FeN_<i>x</i>/FeS_<i>x</i>‑Anchored Carbon Sheet–Carbon Nanotube Composite Electrocatalysts for Oxygen Reduction

Abstract

Even though various Pt-free electrocatalysts\nfor oxygen reduction reaction (ORR) have been introduced, many of\nthem are found to be active only in alkaline conditions. Considering\nNafion, phosphoric acid-doped polybenzimidazole (PBI), and so on as\nthe prominent ionomer membranes, used in the commercially available\npolymer electrolyte membrane fuel cells (PEMFCs), it becomes important\nthat any development on the Pt-free catalysts should ensure the better\nORR performance under acidic conditions. The present work effectively\ntackles this issue, where an ORR-based catalyst could be prepared\nwith simultaneous incorporation of both Fe–N and Fe–S\nactive sites on <i>in situ</i> generated carbon sheets which\nare spatially separated by the carbon nanotube (CNT) network. This\ncatalyst shows ability to perform under both acidic and basic conditions.\nThis has been achieved by growing a polyethylene­dioxythiophene\npolymer network in the presence of CNT and melamine followed by its\npyrolysis under an inert atmosphere. The catalyst formed at 900 °C\n(PMCNT-900) displays 0.94 V onset potential for ORR under acidic electrolyte\nconditions, which corresponds to 60 mV overpotential compared to its\n40 wt % Pt/C counterpart. Interestingly, in single cell demonstration\nof Nafion-based PEMFC with PMCNT-900 as the cathode catalyst, the\nsystem delivered a maximum power density (PD) of 500 and 275 mW/cm² at 60 °C under H₂–O₂ and\nH₂–air feed conditions, respectively. On the other\nhand, in a single cell test in the anion exchange membrane fuel cell\n(AEMFC) mode, a maximum power density of 65 mW/cm² at 50\n°C could be achieved with the same cathode catalyst, which is\na comparable value obtained while employing Pt/C as the cathode. These\nresults, thus, infer to the efficiency of the catalyst to facilitate\nORR under the extreme pH conditions, and particularly its performance\nunder acidic condition reveals its prospect as a potential Pt-free\nelectrocatalyst to serve in the Nafion-based systems.