Single Atomic Iron Site Catalysts via Benign Aqueous Synthesis for Durability Improvement in Proton Exchange Membrane Fuel Cells

Abstract

Atomically-dispersed iron-nitrogen-carbon (Fe–N–C) catalysts have arisen as promising candidates for replacing the costly precious metal catalysts in fuel cells but still face some grand challenges, such as insufficient site density and durability. Herein, we report a self-assembly method in an aqueous solution to develop an atomically-dispersed iron catalyst with high oxygen reduction reaction (ORR) activity and stability in acidic electrolytes. As determined by high-resolution transmission electron microscopy (HR-TEM), X-ray absorption spectroscopy (XAS), and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), this benign aqueous synthesis strategy facilitates the formation of homogeneous atomic nitrogen-coordinated iron sites embedded in a popcorn-like porous graphitic carbon matrix. These catalyst properties contribute to the improved ORR kinetic current density and mass transport. By controlling synthesis chemistry, the correlation between structure and property is systematically investigated. The iron content is the most critical material property and can regulate site density and graphitic carbon structures in the catalyst, impacting catalytic activity and stability. The enhanced performance and durability were examined in both acidic aqueous electrolytes and membrane electrode assemblies.

Chemical Vapor Deposition for N/S-Doped Single Fe Site Catalysts for the Oxygen Reduction in Direct Methanol Fuel Cells

“…Figure d shows the Fe K-edge XANES spectra of the Fe­(Fc)–N/S–C catalyst and reference samples. XANES spectra exhibit the intensity of the line for the Fe­(Fc)–N/S–C situated between those for the FeO and Fe 2 O 3 , further revealing its typical electronic structure (Fe δ+ , 2 < δ < 3), similar to other Fe–N–C catalysts. ,,, The Fourier-transformed (FT) k 3 -weighted EXAFS spectra of the R space for the Fe­(Fc)–N/S–C exhibit a dominant peak at 1.45 Å. No characteristic distance of Fe–Fe coordination is observed, indicating that the atomically dispersed Fe sites are primary species in the catalyst (Figure e). , According to the fitting, we propose a possible Fe coordination consisting of three nitrogen atoms and one sulfur atom (Figure f and Table S2).…”

“…XANES spectra exhibit the intensity of the line for the Fe(Fc)−N/S−C situated between those for the FeO and Fe 2 O 3 , further revealing its typical electronic structure (Fe δ+ , 2 < δ < 3), similar to other Fe−N−C catalysts. 9,36,51,52 The Fourier-transformed (FT) k 3 -weighted EXAFS spectra of the R space for the Fe(Fc)−N/S−C exhibit a dominant peak at 1.45 Å. No characteristic distance of Fe−Fe coordination is observed, indicating that the atomically dispersed Fe sites are primary species in the catalyst (Figure 3e).…”

Tuning Two‐Electron Oxygen‐Reduction Pathways for H2O2 Electrosynthesis via Engineering Atomically Dispersed Single Metal Site Catalysts

“…Furthermore, carbon-based atomically dispersed metal site catalysts have proven to be an effective platform for exploring the synergistic effects between active sites and adjacent atomic configurations. [34,[42][43][44][45][46][47][48] A variety of transition metals (e.g., Fe, Co, Ni, Mn, Cu, Cr, or Ce) stabilized into nitrogen-doped carbon (M-N-C) materials with well-defined architectures and activated metal centers have emerged as a promising formulation to design electrocatalysts for many critical reactions such as oxygen, [49][50][51][52][53][54][55][56][57][58][59][60] nitrogen, [61][62][63][64][65] and CO 2 reduction reactions. [64,[66][67][68][69][70][71] The structurally defined M-N-C configurations allow rational regulation of active site structures, e.g., the optimization of coordination environments, surface adsorption properties, geometric designs at the atomic level, etc.…”

PGM-Free Oxygen-Reduction Catalyst Development for Proton-Exchange Membrane Fuel Cells: Challenges, Solutions, and Promises

“…We have developed a novel acid protection method to synthesize Mn−N−C catalyst in an environmentally friendly aqueous solution to address this issue (Figure 5d). 15 Importantly, this approach yields a catalyst with ultrahigh surface area (>1500 m 2 g −1 ) to host sufficient MnN 4 moieties in the partially graphitized carbon structures. As expected, the obtained Mn−N−C catalyst exhibits a much-increased MnN 4 active site density and demonstrates greatly enhanced ORR performance compared to our previous results.…”

“…Meanwhile, Dodelet et al have discovered that zeolite imidazole frameworks (ZIF-8s) can be effective precursors to synthesize advanced Fe–N–C catalysts . Inspired by the pioneering work, we comprehensively explore the zinc-based ZIF-8s through chemically doping/adsorbing active metal ions (e.g., Fe, Co, Mn, and Ni) into size/shape-tunable ZIF-8 nanocrystals and their derived materials, aiming to synthesize single metal site M–N–C catalysts with increased site density and optimal coordination/structure. ,− Compared to other studied precursors, ZIF-8s can host more single metal sites with adequate N ligation and form MN 4 sites in microporous carbon during controlled thermal activation. In particular, due to the facile zinc evaporation from ZIF-8s around 900 °C, the derived carbon in catalysts often contain high surface areas, dominant micropores, and sufficient nitrogen dopants.…”