Non-Pt Catalysts For Low Temperature Hydrogen-Oxygen Fuel Cells

Low temperature hydrogen-oxygen fuel cells,especially proton exchange membrane fuel cell?PEMFC?and alkaline polymer electrolyte fuel cell?APEFC?have such advantages as high energy transfer efficiency,without pollution,quick start at room temperature and long life.They do have good application prospect and make a difference in facing energy crisis and environmental issues.However,the fact that electrode reactions at cathode and anode rely heavily on precious metal platinum?Pt?makes low temperature hydrogen-oxygen fuel cells have difficulty in large scale commercialization.The main difference between PEMFC and APEFC is the pH of their operation environments.PEMFC works in acid.Oxygen reduction reaction?ORR?at the cathode of PEMFC has sluggish electrochemical dynamics,which is the main reason for high cost of PEMFC.In the past decades,few catalysts without Pt?Pt-free?have been developed to match Pt-based catalysts in acid.In contrast,some Pt-free catalysts can match Pt-based catalysts in alkaline solutions.Hence,ORR of APEFC which operates in base can apply Pt-free catalysts.But,the activity of Pt-based catalyst for hydrogen oxidation reaction in alkaline electrolyte is 2³ orders of magnitude lower than that in acid electrolyte.So in alkaline electrolyte,anodes require much more Pt for HOR,increasing the cost of APEFC.Above all,two very important points in large scale commercialization of low temperature hydrogen-oxygen fuel cells are:1)developing Pt-free catalysts for ORR with high performance in acid;2)exploiting alternatives of Pt-based catalysts for HOR in base.Therefore,we have carried out the following two aspects research work:?1?Directing at developing Pt-free ORR catalysts with high performance in acid,Fe-N-C catalysts were paid more attention for its high activity,easy preparation and low cost.We investigated the relationship between the molecular structures of PANI and morphology,properties and active sites formation of the final Fe-N-C catalysts.The content of quinoid rings?QR?in PNAI was controlled by varying molar ratios of ammonium persulfate?APS?to aniline monomers?AN?.With the increase of APS/AN ratio,the QR content of PANI increased.After pyrolysis the composite of ferric chloride and PANI in N₂ at 900?,the target catalysts were obtained.The morphologies of the final catalysts showed a structural evolution from bulk,nanorods to flower-like structure with increase of QR in PANI precursors.The ORR activity,however,showed volcano-relationship depending on the QR content in PANI precursors.The QR benefits the Fe coordination and Fe-Nx active sites formation during the pyrolysis.However,when APS was excessive,except more QR produced,accompanying formed oligomers decreased the thermal stability and the amount of active sites.Specifically,when APS/AN=2.5,the derived PANI with moderate QR content benefited to produce Fe-N-C catalyst with most Fe-Nx sites,high specific surface area,and consequently the maximum ORR activity.?2?Aim at the problem that the Pt-based catalysts for HOR declined significantly in base,we produced new HOR catalysts which have a honeycomb like porous morphology derived from ruthenium?Ru?and iron?Fe?,with phytic acid as complexing agent.Ru was selected as precursor for the reason that it has enough HOR activity,and it is relatively cheap?the price of Ru is only about 4%of Pt?and CO anti-poisoning.The study found that in comparison to Ru,Fe had stronger complexation ability to P to form FeP,thus freed Ru.The catalyst had some HOR catalytic activity in base.After acid leaching,the HOR activity of the catalyst increased due to the removal of FeP and lowered hydrogen binding energy.