Synthesis and Characterization of Nitrogen Doped Reduced Graphene Oxide Based Cobalt-ZIF-8 Catalysts for Oxygen Reduction Reaction

Abstract

Non-Platinum Group Metal (non-PGM) catalysts have recently attracted huge attention for use in the Polymer Electrolyte Membrane Fuel Cell (PEMFC) vehicles, as the demand for alternative and sustainable transportation is increasing exponentially. Carbon-based catalysts have advantages such as relative ease of manufacturing, low cost of synthesis, high electrochemical durability, and high electrical conductivity which makes them a potential alternative. While there are several ways to improve the catalytic activity of carbon catalysts, one promising technique is doping heteroatoms into the lattices of the sp2 carbon structure. The most commonly used doping is by nitrogen (N-doping) where a nitrogen precursor is used to incorporate N atoms to help improve electrochemical performances. Transition Cobalt-Nitrogen-Carbon (Co-N-C) catalysts show promising catalytic performance and durability due to their fast electron transfer rates leading to increased kinetic reaction rates. In this paper, we report the synthesis and characterization of Nitrogen-doped Graphene/Graphene oxide with Metal-Organic Framework (MOF) material using wet chemical and pyrolysis techniques for the development of an efficient catalytic material to tackle the sluggish cathodic Oxygen Reduction Reaction (ORR) in fuel cells (FCs). Transition metal, Cobalt is doped into Zeolite Imidazole Frameworks (ZIF-8) using Cobalt Nitrate as precursor to enhance the mass and kinetic rates of the ORR reaction at the cathode. The synthesized metal-doped MOF catalysts (Co-ZIFx) are further pyrolyzed by one-step pyrolysis in an argon atmosphere at 600oC for 2 hours. These catalysts are produced with varying amounts of precursor metals (different molar ratios of metal precursor to ZIF-8 such as 0.6, 0.9 and 1.2 respectively; x denotes molar ratio values) to analyze the effects of each element proportion on the electrochemical performance. The pyrolyzed electrocatalysts (named Co-ZIFx-600 respectively) were further synthesized chemically using nitrogen-doped graphene (N-Gr) catalysts through wet chemical synthesis procedure and are named as N-Gr-Co-ZIFx and N-Gr-Co-ZIFx-600 (x changes with the molar ratios of metal precursor). Doping of graphene carbon lattice with nitrogen heteroatoms has proven to influence charge densities and adsorption mechanism which further enhances the catalytic activity. Nitrogen loading effects were also analyzed to study the electronic and electrochemical behavior of individually varied graphene materials. Various characterization techniques, viz., FTIR, Raman spectroscopy, XRD, SEM and EDS were performed to analyze various physical and chemical properties affecting the electrochemical performances. FTIR of N-Gr-Co-ZIF-600 catalysts showed the presence of cobalt-doped ZIF-8 peaks around 420 cm-1 and the presence of nitrogen bonds at a stretching frequency of around 3000 cm-1. XRD results showed the formation of crystalline phases of the Cobalt-doped ZIF-8 with a (100) orientation. Further, it was observed that the crystallinity was affected with an increase in the metal loading. SEM examination of the Cobalt-doped ZIF-8 samples showed an increase in grain size from around 100 nm to 2 μm with an increase in cobalt composition. EDS results of the catalysts indicated uniform distribution of the various elements such as carbon, nitrogen, cobalt, oxygen, and zinc (from ZIF-8) in varying proportions as we alter the metal precursor doping in ZIF-8. Cyclic Voltammetry is performed in 0.1M HClO4 aqueous solution to evaluate the electrochemical performances of the synthesized catalysts. It is found that the addition of the transition metal plays a major role in the enhancement of electrochemical performance compared to that of non-transition metal carbon-based catalysts. the N-Gr-Co-ZIFx-600 catalyst showed relatively better ORR activity in acidic media with onset and half-wave potentials only 24 and 51 mV lower than those of the Pt/C catalyst. Overall, the synthesized N-Gr-Co-ZIFx-600 is a potential candidate as a high activity, stable, and low-cost ORR catalyst for FCs. All the results of the characterizations will be discussed in detail for different metal precursor loadings (0.6, 0.9 and1.2 molar ratios) of different MOF derived carbon-based electrocatalysts.