Silver-based Oxygen Reduce Electrocatalysts:Performance And Reaction Mechanism

Energy crisis is an important problem that the development of the world has faced since new century.Traditional fossil energy resources are finite,and a large amount of carbon emissions are inevitable during use.The development of new types of clean energy for energiewende is the key to solving energy problems.The electric vehicle market is an important area of the new energy industry.The novel energy devices such as low-temperature hydrogen fuel cells and metal-air batteries are considered as the ideal candidate technologies for powering zero-tailpipe-emission electric vehicles.Oxygen reduction reaction(ORR)is one of the important electrode reactions in these energy devices,and it greatly determines the output power of the device.Currently,Pt-based materials are the state of the art and the most widely used electrocatalysts for ORR.Platinum is a sort of rare metal with limited reserves,and is expensive.The dependence on Pt-based catalysts greatly restricts the development of such new energy devices.Ag is one of the best platinum group metal(PGM)-free catalysts for the ORR in base,and the price of silver is only 1/50 of platinum.Using Ag-based materials instead of Pt can significantly reduce the cost of catalysts.It has broad application prospects in the energy devices(such as:hydroxide exchange membrane fuel cells,metal-air batteries,etc.)operating in alkaline conditions.However,the development of Ag-based ORR catalysts still faces some problems.Firstly,the ORR activities of reported Ag-based catalysts are still lower than that of Pt.Secondly,there is still a lack of a rule to guide the design of highly active Ag-based ORR catalysts.Thirdly,Ag-based nanocatalysts(≤20 nm)is vulnerable to Ostwald Ripening(agglomeration)during the electrochemical reaction process,which can result in a fast loss of the catalytic activity.Fourthly,Ag-based materials have limited oxygen evolution reaction(OER)activity and cannot be used as ORR/OER bi-functional catalysts.Aiming at these problems,in this thesis,we attempt to understand the mechanism of ORR on Ag and to determine the rate-limiting step(RDS).By increasing the oxyphility of the Ag-based catalyst surface,we developed a highperformance Ag-based bimetallic catalyst,and achieved excellent device performance in the actual fuel cell.Further through the optimization of the nanostructure,we improved the stability of Ag-based nanomaterial,a high efficiency and reliability Ag-based ORR catalyst is prepared.In addition,we synthesized a multi-element Ag-based ORR/OER bifunctional nanocatalyst.A rechargeable Zn-air battery(ZAB)is fabricated by using the catalyst as the air electrode.The battery has a high energy efficiency and excellent stability.The specific research contents are as follows:Firstly,we study the ORR activity of Ag and the corresponding H2O2 yield in 50 electrolytes with different pHs.We observed the ORR activity of Ag has been influenced by the anions and pHs in different buffer solutions.We further employed the electrochemical in-situ attenuated total reflection surfaceenhanced infrared absorption spectroscopy(ATR-SEIRS),the*O2-,*OOH and*HOOH intermediates was observed experimentally at the ORR condition.Based on these results,we proposed the ORR mechanism on Ag and determined the RDS.Based on the Butler-Vomer equations,the relationship equation was proposed between H2O2 yield with pH,E.We simulate the H2O2 yield by the relationship equation,and it matches well with the measured curve,demonstrating the proposed mechanism is reasonable.Combining with the ORR mechanism on Ag,we propose that the key to improving the ORR activity of Ag is increasing the O2 adsorption energy(ΔGM-O)on the surface.Secondly,the pristine 3d transition metals(Mn,Co,Fe,Ni,Cu)have a stronger oxyphility than Ag.The oxyphility increases along with the decrease of the atomic number of the 3d transition metals.We further find that cooperation with oxyphilic metal can enhance the ΔGM-O,which accelerated the reaction rate of the first step of ORR on Ag,thus improve the ORR activity of Ag.A series of Ag-M(M:Mn,Fe,Co,Ni,Cu)bimetallic catalysts were synthesized,their ORR activities are found to be higher than pristine Ag,and correlated to the oxygen binding energy of the additional metal.Mn has the strongest oxygen binding in the 3d transition metals,and it provides the highest ORR activity when cooperated with Ag.The Ag-Mn has an ORR E1/2 of 0.893 V(vs.RHE)and its ORR activity is even higher than that of Pt/C in 1 M KOH with calculated the specific activities of the catalysts.Similar HEMFC performance was achieved by using Ag-Mn as the cathode catalysts to that using Pt,the peak power density(PPD)reached 1.84 W cm-2,demonstrating its potential to replace the costly Pt catalyst in HEMFCs.Thirdly,Ag-based nanocatalysts are vulnerable to Ostwald Ripening during the electrochemical reaction process,which can result in a fast attenuation of the catalytic activity.This phenomenon will become more intense as the size(<20 nm)of Ag nanoparticles(NPs)decreases.To solve this problem,we have developed a novel Cu@Ag-Mn core-shell NPs as cathode catalysts for HEMFCs.Compared with Ag,Ag-Mn bimetallic shell has better ORR activity,and the alloying of Cu@Ag significantly improves the stability of Ag-based NPs.The Cu@Ag-Mn/C has highly efficient ORR catalytic activity,the optimal ORR El/2 of 0.853 V(vs.RHE)in 0.1 M KOH.And the catalytic activity remained almost unchanged after 30 K cyclic voltammetry(CV)cycles in accelerated ageing test.Fourthly,multi-elemental nanoparticles(MENPs)provide the possibility to integrate multiple catalytic functions from different elements into one NP.However,it is difficult to synthesize Ag-based MENPs with transition metals like Ni and Fe because of the strong phase segregation between Ag and the other metals.Here,we show that non-metal element P can help the amalgamation of Ag with other metals.Ag-Ni-Fe-P MENPs are successfully synthesized by a solution-phase chemistry,and they demonstrate excellent bi-functional ORR/OER catalytic activities(the potential gap of the potential at 10 mA·cm-2 for OER(E10,OER)and E1/2,ORR is 630 mV).More important,the synergistic effect from the MENPs endow them with even higher ORR or OER activity than the Ag or NiFeP NPs.A rechargeable ZAB is fabricated by using the AgNi-Fe-P MENPs as the air electrode.The battery has an energy efficiency of～60%at 10 mA cm-2.Its performance is almost unchanged during a working period of 250 h,surpassing the Pt/C+IrO2 based battery.