Preparation And Performance Of Fe/Mn Doped Carbon-Based Catalysts For Oxygen Reduction Reaction

Benefiting from high energy density,fast start-up and environmental friendliness,proton exchange membrane fuel cells?PEMFCs?can be widely applied in distributed power station,transportation and potable equipment.The development of catalysts with high-performance and low-cost for sluggish oxygen reduction reaction?ORR?is the key to realize the commercialization of PEMFCs technology.To solve the challenge of insufficient activity and poor stability of current platinum group metal?PGM?free catalysts in acid media,the research in this dissertation focuses on the structure regulation and ORR catalytic performance of zinc-based zeolitic imidazolate framework?ZIF-8?derived carbon.ZIF-NC-Fe catalyst with atomically dispersed and high-density Fe-N₄ center was synthesized by hosting Fe ions into a nitrogen-doped carbon,which was carbonized from ZIF-8 precursors?ZIF-NC?,followed by a controlled thermal activation.With nearly identical carbon structure and nitrogen doping,the thermally-driven evolution of catalytic performance is exclusively originated from the structure evolution of active center.Different from traditional methods,pyrolysis of precursors that contained metal,nitrogen,and carbon sources,this approach is able to deconvolute Fe-N bond formation from carbonization and nitrogen doping,which allows us to study the formation mechanism of efficient Fe-N₄ center.Through correlating the measured ORR activity/stability with Fe-N coordination structures formed at different temperatures,we found that?i?Fe Ox particles can be transformed into atomically dispersed Fe-N₄ centers during thermal activation,increasing the density of active centers;?ii?inactive Fe-Nx configurations formed at room temperature will transform into Fe-N₄ centers with strengthening and shortening Fe-N bond,increasing the intrinsic activity/turnover frequency?TOF?of active center.Molecular dynamics simulation and first-principles calculations revealed that?i?the transformation of Fe Ox to Fe-N₄ is due to the higher thermal stability of Fe-N₄;?ii?Fe-N bond contraction will facilitate the O2 adsorption and subsequent O-O bond breaking process during the ORR.The unique microporous structure and nitrogen doping of ZIF-NC lead to chemical and space confinement effect,which is the key for the formation of high density and atomically dispersed Fe-N₄ center.After the optimization of synthesis conditions,the as prepared ZIF-NC-Fe catalyst showed superior activity: half wave potential?E1/2?in 0.5 mol/L H₂SO₄ reached up to 0.84 V?vs.RHE?,only ?20 m V lower than that of Pt/C;the maximum power density of H2-O2 fuel cell is 0.62 W/cm2,reaching 50% of Pt/C.Mn-ZIF-second catalyst with atomically dispersed and high density Mn-N₄ center was synthesized by a two-step doping/adsorption method,employing ZIF-8 as precursor.In the first step,Mn was introduced into ZIF-8 by chemical doping.The presence of Mn can not only increase the graphitization degree of carbon matrix,but also prevent the collapse of micropore during carbonization,increasing the corrosion-resistance and specific surface area of carbon matrix.Moreover,the morphology and micro-structure of carbon matrix can be tuned by varying the Mn contents in precursor.In the second step,confined adsorption can further increase the density of Mn-N₄ without introducing clusters.After the optimization of synthesis conditions,Mn-ZIF-second catalyst with high activity and stability was obtained for the first time,which showed comparable activity but significantly enhanced stability to Fe-ZIF-second catalyst: E1/2 in 0.5 mol/L H₂SO₄ reached up to 0.80 V?vs.RHE?,only ?20 m V lower than that of Fe-ZIF-second;the loss of E1/2 was only 17 m V after 30,000 cycles accelerated stress test,smaller than that of Fe-ZIF-second catalyst?29 m V?;the maintaining current density was up to 57% after 100 h test at constant potential of 0.8 V,which is much higher than the 39% of Fe-ZIF-second catalyst.The high activity is originated from the atomically dispersed Mn-N₄ center with high density,while the enhanced stability can be attributed to the strengthening Mn-N bonding and corrosion-resistant carbon matrix.Fe-NC@Mn-NC catalyst with core-shell structure was synthesized by coating Mn-ZIF on the surface of Fe-ZIF followed by high-temperature carbonization.After the optimization of metal content and reaction time,the as prepared Fe-NC@Mn-NC catalyst obtained superior stability like that of Mn-NC catalyst,with maintained high activity of Fe-NC catalyst: E1/2 in 0.5 mol/L H₂SO₄ reached up to 0.85 V?vs.RHE?,only ?10 m V lower than that of Fe-NC catalysts without Mn-NC shell;the maintaining current density was up to 77% after 10 h test at constant potential of 0.85 V,which is much higher than the 60% of Fe-NC catalyst.The high activity is originated from the high density and atomically dispersed Fe-Nx center in Fe-NC core,while the enhanced stability is due to the highly graphitized Mn-NC shell.Moreover,the synergetic effect between Fe and Mn in Mn-NC shell can not only increase the activity but also increase the stability of catalyst.