Studi pengaruh fraksi volume penguat nano tembaga (Cu NANO) terhadap sifat mekanis dan konduktivitas listrik pelat bipolar berbasis hybrid nano komposit PP/C - CNT - Cu Nano = Study of influences of addition of volume fraction of nano copper reinforcement to mechanical properties and electrical conductivity of bipolar plate based of hybrid nano composite PP/C-CNT-Cu Nano