Preparation And Investigation Of Carbon-based Non-precious Metal Catalysts Derived From Biomass For Oxygen Reduction/Oxygen Evolution Reactions

The oxygen reduction reaction?ORR?and oxygen evolution reaction?OER?catalysts play critical roles in the large-scale application of various electrochemical energy conversion devices,including fuel cells,electrolysis of water,and metal-air batteries.Currently,noble metals?e.g.,Pt?and metal oxides?e.g.,RuO2 and IrO2?are widely used electrocatalysts for the ORR and OER,respectively.However,noble-metal-based catalysts have the drawbacks of scarcity and prohibitive cost,all of which have seriously hampered the development and commercialization process of these devices.Thus,the development of efficient and inexpensive non-precious metal ORR and OER catalysts is of great significance for promoting commercialization of these devices.Carbon materials have been known as the most promising candidate to replace noble-metal-based catalysts among non-precious metal catalysts,because of their excellent catalytic activity,low cost,good electrical conductivity and stability.The ORR/OER performance of carbon materials is closely connected with their composition and structure.Furthermore,the ORR/OER activity of carbon-based catalysts can be significantly enhanced through properly doping with heteroatoms and constructing appropriate microstructures,such as N dopants,coordination centers,and porous structures.Biomasses owns natural microstructure and containin numerous heteroatoms?e.g.,N,P,and S?.Therefore,biomasses are considered as the ideal precursors for the green synthesis of carbon-based catalysts.In this dissertation we first choose natural biomasses which own rich nitrogen and phosphorus and specific structure as precursors.Then,we design and prepared a series of biomass-derived heteroatom-doped porous carbon for ORR and OER catalysts using hydrothermal,impregnating,and freeze-drying methods.The results of the study are mainly included as follows:?1?A high-performance spherical heteroatom-doped carbon catalyst for oxygen reduction reaction?ORR?,with porous structures and high surface areas,has been prepared through a hydrothermal treatment and subsequent carbonization procedures,by using brewer's yeast powder as the precursor.Electrochemical measurements demonstrate that the catalyst has excellent ORR performance,with a 28 mV more positive half-wave potential?0.861 V vs.RHE?compared to that of commercial 20 wt.%Pt/C?0.833 V vs.RHE?in an alkaline medium.The catalyst also displays remarkable methanol tolerance and notable durability and follows a 4-electron transfer process.The addition of Fe not only induced the formation of spherical morphology,but also increase the proportion of pyridinic N,graphitic N and P-C species as well as specific surface area and pore structure.When the FeCl3/BYP mass ratio is 2:5,the BY-Fe-A catalyst achieves the optimal ORR activity.?2?A Chlorella-derived catalyst with a nestlike framework,composed of bamboolike nanotubes that encapsulate cobalt nanoparticles,has been prepared through a facile pyrolysis process.It achieves perfect bifunctional catalysis both in ORR and OER on a single catalyst.For our optimal catalyst Co/M-Chlorella-900,its ORR half-wave potential is positively shifted by 40 mV compared to that of a commercial Pt/C catalyst,and the overpotential at 10mA cm^-2 for the OER is 23 mV lower than that of a commercial IrO2/C catalyst in an alkaline medium.Furthermore,the catalyst displays better durability than those of Pt/C and IrO2/C catalysts.The introduction of Co not only was crucial for the formation of the special primary?CNTs?and secondary?nest-like framework?structures,but also enhanced high degree of graphitizationy.More importantly,the Co nanoparticles encapsulating in CNTs further improved both ORR and OER performance.The introduction of Co and N not only increased the N content but also enlarged the ratio of pyridinic N and graphitic N simultaneously.In addition,Co/M-Chlorella-900 achieves the highest ORR and OER activity when the precursor contained 0.05 mmol Co.?3?A protein cellulose membrane derived doped carbon catalyst with 3D porous framework structure,coupled with CoP nanoparticles has been prepared through a hydrothermal treatment and subsequent carbonization procedures using cobaltous acetate and phytic acid and protein cellulose membrane as precursor.The introduction of Co and P produced CoP nanoparticles with a particle size ranging from 20 to 50 nm.For our optimal catalyst PCM-Co-P,its ORR half-wave potential is positively shifted by 20 mV compared to that of a commercial Pt/C catalyst,and the overpotential at 10 mA cm^-2 for the OER is 32 mV lower than that of a commercial IrO2/C catalyst in an alkaline medium.The catalyst also displays better durability than those of Pt/C and IrO2/C catalysts.The use of phytic acid not only prevents the growth of the cobalt compound particles,but also effectively anchors the CoP nanoparticles on the doped-carbon skeleton.When precursor contained 0.2 mmol Co,PCM-Co-P achieved the highest ORR and OER activity.?4?A pollen biomass derived doped carbon catalyst with 3D porous framework structure,coupled with FeSe nanoparticles has been prepared through pyrolyzing Fe and Se mixed pollens.Po-Fe-Se not only have large pore structure interconnected,but also make the FeSe nanoparticles homogeneously distributed on the doped carbon skeleton.The half-wave potential of our optimal catalyst towards ORR is positively shifted by 30 mV compared to that of commercial Pt/C catalyst,and the overpotential at 10 mA cm^–2 for the OER is even lower40 mV than that of commercial IrO2/C catalyst.Furthermore,the catalyst displays notable durability.Based on the characterization results,we intend to ascribe the enhanced ORR performance to the coexistence of high content of Fe-N_x,pyridinic N and graphitic N,and OER performance to the sufficient FeSe nanoparticles mounted on the doped carbon materials.In addition,the relationship between the composition and performance of the catalyst was studied.It was found that the doped-carbon plays a critical role in promoting the ORR activity and the FeSe nanoparticles make an important contribution to the OER activity.And the coupling effect between the Fe Se nanoparticles and N-doped carbon led to further improvement in electrochemical performance.When the ratios of Fe:Se is 1:2,Po-Fe-Se achieves the highest ORR and OER activity.