To improve the thermal-to-electrical conversion efficiency of waste exhaust heat at temperatures in the vicinity of 750°C, RTI has combined two different high-performance materials to form a high ZT, hybrid thermoelectric (TE) device. Recently-developed enhanced “TAGS-85”, or e-TAGS, was employed as the p-leg, while the n leg was comprised of improved half-Heusler (HH) material. This hybrid material pair provides a high ZT, lead-free TE material solution for exhaust gas heat recovery for use in vehicle or industrial platforms. The improved HH material employs two novel techniques to reduce thermal conductivity: (1) high-energy milling, and (2) addition of coherent inclusions. Single n-/pcouples were produced that achieved a 9.2% efficiency with a power output of 205mW for Thot = 559°C and ΔT = 523K. This is a significant efficiency improvement at a lower hot side temperature with the hybrid e-TAGS/HH single couple over the performance of a conventional, all HH couple. By optimizing the cross sectional areas of the pellets for equal heat flow, the resulting asymmetric couple achieved 10.5% efficiency with a maximum power output of 317 mW at Thot = 537°C and ΔT = 497°C. A 49-couple hybrid module using other advanced HH materials paired with e-TAGS and operated with Thot up to 600°C reached a maximum efficiency of 10%. The improved module efficiency is believed to be due to both improved materials and optimized cross-sectional area ratios between the n- and p- elements.