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

Abstract

This perspective article formalizes our proposal that the apparent hydrogen binding energy (HBE app ), which takes into account of the water adsorption energy as well as the intrinsic hydrogen binding energy (HBE) on metal surfaces, is the descriptor for hydrogen oxidation/evolution reactions (HOR/HERs). We show that the HBE app , not HBE, is pH dependent and can be determined from the underpotential-deposited hydrogen peaks in cyclic voltammograms for platinum group metals including Pt, Ir, Pd, and Rh, and the decrease of their HOR/HER activities with pH is the result of a strengthened HBE app due to a weakened water adsorption. Also discussed are the needs of further experimental and theoretical work to prove this hypothesis. © The Author Hydrogen oxidation/evolution reactions (HOR/HERs) are important for hydrogen fuel cells, water electrolyzers, and solar hydrogen generators. HOR/HER activities are significantly different on different monometallics in both strong acid and base. This has been explained by invoking the intrinsic hydrogen binding energy (HBE) to metals as the descriptor: correlating HOR/HER activities to the HBE of various monometallics generates a volcano plot in both acid and base. [1][2][3] HOR/HER activities decrease significantly with pH on the platinum group metals (PGMs), including Pt, Ir, Pd and Rh. [4][5][6][7] To explain the pH effect, the pH-dependent HBE, derived from cyclic voltammograms (CVs), has been considered as the descriptor.5-7 An alternative theory claims that there is a change in reaction mechanism in high pH electrolyte as the adsorbed OH (OH ad ) facilitates the removal of H ad and promotes the HOR/HER.8 While the enhancement of HOR/HER activity on PtRu catalyst in base could be attributed to OH ad whose formation may be facilitated by the oxophilicity of Ru, 8 it can also be explained by the weakened HBE of Pt induced by presence of Ru through an electronic effect revealed by the density functional theory (DFT) calculation. 9 The higher HOR/HER activity of fully Ptcovered Ru particles than that of partially covered ones also suggests that the ligand effect rather than bifunctional effect is the main reason for the improved HOR/HER activity of PtRu over Pt catalysts in base.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. 12,13 However, this explanation is not applicable to HOR since H 2 dissociates to form H ad and significant difference in HOR activity between acid and base st...

Interfacing nickel nitride and nickel boosts both electrocatalytic hydrogen evolution and oxidation reactions

“…As estimated from DFT calculations, the free energy change of hydrogen adsorption at the interfacial sites of Ni 3 N/Ni/NF is very close to zero, which is beneficial to hydrogen electrochemistry 66 . Although fully considering the solvent environment and taking into account of water adsorption when calculating the hydrogen adsorption are challenging and beyond the scope of this work, our ongoing work aims to compute the apparent hydrogen adsorption energy, which has been recently proposed as a pH-dependent descriptor for HER and HOR activities 69 .…”

A highly-active, stable and low-cost platinum-free anode catalyst based on RuNi for hydroxide exchange membrane fuel cells

“…the HBE app is close to 0 eV, would give the highest HOR activity. The H upd peak position is related to the real HBE app of the catalysts in the electrolyte 18,42 . However, for Ru-based catalyst, hydroxide may adsorb at low potential, and the peak at low potential is assigned to the exchange reaction of H* to adsorbed OH (OH*) 44,45 .…”

Understanding the pH Dependence of Underpotential Deposited Hydrogen on Platinum

“…where *represents an empty surface site.The pH dependence of the H upd peak indicates that the oxidation of H ad as written in Equation (1) is an incomplete description of the process on the molecular level. We proposed that interfacial water participated in the reaction, [10] as Ha nd H 2 Os hared at least af raction, if not the majority,o ft he adsorption sites on the electrode surface.T hus,both the oxidation and deposition of H upd are displacement reactions in nature,i no ther words, either H upd is displaced by adsorbed water or the reverse,asin Equation (2): [18,19]…”