-based electrodes possess varying valence states, and thus, this strategy provides another way to improve the performance of thermal batteries.
Furthermore, organic electrochemical devices are extensively used in the fields of bioelectronics, energy storage, electrocatalysis, and sensing. These devices operate based on a faradaic process, i.e., involving charger transfer through either oxidation reactions (electron loss) or reduction reactions (electron gain) facilitated by conductive polymers.
Berggren and Malliaras[84] demonstrated a simple metal electrode model that involves capacitive charge transport and storage of opposite charges in two electrode plates. On the other hand, they presented that a Faradaic process occurs with redox reactions on the bipolar plate, enabling charge transport and subsequently leading to a rectangular cyclic voltammetric curve that represented a transient charge current. Contrastingly, the voltammetric curves of the latter yield distinct redox peaks with the presence of a steady-state current. In addition, Horike et al. designed a flexible electrode using a polymer matrix derived from PEDOT: PSS. Their hydrogel samples were composed of Emim:Cl/PVA. While thermal diffusion is the predominant effect observed in this hydrogel, it still holds certain value to reference. The hydrogel, combined with this electrode, exhibited n-type conversion and showed a \(S\) of approximately 1 mV K–1.[85]
Next, the carbon electrode is a carbon-based conductive material made by processing anthracite coal, petroleum coke, graphite, coal asphalt, etc., through molding, roasting, and machining. It is a new, energy-saving, and environmentally friendly material that has gained increasing global usage since the 21st century. Carbon nanotubes (CNTs), discovered in the early 1990s,[86] are seamless, hollow nanoscale tubular structures made of single or multiple layers of graphite carbon, possessing unique physical and chemical properties. CNTs exhibit a range of impressive properties such as metal- or semiconductor-like conductivity, extraordinary mechanical strength, hydrogen storage capacity, broadband electromagnetic wave absorption, and significant adsorption capacity.[87] As a result of these exceptional properties, they hold important application value as energy storage materials, conductive materials, nanoelectronic components, and composite materials. Additionally, owing to their distinctive hollow structure, excellent conductivity, large specific surface area, and ion-permeable pores in electrolytes, coupled with their ability to intertwine and form nanoscale mesh structures, CNTs are often used as electrode materials in double-layer capacitors.
In a study conducted by Liu et al.,[88] a multi-walled CNT (MWCNT)-based ink was prepared with high viscosity and uniformity through ultrasonic treatment. It was demonstrated that chitin nanocrystals (ChNCs) interacted with MWCNT through non-covalent interactions like 𝝅-𝝅 stacking and hydrophobic interactions. The ChNCs/MWCNT (CCNT) ink exhibited excellent stability, with no accumulation even after 3 months. By using CCNT ink, a paper-based TEG was produced utilizing the silk screen printing technique. Further, the CCNT dispersion underwent solvent evaporation, resulting in a self-supporting membrane with an electrical conductivity of up to 1150 S m–1. The TEG was observed to have good biosecurity and flexibility, with the CCNT ink evenly attaching to both the surface and upper inner layers of the cellulose paper. Under a temperature difference of 12 K, the CCNT-based TEG showed efficient conversion of thermal energy into electrical energy, yielding a maximum output voltage of 0.375 mV, with a corresponding temperature difference of 0.7 K.
2.3. Application aspects and experimental scenarios of thermogalvanic hydrogels
2.3.1.Thermal induction self-supply equipments
In recent years, gels have generated significant interest in the fields of self-powering, status detection, and sensors.[98] Additionally, thermoelectric materials, based on thermoelectric principles, have been primarily employed as temperature sensors. However, there has been a growing focus on thermoelectric-based inductors due to their lower cost, simplified manufacturing processes, and ease of obtaining heat sources. They can monitor temperature, movement, and biophysical activities in wearable electronic devices, electronic skin, flexible robots, and other related scenarios.