Carbon Black Supported Highly Stable and Active Electrocatalysts for ORR in Polymer Electrolyte Membrane Fuel Cells

Abstract

Exploring low-cost and highly efficient non-precious metal electrocatalysts toward oxygen reduction reaction is crucial for the development of fuel cells. Pyrolyzed Fe/N/C electrocatalysts have received attention in recent years as a cathode catalyst for oxygen reduction reaction (ORR) in the polymer electrolyte fuel cell (PEFC). These kinds of non-precious metals electrocatalysts had shown high activity and stability in both acidic and alkaline media. [1, 2]. Herein, we organized our significant advances in performance of Fe/N/C catalysts and their applications in low temperature fuel cells: We synthesized N-doped electrocatalysts using melamine as nitrogen precursor, ferric chloride as an iron precursor and oxidized carbon black kJ600 as support by solvothermal method. The final catalyst synthesized at 800°C (SNW-Fe/N/C@800°C) exhibited microporous structure with Brunauer−Emmett−Teller (BET) surface area up to 791 m2g-1 after second heat treatment. The catalyst exhibits high ORR activity (0.64 mA mg-1 at 1 V vs RHE, about half of Pt/C catalyst), stability and low percentage of hydrogen peroxide (% H2O2 < 1.5%) in O2-saturation 0.1 M NaOH solution. The alkaline anion exchange membrane fuel cell (AEMFC) test was performed by using SNW-Fe/N/C@800°C as cathode catalyst demonstrates a high power density of 266 mW cm-2.We study the effect of active nitrogen species on ORR performance through comparison of Fe/N/C electrocatalysts in acidic and alkaline solution. These catalysts are synthesized from different rich-nitrogen polymers. We observed that pyridinic N is very important for oxygen reduction reaction through XPS analysis, especially in alkaline electrolyte. The sensitivity of these three catalysts to acidity/alkalinity highly depends on percentage of pyridinic N and found the reverse order of ORR activity between acidic and alkaline mediaWe designed a facile method for the synthesis of ORR catalysts with iron nanoparticles encapsulated in bamboo like structure of N-doped carbon nanotubes. Finally, pyrolyzed catalyst (Bg-CA-M)-Fe-C-800°C exhibited the efficient ORR performance with the onset potential 1.10 V, half-wave potential 0.93V, 4 electrons transfer number per oxygen molecule and high durability, as well as high ethanol tolerance in alkaline solution. (XPS measurements revealed that ORR activities of catalysts directly correlate with pyridinic and pyrrolic nitrogen active species. The direct ethanol fuel cell (DEFC) with (Bg-CA-M)-Fe-C as cathode catalyst displayed 64 mW cm-2 peak power density.