In our article the possibility of producing gold nanoparticles from electronic waste (metal scrap) containing gold metal has been demonstrated. For you it is a cheap product.
You could use graphene or graphite nano-particles to enhance the thermal conductivity of phase change materials (PCM). Both graphene and graphite are low-cost materials.
You can use silver nanoparticles to enhance the thermal conductivity of phase change materials. A lso you can produce this type of nanoparticles by Laser ablation technique.