Nanostructured
M
nx
O
y as catalyst for Oxygen Reduction Reaction
Authors: Alejandro Muñoz Tabares Jose , Salvador Gian Paolo , Delmondo Luisa , Gerosa Matteo , Sacco Adriano , Garino Nadia , Massaglia Giulia , Quaglio Marzia , Pirri Candido Fabrizio , Chiodoni Angelica 
Published: December 2016

Nowadays the development of new and clean energy resources represents one of the major scientific challenges, due to the growing concerns about global warming. The electrochemical Oxygen Reduction Reaction (ORR) occurring at the cathode of fuel cells and metal‐air batteries is one of the key limits for their further development and requires electrocatalysts to increase its efficiency. Manganese oxides are among the most interesting non‐precious metal‐based catalysts due to their low cost, relatively high abundance, low environmental impact and considerable electrocatalytic activity.

In this work we present nanostructured manganese oxides in the form of xerogels (obtained by means of the sol‐gel plus freeze‐drying techniques) and in the form of nanofibers (obtained by means of the electrospinning technique). They were synthesized by employing manganese acetate as the Mn source and by employing environmental friendly (water is the used solvent) templating agents, such as agar and polyethylene oxide, for xerogels and nanofibers respectively.

To investigate the oxidation process forming the manganese oxides species, structural and morphological characterizations (
in‐situ
X‐ray diffraction, field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM)) were performed on both the nanostructures.

Concerning manganese oxide xerogels,
in situ
X‐ray diffractograms were recorded while heating in air the as‐prepared material from room temperature up to 725 °C, at a heating rate of 2.5°C/min. Figure (1a) shows the phase evolution of the xerogel as a function of the temperature. From this image it can be observed that Mn
3
O
4
nucleates and grows at temperatures between 175 °C and 475 °C, temperature at which the α‐Mn
2
O
3
phase starts to evolve; the latter completes its formation at 700 °C approximately, while Mn
3
O
4
is progressively disappearing. From Rietveld refinements of all diffractograms, major differences were found in the crystal size of the two phases, where the average size for Mn
3
O
4
was 21± 1 nm, while it was 65.8±0.8 nm for α‐Mn
2
O
3
.

By means of bright field TEM and selected area electron diffraction (SAED) techniques, two different morphologies were observed, i.e. nanosized grains and large dendritic structures, composed of rectangular domains of less than 100 nm (Figure 2). The SAED pattern is composed by two different patterns, that are clearly distinguished. The first corresponds to complete and low‐intensity rings, which were indexed as Mn
3
O
4
, and that could,
a priori
, be assigned to the nanosized grains. The second pattern of bright spots was assigned to the large dendritic structure, and indexed as Mn
2
O
3
along the [201] axis zone. The TEM and SAED results are in agreement with the Rietveld analysis.

Regarding manganese oxide nanofibers,
in‐situ
X‐ray diffractograms were recorded while heating in air the as‐prepared material from room temperature up to 725 °C, at a heating rate of 2.5°C/min. Figure (1b) shows the phase evolution of the nanofibers as a function of the temperature. From this image is observed that Mn
3
O
4
starts to nucleate and grow at temperatures between 125 °C and 575 °C, temperature at which the α‐Mn
2
O
3
phase start to evolve, while Mn
3
O
4
is progressively disappearing.

From FESEM and bright field TEM characterizations (Figure 3), it is evident that the nanofibers are composed by nanosized grains of few nanometers.

The catalytic performance of the two nanostructured catalysts were characterized by means of the rotating ring disk electrode technique by using 4‐electrodes measurements. Both xerogels and nanofibers showed extremely good performances for the oxygen reduction reaction, with
n
values between 3.5 and 3.7, meaning a predominant 4‐electrons reduction pathway.

In conclusion, nanostructured xerogels and nanofibers of manganese oxides were prepared through the sol‐gel plus freeze‐drying and the electrospinning techniques respectively, and by using biocompatible and biodegradable precursors. The obtained materials were composed by Mn
3
O
4
, Mn
2
O
3
, or a mix of both, depending on the temperature. Both materials showed good catalytic performance for the ORR, making them good candidates as low‐cost and green catalysts for applications in electrochemical devices.