Abstract
Development of novel energy conversion and storage technologies is highly relevant for the future renewable energy-based economy. Low-temperature polymer electrolyte fuel cells are clean and efficient devices, but their commercialization is hindered by high cost and scarcity of Pt-based cathode catalysts for the electrochemical oxygen reduction reaction (ORR). Among the various non-precious metal catalysts that have been intensively explored through the last decades, transition metal-containing nitrogen-doped carbon nanomaterials have shown the most promising results. One of the most common and simple strategies to prepare these catalysts is high-temperature pyrolysis of organic carbon precursors in the presence of nitrogen and metal sources, where carbonization and N-doping occur simultaneously.1
This work describes preparation of highly active ORR electrocatalysts from 5-methylresorcinol, Co and/or Fe salts and dicyandiamide (DCDA) via a simple one-step pyrolysis procedure at 800 °C. It has been proposed that DCDA forms graphitic carbon nitride below 600 °C, which acts as a reactive template during carbonization of organic precursors, decomposing at above 750 °C and yielding N-doped graphene-like carbon structures.2 The SEM and TEM analysis showed that the materials consist of wrinkled carbon structures doped with nitrogen and metals, and also contain metal nanoparticles encapsulated in N-doped carbon layers. According to XRD analysis, these nanoparticles are mainly composed of Co, Fe/Fe3O4 and FeCo alloy for the CoNC, FeNC and FeCoNC catalysts, respectively. The specific surface area of the materials was between 290 and 410 m2 g−1. The XPS analysis revealed that the pyridinic- N is the most abundant N species in all materials and the catalysts also contain metal-coordinated N centers.
The electrocatalytic activity of the catalysts for ORR was evaluated in alkaline solution using the rotating disc electrode (RDE) method and the optimal ratio of the precursors was determined. The bimetallic catalysts were more active toward ORR than their monometallic counterparts. Treatment of the materials in acid solutions followed by a second pyrolysis slightly increased their ORR activity. The acid-treated bimetallic catalyst (FeCoNC-at) showed a remarkable ORR performance, comparable to that of commercial Pt/C (20 wt%), which can be attributed to the high surface metal and nitrogen contents (Figure 1a). This material also demonstrated a rather high stability in short-time tests (15000 potential cycles) and good tolerance to methanol. The FeCoNC-at catalyst was further tested in anion exchange membrane fuel cell (AEMFC) with hexamethyl-p-terphenyl poly(benzimidazolium) (HMP-PMBI)3 membrane, where a high value of peak power density (Pmax= 415 mW cm–2) was achieved (Figure 1b).
References
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Figure 1