Electrocatalytic Properties Of CNT Supported Metal Phthalocyanine And MnO₂ Dual Catalyst Materials

The issues of energy and environment are becoming a key problem for restricting the economic growth.Most of countries have spent great efforts for the exploration of clean energy.As a new type of energy resource,the fuel cells don't produce pollutants.and havereceived more and more attentions.Remarkably,the direct methanol fuel cells?DMFCs?have drawn much attention due to several advantages,such as high energy conversion efficiency,high reliability,high quality,high efficiency,no pollution,easy to start,no noise,low radiation and so on,etc.Nowadays,the majority of cathode catalyst of DMFCs are based on Pt/C.However,platinum is an expensive and limited natural material.Moreover,the catalytic activity would be dramatically reduced when methanol permeates the cathode,since it would discharge on the Pt/C catalyst.In addition,the concomitantly generated from the discharge process may poison the catalyst.Therefore,the search of oxygen reduction catalysts with high catalytic activity and good resistance to methanol is one of the key research topics for DMFCs.With relatively larger specific area,higher stability and lower electrical resistivity,carbon nanotube?CNT?is fairly applicable carrier of catalyst for the cathode catalysts of fuel cells.Being non-platinum catalysts,metal phthalocyanine compounds possess the advantages of good methanol resistance and low costs,so they are believed to have the potential to replace noble metal catalysts and to serve as a cathode catalyst of DMFCs.CNT supported metal phthalocyanine assemblies were prepared by using an in-situ solid-phase method.Nano MnO₂ was introduced to the CNT supported metal phthalocyanine assemblies by using a continuous variation of quality method.The combined dual catalysts for the oxygen reduction reaction were prepared by mixing CNT supported metal phthalocyanine with nano MnO₂ in different ratios.The main achievements of the dissertation are listed as follows:?-,?-and?-nano MnO₂ were prepared by using a hydrothermal method,and characterized by scanning electron microscope?SEM?and X-ray diffraction?XRD?.Phthalic anhydride-carbamide method was adopted to synthesize CNT supported mononuclear metal phthalocyanine?non-substituted,nitro-substituted,amino-substituted and amide-substituted mononuclear metal phthalocyanine?assemblies,carbon nanotube supported binuclear metal phthalocyanine sharing one benzene ring?non-substituted,nitro-substituted and amide-substituted binuclear metal phthalocyanine sharing one benzene ring?and CNT supported binuclear metal phthalocyanine sharing biphenyl ring?non-substituted,nitro-substituted and amide-substituted binuclear metal phthalocyanine sharing biphenyl ring?.In addition,all the products were characterized by infrared spectra,ultraviolet-visible spectra,XRD and SEM.In order to find the dopant with better performance,the cyclic voltammetric?CV?curves,linear scanning voltammetric?LSV?curves and tafel curves of the three types of nano MnO₂,namely,?-,?-and?-MnO₂ were collected.The results showed that the peak current of?-MnO₂ for oxygen reduction reaction was greater than that of?-MnO₂ and?-MnO₂,and the peak potential displaced 27 mV and 28 mV more positive than?-MnO₂ and?-MnO₂,respectively.Therefore,?-MnO₂ was selected as the dopant substance in this paper.The optimal ratio of CNT supported metal phthalocyanine assembly and nano MnO₂ was determined by comparing the magnitudes of peak currents of the cyclic voltammetric curves of the oxygen reduction reaction on the glassy carbon electrodes modified by CNT supported metal phthalocyanine assembly and nano MnO₂ dual catalysts at ratios of 0:5,1:4,2:3,3:2,4:1 and 5:0.The influence of the central metal ions of metal phthalocyanine,the substituents on the phthalocyanine ring and the size of the conjugated planes on the catalytic activity of the catalyst were studied.It was found that the activity of the same series of metal phthalocyanines,the catalytic activity of dual catalyst was mainly affected by the nature of the central metal ions,and the order of catalytic activity was Fe>Co>Ni>Cu.When the center metal ions of phthalocyanine were the same,the catalytic activities of dual catalysts were mainly affected by the substituents on the phthalocyanine ring.The more electron withdrawing ability of the substituted group led to the higher catalytic activity for the dual catalyst.The larger conjugated system gave the better catalytic activity.The catalytic activity of the binuclear metal phthalocyanine sharing one benzene ring was higher than that of the mononuclear metal phthalocyanine.The catalytic activity of the mononuclear metal phthalocyanine was higher than that of the binuclear metal phthalocyanine sharing biphenyl ring.The dynamic characteristics of dual catalysts with optimal ratio modified glassy carbon electrode for oxygen reduction reaction were determined according to the relationships between the peak current and the scanning rates as well as the square root of scanning rates.The results showed that both the scanning rates and the square root of the scan rates were proportional to the peak currents.Therefore,the oxygen reduction reaction occurring on the dual catalysts modified electrode was controlled by both the adsorption and diffusion processes.The mechanism of the oxygen reduction reaction with the dual catalysts was investigated by employing a rotating disk electrode technique.The results showed that binuclear metal phthalocyanines sharing one benzene ring?including non-substituted,nitro-substituted,and amide-substituted?-MnO₂ showed a one-step,four-electron process for the oxygen reduction reaction,whereas binuclear metal phthalocyanines sharing biphenyl ring?including non-substituted,nitro-substituted,amide-substituted?-MnO₂ mainly exhibited a two-step,two-electron pathway for the oxygen reduction reaction.The catalytic activities of the mononuclear metal phthalocyanines were lower than that of binuclear metal phthalocyanines sharing one benzene ring,and higher than that of binuclear metal phthalocyanine sharing biphenyl ring.Methanol tolerance tests indicated that all the self-prepared dual catalysts with the optimal ratio owned good methanol-tolerant abilities.The durability of the catalysts for the oxygen reduction reaction was determined with chronoamperometry method.The results suggested that the addition of nano-MnO₂ could not improve the durability of the metal phthalocyanine.