How key characteristics of carbon materials influence the ORR activity of LaMnO3- and Mn3O4-carbon composites prepared by in situ autocombustion method

Enhancing Interaction between Lanthanum Manganese Cobalt Oxide and Carbon Black through Different Approaches for Primary Zn-Air Batteries

Due to the need for decarbonization in energy generation, it is necessary to develop electrocatalysts for the oxygen reduction reaction (ORR), a key process in energy generation systems such as fuel cells and metal-air batteries. Perovskite-carbon material composites have emerged as active and stable electrocatalysts for the ORR, and the interaction between both components is a crucial aspect for electrocatalytic activity. This work explores different mixing methods for composite preparation, including mortar mixing, ball milling, and hydrothermal and thermal treatments. Hydrothermal treatment combined with ball milling resulted in the most favorable electrocatalytic performance, promoting intimate and extensive contact between the perovskite and carbon material and improving electrocatalytic activity. Employing X-ray photoelectron spectroscopy (XPS), an increase in the number of M-O-C species was observed, indicating enhanced interaction between the perovskite and the carbon material due to the adopted mixing methods. This finding was further corroborated by temperature-programmed reduction (TPR) and temperature-programmed desorption (TPD) techniques. Interestingly, the ball milling method results in similar performance to the hydrothermal method in the zinc-air battery and, thus, is preferable because of the ease and straightforward scalability of the preparation process.

Photocatalytic Synthesis of Ultrafine Pt Electrocatalysts with High Stability Using TiO2 -Decorated N-Doped Carbon as Composite Support

Commercial Pt/C (Com. Pt/C) electrocatalysts are considered optimal for oxygen reduction and hydrogen evolution reactions (ORR and HER). However, their high Pt content and poor stability restrict their large-scale application. In this study, photocatalytic synthesis was used to reduce ultrafine Pt nanoparticles in-situ on a composite support of TiO2 -decorated nitrogen-doped carbon (TiO2 -NC). The nitrogen-doped carbon had a large surface area and electronic effects that ensured the uniform dispersion of TiO2 nanoparticles to form a highly photoactive and stable support. TiO2 -NC served as a composite support that enhanced the dispersibility and stability of ultrafine Pt electrocatalyst, owing to the presence of N sites and the strong metal-support interaction. Relative to Com. Pt/C, the as-obtained Pt/TiO2 -NC had positive shifts of 44 and 10 mV in the ORR half-wave potential and HER overpotential at -10 mA cm-2 , respectively. After an accelerated durability test, Pt/TiO2 -NC had lower losses in electrochemical specific area (0.7 %) and electrocatalytic activity (0 mV shift) than Com. Pt/C (25.6 %, 22 mV shift). These results indicate that the developed strategy enabled the facile synthesis and stabilization of ultrafine Pt nanoparticles, which improved the utilization efficiency and long-term stability of Pt-based electrocatalysts.

Transition metal oxides with perovskite and spinel structures for electrochemical energy production applications

Transition metal oxide-based materials are an interesting alternative to substitute noble-metal based catalyst in energy conversion devices designed for oxygen reduction (ORR), oxygen evolution (OER) and hydrogen evolution reactions (HER). Perovskite (ABO₃) and spinel (AB₂O₄) oxides stand out against other structures due to the possibility of tailoring their chemical composition and, consequently, their properties. Particularly, the electrocatalytic performance of these materials depends on features such as chemical composition, crystal structure, nanostructure, cation substitution level, e_g orbital filling or oxygen vacancies. However, they suffer from low electrical conductivity and surface area, which affects the catalytic response. To mitigate these drawbacks, they have been combined with carbon materials (e.g. carbon black, carbon nanotubes, activated carbon, and graphene) that positively influence the overall catalytic activity. This review provides an overview on tunable perovskites (mainly lanthanum-based) and spinels featuring 3d metal cations such as Mn, Fe, Co, Ni and Cu on octahedral sites, which are known to be active for the electrochemical energy conversion.

Facile synthesis of multiphase cobalt-iron spinel with enriched oxygen vacancies as a bifunctional oxygen electrocatalyst

The multiphase cobalt-iron spinel was firstly synthesized via a facile cold plasma method and applied as a bifunctional electrocatalyst for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Compared with the single-phase obtained by the traditional calcination method, the CoFe₂O₄ and Co₃O₄ phase were obtained by the plasma method. The multivalence states of cobalt and iron facilitated electron transport in electrochemical reactions. The plasma sample had a small particle size (5 nm) due to the low operation temperature. Notably, electron impact produced more oxygen vacancies and a larger surface area on Co_xFe_yO₄, which increased the active sites and electronic conductivity. Electrochemical investigations indicated that the multiphase spinel obtained with a quasi-four-electron transfer process showed an onset potential of 0.76 V versus the RHE for the oxygen reduction reaction. In the oxygen evolution reaction, the potential of current density at 10 mA cm^-2 was 1.53 V versus RHE. As for the overall electrocatalytic activity, the multiphase spinel had a ΔE (the difference between E₁₀(OER) and E_1/2(ORR)) of 0.89 V, exhibiting greater bifunctional activity than the other prepared catalysts.

In Situ Activation of a Manganese Perovskite Oxygen Reduction Catalyst in Concentrated Alkaline Media

The reaction pathway of the oxygen reduction reaction (ORR) is strongly affected by the electrolytic environment. Meanwhile, the ORR mechanism on transition-metal oxide catalysts has not been studied intensely in very concentrated alkaline solutions that are used in practical metal-air batteries. Herein, we report the in situ activation of ORR catalysis on manganese perovskite in a concentrated alkaline solution, mediated by the spontaneous formation of oxygen vacancy sites. Electrochemical analyses of the (100) epitaxial film electrodes reveal that the exchange current and electron number of the ORR on La_0.7Sr_0.3Mn_0.9Ni_0.1O₃ significantly increase with the duration of the ORR when the KOH concentration is greater than 4 M. However, these values remain unchanged with time at less than 1 M KOH concentration. Operando synchrotron X-ray spectroscopy of the (100) epitaxial film confirmed that La_0.7Sr_0.3Mn_0.9Ni_0.1O₃ involves the oxygen vacancy sites with the reduction of Mn atoms in concentrated KOH solution via the hydroxylation decomposition of perhydroxyl intermediates. Hence, the O₂ adsorption switched from an end-on to a bidentate mode because the cooperative active sites of the oxygen vacancy and neighboring Mn allow bidentate adsorption of the dissolved O₂. Due to the simultaneous interaction with the oxygen vacancy and Mn sites, the O-O bonds are activated and the potential barrier for the electron transfer to adsorbed O₂ is lowered, resulting in a shift in the reaction mechanism from that involving an indirect "2 + 2" transfer pathway to a direct 4-electron pathway.