Impact of Ligand Modification on the Hydrogen Evolution Reaction of Highly Active Silver(I)- and Ruthenium(II)-N-Heterocyclic Carbene-Based Electrocatalysts: Comprehension from the Hydrogen Oxidation Reaction

The development of efficient small-molecule electrocatalysts
for
the hydrogen evolution reaction (HER) is crucial in advanced water
splitting and fuel cell applications. The use of effective molecular
electrocatalysts for HER and allied reactions is limited by a lack
of electrical conductivity, which strongly hinders their activity.
Here, we report a series of N-heterocyclic carbene (NHC)-coordinated
silver(I) (8 and 9) and ruthenium(II) (10 and 11) small-molecule electrocatalysts varied
by the NHC ligand field as highly effective HER catalysts. Silver
complexes 8 and 9 were prepared by the in situ deprotonation of triazolium salts (6 and 7), while ruthenium complexes 10 and 11 were prepared through transmetalation protocol using former
derivatives. Both types of complexes were thoroughly characterized
by various spectral and analytical techniques. Both salts were studied
for their structure using the single-crystal X-ray diffraction technique.
Among others, ruthenium complexes 10 and 11 evidenced an overpotential (η10) of −175
and −209 mV vs RHE to reach a benchmark current density of
10 mA cm–2, while silver derivatives 8 and 9 displayed an overpotential of −291 and
−394 mV vs RHE, respectively. Their respective η50 values are in the range −287 to −496 mV vs
RHE with comparative Tafel slope values (94.3–208.4 mV dec–1) with a decrease in the performance to 265 mV vs
RHE (η10 for 10) over 18 h of HER operation.
Alongside, hydrogen oxidation is evidenced by a significant current
density observed at the platinum ring electrode. Finally, the promising
HER performance of silver and ruthenium-NHC complexes is attributed
to the confined coordination geometry around the metal atom (linear
or three-legged piano stool), the steric bulk offered by the NHC ligand,
surface morphology (well-ordered discrete microgranules/tubes to highly
porous films), and the charge-transfer resistance (Rct: 119.8–191.6 Ω).