Electrolyte Effects on the Stability of Ni−Mo Cathodes for the Hydrogen Evolution Reaction

Abstract

Water electrolysis to form hydrogen as a solar fuel requires highly effective catalysts. In this work, theoretical and experimental studies are performed on the activity and stability of Ni−Mo cathodes for this reaction. Density functional theory studies show various Ni−Mo facets to be active for the hydrogen evolution reaction, Ni segregation to be thermodynamically favorable, and Mo vacancy formation to be favorable even without an applied potential. Electrolyte effects on the long‐term stability of Ni−Mo cathodes are determined. Ni−Mo is compared before and after up to 100 h of continuous operation. It is shown that Ni−Mo is unstable in alkaline media, owing to Mo leaching in the form of MoO42−, ultimately leading to a decrease in absolute overpotential. It is found that the electrolyte, the alkali cations, and the pH all influence Mo leaching. Changing the cation in the electrolyte from Li to Na to K influences the surface segregation of Mo and pushes the reaction towards Mo dissolution. Decreasing the pH decreases the OH− concentration and in this manner inhibits Mo leaching. Of the electrolytes studied, in terms of stability, the best to use is LiOH at pH 13. Thus, a mechanism for Mo leaching is presented alongside ways to influence the stability and make the Ni−Mo material more viable for renewable energy storage in chemical bonds.

Understanding the Hydrogen Evolution Reaction Kinetics of Electrodeposited Nickel‐Molybdenum in Acidic, Near‐Neutral, and Alkaline Conditions

“…However, Sabatier's principle cannot explain why the metal electrocatalysts in alkaline conditions exhibit lower HER activity than in the acidic conditions [21] . Also, while there have been recent advances in the understanding of pH‐dependent HER kinetics for noble metal‐based materials, e. g. Pt, Pd, Rh, [24,25,29] few studies have focused on transition metal based alloys as HER electrocatalysts [30] . Therefore, investigating the pH‐dependent HER kinetics of transition metal‐based alloy electrocatalysts is not only significant for adding new knowledge to the basic hydrogen evolution catalysis, but also essential for the design and development of earth‐abundant electrocatalysts.…”

Basicity and Electrolyte Composition Dependent Stability of Ni‐Fe‐S and Ni‐Mo Electrodes during Water Splitting

“…The observation of the material being unstable, without significantly changing the OER performance led us to believe that the S leaching behaves similar to the Mo leaching observed in previous works . Likewise, we chose to study whether this leaching is dependent on the electrolyte.…”

“…The capacitance is often used as a measure of surface area. We opt, however, to use it as a measure of surface change, as we already found that changing surface composition has a large impact as well . Whereas the activity was not visibly influenced the change in capacitance is.…”

“…Interestingly, when no current is applied while the Ni−Fe−S catalyst is in the electrolyte leaching is approximately double for all three elements present after 24 h. Similarly, when the sample is submerged for 1 week, the leaching is higher for all elements (compared to Table S1) except for Mo, which comes from the Ni−Mo sample also present in this experiment. Mo leaching being increased slightly by the application of current was already found before and thus in line with previous results . This observation strongly suggests that the application of an oxidative potential on the system suppresses the leaching.…”

Defect Engineering of Molybdenum-Based Materials for Electrocatalysis

“…Introducing proper Mo defects into Mo alloys can act in adjusting the catalytic performance of the alloys [107]. For instance, Li et al synthesized a reverse opal-shaped MoC using a hard template method, in which many Mo vacancies were introduced during the template removal process.…”