Atomic-Level Heterometallic Engineering in Single-Crystalline Polyoxometalate-Based MOF for Efficient Solar Photo-Thermo-Electric Conversion

Owing to delocalized free electrons from mixed-valence Mo^V/Mo^VI sites, the resulting molybdenum-based polyoxometalates compounds exhibit broad-spectrum solar absorption and hold significant potential for solar energy utilization. However, in discrete molecular clusters, intervalence charge transfer remains confined within individual polyoxometalate units, restricting carrier mobility to intramolecular hopping. This study pioneers a three-dimensional [Zn₄Mo^V₉Mo^VI₄O₄₀(mbim)₂]_n polyoxometalate-based metal-organic framework (denoted as AHF-Zn₄) that enables intercluster charge delocalization across the extended lattice, significantly enhancing photothermal conversion efficiency. Leveraging crystallographic insights, strategic substitution of 25% Zn²⁺ sites with magnetic metal ions (Co²⁺, Ni²⁺, Mn²⁺) further optimizes electron transport dynamics. X-ray absorption spectroscopy and density functional theory analyses revealed that the unfilled d orbitals of the heterometals provide more active electrons compared to Zn²⁺, facilitating the electron-vibration coupling effect, further increasing the nonradiative relaxation rate. Under 1 sun irradiation, the maximum temperature of AHF-CoZn₃ based MOF can reach approximately 75 °C. Through integration with thermoelectric modules, the evaporator stably achieves an output power of 1088 mW m^-2, enabling continuous power generation via the temperature gradient along the TE modules. This work provides a novel strategy for designing high-performance polyoxometalate-based MOF photothermal materials and demonstrates their potential applications in solar water purification, desalination, and solar thermoelectric power generation.