Iron-Phthalocyanine Based Oxygen Reduction Reaction Electrocatalyst: Structure-Performance Relationship Evolution during Pyrolysis

Abstract

Fuel cells efficiently convert hydrogen into green electricity however the key bottleneck in their deployment lies at the cathode where oxygen reduction reaction (ORR) exhibits sluggish kinetics and requires high loading of precious metals.In this view, Iron-Nitrogen-Carbons (Fe-Nx-Cs) are promising electrocatalysts to replace expensive and scarce noble metals currently used for the ORR. Fe-Nx-Cs are typically synthesized through pyrolysis using iron, nitrogen and/or carbon precursors of diverse nature. However, in general, pyrolysis is a process poorly understood despite recently a couple of manuscripts with in-situ investigations that have been presented [1-2]. The pyrolysis process defines the structural and chemical evolution of the electrocatalyst that ultimately governs its overall electroactivity. Therefore, in this study, the influence of pyrolysis conditions i.e. temperature (200-1000 °C) and atmosphere (Ar or Ar/H2) on the morphological and superficial development of electrocatalyst was elucidated that initially consisted of Iron (II) Phthalocyanine (FePc) mixed with Ketjen Black 600. The morphological and chemical features were then correlated with the electrochemical performance in alkaline and acidic media through a structure-to-property relationship. In-depth analysis showed that pyrolysis conditions significantly affect the nature and proportion of active Fe-based and nitrogen-based moieties that collaboratively determine the pathway and kinetics of the ORR in acid and alkaline media. X-ray diffraction (XRD) showed that reducing atmosphere (Ar/H2) inhibits the crystallographic growth of Fe-containing nanoparticles. In parallel, the neutral atmosphere (Ar) enhanced the aggregation of Fe into oxides as diffraction peaks corresponded to Fe3O4 after a pyrolysis temperature of 600°C. High-resolution-transmission electron microscopy (HR-TEM) showed the formation of nanoparticles and oxides at temperatures above 600°C. X-ray photoelectron spectroscopy (XPS) revealed multitudinous nitrogen-based moieties with varying proportions in the Fe-Nx-C electrocatalysts developed under different pyrolysis conditions. Interestingly, the samples pyrolyzed in Ar/H2 showed an increasing trend of Fe-Nx with respect to a pyrolysis temperature whereas, in Ar-pyrolysed samples, Fe-Nx started plummeting after 600°C. On the other hand, graphitic nitrogen increased prominently after 700°C in both pyrolytic conditions. Using a rotating ring disc electrode (RRDE), the dependence of performance parameters on the chemical and structural features that evolved during the pyrolysis was clarified. Particularly, in acidic media (0.5 M H2SO4), Fe-Nx-C synthesized at 600 ºC under both Ar and Ar/H2 conditions demonstrated higher electrocatalytic activity, with half-wave potentials (E1/2) of ≈ 0.70-0.72 V (vs RHE) while maintaining the direct tetra-electronic ORR (low peroxide production). A similar trend was witnessed in an alkaline media (0.1 M KOH) but with relatively higher performance than in acid media. The highest onset potentials and E1/2 observed in alkaline media were obtained by the Fe-Nx-Cs pyrolyzed at 600°C and 700 ºC in both atmospheres i.e. Ar (E1/2 ≈ 0. 89 V vs RHE) and Ar/H2 (E1/2 ≈ 0. 87 V vs RHE). A prompt reduction in the performance was highlighted at higher pyrolysis temperatures where an increase in peroxide yield might be related to the formation of iron oxide nanoparticles. This work presents a novel insight into the evolution of Fe-Nx-C electrocatalysts during pyrolysis with optimum performance.