[en] Highlights: • Both Yb₂O₃ nanoprecipitates and CNTs are used for grain boundary engineering. • A maximum zT of 1.4 at 875 K and a device ZT of approximately 1.0 are achieved. • A thermoelectric conversion efficiency of 9.3% is realized. There have been few demonstrations that grain boundary engineering results in higher power generation efficiencies despite serious interest in using the strategy to improve zT. Using both Yb₂O₃ nanoprecipitates that decorate grain boundaries and carbon nanotubes to further reduce thermal conductivity and improve mechanical strength, a maximum zT of 1.43 at 875 K and a device ZT of approximately 1.0 in Yb-filled CoSb₃-based nanocomposites are achieved. The preparation procedure of these materials is repeatable in mass production. Thermoelectric power generation modules based on these high-performance materials demonstrate a thermoelectric conversion efficiency of 9.3% under a temperature difference of 558 K. The results highlight nanostructured grain boundary engineering as a strategy to improve conventional thermoelectric materials, and the realistic prospect of large-scale thermoelectric power generation using skutterudite-based nanocomposites is demonstrated.