The structure and electrochemistry of the nickel hydroxide have been studied extensively because of its importance in different technical processes, such as oxygen electro-catalysis, as the active material in nickel battery systems, as electrochromic material and capacitor applications . On the other hand, so as to make use of nickel-oxide electrodes as electrochemical capacitors, a high specific area is necessary for increasing the amount of stored energy. In recent years a great technical achievement has been the preparation of composite materials since these materials are characterized by a controlled degree of porosity. Special interesting result in particular those made up of conductive powders (i.e. nickel, graphite, graphene, nanotubes, etc.) dispersed in an insulating polymeric matrix. With an appropriate percentage of conductive powders, they have a good electrical conductivity and a high developed electro-active surface. Also, a high ionic conductivity is required to allow the electrolyte to penetrate inside the porous network. Nickel hydroxide layers can be prepared by different methods. In basic solutions, the redox processes involving nickel hydroxide and oxi-hydroxide can be described as:

NiOOH lattice Ni(OH)₂ lattice

Nickel+epoxy composites were elaborated by dispersing nickel powder in a polymeric matrix of epoxy resin. Nickel hydroxide was generated on nickel+epoxy surfaces as well as on polycrystalline nickel by means of successive voltammetric cycles and a galvanostatic reduction [2]. The electrochemical response was studied in KOH solutions at several concentrations. Nickel hydroxides grown on nickel+epoxy surfaces exhibit a different potentiodynamic i-E profile and cathodic charges which are largely affected by nickel/epoxy ratio, potential sweep rate and pH solution.

The results revealed that both Ni(OH)₂ and NiOOH present at least, two structural modifications, named a and b in the case of the Ni(OH)₂ while they are designed as g and b in the case of NiOOH. The a-Ni(OH)₂  structure is unstable in KOH concentrated solutions and it progressively transforms into b-Ni(OH)₂, leading to the 50-70 mV positive shift of the anodic peak observed when the electrode is cycled or is allowed to stay in concentrated KOH solutions. Also, during cycling and specially when in a strong alkaline medium, the irreversibly turns into the more expanded . This phase change can be correlated to a 44 % increase in volume, causing mechanical deformation which results in an irreversible damage to the electrode. According to Barnard and Randell [1] the following redox processes can be found: ; and at slightly more positive potentials.

This work is focused on the substrate effect over the ability to accumulate charge and the oxygen evolution reaction, comparing them with those obtained by using polycrystalline nickel [2], in order to further application in power sources and the oxygen evolution electrocatalysis [3].

In these growth conditions, the “coulommetric porosity” of electrodes reveals as a characteristic value related to the structure of nickel+epoxy used as a substrate. At more concentrated solutions, the longer penetration depth of nickel hydroxide inside the porous network of nickel+epoxy leads to higher cathodic charges. However, the ability to accumulate charge is drastically reduced when nickel weight percentage was of 80% wt, even in KOH concentrated solutions. The different potentiodynamic i-E profile of nickel hydroxides grown on nickel+epoxy from those grown on polycrystalline nickel is attributed to a-Ni(OH)₂ stabilization, as a consequence of pH-decrease inside the porous network, having adverse effects on the oxygen evolution. This fact and the higher accumulated charges at concentrated solutions make these electrodes of 87%wt covered with a-Ni(OH)₂ the most suitable for batteries applications.

Table 1. Ni(OH)₂ grown on different electrodes, Q_cat = cathodic charge calculated at v = 50 mV·s^-1,  ρ coulometric roughness. 1 M KOH. T = 298 K

REFERENCES

[1] R. Barnard, C.F. Randell, F.L. Tye, J. Appl. Electrochem. 10 (1980) 109.

[2] J. Agrisuelas, C. Gabrielli, J.J. García-Jareño, D. Giménez-Romero, J. Gregori, H. Perrot, F. Vicente, ECS Transactions, 6 (24) 79-95 (2008).

[3] J.A. Bastos, M.R. Barbosa, J. Gregori, J.J. García-Jareño, F. Vicente, ECS Transactions 6(8) 229-245 (2007).

ACKNOWLEDGEMENTS

Part of this work was supported by FEDER-CICyT CTQ2015-71794-R.