Exploring Selective, Efficient 2 e- and 4 e- Electrochemical Oxygen Reduction Using Electrocatlayts Based on Reduced Graphene Oxide

Abstract

Recently, nitrogen-doped carbon materials, such as N-doped reduced graphene oxide (N-rGO), have been significantly studied due to their outstanding ORR performance, which some studies find rivals that of Pt. Generally the electroreduction of oxygen can proceed through one of two overall reactions: a 4e- process to convert from O2 to H2O, and a 2e- process to form H2O2. There is general agreement that nitrogen-defects selectively catalyze the more thermodynamically favored 4e- mechanism over the 2e-process. However, a distribution of different N-defect sites (pyridinic, pyrollic, graphitic) is typically observed in N-doped carbon materials and there is still no consensus regarding the role of each site on the ORR mechanism. In this study, we use a straightforward, low temperature and pressure procedure to synthesize N-rGO and rGO and compare the resultant materials’ electrocatalytic behavior with rGO and N-rGO synthesized using more traditional high-temperature procedures. Briefly, the low temperature and pressure synthesis was conducted at 100 oC and atmospheric pressure using NH4OH as both the reducing and N-doping agent in a well-dispersed graphene oxide (GO) solution. This method results in materials with a distribution of the three nitrogen defect sites and reasonably high N-doping content (~6 atomic percent), as confirmed by structural analysis including XPS and TGA. The resulting materials could be further annealed at high temperatures (over 400 oC) under inert condition to modify the nitrogen and carbon defect distribution. Similar synthesis procedures were used to prepare non-N-doped rGO samples to elucidate the importance of N-sites. We have characterized electrocatalytic selectivity by monitoring O2 pressure decay in well-stirred, modified electrochemical H-cells with a calibrated, known headspace volume. We also quantify H2O2 formation using a standard iodometric titration on the electrolyte. These techniques provide direct measurements of O2 consumption and H2O2 formation during ORR and are therefore more accurate than traditional rotating ring disk electrode measurements. Both acid (0.1 M H2SO4) and base (0.1 M KOH) conditions have been explored. We have found that certain variants of N-rGO and rGO display exceptionally selective (~100%) and efficient (nearly no onset overpotential) formation of H2O2 in basic conditions. Thermal annealing changes both the N-site distribution and reaction selectivity and efficiency. Furthermore, higher overpotentials for product formation are observed in acid solutions compared to basic solutions. Results will be discussed linking the selectivity and efficiency of the oxygen reduction reaction to the role of nitrogen and carbon defects in N-rGO and rGO.