Abstract
Nickel-based oxides are highly active, cost-effective materials for the oxygen evolution reaction in alkaline conditions. Recent experimental studies have revealed the importance of surface deprotonation and alkali metal cation adsorption on the activity of Ni oxide surfaces, in contact with aqueous alkaline electrolyte. As a first step to elucidate the role of the alkali adsorption for the activity, we performed first-principles electronic structure calculations to address the stable surface structures of β-NiOOH(0001) as a function of the operating conditions in an electrochemical environment. We present a grand-canonical approach to compute the surface Pourbaix diagram of the β-NiOOH/water interface for the processes of deprotonation and alkali metal cation adsorption. The results of this study emphasize the importance of double-layer effects, including the adsorbate-induced change of surface dipole moments and the rearrangement of water molecules due to their strong interaction with the adsorbed species, for the most stable interface structure.
Highlights
Green hydrogen will be essential as a fuel of a defossilized energy economy [1]
Consistent with the recent density functional theory (DFT) calculations in Ref. [17] it was found that deprotonation leads to electron transfer from surface Ni atoms to surface O atoms, which is evidenced by the change in the oxidation state of Ni from Ni3+ to Ni4+
It should be noted that a favourable H-down orientation of water at the surface further increases ∆p toward the surface, which is consistent with the previous study [52]
Summary
Green hydrogen will be essential as a fuel of a defossilized energy economy [1]. Compared with conventional fuels such as coal, natural gas and oil, hydrogen has the highest specific energy [2], and if used in fuel cells it constitutes a clean, 5 efficient and sustainable technology [3]. 35 Recent experimental studies by Diaz-Morales et al and Garcia et al have revealed the importance of surface deprotonation as well as alkali metal cation adsorption on the OER activity of nickel oxyhydroxide (NiOOH) surfaces in contact with the alkaline electrolyte [15, 16]. The mechanism of OER based on deprotonation was investigated on the (0001) facet of NiOOH using density functional theory (DFT), which provided further support for the role of deprotonation in OER activity [17] These calculations suggested that the deprotonation step is the potential determining step with an overpotential of 0.44 V. We employ a first-principles grand-canonical approach to explore the thermodynamic stability of NiOOH/water interfaces for the processes of deprotonation and alkali metal cation adsorption under varying electrochemical conditions with respect to pH and electrode poten tial. We will discuss the crucial importance of double-layer effects such as the adsorbate-induced surface dipole moment as well as the explicit presence of water molecules [19, 20, 21, 22] for the computed surface Pourbaix diagrams
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