Catalysis of the Oxygen Evolution Reaction by 4-10 nm Cobalt Nanoparticles

Unravelling faradaic electrochemical efficiencies over Fe/Co spinel metal oxides using surface spectroscopy and microscopy techniques

Cobalt and iron metal-based oxide catalysts play a significant role in energy devices. To unravel some interesting parameters, we have synthesized metal oxides of cobalt and iron (i.e. Fe₂O₃, Co₃O₄, Co₂FeO₄ and CoFe₂O₄), and measured the effect of the valence band structure, morphology, size and defects in the nanoparticles towards the electrocatalytic hydrogen evolution reaction (HER), the oxygen evolution reaction (OER) and the oxygen reduction reaction (ORR). The compositional variations in the cobalt and iron precursors significantly alter the particle size from 60 to <10 nm and simultaneously the shape of the particles (cubic and spherical). The Tauc plot obtained from the solution phase ultraviolet (UV) spectra of the nanoparticles showed band gaps of 2.2, 2.3, 2.5 and 2.8 eV for Fe₂O₃, Co₃O₄, Co₂FeO₄ and CoFe₂O₄, respectively. Further, the valence band structure and work function analysis using ultraviolet photoelectron spectroscopy (UPS) and core level X-ray photoelectron spectroscopy (XPS) analyses provided better structural insight into metal oxide catalysts. In the Co₃O₄ system, the valence band structure favors the HER and Fe₂O₃ favors the OER. The composites Co₂FeO₄ and CoFe₂O₄ show a significant change in their core level (O 1s, Co 2p and Fe 2p spectra) and valence band structure. Co₃O₄ shows an overpotential of 370 mV against 416 mV for Fe₂O₃ at a current density of 2 mA cm^-2 for the HER. Similarly, Fe₂O₃ shows an overpotential of 410 mV against the 435 mV for Co₃O₄ at a current density of 10 mA cm^-2 for the OER. However, for the ORR, Co₃O₄ shows 70 mV improvement in the half-wave potential against Fe₂O₃. The composites (Co₂FeO₄ and CoFe₂O₄) display better performance compared to their respective parent oxide systems (i.e., Co₃O₄ and Fe₂O₃, respectively) in terms of the ORR half-wave potential, which can be attributed to the presence of the oxygen vacancies over the surface in these systems. This was further corroborated in density functional theory (DFT) simulations, wherein the oxygen vacancy formation on the surface of CoFe₂O₄(001) was calculated to be significantly lower (∼50 kJ mol^-1) compared to Co₃O₄ (001). The band diagram of the nanoparticles constructed from the various spectroscopic measurements with work function and band gap provides in-depth understanding of the electrocatalytic process.

Contribution of the Sub-Surface to Electrocatalytic Activity in Atomically Precise La0.7 Sr0.3 MnO3 Heterostructures

Electrocatalytic reactions are known to take place at the catalyst/electrolyte interface. Whereas recent studies of size-dependent activity in nanoparticles and thickness-dependent activity of thin films imply that the sub-surface layers of a catalyst can contribute to the catalytic activity as well, most of these studies consider actual modification of the surfaces. In this study, the role of catalytically active sub-surface layers was investigated by employing atomic-scale thickness control of the La_0.7 Sr_0.3 MnO₃ (LSMO) films and heterostructures, without altering the catalyst/electrolyte interface. The activity toward the oxygen evolution reaction (OER) shows a non-monotonic thickness dependence in the LSMO films and a continuous screening effect in LSMO/SrRuO₃ heterostructures. The observation leads to the definition of an "electrochemically-relevant depth" on the order of 10 unit cells. This study on the electrocatalytic activity of epitaxial heterostructures provides new insight in designing efficient electrocatalytic nanomaterials and core-shell architectures.

Non-Stoichiometry Induced Exsolution of Metal Oxide Nanoparticles via Formation of Wavy Surfaces and their Enhanced Electrocatalytic Activity: Case of Misfit Calcium Cobalt Oxide

Most heterogeneous catalytic reactions demand high density and yet spatially separated nanoparticles that are strongly anchored on the oxide surfaces. Such nanoparticles can be deposited or synthesized in situ via nonstoichiometric methods. To date, nanoparticles have been exsolved from perovskite oxide surfaces using nonstoichiometric processes. However, the density of the space-separated nanoparticles on the oxide surfaces is still low. And less attention is paid toward the changes that happen to the host during the nanoparticle exsolution process. In this work, we demonstrated in situ exsolution of ultrafine nanoparticles (∼5 nm) of either Co₃O₄ or Ca(OH)₂ via judicious control of nonstoichiometry in a misfit Ca₃Co₄O₉ (CCO). The nanoparticle density over the CCO surface reached as high as 8500/μm², which is significantly higher than previously reported values. High-resolution electron microscopy studies reveal the formation mechanism of Co₃O₄ nanoparticles over CCO, and the formation takes palace via the formation of wavy surfaces on the CCO. Defects caused by the nonstoichiometric synthesis created microstrain within the host CCO, resulting in making the new density of states near the Fermi energy. Further, the exsolution process turned the inert host (CCO) into electrocatalytically active toward water splitting. The nonstoichiometric samples obtained by shorter annealing times showed high electrocatalytic behavior for the hydrogen evolution (HER) and oxygen evolution (OER) reactions. The catalytic activity is further enhanced (reaching overpotential of 320 mV and 410 mV for HER and OER respectively, for a current density of 10 mA/cm²) by removing the surface nanoparticles. The observation indicates that the active sites that are produced during the nonstoichiometric synthesis also present in the bulk of the CCO (host). We believe that similar nonstoichiometric synthesis can be applied to a wide variety of tricomponent systems, and they could endow the hosts with novel properties for applications such as catalysis and thermoelectrics.

Particle size-controlled synthesis of high-performance MnCo-based materials for alkaline OER at fluctuating potentials

Mn and Co containing nanocubes were produced by hydrothermal synthesis. The materials consist of metal spinels and carbonates, where spinels ensure high activity and carbonates contribute to high stability in the oxygen evolution reaction.