The proton reduction and copper deposition in water-in-salt electrolytes with LiCl were studied. Cyclic voltammetry on stationary electrode with various scan rates and linear sweep voltammetry on disk electrodes with different rotation rates were used to determine not only the diffusion coefficients but also the effective concentrations of protons and cupric cations. The suppression of hydrogen evolution limiting current observed in water-in-salt electrolyte was determined to result from the inhibition of proton diffusion. This diffusion inhibition was mainly related to the structural diffusion across hydrogen bond network and is not observed for cupric cations. The effects of alkaline metal cations on the acidity of electrolytes, the diffusivity of protons as well as the copper deposition rate were also studied using various super high concentrations of LiCl, NaCl and KCl.