(a, b) SEM images of Co-OS sphere and Co-OSP sphere. (c) Elemental mapping images of Co, O, S, and P elements. (d) HR-TEM image. (e, f) OER polarization curves and corresponding Tafel slots. (g) The calculated electrochemical C_dl plot. (h, i) HER polarization curves and corresponding Tafel slots. (j) EIS plot. Velocity field distributions of different morphology models for (k) solid sphere, (l) hollow sphere, and (m) ball-in-ball structural sphere. (n) Flow velocity variation along the middle position of ball-in-ball model. Adapted from Ref. [22]. Copyright 2023, Elsevier.