Preparation And Electrochemical Properties Of Carbon Modified Substrates Based Ni,Co,Au Catalysts

The electrode material is one of the key components of fuel cells(FCs),the choice of which decides the performance and applied range of the FCs.Generally,the FCs electrodes are prepared by a "slurry and coating" method,which coat the mixed active material,conductive agent and binder on the metal substrate surface uniformly.Nowadays,the design of electrode substrates and the method of loading active materials on the surface of substrates become research hotspots.Scientists have focused their eyes on the fabrication of three-dimensional(3D)structure based electrodes without binders to overcome the drawbacks of low utilization efficiency and poor electrocatalytic activity of electrodes prepared by the"slurry and coating" method.Currently,with the rapidly developing of wearable electronic devices,as the core component,the design of flexible electrodes has attracted scientists' wide attention.NaBH4 and N2H4 own high hydrogen content,and their storage method is easier than the liquid hydrogen,which has an enhanced safety;H2O2,which can act as both fuel and oxidizer in FCs and only generate H2O and O2 from the electrochemical reaction,is always applied in the electrochemical devices of underwater and aerospace fields.In this paper,conductive Ni foam and flexible sponge,fabric,paper,plastic are employed as the substrates to prepare the electrodes in FCs towards NaBH4,N2H4,H2O2 electro-oxidation and reduction,through coating,dipping and drying,pasting,electro-depositing,chemical reducing,and so on.The physical characterization means,such as scanning electron microscopy(SEM),transmission electron microscopy(TEM),X-ray diffraction(XRD),Raman,Fourier Transform Infrared Spectoscopy(FT-IR),inductive coupled plasma emission spectrometer(ICP),and the electrochemical test methods,such as cyclic voltammetry(CV),linear sweep voltammetry(LSV)and chronoamperometry(CA)are employed to characterize the physical structure and electrochemical property of the electrodes.The as-prepared AB5 type alloy powder was mixed uniformly with multi-walled carbon nanotubes(MWNTs)through mechanical agitation at different mass ratio in a solution containing polytetrafluoroethylene(PTFE)as the binder.The mixture was smeared onto a Ni-foam sheet by a simple "slurry and coating" method as the electrocatalyst for NaBH4 oxidation.The MWNTs not only improve the conductivity of the electrodes,but also act as the hydrogen adsorbent that adsorbs the hydrogen generated from the incomplete electrooxidation and hydrolysis of BH4-.The MWNTs content has a great effect on the electrocatalytic activity of the electrode.The optimized content of MWNTs was found to be 2 wt.%.The steady state current density for NaBH4 electrooxidation at the AB5/MWNTs(2 wt.%)electrode is about twice of that at the AB5 electrode without MWNTs.The utilization efficiency of NaBH4 at the AB5/MWNT5(2 wt.%)electrode reaches 61.5%,which is higher than that at the pristine AB5 electrode(50.9%).Reduced graphene was coated on the skeleton of Ni foam by facile dipping and electroreducing processes,and then reduced graphene network was obtained(RGN).After that,the RGN was employed as a substrate to support Au nanoparticles and served as catalysts towards NaBH4 electrooxidation.RGN improves the dispersity of Au and the whole preparation process doesn't employ any binder.The as-prepared RGN supported Au electrode owns a better electrocatalytic activity than the bare Ni foam based Au.The oxidation current density reaches 500 mA·cm-2 in 2 mol· dm-3 NaOH and 0.1 mol·dm-3 NaBH4 at 0 V.The MWNTs coated sponge(MWNTs/sponge)substrate was prepared by a simple"dipping and drying" method.A novel Ni electrode with a unique 3D porous structure was prepared by electrodeposition of spherical Ni particles onto the MWNTs/sponge and used as the catalysts for NaBH4 electrooxidation.The as-prepared Ni@MWNTs/sponge keeps the 3D porous structure of the sponge,which is beneficial for the diffusion of fuel.Besides,the sponge owns a strong hygroscopicity,ensuring the close contact between the solution and catalysts.All of the above mentioned factors make the Ni@MWNTs/sponge possess a superior catalytic activity towards NaBH4 electrooxidation.The oxidation current density reaches as high as 300 mA·cm-2(20 mA/(cm2·mg))at-0.7 V in 2.0 mol·dm-3 NaOH and 0.1 mol· dm-3 NaBH4,which is significantly higher than that reported for other types of Ni electrodes.It is worth to note that the as-prepared Ni@MWNTs/sponge owns a good flexibility,which can be applied in wearable electronic devices.MWNTs coated fabric with 3D hierarchical structure based transition metals Co and Ni were fabricated by similar "dipping and drying" and electrodeposition processes.Results showed that the as-prepared electrodes exhibited high electrocatalytic activity for NaBH4,H2O2 and N2H4 oxidation.The oxidation current density at Co@MWNTs/Fabric reaches 170 mA·cm-2 at-0.7 V in 1.0 mol·dm-3 NaOH and 0.1 mol·dm-3 NaBH4.When the solution contains 2 mol·dm-3 NaOH and 2.5 mol·dm-3 H2O2,the oxidation current density at Ni@MWNTs/Fabric reaches 720 mA·cm-2 at 0.5 V.The onset potential stayed at around-0.9 V and the oxidation current density reaches as high as 12 mA·cm-2 in the solution containing 1 mol dm'3 NaOH and 20 mmol·dm-3 N2H4 at around-0.80 V,both of which outstrip the previous reports.Paper based Co catalysts were prepared by the pencil drawing and electrodeposition processes.The as-prepared Co@Graphite/Paper and CoPd@Graphite/Paper electrodes own a special 3D structure and exhibit high catalytic activity for H2O2 electrocatalysis in alkaline solution.The oxidation current density at Co@Graphite/Paper reaches 580 mA·cm-2 in 2 mol·dm-3 NaOH and 0.5 mol·dm-3 H2O2 at 0.5 V,and the reduction current density at CoPd@Graphite/Paper reaches-4.30 A·cm-2·mg-1 in 1 mol·dm-3 NaOH and 1.4 mol·dm-3 H2O2 at-0.5 V per unit of Pd.The Co(OH)2 and Co(OH)3 serve as the active sites in the H2O2 electrooxidation.The two paper based electrodes can be used in flexible electronic devices due to their flexibility.The using of paper as the electrode substrate reduces the using of metal substrates and saves the metal resources.Besides,the paper can be degraded and is more environmental.A novel MWNTs/Plastic substrate was first fabricated by adhering MWNTs on a piece of obsoleted plastic bag,and Co nano-thorns were subsequently prepared by a simple electrodeposition method on the MWNTs/Plastic surface to prepare Co@MWNTs/Plastic electrode.The electrode was employed to catalyze NaBH4 electrooxidation and exhibited good electrochemical performance.The oxidation current density at Co@MWNTs/Plastic reaches 110 mA·cm-2 in 3 mol·dm-3 NaOH and 0.1 mol·dm-3 NaBH4 at-0.73 V.The electrochemical reactions at the Co@MWNTs/Plastic electrode include the direct oxidation of hydrogen at metallic Co and MWNTs and the oxidation of BH4-at Co(OH)2.