The p-type TiCoSb-based half-Heuslers are widely studied due to the good electrical transport prop-erties after hole doping,while the pristine TiCoSb is intrinsically n-type.It is thus desired to obtain a compa-rable n-type counterpart through optimization of electron concentration.In this work,n-type Ti0.9-xHfxTa0.1CoSb half-Heuslers were fabricated by arc melting,ball milling,and spark plasma sintering.An optimized carrier concen-tration,together with a decreased lattice thermal conduc-tivity,was obtained by Ta doping at the Ti site,leading to a peak figure of merit(ZT)of 0.7 at 973 K in Ti0.9Ta0.1-CoSb.By further alloying Hf at the Ti site,the lattice thermal conductivity was significantly reduced without deteriorating the power factor.As a result,a peak ZT of 0.9 at 973 K and an average ZT of 0.54 in the temperature range of 300-973 K were achieved in Ti0.6Hf0.3Ti0.1CoSb.This work demonstrates that n-type TiCoSb-based half-Heuslers are promising thermoelectric materials.