Gram-Scale Synthesis of High Surface Area Metal-Free Nitrogen-Doped Carbon Foams for the Oxygen Reduction Reaction

Abstract

Proton exchange membrane fuel cells (PEMFCs) are electrochemical energy conversion devices used for portable, residential and vehicular applications because of their low emissions, high efficiency and quick start-up characteristics. However, PEMFCs use Pt-based electrocatalysts as electrode catalysts. Due to the high cost and limited availability of platinum, research and development of non-precious catalysts is of paramount importance. A promising alternative is nitrogen-doped carbons. Doping graphene with nitrogen alters the chemical structure and modulates the electronic properties, allowing a degree of control over the catalytic properties1 - 4. Here we present the synthesis, characterization and electrochemistry of nitrogen-doped carbon foams. A novel bottom-up chemical synthesis method was developed in our lab 5. Triethanolamine (or any other nitrogen-containing alcohol) is mixed with ethanol in various ratios and reacted with sodium to form a nitrogen-containing alkoxide. In this case, this precursor is pyrolysed under nitrogen at 600˚C to form nitrogen-doped carbon foam mixed with sodium oxide by-product. This by-product is removed by washing with water. The resulting nitrogen-doped carbon foams are then pyrolysed at various temperatures in order to improve the conductivity and vary the nitrogen content. Electron microscopy reveals that the carbon foams have relatively large micron-scale pores separated by monolayer / few-layer graphene-like walls. The samples large surface area varying from ~1000 to 2500 m2/g. The nitrogen content can be varied over a wide range (e.g. <0.5 to 15 at%) by changing the precursor ratios and/or the pyrolysis temperature. In addition, the ratio of e.g. pyridinic to tertiary nitrogen bonding can be tailored by changing the pyrolysis temperature; the relative proportion of tertiary bonded nitrogen increases with increasing temperature. Electrochemical characterization was performed by rotating ring-disk electrode (RRDE) voltammetry. The oxygen reduction reaction (ORR) activity increases with increasing temperature. Detailed analysis of the effect of nitrogen content and type will be presented. In particular, the samples prepared by this method are truly metal-free electrocatalysts and these results give insight into the role of nitrogen in the ORR in non-precious catalysts.