Solid-State N-Type Ionic Thermoelectric Based on Polyanions Enabled by Water State Regulation via MXenes

Adjusting interactions between the polyelectrolyte network and mobile anions/cations is commonly used to develop ion thermoelectric (iTE) materials with a high ionic thermopower and ionic conductivity. However, the development of n-type polyelectrolytes is hindered because cations usually exhibit higher thermophoretic mobility than anions. Here, we demonstrate a high-performance, solid-state n-type iTE based on polyanions by utilizing MXene nanosheets to regulate the water state of the hydrated poly(4-styrenesulfonic acid) (PSSH) network. MXene nanosheets featured negatively charged surfaces and positively charged edges, which allowed them to facilitate the transformation of bound water to intermediate water and promote the dissociation of protons from PSSH in the PSSH/MXene composite. Intermediate water with a low vaporization enthalpy readily evaporated on the hot side of the PSSH/MXene film, causing water and hydronium ions to flow from the cold side to the hot side. This polyanion-based iTE film thus achieved a large thermopower of -15.55 mV/K and a high ionic conductivity of 39.03 S/m at a relative humidity of 40%, leading to a high power factor of 9.44 mW/m·K² at 298 K. Due to the high photothermal efficiency of MXene, a fully-printed flexible device integrated with 30 legs of n-type PSSH/MXene film in series generated a voltage of up to -1.64 V under 0.1 W/cm² simulated sunlight illumination, demonstrating its potential applications in photo-thermoelectric devices. Our strategy of water state regulation presents a promising pathway for developing high-performance n-type iTE materials.