The Effect of Surface Nano and Microstructures on Hydrogen Gas Evolution Behavior on Ni Patterned Electrodes
Authors: Fujimura Tatsuki , Kunimoto Masahiro , Fukunaka Yasuhiro , Homma Takayuki 
Published: December 2020

Highly efficient water electrolysis process is one of the key technologies for hydrogen economy suppressing global warming. Now, alkaline water electrolysis (AWE) and Proton exchange membrane (PEM) electrolysis are regarded as the representative industrial process. The advanced electrode without precious metal or oxide catalysts for hydrogen evolution reaction (HER) in these processes plays an important role to design HER efficiently and economically. Tremendous efforts focusing to unique characteristic catalysts exploration have been engaged world-widely not only tediously and steadily but also with materials informatic-science utilization driven by AI technology. Roughly speaking based on electrochemical engineering aspects, the energy loss in water electrolysis is composed of electrode reaction kinetics governed by catalytic activity, concentration overpotential and ohmic drop in gas-liquid two phase dispersion layer. The concentration overpotential and ohmic resistance are sometimes result in significant energy loss in large scale industrial process.

From the sustainable and economical views toward Hydrogen Economy Society, AWE with less precious catalysts is considered as hydrogen production candidate with cheaper cost and longer durability. Many papers on gas electrode behavior have focused on macroscopic observation, but more fundamental understanding on nucleation and growth behavior of gas bubbles is rather limited due to the experimental difficulties. It is believed that the electrode surface structure tailoring may also provide one of promising methods to improve the catalytic performance as well as advance catalytic HER activity. Especially, nano/micro-scale structures on the electrode surface are suggested to affect the catalyst performance by controlling the nucleation and growth behaviors of bubbles. However, the detailed mechanisms of the effect of surface nano/micro-scale structures on HER and bubble behaviors have not been elucidated. Therefore, to analyze the influence of microstructure and surface wettability of catalytic electrode on bubble behaviors and HER, this study focuses on bubble behaviors on the electrode surface using Ni micro-patterned electrode fabricated by electrodeposition and Pt single crystalline electrodes.

Ni micro-patterned electrodes were electrodeposited in the electrolyte containing 0.05 mol dm-3 NiCl2∙6H2O (pH 2.5) on patterned Cu substrate. Pt and Ag/AgCl electrode were employed as CE and RE, respectively. Surface structures and wettability of prepared electrodes were evaluated by scanning electron microscopy (SEM) and contact angle measurement. Water electrolysis was performed in 1 M KOH and Pt mesh and Hg/HgO. The effect of surface roughness on bubble behaviors was additionally examined in 0.1 M H2SO4 with three electrode system; Pt single crystalline electrode prepared by the Clavier method as WE, carbon rod as CE, and reversible hydrogen electrode as RE, respectively.

Microdot structures, cylinder-, semisphere- and square-like were prepared. Ni coverage ratio of Ni electrodeposited area to Cu substrate, was calculated to correlate surface wettability measured by sessile drop method and Ni microstructure. It was suggested that electrode surface wettability could be controllable by changing Ni coverage and each Ni microdot structure exhibited an apparent catalytic electrode by surface tailoring.

The effect of microstructures and surface wettability on HER performance was then studied during galvanostatic electrolysis at -20 mA cm-2 for 30 min. Total overpotential measured from 200 s to 1800 s seemed to proceed water electrolysis stably. It increased with decreasing surface wettability. This may indicate lower surface wettability derived from Ni microarray structures increases overpotential by accumulating newly evolving H2 bubbles. To further analyze the phenomena, high-speed CCD camera cinematography was applied. The ratio of larger diameter bubble tended to increase with increasing the contact angle. It indicates lower surface wettability would enhance bubble growth behaviors on cathode surface. Moreover, larger diameter bubbles were generated on cylinder-like Ni microdots. These results suggest increasing contact area between bubbles and Ni micropatterned electrode would enhance bubble growth behaviors. The effect of Ni microdot height on bubble nucleation behaviors was also observed. It apparently demonstrated that taller Ni dot height rather increased the nucleation site of bubbles.

In order to additionally investigate the effect of nano-scale roughness tailored by single crystalline growth technique on bubble behaviors, H2 bubble evolution behaviors on Pt single crystalline electrode were cinematographically recorded. No H2 bubbles were generated on smooth surface fabricated by Pt (111) microfacet, suggesting lower nano-scale surface roughness considerably suppressed bubble nucleation on Pt electrode surface.

These results suggested micro and nano structures and surface wettability influenced on bubble nucleation and growth phenomena frequently encountered in industrial electrolysis. Further unknown room still remains to be challenged even in such a rather matured AWE.

This work was partly carried out in Case Western Reserve University. Prof. Daniel Scherson’s supervisions are highly appreciated.