Preparation Of Hierarchical Porous Metal Nanomaterials And Their Oxygen Catalytic Performance

Energy crisis and environmental pollution are becoming more serious than before,which making new energy conversion technologies such as fuel cells and electrolytic water to produce hydrogen are getting more attention.Both the oxygen reduction reaction(ORR)of the fuel cell cathode and the oxygen evolution reaction(OER)of the electrolyzed water anode involve a four-electron transfer process,which limits the whole reaction process due to the lower kinetic rate.At present,the commonly used ORR and OER catalysts are still noble metal catalysts such as Pt?Pd?RuO2?IrO2,etc.The size,morphology and composition of catalyst particles are the key parameters affecting the electrocatalytic activity and stability of the catalytic materials.Recently,nanomaterials with hierarchically porous structure composed of primary structural units such as nanosheets,nanowires,and nanorods have attracted more and more researchers' attention due to their unique structural characteristics,such as more nanopores,larger specific surface area,and three-dimensional interconnect structures.More nanopores can promote the diffusion of target molecules on the electrodes.The larger specific surface area can avoid the use of carbon support,thus eliminating the impact of the corrosion of carbon support on the catalyst performance.At the same time,a large specific surface area can also reduce the use of noble metals.The three-dimensional interconnect structure can effectively reduce the dissolution,migration,aggregation and Ostwald ripening of noble metal nanoparticles.In this paper,small molecule amine reduction method and hard template method were used to prepare metal nanomaterials with fractal multilevel structure,which were used as ORR and OER catalyst respectively in fuel cells and electrolytic water to produce hydrogen.The synthesis method is simple,efficient,green and non-toxic,and has a great application prospect.The main research contents include:(1)Simple and rapid synthesis of high branched Pd nanodandelions(Pd HBDs)catalysts by N,N'-methylenebisacrylamide(MBAA).Pd HBDs is a high dendritic material with hierarchically porous structure composed of 4-5 nm Pd nanowires.In the process of synthesis,MBAA containing amino group and double bond act a coordination agent,reducing agent and morphological guide agent to rapidly synthesize Pd HBDs at room temperature.In alkaline medium,the initial potential of Pd HBDs for oxygen reduction is 1.02 V(vs RHE),and the half-wave potential is 0.863 V(vs RHE),which is much larger than commercial Pd black catalyst.The catalytic process of Pd HBDs is a four-electron transfer pathway,so it has not only good catalytic activity but also high conversion efficiency.Accelerated stability tests in a nitrogen-saturated electrolyte with a half-wave potential decay of only 8.1 mVBy chronoamperometry,the current remained at 77.8%after 20000 s.Both results indicate that Pd HBDs have great electrochemical stability.(2)Porous Pt nanosheets with layered porous structure were prepared by hard template method.The reaction conditions are controlled so that the noble metal precursor forms a coating on the surface of the hard template(NaCl),and then the compound was reduced in a unique gas-solid phase reaction to obtain a loose porous two-dimensional Pt nanosheets.Experiments shown that the ratio of precursor to template has a great influence on the morphology of the catalyst.The electrochemical tests showed that the porous Pt nanosheets greatly improved the catalytic activity and stability of oxygen reduction reaction in alkaline medium.The half-wave potential of the porous Pt nanosheets was 0.896 V(vs RHE)more positive than the 0.882 V(vs RHE)of Pt black.Porous Pt nanosheets have a current density of 5.09 mA cm-2 at 0.9 V(vs RHE),which is 1.8 times that of commercial Pt black.By accelerated stability tests and chronoamperometry tests,we also found that the electrochemical stability and structural stabilityof Pt nanosheets are pretty good.(3)Using NaCl as a hard template,CoCl2 as a precursor,NaH2PO2 as a phosphorus source,NH3 as a reducing gas,and simultaneous reduction and phosphating at 600?,a N,P codoped hierarchically porous Co/CoxMy(M=P,N)Nanosheets was synthesized.Co/CoxMy was applied to the anodic reaction of water electrolysis,and it was found to have excellent oxygen evolution performance.It has low overpotential,high current density,low Tafel slope and good long-term stability,which is better than the comparison sample(Co/CoxPy and Co/CoN)and commercial RuO2 catalyst.It is worth noting that when the current density is 10.0 mA cm-2,the overpotential of Co/CoxMy(M=N,P)is 334 mV,which is far lower than that of RuO2.However,when the potential is 1.65 V(vs RHE),the current density rapidly increases to 100.03 mA cm-2,which is 5.2 times higher than that of commercial RuO2.