Page 74 - Read Online

Choi et al. Energy Mater. 2025, 5, 500106 https://dx.doi.org/10.20517/energymater.2025.50 Page 9 of 28

For p-type i-TE materials, cation-dominated transport mechanisms play a critical role. By leveraging the
inherent negative surface charge of cellulose and optimizing ion-polymer interactions, researchers have
developed efficient strategies to enhance cation transport through structural modifications. One key strategy
involves engineering cellulose to form anion-repelling nanochannels. Li et al. introduced negatively charged
surfaces in cellulose nanofibers to attract Na ions while repelling OH , thereby significantly increasing ion
+
-
[30]
selectivity [Figure 3A] . The optimized NaOH-cellulose electrolyte facilitated ion mobility through well-
ordered nanochannels, achieving a thermal voltage of 24 mV K , more than twice that of conventional
-1
systems. Hu et al. further improved p-type properties by incorporating quaternary ammonium-based
electrolytes, BzMe NOH into the cellulose matrix . This system yielded a Seebeck coefficient of
[74]
3
-1
2.61 mV K , attributed to strong ion-polymer interactions within the cellulose network
[74]
[Figure 3B and C] . Ongoing research continues to focus on improving cation-selective transport by
modifying cellulose structures and optimizing ion-polymer interactions to further enhance TE performance.
While cellulose-based p-type i-TE materials have been extensively studied, efforts to develop n-type i-TE
materials remain at an early stage. Achieving n-type behavior requires promoting anion transport while
suppressing cation transport. This is typically accomplished by modifying the cellulose matrix to increase its
negative surface charge density, which immobilizes cations and facilitates anion diffusion. Liu et al.
demonstrated an n-type cellulose system by incorporating [BMIM][BF ] into a cellulose-nanofiber (CNF)
4
with poly(acrylamide) (PAM) hydrogel . Strong interactions between the BMIM cations and the CNF
+
[75]
component restricted cation mobility, allowing BF anions to dominate thermodiffusion. Optimizing the
-
4
[BMIM][BF ] concentration resulted in the highest anionic diffusion efficiency, while the CNF improved an
4
ionic conductivity to 13.9 mS cm . Wu et al. improved n-type cellulose by introducing Ca ions into
-1
2+
bacterial cellulose (BC) nanochannels. These ions strongly interacted with BC molecular chains to
immobilize Na while facilitating Cl diffusion. The system achieved a Seebeck coefficient of -27.2 mV K
-
+
-1
and an ionic conductivity of 204.2 mS cm [Figure 3D-F] .
-1
[43]
These studies highlight the versatility of cellulose as a platform for both p-type and n-type i-TE materials,
emphasizing the importance of surface charge modulation and polymer-ion interactions in achieving
selective ion transport. Tailoring polymer-ionic conductor interactions enables enhancements in ionic
conductivity, surface charge density, and molecular channel architecture, thereby expanding the design
possibility for high-performance cellulose-based i-TE materials. A summary of recent advancements in
cellulose-based i-TE hydrogels is presented in Table 1 [30,35,37,40,43,47,50,74-77] .
Polyvinyl alcohol
Polyvinyl alcohol (PVA) is a widely studied synthetic polymer for i-TE materials, exhibiting p-type behavior
due to its strong affinity for cations. This characteristic arises from its abundant hydroxyl (-OH) groups,
which enable extensive hydrogen bonding, stabilize the hydrogel matrix, and modulate both ionic diffusion
pathways and water retention . Partial ionization of these hydroxyl groups generates negatively charged
[78]
sites that selectively attract cations, thereby reinforcing p-type conduction. The incorporation of salts or ILs
further promotes thermally induced ion transport, leading to an enhanced Seebeck coefficient. These
structural advantages make PVA a promising candidate for flexible and bio-integrated TE devices.

Recent studies have focused on optimizing cation-dominated transport to enhance the TE performance of
PVA-based materials. By utilizing the high density of hydroxyl groups and strong hydrogen bonding,
researchers have developed strategies to improve cation selectivity and mobility. Chen et al. enhanced H -
+
dominated thermodiffusion in PVA hydrogels by reinforcing hydrogen bonding and crystallinity through
[79]
-
tensile crystallization [Figure 4A-C] . This approach selectively enhanced H mobility while restricting Cl
+