Design And Preperation Of Carbon Nanocomposites Derived From Poly(Amic Acid) For High Efficient Electrocatalysis

As a clean energy source,hydrogen has become an alternative to fossil fuels because of its environmental friendliness and high energy density.Currently,electrolysis of water is a viable method to produce high purity ’green’ hydrogen compared to the conventional methods of methane oxidation and hydrocarbon pyrolysis to produce low purity ’gray’ hydrogen,which requires further purification and carbon emmision.The development of efficient electrocatalysts for hydrogen production remains a great challenge.In this thesis,carbon nanocomposites with different morphologies are prepared by carbonization of polymer precursors and their(hydrogen evolution reaction)HER catalytic properties are investigated.Amine-based monomers of different chemical compositions are polymerized with acid anhydride to form amphiphilic poly(amic acid)(PAA),and then three-dimensional nanoflowers composed of two-dimensional nanosheets are prepared by self-assembly.In addition,the nanoblock is prepared in large quantities by precipitation polymerization.After carbonization,the morphology is inherited due to the unique thermal properties of PAA.Furthermore,carbon materials with different morphologies are used as supports to load ultrafine bimetallic nanoparticles and exhibited good HER catalytic properties.The detailed research contents are as follows:(1)Amphiphilic PAA is prepared by condensation polymerization of p-phenylenediamine and pyromellitic dinahydride anhydride(PMDA),followed by intramolecular cyclization-induced selfassembly method to form three-dimensional nanoflowers composed of two-dimensional nanosheets.And carbon nanoflowers(CNFs)connected by layers of nanosheets are obtained after high temperature carbonization.Furthermore,ultrafine PtCo nanoparticles are deposited on the furface of CNFs to form PtCo/CNFs.The chemical composition,degree of graphitization,morphology and pore structure of PtCo/CNFs are analyzed using X-ray diffraction(XRD),Raman spectroscopy,nitrogen adsorption/desorption,scanning electron microscopy(SEM),transmission electron microscopy(TEM)and X-ray photoelectron spectroscopy techniques.The HER catalytic performance of the catalyst is also tested by disc electrodes and electrochemical workstations.The results show that the specific surface area of PAA nanoflowers increased to 684 m2 g-1 and have a high degree of graphitization after high temperature calcination.The CNFs consisted of carbon nanosheets with homogeneous thickness(15.0 nm±0.5 nm)and are efficiently loaded with ultrafine PtCo bimetallic nanoparticles(1.7±0.5 nm)after the noble metal Pt is used as the catalytic active site and the transition metal Co enhanced the HER catalytic performance of the PtCo bimetallic catalyst through electron transfer.In 1.0 M KOH alkaline electrolyte,PtCo/CNFs shows an overpotential of 27.3 mV at a current density of 10 mA cm-2,which is significantly lower than that of commercial catalyst Pt/C(36.8 mV)and superior to most reported HER electrocatalysts.After normalization,the mass activity of PtCo/CNFs catalysts is 3.1 times higher than that of Pt/C(20 wt%)and comparing the intrinsic catalytic activity,the TOF values are three times higher than the commercial Pt/C(20 wt%)at 100,200 and 300 mV,showing excellent intrinsic activity.After 4000 CV cycles,the overpotential is still lower than that of commercial Pt/C(20 wt%),exhibiting excellent HER catalytic performance and stability.(2)The intramolecular cyclization-induced self-assembly can better control the morphological structure of the assemblies,but it also has the limitation that it is difficult to be prepared on a large scale.Based on this,a precipitation polymerization method is proposed in this section,in which N and S atoms are co-doped in the prepared polymer precursors to produce nanoprecipitates in a simple "onestep" method.The porous nanoassemblies are precipitated from the solution along with the polymerization.And PtNi nanoparticles are loaded on the carbon surface using impregnation method,which is further reduced at high temperature under H2 atmosphere to form a composite catalyst(PtNi/Cs).The chemical composition,porous structure and morphological features are characterized and the HER catalytic performance is also evaluated.The results are as follows:The catalytic performance of PtNi/Cs is better than that of commercial Pt/C(20 wt%)with an overpotential of 26.4 mV and a Tafel slope of 33.9 mV dec-1 at a current density of 10 mA cm-2 in a 1.0 M KOH alkaline electrolyte.And after 4000 cycles,it shows good stability.