An efficient and pH-universal ruthenium-based catalyst for the hydrogen evolution reaction

Abstract

The hydrogen evolution reaction (HER) is a crucial step in electrochemical water splitting and demands an efficient, durable and cheap catalyst if it is to succeed in real applications. For an energy-efficient HER, a catalyst must be able to trigger proton reduction with minimal overpotential and have fast kinetics. The most efficient catalysts in acidic media are platinum-based, as the strength of the Pt-H bond is associated with the fastest reaction rate for the HER. The use of platinum, however, raises issues linked to cost and stability in non-acidic media. Recently, non-precious-metal-based catalysts have been reported, but these are susceptible to acid corrosion and are typically much inferior to Pt-based catalysts, exhibiting higher overpotentials and lower stability. As a cheaper alternative to platinum, ruthenium possesses a similar bond strength with hydrogen (∼65 kcal mol), but has never been studied as a viable alternative for a HER catalyst. Here, we report a Ru-based catalyst for the HER that can operate both in acidic and alkaline media. Our catalyst is made of Ru nanoparticles dispersed within a nitrogenated holey two-dimensional carbon structure (Ru@CN). The Ru@CN electrocatalyst exhibits high turnover frequencies at 25 mV (0.67 H s in 0.5 M HSO solution; 0.75 H s in 1.0 M KOH solution) and small overpotentials at 10 mA cm (13.5 mV in 0.5 M HSO solution; 17.0 mV in 1.0 M KOH solution) as well as superior stability in both acidic and alkaline media. These performances are comparable to, or even better than, the Pt/C catalyst for the HER.

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

“…Recently, Baek and co‐workers have prepared the Ru nanoparticles (NPs) decorated on nitrogenated holey 2D carbon structure (C 2 N) and hybrid comes up with exceptional HER activity by achieving the current density of 10 mA cm −2 at the over potential of 17 mV which is even better than the commercial Pt/C 18. The theoretical studies suggested that when Ru is placed in the holes of C 2 N, the Ru–H 2 O binding energy increases significantly, which results faster water adsorption and accelerates Volmer step.…”

“…However, the differences in H‐binding energy between alkaline and acidic mediums could not be explained and are still not clear. Furthermore, it has also been found that there are few catalyst that perform better in alkaline medium than the acidic one, although majority of catalysts show better activities in acidic than in the alkaline medium,18, 55, 56, 57 suggesting that there are both qualitative and quantitative differences of the reaction mechanisms between these two mediums. Therefore, a thorough understanding of the reaction mechanisms in alkaline medium is significant to ascertain reactive sites, get insight into the reaction mechanism and tailor new materials for HER.…”

“…A moderate adsorption energies for water and hydrogen on the active sites with low attraction to hydroxyl ions is required to proceed the reaction in alkaline medium at lower overpotentials, better efficiencies and stability 18. 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.…”

“…A moderate adsorption energies for water and hydrogen on the active sites with low attraction to hydroxyl ions is required to proceed the reaction in alkaline medium at lower overpotentials, better efficiencies and stability 18. 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.…”

Nickel Hydr(oxy)oxide Nanoparticles on Metallic MoS2 Nanosheets: A Synergistic Electrocatalyst for Hydrogen Evolution Reaction

“…The HER mechanism in acidic electrolytes is generally recognized as a combination of three elementary steps: (1) the Volmer step, described as hydronium (H 3 O + ) discharge and formation of an adsorbed intermediate H ad • on the active site (•) (H 3 O + + • + e − → H ad • + H 2 O); (2) either a following Heyrovsky step (H 3 O + + H ad • + e − → • + H 2 + H 2 O) or Tafel step (2H ad • → 2• + H 2 ) 4. In alkaline media, the HER mechanism is similar to that in acidic conditions except that H ad • is formed from dissociation of water (H 2 O + • + e − → H ad • + OH − ), a step with kinetic rate depending on the binding energies of H 2 O and OH ad on the catalyst surface 8, 9, 33. Thus, the kinetic of HER in alkaline electrolytes depends on both the rate of H ad combination and the rate of H 2 O dissociation 9.…”