Toward Realistic Models of the Electrocatalytic Oxygen Evolution Reaction

The electrocatalytic oxygen evolution reaction (OER) supplies the protons and electrons needed to transform renewable electricity into chemicals and fuels. However, the OER is kinetically sluggish; it operates at significant rates only when the applied potential far exceeds the reversible voltage. The origin of this overpotential is hidden in a complex mechanism involving multiple electron transfers and chemical bond making/breaking steps. Our desire to improve catalytic performance has then made mechanistic studies of the OER an area of major scientific inquiry, though the complexity of the reaction has made understanding difficult. While historically, mechanistic studies have relied solely on experiment and phenomenological models, over the past twenty years ab initio simulation has been playing an increasingly important role in developing our understanding of the electrocatalytic OER and its reaction mechanisms. In this Review we cover advances in our mechanistic understanding of the OER, organized by increasing complexity in the way through which the OER is modeled. We begin with phenomenological models built using experimental data before reviewing early efforts to incorporate ab initio methods into mechanistic studies. We go on to cover how the assumptions in these early ab initio simulations─no electric field, electrolyte, or explicit kinetics─have been relaxed. Through comparison with experimental literature, we explore the veracity of these different assumptions. We summarize by discussing the most critical open challenges in developing models to understand the mechanisms of the OER.

In Situ Formation of Nano Ni-Co Oxyhydroxide Enables Water Oxidation Electrocatalysts Durable at High Current Densities

The oxygen evolution reaction (OER) limits the energy efficiency of electrocatalytic systems due to the high overpotential symptomatic of poor reaction kinetics; this problem worsens over time if the performance of the OER electrocatalyst diminishes during operation. Here, a novel synthesis of nanocrystalline Ni-Co-Se using ball milling at cryogenic temperature is reported. It is discovered that, by anodizing the Ni-Co-Se structure during OER, Se ions leach out of the original structure, allowing water molecules to hydrate Ni and Co defective sites, and the nanoparticles to evolve into an active Ni-Co oxyhydroxide. This transformation is observed using operando X-ray absorption spectroscopy, with the findings confirmed using density functional theory calculations. The resulting electrocatalyst exhibits an overpotential of 279 mV at 0.5 A cm^-2 and 329 mV at 1 A cm^-2 and sustained performance for 500 h. This is achieved using low mass loadings (0.36 mg cm^-2 ) of cobalt. Incorporating the electrocatalyst in an anion exchange membrane water electrolyzer yields a current density of 1 A cm^-2 at 1.75 V for 95 h without decay in performance. When the electrocatalyst is integrated into a CO₂ -to-ethylene electrolyzer, a record-setting full cell voltage of 3 V at current density 1 A cm^-2 is achieved.

Density Functional Theory Study of NiFeCo Trinary Oxy-Hydroxides for an Efficient and Stable Oxygen Evolution Reaction Catalyst

NiOOH and its doped species are widely used as electrocatalysts for the oxygen evolution reaction (OER) in alkaline media. In this work, we carried out comprehensive density functional theory (DFT) simulations of Ni-based electrocatalysts for the OER by applying suitable dopants in β-NiOOH. A range of Fe and Co atoms (%) are employed as doping agents to increase the overall catalytic ability, stability, and feasibility of NiOOH. Our simulations indicate that Ni_88%Fe_6%Co_6%OOH is efficient, stable, and provides more catalytic sites at the surface of resulting catalysts for water adsorption and dissociation, which facilitate the OER. The lower overpotential for the OER is estimated from the higher adsorption energy of water molecule over the surface of Ni_88%Fe_6%Co_6%OOH, followed by other electronic properties such as band structure, electrostatic potential, the density of states, and surface formation energy.

Nickel-Based Bicarbonates as Bifunctional Catalysts for Oxygen Evolution and Reduction Reaction in Alkaline Media

Oxygen electrocatalysis, including the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR), is one of the most important electrochemical processes for sustainable energy conversion and storage technologies. Herein, nickel-based bicarbonates are, for the first time, developed as catalysts for oxygen electrocatalysis, and demonstrate superior electrocatalytic performance in alkaline media. Iron doping can significantly tune the real valence of nickel ions, and consequently tailor the electrocatalytic ability of bicarbonates. Among the nickel-based bicarbonates, Ni_0.9 Fe_0.1 (HCO₃ )₂ exhibits the highest bifunctional catalytic activity, with a potential difference of 0.86 V between the OER potential at a current density of 10 mA cm^-2 and the ORR potential at a current density of -1 mA cm^-2 , which outperforms most of the reported precious-metal-free catalysts. The present work provides new insights into exploring efficient catalysts for oxygen electrocatalysis, and it suggests that, in addition to the extensively studied transition metal hydroxides and oxides, bicarbonates and carbonates also show great potential as precious metal-free catalysts.

Effect of transition-metal-ion dopants on the oxygen evolution reaction on NiOOH(0001)

Iron-doped nickel oxyhydroxide has been identified as one of the most active alkaline oxygen evolution reaction (OER) catalysts, exhibiting an overpotential lower than values observed for state-of-the-art precious metal catalysts. Several computational investigations have found widely varying effects of doping on the theoretical overpotential of the OER on NiOx. Comparisons of these results are made difficult by the numerous differences in the structural and computational parameters used in these studies. In this work, within a consistent framework, we calculate the theoretical overpotentials for reactions occurring on the most stable, basal plane of undoped and doped β-NiOOH. We compare the activities of Fe(iii), Co(iii), and Mn(iii) doping using density functional theory with Hubbard-like U corrections on the transition-metal d orbitals. We compare the effect of surface and subsurface doping in order to establish whether the dopants act as new active sites for the reaction or whether they induce more widespread changes in the material. The results of our study find only a small reduction in the overpotential (∼0.1 and ≤0.05 V when doped in the surface and subsurface layers, respectively) for the three dopants, if doped in the dominant basal plane. This is much less than the reductions of 0.3 V experimentally observed for the most active Fe-doped systems. Furthermore, the magnitudes of reductions in overpotentials for the three dopants are similar. This work therefore disqualifies the possibility of enhancing the activity of the dominant exposed basal plane of β-NiOOH through substitutional doping.

The secret behind the success of doping nickel oxyhydroxide with iron

Discovering better catalysts for water splitting is the holy grail of the renewable energy field. One of the most successful water oxidation catalysts is nickel oxyhydroxide (NiOOH), which is chemically active only as a result of doping with Fe. In order to shed light on how Fe improves efficiency, we perform Density Functional Theory +U (DFT+U) calculations of water oxidation reaction intermediates of Fe substitutional doped NiOOH. The results are analyzed while considering the presence of vacancies that we use as probes to test the effect of adding charge to the surface. We find that the smaller electronegativity of the Fe dopant relative to Ni allows the dopant to have several possible oxidation states with less energy penalty. As a result, the presence of vacancies which alters local oxidation states does not affect the low overpotential of Fe-doped NiOOH. We conclude that the secret to the success of doping NiOOH with iron is the ability of iron to easily change oxidation states, which is critical during the chemical reaction of water oxidation.

Theoretical and experimental investigations of the electronic/ionic conductivity and deprotonation of Ni3−xCoxAl-LDHs in an electrochemical energy storage system

A schematic illustration of the mechanism of enhanced electrochemical performance by doping Co species.

Atomic-level energy storage mechanism of cobalt hydroxide electrode for pseudocapacitors

Cobalt hydroxide is a promising electrode material for supercapacitors due to the high capacitance and long cyclability. However, the energy storage/conversion mechanism of cobalt hydroxide is still vague at the atomic level. Here we shed light on how cobalt hydroxide functions as a supercapacitor electrode at operando conditions. We find that the high specific capacitance and long cycling life of cobalt hydroxide involve a complete modification of the electrode morphology, which is usually believed to be unfavourable but in fact has little influence on the performance. The conversion during the charge/discharge process is free of any massive structural evolution, but with some tiny shuffling or adjustments of atom/ion species. The results not only unravel that the potential of supercapacitors could heavily rely on the underlying structural similarities of switching phases but also pave the way for future material design for supercapacitors, batteries and hybrid devices.