Preparation And Application Of Pd-based Electrocatalysts As Anodes Of A Novel Fuel Cell With Fe(?) As An Oxidant

Direct alcohol fuel cell?DAFC?and direct formic acid fuel cell?DFAFC?have been paid much attention due to their high energy density and easy storage and convenient transportation of fuels.A DAFC?or DFAFC?uses oxygen?or air?as the oxidation agent to carry out the cathode reaction,e.g.,oxygen reduction reaction?ORR?.However,ORR is a sluggish kinetic process even on Pt or Pt-based catalysts that have been considered to be the most efficient electrocatalysts for ORR by far.This leads to the decline of the cell performance and the increase of the cell cost.Although much considerable work on the development of the ORR catalysts has been done,some key issues for ORR are still not well resolved.These severely limit the development of the fuel cells with the ORR as the cathode reaction.Therefore,it is of significance to use alternative oxidation agents to oxygen?or air?.Fe³⁺ ion is another oxidant,which is characterized by its wide material sources,high stability and higher redox potential(Eo(Fe³⁺/Fe²⁺)= 0.771 V vs SHE).In addition,reduction reaction of Fe³⁺ to Fe²⁺ is a relatively easy process on common catalysts,this is because Fe³⁺/Fe²⁺ couple possesses a higher exchanging current density and corresponding electrode process for the redox couple Fe³⁺/Fe²⁺ is a quasi-reversible electrochemical process.Carbon,graphite and platinum have been proved to be the efficient catalysts for Fe³⁺ electro-reduction and the redox couple Fe³⁺/Fe²⁺.In addition,highly efficient electro-oxidation of alcohols is another crucial factor to ensure the normal operation of the DAFC.Pd and Pd-based catalysts have been proved to show high electroactivity for alcohol oxidation in alkaline media.A large number of Pd-based catalysts for alcohol electro-oxidation have been reported,including Au-modified Pd,Pd Ni,Pd Sn et al.All the same,it is significant to develop novel Pd-based catalysts with a high dispersion of the catalyst particles and notable enhancement of the electroactivity for alcohol oxidation.In this work,we constructed a novel fuel cell where anode reaction was alcohol?or formic acid?oxidation in alkaline medium and cathode reaction was Fe³⁺ reduction reaction instead of the conventional ORR.?1?The anode catalysts Pd Sn/b-cd-CNT,Pd Sn/CNT,Pd/b-cd-CNT,Pd/CNT and Pd/ b-cd-C,are prepared by using multi-walled carbon nanotube?CNT??or carbon powder C?and b-cyclodexdrin?b-cd?modified CNT?b-cd-CNT??or b-cd modified C:b-cd-C?as the substrates to immobilize Pd Sn and Pd nanoparticles in glycol solvent.The catalysts were characterized by TEM and XRD,and the electroactivity and stability for C1-C3 alcohol oxidation were tested by cyclic voltammograms and chronamperometry in alkaline media.Results showed that compared to other catalysts,the Pd Sn/b-cd-CNT displays much higher current densities for oxidation of methanol,ethanol,n-propanol and iso-propanol,The biggest oxidation current density was94?145?184 and 87 m A?cm-2,respectively.?2?Fe₃O₄-C composites with different Fe₃O₄-loadings were firstly prepared by coating Fe₃O₄ nanoparticles on carbon powder.Binary Pd Sn nanoparticles were then prepared by Na BH4 reduction method and deposited on Fe₃O₄-C support to obtain the nano-catalysts Pd Sn/Fe₃O₄-C.Transmission electron microscopy?TEM?and X-ray diffraction?XRD?techniques were used to characterize the physical properties of the Pd Sn nano-catalysts.Their electroactivity for ethanol oxidation in alkaline media was studied with cyclic voltammetry?CV?,chronoamperometry?CA?and electrochemical impedance spectra?EIS?.The results show that the Pd Sn nanoparticles are well dispersed on the Fe₃O₄-C support.Among the prepared catalysts?Pd Sn/Fe₃O₄?2%?-C,Pd Sn/Fe₃O₄?5%?-C and Pd Sn/Fe₃O₄?10%?-C?,the Pd Sn/Fe₃O₄?5%?-C catalyst shows the highest catalytic activity for ethanol oxidation,characterized by the ethanol oxidation peak current density of 147 m A cm-2.Also,addition of Fe₃O₄ to Pd nanoparticles improves the electrochemical stability of the catalyst.Compared with the Pd/C,the Pd Sn/Fe₃O₄?5%?-C presents the much lower charge transfer resistance for ethanol oxidation reaction.?3?A novel alcohol fuel cell is constructed by using Fe³⁺ as the oxidation agent instead of the conventional O2.Various alcohols as the fuels are tested,including methanol,ethanol,n-propanol and iso-propanol.In this fuel cell,the anode catalysts tested are Pd Sn/Fe₃O₄?5%?-C? Pd Sn/b-cd-CNT,Pd Sn/CNT,Pd/b-cd-CNT,Pd/CNT,Pd/b-cd-C and commercial Pd/C.Fe³⁺ reduction reaction is carried out on the cathode made of carbon powder.The anolyte?alcohols in 1 mol L-1Na OH?and catholyte(Fe³⁺in 0.5 mol L-1Na Cl)are separated with a Nafion 117 membrane.Open circuit potential?OCP?of the cell with the anode Pd Sn/ b-cd-CNT is 1.14 1.22 V,depending upon different alcohols.The maximum power density?m W cm-2?is 15.2 with methanol fuel,16.1 with ethanol fuel,19.9 with n-propanol and 12.2 with iso-propanol.OCP of the cell with the anode Pd Sn/Fe₃O₄?5%?-C is 1.20 V in ethanol,and the maximum power density is 18.0 m W?cm-2.?4?A new formic acid/iron ion?III?fuel cell was constructed by using formic acid as the fuel and Fe3 + as oxidant.Multi-walled carbon nanotubes?MWCNTs?and?-cyclodextrin??-CD?modified MWCNT??-CD-MWCNT?-supported palladium-based catalysts: Pd/MWCNT,Pd Sn/MWCNT,Pd/?-CD-MWCNT and Pd Sn/?-CD-MWCNT were synthesized and used to electro-catalyze formic acid oxidation.Cyclic voltammograms?CVs?and chronoamperograms?CAs?were applied to investigate their electroactivity for formic acid oxidation in alkaline media.The results showed that addition of suitable amount of tin to palladium and modification of MWCNT with?-CD can promote electrocatalytic activity of Pd/MWCNT catalyst for formic acid oxidation.The formic acid/iron ion?III?fuel cell with the prepared Pd-based catalysts as anode and carbon powder as cathode presents the open circuit voltage of 0.98¹.2 V.The fuel cell with Pd Sn/?-CD-MWCNT as anode,exhibits the maximum current density of 50 m A?cm-2 and the maximum power density of 12.6m W?cm-2,showing a much better cell performance than the fuel cell with Pd/C as the anode.