Facile Metal Coordination of Active Site Imprinted Nitrogen Doped Carbons for the Conservative Preparation of Non‐Noble Metal Oxygen Reduction Electrocatalysts

Abstract

of platinum-based catalysts at the PEFC cathode to accelerate the otherwise sluggish kinetics of the oxygen reduction reaction (ORR). Thus, the development of inexpensive noble metal-free catalysts for the ORR is one of the key research topics for future fuel cell designs. Among various types of non-noble metal catalysts reported so far, one promising class is metal-nitrogen-carbon (Me-N x -C y ) type materials in which transition metals, particularly Fe and Co, are bound to nitrogen doped carbon (NDC). The early reports on the application of such materials date back to the sixties of the last century when Jasinski demonstrated that N-coordinated transition metals can be active sites for the ORR. [1] Similarly, the ORR-activity of metal-free NDCs was shown based on experimental and theoretical results. [2] The moderate acidic performance of NDCs can be enhanced by introducing non-noble metal ions such as iron or cobalt. [3] The research on such non-noble metal catalysts was greatly promoted by the groundbreaking results of competitive activities compared to standard Pt-based catalysts obtained by Lefèvre et al. in 2009. [4] A porous activated carbon was used as a conductive backbone and mixed with phenanthroline as nitrogen and iron(II) acetate as iron source. After heat treatment, first in inert and second in NH 3 -atmosphere, a very active ORR-catalyst in acidic medium was obtained. Later, Wu et al. reported highly active and stable ORR electrocatalysts Iron-or cobalt-coordinated heteroatom doped carbons are promising alternatives for Pt-based cathode catalysts in polymer-electrolyte fuel cells. Currently, these catalysts are obtained at high temperatures. The reaction conditions complicate the selective and concentrated formation of metal-nitrogen active sites. Herein a mild procedure is introduced, which is conservative toward the carbon support and leads to active-site formation at low temperatures in a wet-chemical metal-coordination step. Active-site imprinted nitrogen doped carbons are synthesized via ionothermal carbonization employing Lewis-acidic Mg 2+ salt. The obtained carbons with large tubular porosity and imprinted N 4 sites lead to very active catalysts with a half-wave potential (E 1/2 ) of up to 0.76 V versus RHE in acidic electrolyte after coordination with iron. The catalyst shows 4e − selectivity and exceptional stability with a half-wave potential shift of only 5 mV after 1000 cycles. The X-ray absorption fine structure as well as the X-ray absorption near edge structure profiles of the most active catalyst closely match that of iron(II)phthalocyanine, proving the formation of active and stable FeN 4 sites at 80 °C. Metal-coordination with other transition metals reveals that Zn-N x sites are inactive, while cobalt gives rise to a strong performance increase even at very low concentrations.

Active‐Site Imprinting: Preparation of Fe–N–C Catalysts from Zinc Ion–Templated Ionothermal Nitrogen‐Doped Carbons

“…Fe 2+ was coordinated to 60Li_800(Zn 2+ ) according to our previous procedure, with the difference of using higher Fe 2+ concentrations. [16] Fe 3+ was coordinated using a novel molten salt approach. The coordination was carried out in a FeCl 3 /LiCl mixture at 170 °C (T m = 150 °C).…”

“…[15] The preparation of highly porous Mg-N-C was followed by a low-temperature wet-chemical coordination step to form square-planar FeN 4 sites by either metalation (Equation (1)), transmetalation (Equation (2)), or both. [16] By which reaction the active sites are generated remains an open question to the scientific community.…”

Synthesis and Active Site Identification of Fe−N−C Single‐Atom Catalysts for the Oxygen Reduction Reaction

“…Iron centers were incorporated by stirring the MgN 4 /N 4 imprinted NDC in refluxing FeCl 2 solution for 24 h. The Fe 2+ replaced the Mg 2+ ion due to the formation of more stable Fe complex, or directly coordinate to the free “N x ” sites (Figure C). Nevertheless, it remained unclear whether metalation or transmetalation was directly relevant to the formation of FeN 4 sites …”

“…Nevertheless, it remained unclear whether metalation or transmetalation was directly relevant to the formation of FeN 4 sites. [49]…”

From Molecular Precursors to Nanoparticles—Tailoring the Adsorption Properties of Porous Carbon Materials by Controlled Chemical Functionalization

“…[30a] One approach for enhancing the properties of catalysts for oxygen reduction reaction (ORR) is including biomimetic porphyrine-like structures as the catalytically active site. [31] Mehmood et al used this strategy to embed iron into a nitrogen-doped carbon (NDC) framework, thus using NDCs as solid-state ligands and conductive supports at the same time. [31a] These and many other (bioinspired or non-bioinspired) examples represent the huge potential of chemical functionalization of NCMs for tuning their interaction strength with different guest species.…”