High-entropy alloys (HEAs) show great promise for improving catalyst activity, generating new materials, and accessing novel elemental compositions. However, producing ultra-high-entropy alloy nanoparticles (UHEANPs) containing eight elements or more is extremely challenging, because as the number of components increases, large immiscibilities between components start to overcome the entropic stabilization effects. Additionally, to be used for relevant industrial applications, such UHEANPs should be produced by low-temperature, wet chemical methods, which further complicate the synthesis. In this work, we successfully synthesized single-phase undenary FeCoNiCuRuRhPdAgIrPtAu UHEANPs, 2.2 nm in size, dubbed late-transition metal (LTM)-UHEANPs, containing all of the LTMs except toxic osmium, through a low-temperature approach combining continuous flow mixing and aging in a batch reactor. Despite six out of 11 elements being inactive for the hydrogen evolution reaction (HER), LTM-UHEANPs demonstrated high HER catalytic activity, on par with commercial Pt/C. Annealing treatment increased the average particle size of the LTM-UHEANPs to 4.9 nm, resulting in a drastic 8-fold increase in HER catalytic activity (TOF@25 mV = 4.66 s–1), surpassing previously synthesized combinations of LTM alloys and confirming an unconventional catalytic trend of higher activity for larger particles sizes in HEAs. This trend is attributed to the increased average coordination number of the surface atoms.
Mahin et al. (Sun,) studied this question.