Oxygen electrodes are playing a key role in electrochemical energy conversion devices such as fuel cells and water electrolyzers. In both acidic and alkaline environment, both the oxygen reduction and oxygen evolution reaction (ORR and OER), respectively, are limiting the overall energy/voltage efficiency due to its sluggish kinetics. [1, 2]
Whereas in acidic environment, mainly precious metals are used to catalyze the ORR (e.g., Pt or its alloys) or the OER (e.g., IrO₂ ), the variety of possible catalysts in alkaline electrolyte is significantly increased and also many metal oxide based systems can be employed. Generally the oxygen reduction or evolution mechanisms are only partly understood independent of the electrolyte environment and material used.
In this contribution, some new light will be shed on the electrocatalysis of oxide based systems used in acidic and alkaline environment for the ORR and OER. Similarities and differences as compared to noble metal based systems will be provided supplemented with theoretical considerations.
References
[1] A. Rabis, P. Rodriguez, T.J. Schmidt, ACS Catal., 2012, 2 (5), 864-890
[2] E. Fabbri, A. Habereder, K. Waltar, R. Kötz, T.J. Schmidt, Cat. Sci. Tech., 2014, 4, 3800-3821

Electrochemical water splitting driven by a renewable energy source such as wind or solar, is a promising method to produce hydrogen. Already an important chemical commodity in industry, hydrogen may also potentially be used as a carbon-free fuel. However, the efficiency of electrochemical water splitting is severely limited by the high overpotential costs required for the oxygen evolution half reaction (OER). In order to develop active nonprecious metal-based electrocatalysts for the OER, a better understanding of the activity of transition metal catalysts is needed. Previous studies have shown that depositing transition metal catalysts on metal supports leads to formation of different oxide phases and significantly higher activities.^{1, 2}
In this work we investigate the interaction between manganese oxide (MnO_x) and noble metals to determine the effect on oxidation state and catalytic activity. Beginning with a study on the interaction of MnO_x with gold (Au), we characterize the catalyst using SEM and ex situ L-edge x-ray absorption spectroscopy (XAS) to determine the morphology and oxidation state.³ As a surface sensitive technique, ex situ XAS provides information on the oxidation state of the surface atoms of the MnO_x nanoparticles both alone and in the presence of Au. Electrochemical characterization of this system shows that adding Au to MnO_x greatly increases the activity for the OER and results in an order of magnitude higher turnover frequency compared to MnO_x without Au. Finally, in situ XAS studies provide insight into the oxidation state of the MnO_x both with and without Au under OER operating conditions. Expanding this study to characterize the interaction of MnO_x with other noble metals provides insight to trends with respect to the effect on OER activity.
REFERENCES:
1. Yeo, B.S. & Bell, A.T. Enhanced Activity of Gold-Supported Cobalt Oxide for the Electrochemical Evolution of Oxygen. J Am Chem Soc133, 5587-5593 (2011).
2. Yeo, B.S. & Bell, A.T. In Situ Raman Study of Nickel Oxide and Gold-Supported Nickel Oxide Catalysts for the Electrochemical Evolution of Oxygen. The Journal of Physical Chemistry C116, 8394-8400 (2012).
3. Gorlin, Y. et al. Understanding Interactions between Manganese Oxide and Gold That Lead to Enhanced Activity for Electrocatalytic Water Oxidation. J Am Chem Soc136 (2014).

The oxygen evolution reaction (OER) is regarded as a major bottleneck in the overall water splitting process due to the slow transfer rate of four electrons and the high activation energy barrier for O-O bond formation. For the decades, Ir, Ru and Pt based inorganic materials have presented efficient catalytic activity with high turnover frequency (TOF) under mild conditions. However, its scarcity and high cost still inspire to develop inexpensive and sustainable catalysts. The cubane Mn₄CaO₅ clusters inside biological photosystem II (PS II) can oxidize water using a much smaller overpotential than required by manmade catalysts. Inspired by this attractive feature of Mn₄CaO₅ cluster, intense effort has been dedicated to develop non-precious metal-based water oxidation catalysts. Unfortunately, except for several rare-earth metal-based catalysts, there have been few candidate catalysts that operate under near neutral condition until now.
In this study, we discovered a new Mn-based catalyst that efficiently performs water oxidation catalysis. For conventional manganese oxide catalysts, up to now, the significantly reduced activity under neutral conditions is an unresolved issues. The instability of Mn (III) species at neutral pH is considered as a critical factor in this degradation. Here, we made the breakthrough of a Mn-based water oxidation catalysis that operates under neutral conditions using well-designed monodisperse manganese oxide nanoparticles. Facile surface treatment method was newly adopted and oxidized Mn(III) species were intentionally generated and stabilized on the manganese oxide surface. Various electrochemical methods and in-situ XAS analysis revealed the superior catalytic performance of partially oxidized manganese oxide NCs.

Complex perovskite oxides are considered as the promising candidates of bifunctional catalysts for high catalytic oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). As another class of oxide-based catalyst, Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-d (BSCF5582) is a widely known perovskite oxide as a strong candidate of catalyst in solid oxide fuel cell (SOFC) cathode materials, metal-air batteries and other energy related systems. We report herein the heat treating effect of the BSCF5582 in oxygen atmosphere at 950 °C as a function of annealing time on the electrocatalytic performance, compared with pristine BSCF5582. The pristine BSCF5582 sample was calcined at 1050 °C for 5h in air (BSCF5582), and the heated BSCF5582 sample was prepared by heat-heating the pristine BSCF5582 in oxygen atmosphere at 950°C as a function of annealing time (O₂-BSCF5582). The heat treatment effect of the complex Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-d (BSCF5582) perovskite in oxygen atmosphere at 950°C (O₂-BSCF5582) on the electrocatalytic performances of ORR and OER was investigated. During heat-treatment in oxygen atmosphere, the crystallinity of the overall cubic perovskite structure was enhanced, and, most of all, a nanoscale thick surface layer, which is located between ~30 nm thick amorphous surface layer and particle matrix, was not any more observed. The electrocatalytic performance of BSCF5582 catalyst both in oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) was improved significantly. The mechanism of such microstructural- and electrochemical- improvement was explained by correlating with the effect of monitoring the surface chemistry and structure in this paper.

Perovskite (ABO₃) based mixed metal oxides were widely studied over last several decades as electrocatalysts for high-temperature fuel cell electrodes due to their mixed electronic/ionic conductivities at high temperatures (>700^oC). The perovskites formed of lanthanide (at A-site)-transition element (on B-site) metal complex have been recognized as efficient electrocatalysts for Oxygen Reduction Reactions (ORR) and/or Oxygen Evolution Reactions (ORR) in alkaline electrolyte. However, the electrochemical performance and stability of these catalysts depends on the choice of the type of A- and B-cations. Additionally, oxygen deficiencies created in mixed perovskite-phase (AA&’BB&’O_3-δ) by partial substitution of less valent cations on A- and/or B-site are believed to contribute significantly towards ORR/OER due to their oxygen-exchange redox behavior. In this regard, mixed perovskites formed by praseodymium (Pr) and samarium (Sm) based nickel-cobaltites such as PrNi_xCo_1-xO_3-δ and SmNi_xCo_1-xO_3-δ were considered as electrocatalysts for ORR and OER.
Praseodymium and samarium based mixed-perovskites with x= 0.1, 0.5, 0.9 have been synthesized using a modified nitrate-glycine Pechini method and heat-treated in air at 900°C and 1200°C. X-ray diffraction studies of the resultant materials show that a combination of mixed metal oxide structures such as perovskite, layered Ruddleston-Popper (A₂BO_4-δ) and spinel (ABO₃) depend on heat-treatment temperatures and relative composition of Ni and Co. For example, spinel and individual metal oxides were yielded at x=0.1, whereas a combination of perovskite, layered Ruddleston-Popper phases were developed at x=05 and 0.9. A combination of Temperature Programmed Reduction (TPR) and Thermo-Gravimetric (TG) analysis was further employed to understand the structural changes and accurate oxygen content in these materials as a function of the heat-treatment temperature. Scanning Electron Microscopy (SEM) and BET