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

Abstract

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 Co 3 O 4 or Ca(OH) 2 via judicious control of nonstoichiometry in a misfit Ca 3 Co 4 O 9 (CCO). The nanoparticle density over the CCO surface reached as high as 8500/μm 2 , which is significantly higher than previously reported values. High-resolution electron microscopy studies reveal the formation mechanism of Co 3 O 4 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/cm 2 ) 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.

Exsolution-Driven Surface Transformation in the Host Oxide

“…Consequently, the active reaction zones on the exsolved metal–oxides are not necessarily constrained to the metal nanoparticles but can extend to the oxide surfaces . For example, Roy et al found that the catalytic activity of the exsolved calcium cobalt oxide increased after etching away the surface nanoparticles . This interesting observation indicates that the remaining host oxides after exsolution may be more catalytically active than the exsolved nanoparticles, although we must note that the etching process may introduce potential artifacts to the oxide chemistry.…”

“…20 For example, Roy et al found that the catalytic activity of the exsolved calcium cobalt oxide increased after etching away the surface nanoparticles. 21 This interesting observation indicates that the remaining host oxides after exsolution may be more catalytically active than the exsolved nanoparticles, although we must note that the etching process may introduce potential artifacts to the oxide chemistry. The need to investigate the exsolved host oxide is further seen from recent studies where reduced or invariable performance was observed after exsolution.…”

Cobalt-doped copper vanadate: a dual active electrocatalyst propelling efficient H2 evolution and glycerol oxidation in alkaline water

“…The Tafel slope will be ~120, ~40, or ~30 mV dec -1 if the rate-determining step is a Volmer, Heyrovsky, or Tafel reaction, respectively. 50 This catalyst also showcased the most facile kinetics that was corroborated by the corresponding Tafel slope. The Tafel slopes for Co0%-CuV, Co5%-CuV, Co10%-CuV, and Co20%-CuV are found to be 145 mV/dec, 113 mV/dec, 94 mV/dec, and 120 mV/dec, respectively (Figure 3b).…”

Analysis of Performance Losses and Degradation Mechanism in Porous La2−X NiTiO6−δ:YSZ Electrodes

“…As previously pointed out [ 43 , 44 ], secondary NiO phase could come from a small fraction of unreacted product or, more likely, from the structure ex-solution. Although no diffraction peaks of Ni metal are found, it cannot be totally discarded that the mechanism involves Ni metal ex-solution from the perovskite structure [ 55 , 56 , 57 ] and further oxidation to NiO, rather than direct NiO ex-solution [ 58 ], as described in other perovskite-related compounds. This is a consequence of the charge compensation mechanism due to the cation vacancies creation in the A-site, alternatively to the formation of larger number of oxygen vacancies and the concentration increase of trivalent nickel content from oxidation in the structure.…”