Experimental And Density Functional Theory Investigations Of Fuel Cell Catalysts

The rapid development of the human society has motivated a heavy demand of energy.The traditional fossil fuels have been overspended and on the brink of exhaustion.The development and mass market promotion of new energy resources and improving energy conversion efficiency hence become particularly urgent.Fuel cells,a new efficient power technolygy,can convert chemical energies storaged in chemical compounds such as hydrogen and ethanol into electricity directly.Proton exchange membrane fuel cell(PEMFC)and direct ethanol fuel cell(DEFC)are two eco-friendly fuel cells,which use green renewable energy as the reactant and only produce CO₂ and H₂O.However,the mass commercialization of fuel cells is hindered by a lack of efficient and cheap catalysts.The research and development of fuel cell catalysts for oxygen reduction reaction(ORR)and ethanol oxidation reaction(EOR),were conducted by means of experiments and computations.Oxygen is reduced at the cathode side of the fuel cell,which is also the rate-determining-step of fuel cell reactions.Nanoparticle(NP)electrocatalysts for ORR were studied using experimental method.The influence of the thermal treatment temperatures under hydrogen atmosphere of the catalysts on the structure and the activity towards ORR was examined.Based on high energy-X ray diffraction(HE-XRD)coupled with pair distribution functional(PDF)analyses,and electrochemical ORR activity testing results,Pd Cu/C catalyst thermally calcinated at100?with a single face centered cubic(fcc)structure reveals the highest mass activity for ORR.Pd Cu/C catalysts thermally annealed at temperatures higher than100?exhibit a mixture of fcc and body centered cubic(bcc)structures and a much-decreased ORR activity.Density functional theory(DFT)calculations show a lower activation energy of the O-O bond cleavage on the fcc-structured Pd Cu/C catalyst comparing with that on the bcc-structured Pd Cu/C catalyst.PtPdCu NP catalysts with different compositions were synthesized and prepared by wet chemical method.The correlations between the compostions and the activity of the catalysts were studied.The key influence factors of the stability of the catalysts were studied as well.The results showed that PtPdCu/C catalyst with an atomic ratio of 1:1:3 shows the highest mass activity of 1.66 A/mg _Pt for ORR and no depletion after 50,000 potential cycles.PtPdCu/C catalysts before and after durability test remained an alloy structure with only a small portion of Cu leaching out.The concept of alloy catalytic active site was proposed in contrast to the‘Pt-skin'concept in the ORR community.The existence of Cu in an alloy state on the surface of the catalysts after long-term durability test was confirmed.The addition of Cu would greatly reduce the reaction barriers of the key ORR elementary steps on Pt-based catalysts were further studied by DFT calculations.DEFC shares the same cathodic reaction with PEMFC while EOR occurs at its anode.Currently,EOR is lack of efficient catalysts towards complete oxidation.The development of EOR catalysts is challenged by two major issues,i.e.,the difficulty of C-C bond cleavage and a great likelihood of acetic acid formation.The reaction mechanisms of EOR on Cu(100)and Pd(100)surfaces were studied by DFT calculations.The C-C bond and C-H bond cleavages on the key intermediates and the effect of alkaline media were investigated.Cu(100)surface shows high reaction barriers for the C-C bond cleavages while reveals an excellent catalytic performance for the C-H bond activation.Pd(100)surface shows lower reaction barriers for the C-C bond cleavages than those on Cu(100)surface.However,acetyl prefers to form acetate in alkaline media on both Cu(100)and Pd(100)surfaces,that is,the main product is still acetate.The DFT results provided fundamental basis for the future design of EOR catalysts.