A Challenge to the G ∼ 0 Interpretation of Hydrogen Evolution

Abstract

Platinum is a nearly perfect catalyst for the hydrogen evolution reaction, and its high activity has conventionally been explained by its close-to-thermoneutral hydrogen binding energy (G∼0). However, many candidate non-precious metal catalysts bind hydrogen with similar strengths, but exhibit orders-of-magnitude lower activity for this reaction. In this study, we employ electronic structure methods that allow fully potential-dependent reaction barriers to be calculated, in order to develop a complete working picture of hydrogen evolution on platinum. Through the resulting ab initio microkinetic models, we assess the mechanistic origins of Pt's high activity. Surprisingly, we find that the G∼0 hydrogen atoms are kinetically inert, and that the kinetically active hydrogen atoms have ∆G's much weaker, similar to that of gold. These on-top hydrogens have particularly low barriers, which we compare to those of gold, explaining the high reaction rates, and the exponential variations in coverages can uniquely explain Pt's strong kinetic response to the applied potential. This explains the unique reactivity of Pt that is missed by conventional Sabatier analyses, and suggests true design criteria for non-precious alternatives.

Does a Thermoneutral Electrocatalyst Correspond to the Apex of a Volcano Plot for a Simple Two‐Electron Process?

“…Another possible interpretation of the observed deviation for highly active catalytic materials (DG RI > 0Vat h = 0V) from the common assumption (DG RI = 0Vat h = 0V)isthat the electrode materialsposition in the volcano is affected by its inherent error bars, [42][43][44] which can be determined by modern DFT functionals.T herefore,i tc ould be argued that DG RI -values of about 0.1 eV or 0.2 eV are still reconciled with DG RI = 0V ,t aking the uncertainty of the DFT setup into account. As it was reported that highly active electrocatalysts deviate up to 0.5 eV from the hypothetical DG RI = 0V condition, [20][21][22][23][24][25][26][27] the sensitivity of DFT-calculated free energies cannot entirely account for the right shift of the volcanostop (Figure 3).…”

“…[37] Theabove discussion provides motivation for an alternate interpretation of av olcano plot, which is in line with the recent literature-particularly published within the last year. [20][21][22][23][24][25][26][27]37] While the apex of the volcano is located at DG RI = 0V for the specific case of h = 0V only,a tt ypical reaction conditions (h > 0V)the position of the volcanostop is located at endergonic bonding and directly correlates with the amount of overpotential applied (Figure 3). As ac onsequence,t he traditional understanding of the volcano approach for electrocatalytic processes is reappraised.…”

“…[20][21][22][23][24][25][26][27] Peterson and co-workers investigated the HER on Pt(111), corresponding to the most active HER material, by ab initio calculations and demonstrated that the active hydrogen sites reveal DG RI > 0Vat h = 0V;t hat is, hydrogen is bound endergonically at zero overpotential. [20] As imilar result was reported by Ooka and Nakumara, who used microkinetic modeling to derive ap otential-dependent HER volcano.The accompanied volcano curve indicates that the optimum binding energy of the RI switches from DG RI = 0Vat h = 0V to DG RI > 0Vor DG RI < 0Vat h > 0V , [21] in agreement with an earlier proposal of Zeradjanin and coworkers. [22] Referring to the electrochemical chlorine evolution (CER) and oxygen evolution (OER) reactions,E xner demonstrated that an increase in the applied overpotential shifts the apex of the volcano to an endergonic stabilization (DG RI > 0Vat h = 0V)ofthe corresponding RI;that is,OCl and OOH for the CER and OER, respectively.…”

Overpotential‐Dependent Volcano Plots to Assess Activity Trends in the Competing Chlorine and Oxygen Evolution Reactions

“…It shall be noted that a shift of the volcano's top has been reported by different authors for a variety of electrocatalytic reactions. [47,[66][67][68][69] Kinetic scaling relations [70,71] provide unprecedented insight into the ratedetermining reaction step of electrodes in a homologous series of materials by connecting linear scaling relationships with the experimental Tafel slope. Both approaches have been exerted to the CER and OER over transition metalÀ oxide electrodes: in case of the CER, the concepts of overpotential-dependent Volcano plots [47] and kinetic scaling relations [71] reproduce the higher activity of RuO 2 (110) compared to IrO 2 (110), using the binding energy of oxygen, ΔE O , as descriptor.…”

Recent Progress in the Development of Screening Methods to Identify Electrode Materials for the Oxygen Evolution Reaction

“…Particularly in the last year, several researchers conveyed deviations from the notion of a thermoneutral catalyst at zero overpotential as optimum situation. [17][18][19][20][35][36][37][38][39] This indicates that further progress in the design of sustainable electrode materials on the way toward a sustainable hydrogen economy [40] is not only given by simple thermochemistry models, such as the concept of scaling relations, but also by an in-depth understanding of electrochemical kinetics [41,42] within a class of materials.…”