Ternary Metal Selenides:Structures,Compositions And Activities Toward Oxygen Reduction Reaction

Platinum(Pt)based catalysts have been mainly used as cathode catalysts for oxygen reduction reaction(ORR)in proton exchange membrane fuel cells due to their high activity.Nevertheless,the high cost of these Pt based catalysts is the primary hindrance to their commercialization.It is necessary to develop non-Pt catalysts with low cost,high activity and high stability.Metal selenide has become an alternative candidate owing to its low cost and high tolerance to small organic molecules.However,the activities of metal selenides are lower than that of commercial Pt/C.To further enhance the ORR activity of binary catalysts,introducing the third element to form ternary catalysts becomes one of the most effective approaches.In this work,a simple microwave route was employed to prepare two binary catalysts,namely,carbon supported cobalt selenide(CoSe2/C)and carbon supported nickel selenide(NiSe2/C).The important parameters were explored to optimize their ORR activities and stabilities.Moreover,two ternary catalysts,i.e.,carbon support cobalt nickel selenide((CoxNiy)Se2/C)and carbon support ruthenium cobalt selenide(Ru(CoSe2)/C),were prepared on the bases of the binary catalysts syntheses through microwave or microwave and chemical reduction method.Accordingly,their structures,compositions,electrocatalytic activities toward ORR and stabilities were systematically investigated.The fuel cell performance of Ru(CoSe2)/C was preliminarily studied.It was found that upon the heat treatment at 400 ?,the phase transition from orthorhombic CoSe2 to cubic CoSe2 took place for the microwave-prepared binary CoSe2/C catalyst.The mean crystallite size was 33.9 nm and the potential corresponding to ORR(EoRR)was 0.68 V.The CoSe2/C-400 ? decayed only 5.7%in EORR after the accelerated degradation tests(ADTs)for 500 cycles.The optimized microwave preparation parameters of NiSe2/C catalyst were:using BP2000 as the carbon supporter,Se/Ni ratio of 3,microwave power of 800 W for 3 min without heat treatment.The NiSe2/C exhibited mainly a cubic structure and the mean crystallite size was 32.1 nm.The EORR reached 0.52 V with no loss in EORR after the ADTs for 1000 cycles.The ORR activity of NiSe2/C was poorer than that of CoSe2/C,however,the stability of NiSe2/C was much better than that of CoSe2/C.Based on the optimized microwave preparation of binary NiSe2/C,the ternary(CoxNiy)Se2/C catalysts were synthesized by introducing different amounts of Co element(x,the ratio of Co over the total metals in the catalyst)into NiSe2/C.A new solid solution-like structure with a main cubic phase was formed.By varying from x=0 to x= 1.0,the mean crystallite sizes of(CoxNi1-x)Se2/C reduced from 32.1 nm to 20.6 nm and the EORR values increased from 0.50 V to 0.68 V which was much better than that of NiSe2/C.In additions,at x=0.75,the ternary(Co0.75Ni0.25)Se2/C also exhibited excellent stability(no loss in EORR after the ADTs for 1000 cycles),which was superior to the binary CoSe2/C.The ternary Ru(CoSe2)/C catalysts were also synthesized by introducing a small amount of Ru(5.75 wt%)based on the microwave preparation of CoSe2/C and followed by chemical reduction without or with the heat treatment.Without the heat treatment,the introduction of Ru into binary CoSe2/C formed a core-satellite like structure,in which the Ru nanocrystallites were randomly dispersed around the orthorhombic CoSe2 nanocrystallites.Upon the heat treatment at 400 ?;Ru could readily enter CoSe2 lattice during the phase transition of CoSe2 and the new cubic(RuxCoy)Se2 like nanocrystallites were predominately formed.High activity and excellent stability were achieved for Ru(CoSe2)/C-400 ? with the significantly enhanced Ru utilization.The half-wave potential was 0.68 V,and the normalized degradation was 2.2%after the ADTs for 1000 cycles.Due to the formation of a new phase with the improved ORR active site and structural stability,the electrocatalytic activity and stability of ternary selenide was better than that of binary selenide.The open circuit potential and the half-wave potential of Ru(CoSe2)/C-HT(0.88 V,0.68 V)was 10.0%and 23.6%higher than that of CoSe2/C-HT(0.80 V,0.55 V)respectively.