Three factors make bulk high-entropy alloys as effective electrocatalysts for oxygen evolution

Even in their bulk forms, complex alloys like high-entropy alloys (HEAs) exhibit favorable activity and stability as electrocatalysts for the oxygen evolution reaction (OER). However, the underlying reasons are not yet fully understood. In a family of Mo-doped CrFeCoNi-based HEAs, we have identified three crucial factors that govern their performance: (i) homogeneous solid solution phase of HEAs helps to maintain high-valence states of metals; (ii) surface reconstruction results in a hybrid material comprising amorphous domains and percolated crystalline structures; (iii) diversity of active intermediate species (M–O, M–OOH, and, notably, the abundance of superoxide μ–OO), which display stronger adsorption capacity on the reconstructed surface. These results are revealing due to their resemblance to findings in other families of electrocatalysts for OER, as well as their unique features specific to HEAs. In line with these factors, a CrFeCoNiMo0.2 bulk integrated electrode displays a low overpotential of 215 mV, rapid kinetics, and long-term stability of over 90 d. Bulk HEAs hold great potential for industrial applications.


Figure S1 .
Figure S1.SEM images (top panel) and EDS elemental mapping.(a) CrFeCoNi; (b) CrFeCoNiMo0.1;(c) CrFeCoNiMo0.2;(d) CrFeCoNiMo0.5;(e) CrFeCoNiMo.Note that: the non-uniform distribution in (b) and (c) is not caused by phase separation but by dendrites specific to the cast alloy.This structure is formed due to nonequilibrium cooling and does not change the f.c.c.structure.

Figure S2 .
Figure S2.SEM images and EDS elemental mapping.(a) CrFeCoNiMo0.3.The white dashed circles marked the μ phase.(b) CrFeCoNiMo0.4.It follows that the excessive addition of Mo leads to an increase in the volume fraction of the μ phase.

Figure S3 .
Figure S3.TEM images of CrFeCoNi HEA.(a) TEM bright-field image.The inset is the SAED pattern.(b) HRTEM image.(c) Magnified image of the red dashed box in (b).(d) is the FFT pattern of (c).The results indicate that the CrFeCoNi consisted of a single f.c.c.phase.

Figure S4 .
Figure S4.TEM images of CrFeCoNiMo0.2HEA.(a) TEM bright-field image.The inset is the SAED pattern.(b) and (c) are the HRTEM images of the regions with relatively high and low Mo content in the structure, respectively.(d) and (e) correspond to the enlarged HRTEM image in the red dashed and yellow dashed box in (b) and (c), respectively.f, is the FFT pattern of (d) and (e).Due to the identical FFT pattern of (d) and (e), only one is shown here.(g) and (h) are the HAADF-TEM images.(g) corresponds to high Mo content and (h) corresponds to low Mo contents.

Figure S5 .
Figure S5.TEM images of CrFeCoNiMo HEA.(a) Microscopic morphology of the region containing the μ and f.c.c.phases.(b) HRTEM image.The interface between HEA and μ-phase is marked by the black dashed line.(c) TEM image at the interface between μ and f.c.c.phases.The table in (c) show the atomic ratios of Cr, Fe, Co, Ni, and Mo elements in μ and f.c.c.phases.(d) and (e) are the SAED patterns corresponding to μ and f.c.c.phase in (a), respectively.(f) and (g) are the enlargement of HRTEM images corresponding to μ and f.c.c.phase in (b), respectively.The inset both show the corresponding FFT patterns.(h) The EDX-line analysis of the white lines in (c).(i) TEM image and the corresponding elemental mappings of Cr, Fe, Co, Ni, Mo elements.

Figure S8 .
Figure S8.OER performance comparison and electrochemical stability.(a) The OER preformance comparison showing the overpotential at 10 mA cm -2 with Tafel slope in 1.0 M KOH.(b) OER polarization curves after 5000th CVs of CrFeCoNiMo0.2electrocatalyst.(c) Long-term stability testing of CrFeCoNiMo0.2 at a constant current density of 10 mA cm -2 .The inset shows the SEM image of the surface after stability test.

Figure S12 .
Figure S12.TEM-HAADF images of CrFeCoNiMo electrocatalysts after electrocatalytic.The Cr, Fe, Co, Ni and Mo elements in the μ phase show significant chemical shedding after OER testing.The content of O element on μ phase is much higher than on the substrate.It indicates that μ phase is corroded.

Figure S13 .
Figure S13.The ICP-OES results of each element in the electrolyte.

Figure S14 .
Figure S14.SEM and TEM images of CrFeCoNi after electrochemical activition.(a) and (b) are SEM images at different magnifications.The formation of nanoparticles on the catalyst surface can be clearly observed in (b).(c) TEM images and elemental mapping.

Figure S15 .
Figure S15.SEM images of CrFeCoNiMo0.2after electrochemical activition.From (a) to (f) is the stepwise amplification of the multi-metal oxide formed on the surface.

Figure S16 .
Figure S16.SEM images of the characteristic morphology of multi-metal oxides on the CrFeCoNiMo0.2surface.

Figure S17 .
Figure S17.SEM images of oxides on the surface of CrFeCoNiMo.(a) Distribution of oxides on the surface.(b) Enlarged view of the yellow box in (a).Compared with CrFeCoNiMo0.2, the oxide formed on the surface of CrFeCoNiMo is reduced and unshaped.

Figure S20 .
Figure S20.Multistep OER reactions on different atomic configurations of CrFeCoNiMo0.2.The groups located in the upper part of the model are oxygencontaining intermediates

Table S1 .
The electrocatalytic OER performance of this work and the performance data reported for other catalysts, including high entropy materials, amorphous materials and containing high value metal materials.foroxygen evolution reaction Chem.Eng.J. 423 130168.[42]Li R, Hu B, Yu T, Chen H, Wang Y, Song S 2020 Insights into correlation among surface-structure-activity of cobalt-derived pre-catalyst for oxygen evolution reaction Adv.Sci. 7 1902830. stability