Preparation Of High-performance Pd Electrocatalysts And Their Electrochemical Behavior Toward Formic Acid Oxidation

Direct formic acid fuel cells(DFAFCs),have advantages of high energy conversion efficiency,safe and environmental friendliness,holding great promises in applications of the automobile and portable power sources.However,the low activity and stability of the commonly used anode catalyst Pd/C becomes one of the main obstacles restricting the commercialization of DFAFCs.With advances of nanoscience and technology,various routes for designing and preparing high performance catalysts in nanoscales have been developed.The small-sized Pd nanoparticles with uniform distribution could offer high electrochemical active surface area while improve the utilization of Pd.Additionally,nanostructured Pd could also greatly improves the catalytic properties due to its increased number of atoms on edges or corners.Nevertheless,synthesis of high efficient nanostructured Pd catalysts still faces great challenges in both basic and technical issues.Herein,high performance Pd catalysts are designed and prepared from delicately tunng of crystals size,morphology and structure to enhance the catalytic performance.The transmission electron microscopy(TEM),scanning electron microscopy(SEM),X-ray diffraction(XRD)and energy dispersive X-ray spectroscopy(EDS)are operated to investigate the crystal structure and morphology of prepared catalysts.Meanwhile,electrochemical measurements including cyclic voltammetry(CV)and chronoamperometry(i-t)are also tested to analyze the catalytic behaviors.We demonstrate a universal approach of self-assembling PDDA on graphene to prepare uniform and surface-clean Pd nanocrystals by direct CO reduction with an average size of 3.4 nm.The Pd@PDDA-G catalyst presents higher catalytic performance than that of commercial Pd/C catalyst toward formic acid oxidation.In addition,a facile strategy is introduced to synthesize uniform and highly branched Pd nanodendrites with unique mesopores.The tailored Pd nanodendrites display enhanced catalytic activity and stability toward formic acid oxidation in comparison to Pd/C.The enhanced catalytic performance of prepared Pd nanodendrites could be contributed to their increased number of atoms on edges or corners which could greatly improve the catalytic properties,offering some scientific insights into the catalytic process.The whole thesis includes the following five chapters.1.Literature review.The development of fuel cells,and corresponding principle,classification,performance as well as advantages is briefly summarized.In particular,the catalysts of DFAFCs and their reaction mechanism as well as commonly used preparation method are introduced.In addition,the significance and main contents of this thesis are described.2.Experimental Design,material characterization and electrochemical testing techniques.The experimental equipment and main experimental reagents used in this thesis are summarized.The main physical characterization techniques and electrochemical measurements of prepared catalysts are introduced to investigate their structure and electrocatalytic behaviors,respectively.Additionally,preparation process of working electrode is also depicted.3.Self-assembling PDDA on graphene to surfactant-free synthesize uniform and ultra-small Pd nanocrystals by direct CO reduction for highly efficient electrocatalyst toward formic acid oxidation.Graphene has large specific surface area,superior electrical conductivity and excellent chemical stability.We demonstrate a universal approach of self-assembling PDDA on graphene to prepare uniform and surface-clean Pd nanocrystals by direct CO reduction with an average size of 3.4 nm,and the electrocatalytic behaviors of prepared Pd@PDDA-G towards formic acid oxidation is investigated.Comparing with Pd@G and commercial Pd/C,Pd@PDDA-G catalyst exhibits higher electrocatalytic activity and standard exchange current density,lower Rct as well as better stability,holding great potential as a promising anode catalyst for DFAFCs.The enhancement mechanism could be contributed to synergetic effect between graphene and small-sized Pd nanocrystal,of which graphene has excellent electrical conductivity and porous structure,while Pd nanocrystals present clean surface and uniform dispersion.This work demonstrates that Pd@PDDA-G catalyst is more suitable for using as formic acid oxidation catalyst than commercial Pd/C catalyst while offering some scientific insights into the electrocatysis of formic acid oxidation on a nanostructured electrode.4.Facile synthesis of highly branched Pd nanodendrites with enhanced electrocatalytic performance toward formic acid oxidation.A facile strategy was introduced to synthesize uniform and highly branched Pd nanodendrites(Pd-NDs)with unique mesopores and rough surface,and electrocatalytic behaviors of prepared Pd-NDs towards formic acid oxidation are investigated.It is found that the volume ratio of benzyl alcohol to formic acid plays an important role in the formation of unique nanostructured Pd-NDs,and the concentration of PVP is a significant factor to regulate the crystal size of Pd-NDs.Toward formic acid,the Pd-NDs/C catalyst exhibits higher peak current density,catalytic activity and better stability than Pd/C catalyst.Since this facile synthetic approach involves low-toxicity,it could be extended to prepare other metal-based catalysts for broader energy conversion/storage applications.5.Conclusion and work prospects.Conclusions are made and future works are also proposed.