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Transition metal oxides are attractive noble metal-free catalysts of the oxygen reduction for application at the cathode of alkaline membrane fuel cells or metal-air batteries. However, despite of a rapidly increasing number of publications devoted to the oxygen electrocatalysis on transition metal oxides, a clear picture regarding the relations between their structure and composition on the one hand and electrocatalytic activity on the other hand is lacking. This short review discusses challenges facing researchers seeking to understand electrocatalysis of the oxygen reduction reaction on transition metal oxides.

Manganese oxides are known as active catalysts for the oxygen reduction reaction (ORR) in alkaline media. Being poor electronic conductors, for electrochemical applications, most of Mn oxides are either mixed with or supported on carbon materials, which impart electronic conductivity to the composite electrodes. A number of recent publications reported an immense influence of carbon materials on the electrocatalytic activity of transition metal oxides in the ORR. To better understand the role of carbon, in this work we utilize a wide variety of carbon materials and investigate the ORR on Mn 2 O 3 /carbon composites combining experiment with kinetic modeling. Such an approach allows us to discriminate the contribution of carbon in the ORR activity of composites, underscoring its positive role in improving the oxide utilization, and acting as the ORR co-catalyst, but also its negative role in competing for the adsorption of O 2 . By juxtaposing the electrocatalytic activity with the structural and morphological characteristics carbon materials, we propose some rules of thumb for choosing carbons necessary for creating efficient oxide/carbon ORR electrocatalysts.

We consider compositional and structural factors which can affect the activity of bixbyite α-Mn2O3 towards the oxygen reduction reaction (ORR) and the stability of this oxide in alkaline solution. We compare electrochemistry of undoped, Fe and Al-doped α-Mn2O3 with bixbyite structure and braunite Mn7SiO12 having bixbyite-related crystal structure, using the rotating disk electrode (RDE), the rotating ring-disk electrode (RRDE), and cyclic voltammetry (CV) techniques. All manganese oxides under study are stable in the potential range between the ORR onset and ca. 0.7 V vs. Reversible Hydrogen Electrode (RHE). It is found that any changes introduced in the bixbyite structure and/or composition of α-Mn2O3 lead to an activity drop in both the oxygen reduction and hydrogen peroxide reactions in this potential interval. For the hydrogen peroxide reduction reaction these modifications also result in a change in the nature of the rate-determining step. The obtained results confirm that due to its unique crystalline structure undoped α-Mn2O3 is the most ORR active (among currently available) Mn oxide catalyst and favor the assumption of the key role of the (111) surface of α-Mn2O3 in the very high activity of this material towards the ORR.

Hydrogen peroxide has been detected as a reaction intermediate in the electrochemical oxygen reduction reaction (ORR) on transition-metal oxides and other electrode materials. In this work, we studied the electrocatalytic and catalytic reactions of hydrogen peroxide on a set of Mn oxides, Mn2O3, MnOOH, LaMnO3, MnO2, and Mn3O4, that adopt different crystal structures to shed light on the mechanism of the ORR on these materials. We then combined experiment with kinetic modeling with the objective to correlate the differences in the ORR activity to the kinetics of the elementary reaction steps, and we uncovered the importance of structural and compositional factors in the catalytic activity of the Mn oxides. We concluded that the exceptional activity of Mn2O3 in the ORR is due to its high catalytic activity both in the reduction of oxygen to hydrogen peroxide and in the decomposition of the latter, and furthermore, we proposed a tentative link between crystal structure and reactivity.

Knowledge on the mechanisms of oxygen reduction reaction (ORR) and descriptors linking the catalytic activity to the structural and electronic properties of transition metal oxides enable rational design of more efficient catalysts. In this work ORR electrocatalysis was studied on a set of single and complex Mn (III) oxides with a rotating disc electrode method and cyclic voltammetry. We discovered an exponential increase of the specific electrocatalytic activity with the potential of the surface Mn(IV)/Mn(III) red-ox couple, suggesting the latter as a new descriptor for the ORR electrocatalysis. The observed dependence is rationalized using a simple mean-field kinetic model considering availability of the Mn( III) centers and adsorbate-adsorbate interactions. We demonstrate an unprecedented activity of Mn2O3, ca. 40 times exceeding that of MnOOH and correlate the catalytic activity of Mn oxides to their crystal structure. (C) 2015 Elsevier Ltd. All rights reserved.