Role of Hydroxyl Species in Hydrogen Oxidation Reaction: A DFT Study

Abstract

Slow kinetics of the hydrogen oxidation reaction (HOR) in alkaline electrolyte impedes the development of alkaline fuel cell systems. In this work, density functional theory calculations were used to study the HOR mechanism on several metals (Pt(110), Ir(110), Pd(110), Ni(110), and PtRu(110)), particularly by additionally considering the adsorption of hydroxyl species (OH*) on these metals. We found that the formation of OH* can transfer the potential-determining step of HOR mechanism from the H* oxidation to H 2 O* desorption under remarkably different effects of OH* on H* and H 2 O*. The comprehensive Δ r G−U relational diagrams for HOR/hydrogen evolution reaction show that, apart from the widely accepted activity descriptor, H* adsorption free energy (ΔG H* ), OH* adsorption free energy (ΔG OH* ), and H 2 O* adsorption free energy (ΔG H 2 O* ) also should be involved in predicting the HOR catalytic activity of metal catalysts in alkaline electrolyte. When the OH* formation free energy change (Δ r G OH* = ΔG OH* , at equilibrium potential) is more positive than the H* oxidation free energy change (Δ r G H*→H 2 O* = ΔG H 2 O* − ΔG H* , at equilibrium potential), ΔG H* as the sole descriptor indicates the HOR activity of catalysts due to scarce formation of OH* and a relatively weak H 2 O* adsorption at a relatively low overpotential, which happened in the case of Pt(110) and Pd(110). When Δ r G OH* and Δ r G H*→H 2 O* have little difference as in the case of Ir(110), both OH* formation and H* oxidation affect of the HOR and ΔG OH* and the enhanced ΔG H 2 O* by OH* should be involved in evaluating the HOR activity. In the case of Ni(110), a much lower value of Δ r G OH* than that of Δ r G H*→H 2 O* causes the surface to be mostly blocked by OH*, which suppresses the HOR. The combination of ΔG OH* , ΔG H* , and ΔG H 2 O* gives a more precise and comprehensive description of the HOR mechanism for metallic catalysts at different electrode potentials.

Controlling the Size and Pattern Pitch of Ni(OH)2 Nanoclusters Using Dip-Pen Nanolithography to Improve Water Oxidation

“…The activity of each of the two types of MCS was studied by LSV in M NaClO 4 solutions under various pH conditions (from pH 8 to pH , Ar atmosphere), so as to be able to compare our findings with those reported previously [ 27 , 43 ]. As expected [ 22 , 27 , 40 ], we found that the oxidation process occurs at pH > 10, and that the oxidation current and potential depend both on the pH and on the pattern pitch dimension ( Figure 3 ). The waves between ∼ – V (depending on the conditions) are attributed to the oxidation of Ni(OH) 2 through Reaction ( 3 ).…”

“…We show that MCS electrode can efficiently oxidize water and increase the efficiency of the WSP. The use of DPN to generate the electrode, demonstrated here for the first time, is considerably simpler than other methods [ 20 , 22 , 26 , 27 ]. It enables the patterning of uniform nanoclusters of OER catalysts, such that the process can be controlled by changing the solution pH, the size of the clusters, and the pattern pitch dimensions.…”

“…First, heterogeneous catalysis is favored over homogenous catalysis [ 11 , 13 , 16 , 17 , 18 , 19 ], as the former allows the recycling of the catalyst and renders the issue of catalyst solubility irrelevant [ 16 , 20 ]. Second, the over-potential of the OER—which is usually high—needs to be as low as possible to reduce energy consumption [ 14 , 21 , 22 , 23 , 24 , 25 ]. Third, the catalyst should be inert, such that ligands do not oxidize at a potential that is lower than that of the OER [ 16 ].…”

Non‐Platinum Group Metal Electrocatalysts toward Efficient Hydrogen Oxidation Reaction

“…[ 49–53 ] Besides, Wei and co‐workers studied the role of OH* in the HOR on Pt(110), Ir(110), Pd(110), Ni(110), and PtRu(110) using density functional theory (DFT) calculations. [ 54 ] The authors found that OH* adsorption induces remarkably different effects on H* and adsorbed water molecule (H 2 O*), and the combination of Δ G OH* , Δ G H* , and Δ G H2O* should be evaluated altogether to achieve a more precise and comprehensive understanding on the HOR/HER mechanism. Jia and co‐workers proved that the alkali metal cations (AM + ) including Li + , Na + , K + , Rb + , and Cs + destabilize the OH* in the electric double layer by forming the OH − ‐(H 2 O) x ‐AM + adducts, [ 55 ] which is detrimental to the HOR based on the bifunctional mechanism.…”

Single-atom Pd-Ru pair on Pt surface for promoting hydrogen oxidation in alkaline media

“…Calculation of the H2 molecule was obtained by placing the atom in a 1 nm cube. To obtain the Gibbs Free Energy profiles of the HOR at 0 V and equilibrium potential, we followed the thermodynamic pathways of the HOR as described in the work of Wei and co-workers 30 . The Open Visualization Tool (OVITO) 31 was used to visualize all the configurations and models.…”