Hierarchical carboxymethyl cellulose hydrogel with boosted output power density for low-grade heat energy harvesting

Power density is one of the most critical factors determining the practical applicability of thermoelectric materials. To enhance this property, herein, hierarchically structured hydrogels composed of carboxymethyl cellulose, phytic acid, polyaniline and Fe3+ (CPPF) were prepared via a one-pot approach. At the nanometer scale, the CPPF nanoparticles and nanofibers formed a continuous network structure via multiply covalent and non-covalent interactions. The continuous distribution of polyaniline within the hydrogel matrix facilitates the redox reactions occurring within the hydrogel bulk and enhances ion diffusion, which imparts distinctive electrochemical properties along with superior energy storage and discharge capabilities. The maximum thermopower values of -2.8 mV·K− 1 (n-type) and 3.79 mV·K− 1 (p-type) were achieved. Using a single hydrogel unit for external discharge, the peak energy densities of 274.11 J·m− 2 and 319.83 J·m− 2, alongside peak average power densities of 4.57 W·m− 2 and 5.33 W·m− 2, were attained, higher than the previous literatures. A thermoelectric module comprising 40 p-n legs was constructed and successfully powered a light bulb, demonstrating outstanding potential for real-world applications. In addition, at the micrometer scale, the bamboo-like closed-cell structure exhibits tunable mechanical stiffness and achieving a maximum compressive stress of 6.5 MPa, while the soft variant demonstrated bendable and foldable properties with notable elasticity. The present work demonstrates a novel hierarchically structured hydrogel with both superior energy density and mechanical property, which supplies new ideas for the designation of high-performance hydrogel based thermoelectric materials.