Recognizing the reactive sites of SnFe2O4 for the oxygen evolution reaction: the synergistic effect of SnII and FeIII in stabilizing reaction intermediates

Among the reported spinel ferrites, the p-block metal containing SnFe2O4 is scarcely explored, but it is a promising water-splitting electrocatalyst. This study focuses on the reaction kinetics and atomic scale insight of the reaction mechanism of the oxygen evolution reaction (OER) catalyzed by SnFe2O4 and analogous Fe3O4. The replacement of FeIIOh sites with SnIIOh in SnFe2O4 improves the catalytic efficiency and various intrinsic parameters affecting the reaction kinetics. The variable temperature OER depicts a low activation energy (Ea) of 28.71 kJ mol-1 on SnFe2O4. Experimentally determined second-order dependence on [OH-] and the prominent kinetic isotope effect observed during the deuterium labelling study implies the role of hydroxide ions in the rate-determining step (RDS). Using density functional theory, the reaction mechanism on the (001) surface of SnFe2O4 and Fe3O4 is modelled. The DFT simulated free energy diagram for the reaction intermediates shows an adsorbate evolution mechanism (AEM) on both the ferrites' surfaces where the formation of *OOH is the RDS on SnFe2O4 while *O formation is the RDS on Fe3O4. In contrast to other spinel ferrites, where individual metal sites act independently, in case of SnFe2O4, a synergy between FeIIIOh and the neighbouring SnIIOh atoms is responsible for stabilizing the OER intermediates, enhancing the catalytic OER activity of SnFe2O4 as compared to isostructural Fe3O4.

Highly Accessible Co-Nx Active Sites-Doped Carbon Framework with Uniformly Dispersed Cobalt Nanoparticles for the Oxygen Reduction Reaction in Alkaline and Neutral Electrolytes

Porous carbon materials with nitrogen-coordinated transition metal active sites have been widely regarded as appealing alternatives to replace noble metal catalysts in oxygen-based electrochemical reaction activities. However, improving the electrocatalytic activity of transition-metal-based catalysts remains a challenge for widespread application in renewable devices. Herein, we use a simple one-step pyrolysis method to construct a Co nanoparticles/Co-Nx-decorated carbon framework catalyst with a near-total external surface structure and uniform dispersion nanoparticles, which displays promising catalytic activity and superior stability for oxygen reduction reactions in both alkaline and neutral electrolytes, as evidenced by the positive shift of half-wave potential by 44 and 11 mV compared to 20% Pt/C. Excellent electrochemical performance originates from highly accessible Co nanoparticles/Co-Nx active sites at the external surface structure (this is, exposing active sites). The thus-assembled liquid zinc-air battery using the synthesized electrocatalyst as the cathode material delivers a maximum power density of 178 mW cm-2 with an open circuit potential of 1.48 V and long-term discharge stability over 150 h.

Inverse kinetic isotope effects in the oxygen reduction reaction at platinum single crystals

Although the oxygen reduction reaction (ORR) involves multiple proton-coupled electron transfer processes, early studies reported the absence of kinetic isotope effects (KIEs) on polycrystalline platinum, probably due to the use of unpurified D₂O. Here we developed a methodology to prepare ultra-pure D₂O, which is indispensable for reliably investigating extremely surface-sensitive platinum single crystals. We find that Pt(111) exhibits much higher ORR activity in D₂O than in H₂O, with potential-dependent inverse KIEs of ~0.5, whereas Pt(100) and Pt(110) exhibit potential-independent inverse KIEs of ~0.8. Such inverse KIEs are closely correlated to the lower *OD coverage and weakened *OD binding strength relative to *OH, which, based on theoretical calculations, are attributed to the differences in their zero-point energies. This study suggests that the competing adsorption between *OH/*OD and *O₂ probably plays an important role in the ORR rate-determining steps that involve a chemical step preceding an electrochemical step (CE mechanism).

Revealing the role of ionic liquids in promoting fuel cell catalysts reactivity and durability

Ionic liquids (ILs) have shown to be promising additives to the catalyst layer to enhance oxygen reduction reaction in polymer electrolyte fuel cells. However, fundamental understanding of their role in complex catalyst layers in practically relevant membrane electrode assembly environment is needed for rational design of highly durable and active platinum-based catalysts. Here we explore three imidazolium-derived ionic liquids, selected for their high proton conductivity and oxygen solubility, and incorporate them into high surface area carbon black support. Further, we establish a correlation between the physical properties and electrochemical performance of the ionic liquid-modified catalysts by providing direct evidence of ionic liquids role in altering hydrophilic/hydrophobic interactions within the catalyst layer interface. The resulting catalyst with optimized interface design achieved a high mass activity of 347 A g-1Pt at 0.9 V under H2/O2, power density of 0.909 W cm-2 under H2/air and 1.5 bar, and had only 0.11 V potential decrease at 0.8 A cm-2 after 30 k accelerated stress test cycles. This performance stems from substantial enhancement in Pt utilization, which is buried inside the mesopores and is now accessible due to ILs addition.

Preparation of high-precision CO2 with known triple oxygen isotope for oxygen isotope analysis

The objective of this work is to propose a more effective way to prepare an in-house CO₂ with known triple oxygen isotope compositions. The major experimental steps include: (1) the O₂ is combusted to CO₂ on a graphite rod at 750 °C with Pt-catalyst for 3-4 min; and (2) converted CO₂ is subsequently purified by two cryogenic traps. The results show high reproducibility of δ¹³C and δ¹⁸O values of the converted CO₂ within 0.010-0.020 ‰ and 0.006-0.010 ‰ (1σ, SD), and the identical δ¹⁸O value within error with that of the original O₂. Additionally, we have measured the triple oxygen isotope compositions of converted CO₂ using an O₂-CO₂ Pt-catalyzed oxygen-isotope equilibration method. The measured δ¹⁷O values of CO₂ show high reproducibility within 0.006 ‰ (1σ, SD), and are identical within error with the original O₂ as well. Notably, our experiments also found that the O₂ with heavier oxygen isotope ratios (δ¹⁸O > 40 ‰, VSMOW) might have a lesser conversion efficiency, and this effect, combined with the lighter isotope preferential fractionations during the reaction processes of O₂ to CO and CO to CO₂, may explain the observed lower ¹⁷O/¹⁶O and ¹⁸O/¹⁶O ratios of the converted CO₂ relative to the original O₂.