Spinel‐Structured High‐Entropy Oxide Nanofibers as Electrocatalysts for Oxygen Evolution in Alkaline Solution: Effect of Metal Combination and Calcination Temperature

Abstract

Defect‐engineering is a viable strategy to improve the activity of nanocatalysts for the oxygen evolution reaction (OER), whose slow kinetics still strongly limits the broad market penetration of electrochemical water splitting as a sustainable technology for large‐scale hydrogen production. High‐entropy spinel oxides (HESOs) are in focus due to their great potential as low‐cost OER electrocatalysts. In this work, electrospun HESO nanofibers (NFs), based on (Cr,Mn,Fe,Co,Ni), (Cr,Mn,Fe,Co,Zn) and (Cr,Mn,Fe,Ni,Zn) combinations, with granular architecture and oxygen‐deficient surface are produced by calcination at low temperature (600 or 500 °C), characterized by a combination of benchtop analytical techniques and evaluated as electrocatalysts for OER in alkaline medium. The variation of HESO composition and calcination temperature produces complex and interdependent changes in the morphology of the fibers, crystallinity and inversion degree of the spinel oxide, concentration of the oxygen‐vacancies, cation distribution in the lattice, which mirror on different electrochemical properties of the fibers. The best electrocatalytic performance (overpotential and Tafel slope at 10 mA cm−2: 360 mV and 41 mV dec−1, respectively) pertains to (Cr1/5Mn1/5Fe1/5Co1/5Ni1/5)3O4 NFs calcined at 500 °C and results from the lower outer 3d‐electron number, eg filling closer to its optimal value and higher occupation of 16d sites by the most redox‐active species.

Role of the Microstructure in the Li-Storage Performance of Spinel-Structured High-Entropy (Mn,Fe,Co,Ni,Zn) Oxide Nanofibers

“…In addition, the fit of oxygen longitudinal projection to model profiles (Fig. S5b) reveals that, except for HEO LL , the NFs have a narrow inner channel; its size increases from 20% of the fiber width in mcHEO, to 30% in HEO, in agreement with previous reports on electrospun SHEOs based on different TM combinations 96 and low-entropy oxide NFs, as well. 95 The large temperature gradient along the radial direction experienced by the precursor polymer/TM-acetate NFs during calcination 117 due to the rapid rise in temperature (10 °C min −1 ) and the sintering effects, 95 which favor the development of larger grains at the expense of smaller ones, with a decrease in the specific surface, 118 are responsible for the above described evolution of fiber microstructure and may also affect the electrochemical performance of the fibers.…”

“…42,70,71,[90][91][92] The metal loading in the spinnable precursor solution heavily affects both the diameter of the NFs and the size of the NPs that compose them. 93,94 The postspinning calcination conditions (temperature and duration) strongly influence not only the morphology and crystallinity of the NFs, 83,95,96 but also the concentration of surface oxygen vacancies and defects, 42,92,95,97 the degree of spinel inversion and the distribution of cations in the SHEO lattice. 92,96,97 In this work, electrospun (Mn,Fe,Co,Ni,Zn) SHEO NFs to be used as anode active materials in LIBs are produced via the procedure utilized in a previous work 81 by varying the metal load (19.23 or 38.46 wt% relative to the polymer) in the precursor solution, and temperature and duration of the calcination process (0.5 h at 700 °C, or 2 h at 700 °C followed by 2 h at 900 °C), as schematically depicted in Fig.…”

Biomineralization of Sulfate‐Reducing Bacteria In Situ‐Induced Preparation of Nano Fe2O3‐Fe(Ni)S/C as High‐Efficiency Oxygen Evolution Electrocatalyst

“…High-temperature calcination may lead to a larger BET surface area, but low-temperature calcination may be more favorable for the formation of an appropriate pore structure. [70,71] An appropriately tuned pore structure is vital for the transport of gases and ions during the process of electrolysis OER. A finer pore structure may provide better diffusion pathways for protons and oxygen, promoting the progress of the reaction.…”

Ni and VN Nanoparticles Supported on N-Doped Carbon Layer Containing Ni Single Atoms as Electrocatalyst for the Hydrogen Evolution Reaction

“…Non-precious transition metals like Fe, Co, and Ni have gained much notice owing to the excellent catalytic properties, high abundance, low cost, and high electrical conductivity. − In particular, Ni is located near the top of HER “volcano diagram”, and its Gibbs free energy of adsorption H (Δ G H* ) intermediates is similar to that of Pt. , However, its binding energy to H is stronger than that of Pt, and the H 2 molecules produced are difficult to detach from the Ni surface, resulting in slow HER kinetics . Therefore, the introduction of other active components to form Ni-based heterostructures can be beneficial in optimizing the electronic structure of Ni and consequently increasing its intrinsic activity.…”