The Effect of Fe Dopant Location in Co(Fe)OOH_x Nanoparticles for the Oxygen Evolution Reaction

Abstract

The addition of iron (Fe) can in certain cases have a strong positive effect on the activity of cobalt and nickel oxide nanoparticles in the electrocatalytic oxygen evolution reaction (OER). The reported optimal Fe dopant concentrations are, however, inconsistent, and the origin of the increased activity due to Fe dopants in mixed oxides has not been identified so far. Here, we combine density functional theory calculations, scanning tunneling microscopy, and OER activity measurements on atomically defined Fe-doped Co oxyhydroxide nanoparticles supported on a gold surface to establish the link between the activity and the Fe distribution and concentration within the oxyhydroxide phase. We find that addition of Fe results in distinct effects depending on its location on edge or basal plane sites of the oxyhydroxide nanoparticles, resulting in a nonlinear OER activity as a function of Fe content. Fe atom substitution itself does not lead to intrinsically more active OER sites than the best Co sites. Instead, the sensitivity to Fe promoter content is explained by the strong preference for Fe to locate on the most active edge sites of oxyhydroxide nanoparticles, which for low Fe concentrations stabilizes the particles but in higher concentrations leads to a shell structure with less active Fe on all edge positions. The optimal Fe content thereby becomes dependent on nanoparticle size. Our findings demonstrate that synthesis strategies that adjust not only the Fe concentration in mixed oxides but also its distribution within a catalyst nanoparticle can lead to enhanced OER performance.

Stabilization of a Mn−Co Oxide During Oxygen Evolution in Alkaline Media

“…We expect less than the detection limit of about 50 ppm Fe [101] in the cycled film. The increase in activity with Fe appears to be linear up to the optimal composition [41,54] which spreads much in the range from about 3 to 70 % [57] . The lowest optimal Fe content is 2.8 % Fe (2.8×10 4 ppm) [57] .…”

“…Yet, we cannot exclude minor concentration (mass fraction < 0.1 %) [55,56] and it is unknown if such small concentrations would affect the activity of Co oxides. [57] No substantial morphology differences were observed between the pristine CoO x and (Co 0.7 Mn 0.3 )O x films (Figure S2). Additionally, no significant morphological changes were observed in comparison with the previously reported MnO x .…”

“…The increase in activity with Fe appears to be linear up to the optimal composition [41,54] which spreads much in the range from about 3 to 70 %. [57] The lowest optimal Fe content is 2.8 % Fe (2.8 × 10 4 ppm). [57] In summary, major Fe incorporation could not be detected by EDX nor XAS, and it is unknown if small concentrations of Fe (< 0.1 % mass fraction) [55,56] would significantly affect the activity of Co oxides.…”

MOF-Derived Co and Fe Species Loaded on N-Doped Carbon Networks as Efficient Oxygen Electrocatalysts for Zn-Air Batteries

“…Meanwhile, the wrapping of CNTs for Co nanoparticles could prevent the aggregation and detachment of Co nanoparticles, which are beneficial for the stability of CoNP@FeNC-0.05 under electrochemical reactions [49]. In addition, the introduction of Fe-N sites has an obvious impact on the resulting OER performance of Co-based catalysts [50,51]. (III) Large specific surface area of CoNP@FeNC-0.05 derived from the MOF precursor promotes the exposure of active sites.…”

What X-ray absorption spectroscopy can tell us about the active state of earth-abundant electrocatalysts for the oxygen evolution reaction

“…Other transition metal oxides, in particular those containing Ni, are known to incorporate Fe from alkaline electrolytes, which strongly modifies their electronic and thus catalytic properties. [84][85][86][87] Also, in the absence of Fe, exposure to hydroxide solutions may change the surface and even the bulk phase of transition metal oxides after sufficiently long exposure. 88 Finally, elements may dissolve from the electrocatalyst into the electrolyte changing the composition and thereby other properties, such as redox potentials or the catalytic reaction's overpotential.…”