Microstructure Optimization in the Shear-Exfoliated Bi₆Cu₂Se₄O₆ through Introducing Reduced Graphene Oxide Leads to Wide-Ranged Thermoelectric Performance

Bi₆Cu₂Se₄O₆ is considered as an ideal n-type thermoelectric material to pair with p-type BiCuSeO for preparing oxyselenide-based thermoelectric devices, but its thermoelectric performance is limited by poor electrical conductivity. In this research, the reduced graphene oxide (rGO) nanosheets are introduced into Bi₆Cu₂Se_3.6Cl_0.4O₆ matrix through liquid-phase shear exfoliation to modify the microstructure. rGO can insert into matrix grains as intercalations, or embed into grain boundaries as wetting phase, and prompt grain alignment, which contributes to the significantly enhanced carrier mobility, thus leading to an improvement in electrical conductivity from ≈15 S cm⁻¹ to ≈230 S cm⁻¹ at 303 K. Whereafter, the effective donor dopant Nb is chosen to substitute Bi. The carrier concentration is increased without damaging the carrier mobility, resulting in a further improved electrical conductivity of ≈840 S cm⁻¹ at 303 K. Lattice thermal conductivity is also suppressed owing to the intensive phonon scattering by point defects and grain boundaries. Ultimately, a record-breaking peak ZT ≈0.5 (873 K) and average ZT ≈0.3 (303–873 K) can be achieved in Bi_5.91Nb_0.09Cu₂Se_3.6Cl_0.4O₆ + 0.5% rGO. The microstructure optimization method in this research effectively improves thermoelectric performance, and is anticipated to be applied in other thermoelectrics.