Platinum And Non-Platinum Catalysts Based On Boron Carbide And New Diamond

The platinum supported on a carbon black（Pt/C）catalyst is the most commonly used electrocatalyst in direct methanol fuel cells（DMFC）and has the catalytic activities for the methanol oxidation reaction（MOR）of anode and for the oxygen reduction reaction（ORR）of cathode.However,as a noble metal,Pt has greatly increased the cost of DMFC.At the same time,in the operating environment of DMFC,carbon supports tend to result in the structural collapse and the migration and agglomeration of Pt nanopartricles,which in turn deactivates and reduces the lifetime of Pt/C.Therefore,the preparation of highly stable platinum-and non-platinum-based catalysts presents a key research direction in the field of DMFC.Highly stable Pt-based catalysts are obtained by modifying the support materials and by enhancing their antioxidation capabilities and bonds combined with Pt nanoparticles.Nickel and tungsten carbide supported on new diamond（Ni-WC/nD）was synthesized by using a microwave-assist polyol method that deposited Ni（W）hydroxide（Ni（W）（OH）_x）on carbon black and then heated under N₂ atmosphere at 1300℃.A highly stable Pt-based catalyst called Pt/Ni-WC/nD was obtained by means of the strong antioxidation capability of the new diamond and the strong interaction among Ni,WC,and Pt nanoparticles.Pt/Ni-WC/nD（10 wt%）showed better catalytic activities and stabilities for MOR and ORR compared with 10 wt%Pt/C.Boron carbide（B₄C）was selected as a support material for the Pt-based catalyst because this material mainly comprised covalent bonds.Graphitized B₄C（GB₄C）was obtained by heating B₄C between 1200℃ and 1500℃ for 2 h under a vacuum of 10^-3 Pa.Afterward,a Pt supported on GB₄C heated at 1500℃ was prepared and labelled as Pt/GB₄C-1500.The accelerated degradation test（ADT）revealed that the Pt/GB₄C-1500had an excellent stability that even exceeded that of Pt/C.The catalytic activities and stabilities of the 20 wt%Pt/GB₄C catalyst for MOR and ORR were also better than those of the Pt/C catalyst with the same Pt loading.The N-doped GB₄C（N-GB₄C）catalyst was prepared by heating GB₄C and melamine at 900℃.The N-GB₄C eletrocatalyst for ORR had an onset potential of 870mV under alkaline conditions and a half-wave potential of 40.7 mV,which was slightly lower than that of 20 wt%Pt/C.To further improve the activities of the non-Pt catalyst,GB₄C-supported iron hydroxide was prepared by using the microwave method and then heated with melamine at different temperatures to obtain an iron,nitrogen,and boron co-doped carbon material（Fe-N-B-G）.The Fe-N-B-G eletrocatalyst for ORR had an onset potential of 920 mV under alkaline conditions,while its half-wave potential shifted27 mV right to the 20 wt%Pt/C.Therefore,this electrocatalyst demonstrated better ORR catalytic activities than the 20 wt%Pt/C.After performing the ADT,the catalytic activities of Fe-N-B-G for the ORR were nearly unchanged,thereby indicating the excellent stability of this electrocatalyst.A B-doped titanium dioxide（TiO₂）-modified GB₄C bifunctional electrocatalyst（B-TiO₂/GB₄C）was prepared by using the microwave and heat treatment methods.TiO₂is an efficient photocatalyst that is seldom used in catalyzing ORR.Given that TiO₂reduced the graphitization temperature of B₄C,the latter was decomposed at 700℃ to form B-doped graphite.B was also doped into the structure of TiO₂.The B-TiO₂/GB₄C showed excellent catalytic activities for both ORR and OER compared with noble metal catalysts.