Sdudy On Nitrogen-doped Carbon Supported Transition Metal Electro-catalyst For Oxygen Reduction Reaction

As a kind of clean and high efficiency energy conversion device,proton exchange membrane fuel cells (PEMFCs) have been recognized asa potential future power source for many applications such as thezero-emission vehicles and the stationary or portable power station. Theperformance of PEMFCs is highly dependent on the cathode catalystwhich enhances the oxygen reduction reaction (ORR). As the mostcommon and efficient ORR electrocatalyst, the platinum and its alloysshow excellent electrocatalytic performance. However, the limited supplyand high expense of Pt are the barriers for the large-scale application.Thus, reducing Pt usage or replacing Pt with non-noble metal catalysts iscritical to PEMFCs commercialization. The polypyrrole-modified-carbonsupported transition metal catalysts (Co-PPy/C) have been considered asone of the most promising ORR catalysts in recent years due to their highactivity, low cost and simple synthetic procedures. In this work, a series of non-noble metal catalysts with differentcompositions were synthesized. The morphologies/structures andcompositions were characterized using the powder X-ray diffraction(XRD), the transmission electron microscopy (TEM) images, X-rayphotoelectron spectroscopy (XPS) and inductively coupled plasma (ICP)spectrometric measurements. The ORR catalytic activities of thesynthesized catalysts were also examined using cyclic voltammetry (CV)and rotating disk electrode (RDE) experiments. Furthermore, themorphology/structure-performance relationship has been discussed todistinguish the catalytic active sites and the role of each element in thecatalysts.It is observed that both metallic Co and Co oxide in the catalysts arenot active toward ORR, and they can block the active site to reduce thecatalyst activity. Besides, the carbon-nitrogen structure can not enhancethe ORR. It is strongly believed that the ORR active site in the catalystssynthesized in this paper should be Co-pyridinic-N group. The formationprocess of the ORR active site was also researched. When the carbonsupported polypyrrole is suspended into an aqueous solution of cobaltacetate, polypyrrole on the carbon surface can release proton from N-Hgroup on pyrrole ring and then react with cobalt ion to form theCo-pyrrolic-N structure. During the followed pyrolysis, theCo-pyrrolic-N structure could be transformed into the Co-pyridinic-N structure which should be the ORR active site in the catalyst.It is observed that the cobalt loading has very important effect onboth the composition and the ORR properties of the catalysts. When thecobalt loading is less than1.0wt%, the dominating form of Co should beCo-NX. However, Co-NX, metallic Co and Co oxide particles co-exist inthe catalyst when the loading is higher than1.0wt%. At the Co loading of~1.0wt%, the catalyst gives the best ORR activity, indicating that themost amount of ORR active site is contained in the catalyst. It is believedthat a cobalt loading of1.0wt%should be the balancing point tosynthesize the catalyst with cobalt in the form of only Co-NXstructurewithout metallic cobalt and Co oxide particles, which could give theoptimal ORR activity. When the cobalt loading is lower than1.0wt%,there is not enough cobalt to bond with nitrogen to form the active sites,leading to a decreased ORR activity. On the other hand, when the cobaltloading is larger than1.0wt%, cobalt particles exist in the catalyst andmay cover some of the Co-NXactive sites, leading to a decreased ORRactivity.To verify the mechanism of the formation process of the ORR activesite, another catalyst of (Co-PPy-TsOH/C)Iwas prepared by animpregnation method. For this (Co-PPy-TsOH/C)Isample, ICPmeasurement confirmed that the Co loading is fairly identical to thecatalyst with Co loading of~1.0wt%, further supports that all cobalt are bonded to nitrogen to form Co-N active sites in this catalyst. Regardingthe catalytic behaviors of (Co-PPy-TsOH/C)Iand (1.0wt%-Co-PPy-TsOH/C)Hcatalysts, both their CV curves and RDE datagave almost the same ORR catalytic activity, verifying the mechanism ofthe formation of the ORR active site.