Utilizing electrochemical processes for plastic waste upcycling into valuable chemicals represents an optimal approach for extracting value from waste resources. Herein, we developed a low-cost and large-scale approach based on seawater corrosion engineering to construct amorphous CoFeNi material on nickel foam (A-CoFeNi), which effectively converts ethylene glycol (EG) from waste polyethylene terephthalate (PET) hydrolysate into formate. The optimized A-CoFeNi only necessitated 1.25 and 1.38 V vs. RHE to achieve 10 and 100 mA cm−2, respectively, demonstrating excellent EG oxidation reaction performance. In addition, the application of an ultra-low Pt content coating on A-CoFeNi, resulting in the preparation of a Pt-CoFeNi cathode catalyst, exhibited remarkably low overpotentials of 44 and 122.1 mV, respectively, to acquire current densities of 10 and 100 mA cm−2 for HER in 1 M KOH seawater. In the co-electrolysis system, the Pt-CoFeNi // CoFeNi exhibited excellent Faradaic efficiencies for the simultaneous generation of formate (94.8 %) and H2 (∼100 %). The enhanced activity and stability of this system could be ascribed to CoFeNi trimetallic cooperation and strong electronic interactions. This research provides valuable insights into leveraging seawater corrosion technology for the large-scale synthesis of catalysts, achieving efficient seawater utilization and electro-reforming PET plastic waste into high-value chemicals.