context: The electrocatalytic application of HEA NPs in the selective CO2 reduction reaction (CO2RR) has also been reported recently. For example, Nellaiappan et al. reported quinary fcc-AuPtAgPdCu NPs as an efficient CO2RR electrocatalyst toward CH4 and C2H4 in the low overpotential region. The five elements in the quinary HEA NPs were homogeneously distributed, whose chemical environments adopt metallic characteristics determined by XPS analysis except for Cu; a small amount of Cu2+ content was observed. The quinary fcc-AuPtAgPdCu NPs obtained superior selectivity toward the CO2RR in the low overpotential region, that is, FEmethane = 38.2% and FEethylene = 29.5% at −0.9 VAg/AgCl (−0.3 VRHE) in a CO2-saturated 0.5 M K2SO4 electrolyte (FE: faradaic efficiency). Based on their DFT calculation, the excellent electrocatalytic activity toward the CO2RR has mainly originated from redox-active Cu content in AuPtAgPdCu, while other elements provide an additional synergy in the CO2RR. Also, the long-term stability of the catalyst was verified by a chronoamperometry test for 5 h in a CO2-saturated 0.5 M K2SO4 electrolyte.
answer: CO2-saturated 0.5 M K2SO4