High-nitrogen graphene nanoflakes with iron functionalities: a viable catalyst for PEMFCs

Introduction: A cost-effective alternative catalyst to platinum for the oxygen reduction reaction (ORR) in polymer electrolyte membrane fuel cells (PEMFCs) is iron deposited onto nitrogen-containing graphene. In this work,... [ view full abstract ]

Introduction: A cost-effective alternative catalyst to platinum for the oxygen reduction reaction (ORR) in polymer electrolyte membrane fuel cells (PEMFCs) is iron deposited onto nitrogen-containing graphene. In this work, graphene nanoflakes were grown in-house with high levels of nitrogen (HN-GNFs) and iron functionalities to produce an active catalyst. The iron content was studied structurally and electrochemically.
Methods: HN-GNFs are grown by the plasma decomposition of methane and nitrogen in a two-stage process using a thermal plasma torch. A wet-chemical method is used to incorporate iron nanoparticles into the graphene structure and the iron content is characterized by NAA, SEM, TEM, and XPS. Electrochemical studies by rotating disk electrode (RDE) are performed in different electrolyte solutions to determine catalytic performance towards the ORR.
Results: Iron was successfully introduced at different levels into the HN-GNFs and the weight percent was measured by NAA; Fe-Low (1.25 wt%), Fe-Medium (3.77 wt%), and Fe-High (18.03 wt%). The introduction of iron did not change the qualitative surface conditions of HN-GNF, as verified by SEM. TEM shows iron nanoparticles with diameters between 10 to 35 nm within the graphene sheet layers (figure 1). With the iron imbedded into the HN-GNF, surface conditions are an incorrect indicator of overall iron content. XPS was performed on the limited amount of detectable iron; the deconvolution of the iron signal showed a mix of Fe, Fe(2+), and Fe(3+). An RDE study revealed that the addition of iron did not improve the catalytic performance in alkaline environments, with untreated HN-GNF already showing good activity towards the ORR. Iron proved to be beneficial in acid and neutral environments, with the Fe-Medium catalyst showing the best improvement to onset potential and current density.
Discussion: The iron incorporation method used can be easily scaled to obtain whichever desired iron weight percent, with iron nanoparticles covered by the graphene sheets of the HN-GNFs. Catalysts with varying amounts of iron show no difference structurally to untreated samples while increasing the catalytic activity towards the ORR for acidic environments. The exact iron content still needs to be optimized to produce the best ORR catalyst for PEMFCs.