Synergy between Fe and Ni in the optimal performance of (Ni,Fe)OOH catalysts for the oxygen evolution reaction

Abstract

The oxygen evolution reaction (OER) is critical to solar production of fuels, but the reaction mechanism underlying the performance for a best OER catalyst, Fe-doped NiOOH [(Ni,Fe)OOH], remains highly controversial. We used grand canonical quantum mechanics to predict the OER mechanisms including kinetics and thus overpotentials as a function of Fe content in (Ni,Fe)OOH catalysts. We find that density functional theory (DFT) without exact exchange predicts that addition of Fe does not reduce the overpotential much. However, DFT with exact exchange predicts dramatic improvement in performance for (Ni,Fe)OOH, leading to an overpotential of 0.42 V and a Tafel slope of 23 mV/decade (dec), in good agreement with experiments, 0.3-0.4 V and 30 mV/dec. We reveal that the high spin [Formula: see text] Fe(IV) leads to efficient formation of an active O radical intermediate, while the closed shell [Formula: see text] Ni(IV) catalyzes the subsequent O-O coupling, and thus it is the synergy between Fe and Ni that delivers the optimal performance for OER.

Oxygen Evolution on Metal‐oxy‐hydroxides: Beneficial Role of Mixing Fe, Co, Ni Explained via Bifunctional Edge/acceptor Route

“…The calculated overpotential for NiOOH on the benchmark edge is in good agreement with overpotentials from literature on similar edge systems; 0.52 V, and 0.61 V obtained using PBE+U and RPBE, respectively . Only the overpotential value of Goddard and co‐workers, i. e. 1.22 V (γ‐NiOOH) is significantly different from these values because it was obtained with the hybrid functional B3PW91 instead of a regular GGA or GGA+U. However, for Fe‐doped γ‐NiOOH an overpotential of 0.45 V can be achieved, and further Co‐, Rh‐, or Ir‐doping makes it possible to achieve even lower overpotentials of 0.27, 0.25 and 0.02 V, respectively .…”

“…Previously, the active phase of mixed (Ni,Fe)(O)(OH) systems was identified as γ‐Ni(O)(OH)‐like with a mixed formal oxidation state of +3.6 for the transition metal elements and representative potassium and water intercalated M(O)(OH) systems have been used to describe electrocatalytic cycles computationally . Here, similar models for the pure metal edge sites are used while for the acceptor sites, layered oxy hydroxides are constructed (Figure a–b). Within the edge model systems, we investigated the mononuclear OER behavior of the three geometrically different active sites, indicated with *1, *2 and *3 (Figure a).…”

“…This screening study presents a ‘best case’ scenario based on DFT calculations of neutral reaction intermediates discussed above, and other factors such as the electrical conductivity and activation energy barriers are neglected. As models systems, we use existing edge model systems for γ‐M(O)(OH) (Figure a) where hierarchical, alkali‐ and water intercalation as well as structural features of stacked nanosheets are present since these models are representative at alkaline OER‐conditions …”

Fe‐CoP Electrocatalyst Derived from a Bimetallic Prussian Blue Analogue for Large‐Current‐Density Oxygen Evolution and Overall Water Splitting

“…Water splitting has been considered as a promising technology to produce high‐purity hydrogen,2 a clean energy resource to meet the growing energy demand. However, the practical applications of electrocatalytic water splitting are very limited due to the high overpotentials of two half reactions, the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER), especially for large‐current‐density (>500 mA cm −2 ) OER 3. Hence, enormous research efforts have been devoted to explore water splitting electrocatalysts in order to reduce high overpotentials and prepare high‐efficiency electrocatalysts for HER and OER 4.…”

Fe2+‐Doped Layered Double (Ni, Fe) Hydroxides as Efficient Electrocatalysts for Water Splitting and Self‐Powered Electrochemical Systems

“…Among extensive research in non‐noble electrocatalysts utilized in alkaline condition, Fe‐doped nickel‐based (oxy)‐hydroxides (NiFe‐O x H y ) show impressive oxygen evolution reaction (OER) reactivity with complex catalytic mechanisms . However, for NiFe‐O x H y , it still exhibits large overpotential (η) for practical large‐scale application.…”