Bistable/Oscillatory System Based on the Electroreduction of Thiocyanate Complexes of Nickel(II) at a Streaming Mercury Electrode. Experiment and Simulation

Tuning Electrochemical Bistability by Surface Area Blocking in the Cathodic Deposition of Copper

We report herein a precise control of the electrochemical bistability induced by surface area changes during the cathodic deposition of copper. Small additions of 1,10-phenanthroline (Phen) in the reaction media present an inhibiting effect on the global rate mainly due to the adsorption of protonated Phen. The increase of its concentration favors a shrinkage of the bifurcation (saddle-node) diagram and shifts it to less negative potentials. The dynamic instability is verified by impedance measurements, and a negative impedance is clearly found. We calculated the apparent molar mass of the adsorbents using in situ gravimetric monitoring in the electrochemical experiments, and the results indicate that mass changes occur mainly due to the reduction of copper from bivalent ions dissolved in the reaction media. Importantly, the adsorption of protonated Phen molecules does not show a considerable contribution in mass variations but prevents the formation of a copper course grained morphology over the surface. Imaging analysis indicates finer nodulations at the lower branch compared to the upper branch in the bistability domain. On the basis of these observations, a kinetic mechanism is proposed and a good agreement is obtained between the apparent molar mass extracted from experiments and the theoretical values. Altogether, our results contribute to a detailed physical chemical description of the nonlinear behavior, bringing new insights about this reaction and pointing out the possibility to design switchable surface electrodes by taking advantage of the bistable behavior.

Mechanisms of Oscillations and Formation of Nano-Scale Layered Structures in Induced Co-Deposition of Some Iron-Group Alloys (Ni−P, Ni−W, and Co−W), Studied by an In Situ Electrochemical Quartz Crystal Microbalance Technique

We have investigated mechanisms of oscillations and formation of nano-scale layered structures in induced co-deposition of some iron-group alloys (Ni-P, Ni-W, and Co-W) that have unique properties and are widely used in industries. Detailed in situ electrochemical quartz crystal microbalance (EQCM) experiments have revealed that the electrodeposition (induced co-deposition) of the alloys has negative differential resistances (NDRs), from which the oscillations and the layer-structure formation arise. The NDRs, however, cannot necessarily be seen in current-potential curves owing to overlap of hydrogen evolution current, indicating that the oscillations are of a hidden-NDR (H-NDR) type. The EQCM experiments have also shown that electrolyte components (such as H2PO2- and WO4(2-)) or related species are adsorbed at the electrode (deposit) surface and act as a promoter for the co-deposition reaction and that the NDRs arise from desorption of the adsorbed promoter. Interestingly, the adsorbed promoter is drawn into the deposition reaction itself, thus resulting in the alloy deposits. This mechanism was supported by in situ EQCM investigations of the oscillation as well as Auger electron spectroscopic (AES) analyses of deposits formed during the oscillation. The present work has for the first time clarified a general mechanism for the induced co-deposition reactions of some industrially important iron-group alloys (Ni-P, Ni-W, and Co-W).