Enhancing the Alkaline Hydrogen Evolution Reaction Activity through the Bifunctionality of Ni(OH)₂/Metal Catalysts

Abstract

Active in alkaline environment: The activity of nickel, silver, and copper catalysts for the electrochemical transformation of water to molecular hydrogen in alkaline solutions was enhanced by modification of the metal surfaces by Ni(OH)(2) (see picture; I = current density and η = overpotential). The hydrogen evolution reaction rate on a Ni electrode modified by Ni(OH)(2) nanoclusters is about four times higher than on a bare Ni surface.

Electrocatalysts for Hydrogen Evolution in Alkaline Electrolytes: Mechanisms, Challenges, and Prospective Solutions

“…It has been experimentally proved by developing hybrid catalyst that has an active component for water dissociation. It is found that incorporation of water active catalyst can increase the HER activity of its counterpart (HER active catalyst) 22. For example Markovic group has developed the Ni(OH) 2 decorated Pt catalyst to improve the HER activity of Pt by lowering the energy barriers for water dissociation as shown in Figure 1 13.…”

“…A water dissociation happens at the edges of Ni(OH) 2 clusters and produces the hydrogen intermediates that adsorb on the surface of neighboring Pt active sites and recombine to yield molecular hydrogen. To further confirm the requirement of high energy for additional water dissociation step and eliminate the confusion of Pt, hybrid catalysts based on cost‐effective metals, e.g., Ni, Cu, Ag have been designed and found that after surface modifications by Ni(OH) 2 the performance of these metals also approach to their activity in acidic medium 22. Although this method has enhanced the activity of PGMs and cost effective metals in alkaline medium but have several reservation for practical applications likewise complex synthesis methods, hard to control the interface and stability concerns on longer use.…”

“…Other noble metals, like Ru,70 Au,20, 22 Ir,22 Pd,20 Ag,34 and their alloys with low‐cost transition metals or doped with nonmetal elements were also studied, leading to significant improvements in HER activities in alkaline medium due to their appropriate atomic and electronic structure and the abundance of exposed active sites 34, 71, 72, 73. As a cheaper alternative to Pt, Ru possesses a similar H‐bond energy (≈65 kcal mol −1 ) to Pt, but less studied as a viable HER catalyst 74.…”

“…The theoretical studies have shown that the activity trends in acidic medium are concluded on the basis of hydrogen adsorption (H ad ) at active sites but in alkaline mediums it is controlled by the delicate balance among three important descriptors; (i) the H ad on the surface of catalyst, (ii) the prevention from hydroxyl adsorption (OH ad ) referred as poisoning of active sites, and (iii) the energy requires to dissociate water molecules 18, 19, 20, 21. To confirm the importance of third descriptor, Markovic and co‐workers carried out a series of experiments by modifying the surface of metal with water dissociation catalyst such as Ni(OH) 2 , where Ni(OH) 2 enhanced the dissociation of water molecules to produce H atoms which can then be collected by the metal active centers and recombined into hydrogen 22. Thus, to overcome this additional energy and faster the reaction kinetics, new electrocatalyst that have the ability to dissociate water molecules and moderate affinity for H ad as well as its recombination to yield hydrogen gas need to be developed 11, 23, 24, 25…”

“…The HER in alkaline solution needs extra energy to produce the protons by breaking the water molecule, consequently influencing the overall reaction rates 13, 22. Various theoretical and experimental studies have tried to enlighten the HER mechanisms in alkaline medium and found that HER involves three step under alkaline medium: (i) the Volmer step, (ii) water dissociation, and (iii) creation of a reactive hydrogen intermediates (2H 2 O + M + 2e – → 2M − H ad + 2OH − , M denotes catalyst surface here) that is different from the mechanism in acidic medium (2H 3 O + + 2e − + M → 2M – H ad + 2H 2 O), which normally follow either the Heyrovsky pathway or the Tafel recombination step 37.…”