In Situ Study on Ni–Mo Stability in a Water‐Splitting Device: Effect of Catalyst Substrate and Electric Potential

Abstract

Nickel–molybdenum (Ni–Mo) alloys are well studied as highly effective electrocatalyst cathodes for water splitting. Understanding deactivation pathways is a key to improving the performance of these catalysts. In this study, in situ characterization by UV/Vis spectroscopy and AFM of the morphology and Mo leaching of an Ni–Mo electrocatalyst was performed with the goal of understanding the stability and related Mo leaching mechanism. Switching the potential towards higher overpotentials results in a nonlinear change in Mo leaching. Multiple processes are proposed to take place, such as a decrease in the extent of Mo oxidation at the cathode induced by more strongly reducing potentials, while simultaneously the increase in the local pH at the cathode due to the hydrogen evolution reaction causes more Mo leaching. The change in capacitance of these materials depends strongly on the change in surface composition and not only on the surface area. In situ UV/Vis spectroscopy showed that Mo leaching is a continuous process over the course of 4 h of operation. Finally, the material was deposited on different substrates and the effect on Ni–Mo stability was studied. The substrate has a significant, albeit complex, influence on the stability and activity of Ni–Mo cathodes. In terms of stability in 1 m KOH, Ni–Mo was found to be best deposited on stainless steel substrates operated at low overpotentials, on which it showed nearly no change in capacitance and exhibited low Mo leaching.

Self‐Supported Electrocatalysts for Practical Water Electrolysis

“…However, the primary methods employed to investigate the real active structures for self-catalysts are ex situ ones, which cannot manage to obtain enough and accurate information under a real working environment. In view of this, advanced in situ or operando techniques, including spectroscopic techniques (such as X-ray absorption (XAS), XPS, Raman, Fourier-transform infrared spectroscopy, Mössbauer, nuclear resonant inelastic X-ray scattering, electrochemical impedance spectroscopy and XRD) [253] as well as microscopic techniques (such as atomic force microscopy and TEM) [254,255] should be more applied to get better real-time observation and consequently receive guidance for the design and fabrication of self-supported electrocatalysts for water splitting with high activity, stability, and selectivity.…”

“…[224] Also, modifying porosity and roughness is considered to be key points for the control of the surface wettability of electrodes. [48,255] The surface with superhydrophilicity (when the liquid contact angle of a droplet on a solid surface is less than 5 or 10° in the air) can significantly promote the permeability of electrolyte, ensuring the majority of active sites can be accessed. [32] On the other side, the underwater gasbubble contact angle over 150° means that the self-supported electrocatalyst is superaerophobic, so that the evolved bubbles can depart from the electrode more smoothly.…”

Toward an e-chemistree: Materials for electrification of the chemical industry

“…45 2− leaching in Ni-Mo HER catalysts. [46][47][48] This was possible, as illustrated in Figure 5, by using, a.o., In situ atomic force microscopy (AFM), which was coupled to solution UV-vis spectroscopy.…”

Durability of Mo-Ni Intermetallic Compounds in the Hydrogen Evolution Reaction

“…The ECSA is here based on the transferred charge in the reduction process of NiOOH between 1.2 and 1.5 V vs RHE: Scanning Mo-Ni materials repetitively to potentials of 1.6 V results in molybdenum dissolution, which might increase the ECSA and enrich the surface with nickel. [22] To determine the changes quantitatively, the ESCA was calculated using Equation 1. Equation 1Q is the integrated charge, NA Avogadro's number, F the Faraday constant, n the number of transferred electrons and kmean the mean area of a β-NiOOH unit cell assuming a statistical orientation (details are given in the SI).…”

“…Most studies since then are dealing with electrodeposited coatings of Mo-Ni. [21][22][23] Numerous studies employing nanoparticulate synthesis strategies were published in recent years, reporting highly active and durable intermetallic Mo-Ni compounds. [11,24] Results from these high surface-area catalysts are often hard to compare, especially as the materials involve different supports.…”