Electrodeposition and pyrolysis of Mn/polypyrrole nanocomposites: a study based on soft X-ray absorption, fluorescence and photoelectron microspectroscopies
Novel non-noble metal/polypyrole composites have properties promising to be considered as substitutes of expensive platinum catalysts for the Oxygen Reduction Reaction (ORR) in energy devices, in particular in alkaline fuel cells. Following revelation the catalytic activity of cobalt phthalocyanine and the beneficial effect of pyrolysis at 400–800 °C on its stability and catalytic properties, considerable efforts have been made to explore the chemo-morphological transformations of non-precious metal-nitrogen–carbon composites (M/N/C, M¼Co, Fe, Mn, etc.) during pyrolysis in order to shed light on the modifications leading to the increased electrocatalytic activity. Up to date the majority of the results indicate the formation of MeNx-type moieties as active catalytic sites. Among the different strategies to include nitrogen in the catalyst, electrochemistry offers the possibility of using polypyrrole (PPy) with its dual function as a N-source and an electronically conducting catalyst support for the electrodeposition of metal-containing composite materials.
We used a new approach for the fabrication of MnO2/carbon and metal/PPy electrocatalysts that has allowed creation of more Mn/N/C catalytic active sites during the electrodeposition process. The work, coordinated by Prof. Benedetto Bozzini, is collaboration between the Department of Engineering and Innovation of Salento University, The Institute for Microelectronics and Microsystems, IMM-CNR and Elettra Iaboratory. The investigation combined material fabrication and characterization using clasical electrochemistry methods complemented with imaging and microspectroscopy methods available at the scanning microscopes operated at the TwinMic and ESCAMicroscopy beamlines at Elettra laboratory. In particular, at Elettra by using chemical mapping and X-ray absorption (XAS), fluorescence (XRF) and photoelectron XPS) microspectroscopy we were able to shed light on the evolution of both morphology and chemical composition of synthesized materials following in-situ their evolution under different fabrication conditions.
The representative calibrated Mn XRF images in Fig.1 clearly show the laterally inhomogeneous distribution at mesoscopic and submicrometric scales, achieved under low current densities, while the rationed Mn/O pinpoints the variations in the Mn oxidation state. Comparing the contrast levels of the two maps it is clear that the local concentration and chemical state of Mn are not fully correlated.
Figure 1. Representative XRF maps collected in a low density current region. The maps show the distribution of Mn rationed to the scattering signal (A) and to the Oxygen one (B). The images were acquired at 785 eV with 250 nm spot size over an area of 12μm x 12μm.
The N 1s micro-XPS spectra measured during pyrolysis, shown in Fig. 2, contain features matching the N1s components, reported for metal/nitrogen/carbon ORR catalysts. The pristine sample (spectrum a) is characterized by a broad peak with three components: the most intense at 399.7 eV corresponds to pyrrolic N, the shoulder at 398.2 eV point to the presence of pyridinic or metal-bonded nitrogen, and the shoulder at ca. 402 eV is a contribution of polaronic defects in the PPy matrix. Since the N 1s component at 398.2 eV is absent in the XPS spectrum of pure PPy, our results suggest that the chemical state of N is modified by the presence of Mn in the polymer structure. With increasing temperature (spectrum b)), The gradual changes in the N 1s spectrum (spectrum b and c) with increasing temperature entail attenuation of the N-pyrrolic component and intensity gain of the graphitic and pyridinic/Mn–N features, which dominate the final state (spectrum c).
Figure 2. N 1s micro-XPS spectra measured during pyrolysis. a): pristine sample. b): after 7 h heat treatment at 400 °C; c): at the end of the whole pyrolysis protocol, after cooling down to room temperature. The components at 399.7 eV correspond to N-pyrrolic, the shoulders at 398.2 eV indicate the presence of metal-bonded nitrogen, and the other shoulders at ca. 402 eV are contributions of polaronic defects in the PPy matrix.
These results, obtained using a multi-technique approach which combines in situ and ex situ spectroscopic methods with electrochemical measurements, have provided new molecular-level understanding of the hybrid co-electrodeposition process consisting of electropolymerisation of polypyrrole with incorporation of Mn, which can be used for further improvement of the fabrication methodology.
This research was conducted by the following research team:
- Benedetto Bozzini, Patrizia Bocchetta, Dipartimento di Ingegneria dell'Innovazione, Università del Salento, Lecce, Italy.
- Matteo Amati, Alessandra Gianoncelli, Luca Gregoratti, Hikmet Sezen, Maya Kiskinova, Elettra–Sincrotrone Trieste, Trieste, Italy
- Belen Aleman, IMDEA Materials Institute, Madrid, Spain
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Antonietta Taurino, Institute for Microelectronics and Microsystems, IMM-CNR, Lecce, Italy
Contact persons:
B. Bozzini, P. Bocchetta, B. Aleman, M. Amati, A. Gianoncelli, L. Gregoratti, H. Sezen, A. Taurino and M. Kiskinova, “Electrodeposition and pyrolysis of Mn/polypyrrole nanocomposites: a study based on soft X-ray absorption, fluorescence and photoelectron microspectroscopies”, J. Mater. Chem. A 3, 19155 (2015), DOI: 10.1039/c5ta05572e
Benedetto Bozzini, email:
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