Structural Evolution of Metal (Oxy)hydroxide Nanosheets during the Oxygen Evolution Reaction

Abstract

Metal (oxy)hydroxides (MO H, M = Fe, Co, Ni, and mixtures thereof) are important materials in electrochemistry. In particular, MO H are the fastest known catalysts for the oxygen evolution reaction (OER) in alkaline media. While key descriptors such as overpotentials and activity have been thoroughly characterized, the nanostructure and its dynamics under electrochemical conditions are not yet fully understood. Here, we report on the structural evolution of NiCoO H nanosheets with varying ratios of Ni to Co, in operando using atomic force microscopy during electrochemical cycling. We found that the addition of Co to NiO H nanosheets results in a higher porosity of the as-synthesized nanosheets, apparently reducing mechanical stress associated with redox cycling and hence enhancing stability under electrochemical conditions. As opposed to nanosheets composed of pure NiO H, which dramatically reorganize under electrochemical conditions to form nanoparticle assemblies, restructuring is not found for NiCoO H with a high Co content. NiFeO H nanosheets show high roughness as-synthesized which increases during electrochemical cycling while the integrity of the nanosheet shape is maintained. These findings enhance the fundamental understanding of MO H materials and provide insight into how nanostructure and composition affect structural dynamics at the nanoscale.

Modes of Fe Incorporation in Co–Fe (Oxy)hydroxide Oxygen Evolution Electrocatalysts

“…We found that, upon cycling, the Ni(OH) 2 nanosheet dramatically restructured into a conglomeration of NiO x H y nanoparticles . An analogous study of Co(OH) 2 nanosheets show the material's morphology to be much more stable, even under OER conditions and with electrochemical cycling . The increased morphological stability of the Co versus the Ni (oxy)hydroxide nanosheets is likely related to the relative metal–oxygen bond strengths.…”

“…This is likely because the oxyhydroxide nanosheet building blocks of the CoO x H y film are more stable than those of the NiO x H y film. We have recently collected in situ electrochemical atomic force microscopy data that supports this hypothesis . For example, we synthesized single‐layer Ni(OH) 2 nanosheets and imaged the morphology evolution of a single sheet during the OER.…”

“…We note that this behavior is different from what is observed for the analogous NiO x H y where, upon continued cycling in Fe‐spiked KOH, the Fe cations apparently mix with the Ni species to form mixed Ni(Fe)O x H y nanosheet structures. This difference is explained by the weaker metal–oxygen bond energies in NiO x H y compared to CoO x H y —NiO x H y is more structurally dynamic than CoO x H y and more easily allows cation‐exchange reactions to occur …”

Enhancement of Oxygen Evolution Activity of Nickel Oxyhydroxide by Electrolyte Alkali Cations

“…Nickel-based oxides have been extensively used as catalysts for the oxygen evolution reaction (OER) [1][2][3][4][5][6] because of their high intrinsic activity and stability in alkaline electrolyte. [7][8][9][10][11][12] Understanding the processes and intermediates involved during OER on the nickel oxyhydroxide (NiOOH) surface is of key importance for further improving their performance. TheO ER activity of NiOOH is known to depend on the electrolyte composition, in particular on the Fe content of the electrolyte,the nature of the cation in the electrolyte,and the pH of the electrolyte.…”

Cation‐Modulated HER and OER Activities of Hierarchical VOOH Hollow Architectures for High‐Efficiency and Stable Overall Water Splitting

“…However, the scarcity of noble metals limits the widespread application . Therefore, tremendous efforts have been made to design earth‐abundant electrocatalysts with high activity and stability, such as transition metal chalcogenides, carbides, phosphides, and nitrides for HER, and cobalt‐phosphate, perovskite oxides, and 3d transition metal–based (oxy)hydroxides for OER. Considering the compatibility of the electrocatalysts in the same electrolyte for both HER and OER simultaneously, exploring highly efficient and stable bifunctional electrocatalysts toward overall water splitting is highly desirable.…”