Effect of carbonization conditions of polyaniline on its catalytic activity towards ORR. Some insights about the nature of the active sites

Abstract

N-doped carbon materials were obtained using polyaniline (PANI) as precursor. Heat treatment of PANI and de-doped PANI (PANId) was performed using different temperatures-600 and 800 ºC-. Two different atmospheres were used during the treatment: an inert atmosphere (N 2) and another one consisting on a slightly oxidizing mixture of gases (3000 ppm O 2 in N 2). The prepared materials at 800 ºC showed high values of capacitance, up to 170 and 255 F g-1 in basic and acid electrolytes, respectively, in spite of their low surface area. The electrocatalytic activity towards oxygen reduction reaction (ORR) of all materials was studied in basic and acid media. The heat treated materials at 600 ºC did not show a good electrocatalytic activity due to their poor electrical conductivity. On the other hand, heat-treated materials at 800 ºC showed an enhanced catalytic activity due to their higher conductivity and the presence of nitrogen and oxygen functionalities in the carbon surface. Interestingly, the heat treatment at 800 ºC using a slightly oxidant atmosphere produces carbon materials with much higher ORR activity which seems to be related to the larger amount of N-edge and O-edge sites. Preliminary computational studies suggest that the presence of these nitrogen and oxygen functionalities in the vicinities of the carbon atom improves the catalytic performance of N-doped carbon materials in the ORR and that two adjacent active sites can produce the O 2 reduction to H 2 O through a 4 electrons pathway.

Thermal Stability of Epitaxial Graphene Electrodes for Conductive Polymer Nanofiber Devices

“…Comparison of Figure 3a,b at the same area before and after annealing, shows that the overall shape of the nanofibers is retained; however, both the width and the height of PANI nanofibers are significantly reduced to about 50% after annealing (Figure 3c). This is consistent with previous reports that PANI undergoes dehydrogenation and cross-linking of adjacent chains upon high temperature pyrolysis, and that the weight of polyacetylene (PA) films/fibers [49][50][51] and PANI films/tubes [52][53][54][55][56][57][58] is reduced after pyrolysis while retaining the fibril morphology. I-V characteristics of the PANI nanofibers on G/SiC electrodes before annealing show that the adjacent G/SiC leads are electrically connected due to the PANI nanofibers contacting the two adjacent G/SiC electrodes.…”

“…In the pristine form, the Raman spectra of PANI nanofibers show complex peaks that indicates PANI nanofibers. Raman spectroscopy on the annealed PANI nanofiber bundles shows that the PANI nanofibers become amorphous carbon nanofibers as confirmed by the broad D (1353 cm −1 ) and G bands (1590 cm −1 ) of graphite (Figure 4b) [49][50][51][52][53][54][55][56][57][58][59]. In contrast, the G/SiC remained intact after annealing as shown in Figure 4b.…”

“…Together with the enhanced surface to volume ratio in nanofiber morphology, PANI nanofibers are also promising for gas sensing applications [1,47,48]. Besides, the carbonization of polymers by pyrolysis [49][50][51][52][53][54][55][56][57][58] shows potential for applications such as a fuel cell [53] and catalyst [56,57], and PANI produces nitrogen containing conducting carbons after pyrolysis [52][53][54][55][56][57][58]. On the as-fabricated G/SiC electrode, a suspension of solution containing PANI nanofibers were dispersed (see Method) and we observed that fibers readily form an Ohmic contact to graphene electrodes.…”

Towards understanding the active sites for the ORR in N-doped carbon materials through fine-tuning of nitrogen functionalities: an experimental and computational approach

“…Moreover, experimental studies suggest that edge-type graphitic nitrogen 14,15 and pyrrolic nitrogen species 16,17 can also act as active sites. In recent years, N-C-O species 18,19 have also been proposed as active sites towards ORR due to the large positive charge density of the C atom adjacent to N and O atoms.…”

Doped Nanocarbons Derived from Conducting Polymers toward ORR Electrocatalysts

“…In the conventional way, PANI is prepared by aniline monomer, 1 m HCl, and APS under stirring, which leads to the formation of irregular particulates . Quilez‐Bermejo et al reported the synthesis of PANI using the above‐mentioned method, followed by heat treatment of PANI at different temperatures . Two different atmospheres were used during the treatment: an inert atmosphere of N 2 and another one consisting of a slightly oxidizing mixture of gases (3000 ppm O 2 in N 2 ).…”