Highly Graphitized Fe-N-C Electrocatalysts Prepared from Chitosan Hydrogel Frameworks

Abstract

The development of platinum group metal-free (PGM-free) electrocatalysts derived from cheap and environmentally friendly biomasses for oxygen reduction reaction (ORR) is a topic of relevant interest, particularly from the point of view of sustainability. Fe-nitrogen-doped carbon materials (Fe-N-C) have attracted particular interest as alternative to Pt-based materials, due to the high activity and selectivity of Fe-Nx active sites, the high availability and good tolerance to poisoning. Recently, many studies focused on developing synthetic strategies, which could transform N-containing biomasses into N-doped carbons. In this paper, chitosan was employed as a suitable N-containing biomass for preparing Fe-N-C catalyst in virtue of its high N content (7.1%) and unique chemical structure. Moreover, the major application of chitosan is based on its ability to strongly coordinate metal ions, a precondition for the formation of Fe-Nx active sites. The synthesis of Fe-N-C consists in a double step thermochemical conversion of a dried chitosan hydrogel. In acidic aqueous solution, the preparation of physical cross-linked hydrogel allows to obtain sophisticated organization, which assure an optimal mesoporosity before and after the pyrolysis. After the second thermal treatment at 900 °C, a highly graphitized material was obtained, which has been fully characterized in terms of textural, morphological and chemical properties. RRDE technique was used for understanding the activity and the selectivity of the material versus the ORR in 0.5 M H2SO4 electrolyte. Special attention was put in the determination of the active site density according to nitrite electrochemical reduction measurements. It was clearly established that the catalytic activity expressed as half wave potential linearly scales with the number of Fe-Nx sites. It was also established that the addition of the iron precursor after the first pyrolysis step leads to an increased activity due to both an increased number of active sites and of a hierarchical structure, which improves the access to active sites. At the same time, the increased graphitization degree, and a reduced density of pyrrolic nitrogen groups are helpful to increase the selectivity toward the 4e- ORR pathway.

Synergistic melamine intercalation and Zn(NO3)2 activation of N-doped porous carbon supported Fe/Fe3O4 for efficient electrocatalytic oxygen reduction

“…Moreover, the intensity ratio of D and G band ( I D / I G ) is a vital index to evaluate defect and graphitization degree of the carbon materials. 47 It can be calculated that the value of I D / I G of Fe/Fe 3 O 4 @NC, Fe/Fe 3 O 4 @NC (Zn-free), NC (Fe-free), Fe/Fe 3 O 4 @C (N-free) and Fe 3 O 4 @NC are 0.98, 1.05, 0.96, 1.11 and 1.01, respectively. As a consequence, the graphitization degree of Fe/Fe 3 O 4 @NC is the highest compared with other iron-containing materials, indicating that the addition of zinc nitrate and melamine obviously promotes the formation of graphitization and defect structures of carbon materials, thus endowing these catalysts with good conductivity and stability, which is conducive to improve the electron transfer efficiency in the electrocatalytic ORR process.…”

Sulfur Doping versus Hierarchical Pore Structure: The Dominating Effect on the Fe–N–C Site Density, Activity, and Selectivity in Oxygen Reduction Reaction Electrocatalysis

“…The dispersion of the powder was ensured using a bath sonicator at a controlled temperature. The loading was chosen to be 0.6 mg cm –2 as used in previous works. , …”

“…Despite the economic and technological problems related to the production, transport, and storage of hydrogen, the main FCs problem is on a different aspect: the high cost due to the low kinetic of the cathode reaction, the oxygen reduction reaction (ORR), and thus, the usage of Pt-based materials as catalysts is still required. − With their low cost, high availability, and good tolerance to poisoning, non-precious-metal catalysts (non-PGM) are the best known alternative to Pt. − During past decades, various non-PGM catalysts were investigated: M–N–C based on M–N x sites, non-precious-metal oxide, chalcogenides, and oxynitrides . The most studied are M–N–C, and among them, the most active metal center is Fe, where iron coordinate from two to five nitrogen functional groups, and among the different types of Fe–N x ( x = 1–5), the metal porphyrin-like Fe–N 4 site is considered the most important for its ORR selectivity and activity. − However, different factors need to be considered to reach good performances, including site density, carbon support hierarchical structure, surface chemistry, graphitization degree, etc. − Choosing the right carbon matrix is the turning point to improve catalytic performance; in fact, the increment of the active SD is per se not enough to enhance the activity, but it is necessary to rationally design the textural and porous properties of the carbon matrix to facilitate the mass transport between micropores and the bulk solution. ,, Moreover, it has been also demonstrated that the incorporation of heteroatoms can influence the catalytic performances . The idea underlying the doping process is the capability of heteroatoms to modulate the electronic structure of the carbon plane via the delocalization of the π-electrons when pinned into the carbon framework, improving the catalyst activity .…”

“…The aim of this paper is to understand whether S doping can indeed produce measurable improvements in the formation of Fe–N x active sites, in the catalytic activity and selectivity of the resulting material, and whether this effect prevails on other material properties such as the hierarchical pore structure or the graphitization degree, or even if the improvement is due to indirect effect of sulfur on other properties of materials. To do so, here, we employed S-doped mesoporous carbon (SMC) prepared by hard template approach , and iron phenanthroline that was used as N and Fe precursor for the formation of Fe–N–C sites. , …”

Oxygen Reduction Reaction at Single‐Site Catalysts: A Combined Electrochemical Scanning Tunnelling Microscopy and DFT Investigation on Iron Octaethylporphyrin Chloride on HOPG**

“…Nitrogen atoms not only strongly anchor individual metal centres but also modify the electronic properties of the carbon material, thus altering the catalytic activity. Ternary M−N−C materials (where, for example, M=Co or Fe), which feature atomically dispersed M centres bonded to neighbouring N atoms, have been intensely investigated as promising candidates to replace Pt in a variety of electrocatalytic reactions, such as the O 2 and CO 2 reduction reactions [3–6] . Notwithstanding the high research interest, atomically dispersed M−N−C materials have been systematically synthetized and investigated only very recently.…”