Preparation Of Carbon-based Oxygen Reduction Reaction Electrocatalysts Via Carbonization Of Condutive Polymers

Fuel cells have been recognized as one of the most promising energy conversion devices due to their high power density and environmentally benign nature;however,the performances of fuel cells are largely limited by the slow kinetics of oxygen reduction reaction(ORR)at the cathode.To solve this problem,noble metals have been used as ORR electrocatalysts owing to their high activity;but the high cost,low poison resistance and unsatisfactory durability restrict their widespread applications.To overcome this issue,inexpensive ORR catalysts have been intensively explored,among which heteroatom-doped and conductive carbon materials showed satisfactory electrocatalytic behavior.Although much progress has been made,how to achieve sufficient carbon-based ORR catalysts with a fine control over dopants and architecture is still a huge challenge.In this dissertation,one-dimensional carbon nanotubes(CNTs)and two-dimensional layered double hydroxides(LDHs)have been respectively used as template to induce polymerization of pyrrole,followed by a further pyrolysis treatment to obtain uniformly doped carbon-based electrocatalysts toward ORR.The nanostructure and composition of carbon-based catalysts were studied;their electrocatalytic performance for ORR was investigated and the structure-property correlation was revealed.This work provides a facile synthesis method for the preparation of heteroatom-doped carbon nanomaterial for efficient ORR process,which can be potentially used in fuel cells.The details are as follows:(1)One-dimensional carbon nanotubes(CNTs)induced polymerization of pyrrole followed by pyrolysis to obtain nitrogen-doped carbon electrocatalysts.We report the preparation of N-doped CNT material via a facile two-step route:synthesis of a CNTs@polypyrrole(PPy)composite with a core-shell structure,followed by a further pyrolysis treatment for homogeneous N doping.The resulting product possesses a hierarchical mesoporous structure(mainly centered at 2-3 nm and 30-40 nm).The as-obtained NCNTs exhibit excellent performance in electrocatalytic oxygen reduction with an onset potential of 0.95 V vs.RHE,a diffusion-limited current of 6.82 mA cm-2 and excellent stability in alkaline media,much superior to commercial Pt/C electrocatalyst.Moreover,the content of nitrogen can be tuned easily by changing the coating amount of PPy in the CNTs@PPy core-shell precursors.The influences of graphitization degree and nitrogen content were also investigated.With the increase of graphitization degree,the ORR activity of catalysts is proportional to the summation of pyri-N and grap-N,indicating that both pyridinic N and graphitic N contribute to the ORR activity.A further study on the type of doped nitrogen(pyridinic,and graphitic N)reveals that the pyridinic-N in NCNTs plays a key role in promoting the ORR performances.(2)Two-dimensional layered double hydroxides(LDHs)induced polymerization of pyrrole followed by pyrolysis to obtain metal/heteroatom-doped carbon electrocatalysts.CoAl-LDH nanoplatelets were firstly synthesized by urea method,followed by a surface-induced polymerization of pyrrole;subsequently,a pyrolysis treatment was performed to obtain the final carbon nanosheets with a homogeneous doping.The resulting Co supported and S,N co-doped carbon nanoplates(Co,S,N-CNP)exhibit satisfactory performance in electrocatalytic oxygen reduction with an onset potential of 0.92 V vs.RHE,a diffusion-limited current of 4.68 mA cm-2.In addition,both an extraordinary long-term stability and a strong tolerance against methanol corrosion are also obtained.The effect of LDHs precursor and heteroatoms on the ORR performance of catalysts was further investigated.It is found that the immobilization of Co or Fe on carbon-based catalysts shows enhanced acticity toward oxygen reduction.Compared with the N-doped carbon materials,the introduction of S can improve the electrocatalytic activity and stability.This method can be extended to the synthesis of other metal-loaded and heteroatom-doped carbon catalysts,which would show potential applications in fuel cells and metal-air batteries.