Elemental Stability in Mixed Noble and Non-Noble Metal High Entropy Alloy Nanoparticle Electrocatalysts

High entropy alloys (HEAs) exhibit unique physical and chemical properties that distinguish them from conventional alloys, which makes them highly attractive for electrocatalysis. Their enhanced corrosion resistance is often attributed to the high entropy of mixing; however, predicting and evaluating the stability of individual elements within HEAs remain challenging. Conventional benchmarking approaches, such as accelerated stress tests and ex situ characterization, provide only indirect insights into degradation pathways and lack time-resolved element-specific information. In this work, we combine a scanning flow cell (SFC) with online inductively coupled plasma mass spectrometry (ICP-MS) to enable real-time quantification of metal dissolution from HEA nanoparticle catalysts during electrochemical measurements. This approach allows simultaneous electrochemical characterization and detection of trace metal dissolution at the picogram-per-square-centimeter level. The stability of a representative noble-non-noble PdCuPtNiCo HEA is systematically compared with that of bimetallic Pt-based alloys (PtPd, PtNi, and PtCo) in both acidic and alkaline electrolytes. All alloys exhibit high stability in the potential region for the oxygen reduction reaction (ORR), irrespective of pH. However, at higher anodic potentials in acidic media, Pd induces pronounced destabilization in its alloys, rendering it a surprisingly detrimental component of the HEA. In contrast, the enhanced stability of non-noble metals under alkaline conditions suppresses dissolution in the PdCuPtNiCo HEA even at a significantly higher potential, resulting in exceptional stability across a wide potential window. These findings underscore the importance of knowledge-driven materials design when combining noble and non-noble elements in HEAs. Overall, HEA formation enhances electrochemical stability compared to corresponding bimetallic systems: Pt is stabilized when alloyed with Ni or Co, whereas Pd introduces instability within multicomponent compositions.