The role of metal hydroxides in NiFe deposition

In this work, the role of hydroxyl compounds during the anomalous NiFe electrodeposition is reinvestigated. It is found that the manner in which the boundary conditions are incorporated in existing NiFe models that lire based on the chemical equilibrium of species in solution, is not correct and leads to unphysical situations. In order to alleviate this problem, a kinetic description of the homogeneous reactions in the solution is used to calculate the concentration profiles of the ionic species near electrodes. This shows that even the fastest chemical reactions are nor in chemical equilibrium in the vicinity of the electrode. Also, the amount of generated hydroxyl ions during proton reduction and hence the extent to which metal ions are hydrolyzed, is far less than assumed in existing models. This result is confirmed experimentally by polarization measurements. It is shown that the Ni-H-2 and Fe-H-2 systems exhibit anomalous codeposition behavior as well and that inhibition of the Ni and H reduction in the Ni-Fe-H-2 system cannot be explained in terms of metal hydrolysis reactions. Therefore. it is concluded from both the theoretical calculations and the experimental findings, that metal hydrolysis does not play a determining role in anomalous NiFe codeposition. (C) 2000 The Electrochemical Society. S0013-4651(00)01-010-7. All rights reserved.