ZrNiSn-based half-Heusler compounds are promising thermoelectric materials for mid-to-high temperature power generation, but their widespread application is hindered by the costly reliance on hafnium (Hf) for performance optimization. Herein, we present a cost-effective, Hf-free strategy that simultaneously enhances the power factor and figure of merit (zT) in n-type ZrNiSn via grain-boundary-engineered modulation doping. Metallic tungsten (W) nanoparticles are introduced as a secondary phase, where they spontaneously segregate to grain boundaries during consolidation. This microstructure creates a network of internal heterojunctions that perform two synergistic functions: Charge injection from the low-work-function W elevates electrical conductivity, while interfacial energy-filtering barriers enhance the Seebeck coefficient. Consequently, the optimized composite achieves a peak power factor of 44 μW·cm⁻¹·K⁻² and a zT of 0.76 at 910 K. Beyond intrinsic transport improvements, the practical viability of this material system is validated at the device level. When integrated into a thermoelectric unicouple, the material delivers an exceptional power density of 2.1 W·cm⁻² and a conversion efficiency of 5.2% at a temperature difference of 307 K. Crucially, this approach eliminates expensive Hf, reducing material costs by over 95%. This work establishes grain-boundary modulation doping as a cost-effective route to enhance the power factor and zT of Hf-free ZrNiSn, offering a pragmatic balance between performance and economic viability for mid-temperature waste-heat recovery.