Preparation Of Supported Metal Catalysts And The Electrocatalytic Performance For Hydrazine Electrooxidation

Direct hydrazine fuel cell employing hydrazine as fuel has gained a great deal of attention due to its several attractive advantages.Firstly,only nitrogen and water without greenhouse gas are produced in the process of hydrazine electrooxidation.Therefore,DHFC achieves a zero pollution emission.Secondly,hydrazine electrooxidation does not produce species which can poison the electrocatalysts(such as CO).Thirdly,DHFC has high theoretical voltage and power density.However,hydrazine electrooxidation has a sluggish kinetics and the electrocatalytic activities of catalysts remain to be enhanced,which are the key problems for DHFC.Therefore,it is necessary to develop catalysts with high electrocatalytic activities for hydrazine electrooxidation.Noble metal catalysts exhibit excellent electrocatalytic performance for hydrazine electrooxidation.However,high cost and limited reserves of noble metal catalysts have hindered their wide application.To solve these problems,several highly active and economic supported metal catalysts were prepared and used as catalysts for hydrazine electrooxidation in this paper.The surface morphology and phase composition of electrode materials are characterized by X-ray diffraction spectroscopy(XRD),scanning electron microscope(SEM)and transmission electron microscopy(TEM).The electrooxidation behaviors of hydrazine on the electrode in alkaline solution are investigated by cyclic voltammetry,linear sweep voltammetry,chronoamperometry and electrochemical impedance spectroscopy.The Ag/CFC electrode was prepared by electrodeposition of Ag particles directly on carbon fiber cloth(CFC)by a square wave potential electrodeposition method.The morphology of silver particles was controlled by adjusting the electrodeposition parameters including oxidation potential,reduction potential,deposition frequency and deposition time.SEM measurement showed that Ag displays microspheric structure and the mean particle size is around 200 nm.The electrocatalytic performance of the Ag/CFC electrode for hydrazine electrooxidation was investigated.It was found that the Ag/CFC electrode possessed high electrocatalytic activity toward hydrazine electrooxidation.An oxidation peak potential of-0.2 V and an oxidation peak current density of 30 mA·cm-2 were achieved.The kinetics of hydrazine electrooxidation on the Ag/CFC electrode was studied.The results showed that the hydrazine electrooxidation on the Ag/CFC electrode is an irreversible process and controlled by a diffusion process.Hydrazine electrooxidation on the Ag/CFC electrode proceeds via a four electron process.The reaction order of hydrazine electrooxidation on the Ag/CFC electrode is one in respect to hydrazine.Raising temperature is beneficial to the hydrazine electrooxidation on the Ag/CFC electrode.However,the reaction mechanism of hydrazine electrooxidation on the Ag/CFC electrode does not vary with temperature.Hydrazine electrooxidation on the Ag/CFC electrode was much easier at a more positive potential where the species AgOHads is involved.The Co/CFC electrode was prepared by electrodeposition of Co particles directly on CFC via a square wave potential electrodeposition method.SEM measurement showed that Co presents as nano-sheets and micro-particles intersected mutually.The nano-sheets have a thickness of around 250 nm.The size of the micro-particles is up to around 10 ?m.The electrocatalytic activity of the Co/CFC electrode toward hydrazine electrooxidation was investigated.The results indicated that the onset potential for hydrazine electrooxidation on the Co/CFC electrode was-1.1 V.A shoulder oxidation peak at-0.87 V and a strong oxidation peak at-0.75 V were achieved,respectively.The oxidation peak at-0.87 V can be ascribed to the electrooxidation of hydrogen produced by the decomposition of hydrazine.The oxidation peak centered at-0.75 V can be attributed to the direct electrooxidation of hydrazine.Hydrazine electrooxidation on the Co/CFC electrode proceeds via both direct and indirect pathways.The kinetics study showed that hydrazine electrooxidation on the Co/CFC electrode is an irreversible process and controlled by a diffusion process.The reaction order of hydrazine electrooxidation on the Co/CFC electrode is one in respect to hydrazine.A three-dimensional porous copper film was directly deposited on Cu foam by an electrodeposition method using hydrogen bubbles as dynamic template(Cu/Cu foam).SEM measurement showed that pore diameter of copper film is about 80?150 ?m.The mean width of copper film walls is about 40 ?m.The copper film walls are composed of numerous interconnected pine-like dendrites.The individual dendrite is made up of branches with the length of approximately several hundreds nanometer to several micron.The electrocatalytic activity of the Cu/Cu foam electrode toward hydrazine electrooxidation was investigated.It was found that compared with Cu foam,hydrazine electrooxidation on the Cu/Cu foam electrode showed that the onset oxidation potential displayed a?100 mV negative shift,the current density at-0.6 V raised about 14 times,the apparent activation energy and the charge transfer resistance reduced significantly,indicating that the Cu/Cu foam electrode decreased the overpotential of hydrazine electrooxidation effectively and the hydrazine electrooxidation was more easily.The Cu/Cu foam electrode possessed the superior electrocatalytic activity toward hydrazine electrooxidation over Cu foam.Hydrazine electrooxidation on the Cu/Cu foam electrode proceeds via a four-electron process,which indicated hydrazine electrooxidation was almost completely achieved and the use efficiency of hydrazine was high.A series of multiwall carbon nanotubes(MWCNTs)supported Pd-Co catalysts(Pd-Co/MWCNTs)with different mass ratios of Pd to Co were prepared by chemical reduction method.The electrocatalytic activities of the Pd-Co/MWCNTs catalysts toward hydrazine electrooxidation were investigated.It was found that the Pd-Co/MWCNTs catalyst with a mass ratio of 1:1 for Pd:Co showed superior electrocatalytic performance for hydrazine electrooxidation over both the Pd/MWCNTs catalyst and the Co/MWCNTs catalyst,which can be attributed to the larger electrochemical active surface area of the Pd-Co(1:1)/MWCNTs catalyst.The transfer electron number for hydrazine electrooxidation on the Pd-Co(1:1)/MWCNTs catalyst is four,suggesting the high use efficiency of hydrazine fuel.