Modification of graphene oxide as efficient catalyst support for polymer electrolyte membrane fuel cells

Since 2004, graphene has gained immense popularity due to its unique properties and structure but graphene as a bulk material, rarely depicts surface properties needed for energy conversion and storage owing to the irreversible aggregation of the nanosheets. Electrocatalysts are often synthesized on supports by solution based methods that require the supports to have high surface area with excellent solubility and processibility in solvents e.g. organic solvents for uniform dispersion and growth of ultra-small platinum nanoparticles for high electrocatalytic activity in PEM fuel cells. As a remedy, graphene oxide can act as a precursor which has many chemically reactive oxygenated functional groups at edges and basal planes giving an opportunity for covalent and non-covalent functionalization with many functional materials like organic compounds, biomaterials, polymers and nanoparticles. As a result, functionalized graphene oxide (f-GO) can have many applications because of their interesting properties such as chemical stability, tunable modification, high electrical conductivity and multifunctional structures. In this research, the successful modifications of graphene oxide via covalent functionalization with novel functional reagents p-phenylenediamine (GO-NH2)and Gly-Arg-Gly-Asp-Ser-Pro (GO-RGD) was achieved to overcome the graphene issues mentioned before while imparting desirable properties, as evidenced by the different characterization techniques employed such as FTIR, TGA, XRD and Raman. Subsequently, these f-GO were utilized as effective electrocatalyst supports for anchoring platinum nanoparticles using the well-known polyol reduction process along with Vulcan XC-72, GO and N doped GO for comparison purposes and further characterized by XRD, TGA, TEM and CV. Pt/GO-RGD showed the highest hydrogen adsorption/desorption peaks with large areas under them indicating increased number of available Pt catalyst active sites for charge transfer which in turn gave high ECSA value of 147 m2/g about 83% higher than synthesized Pt/Vulcan XC-72 (80 m2/g) and 48% higher than Pt/GO (99 m2/g) which can be attributed to smaller Pt nanoparticles’ size with homogeneous distribution on the support material with large surface area to anchor metal nanoparticles. While the runner-up in ECSA values was Pt/GO-NH2 (130 m2/g) due to small Pt nanoparticles’ size (3.9 nm) with excellent dispersion as evidenced by TEM which increased the Pt utilization. These results showed that the as-synthesized Pt/f-GO are promising electrocatalysts for high performance PEM fuel cells which would be the future task of our investigation. This concluded the first part of our work. Secondly, graphene oxide was modified via non-covalent functionalization with incorporation of polypyrrole/carbon nanocomposite by solution intercalation to yield hybrid nanocomposite. Subsequently, Pt nanoparticles were reduced on the catalyst supports i.e. graphene oxide (GO), polypyrrole/carbon black nanocomposite (PPy/CB) and polypyrrole/carbon black/graphene oxide hybrid nanocomposite by three different methods based upon modified polyol process to see their effects on the size of Pt nanoparticles, their dispersion on supports and electrocatalytic activity by employing different characterization techniques such as XRD, TGA, TEM, and CV. Method 2 i.e. modified polyol process with FeCl3 proved to be the best among the three compared methods owing to the utilization of FeCl3 which acted as an effective stabilizer and retardant in the Pt seed growth leading to smaller particle size with high surface area available for electrocatalytic activity. While the hybrid nanocomposite support i.e. Pt/PPy/CB/GO synthesized by the method 2 showed the highest ECSA value of 153 m2/g about 30% higher than Pt/PPy/CB (117 m2/g) and 20% higher than Pt/GO (128 m2/g). The improved electrocatalytic activity of Pt loaded hybrid nanocomposite as compared to other supports could be explained due to the increased electrical conductivity, increased number of charge carriers with their free mobility. Since the oxygen containing groups which acted as insulators, were partially reduced during Pt synthesis on the hybrid nanocomposite, which resulted in less resistance for electron and mass transfer. In addition to this, the inclusion of PPy/CB into layers of GO inhibited the aggregation of graphene sheets during partial reduction of GO which in turn provided large surface area for uniformly distributing Pt nanoparticles on the heterogeneous nucleation sites formed by the PPy/CB. Thus Pt loaded hybrid nanocomposite synthesized by method 2 could serve as potential candidate for PEM fuel cell application which would be our future task of investigation. This concluded the second part of our work