Metal hydroxides, oxides, and related compounds, particularly those containing Ni and Fe, have received much attention for oxygen evolution reaction (OER) catalysts because they have produced some of the highest reported OER rates and are chemically stability in alkaline media.  Electrode platforms based on Ni and Fe are attractive because they exclude expensive precious metals, and thin films of these metals can be coupled with light-absorbing materials in photoelectrochemical energy conversion devices used in water electrolysis.

Our group has been investigating growth of thin metal films through self-terminated deposition reactions, i.e., wet atomic layer deposition. These reactions can be coupled with potential modulation to deposit films of any desired thickness [1].  Similarly, self-termination behavior has been recently uncovered for iron-group metals albeit through a different mechanism than that reported for Pt.  Ni growth stops upon the onset of H₂O reduction whereas formation of a saturated H_upd  monolayer is responsible for termination of  Pt deposition.  Possible explanations for Ni growth termination include formation of a passivating surface-bound OH^– species formed upon H₂O reduction or cross reactions between the egress of OH^– and the incoming Ni²⁺ species to form Ni(OH)₂  and related products.

Here, we employed ultramicroelectrodes (UME’s) and scanning electrochemical microscopy (SECM) to further probe electrodeposition of thin metal films of Ni on Au electrodes and elucidate possible mechanisms for self-termination.  Benefits of using UME’s include minimized solution resistance and well-controlled mass transport.  Voltammetry of Au UME’s in electrolytes containing Ni²⁺ revealed a sharp, current spike at the onset of H₂O reduction, demonstrating the first observed electrochemical signal corresponding to a metal self-termination process.  The sharpness of this feature is reminiscent of a phase change or the initiation of a catalytic process.  One explanation is that a highly-active species for the HER is generated upon H₂O reduction in the presence of Ni²⁺;  it is well-known that  hydroxy-nickel species form during Ni²⁺ reduction due to a rise in pH, and Ni(OH)₂catalyzes the HER.  Generation/collection SECM, where a second Pt UME was used as the collector electrode, indicated that H₂is generated during the process giving rise to this cathodic spike.

Electrochemical results will be compared with optical characterization as well as SEM, EDX, and Raman spectroscopy to characterize electrodeposited films.  Ni films deposited with only concurrent proton reduction were silvery, bright, and continuous;  however, at potentials near the onset of H₂O reduction, the Au was covered with a patchy, thin metallic film.  These results agree with those at larger, cm-sized electrodes.  At sufficiently negative potentials under H₂O reduction, a thick, dull gray ring formed on the glass sheath surrounding the Au disk electrode, suggesting a transport-dependent phenomenon in the diffusion layer, e.g., precipitation of hydroxy-nickel species, surrounding the UME disk.  At even higher overpotentials, hydrogen bubbles formed, destroying the Ni films.

[1] Y. Liu et al,. Science, 338, 1327 (2012)       

 N.L.R.  acknowledges a National Research Council (NRC) Postdoctoral Research Assistantship.