Emergence of Temporal Instabilities during Electro-Dissolution of Nickel-Iron Alloy
Authors: Pereira dos Santos Caio Guilherme , Machado Eduardo , Kiss Istvan Z , Nagao Raphael 
Published: July 2021

Dynamics instabilities are often observed in many systems, such as biological, chemical, physical, and others. Some very commons examples are the circadian rhythms that regulates biological process during the rotation of the Earth every 24 hours, and dunes formations where the topographical structures are built with large masses of sand accumulated. These instabilities can be classified as spatial, where the vicinity affect the emergence of instabilities, and temporal, when the instabilities are time dependent. In chemical systems there is a well-known Belousov-Zhabotinsky reaction where the instabilities rise between citric acid, bromate ions and ceric ions (or similar set of substances) and is possible to emerge spatial and temporal instabilities.
Among many chemical systems the electrochemical has been of great interest due to the fine adjustment that is possible by the potential or current control. Another advantage is that electrochemical setup is relatively simple, formed in most cases by a working electrode, counter-electrode and reference electrode emerging from an electrolyte inside a container. Anodic dissolution of metals has been studied since 1828, with the reports by Fechner. Many studies were performed using different metals like Cu, Fe, Ni, among others, as electrochemical oscillators using single or multielectrode configuration coupled under different network topologies, where the latter configuration has been made often using similar electrodes. In this present work, we report experimental studies of the coupling between two different electrochemical oscillators during the electro-dissolution of Ni and Fe, not as two separated electrodes but like an alloy.
To perform the experiments was utilized a glass electrochemical cell with 3 electrodes, a platinum sheet as counter-electrode, a saturated calomel electrode as reference electrode, and a Ni-Fe(80-20) alloy embedded in Teflon with a cylindrical shape and a diameter of 0.5 cm as working electrode. For comparison, all experiments were performed also using a pure Ni and a pure Fe with the same geometry of the alloy as working electrode. All experiments were carried out using H2SO4 1 mol L-1 as electrolyte and before each electrochemical experiment the working electrode was polished to ensure that the surface will be the same, i.e., the absent of oxides and other substances on the surface. The phenomenon observed during the experiments is described in terms of stationary and dynamic electrochemical techniques such as chronoamperometry and cyclic voltammetry, varying the external resistance Rext of the system. For each Rext applied there is a different potential range where the oscillations occur during cyclic voltammetry and by different Rext applied was built a bifurcation diagram (Rext vs. E). In each region of oscillations was performed chronoamperometry experiments, choosing different potential applied values, where is possible to note the electrode potential change, revealing that occurs a separation of the activation and passivation processes during an oscillatory cycle.
Comparing with the Ni and Fe pure electrodes, these results suggest that there is a change in the mechanism during the emergence of the oscillations in the Ni-Fe alloy. For the alloy at low potentials, there is a mixed contribution from both Ni and Fe dissolution on the oscillations, that occurs before the Flade potential, while the pure Ni dissolution occurs in the transpassive domain with range of ~1.2 to 1.5 V vs. SCE (both with Rext 392 Ω cm2) and the pure Fe dissolution ~0.4 to 0.6 vs. SCE (without Rext). The mixed contribution produced by Ni and Fe in the alloy during the oscillatory electro-dissolution are caused because of their oxides and salts present in the passivation step, which impacts on the instabilities generated during the nonlinear dynamics of the system. With these results we can conclude that a small amount of Fe (only 20%) on the Ni electrode at electro-dissolution process is enough to induce a change in the mechanism during the emergence of oscillations, suggesting that a new process is taking place, from high potentials for Ni (~1.2 to 1.5 V) to lower overpotentials for Ni-Fe alloy (~-0.1 to 0.1 V). Also, there is a suppression of the transpassive state, i.e, is observed in pure Ni and absent in the Ni-Fe alloy. The Figure compares the shape and period of the oscillations for each electrode at some fixed potentials, Ni (a), Ni-Fe (b), and Fe (c).
As perspectives of the work we are working to observe the instabilities using other conditions to be able to understand more deeply the mechanism found by the Ni-Fe alloy. We also intend to develop simulations to describe this phenomenon more accurately.

Figure 1