Understanding the Oxygen Evolution Reaction on a Two‐Dimensional NiO₂ Catalyst

Abstract

Solar water splitting is a promising technology to store energy in chemicals but the technology suffers from a lack of efficient catalysts. Nickel oxyhydroxide (NiOOH) is an outstanding catalyst for the oxygen evolution reaction (OER). NiOOH is composed of partially hydrogenated nickel oxide (NiO2) two‐dimensional (2D) sheets that are stacked together. Owing to recent advancements in fabrication it is possible to isolate a single layer of the material. Such 2D catalytic systems are extremely promising, especially due to the wider exposed surface. However their reactivity is not completely understood, and density functional theory (DFT) is an appropriate tool to address that issue. In the current paper we use DFT to model several OER mechanisms. We show that the catalytic activity of these 2D materials is not limited to the defect sites and to the edges, as usually assumed, but rather many reaction centers are available on the NiO2 surface itself. In addition, we will provide evidence for the inhibiting effect of hydrogen ad‐atoms on such layers, thus supporting the necessity of operating in highly basic conditions. The new insights that we are providing here on NiO2 reactivity, and the detailed understanding of the surface mechanisms regarding OER processes, will be key for further developments of 2D tailored catalysts. Indeed, as a main outcome of our work, NiO2 sheets are found to be significantly more reactive than bulk NiOOH itself.

Radially Aligned Hierarchical Nickel/Nickel–Iron (Oxy)hydroxide Nanotubes for Efficient Electrocatalytic Water Splitting

“…The setup details are thoroughly addressed in the Experimental Section. The strongly alkaline media may to some extent bring about deprotonation of (oxy)­hydroxide layers, which has been recently reported to be advantageous to promote eletrocatalytic reactivities. , The performance of individual components, including neat (oxy)­hydroxide, neat Ni, and benchmark noble-metal catalysts, Pt for HER and RuO 2 for OER, were also studied for comparison purposes. Figure a depicts the collected typical OER linear sweep voltammetry (LSV) curves at a rate of 5 mV s –1 .…”

Intrinsic Electrocatalytic Activity for Oxygen Evolution of Crystalline 3d‐Transition Metal Layered Double Hydroxides

“…[23] On the other hand, the active phases and catalytic mechanisms are still under debate even for classic OER catalysts such as IrO x . [24] Fora lkaline OER on transition metal layered double hydroxides (LDH), apart from the promoting effects of Fe in NiFea nd CoFe LDHs, [15,25] ag eneral understanding has not been reached. Forexample,itisunclear whether the OER trends that were established on those two most active catalysts can be extended upon introduction of ad ifferent element into the same hosts.I ti sa lso unknown whether those advanced designing and screening methods,w hich were developed on the basis of diverse model systems,a re applicable to more specific and more realistic OER catalysts.Such afundamental understanding is not only important for providing scientific insights to rationalize experimental trends,but is also crucial for establishing amore complete picture of OER mechanisms that can be used to calibrate the above designing methods and theories.These achievements would permit feasible design of OER catalysts with performance beyond the state-of-the-art catalysts.…”

In-situ structure and catalytic mechanism of NiFe and CoFe layered double hydroxides during oxygen evolution

“…Density functional theory (DFT) calculations allow us to examine all of the above hypotheses and to extract atomic-scale details by screening suitable candidate phases and comparing their relative stability with that of known phases. Although significant efforts have been made, particularly on the modeling of the electronic structure effects and catalytic mechanism of Nibased catalysts for OER 4,[10][11][12]33,[35][36][37][38][39][40] , such a screening and comparison has not yet been rigorously carried out because of the structural complexity of the active phases. Indeed, even the atomic-scale structure of the γ-NiOOH phase itself is still unclear 4,33,38 .…”

“…The lack of these atomic-scale details has, in turn, made it highly challenging to choose appropriate models for DFT-based mechanistic studies 4,38,41 . Hence, a variety of structures have been employed in the modeling, including those that resemble as-synthesized precursor phases 37 , NiO 38 , twodimensional single layer (oxy)hydroxides 35,39 , β-MOOH analogs 4,[10][11][12]42 , and γ-NiOOH analogs 33,40,[43][44][45] with or without Fe dopants. Although significant efforts have been made to explain the high activity of MFe LDHs, the diversity of studies suggests that large uncertainties exist concerning the relationship between the active site structure and the catalytic mechanism.…”