First‐principles study of surface segregation in bimetallic Ni₃M (M = Mo, Co, Fe) alloys with chemisorbed atomic oxygen

Abstract

Segregation at metal alloy surfaces has an important impact on their catalytic and chemical properties. We have performed density‐functional theory calculations to investigate the surface segregation behaviors of Ni3M (M = Mo, Co, Fe) alloys in the presence of chemisorbed atomic oxygen. The calculated results show that the segregation trend at a Ni3M(111) surface can be substantially modified by reactive gaseous environments. At an oxygen coverage of 1/4 ML, both the Ni‐segregated and M‐segregated surfaces are still less stable than the nonsegregated one for Ni3Fe alloy, while an M‐segregated surface is more stable than the nonsegregated one for Ni3M (M = Mo, Co) alloys. Furthermore, the analysis of oxygen adsorption trends and surface electronic structures explains that the surface segregation trend is directly correlated to the surface–adsorbate binding strength. The present study provides valuable insight for exploring practical applications of Ni‐based alloys as hydrogen‐evolution electrodes.

Electrolyte Effects on the Stability of Ni−Mo Cathodes for the Hydrogen Evolution Reaction

“…Thisi sb elievedt ob ear esult of Mo surface segregation,w hich has been described previously. [23,24] In the cases of KOH and LiOH, the Mo/Ni decreases towards the surface, as was also the case for the experiments at pH 14. This means that Mo transport to the surface is less in these cases, as seen by the elemental ratios stabilizing earlier into the material.…”

“…To this end, we studied different commonly used electrolytes—NaOH, KOH, and LiOH—and show that the choice can play a significant role in HER performance . Our calculated energies suggest Ni segregation and Mo vacancy formation to be favorable . Experimentally, by using X‐ray spectroscopy (SEM‐EDX), atomic force microscopy (AFM), X‐ray photoelectron spectroscopy (XPS), inductively coupled plasma atomic emission spectroscopy (ICP‐AES), and UV/Vis spectroscopy, we characterized the stability of Ni−Mo materials in a variety of electrolytes and at various pH values.…”

“…Furthermore, the Mo content is higher near the surface (40 %) than what was found with EDX (25 %), which is effectively a bulk technique for these samples. The observation of Mo being more concentrated on the surface points towards surface segregation, as has been reported before …”

“…In Figure , the XPS depth profiles confirm the earlier statement that lighter alkaline elements infiltrate more easily into the alloy, as Li contents reach 85 % directly on the surface whereas this is 47 % and 20 %, respectively, for Na and K. From these data, the difference in leaching behavior at pH 13 compared with pH 14 can be explained as well: the highest leaching experiment was performed in NaOH and in contrast with KOH and LiOH in this case the Mo/Ni ratio increases instead of decreases. This is believed to be a result of Mo surface segregation, which has been described previously . In the cases of KOH and LiOH, the Mo/Ni decreases towards the surface, as was also the case for the experiments at pH 14.…”

“…[2] Our calculated energies suggest Ni segregation and Mo vacancy formation to be favorable. [23][24][25] Experimentally,b yu sing X-ray spectroscopy (SEM-EDX), atomicf orce microscopy (AFM), X-ray photoelectron spectroscopy (XPS), inductively coupled plasma atomic emission spectroscopy (ICP-AES), and UV/Vis spectroscopy,w ec haracterized the stabilityo fN i ÀMo materials in av ariety of electrolytesa nd at variousp Hv alues. We confirmt hat Mo leaches, as reported by Schalenbache tal., [20] and found that Mo leaches as MoO 4 2À .R oughening and changes in electrocatalytic activity were aresult of Mo leaching.…”