Elucidating Hydrogen Oxidation/Evolution Kinetics in Base and Acid by Enhanced Activities at the Optimized Pt Shell Thickness on the Ru Core

Abstract

Hydrogen oxidation and evolution on Pt in acid are facile processes, while in alkaline electrolytes they are two-orders-of-magnitude slower. Thus, developing catalysts that are more active than Pt for these two reactions is important for advancing the performance of anionexchange-membrane fuel cells and water electrolyzers. Herein, we detail a four-fold enhancement in Pt mass activity that we achieved using single crystalline Ru@Pt core-shell nanoparticles with two-monolayer-thick Pt shells, which doubles the activity on Pt-Ru alloy nanocatalysts. For Pt specific activity, the 2-and 1-monolayer-thick Pt shells, respectively, exhibited an enhancement factor of 3.1 and 2.3 compared to the Pt nanocatalysts in base, differing considerably from the values of 1 and 0.4 in acid. To explain such behavior and the orders-of-magnitude difference in activity on going from acid to base, we performed kinetic analyses of polarization curves over a wide range of potential from -250 to 250 mV using the dual-pathway kinetic equation. From acid to base, the activation free energies increase the most for the Volmer reaction, resulting in a switch of the rate-determining step from the Tafel-to the Volmer-reaction, and a shift to a weaker optimal hydrogen-binding energy. The much higher activation barrier for the Volmer reaction in base than in acid is ascribed to one or both of the two catalyst-insensitive factors -slower transport of OH -than H + in water, and a stronger O-H bond in water molecules (HO-H) than in hydrated protons (H 2 O-H + ).

Co9S8@N,P-doped porous carbon electrocatalyst using biomass-derived carbon nanodots as a precursor for overall water splitting in alkaline media

“…[9][10][11] So far, Pt-based and Ru/Ir-based catalysts have been extensively considered as the most active electrocatalysts for the HER and OER respectively in alkaline media, however, their high costs and source scarcity have limited large-scale production applications. 3,7,[12][13][14] Therefore, searching for cheap, earthabundant and efficient electrocatalysts with high HER and OER activities is still highly desirable.…”

Perspective—Towards Establishing Apparent Hydrogen Binding Energy as the Descriptor for Hydrogen Oxidation/Evolution Reactions

“…10 Elbert et al showed that the activation energy increases the most for Volmer reaction from acid to base, leading to a change of rate-determining step from Tafel step in acid to Volmer step in base as well as the shift in optimal HBE from Pt to Ru@Pt catalysts. 11 For HER, besides the adsorption of H ad , the nature of proton donor (either H 3 O + in acid or H 2 O in alkaline) as well the adsorption of OH ad and spectator species were also invoked in explaining the HER activities of different metals in the whole range of pH. 12 The higher HER rate in acid than in base has been explained by the faster formation of H ad from H + than from H 2 O.…”

Electrocatalysts Prepared by Galvanic Replacement

“…Finally, as far as HOR is concerned, Adzic and co-workers report increased mass and intrinsic activity at Pt submonolayers/shells deposited on Au or Ru substrates or nanoparticles [79,291] via the galvanic replacement of Cu upd layers.…”