The Study On Application Of CoTPP In Catalyst For Oxygen Reduction Reaction

With higher conjugated structure and better chemical stability, Metalloporphyrincomplexes are excellent catalyst for oxygen reduction reaction (ORR) both in acid andalkaline solution. It is generally accepted that metalloporphyrin polymer shows highercatalytic activity for ORR than monomer, and can catalyze oxygen to form water according to4electron process. However, metalloporphyrin polymers are only confined inmetalloporphyrins with easy polymerizing functional groups, such as amino, pyrrolyl, thienyland so on. After heat treatment, the catalytic activity and stability of metalloporphyrins will befurther enhanced. However, in traditional preparation method, the metalloporphyrinconsumption has increased sharply and the efficiency is low, and there is no unifyingunderstanding of the catalytic mechanism and ORR-active sites. As the simplestmetalloporphyrin, tetraphenylporphyrin cobalt (CoTPP) used to catalyze ORR is limited. Inthis work, we innovatively prepared electrocatalysts for ORR from CoTPP by three methods:chemically modified carbon paste electrodes CP/CoTPP/SWCNT, polymer of CoTPP(pCoTPP) on glassy carbon electrode by electrochemical polymerization, and nitrogen richMe-CoTPP/C through pyrolysis. The catalytic activity and mechanism of catalysts preparedthrough different methods was systematically studied. And the structure and morphology wasthoroughly characterized to assess the electrocatalytical ORR-active sites.(1) Combined with the advantage of carbon paste electrode, the chemically modifiedcarbon paste electrodes CP/CoTPP/SWCNT was prepared using CoTPP and single-walledcarbon nanotubes (SWCNT) as modifiers. In0.1mol/L Na2HPO4–NaH2PO4buffered solution(pH=7.0), the effects of SWCNT on electrocatalysis characteristics and mechanism of CoTPPmonomer for ORR were systematically researched. The results have shown that the specialpore structure and distribution of pore diameter can improve the dispersion of the redox activeCoN4center. And the high conductivity of SWCNT might lead to a facile electron delivery tothe active site. Therefore, SWCNT would futher enhance the catalytic activity of CoTPP. Andunder graphite︰SWCNT︰CoTPP︰paraffin＝0.625g︰0.070g︰5mg︰0.3g, the catalyticactivity for ORR was the best. Moreover, the electron transfer number for ORR atCP/CoTPP/SWCNT is calculated to be2.7, which involves a mixing process of two and fourelectron transfer pathways. and the two electron transfer pathway is predominant. The activesite is the Co-N4configuration.(2) The polymer thin film of pCoTPP was obtained from CoTPP monomer by cyclic voltammetric electrochemical polymerization method. At higher potential2.0V vs. Ag/AgCl,the phenyl groups on the periphery of CoTPP ring are oxidized leading to radical-cation(CoTPP2+)·+. The radical cation easily couples with another radical cation, thus forming adimer. Step by step, the pCoTPP film are got following the same principle. From the UV-visspectra, the Soret-band and Q-bands of pCoTPP are red-shifted and their full widths athalf-maximum (FWHM) are wider than CoTPP monomer. It is because pCoTPP has higherconjugated structure than CoTPP. From the SEM images, the pCoTPP film is well dispersedon the surface of ITO glass and exhibits a scaly structure. Due to the synergistic effects of thecentral Co2+and porphyrin ring, the pCoTPP-modified GCE exhibits higher catalytic activityand stability for ORR in oxygen-saturated0.5mol/L H2SO4solution. The electron transfernumber for ORR at pCoTPP film is calculated to be3.8, which involves a mixing process oftwo and four electron transfer pathways, and the four electron transfer pathway ispredominant.(3) As a low price and high nitrogen content raw material, melamine was added into theprecursor to improve the nitrogen content. Nitrogen-rich Me-CoTPP/C catalyst wasinnovatively prepared by pyrolysis of the mixture of CoTPP, melamine and carbon. It wasturned out that when CoTPP︰melamine︰carbon＝0.8mg︰2.1mg︰3.6mg, thenitrogen-rich Me-CoTPP/C catalyst showed the best catalytic activity for ORR, which wascomparable to CoTPP/C catalyst synthesized through traditional method. The amount ofCoTPP in the nitrogen-rich precursor was only12.3%. However, the amount in the traditionalmethod was nearly50%. Therefore, the addition of melamine greatly reduced theconsumption of relatively expensive CoTPP and improved the utilization rate. When theheat-treated temperature was below600°C, Co-N configuration was the catalytically activesite. However, when the heat-treated temperature reached600°C, pyridinic C-N configurationturned out to be the active site. Too high or too low heat-treated temperature and too long ortoo short heat-treated time will directly result in the decrease of the catalytic activity ofMe-CoTPP/C catalyst. The optimal heat-treated condition of preparing the nitrogen-richMe-CoTPP/C catalyst is at600°C for30min. And the electron transfer number for ORR atMe-CoTPP/C catalyst is3.8, which suggests the four electron transfer pathway is dominant.Moreover, the stability and methanol-tolerance of nitrogen-rich Me-CoTPP/C catalyst arebetter than the commercial Pt/C catalyst.