Paolo Nelli

Electrochemical Activation of Raney Nickel Air Electrodes

We study slow and nearly reversible processes of reduction, hydride formation and oxidation in Raney-Ni air electrodes in concentrated KOH solutions. A labile ?-NiH phase is formed upon prolonged cathodic polarization under H2 atmosphere. Repeated cathodic/anodic cycles enhance hydrogen intake and induce surface modifications of the Raney-Ni grains.

Effects of Pressure Modulation on Porous Gas Electrodes

We explore experimentally the coupling between cell voltages at constant current and pressure differentials applied at the gas/electrolyte compartments of a reversible alkaline fuel cell and an electrolyzer system. In the fuel cell mode, higher pressure on the gas side improves the efficiency until gas bubbles evolve into the electrolyte; instead, above a well defined value of overpressure on the electrolyte, the fuel cell voltage drops to zero, as expected. In the electrolyzer mode, an overpressure on the electrolyte decreases the power requirements, mostly due a drop in the overpotential of the H2 electrode. The response of anode and cathode voltages to pressure step consists of a fast component and slow training or relaxation effects.

Charge storage and the oxygen evolution reaction in mixed Ni–Li oxides

Electronic and catalytic properties of mixed oxides of Ni-Li are studied at room temperature with NMR and with electrochemical techniques in alkaline solutions. Our Li(x)Ni(1-x)O (0.04 < or = x < or = 0.2) samples have an f.c.c crystal structure, are in an anti-ferromagnetic phase, and exhibit a substantial conductivity due to a hole-hopping process with a characteristic rate which is essentially independent of composition. Several processes of reduction and oxidation occur in the voltage region between the hydrogen (HER) and oxygen evolution (OER) reactions, which are compared with related phenomena observed in pure Ni electrodes (Ni(OH)2 and NiO). Repeated cycles of reduction and oxidation yield electrodes with a somewhat improved OER activity, and a high charge-storage capacity. Heavy anodic treatments cause irreversible changes of these properties, particularly when coupled with months-long storage in air. The complex behavior of these electrodes is interpreted in terms of formation of a topotactic surface layer with hydrogenated species.

Ni - based Electrodes for Hydrogen and Oxygen Generation

We discuss the effects of treatments (cathodic/anodic conditioning, red ox cycles) upon the electrochemical properties of Ni-based electrodes in concentrated alkaline solutions. We find that daylong treatments greatly enhance the red ox reactions of the Nielectrodes which occur between hydrogen evolution (HER) and oxygen evolution reactions (OER). The effects upon HER and OER are also discussed.

The Apparent Capacitance of the Electrode "Charging" Process

We investigate simple models of the electrode charging (discharging) at constant current i, where the potential (E) is recorded as a function of time, or charge, q = i ? t . Our goal is to assess the advantages of a representation in terms of capacitance,C = dq / dE ,vs. E. Such a plot is related with cyclic voltammetry (c.v.) plot (ic.v. vs E) at a constant rate (? = dE / dt)since, under appropriate circumstances, we should have C = ic.v. / ? = dq / dE . We will discuss the following items: a) origins and suppression of the singularities in the capacitance plots, b) effects of the finite charging currents, c) comparison between c.v. and electrode charging experiments.