Studies On Non-noble Metal Catalysts And Fuel Cells Enhanced By Ion Conduction

Fuel cells are environmental friendliness,with high energy conversion rate and wide application fields,thereby attracting great attention around the world.Performance of fuel cells mainly depends on activity of cathode and anode catalysts.In particular,the kinetics of oxidation reduction reaction(ORR)of cathode play a determining role in power density improvement.Conventional fuel cells use carbon-supported platinum(Pt/C)as cathode catalyst,because Pt has excellent catalytic activity and good stability.However,as a precious metal,platinum is expensive and scarce,leading to the difficulty to be widely used,which limits the fuel cell commercialization.The development of non-noble metal catalysts with high catalytic activity is key to the development of fuel cells.Direct borohydride fuel cell(DBFC)is an alkaline fuel cell with high energy density electromotive force,using non-noble metal as catalyst.However,its large electrode polarization impedes the performance improvement.In this work,DBFC was used as the main research system to study the effect of the active site construction and ion conduction enhancement on the performance of catalysts and fuel cells.Porous carbon materials yield larger specific surface area than traditional carbon carriers,able to support more electrocatalytic active sites.In-situ synthesis of active sites in porous carbon is one of the effective methods to enhance its dispersion and homogeneous distribution,thereby improving the cell performance.Pyrrolic nitrogen converts into active pyridinic nitrogen during carbonization.The coordination of pyrrole and transition metal can be used to form a number of active sites M-Nx.Therefore,pyrrole is an ideal nitrogen source to be used to synthesize ORR catalysts of high performance.In this study,we synthesized non-noble metal porous catalysts using glucose as the carbon source,pyrrole as the nitrogen source,cobalt nitrate as the transition metal source and calcium carbonate as template.By means of electrochemical methods such as constant current discharge,cyclic voltammetry,rotating disk electrode and AC impedance spectroscopy,combined with field emission scanning electron microscopy(SEM),transmission electron microscopy(TEM),fourier transform infrared spectroscopy(FT-IR),X-ray powder diffraction(XRD),nuclear magnetic resonance(NMR),X-ray photoelectron spectroscopy(XPS)and near-edge X-ray absorption fine structure(XANES),the catalytic sites and their structure-activity relationships were studied fundamentally.It is found that the molar ratio of pyrrole to cobalt nitrate significantly affected on the catalytic activity of the catalyst.When the molar ratio of pyrrole to cobalt nitrate reaches 2:1,the resultant catalyst demonstrates high initial reduction potential and peak current density,regardless of acidic or alkaline conditions.Co in catalyst precursor exists in the form of Co(II).The precursor structure of the catalytic active center consists of two nitrogen atoms and one cobalt atom to form N-Co-N bondage.In the process of pyrrole polymerization,the precursor structure was formed synchronously with double-chain structure through binding of cobalt with two long chains of polypyrrole.This structure enhanced the formation of pyridinic nitrogen and Co-Nx during carbonization.The maximum power density of the DBFC with such a cathode catalyst reaches 230 mW·cm-2 at room temperature,which is close to that of the DBFC with 28.6wt%Pt/C.In order to efficiently utilize the abundant active sites in porous catalysts,smooth mass transfer is necessary.Traditional porous catalysts have high mass transfer resistance in the pores.Particularly the ionic conduction in pores is rather poor,leading to the low utilization of active sites and high overpotential.Enhancing the ionic conduction is one of the effective ways to reduce cathodic polarization.Vacuum pouring of cation exchange resin(Nafion)into the inner pores of porous catalysts can enhance cation conduction,thereby improving the utilization of catalytic sites.The product was crushed by ball milling to obtain catalyst particles without resin coated on the outer surface,thereby avoiding the influence of resin on the electronic conductivity.The cathode catalyst performance was significantly improved after vacuum pouring and ball milling treatment.The DBFC reached high power density nearly 300 mW·cm-2 at room temperature,far exceeding the performance of 28.6 wt.%Pt/C catalyst.It is found that ion exchange resin can significantly reduce the polarization of fuel cells by regulation of ion conduction in electrode layer.The anodic reaction of DBFC is a special all-anion electrochemical reaction.The addition of anion exchange resin in anode layer enhances borohydride ions(BH4-)and hydroxide ions(OH-)transfer leading to the fast anodic reaction.At the same time,the generated borate ions(BO2-)diffuse away in time,reducing the anodic polarization.In the cathode layer,the cation exchange resin(Nafion)is used for sodium ions(Na+)conduction,and the anion exchange resin is used for removal of hydroxide ions(OH-)generated by the reaction,which can prevent the accumulation of alkali in the cathode and reduce cathode polarization.At the same time,anion exchange resin can localize HO2-,promoting indirect 4-electron reaction,and increasing the catalytic performance of cathode catalyst.The resultant cell presents an open circuit voltage as high as 1.08V by adding D201 anion exchange resin in anode.The performance of DBFC is improved significantly.Its maxim1un power density reaches 420 mW·cm-2.Ion exchange resin addition can regulate ion conduction of electrode layer in other fuel cells,such as direct hydrazine fuel cell(DHFC).Blending anion exchange resin and cation exchange resin in cathode,and anion exchange resin in anode increases the power density of DHFC by 23%,up to 320 mW·cm-2 at room temperature.This result confirms the important role of ion exchange resin regulation on improving the performance of fuel cells.