Atomically Precise Oxygen Reduction Reaction Catalysts from Simulated Pd Electrodeposition Waste Solutions

Abstract

In this study, ligand-functionalized carbon black and graphene nanoplatelet supports were employed to obtain atomically dispersed Pd-based catalysts through surface coordination of Pd(II) species. A tris(2-aminoethyl)amine (TREN)-based ligand was used as the coordinating moiety. Palladium was sourced from a [Pd(NH3)2(NO2)2] solution, one of the most widely employed Pd salts in the electroplating industry, at concentrations representative of industrial rinsing waters (0.1 g/L) in the presence of a realistic supporting electrolyte (0.1 M Na2SO4). The proposed strategy enables the recovery of Pd(II) from dilute aqueous streams while directly generating low-Pd-content catalysts, which we tested for the oxygen reduction reaction (ORR). Pristine and functionalized carbon substrates, before and after Pd uptake, were characterized by complementary spectroscopic and electrochemical techniques. TEM and XPS analyses revealed that bare carbon black and graphene nanoplatelets predominantly promote the formation of Pd(0) nanoparticles. In contrast, ligand-functionalized surfaces favor the stabilization of supported Pd(II) coordination sites. The Pd(II)-modified materials exhibited enhanced catalytic activity and markedly improved stability compared to catalysts obtained from nonfunctionalized carbon supports. This behavior is attributed to the transition from easily leachable Pd(0) nanoparticles to molecularly defined Pd(II) surface complexes. Rotating ring(-disk) electrode (RDE/RRDE) measurements indicated a predominant four-electron ORR pathway, enabling the selective reduction of oxygen to water with limited hydrogen peroxide formation. By combining dilute waste-inspired Pd sources with a simple and effective carbon functionalization strategy, this approach contributes to the development of circularity of Pd in the electroplating industry and to the achievement of low-cost catalytic materials for alkaline fuel cell technologies.