In Situ Preparation And Characterization Of Porous Carbon For Catalyst

Pt-based catalysts are the most widely used electrocatalysts in proton exchange membrane fuel cells（PEMFCs）.Currently,commercial porous carbon loaded Pt nano-catalysts are mostly used,whose pore structure distribution of carbon carriers strongly affects the catalyst Pt nanoparticle dispersion,catalyst layer ionomer distribution and cell performance at high current density（HCD）.The<2 nm pore structure of carbon carriers is not favorable for oxygen transport,while the>8 nm structure is not favorable for catalyst layer ionomer distribution,which in turn affects catalytic layer proton transport.The pore range of 4-7 nm porous carbon is most suitable for Pt catalyst activity expression.However,the preparation of carbon carriers with the pore structure of 4-7 nm is not easy to achieve,which is a difficult point to reduce the cost of Pt and improve the catalyst activity.In this paper,Ketjen Black EC300J（KB300）,a commercial porous carbon with high specific surface area and high electrical conductivity,was selected as the raw material.Perchloric acid（HClO₄）was used as the oxidizing agent.A tunable porous carbon with a high specific surface area pore structure was prepared.Carbon-supported platinum catalysts were prepared by ethylene glycol reduction and impregnation methods,and the catalyst activity,oxygen accessibility and proton accessibility were investigated.The specific research progress is described as follows.（1）With situ oxidation technique,HClO₄ was used as a pore-forming agent into the micro mesopores of KB300 to generate oxygen in the pores to react with the carbon surface and produce porous carbon with 4-7 nm pore size structure.The control variable method optimized the amount of porator,heat treatment temperature and nitrogen flow to obtain the optimal reaction conditions for sample HSC-a.The same conditions were treated twice to obtain the secondary treated porous carbon HSC-b.The V_{4-7 nm} in HSC-a was increased by 53%compared with that in KB300,and the V_4-7nm in HSC-b was increased by 78%compared with that in KB300.Meanwhile,both V_{<2 nm} and V_{>8 nm} were reduced,indicating that the in situ oxidation technique can produce accessible porous carbon by adjusting the experimental parameters.（2）Accessible porous carbon-loaded Pt catalysts（Pt/HSC-a-PO and Pt/HSC-a-IW）with HSC-a as the carrier were prepared by ethylene glycol reduction and impregnation method for loading Pt particles,respectively.The reproducibility and actual Pt loading of the two Pt-loaded methods were characterized by using ICP.The Pt/HSC-a-PO and Pt/HSC-a-IW catalysts were analyzed by using TEM.the Pt particles of Pt/HSC-a-PO catalyst were mostly distributed on the surface of the carrier,and the Pt particles of Pt/HSC-a-IW catalyst were mostly distributed inside the pores of the carrier.The Pt particles of the catalysts prepared by the two methods were uniformly distributed.（3）The electrochemical activities of Pt/HSC-a-PO and Pt/HSC-a-IW were analyzed by RDE,and both Pt/HSC-a-PO and Pt/HSC-a-IW catalysts have good electrochemical active area（ECSA）,SA and MA.At the same time,the Pt particles were uniformly dispersed in the two methods,and the smaller particle size also improved the ECSA.（4）The proton accessibility of the catalysts was characterized by Electrochemical Impedance Spectroscopy（EIS）at 50%RH,and the proton conductivity of Pt/HSC-a-PO and Pt/HSC-a-IW were smaller.The proton resistivity of Pt/KB300 catalyst layer was maximum 3 times that of Pt/HSC-a-IW,and the proton resistivity of TEC10E30E catalyst layer was 2 times that of Pt/HSC-a-IW times.At 100%RH,the Pt/HSC-a-PO and Pt/HSC-a-IW proton conductivities are smaller,the Pt/KB300 catalyst layer has a maximum proton resistivity approximately 4 times that of Pt/HSC-a-IW,and the TEC10E30E catalyst layer has a proton resistivity 3 times that of Pt/HSC-a-IW.The tests by EIS showed that both catalysts,Pt/HSC-a-PO and Pt/HSC-a-IW consisted of good proton accessibility when using HSC-a as the carrier.（5）The oxygen gain voltage characterizes the O₂ accessibility of the catalyst.The oxygen gain voltage of Pt/HSC-a-PO increased significantly with the increase of current density,and the oxygen accessibility of the Pt/HSC-a-PO catalytic layer was poor.The oxygen gain voltage of Pt/HSC-a-IW is smaller at both low current density（LCD）and high current density（HCD）,indicating that the Pt/HSC-a-IW catalytic layer has better oxygen accessibility.This is due to the fact that the catalyst Pt particles prepared by the impregnation method are loaded inside the pores,and Nafion has little toxic effect on them,while the accessible pore structure meets the oxygen transport and has oxygen accessibility.