Investigation Of Cobalt-based Oxygen Reduction Catalysts And Design & Optimization Of Membrane-free Fuel Cells

Fuel cells have attracted extensive attention due to the non-polluting reaction products and high energy conversion efficiency.However,the high cost of fuel cells has hampered its large-scale application due to the use of precious metal catalysts and proton exchange membranes.The sluggish kinetics of oxygen reduction reaction?ORR?at the cathode is a key factor limiting the development of fuel cell.Noble metal Pt-based catalysts have been recognized as efficient ORR catalysts,but its large-scale application is limited due to the scarce resources and high costs.On the other hand,the proton exchange membrane has high requirements for proton transport,water absorption,corrosion resistance,strength and insulation,resulting in its high cost.Developing high performance and low cost catalysts to replace Pt-based catalyst and eliminating the use of proton exchange membrane is a feasible way to reduce the high cost of fuel cell and promote its practiacl applications.In this work the highly efficient ORR catalysts and the membraneless anaerobic direct methanol fuel cells?DMFC?is demonstrated.The membraneless design is that the proton exchange membrane and air cathode is replaced with an open-spacer and supporting electrolyte and high potential Fe³⁺/Fe²⁺redox species.This eliminates the use of expensive Pt-based catalysts and proton exchange membranes,which finally reduces the cost of fuel cells and improves its performance.The main conclusions are summarized as follows:?1?The flower-like CoS,octahedral CoS₂ and CoS₂/rGO composite catalysts are prepared by one-step hydrothermal method.As a result,the CoS₂ particles are uniformly dispersed on the wrinkled rGO sheet,and the average size of the particles is also significantly reduced.The CoS₂ is higher activity than that of CoS due to the S-S bond.The high electronic cloud density can enhance the oxygen reduction reaction,which attributes to oxygen adsorption on the catalyst surface.The particle size of CoS₂ is decreased when supported on the surface of rGO.And its catalytic active area is greatly enlarged,which attributes that much more active sites is exposed.The rGO supported CoS₂ can be used to reduce the agglomeration between rGO sheets,which improves the catalytic performance.?2?The effect of activity of Ni or Fe doping amount has been investigated.Results show that the 20%doping amount possesses highest ORR activity.The Ni_0.2Co_0.8S₂/rGO and Fe_0.2Co_0.8S₂/rGO catalysts,the particle size is 350 nm and 250 nm,respectively,are prepared by compounding with rGO.The high catalytic activity and good long-term stability of Fe_0.2Co_0.8S₂/rGO are also realized.Further,the results show that the catalyst particle size has a significant effect on the catalytic activity,and the smaller particle size can enhance catalytic activity area and promote ORR performance.?3?The CoS₂/C₃N₄/rGO composite has been prepared while the mass ratio of C₃N₄ to rGO is 1:1.The CoS₂/C₃N₄ particles as spacer are distributed uniformly on rGO support sheets,which can promote the transport of substances and make full use of the active sites on the catalyst surface.Compared to CoS₂/C₃N₄(2.7 mA cm^-2)and CoS₂/rGO(3.8 mA cm^-2),CoS₂/C₃N₄/rGO(4.5 mA cm^-2)has positive onset potential?0.94V vs.RHE?,highest limited current density?comparable to commercial Pt/C?,good long-term stability and four-electron reaction process.Its catalytic activity is better than that of other catalysts.?4?A preliminary exploration is carried out to fabricate the membraneless anaerobic DMFC device,which is to replace the air cathode with high potential Fe³⁺/Fe²⁺redox couple solution,and proton exchange membrane with an open-spacer and supporting electrolyte.The operating parameters affecting performance of fuel cell are optimized to determine its optimal working condition as follows:60?,the flow rate and the concentration of the methanol and the Fe³⁺/Fe²⁺solution are 5 mL min^-1 and 1 mol L^-1,20 mL min^-1 and 2/0.22 mol L^-1,respectively.The loading of the catalyst is 1.5 mg cm^-2.The thickness of the open-spacer is0.8 mm,and the concentration of H₂SO₄ electrolyte is 0.5 mol L^-1.