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.