The role of non-covalent interactions in electrocatalytic fuel-cell reactions on platinum

Abstract

The classic models of metal electrode-electrolyte interfaces generally focus on either covalent interactions between adsorbates and solid surfaces or on long-range electrolyte-metal electrostatic interactions. Here we demonstrate that these traditional models are insufficient. To understand electrocatalytic trends in the oxygen reduction reaction (ORR), the hydrogen oxidation reaction (HOR) and the oxidation of methanol on platinum surfaces in alkaline electrolytes, non-covalent interactions must be considered. We find that non-covalent interactions between hydrated alkali metal cations M(+)(H(2)O)(x) and adsorbed OH (OH(ad)) species increase in the same order as the hydration energies of the corresponding cations (Li(+) >> Na(+) > K(+) > Cs(+)) and also correspond to an increase in the concentration of OH(ad)-M(+)(H(2)O)(x) clusters at the interface. These trends are inversely proportional to the activities of the ORR, the HOR and the oxidation of methanol on platinum (Cs(+) > K(+) > Na(+) >> Li(+)), which suggests that the clusters block the platinum active sites for electrocatalytic reactions.

Direct observation of the oxygenated species during oxygen reduction on a platinum fuel cell cathode

“…This analysis suggests that extending the stability range of the non-hydrated phase will prove beneficial in improving the overall ORR activity. The importance of non-hydrated OH for the ORR at high potentials explains the enhanced ORR activity of Pt in hydrophobic ionic liquids 8 , in the presence of certain aqueous ions 6,7 and on stepped surfaces with narrow terrace width 9,10 . The last is unexpected as step sites strongly bind OH, as indeed observed with a reduced activity for small Pt nanoparticles 27 .…”

“…In general, dehydrated environments 8 will by their very nature limit the hydration of any OH intermediates. Finally, as hydrogen bonding of H 2 O to OH can compete with direct ion-OH interactions 28 , specific ions in the electrolyte could limit the hydration of OH 6,7 . This new insight into the subtle interplay between non-covalent hydrophobic and hydrophilic interactions can inspire the design of new catalysts, not only for the ORR but also for other relevant electrochemical reactions.…”

“…Indeed, based on density functional theory (DFT) simulations of the oxygenated intermediates of the ORR, it has been proposed that the Pt-OH bond strength could be a key descriptor of ORR activity and could therefore guide catalyst design [3][4][5] . However, recent experiments have observed unusual ORR activity in the presence of various ions 6,7 , in dehydrated environments 8 and on stepped surfaces with limited terrace widths 9,10 . These indicate that experiments to observe and characterize OH species directly during the ORR are essential.…”

The correlation of electrochemical and fuel cell results for alcohol oxidation in acidic and alkaline media

“…Therefore, 0.1 M perchloric acid (HClO 4 ) that dissociates only to form weakly adsorbing perchlorate ions has been chosen as a supporting electrolyte in acidic medium. In alkaline medium 0.1 M NaOH or 0.5 M KOH electrolyte is used and these solutions are freshly prepared to avoid gradual dissolution of glass [8] that had been observed to cause changes in voltammetric profiles [9].…”

Nitrogen-doped graphene with enhanced oxygen reduction activity produced by pyrolysis of graphene functionalized with imidazole derivatives

“…In alkaline medium, NaOH or KOH are commonly used as electrolytes. However, it is reported that the non-covalent interactions between the hydrated Na + cations and adsorbed OH species are stronger than with hydrated K + cations so that the OH ad -M + (H 2 O) x clusters block the active sites for the electrocatalytic reaction [42]. In the present thesis, 0.1 M HClO 4 and 0.1M KOH that form weekly adsorbing species were used as electrolytes for the characterization of MOR at PtRu-C catalysts (in acidic condition), and the ORR at N-doped CNMs (in alkaline condition), respectively.…”