In square wave pulse voltammetry for electrodeposition of Fe-Co alloy films, observing higher current densities or more pronounced hydrogen evolution at higher scan rates (e.g., 50 mV/s) in a KOH electrolyte could be attributed to several factors:

· Mass transport limitations: At higher scan rates, the depletion of electroactive species (Fe2+ and Co2+) near the electrode surface occurs more rapidly due to the shorter timescale for diffusion to replenish the consumed ions. This mass transport limitation can lead to an increase in the current density due to the contribution of the hydrogen evolution reaction (HER) as a parallel process.

· Kinetic effects: The faster potential sweep at higher scan rates may not allow sufficient time for the deposition process to reach equilibrium conditions, favoring kinetically controlled reactions like the HER over the metal deposition process.

· Ohmic drop effects: At higher current densities associated with higher scan rates, the potential drop due to the solution resistance (iR drop) becomes more significant, leading to a shift in the effective potential at the electrode surface. This shift can promote the HER over metal deposition.

· Nucleation and growth mechanisms: The higher scan rates may influence the nucleation and growth mechanisms of the Fe-Co alloy films, affecting the surface morphology and potentially favoring the HER on certain surface sites or defects.

· Electrolyte composition: The alkaline KOH electrolyte is known to facilitate the HER, and at higher scan rates, the contribution of the HER may become more pronounced due to the kinetic and mass transport effects mentioned above.

To mitigate the high hydrogen evolution at higher scan rates, you could consider the following strategies:

a) Optimize the electrolyte composition, such as adjusting the concentrations of metal ions, complexing agents, or pH, to suppress the HER while favoring metal deposition.

b) Decrease the scan rate or use a different voltammetric technique (e.g., potentiostatic deposition) to minimize mass transport limitations and kinetic effects.

c) Modify the electrode surface or pre-treatment procedures to promote nucleation and growth of the desired Fe-Co alloy phase over the HER.

d) Investigate the use of alternative electrolytes or additives that can selectively inhibit the HER while promoting metal deposition.

It's important to note that the specific reasons may vary depending on the exact experimental conditions, and a combination of these factors could be contributing to the observed behavior. Further investigation and optimization may be required to achieve the desired electrodeposition characteristics.