Using Scanning Electrochemical Microscopy to Control Boundary Layer Chemistry during Nickel Electrodeposition

Thin films of iron-group metals, e.g., Ni, Fe, and Co, and their alloys and oxides are of interest to a wide range of technical endeavors from microelectronics to low cost energy storage and conversion devices.  For example, Ni is a well-known component of rechargeable battery cathodes and has demonstrated superior oxygen evolution kinetics in alkaline media, making it a popular catalyst for photoelectrochemical cells. Our group has been investigating growth of thin metal films through self-terminated deposition, i.e., wet atomic layer deposition, where film thickness is controlled by potential modulation (Y. Liu et al., Science, 338, 1327 (2012) ).  Thin Ni films can be deposited in a similar, self-terminated manner as reported for Pt by reduction of Ni²⁺ albeit through a different mechanism.  Ni growth stops upon H₂O reduction whereas H⁺ reduction is sufficient to terminate Pt growth through adsorbed H.  However, bulk Ni deposition proceeds at H⁺ reduction potentials.  Possible explanations for Ni termination include formation of surface-bound species that block further deposition or outward flux of OH^– generated during the simultaneous H₂O reduction, hindering transport of Ni²⁺ species to the electrode surface and/or reacting with Ni²⁺, e.g., to form Ni(OH)₂, as the pH increases. 

Here, we used scanning electrochemical microscopy (SECM) to better understand the effect of boundary layer chemistry at the electrode-electrolyte interface during Ni electrodeposition with µm-sized spatial resolution.  Our focus was on differentiating between effects of species adsorbed/bound to the electrode surface and changes in solution chemistry, i.e., pH, on termination as well as using the SECM probe to perturb the solution adjacent to the electrode surface and therefore control Ni deposition.  In our studies at planar, cm-sized Au substrate electrodes, the SECM tip probe blocked diffusion of Ni²⁺ species in the probe-substrate gap, precluding meaningful measurements.  Therefore, we adopted a tip-to-tip configuration, where two disk ultramicroelectrodes (UME’s) are aligned, to enhance flux in the gap.  Because the edge-effect dominates the behavior of UME’s, we first considered Ni deposition at Au UME’s, comparing deposition under two regimes:  H⁺ reduction and H₂O reduction.  Ni films deposited with only H⁺ reduction were silvery, bright, and continuous.  With concurrent H₂O reduction, the Au was covered with a patchy, thin metallic film.  These results agree with those at the larger, cm-sized electrode.  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 surrounding the UME disk. 

Pt probe and Au substrate UME’s were used for SECM measurements.  The potential of the Pt, acting as a local buffering agent, was poised to generate H⁺ via H₂ oxidation, and Ni deposition occurred at Au.  Under buffering conditions during H₂O reduction, a thicker metallic layer formed on the Au and the ring was absent, indicating that pH buffering may lift the termination process. 

On-going experiments include surface analysis, e.g., SEM, EDX, and in situ Raman spectroscopy to characterize the morphology and composition of the electrodeposited films, and fabricating pH probes for the SECM.

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