catalysts Article Electrocatalysis for Oxygen Reduction Reaction on EDTAFeNa and Melamine co-Derived Self-Supported Fe-N-C Materials Mengfan Shen 1,2, Ziwei Meng 2, Tong Xue 2,3, Hongfang Shen 2 and Xiang-Hui Yan 1,2,* 1 Key Laboratory of Powder Material & Advance Ceramics, North Minzu University, Yinchuan 750021, China; [email protected] 2 School of Materials Science and Engineering, North Minzu University, Yinchuan 750021, China; [email protected] (Z.M.); [email protected] (T.X.); [email protected] (H.S.) 3 International Scientific & Technological Cooperation Base of Industrial Waste Recycling and Advanced Materials, North Minzu University, Yinchuan 750021, China * Correspondence: [email protected] and [email protected]; Tel.: +86-951-2067378 Abstract: To explore high-performing alternatives to platinum-based catalysts is highly desirable for lowering costs and thus promoting fuel cell commercialization. Herein, self-supported Fe- N-C materials were prepared by the pyrolysis of dual precursors including EDTA ferric sodium (EDTAFeNa) and melamine (MA), followed by acid-leaching and final annealing. Towards an oxygen reduction reaction (ORR) in 0.1 M KOH, the as-prepared MA/EDTAFeNa-HT2 delivered onset (Eonset) and half-wave (E1/2) potentials of 0.97 and 0.84 V vs. RHE, respectively, identical with that of a state-of-the-art Pt/C catalyst, accompanied with predominantly a four-electron pathway. The introduction of MA and extension of acid-leaching promoted a positive shift of 50 mV for E1/2 relative to that of only the EDTAFeNa-derived counterpart. It was revealed that the enhancement of ORR activity is attributed to a decrease in magnetic Fe species and increase in pyridinic/quanternary nitrogen content whilst nearly excluding effects of the graphitization degree, variety of crystalline Citation: Shen, M.; Meng, Z.; Xue, T.; Shen, H.; Yan, X.-H. Electrocatalysis iron species, and mesoscopic structure. The usage of dual precursors exhibited great potential for the for Oxygen Reduction Reaction on large-scale production of inexpensive and efficient Fe-N-C materials. EDTAFeNa and Melamine co-Derived Self-Supported Fe-N-C Keywords: self-supported Fe-N-C materials; dual precursors; EDTA ferric sodium; melamine; Materials. Catalysts 2021, 11, 623. oxygen reduction reaction https://doi.org/10.3390/catal11050623 Academic Editor: Vincenzo Baglio 1. Introduction Received: 30 March 2021 Owing to a higher over-potential and more complex reaction mechanism for the ca- Accepted: 10 May 2021 thodic oxygen reduction reaction (ORR) than those of the anodic hydrogen oxidation in Published: 12 May 2021 H2-O2 fuel cells during electrocatalysis, the energy conversion efficiency thus primarily depends on its ORR catalysis [1–4]. To pursuit highly efficient Fe-N-C catalysts as alterna- Publisher’s Note: MDPI stays neutral tives for replacing the precious and scarce platinum-based catalysts has been a promising with regard to jurisdictional claims in strategy to reduce the cost and promote the large-scale development of fuel cells [5–7]. published maps and institutional affil- Among various synthetic methods involved in self-supported Fe-N-C catalysts (SSFNCCs), iations. a template-free strategy has gained increasing attention due to fewer preparation proce- dures without the addition of a soft template or removal of a hard template [8–11]. By pyrolyzing a ferrous EDTA chelate, syntheses of SSFNCCs have been reported [12–14]; however, their electrocatalytic performance is still inferior to that of the Pt/C catalyst. Copyright: © 2021 by the authors. These results imply that it is inadequate and even difficult for a single nitrogen-containing Licensee MDPI, Basel, Switzerland. precursor to concurrently control the porous structure, nitrogen content, and coordination This article is an open access article of Fe atoms with N and C, which are key factors affecting ORR performance [15–19]. To distributed under the terms and date, pyrolysis of dual precursors could serve as a promising route to optimize structures conditions of the Creative Commons Attribution (CC BY) license (https:// and properties of the resultant SSFNCCs, thus tailoring the ORR performance [20–24]. Lou creativecommons.org/licenses/by/ et al. [20] reported porous N-doped carbon matrix-supported N-doped carbon nanotube 4.0/). assemblies with embedded iron carbide nanoparticles (Fe3C@N-CNT assemblies), which Catalysts 2021, 11, 623. https://doi.org/10.3390/catal11050623 https://www.mdpi.com/journal/catalysts Catalysts 2021, 11, x FOR PEER REVIEW 2 of 13 Catalysts 2021, 11, 623 2 of 13 [20–24]. Lou et al. [20] reported porous N-doped carbon matrix-supported N-doped car- bon nanotube assemblies with embedded iron carbide nanoparticles (Fe3C@N-CNT as- semblies), which were fabricated via a dual-MOFs pyrolysis route where Fe-based MIL- were88B nanorods fabricated were via afirstly dual-MOFs confined pyrolysis in a Zn-based route where ZIF-8 Fe-basedpolyhedron MIL-88B host. The nanorods as-obtained were firstly confined in a Zn-based ZIF-8 polyhedron host. The as-obtained Fe C@N-CNT Fe3C@N-CNT assemblies was bestowed with well-defined overall morphology,3 mesopo- assemblies was bestowed with well-defined overall morphology, mesoporous structure, rous structure, and highly dispersed small-sized Fe3C nano-crystallites, which contributed and highly dispersed small-sized Fe3C nano-crystallites, which contributed to the higher to the higher ORR activity as compared with the Pt/C catalyst (E1/2 of 0.85 and 0.83 V, ORR activity as compared with the Pt/C catalyst (E of 0.85 and 0.83 V, respectively). respectively). Direct Ar-pyrolysis from a dual nitrogen1/2 precursor of poly-o-phenylenedi- Direct Ar-pyrolysis from a dual nitrogen precursor of poly-o-phenylenediamine (PoPD) amine (PoPD) and melamine (Mela) with FeCl3·6H2O was investigated for the preparation and melamine (Mela) with FeCl ·6H O was investigated for the preparation of Fe/N of Fe/N co-doped carbon materials3 (denoted2 as Fe/oPD-Mela) [25], which possessed a high co-doped carbon materials (denoted as Fe/oPD-Mela) [25], which possessed a high surface surface area and high content of active nitrogen species such as graphitic and pyridinic area and high content of active nitrogen species such as graphitic and pyridinic nitrogens. nitrogens. The as-prepared Fe/oPD-Mela exhibited a four-electron pathway with an im- The as-prepared Fe/oPD-Mela exhibited a four-electron pathway with an impressive pressive electrocatalytic ORR activity in terms of E1/2, outperforming and approaching that electrocatalytic ORR activity in terms of E , outperforming and approaching that of of state-of-the-art Pt/C in alkaline and more1/2 challenging acidic electrolytes, respectively. state-of-the-art Pt/C in alkaline and more challenging acidic electrolytes, respectively. Although dramatic advancements have been achieved, great efforts remain to be made Although dramatic advancements have been achieved, great efforts remain to be made for for further improving ORR performance, clarifying the nature of components in catalysts further improving ORR performance, clarifying the nature of components in catalysts and correspondingand corresponding mechanisms, mechanisms, and facilitating and facilitating the practical the applicationspractical applications of these SSFNCCs of these in fuelSSFNCCs cells. in fuel cells. Herein,Herein, low-costlow-cost EDTAFeNaEDTAFeNa containingcontaining C,C, Fe,Fe, andand NN atomsatoms andand melaminemelamine (MA)(MA) werewere employedemployed asas aa dualdual precursorprecursor toto fabricatefabricate SSFNCCsSSFNCCs viavia facilefacile pyrolysispyrolysis andand acid-acid- leachingleaching forfor ORRORR inin 0.10.1 MM KOH.KOH. AsAs wewe previously reported,reported, SSFNCCsSSFNCCs generatedgenerated fromfrom thethe pyrolysispyrolysis of sole EDTAFeNa showed showed a a low low nitrogen nitrogen content content and and large-sized large-sized crystalline crystalline Fe Feparticles, particles, which which could could be responsible be responsible for relatively for relatively low lowORR ORR activity activity due dueto the to low the den- low x x y densitysity of CN of CN andx FeNand FeNC moietiesxCy moieties involving involving N species. N species. Although Although active sites active in Fe-N-C sites in ma- Fe- x x y N-Cterials materials are still not are stillclear, not the clear, CN theand CNFeNx andC moieties FeNxCy havemoieties been haveproposed been as proposed active sites as active[17,26–28]. sites To [17 ,potentially26–28]. To increase potentially the increasenumbers the of numbersabove-mentioned of above-mentioned active sites, activeN-rich sites,MA was N-rich proposed MA was to proposedproportionally to proportionally combine with combine EDTAFeNa with EDTAFeNa to improve tothe improve percentage the percentageof nitrogen ofspecies nitrogen and species dispersion and dispersionof Fe spec ofiesFe during species pyrolysis. during pyrolysis. In addition, In addition,the acid- theleaching acid-leaching time and time pyrolysis and pyrolysis temperature temperature were were investigated investigated as as well. well. The The resultantresultant SSFNCCsSSFNCCs (denoted(denoted as as MA/EDTAFeNa-HT2) MA/EDTAFeNa-HT2) displayed displayed excellent excellent ORR ORR activity, activity, reflected reflected by theby identicalthe identical Eonset Eonsetand and E1/2 Eto1/2 theto the Pt/C Pt/C catalyst catalyst (0.97 (0.97 and and 0.84 0.84 V, respectively). V, respectively). Meanwhile, Mean- thewhile, MA/EDTAFeNa-HT2