Generally speaking, there are two kinds of proton transfer mechanisms in organic/inorganic
solution systems: vehicle mechanism and structure-diffusion mechanism [22–25]. Vehicle mechanism is reported in traditional KOH electrolytes and is described as the transport of protonated species as
whole molecules (for example: H2O, H3O+) through aqueous media [24]. In this research, proton
transfer in liquid electrolyte is considered as vehicle mechanism. In addition, proton transfer through
the surface solid of the electrodes, is considered as a structure-diffusion mechanism [22]. The proton
hops from site to site along a hydrogen-bonded network over the surface. Diffusion of protonic charges
or protonic defects occurs through the formation and breaking of the hydrogen bonds. Both H bond
types could contribute to structure-diffusion mechanism.
The H-transfer process in additive containing electrolytes during discharge is briefly illustrated in
Figure 15. The solid film over the MgNi alloy and Ni(OH)2 particles is formed during the charge
process or the first few seconds of the discharge process. In the solid film covering the MgNi alloy,
H is covalently or electrostatically bonded to other heteroatoms through bond type 1 or 2. H delivered
through the solid film to the interface of the liquid electrolyte is through site-to-site hopping way
(i.e., structure-diffusion mechanism). The H is the delivered in the electrolyte through vehicle mechanism
with H bond type 1. The H bond type and H-transfer mechanism for the solid film covering the Ni(OH)2
are similar to that of the MgNi alloy. Figure 15 also illustrates the trend of the H concentration
differential from the MgNi alloy to Ni(OH)2 during the discharge process. The H concentration is the
highest at the metal hydride-solid film interface and the H concentration is the lowest at the Ni(OH)2-solid
film interface