Double‐Exchange‐Induced in situ Conductivity in Nickel‐Based Oxyhydroxides: An Effective Descriptor for Electrocatalytic Oxygen Evolution

Abstract

The electrocatalytic oxygen evolution reaction (OER) plays an important role in sustainable energy conversion from water to hydrogen fuel. The most effective non-noble metal catalyst for OER is currently Fe-doped NiOOH (Ni1-xFexOOH), but its overpotential (theoretically calculated η = 0.4) is too large for practical applications. Motivated by our in silico predictions that Ir dopant would lead to a very low overpotential to improve OER activity of Ni-based hydroxides, we report here an experimental confirmation on the altered OER activities for a series of metals (Mo, W, Fe, Ru, Co, Rh, Ir) doped into γ-NiOOH. Most interestingly, we observed that the in situ (intermediate) electrical conductivity for metal doped γ-NiOOH correlates well with the trend in enhanced OER activities. This correlation allows experimental screening of new candidates with fast measurements of intermediate conductivity, while also providing additional information about the catalytic mechanism. To understand the basis of this correlation, we used density functional theory (DFT) calculations to explain the in situ conductivity of the key intermediate states of metal doped γ-NiOOH during OER. The simultaneous increase of OER activity with in situ conductivity was rationalized by their intrinsic connections to the radical character promotion in the metal-oxo bond. This electrical transport characteristic, which was later linked to the double exchange (DE) interactions between adjacent metal ions with various d orbital occupancies, serves as an effective indicator for the key metal-oxo radical character, providing a facile and effective descriptor for the theoretical and experimental guidance in design and screening of efficient OER catalysts.

Principles of Water Electrolysis and Recent Progress in Cobalt‐, Nickel‐, and Iron‐Based Oxides for the Oxygen Evolution Reaction

“…Even though the pure Ni‐based OER electrocatalyst often suffers from bad electrical conductivity, these problems could be resolved by controlling the structural transformation of the starting material into the active Ni oxyhydroxide intermediate or blending them with other hetero elements. [116] Due to their adsorption capability toward Fe species, the OER activity of Ni‐based oxides could be boosted by coupling with iron by forming dynamic active sites. All in all, NiFe‐based oxides are still considered to be promising candidates for use in large‐scale water electrolyzers.…”

Principles of Water Electrolysis and Recent Progress in Cobalt‐, Nickel‐, and Iron‐Based Oxides for the Oxygen Evolution Reaction

“…Even though the pure Ni-based OER electrocatalyst often suffers from bad electrical conductivity,these problems could be resolved by controlling the structural transformation of the starting material into the active Ni oxyhydroxide intermediate or blending them with other hetero elements. [116] Due to their adsorption capability toward Fe species,t he OER activity of Ni-based oxides could be boosted by coupling with iron by forming dynamic active sites.A ll in all, NiFebased oxides are still considered to be promising candidates for use in large-scale water electrolyzers.…”